ANIONIC SURFACTANTS IN SEWAGE: FATE IN UASB PROCESS

Abstract

Surfactants are the most important organic chemicals by volume which are used in cleaning and household laundry detergents. Surfactants are relevant to the environment. Of the various types, the anionic surfactants (AS) are commercially and quantitatively the most important ones. Linear alkylbenzene sulfonate (LAS) is the major anionic surfactant used worldwide in formulations of detergents and household cleaning products. The major portion of AS is discharged into sewage after usage. Its biodegradation and removal has been extensively studied in aerobic sewage treatment plants (STP) like ASP and trickling filters. In last 25 years, anaerobic technology for the treatment of wastewaters has been developed as an alternative to aerobic one. One of the most widely researched configurations is UASB reactor. In last 20 years many full-scale UASB plants have been established throughout the world. In India alone presently >25 UASB based STPs are being used to treat >1000 ML sewage/d. However, inspite of successful fullscale application for over 15 years, fate of AS in the UASB based STPs has not been studied. AS are known to affect aquatic and terrestrial organisms. It is important to know the status of AS in UASB so that effluent could be post-treated to eliminate the associated risk to aquatic environment. Some attempts have been made to study the fate of AS in anaerobic process at bench scale in recent past. Reported results are varying in nature as the experiments and scenarios investigated by the authors were different. It is difficult to draw a definite conclusion about the extent of removal of AS in anaerobic processes. Moreover, actual field conditions are difficult to be ii exactly simulated under laboratory conditions. A need was felt to trace the fate of AS through an actual UASB reactor. Even at larger level, sometime two systems behave differently depending on so many variables over which one has little control. It was decided to carry out the study at five UASB based STPs so that all types ofvariations could be covered. There is also no mention offate ofAS in oxidation ponds (OP) in available literature. It was also decided to include two OPs in the study. Finally, an ASP was also included in the study for the sake of comparison. Application of wet UASB sludges on sludge drying beds (SDBs) is the most widely used method of sludge dewatering in tropical countries. However, neither the concentrations of AS in wet UASB sludges nor in dried-stabilized UASB sludges have been reported. In addition, rates of drying and biodegradation ofAS present in UASB sludges on SDBs have not been evaluated. With the present state of knowledge, it is not possible to assess the impact ofapplication ofUASB sludges on different terrestrial ecosystems. Accordingly, present work was also aimed to study these aspects. Treated sewage is mostly used for irrigation purposes or simply discharged into rivers while sewage sludges are used as soil conditioners in countries having agriculture based economy like India. However, one of the draw-backs of the use is the input of contaminants like anionic surfactants into the aquatic and terrestrial ecosystems. To avoid excessive fertilization and prevent unacceptable environmental effects or accumulation ofcontaminants, the use ofSTP effluents and sewage sludges must be regulated and controlled. Such a regulation should ideally be founded on a risk based procedure. Therefore, an attempt has been made to evaluate the risk generated due to the use of finally treated UASB effluents and dried SDB sewage sludges to aquatic and terrestrial ecosystems respectively. 111 For the ease of presentation the subject matter of the thesis has been divided into following chapters: 1. Introduction 2. Literature Review 3. Materials and Methods 4. Results and Discussion 5. Conclusion References are appended at the end. Data collected during the work has been incorporated as appendix. The objectives of the work along with the justification of the problem undertaken form the subject matter of Chapeter-1: Introduction. Chapter-2 deals with the literature review on the occurrence of AS in raw sewage, and fate in different sewage and sludge treatment processes. Some literature about the UASB based STPs including their applications and performance in India is included. Chapter-3 describes the materials and methods employed. The study was conducted in five concurrent/sequential phases as given below: Phase 1- Study of the fate of AS in UASB and its comparison with ASP and OP based STPs: In order to investigate the fate of AS in five UASB based anaerobic wastewater treatment plants (27, 34, 38, 56, & 70 ML/d), a monthly sampling was carried out for a period of twenty one months (Aug. 2004-Apr. 2006). Samples of raw sewage, UASB effluents and polishing pond effluents were collected. For comparison, 2 OP (9 and 32.5 ML/d) and one ASP (18 ML/d) were also selected and monitoring was carried out for twelve months and one month respectively. Each IV sample was analyzed for filterable and adsorbed AS in addition to conventional pollution parameters. Phase 2- Study of concentration of AS in UASB and dried SDB sludges: Samples of sludges from five UASB and one ASP based STP were also collected along with sewage samples. Sludge samples from UASB based STPs were collected during fifteen months period from Jan. 05 to Mar. 06 while from ASP were collected between Feb. 2005 and Mar. 2005. Samples were analyzed for AS, TS, VSS, and moisture contents. Phase 3- Study of rate of drying and degradation of AS on SDBs: Sludge drying 1 beds located at four of the UASB based STPs (27, 34, 56, and 70 ML/d) were selected to study the temporal variations of different parameters and to evaluate the rate of sludge drying and rate of degradation ofAS on SDBs. Sludge samples were collected at different time intervals during drying cycle from STPs. In addition a bench-scale model of SDB was prepared in laboratory and drying and degradation were studied using anaerobic wet UASB sewage sludge collected from 34 ML/d STP at Noida. Phase 4- Mass-balance of AS at UASB based STPs: Mass balance was carried out for all the five UASB based STPs considering average flows of sewage, wet sludges, and dried sludges. Phase 5- Assessment of risk to (i) aquatic ecosystem due to release of treated sewage and (ii) terrestrial ecosystem due to application of dried UASB sludges: The risk assessment to aquatic and terrestrial environments due to the presence of AS in treated sewage and sludges respectively were evaluated according to the procedure laid down in European Union Technical Guidance Document (EU-TGD, 1996, 2002a). Predicted environmental concentrations (PEC) in rivers and in agricultural fields were calculated from the concentrations of AS in UASB, OP, and ASP effluents and respective dried sludges. Risk was assessed depending on (a) PEC (b) predicted no effect concentration (PNEC) i.e. the concentration below which unacceptable effects on organisms are not likely to occur. The likely risk of AS could be predicted based on the ratio of PEC/PNEC (i.e. RQ, risk quotient). RQ < 1 indicatesno risk. Results and discussion pertaining to different experiments performed have been organized in the fourth chapter. In UASB reactors at four out of five STPs, average removals of AS ranged only from 2 to 18%. At fifth STP, AS were found to increase on an average by around 8%. UASB effluents were found to contain substantial concentrations of AS (4.25-5.91 mg/L). Adsorbed fraction is reduced by around 45% while soluble fraction increases in UASBR. Post-treatment using 1 to 1.6 days detention, anaerobic, non-algal polishing ponds (PP) was found ineffective. In a UASB-PP combine (a) AS are reduced only up to 30%, (b) reduction of adsorbed AS achieved was >80%, and (c) ratio of filterable to adsorbed AS increases from <2 to >11. On the contrary, effluent AS or LAS concentrations recorded in ASP effluents are quite low (<0.2 mg/L). Unlike UASB, LAS or AS removals >99% are achieved. In oxidation ponds also removal efficiency of AS was found to be >85%. Results indicate that in UASBplants >69% of AS present in raw sewage is not removed. It is mainly due to the use of anaerobic process at both secondary (UASB) and tertiary (PP) stages of sewage treatment. Good AS removal could be achieved if anaerobic treatment is followed by aerobic postoreatment. Widely used Indian practice of using anaerobic polishing ponds not only results in inferior effluent quality in terms of AS but also in terms ofBOD, fecal/total coliforms, and nutrients etc. AS concentrations in wet UASB reactor sludges from five STPs were found to range from 4480 to 9233 mg/kgdry wt. (average 7347 mg/kgdry wt.) over a period of VI fifteen months. After drying on SDBs, AS in dried-stabilized sewage sludges averaged 1452 mg/kg dry wt., a reduction of around 80%. This happens to be very close to Danish cut-off value of 1300 mg/kg dry wt. The kinetics of drying followed simple first order reduction of moisture with value of drying constant (kd) = 0.051 d"1. Reduction ofAS also followed first-order kinetics. AS degradation rate constant (kAs) was found 0.034 d"1 and half life of AS as 20 days. The order of rates of removal observed was: kd > kAs > kCoD > k0M (drying >AS degradation > COD reduction > organic matter reduction). At five STPs, on an average 5.15-6.01 kg AS/ML of sewage enters. If the influent fluxes are normalized to 100 units, 11 to 16 units were found to be removed at the stage of PPs, while 1.6 to 17 units were found to be removed at SDBs by aerobic biodegradation. A large fraction of >69 units was estimated to be present in treated sewage. Treated sewage from UASB based STPs when discharged to aquatic ecosystems are likely to generate substantial risk (RQ = 13.3 to 18.2) compared to OP (RQ=2.48 & 12.3) and ASP (RQ= 0.48). Disposal of treated UASB sludges is not likely to cause risk to terrestrial ecosystems. The conclusions derived from the present study form the subject matter of Chapter-5. vn