Controlling Cryptosporidium in vulnerable catchments used for drinking water supply

Abstract

The overall aim of this project was to assess the potential to control Cryptosporidium in vulnerable catchments, using the Louth area in the Anglian Water region as a case study. This was completed through a literature review, a critical analysis of existing data and a four month sampling programme. It was found that Cryptosporidium oocysts which could potentially contaminate humans who get their drinking water from the downstream Covenham Water Treatment Works (WTW), were originating from Louth Sewage Treatment Works (STW). The oocysts from human or sheep hosts were entering Louth STW in the crude sewage. The STW collects and builds up the oocysts in the sludge holding tanks. When dewatering occurs, the sludge holding tanks release a large amount of Cryptosporidium into the STW, which in turn passes through the rest of the works, relatively untreated, into Louth Canal. The oocysts in Louth Canal were abstracted from, and added to, Covenham Reservoir. The oocysts from the reservoir occasionally passed through to Covenham WTW, where there is potential for human contamination. The literature review identified that treatment processes at Louth STW were less effective at oocyst removal than other research has indicated. Trickling filters and humus tanks removed a lower percentage of Cryptosporidium oocysts (17% and 44%) than literature suggested (91%). Overall, it appeared that during the sampling period, the works added 18 oocysts/l, when the influent and the final effluent of the works were compared. This is because of the episodic nature of oocysts and the way that they were being recycled in the works. Oocysts entering Louth STW seemed to be being concentrated in the sludge holding tanks and then recirculated in the sludge supernatant from the dewatering process, back to the primary settlement tanks. This meant that primary settlement at Louth STW was not as effective (-1299%) as literature suggests (54% removal) because of the additional input of oocysts to this treatment process. The concentration increase of Cryptosporidium oocysts within primary settlement tanks has not been observed previously. Not only did this appear at Louth STW, but also at Stamford STW, which was sampled as an additional STW. Because of this research, the operation and monitoring of the sludge at Louth STW is to be further investigated. Additional treatment options are to be considered at the STW and WTW, such as sand filtration at Louth STW and the installation of a permanent UV system or ultrafiltration at Covenham WTW. Further work would be to complete a more in depth analysis of more STWs to determine whether other sites have the same potential to accumulate and release Cryptosporidium. Another area for further study would be to look at the different combinations of treatments that STW use. This would help us understand why there are discrepancies in Cryptosporidium removal rates between sites. This would help to determine the most effective combination of treatment methods for the removal of Cryptosporidium during wastewater treatment.