5 Advice to water utilities
5.1 Introduction
5.1.1 The purpose of this Chapter is to bring together the advice to water utilities contained in this Report for ease of reference. It should be read in conjunction with Chapter 3 and Chapter 4, section 6. The advice covers risk assessment, water treatment, monitoring and communications. Its aim is to minimise the possibility of Cryptosporidium oocysts entering drinking water supplies and to emphasise the need for water utilities to maintain good communications with health authorities and local authorities.
5.2 Risk assessment
5.2.1 The Group concluded that outbreaks of drinking water-related cryptosporidiosis do not just happen. Worldwide there is an increasingly strong correlation between these outbreaks and inadequacies in drinking water treatment. A key element in providing appropriate treatment is the assessment of risk from Cryptosporidium. Risk assessment should be based on a combination of factors including the degree of exposure of the catchment to oocysts, the treatment processes currently in place and the history of cryptosporidiosis in the community. Monitoring systems and water treatment requirements should be reviewed against the level of risk.
Recommendation
5.2.2 The Group recommends that water utilities carry out an assessment of risk from Cryptosporidium for each source and put in place a procedure for updating periodically the review of the risk assessment. Water treatment requirements and monitoring systems should be reviewed against the level of risk.
5.2.3 Water utilities should systematically assess and rank the potential risk of groundwater contamination by Cryptosporidium by application of a tripartite approach which assesses source, catchment and hydrogeological factors (4.8.1).
5.3 Water treatment
5.3.1 As mentioned above, most waterborne outbreaks occurred due to deficiencies in water supply including those in which the treatment was inadequate or the works were operated above design capacity or some part of the treatment was bypassed. As recognised in the earlier Expert Group reports a conventional treatment works (that is, coagulation aided filtration) operated in accordance with good practice, is normally an effective barrier against Cryptosporidium.
5.3.2 The performance of chemical coagulation-based treatment for oocyst removal relies on the ability to maintain a suitable coagulant dose, governed by the raw water quality, particularly colour and turbidity. The subsequent solids-liquid separation processes must then be able to remove the coagulant solids to a very high degree. If a works is operated well in excess of its design capacity, or if some unit processes are by-passed, coagulant dosing and effective removal of the coagulant solids may be impaired, thus compromising the plant performance for oocyst removal. However, some unit processes, such as second stage filters for manganese removal or post-filtration granular activated carbon adsorbers, are not designed for coagulant solids removal and overloading or by-passing of these should not compromise performance for oocyst removal.
5.3.3 The performance of filters is poorest immediately after restart, even if an effective backwash has been applied. Consistent with recommendations in the first two Expert Group reports, attempts should be made to minimise the impact on final water quality of increased turbidity after start up. A number of options are available to achieve this, the most effective being disposal of the first flush (perhaps with recovery for use as backwash water), or recycling to the head of the works. However, this may not be easy to retrofit at some works and more practical alternatives include improved backwash or delayed start (after backwash) of the filters.
5.3.4 United Kingdom Water Industry Research Limited has published a Guidance Manual supporting the water treatment recommendations from the first two reports of the Expert Group (UKWIR 1998). Its objective is to provide guidelines for water treatment works operation to minimise the risk from Cryptosporidium The Expert Group endorses the general approach to water treatment recommended in this manual. The Group also makes the following recommendations on water treatment.
Recommendations
5.3.5 Water treatment works should be designed to handle the typical peak turbidity and colour loadings in the source water.
5.3.6 Water treatment works should be operated at all times in a manner that minimises turbidity in the final water; attention should also be given to other parameters which reflect the performance of chemical coagulation, that is, coagulant metal concentration and colour.
5.3.7 Water treatment works should normally be operated within the design capacity and without by-passing of the solids-liquid separation processes which are responsible for removal of turbidity and coagulant solids; coagulation itself should never be by-passed or compromised.
5.3.8 In the event of an emergency, if it is necessary to overload or by-pass solid-liquid separation processes, a stringent monitoring regimen should be initiated to ensure that turbidity targets indicated in paragraph 5.4.4 below are not exceeded; if there is an indication that these targets will not be achieved, an immediate advice to boil notice should be issued.
5.3.9 For high risk sites, if minimisation of the effects of filter start up on final water quality cannot be achieved through more easily implemented changes (for example improved backwash or delayed start after backwash), modifications to the works should be made to allow the first flush to be run to waste or recycled to the works inlet.
5.3.10 Coagulation/flocculation processes should be checked regularly to meet changing conditions of source water quality and other environmental factors.
5.3.11 Only dedicated washwater mains should be used to carry the returned washwater flow.
5.3.12 Filters should be operated and maintained under optimum conditions with attention to the quality and depth of media and to the operation of the backwashing/air scouring system.
5.3.13 Treatment works staff should be trained to be aware of the potential effect on the final water quality of even very small changes in the catchment or the treatment stream.
5.4 Monitoring
5.4.1 Investigation of waterborne outbreaks has shown that often there was a significant increase in turbidity at the time that the contaminated water was estimated to have entered supply. This is discussed in Chapter 3. The Group has made the following recommendations on monitoring.
Recommendations
5.4.2 Water utilities should check that process monitoring systems are appropriate to the risk at each source.
5.4.3 For all sites at which Cryptosporidiummight be a high risk, as determined by the risk assessment, monitoring should include continuous turbidity measurement on the outlet of each filter and on the final water using instruments capable of detecting changes of less than 0.1 NTU.
5.4.4 Water utilities should define for each of their treatment works the value and duration that constitute a significant deviation in turbidity of the final treated water irrespective of its relationship to the regulatory standard; for example it may be that at large water treatment works alarms should be set to be triggered by any increase in turbidity in the final water of greater than 50% of the normal average or suitably representative level; for small works, the increase of concern would vary and consideration should be given to the impact of the backwashing to individual filters.
5.4.5 Appropriate action procedures to react immediately to turbidity alarms, based on the level of risk and the history of the source/works should be in place; actions might include immediate sampling for Cryptosporidium, isolation of the filter(s) or source or, if suggested by history, the issue of advice to boil.
5.4.6 Good experience has been reported with the operational use of particle counters which, when used in conjunction with turbidity monitors, can provide a more sensitive indication of particle breakthrough.
Recommendation
5.4.7 The Group encourages the use of particle count monitors to provide additional information to that provided by turbidity measurements.
5.4.8 Waterborne outbreaks occur even though oocysts cannot be detected in the water. This supports the general view that the contamination occurs for only a few hours during which time it would be complete chance that routine samples coincided with the event. Random spot sampling is, therefore, unlikely to be effective for operational monitoring.
Recommendations
5.4.9 The Group recommends that water utilities operating sites assessed as being at high risk give consideration to either:
(a) continuous sampling for Cryptosporidium with analysis times linked to turbidity monitoring results; or(b) sampling triggered by turbidity events.
5.5 Local working partnerships
5.5.1 The importance of good local working partnerships has been recognised in both of the earlier Expert Group Reports. Generally good working relationships and practices are in place. However, the Group wishes to emphasise again the importance of both Incident Management Teams and Outbreak Control Teams and of the role of the water utilities in ensuring that the required local advice and support is in place to respond quickly to changing operational circumstances. See also paragraphs 6.3.5 - 6.3.7.
Recommendations
5.5.2 Water utilities should review their working relationships with local health authorities and environmental health officers in the form of Incident Management Teams.Criteria should be established for identifying outbreaks and procedures put in place for activating Outbreak Control Teams.
5.5.3 Water utilities, in liaison with health authorities, should set out criteria for decision-making on the issue and the withdrawal of notice on advice to boil water and review these with experience.
5.5.4 Should there be an outbreak of cryptosporidiosis, the water utility, as a member of the Outbreak Control Team, should encourage the use of good epidemiology recommended in this Report to establish the source of the outbreak, including whether illness is associated with the drinking water supply.
5.5.5 Water utilities should encourage Incident Management and Outbreak Control Teams to review and rehearse regularly the response procedures to incidents and outbreaks.
Reference
UKWIR. (1998) Guidance Manual Supporting Water Treatment Recommendations from the Badenoch Group of Experts on Cryptosporidium(98/DW/06/5). UK Water Industry Research Limited. London, UK.
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