Sewage Fermentation Units to Increase Degradable COD Fraction

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Report no. UWRAA 08

December 1989

Synopsis

Raw sewage, settled sewage, primary sludge and primary sludge mixed with waste activated sludge were investigated as potential sources of volatile fatty acids (VFA) for the biological phosphorus removal process. Investigations were carried out in a pilot scale sedimentation tank, activated primary tank, in daily fed drum reactors and in batch fermenters located at Castle Hill STP.

Only the sludges were found to produce significant increases in VFA concentration, with better results being obtained from primary sludge than the mixed sludge. VFA production was attributed to fermentation of the settleable solids in sewage and results were compared on the basis of a standard concentration of 200 mg/L settleable SS.

Elutriation of VFA’s from sludge solids in a 2800Lprimary sedimentation or activated primary tank did not produce detectable increases (sensitivity: 5 mg/L) in effluent VFA concentration, due to the limited sludge mass collected. However, sludge and supernatant VFA concentrations were observed to equilibrate within 7 days in an off-line primary sedimentation tank. Stable operation of the pilot plant was not achieved due to difficulties in controlling sludge during wastage, transfers and recycling. Sludge solids tended to float after storage for more than two days because of the grease content and gas entrapment. Future applications must have mechanisms for sludge scraping and scum control as well as minimising aeration of the liquid streams.

Acid phase digesters were operated at sludge ages between 1.8 and 6.5 days. The VFA production ranged between 25 and 40 mgVFA/gSS with no systematic improvement with time.

Unstirred batch fermentation of mixed primary and waste activated sludge had a VFA production of 120 mg VFA/gSS over 4 days. Addition of RAS to primary sludge increased the production to 164 mg VFA/gSS over 4 days but gave similar production over 8 days (120 compared with 96 mgVFA/gSS without RAS addition). Stirring batch fermentations of primary sludge at 7 RPM increased the VFA production to between 200 and 312 mg VFA/gSS over 8days. When recalculated on the basis of 200 mg/L settleable solids in the raw sewage, a fermenter generating 150 mg VFA/gSS would provide a biological phosphorus removal plant with an additional 30 mg/L VFA which would be sufficient for removal of 12 mg/L P only if nitrate returned to the anaerobic zone is less than 1 mg/L.

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