May 13, 2019
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Crawford, N. and Contos, A. (2019). How do I ensure my existing recycled water scheme is safe? In: J.B. Rose and B. Jiménez-Cisneros, (eds) Water and Sanitation for the 21st Century: Health and Microbiological Aspects of Excreta and Wastewater Management (Global Water Pathogen Project). (S. Petterson and G. Medema (eds) Part 5: Case Studies), Michigan State University, E. Lansing, MI, UNESCO. https://doi.org/10.14321/waterpathogens.76
|Last published: May 13, 2019|
The objective of this case study was to determine whether public health risks from the local government operated recycled water scheme were appropriately managed through treatment and non-treatment barriers (end user controls), specifically to:
The project took place in the mid north coast of NSW, Australia. Due to historical community pressure against ocean discharge an extensive reuse scheme was developed utilising the effluent from four sewage treatment plants. All plants have primary screening, biological treatment (using an oxidation ditch and clarifier on one plant, and intermittent aeration in the other three). UV and chlorination was used as disinfection in one plant, chlorine in two others and a tertiary maturation pond for the final plant. This recycling scheme now supporting a range of horticultural industries including blueberries, banana and greenhouses for tomatoes and cucumbers. These industries may not have a potable water connection to the town supply. The council (local government), which operates the treatment plants also uses the water to irrigate sports fields and plant nurseries. The region is culturally and linguistically diverse; seasonal workers may be used during harvest.
Figure 1. Drip irrigation of tomatoes (photo provided by Annalisa Contos)
The pathogen log10 requirements for each of the recycled water uses was established using the default values in the AGWR (2006; GWPP case study Australian guidelines for water recycling), considering exposure to food consumers, facility workers and nearby community as appropriate. The AGWR (2006) provide default log10 credits for non-treatment barriers such as 3 log10 reduction for drip irrigation of crops with no ground contact.
Initial process unit verification was conducted using E. coli as a surrogate for bacterial pathogen removal and Clostridium perfringens as a process surrogate for Cryptosporidium. Initial C. perfingens LRVs were lower than expected for the UV unit. We realised 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. Chlorination targets (C.t) were quantified to ensure sufficient disinfection of virus and bacteria.
This project demonstrates how to assess and improve the safety of existing recycled water plants. Plant verification using a combination of indicator organisms and desktop calculations can determine if the water is suitable for the end uses. From this, action plans incorporating short term operational changes (such as increasing chlorination), end user engagement (education and audit) and longer term infrastructure improvements can be implemented to improve water safety.
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The purpose of the case study was to determine whether public health risks from the existing recycled water scheme were appropriately managed.
The scope of the assessment included:
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.
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.
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.
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.
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.
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)