Design and performance of BNR activated sludge systems with flat sheet membranes for solid-liquid separation

Includes bibliographical references.Wastewater treatment technologies have developed out of the need to protect receiving water bodies from the increasingly concentrated municipal and industrial waste streams generated through human activity. Of the methods available to clean wastewaters, biological nutrient removal (BNR) activated sludge (AS) is applied throughout South Africa and internationally as it has many advantages, notably it is cheaper to operate, does not introduce salinity into the water and is a simple and robust process. One of the key steps in the BNR AS process is the separation of biomass from the water. This is traditionally achieved by means of secondary settling tanks (SSTs), however recently the use of membranes for solid-liquid separation has gained popularity for the following reasons: Membranes are able to retain all solids and thus are insensitive to the settling characteristics of sludges, • they can be run at high concentrations and hence smaller reactor volumes are required, • membranes can produce a guaranteed high quality effluent free of pathogens and in some cases viruses too. • Additionally smaller reactor volumes and the obviation of SSTs allow a substantial wastewater treatment plant footprint reduction. Hence the combination of membranes in BNR AS is being increasingly applied. where much research has been conducted on the performance of membranes. The majority of the research has focused on the physical membrane performance, investigating the mechanisms of fouling, or on the membrane biological reactor (MBR) performance in removing organic compounds or nitrogen compounds from wastewater. There are however few case studies investigating BNR using membranes despite speculation that the inclusion of membranes may indeed affect the nature of the activated sludge biomass (Witzig et al., 2002)