Investigating the properties of persistent organic carbon in municipal wastewater treatment biosolids
Wastewater treatment plants (WWTP) are the primary pathway for many organic contaminants such as pharmaceuticals and persistent organic pollutants (POPs) found in domestic, industrial and hospital effluents to reach the environment. These substances can accumulate in WWPT sludges or treated biosolids, which are currently subject to limited environmental monitoring or regulation for organic contaminants. The lack of practical tools for assessing the relative quantities of organic contaminants and the potential for these to transfer into the environment presents a barrier to environmental regulators. The aim of this study was to improve understanding of the persistent organic composition of sludges and biosolids to better inform sludge treatment and disposal guidelines. The study combined conventional test methods in a novel approach to assess the leachable and non-biodegradable fraction of organic carbon in sludges and biosolids to characterise how persistent organic pollutants accumulate in these matrices, and subsequently leach when applied to land. Sludges and biosolids from four municipal WWTP were investigated for desorbable dissolved organic carbon (DDOC) using leaching tests. Leachates were then assessed for their relative quantity of persistent DDOC (PDDOC) using biodegradation experiments and optical properties of both DDOC and PDDOC were examined using UV-Vis and FTIR analysis. Sludges from primary and secondary treatment stages, from both nitrifying and non-nitrifying WWTP, and processed biosolids were compared to identify the effect of treatment on relative quantities of DDOC and PDDOC. This is one of the first studies to quantify leachable and non-biodegradable DOC in municipal WWTP sludges and biosolids, comparing results between treatment plant stages and types. The study found that there was little variability in DDOC for primary sludges but DDOC for secondary sludges varied by degree of nitrification in the WWTP, ranging from 11,760 mg.kg-1 in a nitrifying plant to 33,853 mg.kg-1 in a non-nitrifying plant. Nitrification was found to have a positive impact on reducing DDOC leached from sludges from secondary reatment stages. DDOC in biosolids undergoing thermal treatment, anaerobic digestion (AD) and dewatering were found to vary with DDOC measured at 14,422 mg.kg-1, 22,542-27,862 mg.kg-1 and 26,155-29,983 mg.kg-1 respectively. Sludge treatment was found to reduce DDOC overall, with thermal treatment having the greatest effect followed by AD. The study found that 14-39% of DDOC was found to be persistent with PDDOC values ranging from 4,096 mg.kg-1 in a partially-nitrifying secondary sludge to 7,547 of mg.kg-1 in AD treated biosolids. Concentrations of persistent mobile organic carbon at these levels warrants further consideration by environmental regulators of the potential risk associated with land application of biosolids. The levels of PDDOC were generally higher in biosolids that had undergone further treatment than in untreated primary or secondary sludges. This suggests that sludge treatment processes can result in accumulation of POPs in biosolids and may also enhance desorption potential of POPs. The analysis of optical properties of leachates revealed a similar progression in indicators of biodegradable to non-biodegradable organic compounds between DDOC and PDDOC leachates, with an increase in the ratio of high molecular weight (MW) and aromatic compounds and indicators of functional groups consistent with some common POPs (e.g. alkyl halides, alkyl benzene compounds). The comparison of WWTP type found that non-nitrifying plants had a higher proportion of high MW, aromatic compounds than the nitrifying and partially-nitrifying plants, suggesting that extended aeration and aerobic treatment can increase removal of substances such as PAHs. Overall the study indicates that sludges and biosolids may pose a risk of transfer of POPs into the environment through leaching of PDDOC. Determination of PDDOC could be used as a screening tool for assessing relative POP burden of sludges and biosolids and to assess the overall effectiveness of sludge treatment technologies for reducing POPs prior to land disposal. The novel approach overcomes many of the existing analytical and risk assessment barriers faced by environmental regulators providing an approach that is relatively low cost, accessible and provides a bulk measurement as an indicator of the mobile organic pollutant load.
Biosolids, Sewage, Sludge, Wastewater treatment, Dissolved organic carbon, Persistent organic pollutants, Leaching, FTIR, PDDOC, Environmental risk, Land application, Primary treatment, Secondary treatment, Anaerobic digestion, Thermal treatment
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