The behaviour of pharmaceuticals in anaerobic digester sludge
Student thesis: Doctoral Thesis
Pharmaceuticals are biologically active compounds that may be consumed in hundreds of tonnes per year, and which are excreted into municipal sewerage systems. Many pharmaceuticals persist during sewage treatment, and significant environmental risk has been linked to incomplete removal of pharmaceuticals. Evaluation of this risk is important and should be as representative as possible, taking into consideration all significant exposure routes and removal processes. Sludge treatment processes are of particular interest because they offer a final opportunity for the removal of persistent compounds before the disposal of treated biosolids. During environmental risk assessment, it is currently assumed that anaerobic sludge treatment results in insignificant removal of persistent and adsorptive compounds from sewage sludge. This project was undertaken to address whether this assumption is valid. This thesis describes research into the behaviour and fate of pharmaceuticals in anaerobic digester sludge, and examines how redox conditions influence removal efficiency. Nine commonly used pharmaceuticals (caffeine, cimetidine, fluoxetine, ibuprofen, metformin, naproxen, paroxetine, propranolol and salicylic acid) were selected for use based upon detection levels within sewage sludge and predicted anaerobic biodegradability. Initial experiments were conducted to identify toxicity to anaerobic microorganisms and anaerobic biodegradability. No toxicity to the microorganisms in anaerobic sludge was identified at ≤ 50 mg C L-1for any of the selected pharmaceuticals. The extent of removal in anaerobic sludge and the principal removal mechanism was found to vary between the selected pharmaceuticals. Metformin and salicylic acid were removed by 82 and 93% through mineralisation, respectively; naproxen was completely removed through primary biodegradation; fluoxetine, paroxetine and propranolol were removed by 92 and 96 and 55% due to adsorption, respectively. It was hypothesised that the removal of pharmaceuticals in anaerobic sludge through biodegradation and adsorption would be observed at varying rates under differing redox conditions. Test systems using headspace gas flushing and chemical amendment were evaluated for the control of redox conditions, with Eh measurements and DOC removal being used to indicate the stability of the system and its capacity for biodegradation. The addition of nitrate, sulfate and carbonate resulted in poised Eh values in the region of +200, -200 and -200 mV, respectively and 81, 78 and 74% removal of DOC, respectively. This methodology was subsequently used to evaluate the removal of naproxen, propranolol and fluoxetine under carbonate-, nitrate- and sulfate-amended conditions. Significant differences (p = 0.001) in the removal of naproxen through primary biodegradation were found to exist between different redox conditions. Naproxen was completely removed under control conditions, while 26, 98 and 61% removal was observed under nitrate- sulfate- and carbonate amended conditions, respectively. Some differences were observed in Kd values, however, redox amendment was found to have little influence upon the elimination of propranolol and fluoxetine through adsorption. It was identified from this work that pharmaceuticals within the aqueous phase degraded under anaerobic conditions as predicted, while pharmaceuticals associated with the biosolids were removed through adsorption and present a potential threat to the environment on the disposal of treated biosolids. This work supports the current industry assumption that anaerobic treatment of biosolids offers limited opportunity for the removal of adsorptive pharmaceuticals, and also found no evidence for their enhanced removal under redox-controlled conditions. While research described within this thesis increases knowledge of the behaviour of pharmaceuticals in sewage sludge under anaerobic conditions, data is presented for a limited range of test substances only, and further investigation into the behaviour of additional compound classes is recommended.
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