Effect of turbidity on water disinfection by chlorination with the emphasis on humic acids and chalk.

Chlorine is globally the most widely used chemical for water disinfection. Whereas disinfection efficiency is well known to depend on water pH and temperature, the effect of turbidity is less well studied. Although turbidity is measured online in most drinking water works and most countries where regulations exist have set limits of <1 NTU for water leaving the works, the composition of turbidity is typically unknown. Given the heterogeneous nature of substances contributing to turbidity, the aim of this work was to study the effect of selected compounds on chlorination efficacy. The effect of humic acids and chalk on the inactivation of the indicator bacteria Escherichia coli and Enterococcus faecalis was assessed at neutral pH at different turbidity levels using both plate counting and flow cytometry in combination with membrane integrity staining. For humic acids, a turbidity of 1 NTU (corresponding to 2 mg L-1) was identified as a critical threshold, which when exceeded was found to have a negative impact on chlorine disinfection. Chalk, on the other hand, had no measurable impact up to 5 NTU. The observation applied to both bacterial species with identical conclusions from the two diagnostic methods. Results corroborate that different turbidity causing substances affect chlorination efficiency to very different extents with chlorine demand by organic material probably being the most important determinant. In the case of turbidities >1 NTU, turbidity measurement benefits from the consideration of the organic content as mere NTU values do not allow predicting an impact on chlorination efficiency.

Turbidity composition and the relationship with microbial attachment and UV inactivation efficacy.

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.

When are bacteria dead? A step towards interpreting flow cytometry profiles after chlorine disinfection and membrane integrity staining.

Flow cytometry is increasingly employed by drinking water providers. Its use with appropriate fluorescent stains allows the distinction between intact and membrane-damaged bacteria, which makes it ideally suited for assessment of disinfection efficiency. In contrast to plate counting, the technology allows the visualization of the gradual loss of membrane integrity. Although this sensitivity per se is very positive, it creates the problem of how this detailed viability information compares with binary plate counts where a colony is either formed or not. Guidelines are therefore needed to facilitate interpretation of flow cytometry results and to determine a degree of membrane damage where bacteria can be considered 'dead'. In this study we subjected Escherichia coli and environmental microorganisms in real water to increasing chlorine concentrations. Resulting flow cytometric patterns after membrane integrity staining were compared with culturability and in part with redox activity. For laboratory-grown bacteria, culturability was lost at lower disinfectant concentrations than membrane integrity making the latter a conservative viability parameter. No recovery from chlorine was observed for four days. For real water, loss of membrane integrity had to be much more substantial to completely suppress colony formation, probably due to the heterogenic composition of the natural microbial community with different members having different susceptibilities to the disinfectant.