Impact of cold and dilute sewage on pre-fermentation - a case study

The municipal sewage treatment plant (STP) of the city of Ghent (Belgium) h as to be retrofitted to a 43%-increase in the nitrogen treatment capacity a nd to phosphorus removal. Cold weather, dilute sewage and a critical COD ov er N ratio make the retrofit a challenge for full biological nutrient remov al. The potential for fermentation of primary sludge to alter those critica l feed sewage characteristics was experimentally evaluated. The idea was th at the pinpoint introduction of fermentate could optimise the available rea ctors by achieving high-rate denitrification and enhanced biological phosph orus removal. The fermentation process was evaluated with a bench scale apparatus. At 20 degreesC (heated process), the hydrolysis yield - expressed in terms of sol uble COD - varied from 11% to 24% of the total sludge COD. The fermentation yield expressed in VFA COD varied from 8% to 13% of the total sludge COD. The efficiency of heated fermentation of primary sludge was lower during co ld and wet weather, due to the different sewage characteristics, as a resul t of extended dilution periods and low temperature. The raw sewage, the primary effluent and the fermentate were fractionated a ccording to the requirements for the IAWQ Activated Sludge Model No. 2d. Th e results clearly show that fermentation in the sewer played an important r ole and temperature was the driving parameter for the characteristics of th e dissolved COD. Instead, the weather flow conditions were the driving para meter for the characteristics of the suspended COD. The results of the detailed fractionation were used as background for proce ss evaluation. The final scenario choice for the retrofit depends on a cost -efficiency calculation.