UVB and Dark Disinfection in Wastewater

Author: Yu Lian

Lian, Yu, 2016 UVB and Dark Disinfection in Wastewater, Flinders University, School of the Environment

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Natural wastewater treatment systems, such as wastewater stabilisation ponds (WSPs), provide a promising way to cope with fresh water shortages in the future via the production of reclaimed water. This applies especially in rural communities and developing countries: even in South Australia, over 110,000 people are served by natural wastewater treatment systems. Optimising utilisation of natural self-treatment processes in these systems decreases wastewater treatment cost, energy consumption, chemical by-product production and greenhouse gas (GHG) emissions.

High-rate algal ponds (HRAPs), with their shallower depth, meandering channel and paddle wheel mixing, are one of the outstanding members of the WSP family. Compared to traditional WSPs, HRAPs achieve the same disinfection performance in less time and with less capital costs. Sunlight, particularly UVB, the most damaging wavelength, plays a significant role in the whole disinfection process, with the dark disinfection also contributing. However, there has been a lack of knowledge about the effect of environmental factors on sunlight and on the dark disinfection mechanisms involved in these ponds. The current research’s aim was to model the effects of a series of environmental factors on UVB and dark disinfection in laboratory-based experiments.

Escherichia coli (E. coli) and MS2, being the most common indicators of bacterial and viral pathogens, were used in the experiments. Their comparative sensitivity within different water media was also explored. Several environmentally relevant factors, namely, temperature, dissolved organic matter (DOM), suspended solids (SS), turbidity, chlorophyll a, UVB dose (Jm-2) and UVB dose rate (Wm-2) were assessed for their influence on UVB and dark inactivation. The contribution to MS2 inactivation by reactive oxygen species (ROS), produced by the interaction of UVB with dissolved organic carbon (DOC), was determined by comparison of inactivation rates and log10 reduction values in wastewater in the presence and absence of the ROS quencher L-histidine.

The effect of temperature on E. coli and MS2 inactivation in dark raw wastewater was determined. The dark inactivation rates of MS2 and E. coli increased significantly with increases in temperature from 10ºC to 30ºC in HRAP wastewater. Two models, describing the relationship between temperature and the darkinactivation rates of E. coli and MS2, were provided for further application, with MS2 found to be more sensitive than E. coli in dark incubated wastewater.

In addition, MS2 was also observed to survive longer under UVB irradiance than E. coli. The temperature increase from 10ºC to 30ºC had a significant positive effect on MS2 inactivation but not on that of E. coli. Furthermore, in reverse osmosis (RO) water under the same UVB dose, UVB dose rate had a strong influence on UVB disinfection for both E. coli and MS2. Two distinct regions of the inactivation curve were identified in RO water, where inactivation was UVB dose rate-limited or UVB dose rate-saturated.

MS2 inactivation increased with increasing UVB dose rate in both filtered and wastewater. A mathematical equation was developed to model the relationship, in each water medium, between the UVB dose rates and MS2 inactivation rates. The presence of DOC significantly influenced the MS2 inactivation rates by two mechanisms, firstly, decreasing the MS2 inactivation rate by UVB attenuation and secondly, via the production of ROS. The depression of the inactivation rate in wastewater in the presence of the ROS quencher L-histidine, when compared with those rates measured in its absence, is indicative of the influence of ROS. Uniquely, this study estimated that 10–20% of the MS2 log10 reduction value recorded in raw and 0.22 μm filter wastewater could be attributed to the generation of ROS.

The relevance of the laboratory-acquired MS2 UVB inactivation rate data was clearly demonstrated by comparison with data obtained from a demonstration HRAP treating the wastewater from a rural community in South Australia (SA). Significantly, the laboratory determined MS2 log10 reduction value at 20⁰C, at a UVB dose equivalent to the mean, annual daily dose in SA (39,0373 Jm-2), was 1.46 ± 0.23 compared with the annual mean value of 1.59 ± 0.82 recorded in the HRAP.

The findings of the current research make a significant contribution to understanding the effects of a variety of environmentally relevant factors on UVB and dark disinfection. All results achieved aims which will be utilised in further practical applications, such as improving the design and operation of HRAPs. To the best of the author’s knowledge, this is the first time that the effect of the UVB dose rate on MS2 inactivation, which is fundamental for UV-related research, has been assessed systematically in a range of water media. More generally, the results suggest that both dose and dose rate should be recorded in related solar disinfection research.

Keywords: UVB, dark disinfection, high rate algal pond, wastewater stablization pond

Subject: Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2016
School: School of the Environment
Supervisor: Nancy Cromar