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Keywords:

  • activated sludge process;
  • anionic surfactants;
  • post-treatment;
  • risk assessment;
  • up-flow anaerobic sludge blanket reactor

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Conclusions
  7. Acknowledgement
  8. References

This paper presents the performance of two full-scale up-flow anaerobic sludge blanket–activated sludge process (UASB-ASP)-based sewage treatment plants (STPs) (surface and diffused aeration-based activated sludge processes as post-treatment units). Performance of this combination is compared with UASB–polishing ponds and UASB–ozonation-based STPs. Post-treatment units removed 89 and 92% of anionic surfactants (AS) by surface and diffused aeration, respectively. Finally, 0.61 and 0.23 mg/L of AS were discharged from post-treatment steps after overall reduction of 90–92%. Final concentrations from UASB-ASP-based STPs were low compared with UASB–polishing ponds (3.60–4.91 mg/L) and UASB–ozonation (1.52 and 0.53 mg/L). Overall, UASB-ASP-based STPs were working efficiently for the removal of organics in terms of chemical oxygen demand (COD) (84%) and biochemical oxygen demand (BOD) (93%), but they need further modifications for the removal of AS up to the level of risk quotient [risk quotient (RQ)] ≤ 1 for no risk to aquatic environment.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Conclusions
  7. Acknowledgement
  8. References

Up-flow anaerobic sludge blanket (UASB)-based sewage treatment plants (STPs) are preferred more than the activated sludge process (ASP)-based STPs (Mahmoud 2002). The only drawback is that UASB-based STPs require a post-treatment step because most of the time anaerobic wastewater treatment plants fail to discharge the effluents as per the effluent discharge standards given by the Central Pollution Control Board in India (CPCB 2001), mainly for biochemical oxygen demand (BOD), chemical oxygen demand (COD), pathogens and suspended solids (SS). The largest group among various anionic surfactants (AS) is linear alkylbenzene sulfonate (LAS) and they may enter the aquatic environment when raw or partially treated sewage is discharged. Venhuis & Mehrvar (2004) reported that 0.02–1.0 mg/L LAS in aquatic environment can damage fish gills, cause excess mucus secretion, decrease respiration in the common goby and damage swimming patterns in blue mussel larva. The no-effect concentration of LAS for aquatic organisms is reported to be 0.27 mg/L (van de Plaasche et al. 1999; Belanger et al. 2002; HERA 2007).

More than 900 UASB reactor units are currently operating all over the world (Alves et al. 2000). About 35 UASB-based plants already exist in India, which treat wastewaters from varying sources like distilleries, dairies, pulp mills, pharmaceutical units, starch maize units, textile units, tanneries, sewage, etc (Arceivala 1999). Sato et al. (2006) evaluated the performance of some of the UASB-based STPs in India. Sixteen full-scale UASB reactors and polishing ponds of the overall capacity of 598 m3/day were evaluated under Indian climatic conditions (Sato et al. 2006). The monitoring exercise was based on one-time sampling only for finding the general picture of STPs. These investigations revealed that STPs were unable to produce effluent that conforms to the discharge standards in terms of BOD, SS and faecal coliforms (FC) removal. Therefore, an effective post-treatment step must be employed for following the discharge standard guidelines (CPCB 2001). Studies on treatment of AS in full-scale UASB-based STPs is very limited (Gasi et al. 1991; Mungray & Kumar 2008a). Gasi et al. (1991) studied a UASB reactor (120 m3 cylindrical reactor, pilot scale) with ozonation as a post-treatment step, while Mungray & Kumar (2008a) investigated five full-scale [27, 34, 38, 56 and 70 million litres per day (ML/day) capacity] UASB-based STPs with polishing ponds as post-treatment step.

The present work was started because of the need that the performance study of UASB-ASP combination is feeble (Tawfik et al. 2008), and wherever it is evaluated, AS are not studied. The ASP is considered to be an effective aerobic treatment system for reducing organic load. Therefore, there is a need to develop and check the performance of ASP as a post-treatment of UASB reactors for AS. For this study, two UASB-ASP-based STPs having surface and diffusion-based aeration units with 43 and 100 ML/day capacity, respectively, were available at approachable distance; moreover, UASB–polishing ponds and UASB–ozonation were found ineffective for AS. The effectiveness was checked in comparison with polishing ponds and ozonation-based units and risk generated by the final effluents to the aquatic environment.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Conclusions
  7. Acknowledgement
  8. References

UASB-ASP-based STPs

Two UASB-based STPs followed by ASP-based post-treatment plants of 43 and 100 ML/day capacities in the Gujarat region having surface and diffused aeration-based post-treatment steps, respectively, were selected for the study. Design parameters and dimensions of both the STPs are given in Table 1. The main objective of post-treatment by using aeration method for anaerobically pretreated sewage was to improve the quality of the final effluent for public health protection, environmental consideration and water reuse. A schematic flow diagram of UASB-ASP-based sewage treatment system at both places is given in Fig. 1; accordingly, photographs of both types of aeration units are given in Fig. 2. Samples were collected for the period of 16 weeks for raw sewage, UASB effluent and final effluent (after post-treatment units) as per Fig. 1.

figure

Figure 1. Schematic flow diagram in up-flow anaerobic sludge blanket–activated sludge process (UASB-ASP)-based post-treatment systems at Vadodara and Surat.

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figure

Figure 2. Photographic views of (a) surface aeration and (b) diffused aeration-based post-treatment units at 43 and 100 ML/day up-flow anaerobic sludge blanket–activated sludge process (UASB-ASP)-based sewage treatment plants (STPs), respectively.

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Table 1. UASB-based STPs: dimensions and design parameters
 Installed capacity and locations
  1. BOD, biochemical oxygen demand; COD, chemical oxygen demand; HRT, hydraulic retention time; STP, sewage treatment plant; TSS, total suspended solid; UASB, up-flow anaerobic sludge blanket.

Parameters43 ML/day100 ML/day
Average operating capacity (ML/day)43160
Design parameters  
COD (mg/L)800800
BOD (mg/L) (5 days, 20oC)200250
TSS (mg/L)400300
UASB reactors  
Numbers620
Dimensions (L × W × B) (m3)20 × 24 × 5.120 × 20 × 7.4
HRT (at average flow) (hours)8–98–9
Post-treatment unitSurface aerationDiffused aeration
Numbers24
Dimensions (L × W × B) (m3)52 × 26 × 416 × 60 × 5.5
HRT (at average flow) (hours)63

Analysis

Grab samples were collected from both the STPs considered in this work. At STPs, different numbers of UASB reactors (4 and 20) and aeration units (2 and 4) were installed at 43 and 100 ML/day STPs, respectively. Sewage samples were collected on a weekly basis from combined streams and not from individual reactors. All the samples were collected, preserved and transported as per methods listed in standard methods (APHA 2005).

The AS was measured in samples of sewage as methylene blue active substance (MBAS) as prescribed in standard methods (APHA 2005), while LAS (Hach, USA) was taken as a reference. The AS in nonfilterable residues from sewage samples was extracted by soxhlet extraction technique using methanol (Marcomini & Giger 1987) and then analysed using the MBAS method. The AS concentrations are reported in this paper as milligrams per litre (calculated as LAS, mol. wt., 318). Other conventional pollution parameters were also analysed as per standard methods (APHA 2005). Samples for BOD determination were incubated for 5 days at 20°C. Dissolved oxygen (DO), oxidation–reduction potential (ORP), pH and total dissolved solids were measured by using different portable digital meters (Hach).

Risk assessment

The risk assessment to aquatic environment because of the presence of AS in treated sewage was evaluated according to the procedure laid down in the European Union Technical Guidance Document (EU-TGD 2002). Risk was assessed depending on the risk quotient (RQ) given in Eq. (1) as:

  • display math(1)

where

PEC = 

predicted environmental concentration

PNEC = 

predicted no-effect concentration.

An RQ greater than 1 indicates risk to the aquatic environment. The PEC was measured as an average concentration of AS in the final effluents from different STPs after using a dilution factor of 10. The PNEC was derived by taking no-observed effect concentration (NOEC) of 0.27 mg/L, which has been experimentally found based on long-term laboratory screening tests on broad array of freshwater plants/organisms at different tropic levels(Konemann 1981; McAvoy et al. 1993; Tabor & Barber 1996; van de Plaasche et al. 1999; Belanger et al. 2002; Versteeg & Rawlings 2003; HERA 2007). Because LAS predominates among AS, in this work, AS was determined. The NOEC value reported for LAS (i.e. 0.27 mg/L) was used to calculate PNEC. Assessment factors of 10–1000 have been suggested for the estimation of PNEC values for LAS for different scenario, but it is suggested to take 10 for long-term NOECs for at least three species (normally fish, Daphnia and algae) representing three tropic levels (EU-TGD 2002). This yields the lowest value of PNEC (0.027 mg/L) in receiving water.

Results and discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Conclusions
  7. Acknowledgement
  8. References

Performance of aerobic post-treatment units

The DO, ORP, pH and total suspended solids (TSS) were measured for raw sewage, UASB effluents and final effluents for both the STPs were under consideration. At both places, the trend of the variation of DO, pH, ORP and TSS was almost the same. Figure 3 was prepared by considering the average values of these parameters for both the STPs, and the variation of these monitoring parameters show the performance of the STPs. The DO and ORP (Fig. 3) clearly present that in raw sewage, average DO was 0.13 mg/L and ORP was −278 mV. After the treatment in UASB reactors, DO and ORP of the sewage again decreased because of the anaerobic nature of the reactor. But after the post-treatment step, there is a considerable change in DO and ORP because of the aerobic treatment of UASB effluent. The DO and ORP finally reached up to a level of 3.3 mg/L and 43 mV for final disposal, respectively. Similarly, pH variation (Fig. 3) shows a decrease in pH in UASB reactor effluents because of the production of volatile fatty acids by anaerobic activity. After post-treatment, pH has increased and stabilised up to a level of almost 7.5. The TSS curve (Fig. 3) shows a variation of TSS in the STP, which clearly shows the removal of TSS in UASB reactor. This again decreases because of sedimentation tank in ASP-based post-treatment.

figure

Figure 3. Average dissolve oxygen (DO), pH, oxidation-reduction potential (ORP) and total suspended solids (TSS) variation with their standard deviations for 43 and 100 ML/day up-flow anaerobic sludge blanket (UASB)-based sewage treatment plants (STPs) for various sampling points.

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Figure 4 presents the average AS concentrations in dissolved, adsorbed and in total form with the associated standard deviation for a period of 16 weeks for two STPs of 43 and 100 ML/day UASB-based STPs, respectively, for different sampling locations. If we consider the whole data at both STPs, a range of 3.75–10.26 mg/L was found in raw sewage for AS which is similar to the reported results of 3–21 mg/L by various researchers (Holt & Waters 1995; Adak et al. 2005; Mungray & Kumar 2008b). At a particular STP, average concentration of total AS was 5.85 and 5.91 mg/L, respectively, for the units studied. Concentration of AS in raw sewage depends upon the consumption of cleaning agents and habits of cleaning of the population. Figure 4 clearly indicates that at both the STPs, adsorbed fraction of AS decreased in UASB while dissolved fraction increased. Average values of 5.69 and 5.63 mg/L of total AS were found in UASB effluents. When UASB effluents were sent to the post-treatment units, dissolved fraction of AS decreased excessively and adsorbed fraction of AS has increased. Here, two mechanisms might have taken place, viz.: (a) aerobic biodegradation of AS as described by Haggensen et al. (2002) and Swisher (1987) and (b) adsorption/absorption of AS on excessive biomass. Finally, in the effluent of sedimentation tank, dissolved and adsorbed fractions were decreased, and totals of 0.61 and 0.23 mg/L of AS were discharged from post-treatment steps. Final concentrations from UASB-ASP-based STPs are very low compared with UASB–polishing ponds (3.60–4.91 mg/L) and UASB–ozonation (1.52 and 0.53 mg/L).

figure

Figure 4. Concentration of anionic surfactants (AS) in dissolved, adsorbed and in total form along with their standard deviation for 43 and 100 ML/day up-flow anaerobic sludge blanket–activated sludge process (UASB-ASP)-based sewage treatment plants (STPs). MBAS, methylene blue active substance; Eff., effluents.

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Figure 5 presents the average percentage removal of total AS in UASB reactors, post-treatment units and in overall STP for both the STPs, that is, 43 and 100 ML/day UASB-ASP-based STPs. It is clear from Fig. 5 that in UASB reactors, no removal of dissolved fraction of AS took place. Almost 23% of dissolved fraction increased may be because of the solubilization/hydrolysis of the adsorbed fraction of AS from the UASB reactor. Adsorbed fractions were removed by almost 24 and 29% in both UASB reactors. Because of this increment in dissolved fractions and removal of adsorbed fractions, overall, only 3 and 4.7% of AS were removed in both the UASB reactors. Aerobic environment is the favourable condition for the biodegradation of AS (Haggensen et al. 2002), which can be clearly seen in Fig. 5 for post-treatment units. In post-treatment units, both fractions of AS were removed and even dissolved fraction of AS were removed more (92 and 94%) in comparison with UASB reactors. A total of 89 and 92% of AS were removed in both the post-treatment units. Diffused aeration-based post-treatment seems to be more effective than surface aeration-based treatment because of excessive mixing of biomass throughout the unit and by convective oxygen transfer, which leads to good aerobic degradation of AS in the unit. If both the STPs are considered, total removal of the AS seems to be dependent on post-treatment step only, because there is almost no removal in UASB reactors. Overall, 89.6 and 92.4% of total AS were removed at 43 and 100 ML/day UASB-ASP-based STPs, respectively. Seasonal variation in concentrations of AS was not found noticeable in our previous study (Mungray & Kumar 2008b); therefore, it is not considered in this study.

figure

Figure 5. Percent removal of anionic surfactants (AS) in dissolved, adsorbed and in total form for 43 and 100 ML/day up-flow anaerobic sludge blanket–activated sludge process (UASB-ASP)-based sewage treatment plants (STPs).

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Figure 6 represents the average percent removal of AS, BOD, COD and TSS at 43 and 100 ML/day UASB-ASP-based STPs for UASB reactors, post-treatment step and for overall STPs. As discussed earlier, the anaerobic conditions are not favourable for AS removal, while aerobic conditions are favourable. This clearly indicates that in the UASB reactors, removal of AS was very low compared with other parameters, while in post-treatment unit, AS removal was highest compared with BOD, COD and TSS removal. Although average values of 42 and 35% of BOD were removed in UASB reactors, which are very low compared with the post-treatment step wherein 83 and 89% of BOD were removed. Similarly, COD removal was also less in the UASB reactor and more in the post-treatment step. Percent TSS removal was almost the same in UASB and post-treatment step. This difference of removal in UASB and post-treatment units shows the necessity of the aerobic STPs for UASB-based STPs. The UASBs are high-rate anaerobic reactors; hence, for justifying the discharge standard guidelines (BOD = 30 mg/L, COD = 250 mg/L), post-treatment is a must (CPCB 2001). Overall, BOD removal was higher than the AS removal because of sufficient removal in UASB reactors and at post-treatment step. Finally, 89.6 and 92.4% of AS, 93% BOD in both, 84.3 and 83.4% of COD, and 44 and 54% of TSS were removed from 43 and 100 ML/day UASB-ASP-based STPs, respectively. The performance of both the STPs is almost same and both gave the same kind of percent removal for BOD, COD and TSS.

figure

Figure 6. Percent removal of anionic surfactants (AS), total suspended solids (TSS), chemical oxygen demand (COD) and biochemical oxygen demand (BOD) in up-flow anaerobic sludge blanket (UASB), post-treatment (PT) unit and in overall sewage treatment plant (STP) for 43 and 100 ML/day anaerobic sludge blanket–activated sludge process (UASB-ASP)-based STPs.

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Comparison of UASB-ASP, UASB–polishing ponds and UASB–ozonation

In the present work, three types of post-treatment units were compared for UASB effluents, which are the only available methods until the date for the study of the removal of AS. One UASB–ozonation unit (at two different time intervals and concentrations) was studied by Gasi et al. (1991), and in another study (Mungray & Kumar 2008a), five UASB–polishing ponds-based STPs were reported. The post-treatment unit was a polishing pond of 1–1.6-day detention at all the five STPs. The present study deals with UASB-ASP-based STPs with two types of aeration units (surface and diffused aeration). The treatment steps were similar except the post-treatment units at three locations.

Concentration of AS for UASB reactor effluents and post-treatment effluents at three types of STPs are compared in Table 2. Other parameters like BOD, COD and TSS are also compiled for the sake of comparison of the post-treatment units along with AS. In all the three types of UASB reactor effluents, almost all the parameters were the same but a distinct change in concentration was found after the post-treatment unit. The AS concentrations were removed in the aeration tank unit of post-treatment along with the ozonation unit, but there was negligible change in concentration in polishing pond. Average percent removal of AS, BOD, COD and TSS were calculated for the three types of systems and presented in Fig. 7, which represents the percent removal of the parameters in UASB reactor, post-treatment unit and in overall STP. Only average values for post-treatment steps are calculated and compiled for AS, BOD, COD and TSS for UASB–ozonation-based STP, because Gasi et al. (1991) studied only the post-treatment step, that is, ozonation step for UASB effluents.

figure

Figure 7. Comparison of average percent removals of anionic surfactants (AS), biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solids (TSS) for five up-flow anaerobic sludge blanket–polishing ponds (UASB-PP), two UASB–ozonation (UASB-Ozon) and two anaerobic sludge blanket–activated sludge process (UASB-ASP) for UASB, post-treatment (PT) and in overall sewage treatment plant (STP).

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Table 2. Concentration of AS, BOD, COD and TSS for UASB-ASP, UASB–polishing ponds, UASB–ozonation-based STPs for raw sewage, UASB effluent and final effluent
 STP (ML/day)/locationASBODCODTSSReference
  1. AS, anionic surfactants; BOD, biochemical oxygen demand; COD, chemical oxygen demand; STP, sewage treatment plant; TSS, total suspended solid; UASB, up-flow anaerobic sludge blanket; UASB-ASP, up-flow anaerobic sludge blanket–activated sludge process.

UASB-ASP43Raw sewage5.86223784200Present study
UASB effluent5.69129594148
Final effluent0.6116123113
100Raw sewage5.91240695216
UASB effluent5.63157439179
Final effluent0.451711599
UASB–polishing ponds27Raw sewage5.35170404310Mungray and Kumar (2008a)
UASB effluent5.0565192182
Final effluent4.3332134107
34Raw sewage6.01167446357
UASB effluent5.9176243229
Final effluent4.8338155110
38Raw sewage5.36171281268
UASB effluent4.3754134175
Final effluent3.8328101102
56Raw sewage5.31162383333
UASB effluent5.8156219233
Final effluent4.9129126111
70Raw sewage5.16170463416
UASB effluent4.2564238245
Final effluent3.632160119
UASB–ozonation0.12Raw sewageGasi et al. (1991)
UASB effluent4.634212651
Final effluent1.52217523
0.12Raw sewage
UASB effluent5.34112034
Final effluent0.53205313

If only AS is compared, percent removal of AS in UASB reactors is almost the same in UASB–polishing ponds and UASB-ASP-based STPs (7 and 4%). In post-treatment unit, polishing ponds were found to be insufficient for the removal of AS (15%) when compared with ozonation (90%) and ASPs (91%). Similarly, for the overall STPs, there is a huge difference between the removal of AS for UASB–polishing ponds and UASB-ASP-based STPs (21 and 91%).

Aeration-based post-treatment units removed 89% BOD, which is very high compared with polishing ponds (49%) and ozonation (51%). Consequently, the effluent was also very clear after the overall BOD removal (81 and 93%) for UASB–polishing ponds and UASB-ASP. Similarly, for COD, almost 48 and 31% were removed in UASB reactors for UASB–polishing ponds and UASB-ASP, and 33, 56 and 77% of COD was removed in polishing ponds, ozonation and in ASP-based post-treatment units, respectively. Overall, almost 66 and 84% of COD was removed at UASB–polishing ponds and UASB-ASP-based STPs. The TSS removal in three types of systems was low when compared with other parameters. Post-treatment units removed TSS almost 48, 62, and 35% for polishing ponds, ozonation and aeration-based units, respectively. Overall SS removal was almost 67 and 49% for UASB–polishing ponds and UASB-ASP-based STPs, respectively. It is clear from the above findings that among the three post-treatment units, ASP-based units are more efficient than ozonation and polishing ponds. The AS removal was more than that of BOD, COD and TSS removal in post-treatment units. It clearly shows the effectiveness of the aeration-based post-treatment unit for AS removal.

If UASB-ASP is compared with UASB–polishing ponds and UASB–ozonation, UASB-ASP seems to be better for the removal of AS, organics (COD and BOD) and solids. Polishing ponds are found to be simple, require low maintenance and designed for hydraulic retention time (HRT) of 1 day. Algal growth cannot be expected at pond detention times less than the multiplication rate of algal cells, which is 2–2.5 days at 20°C (Mungray & Kumar 2008b). Because of low algal growth, dissolved oxygen level would also be low, hence less removal of AS and other pollution parameters. While ozonation could be an effective tool for the removal of AS, it would be costly and depends upon the ozonation time period and seems to be not feasible for full-scale operations.

Risk assessment to aquatic environment

The purpose of risk assessment is the overall protection of the aquatic environment. It is generally accepted that protection of the most sensitive species should protect structure and hence function. Based on the final effluent concentrations from the three types of UASB reactors with post-treatment units, a risk to aquatic environment is compared using Eq. (1) for RQ.

Figure 8 is prepared by calculating RQ for various UASB reactor effluents and UASB–post-treatment effluents for UASB–polishing ponds (five), UASB–ozonation (two), and UASB-ASP-based STPs (two). Although UASB reactor effluents are not discharged directly into the environment without post-treatment, the RQs are calculated for comparing the effectiveness of post-treatment units for AS. A risk of adverse effects is indicated when the RQ is above 1 (EU-TGD 2002). It is clear from Fig. 8 that the RQ for all the UASB reactor effluents for the three types of STPs is much higher than 1 (15.7–21.3). It is clear that if such effluent is discharged to the aquatic environment, it will certainly disturb the quality of aquatic system. Hence post-treatment step is a must. When the post-treatment steps were used, the RQ values ranged from 13.3 to 18.2 for polishing ponds, which shows that polishing ponds are also not effective for lowering the risk to aquatic environment. For ozonation, RQs were found to be 5.63 and 1.92, and these values are less than the values compared with polishing ponds, but not less than 1. For ASP, RQs were 2.26 and 1.67 which are better than polishing ponds and ozonation.

figure

Figure 8. Risk quotient (RQ) for up-flow anaerobic sludge blanket (UASB) effluents and UASB–post-treatment (UASB-PT) effluents for three types of post-treatment units. Eff., effluents.

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From the above findings, it is clear that UASB-ASP-based systems could also not remove AS concentration up to a level where RQ ≤ 1 like UASB–polishing ponds and UASB–ozonation, although they removed organic load in terms of COD and BOD efficiently. AS or LAS was evaluated in conventional full-scale ASP-based STPs (Feijtel et al. 1995; Field et al. 1995; Prats et al. 1997; Matthijs et al. 1999; Holt et al. 2003), and RQ were calculated below 1 at all the places (Mungray & Kumar 2008a). This implies that, as a post-treatment unit, ASP could not degrade AS while alone it degraded efficiently. The reason for this result may be that when ASP was used alone, the micro-organisms efficiently degrade the easilybiodegradable organic matter along with AS or LAS which is only aerobically degraded because of HRT of almost 4 days (Rodezno 2004). On the other hand, when it is used as a post-treatment unit with UASB reactor, easily biodegradable organic matter degrades first in UASB reactor and the rest, along with nonbiodegradable organic matter like AS or LAS, are treated by post-treatment unit where aerobic micro-organisms first prefer easily biodegradable organics than not easily biodegradable organic matter like AS or LAS. The HRT values of only 3 and 6 hours in aeration units of post-treatment units would be one of the reasons for not providing sufficient time for aerobic biodegradation and biosorption of AS on to the biosolids. For effective removal of AS, HRT of aeration unit should be increased.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Conclusions
  7. Acknowledgement
  8. References
  • (1)
    In UASB reactors, average removal of AS ranged from 3 to 5% only. In overall UASB-ASP, AS removal was ranged from 89.6 to 92.4% by utilizing aerobic post-treatment systems.
  • (2)
    Better removal efficiencies by both the post-treatment units were found when compared with the earlier post-treatment units of polishing ponds and ozonation for AS.
  • (3)
    The present post-treatment systems (surface aeration and diffused aeration based ASPs) are also not found suitable in lowering the concentration of AS up to the safe level (i.e. RQ of ≤ 1).
  • (4)
    Both the STPs effectively removed organic load in terms of BOD and COD.
  • (5)
    Although the magnitude of the risk was lowered by the comparison of RQ values, it is suggested to increase the HRT of aeration units or some extensive aerobic post-treatment systems are required for reducing RQ value for AS.

Acknowledgement

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Conclusions
  7. Acknowledgement
  8. References

Authors acknowledge the financial support from Sardar Vallabhbhai National Institute of Technology, Surat, through R & D Grant of the Institute.

Footnotes
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References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Conclusions
  7. Acknowledgement
  8. References
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