Assessment of biological nutrients removal from the supernatant originating fromt the anaerobic digestion of the organic fraction of the municipal solid waste

This work critically evaluates the biological processes and technologies applied to remove nitrogen and phosphorus from the anaerobic supernatant produced from the treatment of the organic fraction of municipal solid waste (OFMSW) and from its co-digestion with other biodegradable organic waste (BOW) streams. The wide application of anaerobic digestion for the treatment of several organic waste streams results in the production of high quantities of anaerobic effluents. Such effluents are characterized by high nutrient content, since organic and particulate nitrogen and phosphorus are hydrolysed in the anaerobic digestion process. Consequently, adequate post treatment of this stream is required in order to comply with the existing land application, water reuse and discharge legislation in the European Union (EU) countries. This may include physicochemical and biological processes, with the latter being more advantageous due to their lower cost. Nitrogen removal is accomplished through the conventional nitrification/denitrification, short-cut nitrogen removal (i.e. nitritation/denitritation) and the complete autotrophic nitrogen removal process; the latter is accomplished by the anaerobic ammonium oxidation (anammox) process. As this liquid stream is characterized by low COD/TKN ratio, conventional denitrification/nitrification is not an attractive option; short-cut nitrogen removal and autotrophic nitrogen removal are more promising processes. Both suspended and attached growth processes have been employed to treat the anaerobic supernatant. Specifically, the sequencing batch reactor (SBR), the membrane bioreactor (MBR), the conventional activated sludge (CAS) and the moving bed biofilm reactor (MBBR) processes have been investigated. Physicochemical phosphorus removal via struvite precipitation has been extensively examined. Enhanced biological phosphorus removal from the anaerobic supernatant can take place through the sequential anaerobic/aerobic or anoxic/aerobic processes. More recently, denitrifying phosphorus removal via nitrite or nitrate has been explored. The removal of phosphorus from the anaerobic supernatant of OFMSW is an interesting research topic which has not yet been explored. The anaerobic effluent is often recycled back into the digester; however, this may result in the inhibition of the process due to the accumulation of dissolved ions (i.e. chloride, ammonium) and potentially other substances in the reactor. Consequently, it is possible to recycle only part of the effluent. At the moment standardization in the design of facilities that treat anaerobic supernatant produced from the treatment of OFMSW is still under development. To move towards this direction it is first necessary to assess the performance of alternative treatment options. This work (i) concentrates existing data regarding the characteristics of the anaerobic supernatant produced from the treatment of OFMSW and from their co-digestion with other BOW, (ii) provides data documenting the effect of the anaerobic digestion operating conditions on the supernatant quality and (iii) critically evaluates alternative options for the post-treatment of the liquid fraction produced from the anaerobic digestion process.