Environmentally Sustainable Choices
Abstract
Methodologies for making Environmentally Sustainable Technological Choices
Anaerobic versus Aerobic Biological Treatment of a Dairy Wastewater
A case study is presented herein whereby a dairy wastewater treatment system, consisting of chemical/physical unit operations and anaerobic and aerobic biological treatments, is analysed as consisting of an anaerobic treatment train (AnTT) and an aerobic treatment train (ATT). Both treatment trains are fed with similar wastewater compositions from similar sources. A reasonable comparison of the two approaches was attained with respect to operational aspects, capital and operating costs as well as lifecycle [environmental sustainability] costs. All analyses were based on a per tonne of COD removed basis.
From an operational standpoint, the AnTT was difficult to commission and has been sensitive to excess [toxic] fat loadings. The aerobic reactor has had to cope with nitrate CIP loadings as high as 500 mg/L and poor settling biomass.
Taken over a 30 yr operating period, the capital cost of the AnTT and ATT are nearly equal at NZ$80. The ATT uses more power than the AnTT but the latter is heavily penalised by the high cost of chemicals required to remove fats/proteins ahead of the anaerobic reactor and the much higher cost of disposing of the more obnoxious and unstable DAF float. Overall costs (capital + operating) equated to NZ$410 for the AnTT (incorporating DAF#1) and NZ$250 for the ATT.
Whilst the ATT may be financially preferred in this particular case study, it is penalised environmentally because of its large consumption of energy in the form of electricity. The AnTT process has lower greenhouse gas emissions and a lower net energy demand but is penalised for the high chemical use in the two DAF plants. The DAF sludge is transported some 200 km for land application (compared to 30km for the aerobic sludge) and this reflects poorly on the anaerobic environmental profile. The anaerobic reactor produces around 460,000 m3/y of biogas with a net heating value of approximately 27 MJ/m3. Currently this is flared, providing no useful energy to the AnTT.