VFA Production in Thermophilic Aerobic Digestion of Municipal Sludges

Volatile fatty acids (VFA) are a principal carbon substrate used in biological nutrient removal processes, and an increase in their concentration can enhance process performance. In the autothermal thermophilic aerobic digestion (ATAD) of primary sludge, a reduction in both aeration and retention time had been reported to enhance VFA production. However, smaller wastewater treatment plants, which are suitable for ATAD, often combine sludge in a single digestion process for convenience. Consequently, research was undertaken to determine if a mixture of primary and secondary sludge would increase VFA production, compared to primary sludge alone, in the first stage of ATAD. Secondary sludge was predicted to further enhance VFA production by providing the required substrate "prepackaged" in the correct ratios. The results illustrated that the incorporation of secondary sludge into the primary sludge feed resulted in increased production and accumulation of VFA. The greatest production and accumulation were produced with the digestion of 100% secondary sludge. In addition, analysis of nutrient species confirmed that the mixing of primary and secondary sludge, as well as the ATAD of mixed and secondary sludge, results in the release of stored phosphorus and an increase in ammonia nitrogen.