Fate of cyanobacteria and their metabolites during water treatment sludge management processes
1. Introduction
Cyanobacteria are an issue for water authorities as their persistence in water supplies can potentially cause numerous problems for water treatment plant (WTP) operators. This can include exertion of an additional demand on WTP chemicals and clogging of filters, resulting in reduced filter run-times and increased backwash frequencies. One of the major problems caused by cyanobacteria is the metabolites they produce, in particular, cyanobacterial toxins (cyanotoxins) and compounds which impart tastes and odours. Some of these metabolites account for a large number of consumer complaints due to aesthetic issues; others have the potential to compromise human health.
With the increasing frequency of cyanobacterial detection in water supplies in many parts of the world, coupled with the changing climate and drought, it is imperative that water authorities employ and optimise successful treatment strategies for the effective removal of cyanobacteria and their metabolites. Many drinking water regulations, including the Australian Drinking Water Guidelines, stipulate that it is important to implement multi-barrier treatment options to ensure these contaminants do not reach the customer tap. As control measures in place at WTPs are the last line of defense prior to drinking water distribution, it is crucial that they are assessed accurately and monitored regularly. One specific area where there are significant knowledge gaps is the management of cyanobacterial-laden sludge within WTPs. Currently, there is limited information in the published literature which has studied the fate of such WTP sludge.
During incidents of high cyanobacterial numbers in water sources, it is possible that cyanobacterial cells may accumulate in the sludge generated from coagulation/flocculation/sedimentation processes. If the cyanobacterial-laden sludge is not managed effectively, there is potential for cell lysis, resulting in high concentrations of undesirable metabolites being released into the sludge supernatant. Sludge management varies between WTPs and one of the most common practices is lagoon treatment where the process dewaters sludge to aid in its subsequent disposal. However, little is known as to the fate of cyanobacteria during such processes, although anecdotal evidence suggests that the cyanobacteria lose viability rapidly which can potentially release large quantities of metabolites into the sludge supernatant. This can be problematic if this sludge supernatant (containing such metabolites) is recycled to the head of the WTP, particularly since conventional water treatment (coagulation/clarification/ filtration) is largely ineffective for the removal of extracellular metabolites
Furthermore, some WTPs practice direct filtration, whereby flocs generated after coagulation are not sedimented, rather they are applied directly onto rapid sand filters. Currently, no studies have established what effect filter backwashing procedures have on sand filters laden with cyanobacteria, in particular, if these procedures have the ability to compromise the cell integrity resulting in metabolite release.
The objective of this study was to determine the fate of cyanobacteria and their metabolites during sludge management practices in WTPs after coagulation. In particular, this study focussed on typical sludge management processes, in particular lagoon treatment. The impacts of direct filtration and backwashing on cyanobacterial cell integrity and metabolite release was also investigated. It is envisaged that the findings from this study are likely to facilitate improvements at WTPs with respect to cyanobacterial-laden sludge and/or supernatant recycle management.