Post-treatment of anaerobically digested sewage for nutrient removal

Abstract

Although modern anaerobic sewage treatment systems and in particular the upflow anaerobic sludge blanket reactor (UASB) have demonstrated remarkable performance in tropical and subtropical regions, the effluent quality is generally not compatible with discharge standards in many countries, so post-treatment becomes necessary. In practice, the most widely applied post-treatment options are polishing ponds (PPs) and other aerobic treatment bioreactors. PPs are treatment units treating anaerobically pre-treated wastewater, which distinguishes them from conventional waste treatment ponds, wherein normally raw wastewater is treated. Anaerobic pre-treatment has a very profound influence on the configuration and operation of PPs, as will be shown in this chapter. Other aerobic post-treatments can be realized with conventional activated sludge systems, or more complex technological solutions including membrane bioreactors (MBRs) or trickling filters. The anaerobic–aerobic treatment has important advantages compared to the purely aerobic treatment: the volume and sludge production is much smaller, there is a possibility that the anaerobic–aerobic system operates with energetic self-sufficiency and the concentration of excess sludge is much higher facilitating its processing. An important problem of anaerobic–aerobic treatment is the difficulty of producing a final effluent with low nutrient concentration. Anaerobic sewage treatment reduces the content of biodegradable organic matter in the effluent, limiting in practice the removal of phosphorus to the use of chemical precipitation systems. For the case of nitrogen, the alternatives only go through new processes, which complement the poor capacity of conventional denitrification due to the limited concentration of biodegradable organic matter. This limits in practice, the nitrogen elimination capacity of activated sludge systems or trickling f ilters treating effluents from UASB reactors. A notable breakthrough could occur if reliable partial nitrification anammox processes for the water line were developed, as these are still under development, or systems in which the dissolved methane present in the anaerobic effluents was used to denitrify, as it has already been observed in some MBRs.