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 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.
Download figure to PowerPoint
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 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.
Download figure to PowerPoint
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 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).
Download figure to PowerPoint
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 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.
Download figure to PowerPoint
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 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).
Download figure to PowerPoint
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)/location||AS||BOD||COD||TSS||Reference|
|UASB-ASP||43||Raw sewage||5.86||223||784||200||Present study|
|UASB–polishing ponds||27||Raw sewage||5.35||170||404||310||Mungray and Kumar (2008a)|
|UASB–ozonation||0.12||Raw sewage||–||–||–||–||Gasi et al. (1991)|
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 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.
Download figure to PowerPoint
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.