Inactivation of Cryptosporidium across the Wastewater Treatment Train: Recycled Water Fit for Purpose

This report provides a better understanding of the ‘true effect’ of wastewater treatment train processes on reducing the risk presented by Cryptosporidium oocysts in recycled water.  The fate of Cryptosporidium oocysts was examined along the treatment trains of five (5) Australian wastewater treatment plants in which recycled water is utilised.  The study accurately quantified both the rates of removal and inactivation of oocysts to understand the ‘real risk’ posed by recycled water from these treatment trains.

This report is the culmination of a considerable collaborative research effort between Smart Water Fund, SA Water Corporation, and Water Research Australia that started in 2010.  Phase I of this project optimised a cell culture infectivity assay for Cryptosporidium detection.  A subsequent SA Water funded project extended the findings of Phase I of the Fund’s project to develop a single assay format that provides precise information on Cryptosporidium infectivity and total oocyst numbers.

The Challenge

Cryptosporidium is a problematic organism for the water industry.  The parasite has the potential to persist beyond the wastewater treatment processes and come into contact with the community via water recycling ‘third pipe’ schemes including irrigation of agricultural crops and open space irrigation. Cryptosporidium is highly resistant to chlorine-based disinfection.  Its ever present risk requires additional treatment processes, including alternative disinfection methods such as UV and ozone treatment, and sometimes filtration for effective removal of oocysts.

Understanding the ‘true effect’ of existing wastewater treatment trains in reducing the risk of Cryptosporidium has been incomplete to date.  Risk analysis has relied on the basis of ‘total’ oocyst numbers without considering the infectivity of the oocysts.  This has been due to the absence of a satisfactory tool to measure infectivity.  However, different treatment trains have variable removal rates and as such, only analysing total oocyst numbers could result in an overestimation or underestimation of the Cryptosporidium risk in recycled water.  Therefore both the total rate of removal and inactivation achieved by different wastewater treatment trains provides a more complete understanding of the treatment effectiveness.       

The Project    

This project built on the findings of previous projects to investigate both the rates of removal and inactivation achieved by different wastewater treatment processes.  This study utilised a new single format assay to examine the fate of Cryptosporidium along the wastewater treatment train of five Australian Wastewater Treatment Plants (WWTPs); three (3) in Victoria and two (2) in South Australia.  The WWTPs involved in the study were Western Treatment Plant (Melbourne Water), Altona (City West Water), Mt Martha (South East Water), Aldinga, and Glenelg (SA Water).

Sample grabs from the five WWTPs were taken at numerous points across the treatment train; from covered raw sewage, primary treated wastewater, secondary treated wastewater and post lagoon treated wastewater.  A minimum of six (6) rounds of sampling were conducted at each WWTP between January 2013 and December 2013.  In addition, oocyst inactivation during lagoon treatment was further investigated via pilot chamber studies.  It is worth noting that the early rounds of sampling in Victoria captured data from a large and widespread cryptosporidiosis outbreak.  Importantly, all three Victorian plants achieved removals during this challenge at the higher end of each individual WWTP’s performance.

The Outcomes

This study is the first of its kind to accurately quantify both total Cryptosporidium oocyst numbers and their inactivation across wastewater treatment trains.  In doing so it helps clarify the ‘true effect’ of particular wastewater treatment trains in reducing the risk of Cryptosporidium in recycled water.  For some of the WWTPs involved, this study has identified that the Cryptosporidium risk in their post-treatment wastewater is significantly lower than previously thought.  However for a number of the other WWTPs it is proving to be more challenging to achieve reduced Cryptosporidium density and infectivity. 

Other important conclusions from this study include the finding that oocyst removal rates are seasonally variable.  Greatest removal rates were found to occur in summer and autumn, with poorer rates in winter and spring.  Whilst oocyst density at the WWTPs was found to vary according to the level of Cryptosporidiosis in the community, infectivity of oocysts in raw sewage however was stable throughout the year (though infectivity was substantially lower in South Australia than in Victoria). 

Knowledge generated by this new assessment method has the potential to affect decision-making on future wastewater treatment strategies with regard to Cryptosporidium risk and required treatment processes.  The rich store of data available in this study may also be used to validate future log reduction credits for the use of recycled water which may require less UV treatment and the associated cost reduction that comes from this data.  The data from these treatment plants may also influence future revisions of the recycled water guidelines.

** When reviewing the outputs from this project, be aware there were operational issues that impacted the Mt Martha site. Budget constraints did not allow for a full investigation. It has been suggested that the plant’s recirculation at the time of sampling may not have provided ideal representation of steady-state operation, so please read the report with this in mind. **

Project Information

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  • This new assessment has the potential to affect decision-making on future wastewater treatment strategies with regard to Cryptosporidium risk and required treatment processes.
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