Sidestream treatment for improved BNR process performance

The removal of nutrients from the wastewaters through biological processes is a cost effective and environmentally sound alternative to chemical treatment. The primary driver for the success of the biological nutrient removal (BNR) processes is the availability of suitable carbon sources in the influent wastewater. Unfortunately, in the UK the wastewaters considered being weak for the BNR carbon limited processes and hence many methods have been examined in the past for the enhancement of BNR. In this project an internal carbon source was proposed and examined. The carbon was produced from the disintegration of activated sludge by a mechanical process, which was explored and its impact on the BNR carbon limited processes was evaluated. The equipment used in this study for mechanical sludge disintegration was a deflaker, which was able to cause significant increase in chemical oxygen demand and volatile fatty acids (VFA) in the soluble phase of sludge. Laboratory scale tests revealed that this carbon source can improve the phosphorus release and denitrification process and hence the phosphorus and nitrogen removal. These results led us to investigate the carbon source produced from disintegration in pilot scale and two BNR reactors were used for this purpose. The mechanical disintegration of 5.8% of return activated sludge was able to increase the concentration of VFA in the influent wastewater by 2.5-7 mg l-1 and successfully replace the equivalent amount of acetic acid, which is normally considered to be the best carbon source for biological phosphorus removal. The performance of the test reactor in terms of nitrogen, suspended solids and chemical oxygen demand was also unaffected. In addition, the sludge disintegration affected the bacteria growth yield, which combined with the longer sludge age by 6 days compared to the control reactor caused a 20-26% reduction in sludge production. In order to examine whether this process could be used by the water utilities a cost analysis took place, which revealed that the operational cost of the specific disintegration process and under the conditions examined in this study outweighs the savings from the produced carbon source and reduced amount of sludge.