Effects of sludge settling characteristics in the BNR system

Although the BNR system design assumes no sludge settling problems in the f inal settling tank, the fluctuation of daily loading and up to 40% of anaer obiosis in the BNR system would be considerable factors in determining the sludge settleability. The sludge volume index (SVI) is a classical paramete r to examine the sludge settleability but it has a limited value to evaluat e the overall settling characteristics. The extracellular polymer (ECP) con tent in sludge has long been considered as an indicator of biological flocc ulation in activated sludge process, but the skepticism on analytical accur acy limits the usage in settling study. This study focused on evaluating th e BNR system performance related to the sludge settling characteristics und er the controlled laboratory environment. A 5-stage BNR system (anaerobic-a noxic-3 stage oxic) was operated with the HRTs from 3.9 to 7.6 hours. In or der to determine the sludge settling properties, both SVI and ECP content w ere monitored. The ECP contents in the sludge sample were measured by the s lime-washing step followed by high-G centrifugation and sonication. The gel -electrophoresis was used to detect the DNA in the sonicated samples to det ermine the intracellular polymer contamination. It has been found that the anaerobiosis in the anaerobic and anoxic zone that consisted of 37% of reac tor volume could not deteriorate the sludge settling properties even at the HRT of 3.9 hours. The SVI values of sludge taken from the reactor with the HRT of 7.2 hours averaged to less than 50 mL/gr. It was postulated that a fast settling sludge in the BNR system could not always ensure production o f dear effluent. The sludge with very low SVI might not achieve a complete biological flocculation potential. In addition, the degree of denitrificati on in the BNR system could be related to the sludge settleability.