An Autopsy of Nanofiltration Membrane Used for Landfill Leachate Treatment

Investigation of micropollutants removal from landfill leachate in a full-scale advanced treatment plant in Istanbul city, Turkey

Although the levels of micropollutants in landfill leachate and municipal wastewater are well-established, the individual removal mechanisms and the fate of micropollutants throughout a landfill leachate treatment plant (LTP) were seldom investigated. Therefore, the determination of the removal efficiencies and the fates of micropollutants in a full-scale leachate treatment plant located in the largest city of Turkey were aimed in this study. Some important processes, such as equalization pond, bioreactor, ultrafiltration (UF) and nanofiltration (NF), are being operated in the treatment plant. Landfill leachate was characterized as an intense pollution source of macro and micropollutants compared to other water types. Chemical oxygen demand (COD), NH₃, suspended solids (SS) and electrical conductivity (EC) values of the landfill leachate (and their removal efficiencies in the treatment plant) were determined as 18,656 ± 12,098 mg/L (98%), 3090 ± 845 mg/L (99%), 4175 ± 1832 mg/L (95%) and 31 ± 2 mS/cm (51%), respectively. Within the scope of the study, the most frequently and abundantly detected micropollutants in the treatment plant were found as heavy metals (8 ± 1.7 mg/L), VOCs (38 ± 2 μg/L), alkylphenols (9 ± 3 μg/L) and phthalates (8 ± 3 μg/L) and the overall removal efficiencies of these micropollutants ranged from -11% to 100% in the treatment processes. The main removal mechanism of VOCs in the aerobic treatment process has been found as the volatilization due to Henry constants greater than 100 Pa·m³/mol. However, the molecular weight cut off restriction of UF membrane has caused to less or negative removal efficiencies for some VOCs. The biological treatment unit which consists of sequential anoxic and oxic units (A/O) was found effective on the removal of PAHs (62%) and alkylphenols (87%). It was inferred that both NO₃ accumulation in anoxic reactor, high hydraulic retention time (HRT) and sludge retention time (SRT) in aerobic reactor provide higher biodegradation and volatilization efficiencies as compared to the literature. Membrane processes were more effective on the removal of alkylphenols (60-80%) and pesticides (59-74%) in terms of influent and effluent loads of each unit. Removal efficiencies for Cu, Ni and Cr, which were the dominant heavy metals, were determined as 92, 91 and 51%, respectively and the main removal mechanism for heavy metals has thought to be coprecipitation of suspended solids by microbial biopolymers in the bioreactor and the separation of colloids during membrane filtration. Total effluent loads of the LTP for VOCs, semi volatiles and heavy metals were 1.0 g/day, 5.2 g/day and 1.5 kg/day, respectively. It has been concluded that the LTP was effectively removing both conventional pollutants and micropollutants with the specific operation costs of 0.27 $/(kg of removed COD), 0.13 $/(g of removed VOCs), 0.35 $/(g of removed SVOCs) and 2.6 $/(kg of removed metals).