Year

2012

Degree Name

Master of Engineering - Research

Abstract

There is an increasing concern about the occurrence of micropollutants such as pharmaceuticals and personal care products (PPCPs), industrial precursors and products, pesticides and steroid hormones in freshwater systems worldwide. The removal of these compounds is of great importance to ensure safety of drinking water and environmental protection. A number of studies have reported that white rot fungus (WRF) can degrade a wide range of pollutants which are not amenable to bacterial degradation. Recent studies have focused on the removal of micropollutants by WRF treatment, either by whole-cell fungus, crude extracellular extract or purified enzyme solution. Evidence from the literature indicates that, WRF is capable of removing various micropollutants from aqueous phase, and the removal efficiency is governed by factors such as the physico-chemical properties of micropollutants, fungus strain and enzyme types. This thesis investigates the ability of white rot fungus Trametes versicolor (ATCC 7731) to remove selected micropollutants in pure culture batch tests and by a fungus- augmented membrane bioreactor operated continuously for three months under non-sterile conditions.

The comparison of removal of four micropollutants, namely bisphenol A, diclofenac, carbamazepine, and sulfamethoxazole, by activated sludge and whole cell fungus revealed significantly better fungal removal of bisphenol A and diclofenac and no removal of carbamazepine and sulfamethoxazole, whereas the activated sludge culture achieved low removal of all four compounds. In the crude enzyme extract test, regardless of the presence of the mediator 1-hydroxybenzotriazole (HBT), bisphenol A showed high removal, while carbamazepine showed negligible removal. However, the addition of mediator improved the removal of sulfamethoxazole. Furthermore, the removal of a set of 30 micropollutants, including 12 PPCPs, 3 industrial precursors and products, 6 pesticides, 5 steroid hormones, 3 UV filters and 2 phytoestrogens was studied in a series batch tests with whole cell WRF, chemically inactivated WRF and crude enzyme extract, respectively. Significantly hydrophobic compounds such as 17β - estrodiol 17 - acetate (E2Ac), pentachlorophenol, 4-tert-octylphenol and triclosan were efficiently removed by both live (biosorption and potentially biodegradation) and inactivated (biosorption only) fungus culture; however, their enzymatic degradation confirmed that biodegradation was the main mechanism of their removal from the aqueous phase.

Moderately hydrophobic compounds such as naproxen, formononetin, ibuprofen and estrone (E1), were efficiently removed by the live fungus culture, while relatively lower removal of these compounds by the inactivated culture confirmed biodegradation as the main mechanism for removal. On the other hand, the significantly hydrophilic compounds were removed poorly by the WRF. An overall comparison of micropollutant removal by whole cell fungus and crude enzyme extract indicated that the chemical structures are of great importance for compound removal. Compounds with strong electron withdrawing groups showed particular resistance. Addition of mediator (HBT) improved the removal of the hydrophilic compounds such as salicylic acid, enterolactone and atrazine, and hydrophobic compounds such as oxybenzone, gemfibrozil, pentachlorophenol, since the laccase-HBT system typically gives rise to radicals with higher redox potentials than laccase itself.

In the fungus-augmented MBR, the removal of two compounds, namely, bisphenol A and diclofenac was studied over a period of three months. Total organic carbon (TOC) and total nitrogen (TN) removal remained stable throughout the experimental period, suggesting biological stability. High removal of both micropollutants was achieved initially, however, the removal of diclofenac by the fungal MBR (HRT=1 day, diclofenac loading = 450±90 μg/L.d) gradually dropped from 80% to 40% within three weeks of continuous operation. Interestingly, relatively stable removal of bisphenol A (80-90%) and diclofenac (~55%) was achieved by applying an HRT of 2 days, although their concentration in the feed was increased to keep their respective loadings (475±25 and 345±112 μg/L.d) at levels not significantly different from that at the initial stage. A simple mass balance conducted at the end of operation revealed that a significant portion of bisphenol A (42%) and diclofenac (66%) adsorbed onto MBR sludge underwent biodegradation. It is noteworthy that, loss of extracellular laccase with membrane-permeate was evident, and the level of extracellular laccase activity in the MBR-supernatant gradually diminished to an undetectable level. Although laccase activity in sludge could be detected throughout the operating period, the sludge enzymatic activity profile did not correlate with that of micropollutant removal. Results reported here suggest that while fungal enzyme is indispensable for the degradation of micropollutants, enzymatic activity does not necessarily correlate well with the removal efficiency.

Overall, micropollutant removal by WRF treatment in the batch tests was mainly dependent on their physico-chemical properties such as hydrophobicity and chemical structures. Additionally, the removal of micropollutants by the fungus-augmented MBR was affected by the operating parameters such as micropollutant load and HRT.

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