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Abstract
Turbidity in water can be caused by a range of different turbidity causing materials (TCM). Here the characteristics and attachment of bacteria to TCMs was assessed and the resultant impact on UV disinfection determined. TCMs represent potential vehicles for bacterial penetration of water treatment barriers, contamination of potable supplies and impact on subsequent human health. The TCMs under investigation were representative of those that may be present in surface and ground waters, both from the source and formed in the treatment process. The TCMs were chalk, Fe (III) hydroxide precipitate, kaolin clay, manganese dioxide and humic acids, at different turbidity levels representative of source waters (0, 0.1, 0.2, 0.4, 1, 2, and 5 NTU). Escherichia coli and Enterococcus faecalis attachment followed the order of Fe(III)>chalk, with little to no attachment seen for MnO2, humic acids and clay. The attachment was postulated to be due to chalk and Fe(III) particles having a more neutral surface charge resulting in elevated aggregation with bacteria compared to other TCMs. The humic acids and Fe(III) were the TCMs which influenced inactivation of E. coli and E. faecalis due to decreasing UV transmittance (UVT) with increasing TCM concentration. The presence of the Fe(III) TCM at 0.2 NTU resulted in the poorest E. coli inactivation, with 2.5 log10 reduction at UV dose of 10mJcm−2 (kd of −0.23cm2mJ−1) compared to a 3.9 log10 reduction in the absence of TCMs. E. faecalis had a greater resistance to UV irradiation than E. coli for all TCMs. Effective disinfection of drinking water is a priority for ensuring high public health standards. Uniform regulations for turbidity levels for waters pre-disinfection by UV light set by regulators may not always be appropriate and efficacy is dependent on the type, as well as the amount, of turbidity present in the water.
Highlights
The reason was seen in the high UV254 of these two turbidity causing materials (TCM) which increased with TCM concentration to 38% and 48% UV transmittance (UVT) at 5 NTU turbidity level
The presence of some TCMs substantially reduced the efficacy UV inactivation of E. coli for Fe(III) (β = 0.39) and humic acids (β = 0.27) with some of this attributed to attachment, but the majority due to reductions in UVT
E. faecalis was more resistant to UV inactivation in the presence of TCMs than E. coli suggesting that the impact of TCMs could further increase the recalcitrance of more resistant strains reducing ability of UV to achieve statutory log reductions
Summary
Knowledge of TCM sub-components could question the validity of reported inactivation dose-response relationships of undesired microorganisms (Douterelo et al, 2016; Huber et al, 2011; Templeton et al, 2005) and challenge current guidelines on turbidity levels for treated waters or prior to UV disinfection Another important parameter influencing the inactivation of microorganisms is the potential for them to attach to the surface of particulate TCMs, or be bound together by large organic molecules, complexes of which can aggregate (Hess-Erga et al, 2008; Liu et al, 2013; Mamane, 2008; Wu et al, 2005).
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