SewageFermentation Units to Increase Degradable COD Fraction

Report No WSAA 8

December 1989




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


Only the sludges were found to produce significantincreases in VFA concentration, with better results being obtained from primarysludge than the mixed sludge. VFA production was attributed to fermentation ofthe settleable solids in sewage and results were compared on the basis of astandard 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 detectableincreases (sensitivity: 5 mg/L) in effluent VFA concentration, due to thelimited sludge mass collected. However, sludge and supernatant VFAconcentrations were observed to equilibrate within 7 days in an off-lineprimary sedimentation tank. Stable operation of the pilot plant was notachieved due to difficulties in controlling sludge during wastage, transfersand recycling. Sludge solids tended to float after storage for more than twodays because of the grease content and gas entrapment. Future applications musthave mechanisms for sludge scraping and scum control as well as minimisingaeration of the liquid streams.


Acid phase digesters were operated at sludge agesbetween 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 andwaste 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/gSSover 4 days but gave similar production over 8 days (120 compared with 96 mgVFA/gSS without RAS addition). Stirring batch fermentations of primary sludgeat 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 rawsewage, a fermenter generating 150 mg VFA/gSS would provide a biologicalphosphorus removal plant with an additional 30 mg/L VFA which would besufficient for removal of 12 mg/L P only if nitrate returned to the anaerobiczone is less than 1 mg/L.



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