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Effects on anaerobic digestion of employing polyelectrolytes and ferric chloride as aids to clarification of domestic sewage Warman, Robert W.

Abstract

The advent of high molecular weight polyelectrolytes has stimulated research into their use for BOD reduction in primary and secondary treatment and as sludge dewatering aids in vacuum filtration and centrifugation. Since many of these products have not been approved for use in domestic water supplies because of their possible toxicity to humans, it was felt that they may be toxic to the microorganisms in anaerobic digestion, activated sludge or other biological treatment processes. This research evaluates effects on the anaerobic digestion process. Three continuously mixed laboratory scale anaerobic digesters were operated at 32±1°C with a 30 day hydraulic and solids retention time. Digester number one served as a control and received sludge obtained by sedimentation of domestic sewage without the use of coagulants; number two received sludge obtained using 14 mg/l of a cationic Hercules Incorporated polymer, Hercofloc 814.2, as the coagulant; and number three received sludge obtained using 30 mg/l of ferric chloride as the primary coagulant and 1 mg/l of Hercofloc 836.2 as a coagulant aid. Waste stabilization calculations, based on influent and effluent BOD, COD, and VS results and on methane production, failed to indicate any toxicity or physical inability of anaerobic microorganisms to penetrate the floes formed as a result of the addition of coagulants as aids to sewage sedimentation. Calculations of waste stabilization from BOD₅, BODL , COD, and VS results indicated treatment efficiencies of approximately 81, 70, 62 and 59 percent, respectively, for all three digesters. Results of pH, alkalinity, and volatile acids testing of digester effluents and total gas production from the digesters did not indicate unbalanced treatment due to the presence of coagulants. pH and alkalinity results were, however, consistently higher in the digesters receiving chemically coagulated sludge than in the control digester, signifying a greater buffering capacity against digester upset. The effluent from digester number two was observed to settle more rapidly and leave a clearer supernatant than effluent from either of the other digesters. Subsequent Buchner funnel vacuum filtration tests produced values of specific resistance for effluents from digesters one and three that were 18 times greater than the values obtained for the number two effluent. Although the results were for the mixed digester contents rather than for the settled portion of the effluent, they indicate that little or no additional conditioning would be required prior to vacuum filtration dewatering of effluent from digester number two. A limited number of jar tests and settling column tests using a weak to medium strength domestic sewage, produced BOD₅ and COD removal efficiencies of 50 to 70 percent using 30 mg/l of FeCl₃ in combination with 0.5-1.0 mg/l of anionic Hercofloc 836.2. Similar efficiencies were achieved using 6-14 mg/l of either of the cationic Hercofloc polymers, 812 or 814.2, as the sole coagulant. An economic analysis was carried out comparing the total annual costs of primary and activated sludge treatment methods. Capital costs were amortized over a twenty-five year period at 10 percent per annum and were added to operation and maintenance costs to obtain total costs. The results of this analysis indicated that the maximum economic dosages of Hercofloc 814.2 added continuously to primary plants operating at capacity are 14.9, 10.2, and 6.6 mg/l for 1, 10, and 100 mgd plants, respectively. Similarly for ferric chloride and 0.5 mg/l of Hercofloc 836.2, the corresponding economic dosages of FeCl₃ are 51, 34, and 21.5 mg/l. These dosages are based on the assumption that an adequate degree of treatment can be obtained by chemical precipitation. Within the limitations of the economic analysis and based on jar test results, it was concluded that use of Hercofloc 814.2 as an aid to primary clarification of domestic sewage is not an economically attractive alternative to providing activated sludge treatment unless an adequate degree of treatment can be achieved at dosages somewhat less than 14 mg/l. On the other hand, use of ferric chloride and Hercofloc 836.2 was found to be economically attractive for plants in the 1-10 mgd range.

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