POLISHING OF EFFLUENT FROM UASBR: ORP AS A MONITORING PARAMETER

Abstract

Withthe adventof high rate anaerobic process, several upflow anaerobic sludge blanket (UASB) reactors for the treatment of sewage and industrial wastes have come up in India and other developing countries (Sato et al.2006, Lefebure et al.2006). Anaerobically treated wastewaters generally do not meet effluent discharge standards prescribed for BOD, MPN, suspended solids etc. The nutrient level is also high. Such wastewaters need to be further treated prior to discharge. BOD of the effluent from several UASB based sewage treatment plants (STPs) has been found to be more than 50 mg/L. It appears that BOD can not be anaerobically reduced to 30 mg/L or less. There are several post treatment alternative cited in the literature for improving the quality of anaerobically treated wastewater. Afew STPs have polishing ponds (PP) as one of the alternatives for the post treatment of the effluent from UASB. These polishing ponds are quiescent water bodies with ~ Id detention (HRT) and low aeration constant ka. Effluent samples from PPs have also been found anaerobic. It can be rendered aerobic only when rates of deoxygenation (kb L) is equal to rate of aeration (ka D) where L and D are BOD and oxygen deficit; ka and kb are the aeration and deoxygenation constants. Forsluggish water bodies ka = kb which is around 0.2-0.3 d"1.Therefore, the effluent having D= DOs would be rendered aerobic only when BOD reduces to DOs. Absence or presence of DO describes a system anaerobic or aerobic. A sample can be devoid of DO over a wide range of oxidation reduction potential (ORP) starting from 50 mV to values in the negative range. The anoxic and anaerobic conditions of a treatment system can not be distinguished on the basis of DO. The ORP, however can be used both quantitatively and qualitatively to express the conditions of wastewater as aerobic, anoxic or anaerobic. The ORP of the effluents from UASB varies from -100 to -160 mV. The gap therefore is to assess whether reduced ORP i.e -150 mV is responsible for relatively high BOD (~ 50mg/L) of anaerobically treated effluent i.e to know the effect of ORP on the numerical values of BOD and COD. The other related question is the possibility of using ORP as a monitoring parameter for post treatment. Also, it would be relevant to enquire into the correlation ofORP with other pollution parameters. ORP has been used as a monitoring parameter for the aerobic sludge digestion; nitrification-denitrification and other anoxic processes. It therefore, necessitated to explore the possibility of using ORP along with conventional parameters to evaluate the performance of the post treatment processes ofthe effluent from UASB reactors. Keeping this in view, the objective ofthe present research is to assess the potential ofORP as a monitoring parameter for the post treatment ofanaerobically treated sewage. The scope ofthe work is as under: • Preliminary bench scale batch investigations to assess the potential of ORP and its corelation with other parameters. • Monitoring of field & bench scale UASB based STPs: to evaluate ORP and other pollution parameters suchas BOD, COD, TOC, MPN, nutrients etc. • Evaluation ofORP as a monitoring parameter for three post-treatment alternatives: (i) diffused aeration, (ii) surface aeration and (iii) cascade type aeration. The thesis has beenorganized into five Chapters (1) Introduction (2) Literature Review (3) Materials and Methods in (4) Results and Discussion (5) Conclusion References are appended at the end. Numerical values of the data relevant to the work have been incorporated as appendices. The objective of the work along with the justification of the problem undertaken forms the subject matter of Chapter - 1. Chapter-2 deals with the review of literature on two aspects: (i) ORP as a monitoring parameter for the treatment of wastewater, and (ii) post treatment of anaerobically treated effluent. Overview of three aeration devices used in the present study has also been incorporated in this section. ORP as a controlling and monitoring parameter is well cited for aerobic sludge digestion, nitrification/denitrification in activated sludge process and sequencing batch reactor. .Several options for the post treatment of the effluent frorn anaerobic processes have been highlighted in the second part of the literature review. Energy intensive post treatment methods proposed in literature are the rotating biological contactor (Tawfic et al. 2005); trickling filters (Chernicharo and Nascimento, 2001) and activated sludge process (von Sperling et al., 2001). Various low cost options like waste stabilization ponds ( Dixo et al. 1995), Ghosh et al. (1999); duckweed pond ( van der Steen et al.1999) and down flow hanging sponge system ( Tandulkar et al. 2005 ) are either in use or being explored. Chapter -3 describes the materials and methods employed in the investigations. The experiments were conducted in three phases: Phase 1 Preliminary Investigations: In order to assess the potential of ORP as a monitoring parameter, a series of batch experiments were carried out using BOD bottles as reactors. Experiments were conducted with both soluble, and complex synthetic wastewater such as (i) glucose-glutamic acid (GG) (ii) sucrose-peptone (SP) and (iii) cellulose-sucrose-peptone (CSP). iv In each set, COD was varied from 100 to 500 mg/L with an interval of 100 mg/L. A blank without any substrate was also used with each set. At each COD the ORP was reduced by varying concentrations of sodium sulfide. The ORP along with other pollution parameters were measured. Accordingly, in another set of experiment acetate-acetic acid was used as substrate, and ORP was varied by adding different amounts of Na2S, NaBR} and Na2S03 Phase 2 Monitoring of field and bench-scale STPs -: The experiments of Phase 2 were conducted in two stages. The first stage dealt with the monitoring of five STPs round the year. Samples were collected monthly from STPs at Saharanpur (S-38), Ghaziabad (G-70 & G-56) and Noida (N-34 &N-27). Three set of samples comprising of (i) Influent (I) to UASB i.e. raw sewage, (ii) effluent (Eu) from UASBR, and (iii) effluent (Ep) from polishing pond were collected and characterized for each plant. In the second stage, a 56 L bench-scale UASB reactor was started. The reactor was fed with soluble wastewater (SW1) consisting of sucrose and complex synthetic wastewater (SW2) containing 40% cellulose and 60% sucrose & peptone. Phase 3 In Phase 3, aeration was carried out and ORP as a monitoring parameter was evaluated for three post-treatment alternatives: (i) diffused aeration, (ii) surface aeration and (iii) cascade type aeration. Stage 1: The effluent samples from four STPs namely S-38, G-70, G-56, N-27 & N-34 and SW1 & SW2 of bench-scale UASB were aerated for 2 hours in a batch reactor. ORP and pH were monitored after every five minutes. Samples at regular intervals were drawn and also analyzed for COD, BOD, TOC, NH3 and DO. Diffused aeration of SW2 was also carried out in a continuous manner at a detention period of 10, 20, 60, 90 and 120 minutes. Stage 2: In the second stage batch aeration of effluents was carried out by surface aerator. Samples were analyzed in a manner similar to that ofdiffused aeration. Stage 3: A cascade type aeration device was fabricated using pebbles for providing rough surface. Batch experiments were carried out using effluents from S-38, G-70, G-56, N-27 &N- 34 STPs whereas experiments in continuous mode were done using wastewater (SW2) from bench-scale continuous UASB reactor. The effluent was allowed to flow at a rate of 200 mL/min and 157 mL/min for batch and continuous experiments. Samples were analyzed for all parameters after fall and then intermittently after the detention ofwastewater in a container for two hours. Results and discussion pertaining to different experiments performed have been organized in the Fourth Chapter ofthe thesis. Preview ofthe preliminary investigations is given in the main text and the remaining data has been incorporated as Appendix A. The ORP values of the synthetic wastewater namely GG, SP, CSP and AA were found to be independent ofthe nature of the COD as well as concentration of COD in the range of 100-500 mg/L. Electron transfer was initiated by adding reducing chemicals, like Na2S03, Na2S and NaBFL. Sodium sulfide and sodium borohydride reduced ORP to -120 mV to -300 mV. The COD and BOD showed an inverse relationship with ORP. The inference drawn from these observations is that the CBOD does not increase but the total oxygen demand increases with decrease in ORP. A sample ofwastewater having negative ORP is expected to have higher oxygen demand than the same wastewaterat zero ORP or a positive value of ORP. With this background the monitoring of field STPs and bench-scale STP (treating synthetic wastewater) was taken up. Results ofmonitoring offive full scale UASB based STPs have been presented first. The effluent Ep from polishing pond with one day detention was unable to meet effluent discharge standard of 30 mg/L ofBOD 3at 27°c. The BOD varied from 30 to 45 mg/L and MPN ranged from 106 to 10 9. ORP &COD were found to vary from -100 to - 160 mV and 95 to 450 mg/L respectively. Sulphide was oxidized during BOD analysis, resulting VI inBOD more than the carbonaceous BOD. Suspended solids (SS) concentration in polishing ponds ranged from 100to140mg/L. The removal of SS in the ponds is due to sedimentation. To facilitate the availability of effluent for further treatment as and when required a bench scale reactor of 56.8 L capacity was operated. The ORP of the effluent ranged between - lOOmV to -150 mV. Average COD (COD total), CODs (COD soluble) and ammonical-N were found to be 90, 75 and 80 mg/L respectively. The characteristics of the effluent from bench scale UASB reactor were comparable with that offield samples. Phase 3, Stage 1: In the first stage of phase 3, aeration of effluent from S-38, G-70, N-34, N-27, SW1 and SW2 by sintered disc in batch and continuous reactor is presented. During the batch aeration an instantaneous increase in ORP and DO, followed by relatively slow rate of increase in ORP was noticed in all the cases. The temporal trend, in general, for all the effluents is same. During aeration oxygen demand decreased. The kinetics.of COD reduction conforms to both linear and exponential trend. The standard oxygen transfer efficiency of 0.9% was found which subsequently reduced with time. The volumetric reaeration constant (ka) was found to be 0.06 per minute (3.6 per hour). With 1 mV increase in ORP, CODs & BOD decreased by 0.13 mg/L. Nitrification did not take place although ammonia concentration decreased. In continuous aeration by diffusers ORP increased linearly with detention time. After 140 min detention ORP increased to 122 mV from -100 mV. COD and BOD varied linearly with ORP. With increase in ORP from -100 mV to 120 mV, CODs was reduced from 111 to ~ 55 mg/L (50%). For 1 mV increase in ORP, CODs and BOD decreased by 0.24 and 0.21 mg/L respectively. The continuous aeration was modeled as a completely mixed reactor. The predicted values were in conformity with observed values. The multiple regression analysis of the ORP found significant correlation coefficients for the variable COD, CODi, DOs and DO. vn The following equation accounts for 98 % of the observation. ORP=-126+49.4 _C_ a yDOsJ + 419 Phase 3, Stage 2: In this stage effluent from four STPs namely S-38, N-34, N-27, G-70 and bench-scale UASB were aerated by surface aerators. The observations were similar to that of diffusers i.e. DO and ORP of the effluent increased and COD decreased. However, relative rates of change of these variables were differentfrom what was observed for the diffusers. Aerationby diffusers was more efficient than the surface aerator (impeller type). A distinctive difference in the rate of aeration based on initial COD has been noticed. If COD was less than 150 mg/1 as in case of Eu from S-38 then the initial increase in DO (after 5 min) was more as compared to the effluents having COD in the range from 203 to 236 mg/L. Phase 3, Stage 3: Variation in ORP, DO, BOD, COD etc. in natural fall (cascade aeration) without any mechanical device was subsequently investigated. The increased DOand ORPafter fall reduced during storage. The COD however decreased after fall as well as during storage. Overall COD reduction i.e. during fall and storage ranged from 20-40%. For 100 mV decrease in ORP the COD reduction in continuous and batch was 20 and 36% respectively. The conclusion from the present study and the scope for future investigation form the subject matter of Chapter-5. The polishing ponds having 1 day detention are unable to meet desired standards for disposal. The effluent prior to the discharge should not only haveDO> 4-5 mg/L, BOD < 30 mg/L but also ORP in the + ve range > 120-135 mV. Such an objective has been attained by aerating effluent samples with diffusers, surface aerator and inclined plane. Approximately 20-50% BOD &COD were removed by aeration devices. The land requirement for such system is expected to be nearly 1/10 of the polishing ponds maintained at 1 day detention. viii