How do I ensure my existing recycled water scheme is safe?

What are the end use water quality requirements?

The pathogen log₁₀ requirements (bacteria, virus and protozoa) for each of the recycled water end uses was established using the default values in the AGWR (2006), considering exposure to food consumers, facility workers and nearby community as appropriate. The default assumptions detailed in the AGWR were tested for each end use type. For example, did the uses of the water means pathogen reductions beyond the municipal log₁₀ requirements were required. The AGWR calculates tolerable risk for 10^-6DALYs per person per year, with assumptions on exposure volumes, frequency of exposure and the pathogens present in the source water. The number of times people are exposed under the municipal irrigation end use category were tested and deemed applicable for the sports field in use.

Does treatment meet the end use requirement?

Total required pathogen log₁₀ reduction reductions values were calculated for each recycled water scheme based on the AGWR (2006) default values (for example, secondary treatment has an indicative log₁₀ reduction of 1.0-3.0 for bacterial pathogens).

Process unit verification was undertaken to verify the theoretical AGWR (2006) default values. E. coli was used as a surrogate for bacterial pathogen removal and Clostridium perfringens as a process surrogate for Cryptosporidium. Chlorination targets (C.t) were quantified to ensure sufficient disinfection of virus and bacteria. The free chlorine level to achieve a 4-log reduction for viruses was calculated for recycled water treatment plant, based on achieving a chlorine contact time for coxsackie B virus (Black et al. 2009; Keegan et al. 2012). These log₁₀ reduction treatment values identified for each scheme were then compared see if end use requirements were being met.

Are additional barriers need to meet requirements?

Gaps in treatment log₁₀ reduction requirements were then assessed against non-treatment barriers (e.g. end use controls). Non-treatment barriers were identified and assigned default log₁₀ credits (AGWR 2006) for each exposure pathways (Table 2). A maximum of 3 log₁₀ reduction was used for end use controls.

Verification of non-treatment barriers was undertaken through representative site audits. The audits investigated if the non-treatment barriers were being effectively implemented.

Understand the risks to human health

To further understand the risk to human health from the recycled water scheme and how they were being managed, a risk assessment workshop was held with stakeholders. The risk assessment identified hazards, hazardous events and the barriers implemented to prevent exposure. Effectiveness of the treatment and non-treatment barriers was assessed to determine any areas in which risks to human health may be compromised.

Outcomes

Theoretical log₁₀ credits for treatment barriers were reviewed for each recycled water scheme. Example results for Recycled Water Scheme 1 are shown in Table 3.

Verification monitoring

Three influent samples of C. perfingens and thirteen samples across process units were taken over a two-month period at each plant. The influent samples were consistent with the values provided in the AGWR. The median C. perfingens value was used within the analysis. Log₁₀ pathogen reduction values were calculated across each process step for each sample taken. The time the samples were taken did not account for hydraulic retention times. This resulted in a negative log₁₀ pathogen reduction valueson one occasion.

Initial C. perfingens log₁₀ reduction values were lower than expected for the UV process unit. It was realised that this was because C. perfringens requires a significantly higher UV dose than Cryptosporidium (Smeets et al., 2006). E. coli requires a similar UV dose as Cryptosporidium so was used as the process surrogate for UV. Surrogate virus testing was not undertaken due to both cost and difficulties in ensuring the samples reached a suitable laboratory in time for analysis (the minimum default AGWR value were used in this instance). A summary of verification results for recycled water scheme 1 is shown in Table 4.

For some of the treatment plants, primary and secondary treatment and chlorination processes on average, showed evidence of greater log₁₀ reductions than the indicative values reported in the AGWR (2006). At one plant variability in log₁₀ reduction for filtration indicated that process optimisation may improve log₁₀ reduction efficiencies.

Chlorination targets at the treatment plants were increased to ensure sufficient disinfection of virus and bacteria. A summary of treatment log₁₀ reduction values is shown in Table 5.

A comparison of log₁₀ reduction values against end use water quality requirements found that non-treatment barriers (such as drip irrigation) were necessary to ensure the public health risk were managed for all the schemes. Example graphs of protozoan treatment log₁₀ reduction values are shown in Figure 2.

Figure 2. Example protozoan pathogen treatment and non-treatment barriers and end-user requirements (figure provided by Atom Consulting with permission)

Risk assessment

The risk assessment workshop reviewed and assessed the adequacy of treatment and non-treatment barriers to prevent significant hazards from the recycled water system. The workshop identified where additional barriers, process optimisation, additional maintenance and/or increased end user training was needed. In particular, it was recommended that audits be undertaken to ensure that non-treatment barriers are being effectively implemented.

Implications for practitioners

Adequacy of implementation of non-treatment (end use) barriers was identified as a key issue for this scheme. A transient worker population at the end use sites means that training (in the form of workplace inductions) must be ongoing. Appropriate governance, in the form of end user agreements are key to ensuring that end users successfully implement the required controls, both operationally and in support areas (e.g. training and maintenance). An auditing scheme undertaken by the council is essential in order to verify that there is end user compliance and ongoing awareness of scheme hazards and the potential risk to human health. Consideration should also be given to appropriately documenting approved end use types and strategies for assessing changes to future end use types.

Treatment verification using a combination of indicator organisms and desktop calculations can determine if the water is suitable for the end uses. Verification monitoring should be undertaken that is appropriate to the treatment processes in place and with consideration given to the financial costs of tests, using data available in the literature where appropriate. Verification testing can identify areas where process optimisation or longer-term infrastructure improvements can be implemented to improve water safety.

Implication for risk management

This project demonstrates how to assess and improve the safety of existing recycled water plants within the context of the AGWR(2006). Existing recycled water schemes while designed to meet chemical water quality standards may not have been designed for pathogen reduction.

Successful risk management includes having access to accurate and appropriate information regarding how the scheme is being implemented in practice. Appropriate stakeholders are needed for input into the risk assessment process. Existing schemes may no longer be operated as per the original design. Operational knowledge is essential in understanding how processes work when assessing the effectiveness of barrier implementation. On site verification of the treatment process and end user sites is essential.