TREATMENT OF SUGAR INDUSTRY WASTES USING UPFLOW ANAEROBIC SLUDGE BLANKET (UASB) PROCESS

Abstract

High BOD wastewaters from cane sugar mills are among the major polluters of water bodies in India. The problem is compounded due to seasonal nature of the industry and lean water flow in rivers during crushing season. Aerobic methods are not economically feasible for treatment of these wastes, and conventional anaerobic processes are not considered favourable as they require long detention periods necessitating large reactor volumes and associated operational problems. Sanitary disposal of sugar mill waste in India has thus proved to be a difficult problem for want of a suitable treatment method. Work presented in the thesis pertains to the treatment (laboratory scale) of sugar industry wastes by UPFLOW ANAEROBIC SLUDGE BLANKET (UASB) process. The UASB reactors work at hydraulic retention times (HRT) of the order of 3-6 hours and solids retention time (SRT) in the range of 200- 300 days. The work was carried out in two phases. In the first, synthetic wastewater and in the second, actual sugar mill effluent was used. As sugar mill effluents are deficient in nutrients (N and P), nitrogen and phosphorous were added to the feed. By-product methane was captured. Organization of the Thesis The thesis has been organized in nine chapters. Chapters 1 to 5 are devoted to the review of relevent literature,and chapter 6 gives the materials and methodology used in the experimental work. The data obtained from the studies have been presented in chapter 7. Discussions of the results are incorporated in the same chapter. The analysis of results has been per formed to give information about potential, performance, kinetics and control of the process. Development of granular sludge, a key factor in the UASB process is also discussed in this chapter. Tentative cost analysis of the process has been given in chapter 8. Conclusion drawn from the present investigation, (ii) and recommendations for future work have been incorporated in Chapter 9. Experimental Work Continuous flow experiments were conducted at a liquid temperature of 30 ± 1 C in three identical reactors of 11.0 1 capacity and designated as Rj, R2 and Ry Net height of the reactors was 118 cm, and cross-section 10cm x 10cm. The reactors were equiped with gas solids separators and gas holders. Digested sludge from a nearby Drugs and Pharmaceutical industry effluent treatment plant was used for seeding the reactors. Synthetic waste was prepared daily by dissolving cane sugar molasses in tap water and fed continuously using three peristaltic pumps. Urea and ammonium phosphate were added to the feed as sources of nitrogen and phosphorous. Experimental data was recorded regularly for .535 days. Reactors R{ and R2 were started at 24 h HRT with an organic loading T I of 2 kg COD m" d" and sludge loading of 0.3 kg COD kg VSS"1 d"1. COD:N:P ratio of 300:10:1 in R{ and 300:5:1 in R2 were maintained. The reactors were operated at organic loadings from 0.6-3.15 kg COD m"3 d"1 and HRT 24-12 h for 250 days (low loading range). Although COD removed was 80-85%, stable digestion environment could not be maintained under varying loading conditions. Due to low gas production rates, mixing of the digester contents was not satisfactory. Granular sludge, an important feature of the UASB process could not be developed under low loading range. The loadings in reactors RJ and R2 were changed after the 252nd day. By keeping the HRT at 6 h, organic loading was suddenly increased in Rl and gradually increased in R" . At higher loadings gas production increased (iii) and consequently, good mixing could be achieved. COD removal efficiency improved (to %90%) and sufficient buffering capacity developed through conver sion of feed nitrogen to ammonical form. Sludge in R{ was converted to granular form 24 days after shock loading the reactor. Sludge bed in R2 developed into flocculant mass with large, loose granules. Reasons for the development of different type of sludges could be the loading pattern followed and the nutrient ratios adopted. The third reactor, R3 was started at organic loading of 1.5 kg COD m' d" and sludge loading of 0A5 kg COD kg VSS"1 d"1, and the loading was increased to 4.5 - 5 kg COD m"3 d"1 after 3 weeks. Granular sludge developed in 6 weeks after start-up. Nutrient ratio was maintained at 300:10:1. Observation of the sludge under a scanning electron microscope at regular intervals revealed that there was a progressive enrichment of the sludge bed with microbial mass. Amatured sludge consisted of about 70 g f1 suspended solids with 80% volatile fraction, of which 14-17% was nitrogen. The sludge granules at this stage had a settling velocity of 100 cm min"1. Overall sludge production rate was about 4% (range 3-5%) of the COD removed. Once the sludge was well acclimatized, organic loadings upto 9 and 2 I 12-13 kg COD m" d~ , and sludge loadings upto 1.25 kg COD kg VSS"1 d"1 could be satisfactorily applied in flocculant and granular beds respectively, at 3-6 h HRT. Surface loading (hydraulic) varied from 0.0^75 m h"J at 24 h to 0.38 m h at 3 h HRT. At the loading rates applied, reactors performed well at COD concentrations of ^50-2500 mg I . Performance of the reactors was good both in terms of COD removal and gas production. Above 90% removal of COD, and a gas production of 0.35-0.5 m of bio-gas kg COD applied"1 (upto 5 m3 m"3 reactor vol. d"1) (iv) with a methane content upto 75% could be achieved when nearly half the reactor was filled with rich acclimatized sludge. The treated effluent carried about 20-70 mg 1" (on an average 30-40 mg f1) suspended solids, and about 80% of the nitrogen input, mostly in ammonical form at various stages of operation. In the final phase of the studies, COD of actual sugar mill effluent varied from 500 to 1600 mg l"1 over a 3 weeks period. COD removal upto 95% and bio-gas production of about 0.45 m3 kg COD applied-1 (2-3 m3 m~3 reactor vol. d ) having a methane content of 80-85% were achieved. Organic loading varied from 3 to 13 kg COD m"3 d"1. HRTs maintained were 3 h and 4 h ; and nutrient ratio was 300:10:1. Atentative cost analysis has revealed that a sugar mill in Western U.P., with a crushing capacity of 3000 tonnes d-1 has to invest Rs. 750000 to install a UASB plant for treating its effluent. Land requirement for the installation is less than a hectare even after accounting for future expansion. The mill can earn a net profit of Rs. 500000 by utilization of bio-