Mass balance of anionic surfactants through up-flow anaerobic sludge blanket based sewage treatment plants

Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 254–260

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Mass balance of anionic surfactants through up-flow anaerobic sludge blanket based sewage treatment plants Arvind Kumar Mungray a,∗ , Pradeep Kumar b a b

Department of Chemical Engineering, S.V. National Institute of Technology, Surat 395007, India Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India

a b s t r a c t The outcome of a 21-month monitoring study on anionic surfactants (AS) at five (27–70 ML/d) up-flow anaerobic sludge blanket (UASB) based sewage treatment plants (STPs) is described. The average removals of AS were around 8–30%. Appreciable concentrations of AS were being discharged to the watercourse (average 4.30 mg/L; range 3.60–4.91 mg/L). On an average dried sludge contained 1452 mg AS/kg dry weight. Mass balance at three STPs indicated that, AS load of the order of 5–17% and ≈12% is removed by adsorption in UASB reactors and polishing ponds (PP) respectively. Biodegradation of AS under anaerobic conditions in UASB reactors and PP does not seem to take place. In the sludge stream, appreciable biodegradation (≈46%) of adsorbed AS under aerobic conditions on the sludge drying beds takes place. If influent AS mass flux is normalized to 100 units, than average of ≈74 and ≈7 units are discharged with treated effluent and dried sludge respectively, while 12 and 6 units are adsorbed/settled in PP and aerobically biodegrade on sludge drying beds respectively. At two STPs (34 and 56 ML/d), the filterable fluxes in UASBR increased so that the mass balance could not be computed. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Anionic surfactants; Up-flow anaerobic sludge blanket reactor; Anaerobic digestion; Mass balance; Sludge drying beds

1.

Introduction

Anionic surfactants (AS) are the primary cleaning agents used in laundry and cleaning products. Linear alkylbenzene sulfonate (LAS) is most widely used AS. The key for protecting the environment from the negative impact of down-thedrain-chemicals is the treatment of the wastewater/sewage. Activated sludge is most widely used treatment process. Sorption of AS onto the biosludge and biodegradation under aerobic conditions result in 98–99% removal (Cavalli et al., 1993; Water and Feijtel, 1995; Matthijs et al., 1999; Holt et al., 2003). Under aerobic conditions, total mineralization of LAS proceeds through degradation of the alkyl group via ␻oxidation, ␤-oxidation, desulfonation and finally degradation of the phenyl ring (Haggensen et al., 2002). Biodegradation under anaerobic conditions has been believed not to occur for a long time. LAS degrade very slowly, or that, until now, it has widely been believed that no degradation takes place under anaerobic conditions. However, recently indications of

∗

anaerobic biodegradation of LAS have been reported (Denger and Cook, 1999; Angelidaki et al., 2000; Mogensen and Ahring, 2002; Sanz et al., 2003; Lobner et al., 2005) based on studies conducted in pilot and bench-scale UASB reactors. It is concluded that under certain specific conditions AS are biodegraded without the presence of oxygen. However, metabolic pathways of anaerobic biodegradation are yet to be identified and understood. The amount of surfactants present in the dried sludge is highly dependent on the process used for sludge processing i.e. aerobic or anaerobic. The most important parameter in limiting the surfactant content in the finally dried sludge is the aerobic condition during processing. Biodegradation of AS under anaerobic methanogenic conditions in sludge treatment has not yet been demonstrated. Anaerobically digested sludge contains on an average 10,500 (±5200) mg LAS/kg dry wt. while aerobically digested sludges contain on an average only 150 (±120) mg LAS/kg dry wt. (McAvoy et al., 1993).

Corresponding author. Tel.: +91 261 2201642/919904173019 fax: +91261 2227374/2228394. E-mail addresses: [email protected], [email protected] (A.K. Mungray), [email protected] (P. Kumar). Received 1 March 2009; Received in revised form 27 March 2009; Accepted 31 March 2009 0957-5820/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2009.03.004

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Fig. 1 – Locations of investigated STPs with their treatment capacities. Recently, sixteen full-scale UASB reactors with a total installed capacity of 598 ML/d have been constructed in North India in towns situated along river Yamuna and its tributary Hindon (Fig. 1). Their treatment capacities vary from 10 to 78 ML/d. All were almost identically designed and constructed around the same time (September 1998–January 2004). Hydraulic detention time (HRT) adopted in the design ranged from 8.4 to 10.9 h for the UASB reactors and 1.0–1.6 days for the polishing ponds (PP). However, in-spite of the fullscale application of UASB since over fifteen years, data on the removal or biodegradation of AS in a field UASB reactor is not available. Whatever little work has been done on the removal of AS in a UASB reactor has been carried on laboratory or pilot

scale reactors. These studies have been carried out under laboratory conditions which on occasions do not take into account or do so only in part, the actual situation in which AS find themselves after being discharged into the environment. This paper presents the results of a study of the removal/mass balance of AS at the five full-scale UASB-PP based STPs.

2.

Materials and methods

2.1.

Site descriptions

Five UASB based STPs, one in Saharanpur (29◦ 58 N, 77◦ 23 E) of 38 ML/d capacity, two in Noida (28◦ 20 N, 77◦ 30 E)

Fig. 2 – Typical flow diagram of UASB based STPs studied along with sampling locations. Table 1 – Dimensions and other details UASB reactors. Parameters/design flow UASB reactors numbers Dimensions, L × W × D (m) (each) Effective depth (m) Effective volume of reactors (m3 ) HRT (at average flow) (h) Average operating flow (ML/d)

27 ML/d

34 ML/d

38 ML/d

56 ML/d

70 ML/d

3 24 × 28 × 6.10 5.55 ≈11,200 9.9 25.4

4 24 × 24 × 6.25 5.90 ≈13,600 9.6 31.6

4 28 × 24 × 6.05 5.55 ≈15,000 9.4 30.6

4 32 × 32 × 6.10 5.60 ≈23,000 9.8 42.4

4 32 × 40 × 6.38 5.88 ≈30,000 10.3 59.5

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Table 2 – Dimensions and HRT of polishing ponds (PP). Parameters

27 ML/d

34 ML/d

Polishing ponds numbers Surface area, L × W (m) (each) Effective depth (m) Total volume of ponds (m3 ) HRT (at average flow) (d)

2 110 × 120 1.6 42,000 1.6

2 237.4 × 55.1 1.3 34,000 1

of 27 and 34 ML/d capacities, and two in Ghaziabad (28◦ 40 N, 77◦ 28 E) of 56 and 70 ML/d capacities were selected for the study (Fig. 1). A general schematic flow diagram of the combined UASB-PP system for all STPs is shown in Fig. 2 and their main characteristics are summarized in Tables 1–3 for UASB reactors, polishing ponds and sludge drying beds respectively. The combined (UASB-PP) systems were designed to handle 200 mg/L of influent biochemical oxygen demand (BOD, 5 day, 20 ◦ C) and 400 mg/L of influent suspended solids (SS) for meeting the required Indian Standards of 30 mg/L of BOD and 50 mg/L of SS in the final effluent. Sewage after preliminary treatment (screening and grit removal) is uniformly distributed at the bottom of the UASB reactors. The UASB effluents are discharged to 1–1.6 day detention polishing ponds for the treatment. In Noida, Saharanpur and Ghaziabad, sewage reaches STPs after multistage pumping because of the flat topography of these cities. At every stage sewage is detained for a short time is mixed up in a sump. It is finally collected at the main pumping stations (MPS) just ahead of STPs from where it is pumped round the clock at a more or less uniform rate. It flows through the STPs (Fig. 2) by gravity. Prior to Aug. 2004, 2-h samples were composited at the plants (based on the 2-h treatment plant flow) to prepare a 24-h flow-weighted composite sample. Composite samples were analyzed along with 2-h grab samples. This exercise was repeated twice at each STP. Results are not reported in this paper. Distinct diurnal variations in the raw sewage, UASB effluent and final effluent characteristics were not observed, presumably due to a general leveling effect in the collection system (due to multistage pumping) and treatment plant (due to long retention times in UASBRs and PPs). Based on this, it was subsequently decided to collect grab samples during the present study. Grab samples of UASB reactor, and drying bed sludges were also collected in polyethylene containers. It was felt that grab samples of sludges were adequate due to long solids residence times in the UASB reactors (designed solid retention times: 38–52 days) and on sludge drying beds (7–30 days). Sampling locations are marked in Fig. 2. At the STPs a different number of UASB reactors and sludge drying beds (SDBs) are installed (UASB reactors 3–4 and SDBs 10–24) while the number of polishing ponds installed is fixed at two irrespective of plant capacity. Samples were collected from combined streams and not from individual reactors. Sewage and sludge samples were collected around the tenth of every month.

2.2.

38 ML/d

56 ML/d

70 ML/d

2 12,700 1.5 38,000 1

2 180 × 120 2.0 86,000 1.5

2 190 × 144 1.75 96,000 1.4

Analytical methods

Anionic surfactants were measured in samples of sewage and sludges as MBAS (methylene blue active substances) as prescribed in Standard Methods (APHA, 1998, 2005). Linear alkylbenzene sulfonate (Hach Company, USA) was taken as a reference. Anionic surfactants in sludges and non-filterable residues from sewage samples were extracted by soxhlet extraction technique using methanol (Marcomini and Giger, 1987) and than analyzed using MBAS method. All AS concentrations are reported in this paper as “mg MBAS/L (calculated as LAS, mol. wt., 318)”. Other conventional pollutional parameters were also analyzed as per Standard Methods (APHA, 1998, 2005). Dissolved oxygen (DO) was measured by using a DO meter (Senso Direct OX 24, Aqualytic, Germany). Likewise pH was measured by using pH meter (Thermo Orion, USA). A spectrophotometer (DR/4000, Hach Company, USA,) was used for colorimetric measurement. Wastewaters and sludges were collected over a period of twenty-one months (August 2004–April 2006) covering winter, summer, and rainy seasons.

3.

Results and discussion

3.1.

Mass balance at UASB based STPs

Throughout the study, incoming sewage flow (average operating flow: Table 1) was recorded to be less than the design flow at all the STPs. Average composition of raw sewage at five UASB based STPs is given in Table 4. It represents a typical Indian sewage. Distinct seasonal trends in AS concentrations were not noticed at any of the STPs during the study period from August 2004 to April 2006 in spite of variation in raw sewage temperature from 12 to 36.6 ◦ C (25 ± 5.23 ◦ C). Only fecal coliform levels in raw sewage were found to reduce to 75 × 105 in summer from maximum of 46 × 1012 in winter. It appears AS concentrations do not correlate well with sewage temperature. Accordingly, the entire data for the study period was processed together. Total, filterable, and particulate concentrations of AS (average ± standard deviation) in raw sewage, UASB effluent and final treated sewage for all the five STPs is shown in Fig. 3. Wide variations in total AS concentrations in influent ranging from 2.18 to 9.82 mg/L were noticed. The filterable fraction of AS ranged from 1.25 to 6.93 mg/L (mean 3.52 mg/L and standard deviation = ±0.79 mg/L), while the particulate

Table 3 – Dimensions and other details of sludge drying beds (SDB). Parameters

27 ML/d

34 ML/d

Sludge drying beds numbers Dimensions, L × W (m) (each) Depth of sludge application (m) Average rate of UASB wet sludge application (m3 /d) Average dried sludge (m3 /d)

10 25 × 15.4 0.3 14.5 2.07

16 13.4 × 22.7 0.3 62 23.8

38 ML/d

56 ML/d

20 25 × 14 0.3 41.2 10.3

24 30 × 15 0.3 89 24.7

70 ML/d 16 35.5 × 23.6 0.3 101 25

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Table 4 – Average composition of the raw sewage (August 2004–April 2006) at UASB based STPs. Range

(mean ± SD)

12.0–36.6 6.90–7.9 196–510 101–301 109–272 51–197 162–683 102–381 75 × 105 –46 × 1012 75 × 105 –46 × 1012 0 1.25–6.93 0.11–4.42

(25 ± 5.23) (7.37 ± 0.21) (337 ± 73.04) (191 ± 51.15) (168 ± 28) (82.8 ± 19.50) (398 ± 116.4) (184 ± 56) (16 × 1012 ± 11 × 1012 ) (15 × 1012 ± 11 × 1012 ) 0 (3.52 ± 0.79) (1.93 ± 0.81)

Parameters ◦

Sewage temperature ( C) pH Suspended solids (mg/L) Volatile suspended solids (mg/L) Total BOD (mg/L) Filterable BOD (mg/L) Total COD (mg/L) Filterable COD (mg/L) Total coliform (MPN/100 ml) Fecal coliform (MPN/100 ml) Dissolved oxygen (mg/L) Filterable AS (mg/L) Particulate AS (mg/L)

fraction varied from 0.11 to 4.42 mg/L (mean = 1.93 mg/L and standard deviation = 0.81 mg/L). The filterable fraction of AS was found to dominate; at Saharanpur plant on an average the filterable fraction was found to be more than even double that of the adsorbed fraction. After the treatment in the UASB reactors, on average UASB effluents contained 4.25–5.91 mg/L AS at all STPs. Gasi et al. (1991) also found UASB effluent from a 120 m3 cylindrical UASBR in Brazil rich in AS concentration. They found it to have 4.63–5.30 mg/L AS as MBAS. UASB reactors contain very large amount of SS (≈92,000 mg/L) which provides a large surface area for filterable and non-filterable AS to adhere to. In a UASBR, biosolids are retained for 30–50 days (i.e. solids retention time, SRT) while hydraulic retention time (HRT) is maintained only between 8 and 12 h. In most of the cases an increase in filterable fractions in UASB effluents compared to raw sewage was noticed (Fig. 3). DeLeenheer (2004) has reported that AS are released due to desorption or solubilization from the sludge bed to the effluent. At STPs of 27, 34, 38, and 70 ML/d capacities, although filterable fraction of AS increases, but this increase is compensated by decrease in particulate fraction resulting in overall reduction of total AS compared to raw sewage. At STP with flow rates of 56 ML/d, concentrations of total AS in UASB effluents were consistently found to be more than concentrations in raw sewage. Possible reasons are discussed later. In UASBRs, compared to the average removal of 45% of particulate AS, the filterable fraction of AS was found to increase rather then decrease except at the 38 ML/d capacity STP at Saharanpur where 7% reduc-

tion was observed. Overall average AS reductions in UASBRs ranged only from 2% to 18% (mean ≈11%). UASB effluents are discharged to polishing ponds where it is retained for up to 1.6 days. Average removals of BOD and AS in polishing ponds were found to be ≈49% and ≈15.2% respectively. Similar to the UASBRs, total AS removal was low at only 12.8–18.3%. Polishing ponds were also found anaerobic during the sampling period because of the hydraulic retention time (HRT) was ranging from 1 to 1.6 days and within this HRT, DO and algae were found absent in the pond effluents. In UASB-PP system, AS were removed from 8% to 30% at the five STPs. It did not match with the BOD removal of 78–84%. The UASB-PP effluents discharge substantial concentrations of AS ranging from 3.60 to 4.91 mg/L to water bodies. The UASB waste sludge is discharged for maintaining the desired food to micro-organism ratio, so that the reactor can perform satisfactorily (Fig. 2). Sand drying beds are used at the STPs where this study was carried out for the natural drying of waste sludges. Average concentration of AS in wet UASB and finally dried SDB sludges along with error bars for five treatment works over a period of twenty-one months are shown in Fig. 4. UASB wet sludge had considerable amounts of AS varying from 4480 to 9233 mg/kg dry wt. Average ranged from 5982 to 7997 mg/kg dry wt. for the five STPs. However, anaerobic digester sludges have been reported to have AS concentrations of 10,500 ± 5200 mg/kg dry wt. (McAvoy et al., 1993). The AS concentration in dried-stabilized sludge was found to range widely from 336 to 5880 mg/kg dry wt. with an overall average of 1452 mg/kg dry wt. After drying on SDBs, on an average

Fig. 3 – UASB based STPs: Average MBAS concentration (along with error bars) in raw sewage, UASB effluent, and in finally treated effluent at different UASB-PP based STPs.

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Fig. 4 – Average particulate AS in UASB reactor waste sludge (I bar in each set), and final sludge (II bar in each set). Values in parenthesis are installed capacities of UASB based STPs in ML/d. the AS reduction was found to be ≈80% (7347–1452 mg/kg dry wt.). Application of anaerobic wet sludges on SDBs not only resulted in drying and volume reduction but also in considerable elimination of AS ranging from ≈66% to ≈85%. To carry out mass-balances in UASB based STPs, filterable, particulate and total MBAS concentrations were calculated in terms of kg/d for five STPs (Fig. 5). The average flows of wastewater, UASB sludge and dry sludges were estimated at plants (Tables 1 and 3). To simplify and for better understanding the mass balances, the total influent concentrations were normalized to 100 units. Over the period of twenty-one months, raw sewage average AS concentration varied from 5.16 to 6.01 mg/L at all five STPs. The STPs were commissioned 6 years before the start of the study and were not receiving flows for which they were originally designed. Based on their operating capacities (Table 1), total mass fluxes of AS were found from 136 to 307 kg/d in which the filterable flux were more than the particulate flux at all STPs. In line with previous work, biodegradation of AS under anaerobic conditions did not take place (Scott and Jones, 2000; Ying, 2006). Under anaerobic conditions where AS is used as a sole substrate, it can be degraded (Sanz et al., 2003). Easily digestible substrate in raw sewage was more then the sufficient as unutilized organics (BOD 31–116 mg/L) were found in the UASB effluents. Under these conditions, anaerobic microorganisms will not utilize AS; it appeared that only sorption of AS took place. As raw WW enters into UASB reactors, part of adsorbed solids are trapped in the sludge bed/blanket or settle in the reactor while filterable surfactants find new sites for adsorption as an average of ≈92,000 mg/L TSS was usually found to be present in the UASB reactors compared to on an average of 196–510 mg/L TSS in raw WW. This mechanism is well supported by Karim and Gupta (2002), who stated that microbial biomass has sufficient sorption sites such as cell walls, cell membranes, cell cytoplasm and extracellular polymeric substances. In a biological treatment system pollutants may get sorbed to biomass as they remain in contact with each other for a sufficiently long time. From Fig. 5, it appeared that almost 25 kg/d of the particulate fraction was trapped in biomass except at 70 ML/d UASBR where it was 70 kg/d. The filterable fractions increased in the anaerobic environment inside the UASBRs. This increase was found to be around 17, 16, and 24 kg/d at 27, 70, and 34 ML/d UASBRs, while at 38 ML/d STP filterable AS were found to reduce by 9 kg/d. The maximum increase of 47 kg/d was observed at 56 ML/d UASBR. Filterable fluxes increased because of the desorption of the particulate fraction in biomass and also solubilization/minerilazation of particulate fractions at the

Fig. 5 – Mass balance (kg/d) of AS in {(a) 2 ML/d, (b) 38 ML/d, (c) 70 ML/d, (d) 34 MLd, and (e) 56 ML/d capacity} UASB based STPs (f, p, and T: filterable, particulate and total AS) (Numbers in paranthesis and circles indicate mass flow of AS in WW or sludge streams, and removed at a particular stage respectively).

Process Safety and Environmental Protection 8 7 ( 2 0 0 9 ) 254–260

reactors HRTs. According to Flemming (1995), biofilms represents a dynamic system in which the various components are synthesized, assembled, modified and finally broken down by autolysins and sloughed off in to the environment. They contributed to the remobilization of the sorbed substances. Similar mechanism is also described by Cowan et al. (1993) for activated sludge process (ASP) based STPs. They stressed that the total amount of LAS, i.e. dissolved plus sorbed, that is available for biodegradation and the time for biodegradation is equal to the hydraulic retention time (HRT) as in ASP. On the contrary in UASBRs, SRT  HRT, resulted in solubilization/hydrolysis of AS. Hand and Williams (1987) found that the desorption of LAS from river sediment was rapid and equilibrium reached within 3–8 h and was nearly 100% reversible. By the combination of the above mechanisms total removal of AS were found 8, 30 and 54 kg/d in 27, 38 and 70 ML/d UASBRs respectively. Almost the same amount (kg) of AS was found in UASB waste sludges per day in these reactors. In the other two reactors i.e. 34 and 56 ML/d, the filterable fluxes increased so that the mass balance of these two could not be completed correctly. The reasons for this excessive increment are not clearly known. One possibility for this excessive increase may be because of the production of extracellular polymers (ECPs). Shin et al. (2001) described that ECPs are generally found in biological wastewater treatment processes and they carry a negative charge. Sutherland (1984) also concluded that ECPs may contain anionic groups such as carboxyl, phosphoryl, and sulfate groups. ECPs have been believed to mediate the linkages in biofilms, flocs and granules (Jia et al., 1996). Morgan et al. (1990) analyzed 10–20 mg ECP/(g SS) in anaerobic sludges including UASB granules. The second possibility may be the production of biosurfactants. According to Youssef et al. (2004) and Anna et al. (2002) biosurfactants are a diverse group of surface-active chemical compounds having amphiphilic molecules with both hydrophilic and hydrophobic regions. They are produced by wide variety of micro-organisms. The type of biosurfactants include lipopeptides synthesized by many bacilli and other species, glycolipids synthesized by Pseudomonas species and Candida species, phospholipids synthesized by Thiobacillus thiooxidans, polysaccharide-lipid complexes synthesized by Acinetobacter species, or even the microbial cell surface itself. Although all five STPs were working on the same technology and treating municipal sewage, results differ considerably. Overview of the possibilities demonstrated that many questions are still open as far as the understanding of the sorption mechanism is concerned. Research is required as biomass is still taken as a black box. The overall surfactant binding capacity of biofilms remains unclear as well as their remobilization potential. High rate anaerobic digesters, such as the UASB reactors, have been found to be quite effective in treating various organic wastewaters. But for the case of anionic surfactants, there is a demand for further studies in biofilm desorption, and generation of ECPs and biosurfactants so that behaviour and mass balance at 34 and 56 ML/d STPs could be understood clearly (Fig. 5; d and e). It is clear from the Fig. 5 (a, b, and c), that if 100 units of AS enter in the UASBRs, 82–94 units of AS are discharged with secondary effluents. 5.4–17 units reappear in UASB wet sludges after adsorption on biomass. Only ≈1 to ≈2 units are really degraded in reactors and around 6–18 units are totally removed in UASBRs.

3.2.

259

Mass balance at polishing ponds

Polishing ponds are used for polishing the effluents of UASB reactors, i.e. for maintaining the desired effluent standards. Total of 128–253 kg/d of AS entered in PPs for tertiary treatment. Both filterable and particulate forms were reduced at this stage. The particulate AS were removed more then the filterable forms. No increments in the filterable fluxes were found as observed in case of UASB reactors. Polishing ponds have been designed for HRTs of 1–1.6 days. Algal growth cannot be expected at pond detention time < the multiplication rate of algae cells (2–2.5 days at 20 ◦ C). DO was also absent in pond effluents through out the study period. Based on the parametric study, ponds could be classified as 1–1.6 day detention, non algal, shallow, anaerobic ponds. At this detention time, only adsorption of AS followed by settling of particulate matter in PPs was expected. This resulted in 14–37 kg/d of particulate flux removal at PPs at five STPs. Very less removal (0–3 kg/d) was found in the filterable fluxes, because PPs are also anaerobic ponds. This minor removal may be due to anaerobic biodegradation of AS. This is because, PP influent contained on an average 54–76 mg/L of BOD. Under such conditions, there is a possibility that anaerobic biodegradation of not so readily biodegradable organics like AS may occur. The predominant mechanism was the adsorption/settling which was responsible for the removal of 11–16 units at all five STPs at PPs. Finally 69–93 units of AS were discharged to the aquatic environment by the UASB-PP system.

3.3.

Mass balance at sludge drying bed

UASB waste sludges were spread on the beds in a thin layer of about 10–20 cm on a periodic basic. A large variation was found in the sludge fluxes. 7.4–58 kg/d of AS reached at the SDBs for natural drying. Moisture content reduced with time and cracks were formed with the reduction of moisture. Cracks increases in width and depth and air reaches even at deepest layer of sludge. Sometimes tilling/milling operations help in distribution of oxygen through different layers of particularly dried sludges and under such conditions aerobic degradations speeds up. By this mechanism, SDBs are classified as aerobic reactors where AS degraded by the known mechanisms (Haggensen et al., 2002). Due to aerobic biodegradation only 1.6–17 units were removed at SDBs at five STPs. Large variations were found at 27 ML/d and 70 ML/d STPs. A number of reasons have been reported in literature for large variations. Firstly rainfall, wind, relative humidity and ambient air temperature during dewatering generally result in such variations and prolong the dewatering process. Secondly, the proper maintenance of the drainability of the sand beds is must, otherwise it leads to decreased rate of infiltration and reduced rate of sludge dewaterability. That is why 4–11 units were discharged from the SDBs to the terrestrial environment.

4.

Conclusions

Mass balance for anionic surfactants was carried out at five UASB based STPs. Wide variations in total AS concentrations in influent ranging from 2.18 to 9.82 mg/L were noticed. Removal of AS ranged from 8% to 30%. Removal of AS could mainly be attributed to the adsorption on inert and bio-solids. Biodegradation of AS under anaerobic conditions at secondary (UASB reactors) and tertiary (polishing ponds) stage of treat-

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ment does not seem to occur. Appreciable concentrations of AS were finally being discarded to the watercourse (average 4.30 mg/L; range 3.60–4.91 mg/L). In the sludge stream appreciable biodegradation (≈46%) of adsorbed AS on the sludge drying beds took place under aerobic conditions.

Acknowledgements The study on “Evaluation of performance of UASB based STPs” was financially supported by Ministry of Environment and Forest (MOEF), Govt. of India (GOI), India. Authors would also like to offer their sincere appreciation and thanks to Dr. Indu Mehrotra, Professor and Coordinator of the project for her many helpful comments and suggestions. Assistance by A.K. Srivastava, A. Bharti, T. Parveen, and A. Hussain, Project Fellows during sampling and analysis is acknowledged. Authors also wish to thank the technical staff at all STPs. First author received fellowship from Ministry of Human Resources Development (MHRD), GOI, India.

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