Assessment of sludge characteristics from a Biological Trickling Filter (BTF) system

Journal of Water Process Engineering 22 (2018) 172–179

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Assessment of sludge characteristics from a Biological Trickling Filter (BTF) system

T

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Jason. B.K. Park , Chris. C. Tanner, Rupert. J. Craggs National Institute of Water and Atmospheric Research Ltd (NIWA), P. O. Box 11-115, Hamilton, New Zealand

A R T I C L E I N F O

A B S T R A C T

Keywords: Biological Trickling Filter (BTF) Sludge treatment Sludge dewatering Sludge mineralisation Sludge settling velocity

This study investigates settling and drainage properties of Biological Trickling Filter (BTF) sludge for dewatering and mineralisation potentially using natural Sludge Treatment Wetlands (STWs). Sludge settling experiments were conducted to determine solid removal efficiency using 1-L Imhoff cones (after 1 hour) and its effect on the removal of particulate components such as BOD5, TN, and TP was investigated. Drainage experiments were also conducted in the laboratory to measure the Specific Resistance to Drainage (SRD, describing sludge dewaterability) and sludge settling velocity to determine gravitational drainage of the BTF sludge. Excellent solid removals (68–95%) were achieved after 1 hour settling of BTF samples, reducing BOD5, TN and TP concentrations by 72%, 48% and 55% respectively. Dewatering experiments showed that while the settling velocity of the sludge (6.2 × 10−4 m/s) was ∼15–50 times faster than that of typical activated sludge (in the order of 10−5 m/ s), the SRD of the sludge (∼6 × 1010 m/kg) was ∼2–15 times higher than that of activated sludge. Anaerobic conditions in the sludge and vigorous mixing greatly increased the SRD (up to 3.4 × 1011 m/kg), increasing the time for dewatering. This result suggests that while the sludge can be easily removed from BTF effluent by simple gravity sedimentation, dewatering of the sludge may be much slower than for typical activated sludge. Sludge Treatment Wetlands (STWs) combining sludge drying beds with vertical flow constructed wetlands could be applicable for dewatering and mineralisation of the BTF sludge. However, the STWs may require a larger land area than one treating activated sludge due to the lower dewaterability.

1. Introduction Since wastewater treatment processes typically produce large amounts of sludge as a waste or by-product, the processing and disposal of the waste sludge can be one of the most complex and expensive problems for wastewater treatment plants [1–5]. Therefore, sludge treatment and disposal are one of the key issues to achieve efficient and effective wastewater management. The solid content of waste sludge may vary considerably depending on the characteristics of the sludge, the sludge removal, pump type, and the method of operation. Thus, sludge thickening increasing the solids content is beneficial to reduce the cost for handling, transportation, and final disposal [1,6]. Sludge thickening is generally achieved by physical processes such as gravity settling, flotation, and centrifugation, but natural processes such as evaporation, evapotranspiration, and percolation may be also used for sludge thickening and drying [1,7,8]. Sludge Treatment Wetlands (STWs) or Sludge Treatment Reed Beds (STRBs) combining traditional sand sludge drying beds with vertical flow planted constructed wetlands can be an effective and economically

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viable option for sludge thickening and drying [9,4,3,10,11]. Sand drying beds have been traditionally used for sludge dewatering in small to medium size wastewater treatment plants, while the use of constructed wetlands (CWs) for sludge dewatering has increased over the last ∼20 years [7,4,5]. The STWs have been widely applied in European countries [12,4,10] particularly in Denmark where > 100 fullscale systems are currently being operated [7]. However, they remain to be applied in New Zealand climatic conditions and particularly for the treatment of Biological Trickling Filter (BTF) sludge. Since STWs mainly rely on natural processes for sludge dewatering and mineralization [13,7,5,14], they are less expensive and easier to operate compared to the mechanical and chemical sludge treatment systems [15,2]. However, the most common issue of the STWs is filter clogging of the wetland beds due to accumulation and compaction of sludge solids on the surface of the beds, resulting in poor sludge dewatering [16,17,8,18]. Therefore, to understand the characteristics of activated sludges in terms of sludge settling velocity, time of drainage, and compressibility during gravity dewatering, Dominiak et al. [3] developed a laboratory analytical method called the Specific Resistance

Corresponding author. E-mail address: [email protected] (J.B.K. Park).

https://doi.org/10.1016/j.jwpe.2018.02.006 Received 16 November 2017; Received in revised form 2 February 2018; Accepted 5 February 2018 2214-7144/ © 2018 Elsevier Ltd. All rights reserved.

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settled sludge were also determined according to APHA [22]. When the settled sludge was removed, the supernatants remaining in the three Imhoff cones were combined into a 5 L container, and then a 1 L homogenized sample was taken for the measurement of Biochemical Oxygen Demand (BOD5), Total-N (TN) and Total-P (TP) according to APHA [22] to investigate the effect of solid settling on the removal of BOD5, TN and TP. During the experiments conducted in June 2015 (Experiment 3), dissolved nutrients including ammoniacal-N (NH4+-N), nitrate-N (NO3−-N), dissolved reactive phosphorus (DRP), and E.coli were also measured. The concentrations of the water quality parameters in the supernatants were compared with those in the initial BTF effluent to determine the effect of sludge settling.

to Drainage (SRD) describing the dewaterability of sludge. Biological Trickling Filter (BTF) processes have been successfully used for domestic wastewater treatment over the last few decades [19]. The systems typically consist of fixed bed of rocks, gravel, or plastic media over which wastewater flows downward and grow a layer of microbial biofilm on the media [1]. The removal of pollutants from the wastewater stream involves both absorption and adsorption of organic compounds and some inorganic species such as nitrite and nitrate ions by the layer of microbial biofilm in aerobic conditions [1,20]. As the biofilm layer thickens, it eventually sloughs off into the effluent and subsequently forms part of the secondary sludge. Typically, a clarifier (or sedimentation tank) is required for the separation and removal of the secondary sludge in the BTF effluents. However, little information about the characteristics, handling and appropriate disposal options of the BTF sludge is currently available in the literature. Thus, this paper investigates the characteristics of the secondary sludge produced from a Biological Trickling Filter (BTF) system currently treating domestic wastewater in Gisborne, New Zealand in terms of sludge settling and drainage properties to assess the efficacy of STW treatment of the waste sludge in New Zealand climatic conditions.

2.3. Measurement of specific resistance to drainage (SRD) 2.3.1. BTF sludge sampling and pre-treatment About 120 L of BTF effluent was collected using six 20 L containers from the BTF effluent sump during the flushing cycle period and settled for about 30 min to increase the solid concentration up to ∼2% (20 g TS/L). Approximately 20 L of the concentrated BTF effluent sludge was collected after decanting off the supernatants for the use in the laboratory SRD experiments. In Experiment 1, to investigate the influence of pre-treatments on BTF sludge SRD and solid settling velocity, a batch of the untreated (fresh) BTF sludge was divided into three sub-samples (∼7 L each). One sample (untreated sludge) was used directly for the measurement of SRD, the second was stored anaerobically overnight at a room temperature, and the third was intensively mixed at ∼800 rpm using an overhead stirrer (mimicking sludge pumping through a long pipeline) overnight until use. In Experiment 2, the pre-treatment conditions were unchanged (untreated, anaerobically stored, and vigorously mixed), but air was added to one sub-sample (∼7 L) of the BTF sludge with gentle mixing (∼200 rpm) to investigate the effect of aeration on the sludge quality (Supplementary Fig. S2). An aquarium pump was used for the aeration and maintained the sludge dissolved oxygen (DO) concentration at ∼5 mg/L until use.

2. Materials and methods 2.1. Description of the study area The Gisborne District Council (GDC), New Zealand has operated a Biological Trickling Filter (BTF) system (38°39′56.7″S 178°00′13.4″E) for the treatment of domestic wastewater since 2011. During the 3.5year monitoring period (from January 2011 to June 2014), the BTF system has produced approximately 12,000 m3 of BTF effluent, 2.2 tonnes total suspended solids (TSS) of sludge, 1.2 tonnes of BOD5, and 0.29 tonnes of TN per day. The BTF system consists of a fixed bed of plastic media (having a high surface area to volume) over which wastewater flows downward and grow a biofilm layer; aerobic conditions are maintained by forcedair flowing through the bed. Passage of the wastewater over the media also provides dissolved oxygen which the biofilm layer requires for nitrifying ammoniacal-N to nitrate-N (nitrification) and the removal of organic compounds (BOD5) [19,21]. As the biofilm layer thickens, it eventually sloughs off into the liquid flow and subsequently produce the secondary sludge. Daily flow of the BTF effluent and concentrations of key wastewater contaminants such as TSS, BOD5, COD, and nitrogen and phosphorus species are summarized in the supplementary information (Supplementary Table S1).

2.3.2. SRD experiment set-up SRD experiments that were previously developed by Dominiak et al. [3] were conducted on the BTF effluent settled sludge in March and June 2015 (Experiments 1 and 2). The experimental set-up consisted of a transparent glass cylinder (inner diameter: 80 mm; area: 0.005 m2), a mesh funnel, and a metal clamp (holding the transparent glass cylinder and funnel), a filter, and a laptop computer connected to a video camera (Fig. 1). The filter was mounted on the funnel inside the transparent glass cylinder, which was then placed on a 1 L measuring cylinder to collect the drainage. Whatman 41 paper (Whatman, UK, porosity: ∼20–22 μm) was used as the filter as recommended by Dominiak et al. [3]. A sludge sample of a given volume (400, 500, or 600 ml) was loaded into the transparent cylinder and left to drain by gravity into the measuring cylinder. The volume of drainage was measured at one minute intervals until the draining ceased. The video camera was used to film the sludge dewatering until the experiment was complete.

2.2. Measurement of solid settling efficiency BTF effluent solids settling experiments were conducted three times in three seasons (Experiment 1: Summer, December 2014; Experiment 2: Autumn, March 2015; Experiment 3: Winter, June 2015). For each laboratory experiment, composites of three 10 L samples of BTF effluent were taken from the effluent sump (weir) during both normal operation and flushing cycle periods. The normal BTF operation period sample was a composite of three samples taken 1 h before the flushing cycle commenced, and 1 and 2 h after the flushing cycle ceased. The flushing cycle period sample was a composite of three samples taken 5, 30 and 55 min (initial, middle and final) after the flushing cycle started. 1-h solid settling efficiencies were measured for the BTF effluent samples using 1 L Imhoff cones (in triplicate) under laboratory conditions. Water samples (50 ml) were taken using a syringe from the mid depth of the Imhoff cone (∼450 ml depth) after 1 h settling and used for the measurement of Total/Volatile Suspended Solids (TSS/VSS) according to APHA [22], which were then compared with the initial TSS/VSS to determine the solid settling efficiency. The settled sludge at the bottom of the Imhoff cones was also collected using a tap (at the bottom of the cone) and the percentage solids (as% total solids) of the

2.3.3. Determination of settling velocity and specific cake resistance Calculations of the settling velocity and SRD were based upon the changes of clear water phase height (Δh) over the sludge drainage process, which initially increases linearly until time t1 (Period 1), and then decreases until time t2 (Period 2). The settling velocity was determined as the slope of the line fitted to the data from Period 1 (according to Eq. (1)).

vs = 173

Δh t

(1)

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Fig. 1. Laboratory experiment set-up for the sludge SRD measurement.

The difference between liquid level and sludge blanket level (Δh) as a function of time was determined by image analysis and used to calculate SRD and sludge settling velocity (Supplementary Fig. S1). Evaluation of the replicability of the test was also carried out by measuring the SRD of a sample of untreated (fresh) activated sludge from the Hamilton WWTP (∼5 g TS/L) three times. The mean SRD was 3.5 × 1010 m/kg with a standard deviation ∼5% of the mean value, showing that the experimental technique was reliable.

Where; Δh: the height of the clear water phase (i.e., the difference between liquid level and sludge blanket level) t: a given time Sludge compression was irreversible, so no cake swelling was observed during the drainage experiment even through the pressure declines during Period 2 as described by Dominiak et al. [3]. During Period 2, ln(h(t)/h*) was plotted as a function of t-t1 giving a straight line (as shown in the Supplementary Fig. S1) and the slope (e.g. a: 7 × 10−4 in Supplementary Fig. S1) was fitted into Eq. (2) to determine the SRD.

SRD =

ρg ch 0 μ × 7 × 10−4

2.3.4. BTF SRD experimental conditions In Experiment 1, a series of mass loadings (0.24–1.2 kg/m2) and volumetric loadings (0.05 up to 0.15 m3/m2) were tested to investigate the influence of sludge mass and volumetric loading on SRD, solid settling velocity and time of drainage. Results for the anaerobically stored and sheared sludge at each mass and volumetric loading were compared to those of the untreated sludge. Experiment 2 was a repeat of Experiment 1 but with four treatments (untreated, aerated, anaerobically stored, and sheared) compared at each of the mass and volumetric loadings. Sludge solids and volumetric loadings and pre-treatment conditions for SRD experiments 1 and 2 are summarized in Table 1.

(2)

Where: ρ: Density of the filtrate (kg/m3) g: Gravitational acceleration (m/sec2) c: Initial solid concentration of the given sludge (g/m3) h0 Initial level of the suspension (m) μ: filtrate viscosity (kg/m/sec).

Table 1 Sludge solids and volumetric loadings and pre-treatment conditions for SRD Experiments 1 & 2.

Test Test Test Test Test Test Test Test Test

1 2 3 4 5 6 7 8 9

Solid loading (kg/m2)

Volumetric loading (m3/m2)

Untreated

Anaerobically stored

Anaerobically sheared

Aerated (Expt. 2 only)

0.24 0.30 0.36 0.40 0.50 0.60 0.80 1.00 1.20

0.05 0.10 0.15 0.05 0.10 0.15 0.05 0.10 0.15

√ √ √ √ √ √ √ √ √

√ √ √ √ √ √ √ √ √

√ √ √ √ √ √ √ √ √

√ √ √ √ √ √ √ √ √

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Table 2 TSS concentrations of BTF effluent (for both the normal operation and flushing cycle period) and settled supernatant after 1 h settling in Imhoff cones, and% solids removal efficiency (values are means of triplicate samples ± standard deviation).

Experiment 1 (Dec 14) Experiment 2 (Mar 15) Experiment 3 (Jun 15)

TSS in BTF effluent (g/m3)

TSS in supernatant after 1 h settling (g/m3)

% solids removal efficiency

Normal

Flushing

Normal

Flushing

Normal

Flushing

230.0 ± 33.0 98.0 ± 14.0 72.0 ± 16.0

884.7 ± 468.5 565.3 ± 288.3 436.0 ± 96.2

30.7 ± 4.8 23.8 ± 5.7 22.0 ± 2.9

37.6 ± 12.6 29.0 ± 12.2 64 ± 5.6

75.6 ± 20.3 74.8 ± 9.7 68.5 ± 6.9

95.5 ± 0.7 94.5 ± 1.4 85.1 ± 1.9

Fig. 2. % solids removals after 1 h settling in Imhoff cones for the normal operation and flushing cycle BTF effluent in Experiments 1–3 (a); Mean ± s.d.% solids removal from normal operation and flushing cycle period BTF effluent in Imhoff cones over a 1 h settling period (b and c).

Table 3 Removal of Total BOD5, Total-N (TN) and Total-P (TP) by 1 h gravity settling of BTF effluent from normal operation and flushing cycle periods in Experiments 1–3 (values are means of triplicate samples ± standard deviation). BTF effluent (g/m3)

Supernatant after 1 h settling (g/m3)

% removal

TBOD5

Normal

Flushing

Normal

Flushing

Normal

Flushing

Expt 1 (Dec 14) Expt 2 (Mar 15) Expt 3 (Jun 15) Total-N (TN) Expt 1 (Dec 14) Expt 2 (Mar 15) Expt 3 (Jun 15) Total-P (TP) Expt 1 (Dec 14) Expt 2 (Mar 15) Expt 3 (Jun 15) E.coli (MPN/100 ml) Expt 3 (Jun 15)

140.0 ± 20.0 59.0 ± 16.6 62.0 ± 14.9

356.7 ± 129.0 205.7 ± 202.3 193.0 ± 136.1

17.0 ± 1.0 32.7 ± 11.8 45.0 ± 4.0

23.7 ± 3.5 33.7 ± 2.5 54.7 ± 20.5

87.6 ± 2.5 46.0 ± 9.3 25.5 ± 13.1

93.0 ± 1.4 61.7 ± 40.0 57.0 ± 32.0

34.3 ± 4.0 21.3 ± 9.9 20.0 ± 1.0

68.7 ± 22.3 44.3 ± 21.0 36.7 ± 16.3

21.0 ± 1.7 18.0 ± 8.7 17.3 ± 1.2

25.3 ± 0.6 16.0 ± 3.6 24.0 ± 1.0

38.7 ± 2.0 16.5 ± 4.4 13.4 ± 3.0

60.8 ± 10.4 56.2 ± 25.2 27.1 ± 25.5

7.7 ± 0.8 5.2 ± 1.2 5.0 ± 0.3

16.0 ± 4.3 13.2 ± 6.4 7.7 ± 2.6

4.6 ± 0.0 4.3 ± 1.2 4.5 ± 0.4

4.9 ± 0.1 4.0 ± 0.2 4.9 ± 0.4

39.8 ± 6.3 20.6 ± 3.6 9.4 ± 3.1

68.0 ± 7.6 63.5 ± 20.0 32.1 ± 20.9

1.9 × 106 ± 8.3 × 105

2.5 × 106 ± 2.3 × 106

1.3 × 106 ± 5.3 × 105

2.1 × 106 ± 1.9 × 106

33.3 ± 1.7

17.4 ± 2.2

3. Results and discussion

TSS concentration was almost comparable to that achieved in the normal operation BTF effluent (∼30 mg/L) even though the initial TSS concentration of the flushing cycle BTF effluent was ∼4 fold higher than that of the normal operation BTF effluent. These results suggest that short hydraulic retention time (HRT) solids settling basins could be effectively used to remove the sludge from the BTF effluent. BTF effluent TSS concentrations declined to ∼72 g/m3 during the winter Experiment 3 (Table 2) and solid settling efficiencies were also reduced (68% and 85% for the normal operation and flushing cycle periods respectively). The lower BTF effluent TSS concentration in winter was due to reduced microbial activity and growth at lower temperatures. Moreover, as water temperature decreases (Experiment 1: 21 °C; Experiment 3: 14.9 °C), the solid settling velocity declines [1,23], which may have contributed to the lower solid settling efficiencies in Experiment 3.

3.1. BTF solid setting efficiencies The results of BTF solid settling efficiencies during both normal operation and flushing cycle periods are summarized in Table 2. Excellent solid removal was achieved with both the normal operation and flushing cycle BTF effluent in all three experiments (Fig. 2a), with the majority of settleable solids removed within ∼10 min of the experiment. Percent removal of solids were 48, 63, 69 and 77% for the normal operation BTF effluent and 77, 84, 87 and 92% for the flushing cycle period BTF effluent after 5, 10, 30, and 60 min of settling respectively (Fig. 2b&c). In Experiment 1, > 95% solid removal efficiency was achieved with the flushing cycle BTF effluent (initial TSS concentration of ∼885 g/m3 and settled TSS concentration ∼38 g/m3). The settled 175

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Approximately 11, 18 and 4 L of sludge (∼1–2% solids) were collected per 1 m3 of BTF effluent settled during the summer, autumn and winter experiments respectively. The settled sludge was about 2% solids for both the normal operation and flushing cycle periods in Experiment 1, but was lower (1.1 and 1.7% for the normal operation and flushing cycle periods respectively) in Experiment 3 (Table 4). The decrease in the solids content of the settled sludge was probably due to the lower solid settling efficiency in Experiment 3 as previously discussed in Section 3.1. However, the solids content of the BTF sludge (1.1–2.3% solids) is higher than that of typical waste activated sludge (0.8–1.2% solids) reported in the literature [28,1,29,20]. The higher solids content of the settled BTF sludge implies less volume of BTF sludge to be treated possibly using relatively simple sludge treatment processes such as sludge drying beds or constructed wetlands. Dominiak et al. [3,8] and Christensen et al. [30] reported that increases in solid concentration (load) of sludge at a constant volumetric load increased the height of the sludge cake, resulting in an increase in dewatering time. Moreover, time of drainage increases exponentially with volumetric load of sludge (m3/m2) compared linearly with solid load since the hydrostatic pressure on the sludge cake increased [30]. Therefore, the previous studies suggest that the solid content of the BTF sludge may need to be diluted to reduce the solid load of sludge to achieve sufficient sludge dewatering in the STWs. The organic content of the settled sludge was relatively high (∼73–86%) in all three experiments (Table 4), indicating that BTF sludge is readily-biodegradable and should be easily stabilized and mineralized when discharged to STWs.

3.2. Removal of TBOD5, TN, TP and E.coli by gravity sedimentation Gravity settling effectively removed TBOD5, TN and TP from the BTF effluents (Table 3). For example, excellent removal of TBOD5 (93%), TN (61%) and TP (68%) was achieved for the flushing cycle period BTF effluent in Experiment 1. Like the TSS removal described above, the TBOD5, TN and TP concentrations in the supernatants of the settled flushing cycle period BTF effluent were only 24, 25, and 4.9 g/ m3 respectively, even though the initial concentrations of these water quality parameters were ∼2.5 fold higher than those of the normal operation period BTF effluent in Experiment 1. This result implies that the particulate forms of TBOD5, TN and TP associated with the sludge were effectively removed by the simple gravity sedimentation, while ‘soluble’ BOD5, ‘dissolved’ inorganic nitrogen (such as ammoniacal-N, nitrate-and nitrate-N) and phosphate (PO4-P) concentrations did not change between the flushing cycle and normal operation BTF effluent. The relatively poor removal of TBOD5, TN and TP achieved in Experiment 3 (Table 3) was mainly due to the decrease in the solid removal efficiency. For example, during the normal operation period in Experiment 3, only 25, 15, and 9% removal of TBOD5, TN and TP were achieved by the settling compared to 88, 39, and 40% removal respectively in Experiment 1. The lower microbial activity in winter (Experiment 3) reduced the performance of the BTF system [24,1,20], resulting in higher TBOD5 concentrations (∼45 and ∼55 g/m3) in the supernatants of the normal operation and flushing cycle period BTF effluents respectively than in the previous experiments. 1 h solids settling did not change (or slightly increased) the BTF effluent supernatant concentrations of dissolved nutrients including ammoniacal-N, nitrateN and DRP in Experiment 3. For example, nitrate-N and DRP concentrations remained at 3.3 and 4.6 mg/L before and after the settling, while ammoniacal-N concentration increased slightly from 12.9 to 13.5 mg N/L after the 1 h settling for the normal operation period. The E.coli level in the BTF effluent (∼2 × 106 MPN/100 ml) was similar to that of typical raw domestic wastewater in New Zealand [25–27] and little E.coli removal (only ∼17–33% reduction) was achieved by the solid settling in Experiment 3 (Table 3).

3.4. BTF sludge quality The specific resistance to drainage (SRD) for the untreated (fresh) BTF sludge was ∼6 × 1010 m/kg, which is about 2–15 times higher than that of typical activated sludges from Danish [8] and Hamilton WWTPs (4 × 109–4.2 × 1010 m/kg) (Fig. 3). However, as shown in Fig. 4, the settling velocity of the BTF sludge (6.2 × 10−4 m/s) was ∼15–50 times higher than those of activated sludge (in the order of 10−5 m/s). These results suggest that while the sludge (biosolids) can be easily removed from the BTF effluent by simple gravity sedimentation since the majority of settleable solids were removed within only ∼10 min (Fig. 2), drainage of the sludge would take longer than for typical activated sludge. Anaerobic storage or intensive shearing (i.e. pumping) of the BTF effluent sludge both greatly increased SRD to 2.4 × 1011 and 3.4 × 1011 m/kg respectively (Fig. 3), resulting in an increase in the time of drainage (Fig. 5b). The effect of pre-treatments such as anaerobic storage and shearing on the SRDs was previously investigated by Dominiak et al. [3,8] in the laboratory. They found that 24 h anaerobic storage and vigorous shearing at 800 rpm both significantly increased the SRD of activated sludge from Danish WWTPs. Shearing and anaerobic conditions cause deflocculation of activated sludge, resulting in floc fragmentation and liberation of small aggregates [31,3,8]; [32]. Therefore, the higher SRDs observed with deflocculated sludge may be attributed to small particles clogging or blinding the filter, causing slower water flow and more significant cake compression due to higher liquid pressure. The drainage properties (SRDs and the time of drainage) of three different volumes (400, 500, and 600 ml, or volumetric loads 0.08, 0.10 and 0.12 m3/m2) of BTF sludge from the same sample were tested. SRD increased linearly with increasing volumetric load (Fig. 5a), however, the time of drainage correlated with the volumetric load squared (Fig. 5b). Moreover, the increasing the TSS concentration (solid load) of the BTF sludge from 3 to 11.5 g/m3 linearly increased both the SRD and time of drainage (Fig. 5c&d) as the sludge height (cake compressibility) increased. Moreover, anaerobic storage and shearing also increased SRD and time of drainage significantly (Fig. 5a&b). Christensen et al. [30] reported that as the volumetric load increased, the hydrostatic

3.3. BTF sludge characteristics Characteristics of the BTF sludge in Experiments 1–3 including settled sludge volume (L sludge/m3), solids content (%TS), and organic content (% VSS in TSS) are summarized in Table 4. The amount of settled sludge and% solids content varied depending on the TSS concentration in the BTF effluent, solid settling efficiency, and the operation of the BTF system (either normal operation or flushing cycle). Table 4 Characteristics of BTF sludge including settled sludge volume (litre/m3), % solids content and organic content (% VSS in TSS). Expt 1 (Dec 14)

Expt 2 (Mar 15)

Expt 3 (Jun 15)

Normal operation period Settled sludge volume (L sludge/m3) % solids content (as TS) Organic content (% VSS in TSS) Settled sludge volume (L sludge/m3) % solids content (as TS) Organic content (% VSS in TSS) Average settled sludge volume over 24 h (L/m3)

9.9 ± 2.6

16.3 ± 0.4

3.9 ± 1.4

2.1 ± 0.4 73.0 ± 0.6

1.8 ± 0.2 85.5 ± 5.2

1.1 ± 0.1 80.5 ± 4.9

Flushing cycle period 43.4 ± 18.4 47.2 ± 24.3

13.7 ± 5.7

2.3 ± 0.2 74.4 ± 0.9

2.2 ± 0.1 89.0 ± 2.9

1.7 ± 0.1 86.3 ± 1.7

11.4

17.6

4.3

176

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Fig. 3. Specific resistance to drainage (SRD) of BTF sludge compared with those of activated sludge (AS) from Danish [8] or Hamilton WWTPs.

and ‘anoxic’ conditions may cause less damage to sludge quality than shear and anaerobic conditions.

pressure on the settled sludge cake increases; therefore, the increase in SRD is a result of cake compression. This suggests that both volumetric and solid loads of the sludge may need to be determined carefully to achieve sufficient sludge dewatering in the STWs [30,3,33,34].

4. Conclusions This study showed that excellent solid removals (68–95%) were achieved after 1 h settling of the Biological Trickling Filter (BTF) sludge samples, reducing total BOD5, TN and TP concentrations by 72%, 48% and 55% respectively. Moreover, the settling velocity of the BTF sludge (6.2 × 10−4 m/s) was ∼15–50 times faster than that of typical activated sludge (in the order of 10−5 m/s). The results suggest that the BTF sludge can be easily removed from the effluent by gravity sedimentation and a simple sedimentation tank (or clarifier) can be used for the efficient separation and subsequent removal of the secondary sludge. Laboratory dewatering experiments showed that the Specific Resistance to Drainage (SRD, describing sludge dewaterability) of the BTF sludge (∼6 × 1010 m/kg) was ∼2–15 times higher than that of activated sludge (4 × 109–4.2 × 1010 m/kg), indicating that BTF sludge dewatering was much slower than for typical activated sludge. Pretreatments such as storage under anaerobic conditions and intensive shearing (mixing) greatly increased the SRD (up to 3.4 × 1011 m/kg), resulting in an increase in the time for dewatering. However, oxygen addition (∼5.0 mg O2/L) to the sludge was found to improve sludge dewatering. This study suggests that Sludge Treatment Wetlands (STWs) combining sludge drying beds with vertical flow constructed

3.5. Improving sludge quality Aeration of BTF sludge (D.O concentration: ∼5 mg/L) improved sludge drainability (Fig. 6). The SRD of aerated BTF sludge was only 1.3 × 1010 m/kg at a volumetric load of 0.08 m3/m2 (Experiment 2), which is about 3–18 times lower than that of the untreated (4.5 × 1010 m/kg), anaerobically stored (1.7 × 1011 m/kg) and anaerobically sheared sludge (2.5 × 1011 m/kg). Moreover, increasing the volumetric load of aerated BTF sludge from 0.08 to 0.12 m3/m2 did not significantly increase SRD (1.3 × 1010 to 3 × 1010 m/kg) or time of drainage (although it more than doubled from 0.3 to 0.8 h) compared to the untreated, anaerobically stored and sheared sludges (Fig. 6). Dominiak et al. [3] also found that extended aeration (∼6 h) of activated sludge improved drainability by promoting flocculation. Some practical methods for overcoming the difficulties with sludge dewatering have been proposed previously such as addition of calcium carbonate or nitrate [8]. For example, addition of nitrate-N (∼15 mg N/L) into a pumped stream of waste activated sludge to maintain the nitrate concentration at 5–7 mg N/L resulted in an improvement of the sludge drainage in the STWs [8], suggesting that shear

Fig. 4. Settling velocity of BTF sludge (a) compared with those of activated sludge from Danish [8] and Hamilton WWTPs (b).

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Fig. 5. The effect of increasing volumetric loading on SRD (a) and the time of drainage (b); the effect of increasing solids loading on SRD (c) and the time of drainage (d).

Fig. 6. The effect of aeration on SRD (a) and time of drainage (b).

Advisory Group (WTAG) members (Bruce Duncan, Beven Turnpenny, Peter Williamson and Murray Palmer), and Jacqui Horswell at Institute of Environmental Science and Research ltd (ESR). Their collaborative and supportive contributions were essential for achieving the goals outlined the research.

wetlands could be applicable for dewatering and mineralisation of the BTF sludge. However, the STWs may require a larger land area than one treating activated sludge due to the lower dewaterability. Acknowledgements

Appendix A. Supplementary data

We would like to acknowledge the valuable inputs of Gisborne District Council staff (particularly David Wilson, Robson Timbs, David Viggars, Helen Churton, and Tracey Panton), and Wastewater Technical

Supplementary data associated with this article can be found, in the 178

Journal of Water Process Engineering 22 (2018) 172–179

J.B.K. Park et al.

online version, at https://doi.org/10.1016/j.jwpe.2018.02.006. [18]

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