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Effects of Additives on Volatile Fatty Acids Production in Anaerobic Digestion

By Prasanna Rao

Laboratory-scale experiments were conducted to study the effect of additives on the process of anaerobic digestion in general and on the production of volatile fatty acids in particular. Twelve reactors with a working volume of 3-liters were operated as anaerobic digesters and were loaded daily with 150 ml of the primary sludge with an average volatile solids content of about 3%. Five commercial additives - Biocope, Medina d-part, MPC, Alken Murrary and Biosep - were added in duplicates to the total of ten reactors and two reactors were operated as control. The influent and effluent from each reactor were analyzed for total volatile solids, volatile fatty acids, and pH.

The volatile fatty acids were analyzed by using gas chromatography. The addition of Biocope results in the greatest reduction of the volatile solids of 47.5%. This was 8.1% greater (P< 0.05) than the reduction of volatile solids in control reactors, which did not receive any additives. Acetic and propionic acids were the only major acids present in the reactors receiving Biocope and Medina d-part. The reactors receiving MPC, Alken Murray and Biosep produced considerable amounts of six major fatty acids. The reactors receiving MPC developed unbalanced conditions and hence resulted in the lowest destruction in volatile solids and highest concentrations of volatile fatty acids. It was found that only Biocope affected the anaerobic digestion process in a significant manner by causing the highest destruction in volatile solids. A cost analysis was performed for a wastewater treatment plant using Biocope as an additive, which gave a benefit to cost ratio of 2.27, indicating the potential economic advantages associated with its use.

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