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Effectiveness of sand media filters for removing turbidity and recovering dissolved oxygen from a reclaimed effluent used for micro-irrigation
Agricultural Water Management 111 (2012) 27–33
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Effectiveness of sand media filters for removing turbidity and recovering dissolved oxygen from a reclaimed effluent used for micro-irrigation M. Elbana, F. Ramírez de Cartagena, J. Puig-Bargués ∗ Department of Chemical and Agricultural Engineering and Technology, University of Girona, Carrer de Maria Aurèlia Capmany 61, 17071 Girona, Spain
a r t i c l e
i n f o
Article history: Received 16 January 2012 Accepted 24 April 2012 Available online 20 May 2012 Keywords: Filtration Removal efficiency Clogging Filter ripening Backwashing Drip irrigation
a b s t r a c t Sand media filters are among the most common filters used in micro-irrigation systems, especially for filtering waters with large amounts of organic contaminants like reclaimed effluents. An experiment was conducted for 1620 h between August 2007 and September 2008 using a reclaimed effluent to evaluate the efficiency of sand filters with sand effective diameters of 0.32, 0.47, 0.63 and 0.64 mm in decreasing turbidity and improving dissolved oxygen concentration. In addition, this study strived to determine the filter ripening period (i.e. the time after backwashing when the filtered effluent has the lowest quality) and the effect of filter backwashing on filtration efficiency. Depending on the sand effective size, the sand filter achieved turbidity reductions of between 59.6 and 85.4% and dissolved oxygen recoveries from 4.5 to 15.7%. During the experiment the filter ripening period was 15 min. Overall, the results support the idea that a daily backwashing is a good maintenance practice since it reduces inefficient backwashings and increases dissolved oxygen, which is interesting when hypoxic water is used for irrigation. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In micro-irrigation systems, emitter and filter clogging are two substantial problems, especially when treated wastewater of poor quality is used since it contains high concentrations of dissolved and suspended solids (Bucks et al., 1979; Ravina et al., 1997). Sand media filters are among the most common filter types used in micro-irrigation systems, especially when irrigating with effluents (Burt and Styles, 2007). They are always recommended when algae (Naghavi and Malone, 1986) or other organic contaminants are present (Haman et al., 1994). In addition, sand media filters can guarantee the best emitter performance with poorly treated wastewater (Ravina et al., 1997; Capra and Scicolone, 2007). The media grain size, which affects the efficiency of filtration (AWWA, 2003), is considered to be an indicator of the size of the particles that could be removed by the media (Haman et al., 1994). The finer the sand, the smaller the particle size removed, and the better the quality of the filtration process, although a smaller size requires more frequent cleaning (Pitts et al., 1990; Haman et al., 1994; Phillips, 1995). Sand effective diameter (de ) and its uniformity coefficient (UCs ) are two indicators of the sand filter grain size. The sand effective diameter is defined as the size of the screen opening which will allow 10% of the total sand sample mass to pass (Haman et al.,
∗ Corresponding author. Tel.: +34 972 41 84 59; fax: +34 972 41 83 99. E-mail address: [email protected] (J. Puig-Bargués). 0378-3774/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agwat.2012.04.010
1994). The UCs is the ratio between the screen pores that let pass 60 and 10% of the sand. A uniform sand media has a low UCs value while a good graded media is characterized by a high UCs value (Burt and Styles, 2007). For irrigation sand filters, a UCs of 1.5 is recommended by Haman et al. (1994), although other researchers suggested working with lower UCs (Burt and Styles, 2007). Suspended materials trapped by the filter decrease the water flow rate across the filter and eventually the sand media filter must be cleaned by backwashing, which is a critical part of media filter operation and performance (Nakayama et al., 2007). Automatic backwashing can be controlled by time and/or by head loss across the filter. However, when sand media filters are not flushed frequently enough, large and interconnected pores called “rat holes” can form in the sand media and decrease the filter performance (Nakayama et al., 2007). When this happens, head loss across the filter is low and cannot be used as a guide for backwashing. Therefore, filters should be backwashed frequently (Pitts et al., 1990). Enciso-Medina et al. (2011) pointed out that inadequate sand filter backwashing intervals and durations caused poor performance in subsurface drip irrigation systems. The time span that an effluent of degraded water quality passes through a filter immediately after being backwashed is called filter ripening. Amirtharajah (1985) showed that more than 90% of the particles passing through a well-operating filter did so during the ripening period. Thus, filter ripening is a period during which emitter clogging is likely to occur as filter protection is not as effective as it should be. This ripening period has been studied in depth. Scientists have focused primarily on effluent turbidity (Amirtharajah,
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1985; Amburgey and Amirtharajah, 2005) and on particle size distribution (Darby and Lawler, 1990) as principal indicators of filter ripening. Nevertheless, the concentration of dissolved oxygen (DO) in irrigation water, which can be an important limiting factor, especially in intensive agriculture systems (Raviv et al., 2004; Bhattarai et al., 2005; Marfà et al., 2005), has not been studied to date in a filtered effluent during the ripening period. The shortage of DO in the irrigation water can lead to root oxygen deficiency, which results in agronomic problems such as crop stress, slow plant growth, low yields (Bhattarai et al., 2008) and increase in disease (Chérif et al., 1997). The present study focuses on investigating the effect of sand media filter backwashing on effluent turbidity and DO during the ripening period. In addition, it studies the influence of sand effective diameter on the filtration efficiency.
which has a negative impact on filtrate turbidity and on filter run time (Cleasby, 1990). Efficient and inefficient backwashings were identified in accordance with the following head loss thresholds across the filter after a backwashing was carried out: 20–40 kPa for efficient backwashings and greater than 40 kPa for the inefficient ones. These thresholds agree with the 28–40 kPa range pointed out by Duran-Ros (2008) for clean media filters. 2.2. Applied effluent A reclaimed effluent from the wastewater treatment plant (WWTP) in the municipality of Celrà (Province of Girona, Catalonia, Spain) was applied. This WWTP biologically treats the urban and industrial wastewater of the municipality to primarily remove nitrogen and phosphorus (ACA, 2010). The effluent was obtained by filtering the sludge through a disc filter with a 130 m filtration level and treatment by ultraviolet radiation.
2. Material and methods 2.3. Sand media characteristics 2.1. Filter installation and operation A sand filtration unit consisting of two sand filters (Regaber, Parets del Vallès, Spain) in parallel was installed. Each sand filter had an internal diameter of 0.508 m, a filtration surface of 1963 cm2 and a mean height of 0.50 m. Both sand filters were filled with 175 kg of sand as a single filtration layer. Two MBS 4010 (Danfoss, Nordborg, Denmark) pressure transmitters with flush diaphragms measured the pressure at the inlet and outlet of the sand filtration unit, respectively, and allowed determination of the head loss. The flow at the filtration unit outlet was measured by an MP-400-CB (Comaquinsa, Llinars del Vallès, Spain) electromagnetic flowmeter. These devices were connected to a supervisory control and data acquisition (SCADA) system, which allowed for filter scheduling and the collection of filter performance data every minute (DuranRos et al., 2008). The average registered sand filter surface flow rate (i.e. effluent flow rate divided by filter surface area) during the experiment was between 13.81 and 15.44 l s−1 m−2 . This rate was within the range 10.17–16.94 l s−1 m−2 recommended by Pitts et al. (1990), Haman et al. (1994) and Phillips (1995) and less than the average of 17 l s−1 m−2 (Abbott, 1985) necessary to avoid the formation of channels in the sand media bed or the hydraulic movement of contaminants through it. The filter operation time varied between 6 and 12 h per day, with minor interruptions of a few days primarily due to system maintenance and operational problems such as a lack of effluent or reclaimed effluent, a power failure after a thunderstorm and a breakdown of some sensors. The experiment lasted 1620 h, divided into three filtration periods of 540 h each. The first period began on August 3 and ended on December 7, 2007; the second one lasted from March 11 to May 26, 2008; and the third one from June 30 to September 8, 2008. The sand media was changed only once during the experiment – before starting the third period (after 1080 h of operation) – as recommended by the manufacturer. Filters were backwashed various times at the beginning of the experiment and after changing the sand media (operation hour 1080) to get rid of the finest particles, which could cause future clogging problems. Furthermore, filters were automatically backwashed for 90 s when the head loss across them reached 50 kPa, as recommended by Ravina et al. (1997). The number of efficient and inefficient backwashing cycles was classified on the basis of the filter capacity to recover the pressure loss across it. While efficient backwashings remove most of the trapped particles in the sand media, allowing the normal development of the subsequent filter run, inefficient backwashings do not release most of the particles retained (Duran-Ros et al., 2009a). Filters with inefficient backwashings tend to accumulate aggregates of the suspended matter,
The sand effective diameter (de ) and its uniformity coefficient (UCs ) were determined at the beginning of the first filtration period, at the end of the second one and before the third period started and again after it ended. Sand effective diameter was determined through homogeneous and representative sand samples. Four homogeneous and representative 250 g samples were taken at each sampling event, dried and sieved through twelve stainless steel screens ranging from 0.06 to 1.20 mm pores. Then, each screen was weighed to determine the retained sand and the particle size distribution curve, which allowed the computation of de and UCs . 2.4. Assessment of filter efficiency Measurements of pH, temperature, DO, turbidity and electrical conductivity (EC) at the filter inlet were performed using Endress + Hauser (Nesselwang, Germany) sensors (Orbisint CPS11D, OxyMax W COS61, TuriMax W CUS 31 and ConduMax W CLS 21, respectively) and transmitters (CPM253, COM253, CUM253 and CLM253, respectively). At the filter outlet, only DO and turbidity were monitored as the other parameters were not expected to be different. The same type of sensors and transmitters installed at the filter inlet were used. Only the turbidity sensor needed to be washed periodically with potable water. This was done either with a SCADA system or manually. When the automatic washing did not remove some particles from the sensor, the turbidity meter was washed manually with potable water. The effectiveness of a filter (removal efficiency) was computed as a measure of the ability of the sand media filter to remove suspended particles from the inlet effluent. Filter removal efficiency was determined for both effluent turbidity and DO using the formula: y − yo REy = i × 100 (1) yi where REy is the removal efficiency for the physical parameter with y being turbidity or DO, yi being the physical parameter value before filtering and yo its value after filtration. 2.5. Data treatment and statistical analysis Filter run time, filtered flow, inlet and outlet filter pressure, inlet and outlet effluent parameters, number of automatic backwashings and backwashing water volume were recorded every minute by a SCADA system previously developed (Duran-Ros et al., 2008) and adapted to this experiment. The collected data were used to compute the average inlet and outlet turbidity and DO for each 10 h
M. Elbana et al. / Agricultural Water Management 111 (2012) 27–33 Table 1 Sand effective diameter (de ) and uniformity coefficient (UCs ) during the experiment. Period
Accumulated operation hours of sand media
de , mm
UCs
Before the 1st irrigation period After the 2nd irrigation period Before the 3rd irrigation period After the 3rd irrigation period
0 1080 0 540
0.47 0.64 0.32 0.63
1.81 1.40 3.17 1.73
of operation (i.e. average from 600 values). Removal turbidity and DO efficiencies were calculated using Eq. (1) for each 10 h of operation. Head loss analysed after the backwashings allowed them to be classified as efficient or inefficient, as detailed previously in Section 2.1. The effects of de and of filter backwashing on filter efficiency over time were also studied. However, for these purposes, filter runs shorter than 240 min were not considered in order to avoid repetitions of outlet parameter data before filter backwashing occurred. Thus, only the data from 67 filter runs were used to compute RETurbidity and REDO 10 min before and 10, 15, 30, 45, 60 and 120 min after the backwashing took place. To compare average inlet and outlet turbidity and DO for the different filtration periods, Duncan’s multiple range test was applied. The effect of de and time before and after backwashings on RETurbidity and REDO was analysed using a multivariate general linear model and Duncan’s pairwise comparison to identify means that were different. Statistical analysis was carried out using the SPSS statistical program (SPSS Inc., Chicago, Illinois, USA) and results were checked at a significance level of 0.05. 3. Results and discussion 3.1. Sand filter characteristics The sand media effective diameter and its uniformity coefficient are shown in Table 1. The UCs were slightly greater (1.81 before the first period and 1.73 after the third one) or smaller (1.40 after the second irrigation period) than 1.5, as recommended by Haman et al. (1994) and Burt and Styles (2007) for micro-irrigation sand media filters. At the end of the second irrigation period, and after 1080 h of operation, the sand was changed following the manufacturer’s recommendation. The new sand raised the UCs to 3.17, which was outside the recommendations for irrigation sand filters, but within USEPA (2002) recommendation (UCs < 4) for intermittent sand filters used for wastewater treatment. The larger the UCs , the less 100
uniform the sand, and the smaller sand particles can fill the spaces between the larger particles, making filter clogging easier at higher loading rates (Darby et al., 1996). During the operation time the UCs decreased while the sand effective diameter increased, which coincides with the results of Duran-Ros (2008), who working with a similar reclaimed effluent found after 1000 h of operation a decrease in UCs from 2.39 to 1.65 and an increase in de from 0.27 to 0.52 mm. The sand particle size distribution curves (Fig. 1) show that there were less fine sand grains at the end of the analysed filtration periods. Thus, while the screen size opening where 60% of the sand sample passes changed slightly (from 0.85 to 0.90 mm and from 1.10 to 1.01 mm in both periods considered), the screen size opening with 10% throughfall (effective sand size) increased with 0.17 and 0.31 mm, respectively, at the end of these periods. Sand particles smaller than 0.63 mm were reduced by 21.6 and 47.0% at the end of the second and third filtration period, respectively, whereas sand particles between 0.75 and 2 mm were only reduced by 6.2 and 41.9%. These particles were lost during the initial backwashings carried out when new sand was used and also during the normal filter operation and backwashing. The most problematic particles are those with diameters close to the narrowest openings of the emitter, which are usually in the 0.7–1.0 mm range, because they can clog these emitters. Gilbert et al. (1982) recommended installing screen filters after sand media filters to remove suspended solids released from them. This could help to prevent the fine sand media particles from escaping to the lateral pipes and causing emitter clogging problems during the irrigation. In our case, as the maximum sand grains are released in the initial backwashings when new sand is used, the backwashing water was diverted and not used for irrigation. However, as some sand particles can accidentally be introduced in the drip lines, dripline flushing need to be carried out periodically to remove them and help to reduce clogging (Puig-Bargués et al., 2010a,b). 3.2. Effect of sand filter on effluent quality The average and standard deviation of the effluent relevant parameters for each filtration period at both the filter inlet and outlet and the significant difference between them are presented in Table 2. The sand filter media significantly increased (P < 0.05) the concentration of dissolved oxygen in the outlet effluent during the first and third filtration periods, with an average increase of 0.15 and 0.39 g m−3 respectively. Maestre-Valero and Martínez-Álvarez (2010), studying the DO in a drip irrigation system using hypoxic 100
a
b
90
80
Accumulated sand mass, %
Accumulated sand mass, %
90
70 60 Before Aer
50 40
80 70 60 50
30
20
20
10
10 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Screen pore diameter, mm
1.6
1.8
2.0
Before Aer
40
30
0
29
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Screen pore diameter, mm
Fig. 1. Particle size distribution of sand media (a) before the first filtration period and after the second one, and (b) before and after the third filtration period.
2.0
30
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Table 2 Average and standard deviation of effluent parameters for each filtration period at both filter inlet and outlet registered by the SCADA system. Different letters show significant differences (P < 0.05) between inlet and outlet parameters for the same filtration period (N = 54 for each period).
Parameter −1
Flow rate, l s DO, g m−3 Turbidity, FNU EC, dS m−1 pH Temperature, ◦ C
1st filtration period (0–540 h)
2nd filtration period (540–1080 h)
3rd filtration period (1080–1620 h)
Inlet
Outlet
Inlet
Outlet
Inlet
4.15 ± 0.57a 3.18 ± 2.00b – – –
2.73 3.40 9.90 4.80 7.96 18.2
3.37 ± 1.05a 3.73 ± 3.80b – – –
2.71 3.01 8.63 4.54 – 27.0
3.03 4.00 10.80 5.63 7.33 21.1
± ± ± ± ± ±
0.20 0.55b 8.17a 0.98 0.98 3.4
water from a covered agricultural reservoir (DO < 1.08 g m−3 ), found DO increases of 0.25 g m−3 after pumping and 1.56 and 3.15 g m−3 through a laminar and a turbulent emitter, respectively. Because the sand irrigation filter is closed and pressurized, it does not allow contact with oxygen as emitters do. MaestreValero and Martínez-Álvarez (2010) attributed the DO increase in the pumping to minor imperfections that result in air intrusions. Although this could explain some of the DO increase in the sand filter, the main reason is related to backwashing. Backwashing removes organic particles retained in the filter, which allow microbial growth and reduce DO concentrations. As sand micro-irrigation filters are mainly used to remove those particles that could clog the emitters, they are not specifically designed for water treatment, and do not achieve important DO recoveries or reductions of other related parameters – such as chemical and biochemical oxygen demand – that other equipment like intermittent sand filters (USEPA, 2002) do. On the contrary, there was an average DO reduction of 0.03 g m−3 in the second filtration period, but this difference was not statistically significant (P = 0.780). In this period, the water temperature was lower than in the other periods. Although cold water holds more oxygen than warm water, DO inlet values were not the highest in this filtration period because effluent parameters depended on raw wastewater characteristics and WWTP operation efficiency at that moment. The differences in DO removal are attributed to bad filter performance because the sand media was at the end of its useful life, according to the manufacturer. The filter also achieved a drop in turbidity removal in this second period, supporting the idea of a worst sand media performance between 540 and 1080 h of operation. On the other hand, there was a significant decrease in effluent turbidity after filtration (P < 0.05) for the three different periods, achieving average turbidity removals of 70.6%, 62.3% and 75.6% through the three filtration periods. This indicates the effectiveness of the sand filtration unit in reducing the total suspended matter in the outlet effluent.
± ± ± ± ± ±
0.13 0.89a 6.60a 1.04 0.26 2.06
0.22 1.02b 7.71a 0.55
± 1.1
3.40 ± 0.95a 2.11 ± 1.82b – – –
100 a 80 0.32
0.47
0.64 b
b
-10
RETurbidity , %
10
REDO, %
± ± ± ±
The average and standard error of DO and turbidity removal efficiency for different sand effective diameters (de ) is illustrated in Fig. 2. There were missed data for the 0.63 mm de after 1270 h of filtration due to a DO sensor failure. Therefore the removal efficiency for DO was examined with the determined de only at the start of the experiment, after the end of the second filtration period and before the start of the third one. The DO removal efficiency with de of 0.32 mm was statistically different (P < 0.05) from those achieved with de of 0.47 and 0.64 mm, which were not significantly different from each other (Fig. 2). The negative removal efficiency for effluent DO, which is in fact a DO recovery, was higher for the smallest de (0.32 mm) than for the other two de of 0.47 and 0.64 mm. In general, DO removal efficiencies increase, hence DO recoveries decrease with larger de (Ives, 1980). Thus, the smaller the sand effective diameter, the higher the concentration of DO in the filtered effluent. This is favourable for the plant. A low DO concentration in irrigation water could have critical consequences because it causes root oxygen deficiency, which in turn can result in agronomic problems (Maestre-Valero and Martínez-Álvarez, 2010), but a well-designed drip irrigation system can oxygenate the water so that it does not reach critical thresholds (Morard, 1995). On the other hand, the four different sand effective diameters were efficient in removing most of the turbidity from the applied effluent (Fig. 2). The de of 0.63 mm achieved the highest removal efficiency for effluent turbidity (85%), but even the 0.64 mm de achieved a high efficiency (65%). However, as discussed previously, besides the effect of de , the sand media performance can worsen at the end of its useful life, as it happened with 0.64 mm de . Although Naghavi and Malone (1986) pointed out that effluent quality deteriorated when sand with a median grain size diameter of 0.34 mm is used for the filtration bed, our study did not show a significant difference in turbidity removal efficiencies between sand effective diameters of 0.32, 0.47 and 0.64 mm. Removal efficiencies for turbidity (60–85%) were consistent with the results of previous studies. Nakhla and Farooq (2003)
20
0
Outlet
60
b
b
b
40 20
-20
a
d e, mm
0
0.32
0.47
0.63
0.64
de , mm Fig. 2. Average and standard error of DO (REDO ) (N = 19, 52 and 56) and turbidity (RETurbidity ) (N = 27, 52, 56 and 27) removal efficiencies for each 10 h of operation. Different letters mean significant differences (P ≤ 0.05) among sand effective diameters.
M. Elbana et al. / Agricultural Water Management 111 (2012) 27–33
3.3. Effectiveness of the backwashing processes The number of efficient and inefficient backwashings, the volume and percentage of filtered effluent and the volume of water consumed during backwashing for each period are given in Table 3. The total number of backwashing cycles was almost doubled, from 105 during the first filtration period to 202 during the second one. In the third filtration period, with sand of smaller de and greater UCs , the number of backwashings rose to 372. As the backwashing water consumption (determined as the ratio between filtered water consumed for backwashing and the total filtered water consumed for both backwashing and irrigating) is dependent on the number of backwashings carried out (Duran-Ros et al., 2009b), it increased from 1.3% for the first period to 5.7% for the third. This agrees with the results of Duran-Ros et al. (2009b), who observed an increase from 1.1 to 5.0% in backwashing water consumption when the number of filter runs increased from 219 to 645 using a sand filtration media. The 1.3% backwashing water consumption during the first period (de = 0.47 mm) was similar to that observed by Duran-Ros et al. (2009b) with sand with de = 0.40 mm (1.1%), but smaller than the 3.0% observed by Tajrishy et al. (1994) with de = 0.45 mm. However, backwashing water consumptions, except for the first filtration period, were greater than 0.5–1.5% observed by Ravina et al. (1997), but the effective diameter was 1.0 mm, which was greater than in the present work (Table 1). The larger de , the faster the water moves through the sand and the more water can be filtered, which means less backwashings are needed. Backwashing frequency does not only depend on sand media characteristics. It is also greatly influenced by filtering water characteristics, mainly particle load and filtration flow (Capra and Scicolone, 2007). Fig. 3 shows the inlet effluent quality presented as percentage of the operation time for intervals of turbidity of 10 FNU during the three filtration periods. During the second filtration period, the percentage of time when turbidity was ≥50 FNU was 7%, which was greater than the first one (5%) and the third one (2%). This means that during the second period, the inlet effluent experienced temporally high turbidity events, related to high suspended solid loads, that easily clog the filter. In addition, the average volume of filtered effluent per filter run was higher during the first (50.1 m3 ) filtration period than during the second (18.3 m3 ) (Table 3). Thus, the increase in filter backwashings during the second period was primarily due to the sand media being clogged by trapped particles
Third Period (1080 - 1620 h)
Turbidity, FNU
50
Second Period (540 - 108 0 h) First period (0 - 540 h)
40 - 50 30 - 40 20 - 30 10 - 20 0 - 10 0
20
40
60
80
10 0
Percentage of oper aon me Fig. 3. Percentage of operation time for each interval of the inlet effluent turbidity during the three filtration periods.
which were observed while changing the sand media after the end of the second filtration period. Despite the inlet effluent having the lowest turbidity values higher than 10 FNU (Fig. 3), the number of backwashings was the highest during the third filtration period, the main cause being that de was smaller (0.32 mm) at the beginning of the third than during the first and second filtration period (Table 1). According to Haman et al. (1994), the smaller the effective diameter, the higher the clogging possibility because media filters are not able to remove more particles and, consequently, both the total number of backwashing cycles and the volume of water consumed for backwashing increase. On the other hand, the number of inefficient backwashings decreased through the experiment from 39.0% (first period) to 7.8% (third one). After an inefficient backwashing, head loss values across the filter are higher than normal, so the filter run will be shorter. An inefficient backwashing shows that the filter cleaning was not done properly. The number of inefficient backwashings increased with the average volume of filtered effluent. Thus, in the first period, the average filtered effluent volume (i.e. between two backwashings) was 50.1 m3 /cycle and 39% of the backwashings were inefficient, while in the third filtration period the average filtered effluent volume was 10.1 m3 /cycle and the inefficient backwashings were only 7.8%. When more water is filtered, more particles are trapped within the sand filter and their release by backwashing is made more difficult. The results support the recommendation of Burt and Styles (2007) of carrying out at least a daily backwashing, which has been verified as a practice that helps to assure good emitter performance (Enciso-Medina et al., 2011). 3.4. Effect of filter backwashing on filtration efficiency The statistically significant differences between the mean reduction in effluent turbidity (RETurbidity ) and DO (REDO ) removal efficiency 10 min before and 10, 15, 30, 45, 60 and 120 min after filter backwashing are shown in Figs. 4 and 5, respectively. Results
70
a
66
RETurbidity , %
reported a 33–56% turbidity removal efficiency using coarse sand filters (de = 0.50 mm) and a 40–62% removal efficiency for turbidity with fine media sand filters (de = 0.30 mm) when using an effluent with a turbidity of 0.20–0.95 formazin nephelometric units (FNU). Duran-Ros et al. (2009b), who used two reclaimed effluents with inlet turbidity of 6.76 and 4.08 FNU, respectively, observed a turbidity reduction of 57.6 and 66.4% when filtering with de of 0.40 and 0.27 mm, respectively. Total suspended solids (TSS) removal, which is highly correlated with turbidity, was studied among others by Tebbutt (1971) and Ives (1980), who found a better improvement in TSS removal with finer media, i.e. smaller de . Nevertheless, Naghavi and Malone (1986), who studied algal removal by fine sand/silt filtration with five fine median sand sizes that varied from 0.064 to 0.335 mm, found that increasing the median grain size diameter from 0.064 to 0.200 mm did not have a significant effect on the effluent quality. They also found that the effluent quality deteriorated when sand with a median grain size diameter of 0.335 mm was used for the filtration bed. Conversely to those results, Adin and Elimelech (1989) found that removal efficiency of TSS was increased with greater sand effective diameters. However, they studied three sand media filters with higher de (0.70, 0.84 and 1.20 mm), than those used by Nakhla and Farooq (2003), Duran-Ros et al. (2009b) and in the current study.
31
62 58
ab
ab
ab
ab
ab b
54 50 46
Fig. 4. Average and standard error for turbidity removal efficiency (RETurbidity ) at 10 min before backwashing and 10, 15, 30, 45, 60 and 120 min after it (N = 67). Different letters show significant differences (P ≤ 0.05).
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Table 3 Types of backwashings, backwashing water consumption, average filtered effluent per filter run and total filtered effluent for each filtration period. Parameter
1st filtration period (0–540 h)
2nd filtration period (540–1080 h)
3rd filtration period (1080–1620 h)
Total number of backwashings Number of efficient backwashings Inefficient backwashings, % Backwashing water consumption, % Average filtered effluent per filter run, m3 Total filtered effluent, m3
105 64 39.0 1.3 50.1 5836.9
202 158 21.8 3.1 18.3 5704.5
372 343 7.8 5.7 10.1 5484.7
were used to determine the duration of filter ripening and whether there was a negative impact on RETurbidity and REDO during it. The removal efficiency of effluent turbidity decreased significantly from 63.5% at 10 min before backwashing to 54.2% at 10 min after it (Fig. 4). This means that during the first 10 min after backwashing the filter did not retain as many solids as the almost clogged filter. Actually, the filtration process becomes more efficient with the passage of time because smaller particles can be filtered out as the flow passages become smaller (Nakayama et al., 2007) due to the trapped particles. Thus, at 15 min after backwashing, the filter regained its capacity to remove an average of 60.6% of the suspended solids from the filtered effluent. This coincided with what the USEPA (1998) pointed out, that effluent quality attains normal values within 15 min after filter backwashing. These results demonstrate that filter ripening during the experimental time did not exceed 15 min. There was no significant difference between RETurbidity during the studied times of 15, 30, 45, 60 and 120 min after backwashing. Thus, a good management practice would consist of only using filtered effluent for micro-irrigation at least 15 min after a good filter backwashing is carried out. However, each particular effluent characteristic should be considered because, depending on the type of particles present, solid detachment from the filter cake (Adin and Alon, 1986) or different size particle removal (Puig-Bargués et al., 2005) can occur. On the other hand, the negative removal or recovery efficiency for effluent DO values (Fig. 5) increased significantly (P ≤ 0.05) after filter backwashing. DO recovery was of 2.32% before backwashing and increased to an average of 6.36% in the 5–120 min studied time interval after backwashing. There were no significant differences between REDO at the different investigated times after the backwashing. Although no other studies exist in the literature dealing with DO evolution in sand media filters for irrigation, the research carried out in wastewater treatment processes is highly dependent on aerobic or anaerobic conditions (Tanwar et al., 2008) and is not directly comparable to our study. Available organic nutrients in sand filters promote microbial activity (Nakayama et al., 2007), which tends to lower the levels of DO within the filter (Bouwer and Zehnder, 1993). In our experiment, microbial activity in the filter was not exacerbated since the reclaimed effluent did not have an important organic pollutant load and the residence time was low (<2 min). Belkin et al. (2012) observed that, after backwashing in multi-layered rapid sand filters, DO levels increased from 5 to 11 h, but after 42 h they decreased again. Sand filter runs for a
REDO, %
20 10 0 -10
b a
a
a
a
a
a
-20
Fig. 5. Average and standard error for DO removal efficiency (REDO ) at 10 min before backwashing and 10, 15, 30, 45, 60 and 120 min after it (N = 67). Different letters show significant differences (P ≤ 0.05).
micro-irrigation system are shorter but the DO evolution pattern seems to be similar. Such DO recovery corresponds to a decrease in bacterial respiration activity due to the depletion of organic nutrients (Tanwar et al., 2008; Belkin et al., 2012) caused by backwashing. Amirtharajah (1985) found that the initial degradation of effluent quality is due to the backwash water remnants within the media and the backwash water remaining in the filter. As backwashing causes turbulence in the sand, some of the finer sand particles of the filter escape with the effluent during the backwashing and cause the noticed increase in effluent turbidity after backwashing (Pitts et al., 1990). This also explains the increase of sand effective diameter during filter operation. According to Nakayama et al. (2007), these fine particles may travel through the filters as individual particles, but then flocculate or become attached to organic residues and eventually become large enough to clog emitters. After backwashing, the filter was clear and apparently no particles were trapped. Retained particles could have helped to improve the removal efficiency of particles, resulting in less outlet turbidity but, on the other hand, they would have contributed to microbial activity, thereby reducing DO recovery, as observed before backwashing was carried out. Besides, inefficient backwashings that cause short filter runs are reduced when more backwashings are carried out because the filter cleaning is easier and could be more effective. 4. Conclusions The presented work aimed to determine the behaviour of a sand media filter during the filtration process and the influence of its backwashing process on the filtration efficiency during the ripening period. It also strived to determine the effect of sand effective diameter on the filtered effluent quality. Smaller sand media particles are released during automatic backwashing and filter operation. Accurate filter management is needed, especially if finer particles have a size that can clog drip irrigation emitters. It is highly recommended to conduct several initial backwashings when new sand media are used and to divert the initial backwashing water. The sand media filter improved the filtered effluent values of turbidity and dissolved oxygen. Effluent turbidity was reduced around 60% with effective sand diameters of 0.32, 0.47 and 0.64 mm, and about 85% when the effective sand diameter was 0.63 mm. Recovery of dissolved oxygen in the sand filter was quite moderate, having a maximum value of 15.7% for a sand effective diameter of 0.32 mm. However, DO recovery seems to be more dependent on the number of backwashings carried out because they reduce organic load in the filter, lowering microbial activity and oxygen consumption. The DO recovery in the sand filter for the micro-irrigation system is not as quantitatively important as it is in the emitters, but it helps avoid the agronomic problems caused by hypoxic water. The sand filter ripening period was 15 min during the experiment. Backwashing had an opposite effect for turbidity and dissolved oxygen. After backwashing, turbidity reductions are still important (about 54%) but smaller than before backwashing
M. Elbana et al. / Agricultural Water Management 111 (2012) 27–33
because there are no trapped particles that reduce water passages and improve solid retention. On the contrary, the effluent DO recovery increased significantly after the backwashing and remained almost stable during the investigated period of 120 min due to the removal of microbial substrate. Overall, and despite the higher water consumption that a daily backwashing causes, the results support the idea that it is a good maintenance practice for micro-irrigation sand filters. Acknowledgements The authors would like to thank the Spanish Ministry of Science and Innovation for their financial support through grant CGL200502201/HID. M. Elbana also acknowledges a research fellowship from the Spanish Ministry of Science and Innovation. The valuable contributions of two anonymous reviewers are acknowledged. References Abbott, J.S., 1985. Emitter clogging – causes and prevention. ICID Bulletin 34 (2), 11–20. ACA, 2010. Fitxes de les estacions de depuració d’aigües residuals: Sistema de sanejament: Celrà. Agència Catalana de l’Aigua, Generalitat de Catalunya, Barcelona. Adin, A., Alon, G., 1986. Mechanisms and process parameters of filter screens. Journal of Irrigation and Drainage Engineering-ASCE 112 (4), 293–304. Adin, A., Elimelech, M., 1989. Particle filtration for wastewater irrigation. Journal of Irrigation and Drainage Engineering 115 (3), 474–487. Amburgey, J.E., Amirtharajah, A., 2005. Strategic filter backwashing techniques and resulting particle passage. Journal of Environment Engineering 131 (4), 535–547. Amirtharajah, A., 1985. The interface between filtration and backwashing. Water Research 19 (5), 581–588. AWWA, 2003. Principals and Practices of Water Supply Operations: Water Treatment, Third ed. American Water Works Association, Denver, Colorado. Belkin, N., Bar-Zeev, E., Berman, T., Berman-Frank, I., 2012. Two innovative devices for depth sampling in granular filtration systems. Desalination 286, 115–119. Bhattarai, S.P., Su, N., Midmore, D.J., 2005. Oxygation unlock yield potentials of crops in oxygen limited soil environment. Advances in Agronomy 88, 313–377. Bhattarai, S.P., Su, N., Midmore, D.J., Pendergast, L., 2008. Yield, water-use efficiencies and root distribution of soybean, chickpea and pumpkin under different subsurface drip irrigation depths and oxygation treatment in vertisols. Irrigation Science 26 (5), 439–450. Bouwer, E.J., Zehnder, A.J.B., 1993. Bioremediation of organic compounds – putting microbial metabolism to work. Trends in Biotechnology 11 (8), 360–367. Bucks, D.A., Nakayama, F.S., Gilbert, R.G., 1979. Trickle water quality and preventive maintenance. Agricultural Water Management 2 (2), 149–162. Burt, C.M., Styles, S.W., 2007. Drip and Micro Irrigation Design and Management. Irrigation Training and Research Center (ITRC), California Polytechnic State University, San Luis Obispo, CA. Capra, A., Scicolone, B., 2007. Recycling of poor quality urban wastewater by drip irrigation system. Journal of Clean Products 15 (16), 1529–1534. Chérif, M., Tirilly, Y., Bélanger, R.R., 1997. Effect of oxygen concentration on plant growth, lipidperoxidation, and receptivity of tomato roots to Pythium F under hydroponic conditions. European Journal of Plant Pathology 103 (3), 255–264. Cleasby, J.L., 1990. Filtration. In: American Water Work Association Water quality and Treatment: A Handbook of Public Water Supplies, Fourth ed. McGraw-Hill, Inc, New York. Darby, J.L., Lawler, D.F., 1990. Ripening in depth filtration. Environmental Science and Technology 24 (7), 1069–1079. Darby, J., Tchobanoglous, G., Asri Nor, M., Maciolek, D., 1996. Shallow intermittent sand filtration: performance evaluation. Small Flows Journal 2 (1), 3–15. Duran-Ros, M., 2008. Filtración y uniformidad de riego en los sistemas de microirrigación con efluentes de estaciones depuradoras de aguas residuales. Ph.D. Thesis. Universitat de Lleida, Lleida. Duran-Ros, M., Puig-Bargués, J., Arbat, G., Barragán, J., Ramírez de Cartagena, F., 2008. Definition of SCADA system for a microirrigation network with effluents. Computer and Electronics in Agriculture 64 (2), 338–342.
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Duran-Ros, M., Puig-Bargués, J., Arbat, G., Barragán, J., Ramírez de Cartagena, F., 2009a. Performance and backwashing efficiency of disc and screen filters in microirrigation systems. Biosystem Engineering 103 (1), 35–42. Duran-Ros, M., Puig-Bargués, J., Arbat, G., Barragán, J., Ramírez de Cartagena, F., 2009b. Effect of filter, emitter and location on clogging when using effluents. Agricultural Water Management 96 (10), 67–79. Enciso-Medina, J., Multer, W.L., Lamm, F.R., 2011. Management, maintenance and water quality effects on the long-term performance of subsurface drip irrigation systems. Applied Engineering and Agriculture 27 (6), 969–978. Gilbert, R.G., Nakayama, F.S., Bucks, D.A., French, O.F., Adamson, K.C., Johnson, R.M., 1982. Trickle irrigation: predominant bacteria in treated Colorado River water and biologically clogged emitters. Irrigation Science 3 (2), 123–132. Haman, D.Z., Smajstrla, A.G., Zazueta, F.S., 1994. Media Filters for Trickle Irrigation in Florida. AE57, Agricultural and Biological Engineering Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL. Ives, K.J., 1980. Deep bed filtration: theory and practice. Filtration and Separation 17 (2), 157–166. Maestre-Valero, J.F., Martínez-Álvarez, V., 2010. Effect of drip irrigation systems on the recovery of dissolved oxygen from hypoxic water. Agricultural Water Management 97 (11), 1806–1812. Marfà, O., Cáceres, R., Guri, S., 2005. Oxyfertigation: a new technique for soilless culture under Mediterranean conditions. Acta Horticulturica 697, 65–72. Morard, P., 1995. Etude de l’oxygenation du systeme racinaire. In: SARL (Ed.), Les cultures vegetables hors-soil. Agricoles SARL, Agen, France, pp. 245–252. Naghavi, B., Malone, R.F., 1986. Algae removal by fine sand/silt filtration. Water Resources 20 (3), 377–383. Nakayama, F.S., Boman, B.J., Pitts, D.J., 2007. Maintenance. In: Lamm, F.R., Ayars, J.E., Nakayama, F.S. (Eds.), Microirrigation for Crop Production: Design, Operation, and Management, Developments in Agricultural Engineering, vol.13. Elsevier, Amsterdam, pp. 389–430. Nakhla, G., Farooq, S., 2003. Simultaneous nitrification-denitrification in slow sand filters. Journal of Hazardous Materials 96 (2-3), 291–303. Phillips, K.P., 1995. Long-term operation of microirrigation filtration systems; success, failures and operational solutions. In: Proceedings of the Fifth International Irrigation Congress, Orlando, FL, pp. 579–585. Pitts, D.J., Haman, D.Z., Smajstrla, A.G., 1990. Causes and prevention of emitter plugging in microirrigation system. Bulletin 258. Florida Cooperative Extension Service, Institute of Food and Agriculture Science, University of Florida, Gainesville, FL. Puig-Bargués, J., Barragán, J., Ramírez de Cartagena, F., 2005. Effluent particle removal by microirrigation system filters. Spanish Journal of Agricultural Research 3 (2), 182–191. Puig-Bargués, J., Arbat, G., Elbana, M., Duran-Ros, M., Barragán, J., Ramírez de Cartagena, F., Lamm, F.R., 2010a. Effect of flushing frequency on emitter clogging in microirrigation with effluents. Agricultural Water Management 97 (6), 883–891. Puig-Bargués, J., Lamm, F.R., Trooien, T.P., Clark, G.A., 2010b. Effect of dripline flushing on subsurface drip irrigation systems. Transactions on ASABE 53 (1), 147–155. Ravina, A., Paz, E., Sofer, Z., Marcu, A., Shisha, A., Sagi, G., Yechialy, Z., Lev, Y., 1997. Control of clogging in drip irrigation with stored treated municipal sewage effluent. Agricultural Water Management 33 (2–3), 127–137. Raviv, M., Wallach, R., Blom, T.J., 2004. The effect of physical properties of soilless media on plant performance – a review. Acta Horticulturica 644, 251–259. Tanwar, P., Nandy, T., Ukey, P., Manekar, P., 2008. Correlating on-line monitoring parameters, pH, DO and ORP with nutrient removal in an intermittent cyclic process bioreactor system. Bioresource Technology 99 (16), 7630–7635. Tajrishy, M.A., Hills, D.J., Tchobanoglous, G., 1994. Pretreatment of secondary effluent for drip irrigation. Journal of Irrigation Drainage Engineering 120 (4), 716–731. Tebbutt, H.Y., 1971. An investigation into tertiary treatment by rapid sand filtration. Water Research 5 (3), 81–92. USEPA, 1998. Handbook Optimizing Water Treatment Plant Performance Using the Composite Correction Program. United States Environmental Protection Agency, Cincinnati, OH. USEPA, 2002. Onsite wastewater treatment systems technology. Fact Sheet 10. Intermittent sand/media filters. In: Onsite Wastewater Treatment System Manual. EPA/625/R-00/008. United States Environmental Protection Agency, Office of Water, Washington, D.C, pp. 53–59.
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Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat
Effectiveness of sand media filters for removing turbidity and recovering dissolved oxygen from a reclaimed effluent used for micro-irrigation M. Elbana, F. Ramírez de Cartagena, J. Puig-Bargués ∗ Department of Chemical and Agricultural Engineering and Technology, University of Girona, Carrer de Maria Aurèlia Capmany 61, 17071 Girona, Spain
a r t i c l e
i n f o
Article history: Received 16 January 2012 Accepted 24 April 2012 Available online 20 May 2012 Keywords: Filtration Removal efficiency Clogging Filter ripening Backwashing Drip irrigation
a b s t r a c t Sand media filters are among the most common filters used in micro-irrigation systems, especially for filtering waters with large amounts of organic contaminants like reclaimed effluents. An experiment was conducted for 1620 h between August 2007 and September 2008 using a reclaimed effluent to evaluate the efficiency of sand filters with sand effective diameters of 0.32, 0.47, 0.63 and 0.64 mm in decreasing turbidity and improving dissolved oxygen concentration. In addition, this study strived to determine the filter ripening period (i.e. the time after backwashing when the filtered effluent has the lowest quality) and the effect of filter backwashing on filtration efficiency. Depending on the sand effective size, the sand filter achieved turbidity reductions of between 59.6 and 85.4% and dissolved oxygen recoveries from 4.5 to 15.7%. During the experiment the filter ripening period was 15 min. Overall, the results support the idea that a daily backwashing is a good maintenance practice since it reduces inefficient backwashings and increases dissolved oxygen, which is interesting when hypoxic water is used for irrigation. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In micro-irrigation systems, emitter and filter clogging are two substantial problems, especially when treated wastewater of poor quality is used since it contains high concentrations of dissolved and suspended solids (Bucks et al., 1979; Ravina et al., 1997). Sand media filters are among the most common filter types used in micro-irrigation systems, especially when irrigating with effluents (Burt and Styles, 2007). They are always recommended when algae (Naghavi and Malone, 1986) or other organic contaminants are present (Haman et al., 1994). In addition, sand media filters can guarantee the best emitter performance with poorly treated wastewater (Ravina et al., 1997; Capra and Scicolone, 2007). The media grain size, which affects the efficiency of filtration (AWWA, 2003), is considered to be an indicator of the size of the particles that could be removed by the media (Haman et al., 1994). The finer the sand, the smaller the particle size removed, and the better the quality of the filtration process, although a smaller size requires more frequent cleaning (Pitts et al., 1990; Haman et al., 1994; Phillips, 1995). Sand effective diameter (de ) and its uniformity coefficient (UCs ) are two indicators of the sand filter grain size. The sand effective diameter is defined as the size of the screen opening which will allow 10% of the total sand sample mass to pass (Haman et al.,
∗ Corresponding author. Tel.: +34 972 41 84 59; fax: +34 972 41 83 99. E-mail address: [email protected] (J. Puig-Bargués). 0378-3774/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agwat.2012.04.010
1994). The UCs is the ratio between the screen pores that let pass 60 and 10% of the sand. A uniform sand media has a low UCs value while a good graded media is characterized by a high UCs value (Burt and Styles, 2007). For irrigation sand filters, a UCs of 1.5 is recommended by Haman et al. (1994), although other researchers suggested working with lower UCs (Burt and Styles, 2007). Suspended materials trapped by the filter decrease the water flow rate across the filter and eventually the sand media filter must be cleaned by backwashing, which is a critical part of media filter operation and performance (Nakayama et al., 2007). Automatic backwashing can be controlled by time and/or by head loss across the filter. However, when sand media filters are not flushed frequently enough, large and interconnected pores called “rat holes” can form in the sand media and decrease the filter performance (Nakayama et al., 2007). When this happens, head loss across the filter is low and cannot be used as a guide for backwashing. Therefore, filters should be backwashed frequently (Pitts et al., 1990). Enciso-Medina et al. (2011) pointed out that inadequate sand filter backwashing intervals and durations caused poor performance in subsurface drip irrigation systems. The time span that an effluent of degraded water quality passes through a filter immediately after being backwashed is called filter ripening. Amirtharajah (1985) showed that more than 90% of the particles passing through a well-operating filter did so during the ripening period. Thus, filter ripening is a period during which emitter clogging is likely to occur as filter protection is not as effective as it should be. This ripening period has been studied in depth. Scientists have focused primarily on effluent turbidity (Amirtharajah,
28
M. Elbana et al. / Agricultural Water Management 111 (2012) 27–33
1985; Amburgey and Amirtharajah, 2005) and on particle size distribution (Darby and Lawler, 1990) as principal indicators of filter ripening. Nevertheless, the concentration of dissolved oxygen (DO) in irrigation water, which can be an important limiting factor, especially in intensive agriculture systems (Raviv et al., 2004; Bhattarai et al., 2005; Marfà et al., 2005), has not been studied to date in a filtered effluent during the ripening period. The shortage of DO in the irrigation water can lead to root oxygen deficiency, which results in agronomic problems such as crop stress, slow plant growth, low yields (Bhattarai et al., 2008) and increase in disease (Chérif et al., 1997). The present study focuses on investigating the effect of sand media filter backwashing on effluent turbidity and DO during the ripening period. In addition, it studies the influence of sand effective diameter on the filtration efficiency.
which has a negative impact on filtrate turbidity and on filter run time (Cleasby, 1990). Efficient and inefficient backwashings were identified in accordance with the following head loss thresholds across the filter after a backwashing was carried out: 20–40 kPa for efficient backwashings and greater than 40 kPa for the inefficient ones. These thresholds agree with the 28–40 kPa range pointed out by Duran-Ros (2008) for clean media filters. 2.2. Applied effluent A reclaimed effluent from the wastewater treatment plant (WWTP) in the municipality of Celrà (Province of Girona, Catalonia, Spain) was applied. This WWTP biologically treats the urban and industrial wastewater of the municipality to primarily remove nitrogen and phosphorus (ACA, 2010). The effluent was obtained by filtering the sludge through a disc filter with a 130 m filtration level and treatment by ultraviolet radiation.
2. Material and methods 2.3. Sand media characteristics 2.1. Filter installation and operation A sand filtration unit consisting of two sand filters (Regaber, Parets del Vallès, Spain) in parallel was installed. Each sand filter had an internal diameter of 0.508 m, a filtration surface of 1963 cm2 and a mean height of 0.50 m. Both sand filters were filled with 175 kg of sand as a single filtration layer. Two MBS 4010 (Danfoss, Nordborg, Denmark) pressure transmitters with flush diaphragms measured the pressure at the inlet and outlet of the sand filtration unit, respectively, and allowed determination of the head loss. The flow at the filtration unit outlet was measured by an MP-400-CB (Comaquinsa, Llinars del Vallès, Spain) electromagnetic flowmeter. These devices were connected to a supervisory control and data acquisition (SCADA) system, which allowed for filter scheduling and the collection of filter performance data every minute (DuranRos et al., 2008). The average registered sand filter surface flow rate (i.e. effluent flow rate divided by filter surface area) during the experiment was between 13.81 and 15.44 l s−1 m−2 . This rate was within the range 10.17–16.94 l s−1 m−2 recommended by Pitts et al. (1990), Haman et al. (1994) and Phillips (1995) and less than the average of 17 l s−1 m−2 (Abbott, 1985) necessary to avoid the formation of channels in the sand media bed or the hydraulic movement of contaminants through it. The filter operation time varied between 6 and 12 h per day, with minor interruptions of a few days primarily due to system maintenance and operational problems such as a lack of effluent or reclaimed effluent, a power failure after a thunderstorm and a breakdown of some sensors. The experiment lasted 1620 h, divided into three filtration periods of 540 h each. The first period began on August 3 and ended on December 7, 2007; the second one lasted from March 11 to May 26, 2008; and the third one from June 30 to September 8, 2008. The sand media was changed only once during the experiment – before starting the third period (after 1080 h of operation) – as recommended by the manufacturer. Filters were backwashed various times at the beginning of the experiment and after changing the sand media (operation hour 1080) to get rid of the finest particles, which could cause future clogging problems. Furthermore, filters were automatically backwashed for 90 s when the head loss across them reached 50 kPa, as recommended by Ravina et al. (1997). The number of efficient and inefficient backwashing cycles was classified on the basis of the filter capacity to recover the pressure loss across it. While efficient backwashings remove most of the trapped particles in the sand media, allowing the normal development of the subsequent filter run, inefficient backwashings do not release most of the particles retained (Duran-Ros et al., 2009a). Filters with inefficient backwashings tend to accumulate aggregates of the suspended matter,
The sand effective diameter (de ) and its uniformity coefficient (UCs ) were determined at the beginning of the first filtration period, at the end of the second one and before the third period started and again after it ended. Sand effective diameter was determined through homogeneous and representative sand samples. Four homogeneous and representative 250 g samples were taken at each sampling event, dried and sieved through twelve stainless steel screens ranging from 0.06 to 1.20 mm pores. Then, each screen was weighed to determine the retained sand and the particle size distribution curve, which allowed the computation of de and UCs . 2.4. Assessment of filter efficiency Measurements of pH, temperature, DO, turbidity and electrical conductivity (EC) at the filter inlet were performed using Endress + Hauser (Nesselwang, Germany) sensors (Orbisint CPS11D, OxyMax W COS61, TuriMax W CUS 31 and ConduMax W CLS 21, respectively) and transmitters (CPM253, COM253, CUM253 and CLM253, respectively). At the filter outlet, only DO and turbidity were monitored as the other parameters were not expected to be different. The same type of sensors and transmitters installed at the filter inlet were used. Only the turbidity sensor needed to be washed periodically with potable water. This was done either with a SCADA system or manually. When the automatic washing did not remove some particles from the sensor, the turbidity meter was washed manually with potable water. The effectiveness of a filter (removal efficiency) was computed as a measure of the ability of the sand media filter to remove suspended particles from the inlet effluent. Filter removal efficiency was determined for both effluent turbidity and DO using the formula: y − yo REy = i × 100 (1) yi where REy is the removal efficiency for the physical parameter with y being turbidity or DO, yi being the physical parameter value before filtering and yo its value after filtration. 2.5. Data treatment and statistical analysis Filter run time, filtered flow, inlet and outlet filter pressure, inlet and outlet effluent parameters, number of automatic backwashings and backwashing water volume were recorded every minute by a SCADA system previously developed (Duran-Ros et al., 2008) and adapted to this experiment. The collected data were used to compute the average inlet and outlet turbidity and DO for each 10 h
M. Elbana et al. / Agricultural Water Management 111 (2012) 27–33 Table 1 Sand effective diameter (de ) and uniformity coefficient (UCs ) during the experiment. Period
Accumulated operation hours of sand media
de , mm
UCs
Before the 1st irrigation period After the 2nd irrigation period Before the 3rd irrigation period After the 3rd irrigation period
0 1080 0 540
0.47 0.64 0.32 0.63
1.81 1.40 3.17 1.73
of operation (i.e. average from 600 values). Removal turbidity and DO efficiencies were calculated using Eq. (1) for each 10 h of operation. Head loss analysed after the backwashings allowed them to be classified as efficient or inefficient, as detailed previously in Section 2.1. The effects of de and of filter backwashing on filter efficiency over time were also studied. However, for these purposes, filter runs shorter than 240 min were not considered in order to avoid repetitions of outlet parameter data before filter backwashing occurred. Thus, only the data from 67 filter runs were used to compute RETurbidity and REDO 10 min before and 10, 15, 30, 45, 60 and 120 min after the backwashing took place. To compare average inlet and outlet turbidity and DO for the different filtration periods, Duncan’s multiple range test was applied. The effect of de and time before and after backwashings on RETurbidity and REDO was analysed using a multivariate general linear model and Duncan’s pairwise comparison to identify means that were different. Statistical analysis was carried out using the SPSS statistical program (SPSS Inc., Chicago, Illinois, USA) and results were checked at a significance level of 0.05. 3. Results and discussion 3.1. Sand filter characteristics The sand media effective diameter and its uniformity coefficient are shown in Table 1. The UCs were slightly greater (1.81 before the first period and 1.73 after the third one) or smaller (1.40 after the second irrigation period) than 1.5, as recommended by Haman et al. (1994) and Burt and Styles (2007) for micro-irrigation sand media filters. At the end of the second irrigation period, and after 1080 h of operation, the sand was changed following the manufacturer’s recommendation. The new sand raised the UCs to 3.17, which was outside the recommendations for irrigation sand filters, but within USEPA (2002) recommendation (UCs < 4) for intermittent sand filters used for wastewater treatment. The larger the UCs , the less 100
uniform the sand, and the smaller sand particles can fill the spaces between the larger particles, making filter clogging easier at higher loading rates (Darby et al., 1996). During the operation time the UCs decreased while the sand effective diameter increased, which coincides with the results of Duran-Ros (2008), who working with a similar reclaimed effluent found after 1000 h of operation a decrease in UCs from 2.39 to 1.65 and an increase in de from 0.27 to 0.52 mm. The sand particle size distribution curves (Fig. 1) show that there were less fine sand grains at the end of the analysed filtration periods. Thus, while the screen size opening where 60% of the sand sample passes changed slightly (from 0.85 to 0.90 mm and from 1.10 to 1.01 mm in both periods considered), the screen size opening with 10% throughfall (effective sand size) increased with 0.17 and 0.31 mm, respectively, at the end of these periods. Sand particles smaller than 0.63 mm were reduced by 21.6 and 47.0% at the end of the second and third filtration period, respectively, whereas sand particles between 0.75 and 2 mm were only reduced by 6.2 and 41.9%. These particles were lost during the initial backwashings carried out when new sand was used and also during the normal filter operation and backwashing. The most problematic particles are those with diameters close to the narrowest openings of the emitter, which are usually in the 0.7–1.0 mm range, because they can clog these emitters. Gilbert et al. (1982) recommended installing screen filters after sand media filters to remove suspended solids released from them. This could help to prevent the fine sand media particles from escaping to the lateral pipes and causing emitter clogging problems during the irrigation. In our case, as the maximum sand grains are released in the initial backwashings when new sand is used, the backwashing water was diverted and not used for irrigation. However, as some sand particles can accidentally be introduced in the drip lines, dripline flushing need to be carried out periodically to remove them and help to reduce clogging (Puig-Bargués et al., 2010a,b). 3.2. Effect of sand filter on effluent quality The average and standard deviation of the effluent relevant parameters for each filtration period at both the filter inlet and outlet and the significant difference between them are presented in Table 2. The sand filter media significantly increased (P < 0.05) the concentration of dissolved oxygen in the outlet effluent during the first and third filtration periods, with an average increase of 0.15 and 0.39 g m−3 respectively. Maestre-Valero and Martínez-Álvarez (2010), studying the DO in a drip irrigation system using hypoxic 100
a
b
90
80
Accumulated sand mass, %
Accumulated sand mass, %
90
70 60 Before Aer
50 40
80 70 60 50
30
20
20
10
10 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Screen pore diameter, mm
1.6
1.8
2.0
Before Aer
40
30
0
29
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Screen pore diameter, mm
Fig. 1. Particle size distribution of sand media (a) before the first filtration period and after the second one, and (b) before and after the third filtration period.
2.0
30
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Table 2 Average and standard deviation of effluent parameters for each filtration period at both filter inlet and outlet registered by the SCADA system. Different letters show significant differences (P < 0.05) between inlet and outlet parameters for the same filtration period (N = 54 for each period).
Parameter −1
Flow rate, l s DO, g m−3 Turbidity, FNU EC, dS m−1 pH Temperature, ◦ C
1st filtration period (0–540 h)
2nd filtration period (540–1080 h)
3rd filtration period (1080–1620 h)
Inlet
Outlet
Inlet
Outlet
Inlet
4.15 ± 0.57a 3.18 ± 2.00b – – –
2.73 3.40 9.90 4.80 7.96 18.2
3.37 ± 1.05a 3.73 ± 3.80b – – –
2.71 3.01 8.63 4.54 – 27.0
3.03 4.00 10.80 5.63 7.33 21.1
± ± ± ± ± ±
0.20 0.55b 8.17a 0.98 0.98 3.4
water from a covered agricultural reservoir (DO < 1.08 g m−3 ), found DO increases of 0.25 g m−3 after pumping and 1.56 and 3.15 g m−3 through a laminar and a turbulent emitter, respectively. Because the sand irrigation filter is closed and pressurized, it does not allow contact with oxygen as emitters do. MaestreValero and Martínez-Álvarez (2010) attributed the DO increase in the pumping to minor imperfections that result in air intrusions. Although this could explain some of the DO increase in the sand filter, the main reason is related to backwashing. Backwashing removes organic particles retained in the filter, which allow microbial growth and reduce DO concentrations. As sand micro-irrigation filters are mainly used to remove those particles that could clog the emitters, they are not specifically designed for water treatment, and do not achieve important DO recoveries or reductions of other related parameters – such as chemical and biochemical oxygen demand – that other equipment like intermittent sand filters (USEPA, 2002) do. On the contrary, there was an average DO reduction of 0.03 g m−3 in the second filtration period, but this difference was not statistically significant (P = 0.780). In this period, the water temperature was lower than in the other periods. Although cold water holds more oxygen than warm water, DO inlet values were not the highest in this filtration period because effluent parameters depended on raw wastewater characteristics and WWTP operation efficiency at that moment. The differences in DO removal are attributed to bad filter performance because the sand media was at the end of its useful life, according to the manufacturer. The filter also achieved a drop in turbidity removal in this second period, supporting the idea of a worst sand media performance between 540 and 1080 h of operation. On the other hand, there was a significant decrease in effluent turbidity after filtration (P < 0.05) for the three different periods, achieving average turbidity removals of 70.6%, 62.3% and 75.6% through the three filtration periods. This indicates the effectiveness of the sand filtration unit in reducing the total suspended matter in the outlet effluent.
± ± ± ± ± ±
0.13 0.89a 6.60a 1.04 0.26 2.06
0.22 1.02b 7.71a 0.55
± 1.1
3.40 ± 0.95a 2.11 ± 1.82b – – –
100 a 80 0.32
0.47
0.64 b
b
-10
RETurbidity , %
10
REDO, %
± ± ± ±
The average and standard error of DO and turbidity removal efficiency for different sand effective diameters (de ) is illustrated in Fig. 2. There were missed data for the 0.63 mm de after 1270 h of filtration due to a DO sensor failure. Therefore the removal efficiency for DO was examined with the determined de only at the start of the experiment, after the end of the second filtration period and before the start of the third one. The DO removal efficiency with de of 0.32 mm was statistically different (P < 0.05) from those achieved with de of 0.47 and 0.64 mm, which were not significantly different from each other (Fig. 2). The negative removal efficiency for effluent DO, which is in fact a DO recovery, was higher for the smallest de (0.32 mm) than for the other two de of 0.47 and 0.64 mm. In general, DO removal efficiencies increase, hence DO recoveries decrease with larger de (Ives, 1980). Thus, the smaller the sand effective diameter, the higher the concentration of DO in the filtered effluent. This is favourable for the plant. A low DO concentration in irrigation water could have critical consequences because it causes root oxygen deficiency, which in turn can result in agronomic problems (Maestre-Valero and Martínez-Álvarez, 2010), but a well-designed drip irrigation system can oxygenate the water so that it does not reach critical thresholds (Morard, 1995). On the other hand, the four different sand effective diameters were efficient in removing most of the turbidity from the applied effluent (Fig. 2). The de of 0.63 mm achieved the highest removal efficiency for effluent turbidity (85%), but even the 0.64 mm de achieved a high efficiency (65%). However, as discussed previously, besides the effect of de , the sand media performance can worsen at the end of its useful life, as it happened with 0.64 mm de . Although Naghavi and Malone (1986) pointed out that effluent quality deteriorated when sand with a median grain size diameter of 0.34 mm is used for the filtration bed, our study did not show a significant difference in turbidity removal efficiencies between sand effective diameters of 0.32, 0.47 and 0.64 mm. Removal efficiencies for turbidity (60–85%) were consistent with the results of previous studies. Nakhla and Farooq (2003)
20
0
Outlet
60
b
b
b
40 20
-20
a
d e, mm
0
0.32
0.47
0.63
0.64
de , mm Fig. 2. Average and standard error of DO (REDO ) (N = 19, 52 and 56) and turbidity (RETurbidity ) (N = 27, 52, 56 and 27) removal efficiencies for each 10 h of operation. Different letters mean significant differences (P ≤ 0.05) among sand effective diameters.
M. Elbana et al. / Agricultural Water Management 111 (2012) 27–33
3.3. Effectiveness of the backwashing processes The number of efficient and inefficient backwashings, the volume and percentage of filtered effluent and the volume of water consumed during backwashing for each period are given in Table 3. The total number of backwashing cycles was almost doubled, from 105 during the first filtration period to 202 during the second one. In the third filtration period, with sand of smaller de and greater UCs , the number of backwashings rose to 372. As the backwashing water consumption (determined as the ratio between filtered water consumed for backwashing and the total filtered water consumed for both backwashing and irrigating) is dependent on the number of backwashings carried out (Duran-Ros et al., 2009b), it increased from 1.3% for the first period to 5.7% for the third. This agrees with the results of Duran-Ros et al. (2009b), who observed an increase from 1.1 to 5.0% in backwashing water consumption when the number of filter runs increased from 219 to 645 using a sand filtration media. The 1.3% backwashing water consumption during the first period (de = 0.47 mm) was similar to that observed by Duran-Ros et al. (2009b) with sand with de = 0.40 mm (1.1%), but smaller than the 3.0% observed by Tajrishy et al. (1994) with de = 0.45 mm. However, backwashing water consumptions, except for the first filtration period, were greater than 0.5–1.5% observed by Ravina et al. (1997), but the effective diameter was 1.0 mm, which was greater than in the present work (Table 1). The larger de , the faster the water moves through the sand and the more water can be filtered, which means less backwashings are needed. Backwashing frequency does not only depend on sand media characteristics. It is also greatly influenced by filtering water characteristics, mainly particle load and filtration flow (Capra and Scicolone, 2007). Fig. 3 shows the inlet effluent quality presented as percentage of the operation time for intervals of turbidity of 10 FNU during the three filtration periods. During the second filtration period, the percentage of time when turbidity was ≥50 FNU was 7%, which was greater than the first one (5%) and the third one (2%). This means that during the second period, the inlet effluent experienced temporally high turbidity events, related to high suspended solid loads, that easily clog the filter. In addition, the average volume of filtered effluent per filter run was higher during the first (50.1 m3 ) filtration period than during the second (18.3 m3 ) (Table 3). Thus, the increase in filter backwashings during the second period was primarily due to the sand media being clogged by trapped particles
Third Period (1080 - 1620 h)
Turbidity, FNU
50
Second Period (540 - 108 0 h) First period (0 - 540 h)
40 - 50 30 - 40 20 - 30 10 - 20 0 - 10 0
20
40
60
80
10 0
Percentage of oper aon me Fig. 3. Percentage of operation time for each interval of the inlet effluent turbidity during the three filtration periods.
which were observed while changing the sand media after the end of the second filtration period. Despite the inlet effluent having the lowest turbidity values higher than 10 FNU (Fig. 3), the number of backwashings was the highest during the third filtration period, the main cause being that de was smaller (0.32 mm) at the beginning of the third than during the first and second filtration period (Table 1). According to Haman et al. (1994), the smaller the effective diameter, the higher the clogging possibility because media filters are not able to remove more particles and, consequently, both the total number of backwashing cycles and the volume of water consumed for backwashing increase. On the other hand, the number of inefficient backwashings decreased through the experiment from 39.0% (first period) to 7.8% (third one). After an inefficient backwashing, head loss values across the filter are higher than normal, so the filter run will be shorter. An inefficient backwashing shows that the filter cleaning was not done properly. The number of inefficient backwashings increased with the average volume of filtered effluent. Thus, in the first period, the average filtered effluent volume (i.e. between two backwashings) was 50.1 m3 /cycle and 39% of the backwashings were inefficient, while in the third filtration period the average filtered effluent volume was 10.1 m3 /cycle and the inefficient backwashings were only 7.8%. When more water is filtered, more particles are trapped within the sand filter and their release by backwashing is made more difficult. The results support the recommendation of Burt and Styles (2007) of carrying out at least a daily backwashing, which has been verified as a practice that helps to assure good emitter performance (Enciso-Medina et al., 2011). 3.4. Effect of filter backwashing on filtration efficiency The statistically significant differences between the mean reduction in effluent turbidity (RETurbidity ) and DO (REDO ) removal efficiency 10 min before and 10, 15, 30, 45, 60 and 120 min after filter backwashing are shown in Figs. 4 and 5, respectively. Results
70
a
66
RETurbidity , %
reported a 33–56% turbidity removal efficiency using coarse sand filters (de = 0.50 mm) and a 40–62% removal efficiency for turbidity with fine media sand filters (de = 0.30 mm) when using an effluent with a turbidity of 0.20–0.95 formazin nephelometric units (FNU). Duran-Ros et al. (2009b), who used two reclaimed effluents with inlet turbidity of 6.76 and 4.08 FNU, respectively, observed a turbidity reduction of 57.6 and 66.4% when filtering with de of 0.40 and 0.27 mm, respectively. Total suspended solids (TSS) removal, which is highly correlated with turbidity, was studied among others by Tebbutt (1971) and Ives (1980), who found a better improvement in TSS removal with finer media, i.e. smaller de . Nevertheless, Naghavi and Malone (1986), who studied algal removal by fine sand/silt filtration with five fine median sand sizes that varied from 0.064 to 0.335 mm, found that increasing the median grain size diameter from 0.064 to 0.200 mm did not have a significant effect on the effluent quality. They also found that the effluent quality deteriorated when sand with a median grain size diameter of 0.335 mm was used for the filtration bed. Conversely to those results, Adin and Elimelech (1989) found that removal efficiency of TSS was increased with greater sand effective diameters. However, they studied three sand media filters with higher de (0.70, 0.84 and 1.20 mm), than those used by Nakhla and Farooq (2003), Duran-Ros et al. (2009b) and in the current study.
31
62 58
ab
ab
ab
ab
ab b
54 50 46
Fig. 4. Average and standard error for turbidity removal efficiency (RETurbidity ) at 10 min before backwashing and 10, 15, 30, 45, 60 and 120 min after it (N = 67). Different letters show significant differences (P ≤ 0.05).
32
M. Elbana et al. / Agricultural Water Management 111 (2012) 27–33
Table 3 Types of backwashings, backwashing water consumption, average filtered effluent per filter run and total filtered effluent for each filtration period. Parameter
1st filtration period (0–540 h)
2nd filtration period (540–1080 h)
3rd filtration period (1080–1620 h)
Total number of backwashings Number of efficient backwashings Inefficient backwashings, % Backwashing water consumption, % Average filtered effluent per filter run, m3 Total filtered effluent, m3
105 64 39.0 1.3 50.1 5836.9
202 158 21.8 3.1 18.3 5704.5
372 343 7.8 5.7 10.1 5484.7
were used to determine the duration of filter ripening and whether there was a negative impact on RETurbidity and REDO during it. The removal efficiency of effluent turbidity decreased significantly from 63.5% at 10 min before backwashing to 54.2% at 10 min after it (Fig. 4). This means that during the first 10 min after backwashing the filter did not retain as many solids as the almost clogged filter. Actually, the filtration process becomes more efficient with the passage of time because smaller particles can be filtered out as the flow passages become smaller (Nakayama et al., 2007) due to the trapped particles. Thus, at 15 min after backwashing, the filter regained its capacity to remove an average of 60.6% of the suspended solids from the filtered effluent. This coincided with what the USEPA (1998) pointed out, that effluent quality attains normal values within 15 min after filter backwashing. These results demonstrate that filter ripening during the experimental time did not exceed 15 min. There was no significant difference between RETurbidity during the studied times of 15, 30, 45, 60 and 120 min after backwashing. Thus, a good management practice would consist of only using filtered effluent for micro-irrigation at least 15 min after a good filter backwashing is carried out. However, each particular effluent characteristic should be considered because, depending on the type of particles present, solid detachment from the filter cake (Adin and Alon, 1986) or different size particle removal (Puig-Bargués et al., 2005) can occur. On the other hand, the negative removal or recovery efficiency for effluent DO values (Fig. 5) increased significantly (P ≤ 0.05) after filter backwashing. DO recovery was of 2.32% before backwashing and increased to an average of 6.36% in the 5–120 min studied time interval after backwashing. There were no significant differences between REDO at the different investigated times after the backwashing. Although no other studies exist in the literature dealing with DO evolution in sand media filters for irrigation, the research carried out in wastewater treatment processes is highly dependent on aerobic or anaerobic conditions (Tanwar et al., 2008) and is not directly comparable to our study. Available organic nutrients in sand filters promote microbial activity (Nakayama et al., 2007), which tends to lower the levels of DO within the filter (Bouwer and Zehnder, 1993). In our experiment, microbial activity in the filter was not exacerbated since the reclaimed effluent did not have an important organic pollutant load and the residence time was low (<2 min). Belkin et al. (2012) observed that, after backwashing in multi-layered rapid sand filters, DO levels increased from 5 to 11 h, but after 42 h they decreased again. Sand filter runs for a
REDO, %
20 10 0 -10
b a
a
a
a
a
a
-20
Fig. 5. Average and standard error for DO removal efficiency (REDO ) at 10 min before backwashing and 10, 15, 30, 45, 60 and 120 min after it (N = 67). Different letters show significant differences (P ≤ 0.05).
micro-irrigation system are shorter but the DO evolution pattern seems to be similar. Such DO recovery corresponds to a decrease in bacterial respiration activity due to the depletion of organic nutrients (Tanwar et al., 2008; Belkin et al., 2012) caused by backwashing. Amirtharajah (1985) found that the initial degradation of effluent quality is due to the backwash water remnants within the media and the backwash water remaining in the filter. As backwashing causes turbulence in the sand, some of the finer sand particles of the filter escape with the effluent during the backwashing and cause the noticed increase in effluent turbidity after backwashing (Pitts et al., 1990). This also explains the increase of sand effective diameter during filter operation. According to Nakayama et al. (2007), these fine particles may travel through the filters as individual particles, but then flocculate or become attached to organic residues and eventually become large enough to clog emitters. After backwashing, the filter was clear and apparently no particles were trapped. Retained particles could have helped to improve the removal efficiency of particles, resulting in less outlet turbidity but, on the other hand, they would have contributed to microbial activity, thereby reducing DO recovery, as observed before backwashing was carried out. Besides, inefficient backwashings that cause short filter runs are reduced when more backwashings are carried out because the filter cleaning is easier and could be more effective. 4. Conclusions The presented work aimed to determine the behaviour of a sand media filter during the filtration process and the influence of its backwashing process on the filtration efficiency during the ripening period. It also strived to determine the effect of sand effective diameter on the filtered effluent quality. Smaller sand media particles are released during automatic backwashing and filter operation. Accurate filter management is needed, especially if finer particles have a size that can clog drip irrigation emitters. It is highly recommended to conduct several initial backwashings when new sand media are used and to divert the initial backwashing water. The sand media filter improved the filtered effluent values of turbidity and dissolved oxygen. Effluent turbidity was reduced around 60% with effective sand diameters of 0.32, 0.47 and 0.64 mm, and about 85% when the effective sand diameter was 0.63 mm. Recovery of dissolved oxygen in the sand filter was quite moderate, having a maximum value of 15.7% for a sand effective diameter of 0.32 mm. However, DO recovery seems to be more dependent on the number of backwashings carried out because they reduce organic load in the filter, lowering microbial activity and oxygen consumption. The DO recovery in the sand filter for the micro-irrigation system is not as quantitatively important as it is in the emitters, but it helps avoid the agronomic problems caused by hypoxic water. The sand filter ripening period was 15 min during the experiment. Backwashing had an opposite effect for turbidity and dissolved oxygen. After backwashing, turbidity reductions are still important (about 54%) but smaller than before backwashing
M. Elbana et al. / Agricultural Water Management 111 (2012) 27–33
because there are no trapped particles that reduce water passages and improve solid retention. On the contrary, the effluent DO recovery increased significantly after the backwashing and remained almost stable during the investigated period of 120 min due to the removal of microbial substrate. Overall, and despite the higher water consumption that a daily backwashing causes, the results support the idea that it is a good maintenance practice for micro-irrigation sand filters. Acknowledgements The authors would like to thank the Spanish Ministry of Science and Innovation for their financial support through grant CGL200502201/HID. M. Elbana also acknowledges a research fellowship from the Spanish Ministry of Science and Innovation. The valuable contributions of two anonymous reviewers are acknowledged. References Abbott, J.S., 1985. Emitter clogging – causes and prevention. ICID Bulletin 34 (2), 11–20. ACA, 2010. Fitxes de les estacions de depuració d’aigües residuals: Sistema de sanejament: Celrà. Agència Catalana de l’Aigua, Generalitat de Catalunya, Barcelona. Adin, A., Alon, G., 1986. Mechanisms and process parameters of filter screens. Journal of Irrigation and Drainage Engineering-ASCE 112 (4), 293–304. Adin, A., Elimelech, M., 1989. Particle filtration for wastewater irrigation. Journal of Irrigation and Drainage Engineering 115 (3), 474–487. Amburgey, J.E., Amirtharajah, A., 2005. Strategic filter backwashing techniques and resulting particle passage. Journal of Environment Engineering 131 (4), 535–547. Amirtharajah, A., 1985. The interface between filtration and backwashing. Water Research 19 (5), 581–588. AWWA, 2003. Principals and Practices of Water Supply Operations: Water Treatment, Third ed. American Water Works Association, Denver, Colorado. Belkin, N., Bar-Zeev, E., Berman, T., Berman-Frank, I., 2012. Two innovative devices for depth sampling in granular filtration systems. Desalination 286, 115–119. Bhattarai, S.P., Su, N., Midmore, D.J., 2005. Oxygation unlock yield potentials of crops in oxygen limited soil environment. Advances in Agronomy 88, 313–377. Bhattarai, S.P., Su, N., Midmore, D.J., Pendergast, L., 2008. Yield, water-use efficiencies and root distribution of soybean, chickpea and pumpkin under different subsurface drip irrigation depths and oxygation treatment in vertisols. Irrigation Science 26 (5), 439–450. Bouwer, E.J., Zehnder, A.J.B., 1993. Bioremediation of organic compounds – putting microbial metabolism to work. Trends in Biotechnology 11 (8), 360–367. Bucks, D.A., Nakayama, F.S., Gilbert, R.G., 1979. Trickle water quality and preventive maintenance. Agricultural Water Management 2 (2), 149–162. Burt, C.M., Styles, S.W., 2007. Drip and Micro Irrigation Design and Management. Irrigation Training and Research Center (ITRC), California Polytechnic State University, San Luis Obispo, CA. Capra, A., Scicolone, B., 2007. Recycling of poor quality urban wastewater by drip irrigation system. Journal of Clean Products 15 (16), 1529–1534. Chérif, M., Tirilly, Y., Bélanger, R.R., 1997. Effect of oxygen concentration on plant growth, lipidperoxidation, and receptivity of tomato roots to Pythium F under hydroponic conditions. European Journal of Plant Pathology 103 (3), 255–264. Cleasby, J.L., 1990. Filtration. In: American Water Work Association Water quality and Treatment: A Handbook of Public Water Supplies, Fourth ed. McGraw-Hill, Inc, New York. Darby, J.L., Lawler, D.F., 1990. Ripening in depth filtration. Environmental Science and Technology 24 (7), 1069–1079. Darby, J., Tchobanoglous, G., Asri Nor, M., Maciolek, D., 1996. Shallow intermittent sand filtration: performance evaluation. Small Flows Journal 2 (1), 3–15. Duran-Ros, M., 2008. Filtración y uniformidad de riego en los sistemas de microirrigación con efluentes de estaciones depuradoras de aguas residuales. Ph.D. Thesis. Universitat de Lleida, Lleida. Duran-Ros, M., Puig-Bargués, J., Arbat, G., Barragán, J., Ramírez de Cartagena, F., 2008. Definition of SCADA system for a microirrigation network with effluents. Computer and Electronics in Agriculture 64 (2), 338–342.
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