Effect of various design and operation parameters on performance of pilot-scale Sludge Drying Reed Beds

Ecological Engineering 38 (2012) 65–78

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Effect of various design and operation parameters on performance of pilot-scale Sludge Drying Reed Beds Alexandros I. Stefanakis, Vassilios A. Tsihrintzis ∗ Laboratory of Ecological Engineering and Technology, Department of Environmental Engineering, School of Engineering, Democritus University of Thrace, 67100 Xanthi, Greece

a r t i c l e

i n f o

Article history: Received 14 April 2011 Received in revised form 9 September 2011 Accepted 28 October 2011 Available online 26 November 2011 Keywords: Sludge treatment wetlands Dewatering Mineralization Sludge loading rate Resting period Vegetation Porous media Aeration Nutrients Chromium

a b s t r a c t Thirteen pilot-scale Sludge Drying Reed Bed units have been constructed and operated under various design and operational characteristics. All the beds included a cobbles bottom layer, where perforated PVC aeration tubes were placed. One bed did not contain aeration tubes. Two gravel layers were placed above the cobbles layer. The setup included planted beds with common reeds and control units. Three sludge loading rates were examined: 30, 60 and 75 kg dm/m2 /yr. The dewatering efficiency of the planted units exceeded 95% volume reduction. The final dry matter content varied between 50 and 64%, depending on the applied SLR. Mineralization of the residual sludge also took place. VS content of the planted units decreased up to 20%, while nutrient concentration was also reduced. The presence of reeds, aeration tubes and a fine-grained material improved the system efficiency, while the material composition did not have any effect on efficiency. Moreover, the units were able to treat sludge with high concentration of Cr. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Sludge produced from biological wastewater treatment facilities (BWTF) still remains a difficult problem to address. The main issues involved are the large amounts of produced surplus activated sludge (SAS) on a daily basis, as also the high costs for construction and operation of sludge treatment facilities (Campbell, 2000). Sludge treatment and management aims at both decreasing the water content and stabilizing the sludge. Sludge Drying Reed Beds (SDRBs) for SAS treatment is a relatively new technology developed in the last two decades. It is an effective, economical (i.e., of low investment, operation and maintenance cost, and low energy consumption), environmentally friendly, and technically efficient technology (Hardej and Ozimek, 2002; Peruzzi et al., 2010). In SDRBs, the sludge is applied to a growing stand of reeds under controlled conditions. The method relies on the exploitation of the transpiring and aerating capabilities of the reeds (Kengne et al., 2008), which absorb moisture and nutrients from the sludge for their growth. Additionally, the

sludge is dewatered by drainage through the underlying gravel layers, and evaporation from the sludge surface (Stefanakis and Tsihrintzis, 2011). In the long run, the sludge is oxidized and its organic matter content decreases. The final solids content of the dewatered sludge can build up to 40% (Nielsen, 2003). SDRBs have been used in Denmark (Nielsen, 2003; Nielsen and Willoughby, 2005), France (Liénard et al., 1995), Belgium (De Maeseneer, 1997), the UK (Edwards et al., 2001; Cooper et al., 2003), Spain (Uggetti et al., 2009,2010), Greece (Stefanakis et al., 2009; Melidis et al., 2010), Italy (Bianchi et al., 2010), and the USA (Burgoon et al., 1997; Kim and Smith, 1997; Begg et al., 2001). The aim of this study was to construct, operate and examine various pilot-scale SDRBs through parallel experiments, present their effectiveness in sludge dewatering and mineralization, and specify appropriate design and operation parameters for optimum performance under Mediterranean climate conditions.

2. Materials and methods 2.1. Experimental setup

∗ Corresponding author. Tel.: +30 25410 79393; fax: +30 25410 79393; mobile: +30 6974 993867. E-mail addresses: [email protected], [email protected] (V.A. Tsihrintzis). 0925-8574/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2011.10.003

Thirteen similar pilot-scale SDRB units (S1–S13) were constructed and operated in our open-air laboratory (41◦ 08 47 N, 24◦ 55 09 E). Each unit was a plastic cylindrical tank of diameter

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Fig. 1. (a) Layout of the experimental set up; (b) schematic section of the pilot-scale SDRB units; (c) general picture of the units; (d) bottom cobbles layer and aeration tubes; (e) medium gravel layer; (f) view of the sludge layer surface (unplanted unit); (g) a planted unit; (h) close view of reed stems arising from the sludge layer.

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Table 1 Pilot-scale SDRB unit construction and operation characteristics (Stefanakis and Tsihrintzis, 2011). Unit

Meso-porous media a

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 a b

Origin

Size

R Q R R R R R R R R R R R

Fine Fine Fine Fine Fine Fine Fine Fine Fine Fine Coarse Fine Fine

Plant species

Aeration tubes

Cr added

SLR (kg dm/m2 yr)

Reed Reed Cattailb No Reed Reed Reed Reed Reed Reedb Reed Reed Reed

Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes

No No No No No No Yes Yes Yes No No No No

75 75 75 75 30 60 75 30 60 75 75 30 75

R, river bed; Q, quarry. Initially planted; after first year plants dried up.

0.82 m and height 1.5 m (surface area of 0.57 m2 ). Table 1 summarizes the characteristics of each unit (Stefanakis and Tsihrintzis, 2011). Fig. 1 presents a schematic layout of the facility and a cross-section of the pilot-scale units, as also photos taken from the construction and operation phase of the beds. Briefly, all units had a drainage layer made of cobbles (D50 = 90 mm), placed at the bottom of each unit. The drainage layer contained aeration pipes made of PVC (50 mm in diameter), perforated only within the drainage layer. One unit (S10) did not have aeration tubes. Above the cobbles layer, a 15-cm thick layer of medium gravel (D50 = 24.4 mm) and a 15-cm thick layer of fine gravel (D50 = 6 mm) were placed in most of the units. One unit (S11) included an extended cobbles layer (25-cm thick) and only a 15-cm thick layer of medium gravel. Two different porous media were used: one obtained from a river bed in the area (igneous rock: Si 28.50%, Al 7.95%, Fe 4.22%, Ca 3.62%, Mg 1.76%, P 0.11%) was used in most units, and one from a quarry (carbonate rock: Si 3.39%, Al 0.90%, Fe 0.82%, Ca 27.20, Mg 4.53%, P 0.03%) was used in unit S2. The river bed material is rich in Fe, Al and Ca. The carbonate material mainly contains Ca, and is relatively poor in Fe and Al. Two plant species were used: common reeds (Phragmites australis) in most units and cattails (Typha latifolia) in one unit (S3). Reeds were planted within the gravel layers. One unit (S4) was kept unplanted. This setup allows for the evaluation of the effect of various design parameters on SDRB system performance (Stefanakis and Tsihrintzis, 2011). The units received three different sludge loading rates (SLR): low, medium and high SLR (30, 60 and 75 kg dry matter/m2 yr, respectively; Table 1). In three units, with different SLR, chromium was added. Units S1–S11 were constructed and planted in early June 2007 and units S12 and S13 one year earlier (May 2006). The sludge was produced and transported every time from the BWTF of the municipality of Komotini, Rhodope Province, Greece, and was introduced to the units in loading cycles; a feeding period of 7 days in daily equal portions, followed by a resting period mainly of 1–3 weeks, depending on the season. Table 2 presents the sludge loading timetable and other data during the entire monitoring period. The sludge loadings for units S1–S11 started in October 2007 (Stefanakis and Tsihrintzis, 2011) and lasted for 2.5 years (October 2007–April 2010). After April 2010, the resting phase started and lasted for another 6 months (till late October 2010; Table 2), during which the unit monitoring continued with samples taken once a month. At the end of each resting period, the height of the residual sludge layer was measured. For units S12 and S13, sludge loading started in October 2006. The performance of these two units till December 2008 has been presented by Stefanakis et al. (2009). The loading of unit S13

stopped in June 2008, while the loading of unit S12 continued till October 2010. In order to obtain a uniform data set for all loaded units, this paper presents the performance of units S1–S12 from October 2007 till October 2010. Thus, units S1–S11 operated for three years and unit S12 for four years, which provides additional information about the dewatering and mineralization progress. For unit S13, results presented here originate only from the extended resting phase (no sludge loadings) of this unit (June 2008–October 2010). The fact that unit S13 was kept unloaded, helped testing the mineralization of the sludge layer under no loadings. The different mineralization degrees were investigated by comparing the loaded unit S1 and the unloaded unit S13. 2.2. Sampling and analytical methods – statistical analyses Residual sludge samples were collected at the end of each resting period (i.e., exactly before the application of the new sludge loading) using a core sampler, in order to sample the entire depth of the sludge layer. To obtain a more representative residual sludge sample, all samples were taken from a minimum of two different points from each bed surface, and were analyzed for TS (indicator of dewatering efficiency), VS (indicator of mineralization), TKN, NO3 − -N and NO2 − -N and TP according to Standard Methods (APHA, 1998). Plants were harvested once a year (during winter) and the total produced above bed surface dry biomass was weighted. Harvested biomass samples were analyzed for TKN, NO3 − -N and NO2 − -N and TP, while for four units separate analyses were carried out on leaves and stems (Table 3). Plants in units S3 (cattails) and S10 (reeds; no aeration tubes), both with high SLR, did not adjust to the specific environment of the units, were shocked and dried up during the first summer of operation (year 2008). After plant death, both units continued to operate under similar conditions with unplanted unit S4. Water samples for the characterization of the drained fluid were also taken at various time-points (10 min, 1 h, 1 d and 2 d) after sludge application. Samples were analyzed for TSS, VSS, COD, NH4 + -N, SO4 −2 and PO4 3− -P according to Standard Methods (APHA, 1998). For pH and EC measurement, WTW Inolab series instruments were used. Meteorological data (air temperature, precipitation depth, atmospheric pressure, air humidity, wind velocity and direction) were recorded on site for the entire operation period, at a 5-min time interval, using an ELE MM900/950 station. Paired t-test (95% confidence interval of the difference) was used to examine if the differences between the constituent content (TS,

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Table 2 Timetable of loading cycles, influent surplus activated sludge (SAS) characteristics, and meteorological data for the first (October 2007–October 2008), second (October 2008–October 2009) and third (October 2009–October 2010) year of operation. Datesa

Feeding week + resting weeks (total cycle days)

Influent SAS characteristics Parameter

Mean ± sd (N = 32)

May–October 2006 1/10/2006–28/5/2007 29/5/2007–30/9/2007 May–October 2007 01/10/2007–28/10/2007 29/10/2007–02/6/2008 03/6/2008–30/6/2008 31/6/2008–11/8/2008 12/8/2008–25/8/2008 26/8/2008–06/10/2008 07/10/2008–01/12/2008 02/12/2008–18/1/2009 19/1/2009–16/3/2009 17/3/2009–4/5/2009 05/5/2009–01/6/2009 2/6/2009–13/7/2009 14/7/2009–31/8/2009 1/9/2009–26/10/2009 27/10/2009–09/12/2010 10/12/2009–01/2/2010 02/2/2010–5/4/2010 01/4/2010–04/11/2010

Commissioning phase for units S12 and S13 1 + 3 (28) 1 + 1 (14) Commissioning phase for units S1–S11 1 + 1 (14) 1 + 3 (28)a 1 + 1 (14) 1 + 2 (21) 1 + 1 (14) 1 + 2 (21) 1 + 3 (28) 1 + 6a (49) 1 + 3 (28) 1 + 6 (49) 1 + 3 (28) 1 + 2 (21) 1 + 6* (49) 1 + 3 (28) 1 + 5 (42) 1 + 7 (56) 1 + 8 (64) Resting phase

TS (%) VS (%TS)

3.1 ± 0.73 73.7 ± 3.2

TKN (mg/g dm) TP (mg/g dm)

54.8 ± 7.2 12.3 ± 3.3

NO3 − -N (mg/g dm)

0.69 ± 0.37

NO2 − -N (␮g/g dm)

3.57 ± 1.16

a

Air temperature (◦ C) Year 1 Year 2 Year 3 Mean Precipitation (mm) Year 1 Year 2 Year 3 Mean

Mean 16.2 16.2 16.5 16.3

Min −6.6 −2.5 −9.4 −6.2

Max 39.5 38.0 37.1 38.2

Total 590.5 784.8 659.4 678.1

Frequency (days/yr) 82 122 142 115

Loading of S13 stopped in June 2008.

Table 3 Sludge volume reduction, TS and VS statistical data during loading (L-phase: October 2007–April 2010) and resting (R-phase: April 2010–October 2010) phases for all units, and mean values for those receiving high (S1, S2, S7 and S11), medium (S6 and S9) and low (S5, S8 and S12) SLR, and the unplanted units (S3, S4 and S10). Total volume of applied sludge S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12

*

S13** High SLR Medium SLR Low SLR Unplanted (high SLR) * **

L cm L cm L cm L cm L cm L cm L cm L cm L cm L cm L cm L cm L cm L cm L cm L cm L cm

2611 493 2611 493 2611 493 2611 493 1085 205 2102 397 2611 493 1085 205 2102 397 2611 493 2611 493 1709 323 2054 388 2611 493 2102 397 1085 205 2611 493

Final residual sludge volume 71.6 13.5 87.5 16.5 249.1 47.0 257.1 48.5 37.1 7.0 71.5 13.5 90.1 17.0 42.4 8.0 68.9 13.0 254.4 48.0 79.5 15.0 37.1 7.0 39.8 7.5 82.2 15.5 70.2 13.3 39.8 7.5 253.5 47.8

Loading of unit S12 started in October 2006. Data presented here are from the period October 2007–October 2010, as for units S1–S11. For S13 unit, only data from its resting phase are presented (June 2008–October 2010).

Sludge volume reduction (%) 97.3 96.7 89.9 89.8 96.6 96.6 96.6 96.1 96.7 89.8 97.0 97.8 98.1 96.9 96.7 96.4 89.8

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Fig. 2. (a) Monthly precipitation depth and solar energy flux; (b) daily air temperature; (c) variation of mean residual sludge layer depth in planted units receiving high (S1, S2, S7 and S11), medium (S6 and S9) and low (S5, S8 and S12) SLR, and the unplanted units (S3, S4 and S10).

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VS, TKN, TP, nitrite and nitrate) of pairs of units were statistically significant. Tests were carried out using MINITAB® Release 14.0. 3. Results and discussion 3.1. Overall SDRB performance 3.1.1. Meteorological conditions Fig. 2a presents the monthly rainfall depth and the average daily solar energy flux and Fig. 2b the daily air temperature for the entire monitoring period (October 2007–October 2010). Table 2 also contains temperature and rainfall data. For the study period (three years), the average annual temperature value was 16.3 ◦ C, with respective mean values of 26.3 ◦ C and 6.7 ◦ C for the warm and cold seasons. The average annual rainfall depth was 678 mm, with an average annual rain frequency of 115 days/year. Mean rainfall depths for the warm and cold seasons were 70.3 and 38.9 mm, respectively. The maximum measured solar energy value and the heavier daily rainfall incident recorded were 1203.0 W/m2 and 69.0 mm, respectively. These values are typical of Mediterranean climate. 3.1.2. Influent SAS quality and loading rate Table 2 presents various physico-chemical characteristics of the inflowing SAS. The sludge loads applied to units S1–S11 after 2.5 years of operation (October 2007–April 2010) were 2.0, 4.0 and 4.9 m3 /m2 for the low, medium and high SLR (30, 60 and 75 kg dm/m2 /yr, respectively). Using a mean TS content of 3.1% of the SAS, the respective loads applied were 67.7, 131.1 and 162.9 kg TS/m2 . Unit S12 (low SLR) operated for 3.5 years (October 2006–April 2010) and received 3.2 m3 /m2 or 106.9 kg TS/m2 , and unit S13 (high SLR), which operated for 1.5 year (October 2006–June 2008) and then rested, received 4.1 m3 /m2 or 136.2 kg TS/m2 . The mean daily loadings, depending on the SAS TS content, were 6.8, 13.2 and 16.3 L for the low, medium and high SLR, respectively. Considering a sludge production of 16 kg TS/pe/yr during primary wastewater treatment (Uggetti et al., 2010), the surface area for the units receiving low, medium and high SLR corresponded to 1.7, 3.2 and 4.0 pe/m2 . These values are within the range of 1.5–4.0 pe/m2 reported by De Maeseneer (1997). 3.1.3. Dewatering efficiency The total sludge volumes applied to units S1–S11 were 1085, 2102 and 2611 L for low, medium and high SLR, respectively, and to units S12 and S13 1709 and 2054 L (low and high SLR, respectively; Table 3). The dewatering efficiency was calculated based on the sludge volume decrease during the resting periods of the loading and the resting phases. Removal of water content in all planted units exceeded 95% (Table 3). Respective reported values varied from 91% to 98% (Burgoon et al., 1997; Cooper et al., 2003; Begg et al., 2001; Nielsen, 2003). The highest removal was observed in unit S12 (97.8%), which received low SLR and operated for 3.5 years. Fig. 2c presents the development of the residual sludge layer height during the entire monitoring period (loading and resting phase) for the units receiving the three SLRs and for the unplanted units. During the 2.5 years of loadings, the mean accumulated sludge ranges from 8.0 cm in the units receiving low SLR (S5, S8 and S12), to 14.0 and 18.8 cm in the units with medium (S6 and S9) and high SLR (S1, S2, S7 and S11), respectively, while higher values were observed in the unplanted units (S3, S4 and S10; 34.4 cm). This chart is a first indication that dewatering is a year-round process, affected by seasonal variations. The average annual solids accumulation reaches 7.5, 6.1 and 3.0 cm/yr for units with high, medium and low SLR, and 19.0 cm/yr for the unplanted units. The values for

the planted beds are below the range of maximum 10–14 cm/yr reported in the literature in colder climates (Begg et al., 2001; Nielsen and Willoughby, 2005), although treating a higher SLR (75 kg dm/m2 /yr) than the maximum proposed (60 kg dm/m2 /yr). If the loadings continued, and assuming a freeboard of 80 cm, solids accumulation could possibly last for 10–13 years in the beds with high and medium SLR and more than 26 years in the units with low SLR, before emptying becomes necessary; these values are close to those reported (8–12 years) in the literature (Nielsen, 2003). For the unplanted units, loadings would last for 4.5 years. The dewatering efficiency is associated with the Total Solids (TS) content in the residual sludge. Fig. 3a presents mean TS content in the residual sludge of each pilot-scale unit during the loading phase (2.5 years) and the resting phase (6 months). TS content is affected by the SLR. Units receiving the low (S5, S8 and S12), medium (S6 and S9) and high (S1, S2, S7 and S11) SLR had a mean TS content of 30.4%, 24.3% and 22.3%, respectively, during the loading phase (Fig. 3a). These values are comparable to respective efficiencies of conventional dewatering with mechanical equipment (Zwara and Obarska-Pempkowiak, 2000). Unplanted units (S3, S4 and S10) showed the lower TS content (17.4%). Fig. 4 contains charts of solids and various constituent contents for the entire monitoring period. The vertical dash line to the right of the graphs separates the loading and resting phases. The comparison of Fig. 2c (sludge layer variations) and Fig. 4a (TS content variations) reveals a parallel behavior of sludge layer height and TS variations, since both are affected by season. Higher TS values (74.6%, 65.6% and 85.5% for units with high, medium and low SLR, respectively) were observed during spring and summer months due to increased water losses through evapotranspiration (ET) (Uggetti et al., 2009; Stefanakis and Tsihrintzis, 2011). During the loading phase, a resting period of 1 or 2 weeks was applied in the warmer months and of 3 weeks in the colder months (Table 2). This regime assisted the dewatering process, since at higher temperatures this process proceeds more rapidly compared to winter. The resting period duration is also very important, since it prevents the biological clogging of the bed by restoring aerobic conditions within the system, thus helping the decomposition of the accumulated organic matter (Kadlec and Wallace, 2009). Extended resting periods up to 7 weeks were also applied at different seasons (Table 2). A longer resting period during the winter months did not appreciably improve the residual sludge characteristics and the TS content was not highly increased, due to increased precipitation. However, in the summer, the extended resting period increased appreciably the dry matter content, but also tested the tolerance of the plants. Harvested biomass densities show a gradual increase year by year (Table 4). As mentioned, the role of reeds is essential in the dewatering process, and the denser a reed cluster is, the higher the ET values are (Stefanakis and Tsihrintzis, 2011). In April 2010, sludge feeding was stopped and the units were left for 6 months. The drying process was completed during the resting phase. The residual sludge height was decreased in all units from 38.4% to 46.3%, depending on the SLR (Fig. 2c), since water was further removed and the solids content increased. The mean TS content in all units was nearly double compared to the loading phase and reached 50.1%, 58.8% and 64.8% for units receiving high, medium and low SLR, respectively (Fig. 3a), and 31.1% for the unplanted units. The last samples taken in early September had the highest TS (69.2%, 82.0% and 90.4% for high, medium and low SLR, respectively). 3.1.4. Organic matter removal Fig. 3b presents the Volatile Solids (VS) content. Starting from a value of 73.7%TS in the SAS, the VS content was decreased in all units, as also reported by Uggetti et al. (2009). Mean VS values

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Fig. 3. Content of residual sludge: (a) TS; (b) VS; (c) TKN; (d) NO3 − -N; (e) NO2 − -N; (f) TP during loading (L-phase: October 2007–April 2010) and resting (R-phase: April 2010–October 2010) phases for all units, and mean values for those receiving high (H; S1, S2, S7 and S11), medium (M; S6 and S9) and low (L; S5, S8 and S12) SLR, and the unplanted units (Un.; S3, S4 and S10).

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Fig. 4. Time series charts for sludge content of: (a) TS; (b) VS; (c) TKN; (d) NO3 − -N; (e) NO2 − -N; (f) TP.

1.28 1.31 1.04

1.84 3.43 4.31

1.70 1.85 2.05

1.32 1.99 7.16 7.13

0.83 0.17 0.52 0.61 0.52 0.48 0.37

2.18 6.66 5.21

1.61 3.02 2.74

0.06 0.16 0.38

2.75 4.19 3.83

2.53 3.68 3.25

0.68 0.25 0.41 0.53 4.50 1.99 5.10 4.64 1.96 3.31 4.04 3.82

5.0 15.9 21.6 20.3 0.87 0.29 0.46 0.38 1.14 1.20 3.40 3.26 1.25 1.21 3.11 2.41 3.3 14.5 21.0 19.8 1.19 0.26 0.48 0.12 1.29 2.82 3.22 2.83 1.02 1.63 1.37 8.7 – – – – – – – – – – – – – – – – L, leaves; S, stems; L + S, leaves and stems (total biomass).

NO2 − -N (␮g/g)

NO3 − -N (mg/g)

TP (mg/g)

TKN (mg/g)

a

1.45 1.92 1.87

0.86

1.09 1.72 1.71

1.27 2.86 2.64

0.22 0.21 0.29 1.66

2.65 5.42 4.92

22.7 26.6 23.4

0.25 0.29 0.62

0.33

5.6 17.1 16.7

L+S

5.8 – – – 0.36 – – – 1.85 – – – 1.05 – – – 4.6 13.2 25.2 22.1 0.29 0.12 0.56 0.26 1.22 1.79 5.59 6.16 1.15 1.74 2.37 1.48

L+S La

Jan 2008 Jan 2009 Jan 2010 Jan 2011 Density (g/m2 )

Jan 2008 Jan 2009 Jan 2010 Jan 2011 Jan 2008 Jan 2009 Jan 2010 Jan 2011 Jan 2008 Jan 2009 Jan 2010 Jan 2011 Jan 2008 Jan 2009 Jan 2010 Jan 2011

119.3 363.2 1496.5 701.8

5.2

Sa

100.0 466.7 1078.9 736.8

121.1 – – –

L+S

– – – –

L+S

112.3 377.2 680.7 438.6

L+S

L

10.8 18.6 19.9

5.9

S

7.7 7.6 7.7

L

22.3 18.4 19.2

6.9 10.3 12.1 0.10 0.45 0.40 0.36 1.35 1.92 4.63 3.93 0.96 1.59 1.48 2.37

15.1 32 30.8

8.5 L S 3.7

152.6 326.3 886.0 626.3 257.9 198.2 400.0 614.0 200.0 398.2 1354.4 1403.5 71.9 419.3 721.1 526.3

1.19

S

5.7 7.6 9.0

0.07 0.37 0.40

L+S

13.1 16.2 13.2 12.8 0.26 0.53 0.35 0.36 2.93 3.68 2.43 2.01 1.72 2.02 2.59 2.25 3.9 8.8 15.9 16.7 0.77 0.53 0.44 0.49 0.72 2.65 5.28 5.25 1.17 1.10 4.06 3.62

L+S L+S

101.8 452.6 1152.6 1052.6 149.1 -

S11 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1

Unit Date Parameter

Table 4 Harvested biomass densities and nutrient analyses.

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during the loading phase (2.5 years) for units with high (S1, S2, S7 and S11), medium (S6 and S9) and low (S5, S8 and S12) SLR and the unplanted units (S3, S4 and S10) were 65.1, 62.4, 61.6 and 66.4%TS (Fig. 3), respectively. As it is seen, lower sludge loadings resulted in lower VS content. These values correspond to a respective mean VS removal of 11.7%, 15.3%, 16.5% and 9.9%. VS variation (Fig. 4b) implies that mineralization takes place gradually with time. At the last sampling before the resting phase (April 2010), after 2.5 years of continuous loadings, the VS content in units with high, medium and low SLR was 59.9%, 59.2% and 58.7%TS (respective decrease of 18.7%, 19.7% and 20.4%), which are similar to those reported for aerobic or anaerobic digestion (50–65%; Metcalf and Eddy, 2003), indicating that SDRBs demonstrate a comparable performance, not only for sludge dewatering and drying, but also for sludge mineralization. A slight seasonal effect is also present. Spring and summer months appear to favor a more intense decrease of the VS content. This has to be attributed to plant activity; plants transfer oxygen to the bed through their root system, thus enhancing the organic matter decomposition by bacteria (Uggetti et al., 2010). The VS content was further decreased during the resting phase. Mean VS for units with high, medium and low SLR was 56.8%, 57.5% and 52.1%TS (Fig. 3). It is noticeable that the last samples taken in early September 2010 had the highest TS content (69.2%, 82.0% and 90.4% for high, medium and low SLR, respectively), and the lowest VS content measured (51.6%, 55.2% and 46.1%TS).

3.8 – – – 0.79 – – – 0.85 – – – 1.25 – – –

445.9 770.8 857.9 621.0

S12

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3.1.5. Nutrients in residual sludge Mean residual sludge nutrient concentrations are presented in Fig. 3c–f and respective variations during the entire monitoring period in Fig. 4c–f. The nutrient content in the residual sludge was reduced in all units. TKN concentration was decreased from 54.8 mg TKN/g dm in the SAS (Table 2) to mean 35.1, 35.3 and 31.5 mg TKN/g dm for the units with high (S1, S2, S7 and S11), medium (S6 and S9) and low (S5, S8 and S12) SLR during the loading phase, respectively. These values correspond to a respective reduction of 35.3%, 34.9% and 42.0%. TKN removal in the unplanted units (S3, S4 and S10) was only 24.3%. TKN mean reduction was accompanied by a simultaneous increase in nitrate production within all units. As Fig. 4c shows, TKN concentration in the residual sludge decreases gradually with time, while NO3 -N shows a gradual increase (Fig. 4d). This means that nitrification takes place within the beds, which implies the presence of aerobic microzones, especially near the plant roots. However, since the nitrate production was not as high as TKN reduction, it can be assumed that ammonification of organic nitrogen also takes place. Oxygen transfer for these processes is accomplished through root transport, aeration pipes and air trapping during the sludge feeding. On the other hand, increase of nitrite concentrations (Fig. 3d) compared to SAS, indicates that denitrification also takes place. Fig. 4e shows slight variations of NO2 -N production during the entire operation period. The simultaneous presence of microzones dominated by anaerobic conditions was also implied by gas bubbles observed on the sludge surface. The TP content was also decreased in the residual sludge. From the mean initial concentration of 12.3 mg TP/g dm in the SAS (Table 2), respective mean concentrations in the residual sludge during the loading phase (2.5 years) were 7.3, 7.4 and 7.1 mg TP/g dm for units receiving high, medium and low SLR, while in the unplanted units (S3, S4 and S10) the TP content was 7.8 mg/g dm (Fig. 3f). Average removal rates during the loading phase were 42.1%, 40.9% and 42.7% for units with high, medium and low SLR, respectively, and 38.6% for the unplanted units. Fig. 4f shows that the decrease of TP concentration in the residual sludge was constant, even when the SAS applied had higher TP content.

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The differences between the various units are smaller compared to nitrogen, probably because the main phosphorus removal mechanism (sorption onto substrate) is present in all units to the same – more or less – extent. The nutrient content also decreased during the resting phase. Mean TKN concentration was reduced about 14.3%, 10.8% and 15.9% for units with high, medium and low SLR compared to the loading phase (Fig. 3c), as also implied by the simultaneous increase of nitrate (Fig. 4e). The dewatering process continued through reed activity and direct evaporation from the sludge layer surface, since the resting phase took place during the spring and summer months (high temperatures). Under these conditions, the sludge layer surface was rapidly dried and showed cracks and intense harshness, resulting in direct exposure to the atmosphere of large residual sludge parts. Thus, more oxygen was diffused within the residual sludge layer. Another indication for this is the nitrite decrease by 12.5–76.5% during the resting phase. On the other hand, the TKN concentration was practically the same for both phases in the unplanted units (Fig. 3c). Due to the great thickness of the sludge layer in these units (Fig. 2c), oxygen transfer within the sludge layer was more limited, which favored the creation of anaerobic conditions, as indicated by increased nitrite during the resting phase in these units (Fig. 3e and Fig. 4e). TP concentrations between the two phases were only slightly different for all units (Fig. 3f). Since there was no sludge feeding, phosphorus adsorption onto the substrate was limited, and TP concentration of the residual sludge remained more or less steady. 3.1.6. Drained water quality The determination of the drained water quality is important, since the handling of the drained water includes pumping and treatment in the nearby BWTF or recirculation on the SDRB unit surface. Quality characteristics of the drained water were determined during the first two years of operation. Fig. 5 presents charts of the variation of different parameters measured in the influent SAS and in the drained water at each time-point (10 min, 1 h, 2 h, 1 d and 2 d). It has to be mentioned that after the first two years, and when a sufficient sludge layer (with more than 20% TS) was created, the volume of drained water was limited and, especially in units with low SLR, drainage practically used to stop in 1–2 days after sludge application. Usually, after heavy rainfall incidents, the drained water volume was increased. This excellent dewatering efficiency implies that clogging of the beds did not take place. COD in drained water was rapidly decreased. Reduction exceeded 85%, 10 min after sludge application, and continued over time, with only small variations in the units (Fig. 5a). This has also been observed by others (Wang et al., 2009). After 2 days, the COD decrease reached 92.0%, while in the units with low SLR (S5, S8 and S12) the decrease was over 97.0%. These indicate that the major portion of the influent organic matter remains within the bed. Additionally, occasional BOD5 measurements showed that the biodegradable organic matter was always below 10–15% of the COD. A decrease was also observed in NH4 + -N concentrations, starting from 67 to 90% in the first 10 min. At the same time point, a high increase of nitrate appeared in all units (Fig. 5c). This indicates that rapid nitrification takes place exactly after sludge feeding. NH4 + -N concentration showed variations with time (Fig. 5b), while nitrate production dropped from the concentration in 10 min but remained at high levels. This probably means that additional ammonia nitrogen is produced through the ammonification process. Units S10 showed the highest NH4 + -N concentration, which implies that under lower oxygen amounts (absence of aeration pipes), higher NH4 + -N concentration leaves the system through drainage. An interesting observation was the sharp nitrite increase

at 1 day after SAS application (Fig. 5d). If coupled with the lowest nitrate concentration at the same time point, it seems that denitrification takes place after 1 day, especially in units with high and medium SLR. It is possible that the thicker sludge layer in these units allows for the creation of additional anaerobic sites within the bed. Moreover, the highest nitrite production was observed in units S7, S8 and S9, which were all receiving additional Cr concentration. This is an indication that the presence of chromium possibly limits the extension of the root systems, thus lower oxygen is transferred within the bed, allowing this way for the creation of additional anaerobic microzones compared to the other units. On the whole, NH4 + -N reduction was higher in the units receiving low SLR (87% after 2 days) and lower in the unplanted units (63.9%), which again indicates the aerating capabilities of the reeds. Ortho-phosphate concentrations showed a decrease in the drained water between 69 and 73% at 10 min, which reached higher values (over 90%) after 2 days (Fig. 5e). Removal in the unplanted units was always lower compared to the planted units, which indicates that without reeds more phosphorus leaves the system through the drained water. These differences were lower during the first day (6–10%) but increased after the second day (15–20%). Planted units possess a lower hydraulic conductivity, which favors the slower filtration of the fluid downwards, offering a greater contact time with the substrate, thus enhancing phosphorus adsorption compared to the unplanted units. In the case of sulphate, during the first 2 h after sludge application, units with low SLR and unplanted units showed higher initial reductions. After 2 h, the sulphate decrease became more intense (Fig. 5f), which implies that anaerobic microzones are created with time within the bed. The unplanted units showed the greatest sulphate reduction, which again indicates the lower oxygen levels compared to the planted units. TSS and VSS concentrations were rapidly decreased, even after 10 min. After 2 days, the draining water was practically free of suspended solids (Fig. 5g and h), which means that the entire solids amount was retained within the beds. This also holds for the unplanted units (S3, S4 and S10), although the solids reduction proceeds relatively slower compared to the planted units. Regarding pH values, a decrease was observed during the first hour after sludge application. This decrease could be correlated with the intensive nitrification during the same time points (Fig. 5c), which produces nitrate and H+ , thus leading to a respective decrease of the pH. After that point, increase of pH values (Fig. 5i) can be connected with the respective increase of EC (Fig. 5j), since interactions between the substrate material and the attached biofilm result in salt release from the media to the fluid (Kadlec and Wallace, 2009). 3.2. Effect of sludge loading rate (SLR) and final resting phase Units S1, S5 and S6 received high, medium and low SLR (75, 30 and 60 kg dm/m2 /yr, respectively; Table 1), while design characteristics were the same. All units showed similar dewatering efficiency, with a volume reduction that exceeded 96% (Table 3). Unit S1 had the lowest TS content and unit S5 the highest TS content among these three units (Fig. 3a). The VS content showed a decrease with SLR (Fig. 3b) which also holds for nutrients (TKN and TP; Fig. 3c–f). Higher differences occurred between units with low and high SLR. Statistical analysis showed that the differences in TS and TKN content between pair of units (S1–S5 and S5–S6) are significant (p < 0.05), while for VS content significant differences were found for the pair S1–S5 only (high and low SLR). The comparable dewatering efficiency of the three units is an indication that the systems, under the particular climate setting, possibly possess an even greater dewatering capacity, which means that even higher SLR

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Fig. 5. Drained water parameter variations at various time points: (a) COD; (b) NH4 + -N; (c) NO3 − -N; (d) NO2 − -N; (e) PO4 −3 -P; (f) SO4 −2 ; (g) TSS; (h) VSS; (i) pH; (j) EC.

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than those tested could be applied. However, results showed that the residual sludge characteristics are improved as the level of the SLR decreases. In all cases, the final product (after the resting phase) exceeded the maximum reported TS content of 40% (Nielsen, 2003) and reached even a 90% TS content for units with low SLR, with simultaneous decrease of the nitrogen and phosphorus content. It is noticeable that the dewatering efficiency was excellent even for the units receiving a higher SLR (75 kg dm/m2 /yr) than the maximum proposed (50–60 kg dm/m2 /yr; Nielsen, 2003); at least this is valid in the Mediterranean environment. As mentioned, unit S13 remained unloaded for more than 2 years (June 2008–October 2010). The importance of the resting phase (no loadings) is clearly depicted by the comparison of unloaded unit S13 with loaded unit S1, both with high SLR. Unit S13 had higher mean TS content of 53.1% (Fig. 3a), while unit S1, with only 6 months resting, showed lower mean TS content (46.2%) during its resting phase. The highest difference occurred at the last sampling (November 2010), when TS in S13 reached 91.0% under high temperatures (69.2% in S1). Additionally, the VS content in unit S13 was always 2–7% lower than in unit S1. Nutrient content (TKN and TP) was also lower in unit S13. Statistical analyses showed that the TS, VS, TKN and nitrate differences of units S1 and S13 were significant (p < 0.05). These results reveal the importance of the resting phase and confirm that, during this phase, the residual sludge improves its final characteristics. The proposed period for the application of the resting phase under the Mediterranean climate is to start in late spring (i.e., in late April to mid-May) and end in late summer (i.e., late August), with a mean duration of about 4 months. 3.3. Effect of vegetation As mentioned, cattails in unit S3 were shocked and dried during the first summer of operation (2008); therefore, their function cannot be evaluated. Dieback signs for cattails were also reported by Koottatep et al. (2005) at the beginning phase of a system operating under tropical conditions. This implies that cattails should probably be lightly loaded during their first growing season, since they seem to be more sensitive to the initial sludge loads. The comparison of the planted unit S1 with the unplanted units S3, S4 and S10 (all with same design characteristics and receiving same SLR) showed that vegetation is a key component in effective sludge dewatering. The presence of reeds improved the dewatering efficiency by about 7.5% (Table 3). TS content was always higher in the planted unit during the loading (23.8% vs 17.4%; Table 3), and the resting phase (46.2% vs 31.1%). This should be attributed to the high transpiration capacity of the reeds (De Maeseneer, 1997). Indeed, cumulative evapotranspiration value for unit S1 was found increased by 35% compared to the unplanted units (Stefanakis and Tsihrintzis, 2011), showing that the presence of plants enhances the dewatering process. Additionally, unit S1 showed a faster mineralization, since the VS content was always 5–6% lower compared to the unplanted units during the entire monitoring period. The same also holds for nutrient content. This was also confirmed by the higher organic matter, nitrogen and phosphorus concentrations measured in the drained water of the unplanted units (Fig. 5). Sampling also showed that the lower part of the sludge layer in planted beds was brown, indicating aerobic conditions, while in the unplanted beds the same layer was black (indicating anaerobic conditions), as also observed by Uggetti et al. (2009) and Melidis et al. (2010). Improved performance in planted beds has also been reported in other studies (De Maeseneer, 1997; Liénard et al., 1995; Edwards et al., 2001; Cooper et al., 2003; Wang et al., 2009). The fact that reeds improve the performance of the system was also confirmed

by statistical analysis. The differences of all parameters (TS, VS, TKN, nitrate, nitrite) between the pairs of the S1–S3 and S1–S4 units were found to be significant (p < 0.05). The lack of statistical significance for phosphorus implies that the main removal mechanism is adsorption onto the substrate. Furthermore, the similar performance of the unplanted units S4 and S3, with simultaneous lack of statistically significant differences, confirms that the experimental conditions were comparable within the beds. Reeds utilize sludge nutrients (nitrogen and phosphorus) for their growth. However, plant uptake was estimated to account only for less than 1% of the total nitrogen and phosphorus loading (Table 4). This value is lower than a respective value of 5%, reported by Koottatep et al. (2004) for very high SLR (250 kg TS/m2 /yr) in a tropical climate. Panuvatvanich et al. (2009) reported plant uptake of 0.2% for the same SLR of 250 kg TS/m2 /yr. An interesting observation is that the major portion of the accumulated nitrogen in plant biomass was concentrated in the leaves (Table 4), no matter what the SLR was. Although leaves accounted only for 25–30% of the total produced biomass, they accumulated more than 60% of the total accumulated nitrogen and phosphorus in the aboveground biomass. Similarly, Ozimek et al. (2001) reported that 50% of the accumulated nitrogen was located in the leaves.

3.4. Effect of porous media origin and thickness As mentioned, units S1 and S2 contained materials of different origin but of the same size: igneous and carbonate, respectively (Table 1). Otherwise, the units were similar in design characteristics and received the same SLR. Both units showed comparable dewatering efficiencies (Table 3). Slight differences occurred in TS, VS, TKN and TP content (Fig. 3). Moreover, nitrogen and phosphorus concentrations in drained water were comparable. The lack of significant differences between the two units implies that the material does not significantly alter the system performance. The material in unit S1 was fine-grained and in unit S11 coarsegrained, both of the same origin. Other design characteristics and SLR were the same. The dewatering efficiency of both units exceeded 97% (Table 3). Higher porosity (no fine gravel layer) of S11 unit, thus larger pore volume, resulted in higher drained water volume and limited evapotranspiration (Stefanakis and Tsihrintzis, 2011). Drained water of unit S11 contained higher COD, nitrogen and phosphorus concentrations. Statistical analysis did not reveal significant differences for the various parameters (TS, VS, TKN and TP). On the whole, the fine-grained material (unit S1) appears to be more appropriate, since the drained water was of better quality and the water volume to be handled was smaller.

3.5. Effect of aeration tubes Unit S10 contained no aeration tubes and was unplanted, since reeds dried during the first summer of operation. Compared to the other unplanted units (S3 and S4), which contained aeration tubes, the dewatering efficiency was similar (89.8%; Table 3). TS and VS content were also similar in these three units, as also the nutrient content (Fig. 3). These imply that the presence of the tubes and the better aeration of the bed do not directly improve the dewatering process, as also observed by Edwards et al. (2001). Heinss and Koottatep (1998) also reported plant death in beds without bottom ventilation tubes. It can be stated, that the enhanced bed aeration assists plant growth, and thus, improves indirectly the dewatering efficiency in planted beds.

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3.6. Effect of chromium content Units S7, S8 and S9 received higher Cr concentration (5.0 mg/g dm), compared to the original concentration in the SAS (mean 0.28 mg/g dm). The dewatering efficiency of these units was not altered and remained above 96% (Table 3), compared to respective units with the same SLR (S1, S5 and S6). TS content in the units with extra chromium concentrations was about 6.8–10% lower during the loading phase (Fig. 3). Differences in TS of unit pairs with the same SLR revealed statistical significance. Regarding the VS content, the differences were smaller (1–2%). Slight alterations also occurred in the nutrient concentrations of the units receiving extra chromium, with respective units of the same SLR (Fig. 3). Furthermore, the produced biomass was altered, mainly for units S8 and S9 (low and medium SLR, respectively). Indeed, these two units showed lower ET values compared to respective units receiving the normal Cr concentration, while unit S7 (high SLR) showed a comparable ET value with unit S1 (Stefanakis and Tsihrintzis, 2011). This probably means that the Cr effect is more or less compensated by the higher SLR (more water volume available), while under lower SLR the presence of Cr possibly limits the extension of the root systems. Limited common reed growth was reported by Mant et al. (2006) for a system treating tannery wastewater with Cr concentration of 10 mg/L. 4. Summary and conclusions Thirteen pilot-scale SDRBs were constructed and operated for three years. Experimental results showed the effectiveness of SDRBs, showing a water volume decrease up to 97.0% and a final TS content of the planted units above 45%. These efficiencies are comparable to those of conventional dewatering methods. Residual sludge shows a gradual mineralization, since TP and TKN concentration were significantly decreased compared to the influent SAS. The experimental data obtained from the current study indicate that the design of SDRBs should consider the following for optimum performance: • The systems are capable of treating effectively even higher SLR than the one tested here (75 kg dm/m2 /yr), i.e., probably up to 85–90 kg dm/m2 /yr. The maximum recommended area is 4 pe/m2 for high SLR. • Under the SLRs tested, the system lifecycle prediction is for at least 10 years before emptying the beds, assuming a freeboard of 80 cm above the porous media. • The length of the resting periods between sludge loadings should be based on season. For Mediterranean climate, the recommendation is one week loading and three weeks resting in the winter, and one (for lower SLR) or two (for higher SLR) weeks resting in the summer. The increase in sludge loading frequency in the summer is important for plant survival. • The final resting phase should be during summer months (higher temperatures). Duration of at least 4 months appears to be adequate, for the residual sludge to obtain its near to final characteristics (TS above 65% and VS below 55%TS). • The presence of plants significantly improves the overall performance of the system. Common reeds appear to be a more appropriate plant species. Cattails should be lightly loaded during the initial growth to avoid plant death. • The use of porous media of different origin did not improve system performance. However, material thickness is important, and a fine-grained media appears to be more appropriate. The recommended bed setup is 3 layers of fine and medium gravel and cobbles, each 15 cm thick.

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• The presence of aeration tubes, placed in the lower porous media layer, allows for better substrate aeration and supports the viability of the reeds. • The beds proved to be appropriate for phytoremediation of sludge containing high concentrations of Cr, e.g., industrial sludge, at high loading rates. Plants seem to tolerate high metal concentration without apparent significant toxicity signs (e.g., yellowing, wilting), when adequate water volume is available. Acknowledgements The study was funded by the General Secretariat of Research and Technology (GSRT) of Greece, as part of the project “Integrated Management of Sludge from Wastewater Treatment Facilities, and Wastewater Treatment Using Natural Systems”, Operational Program of the Region of East Macedonia – Thrace, 2000–2006. References APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, USA. Begg, J.S., Lavigne, R.L., Veneman, P.L.M., 2001. Reed beds: constructed wetlands for municipal wastewater treatment plant sludge dewatering. Water Sci. Technol. 44 (11–12), 393–398. Bianchi, V., Peruzzi, E., Msciandaro, G., Ceccanti, B., Mora Ravelo, S., Iannelli, R., 2010. Efficiency assessment of a reed bed pilot plant (Phragmites australis) for sludge stabilization in Tuscany (Italy). Ecol. Eng. 37 (5), 771–778. Burgoon, P.S., Kirkbride, K.F., Henderson, M., Landon, E., 1997. Reed beds for biosolids drying in the arid NW United States. Water Sci. Technol. 35 (5), 287–292. Campbell, H.W., 2000. Sludge management – future issues and trends. Water Sci. Technol. 41 (8), 1–8. Cooper, P., Cooper, D., Edwards, J., Biddlestone, J., 2003. Treatment of sludges from sewage and agricultural wastewaters in Sludge Drying Reed Beds. In: Vymazal, J. (Ed.), Wetlands – Nutrients, Metals and Mass Cycling. Backhuys Publishers, Leiden, The Netherlands, pp. 201–213. De Maeseneer, J.L., 1997. Constructed wetlands for sludge dewatering. Water Sci. Technol. 35 (5), 279–285. Edwards, J.K., Gray, K.R., Cooper, D.J., Biddlestone, A.J., Willoughby, N., 2001. Reed bed dewatering of agricultural sludges and slurries. Water Sci. Technol. 44 (11–12), 551–558. Hardej, M., Ozimek, T., 2002. The effect of sewage sludge flooding on growth and morphometric parameters of Phragmites australis (Cav.). Trin. Ex Steudel. Ecol. Eng. 18, 343–350. Heinss, U., Koottatep, T., 1998. Use of Reed Beds for Faecal Sludge Dewatering – A Synopsis of Reviewed Literature and Interim Results of Pilot Investigations with Septage Treatment in Bangkok, Thailand. Asian Institute of Technology Bangkok, Thailand – Urban Env. Engineering & Management Program. Kadlec, R.H., Wallace, S.D., 2009. Treatment Wetlands, second ed. CRC Press, New York, USA. Kengne, I.M., Akoa, A., Soh, E.K., Tsama, V., Ngoutane, M.M., Dodane, P.-H, Koné, D., 2008. Effects of faecal sludge application on growth characteristics and chemical composition of Echinochloa pyramidalis (Lam.) Hitch. and Chase and Cyperus papyrus L. Ecol. Eng. 34 (3), 233–242. Kim, B.J., Smith, E.D., 1997. Evaluation of sludge dewatering reed beds: a niche for small systems. Water Sci. Technol. 35 (6), 21–28. Koottatep, T., Polprasert, C., Nguyen, T.K.O., Montangero, A., Doulaye, K., Strauss, M., 2004. Sludges from on-site sanitation systems – low-cost treatment alternatives. In: Proceedings of 9th International IWA Specialist Group Conference on Wetland Systems for Water Pollution Control, Avignon, France, 27 September–1 October. Koottatep, T., Surinkul, N., Polprasert, C., Kamal, A.S.M., Koné, D., Montangero, A., Heinss, U., Strauss, M., 2005. Treatment of septage in CWs in tropical climate: lessons learnt after seven years of operation. Water Sci. Technol. 51 (9), 119–126. Liénard, A., Duchène, P., Gorini, D., 1995. A study of activated sludge dewatering in experimental reed-planted or unplanted sludge drying beds. Water Sci. Technol. 32 (3), 251–261. Mant, C., Costak, S., Williamsk, J., Tambourgik, E., 2006. Phytoremediation of chromium by model constructed wetland. Bioresour. Technol. 97, 1767–1772. Melidis, P., Gikas, G.D., Akratos, C.S., Tsihrintzis, V.A., 2010. Dewatering of primary settled urban sludge in a vertical flow wetland. Desalination 250, 395–398. Metcalf, Eddy, 2003. Wastewater Engineering: Treatment and Reuse, 4th ed. McGraw-Hill, New York. Nielsen, S., 2003. Sludge Drying Reed Beds. Water Sci. Technol. 48 (5), 101–109. Nielsen, S., Willoughby, N., 2005. Sludge treatment and drying reed bed systems in Denmark. J. Water Environ. Manage.: WEJ 19 (4), 296–305. Ozimek, T., Obarska-Pempkowiak, H., Cytawa, S., 2001. The effect of secondary sewage sludge on retention of nitrogen and phosphorus in Phragmites australis growing in constructed wetlands. In: Vymazal, J. (Ed.), Transformations of

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Nutrients in Natural and Constructed Wetlands. Backhuys Publishers, Leiden, The Netherlands, pp. 177–186. Panuvatvanich, A., Koottatep, T., Kone, D., 2009. Influence of sand layer depth and percolate impounding regime on nitrogen transformation in vertical-flow constructed wetlands treating faecal sludge. Water Res. 43, 2623–2630. Peruzzi, E., Msciandaro, G., Macci, C., Doni, S., Mora Ravelo, S.G., Peruzzi, P., Ceccanti, B., 2010. Heavy metal fractionation and organic matter stabilization in sewage sludge treatment wetlands. Ecol. Eng. 37 (5), 779–785. Stefanakis, A.I., Tsihrintzis, V.A., 2011. Dewatering mechanisms in pilot-scale Sludge Drying Reed Beds: effect of design and operational parameters. Chem. Eng. J. 172, 430–443. Stefanakis, A.I., Akratos, C.S., Melidis, P., Tsihrintzis, V.A., 2009. Surplus activated sludge dewatering in pilot-scale Sludge Drying Reed Beds. J. Hazard. Mater. 172, 1122–1130.

Uggetti, E., Llorens, E., Pedescoll, A., Ferrer, I., Castellnou, R., García, J., 2009. Sludge dewatering and stabilization in drying reed beds: characterization of three full-scale systems in Catalonia, Spain. Bioresour. Technol. 100 (17), 3882–3890. Uggetti, E., Ferrer, I., Llorens, E., García, J., 2010. Sludge treatment wetlands: a review on the state of the art. Bioresour. Technol. 101, 2905–2912. Wang, R., Korboulewsky, N., Prudent, P., Baldy, V., Bonin, G., 2009. Can vertical-flow wetland systems treat high concentrated sludge from a food industry? A mesocosm experiment testing three plant species. Ecol. Eng. 35, 230–237. Zwara, W., Obarska-Pempkowiak, H., 2000. Polish experience with sewage sludge utilization in reed beds. Water Sci. Technol. 41 (1), 65–68.