Year

2012

Degree Name

Master of Engineering

Department

School of Civil, Mining and Environmental Engineering

Abstract

The occurrence of trace organics such as pesticides, pharmaceutically active compounds, natural and synthetic hormones as well as various industrial compounds in the aquatic environment is of great concern due to their potential adverse effects on human health and those of other biota. Therefore, the removal of these compounds from wastewater is an important consideration to ensure safe drinking water and better protect the environment. In the literature, several techniques have been explored for trace organics removal, namely, conventional activated sludge (CAS), membrane bioreactors (MBRs), nanofiltration/reverse osmosis membrane filtration (NF/RO) and adsorption; however a universal end-of-pipe treatment process is yet to be established.

Evidence from the literature indicates that neither MBR nor activated carbon on its own can adequately remove all trace organics of concern. This thesis investigates sequential and simultaneous application of activated carbon adsorption with MBR treatment for an enhanced removal of trace organic contaminants. A set of 22 compounds representing four major groups of trace organics including 11 pharmaceutical and personal care products, 2 pesticides, 4 industrial chemicals and their metabolites and 5 steroid hormones was selected for this investigation. Various investigations were conducted during the continuous operation of a laboratory-scale MBR system for a total of 306 days. This thesis focuses on 93 days of operation of a combined MBR with granular activated carbon (MBR - GAC system) followed by 100 days of operation of the MBR after direct addition of powdered activated carbon (PAC) into it.

The MBR showed stable and high performance with respect to all key basic water quality parameters (e.g., TOC, TN and turbidity) and operating parameters (e.g., pH, and MLVSS/MLSS ratio). It was confirmed that MBR treatment can effectively remove hydrophobic (i.e., compounds having a distribution coefficient, Log D >3.2) and readily biodegradable trace organic compounds. The reported data also highlighted the limitation of MBR in removing hydrophilic and persistent compounds such as metronidazole, ketoprofen, carbamazepine, diclofenac, and fenoprop.

GAC post-treatment was observed to complement MBR treatment to obtain initially high overall removal of less hydrophobic and biologically persistent trace organic GAC post-treatment was observed to complement MBR treatment to obtain initially high overall removal of less hydrophobic and biologically persistent trace organic

The removal of the 22 selected trace organic contaminants by MBR treatment was enhanced after direct addition of PAC into it. The high degree removal (95%) of the hydrophobic and readily biodegradable compounds continued to be achieved in PAC – MBR system. An immediate increase in removal efficiency of biologically persistent hydrophilic compounds (metronidazole, fenoprop, naproxen, ketoprofen, diclofenac, and carbamazepine), which showed low removal by MBR- only treatment, was observed in the PAC – MBR system. However, within approximately three weeks the removal efficiency dropped down to the level achieved before the addition of PAC. The removal efficiency of these compounds could be recovered by adding a second dose of PAC, raising the PAC concentration in the MBR to 0.5 g/L. The removal of the above mentioned six persistent compounds did not drop below 60 % even after one mont h (metronidazole 73 %, fenoprop 59 %, naproxen 93 %, ketoprofen 91 %, diclofenac 71 %, and carbamazepine 87%). However, except for ketoprofen and carbamazepine, the removal efficiency of the other four problematic compounds further diminished gradually, indicating that withdrawal of spent PAC and replenishment of fresh PAC would be required to achieve more stable performance.

Overall, both simultaneous application of PAC within MBR and sequential application of GAC adsorption following MBR treatment process are potential treatment processes to enhance removal of trace organic contaminants. Based on a simple cost-benefit analysis from the performance stability and activated carbon usage points of view, of the two processes, simultaneous application of PAC within MBR appears to be a better option than sequential application of GAC following MBR treatment.

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