Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | APRIL 2014 | 41 In the know Technically speaking: Flooded sewerage completed before recommissioning to avoid further pollution aer they are started. Recovery of package treatment plants may take longer if the biofilms on the rotating biological contactors (RBCs) or the activated sludge mixed liquors have been washed away. In both cases, recovery will be impaired by the inappropriate application of chemicals in the clean-up process. Designing in resilience The use of septic tanks in large parts of the Somerset Levels is unlikely to comply with good design practice. Guidance on the design of drainage fields from septic tanks (Building Regulations Approved Document H) states that they should not be used where the groundwater will rise to higher than 1m below the invert of the distribution pipes. In the Somerset Levels, this may be the case in large parts of a normal winter and the drainage field is un- likely to provide adequate attenuation of the septic tank effluent. Where package wastewater treat- ment is used, these plants need to be resilient to flooding, but there is no point in having a resilient treatment system unless the drainage system it is connected to is sealed from the floodwater. Sealing the drainage system would be relatively straightforward using establish lining systems and isolating open points such as gully boxes. If this were done then perhaps a septic tank connected to an above- ground secondary treatment system would be one answer. The septic tank could be provided with additional storage so that the outlet can be sealed and the tank can function as a cesspool for the duration of the flooding with periodic emptying. The tank would need to be designed to resist floatation if emptied, while the area was flooded. There are other factors to consider in developing an alternative approach to treating sewage from small communities in flood-prone areas, but the septic tank is likely to be a more resilient component than package treatment plants that depend on power. In any case, better consideration must be given to design and operation of sewerage systems in those parts of the country that are more susceptible to flooding events. Further information is available from mark.smith@wrcplc.co.uk ● Current guidance for dealing with clean-up of households that have been flooded by sewage is available from the Health Protection Agency. ● WRc, in association with water companies, developed a tool for managing risks that also covered outdoors, where gardens, playing fields, amenity areas and arable crops had been affected. ● The WRc Portfolio Project CP108 - Reassuring Public Confidence a er Sewage Flooding (2005) was developed to allow water companies to make their own risk assessments based on robust evidence that would demonstrate due diligence on their part. ● Guidance was provided on how to manage the risks and also highlighted the need to avoid the 2009, it was found that the drains were heavily silted up, which was the main hurdle to a quick reinstate- ment of a functioning sewerage system. ● Septic tanks are likely to still be operational, in as much as they will settle solids and biological activ- ity will continue, but the head space will be flooded. If there is still flow from the source, raw or partly treated sewage will be flowing directly to surface water. ● Small package WwTPs may be non-operational due to power loss with sewage flowing to the plant passing through untreated and contaminating sur- face water. ● There is a large possibil- ity that there will be addi- tional contamination from agricultural waste and activities; this is likely to be dominated by livestock waste, but could also include chemicals such as pesticides and fertilis- ers (due to inappropriate storage). ● As the flood water is drained and pumped away, the sewerage system will naturally regain a degree of functionality and the focus will shi to remedia- tion and reinstatement. aggressive use of clean- up chemicals that can represent a greater environmental risk than the original biological contamination. ● O en, it was best to wait until the affected area was judged to be safe. ● Current understanding indicates that land exposed to sewage is likely to be safe for livestock a er about three weeks. ● WRc is undertaking work to bring the model up to date with the latest available scientific information and to extend its capability to cover risks posed to livestock and from potentially harmful chemicals is on-going - WRc Portfolio Project CP467 - Defining & Managing the Health Risks from Sewage Flooding. ● With such large floods, it is safe to assume that the foul drainage system package WwTPs and septic tanks will be flooded with water. ● Flow through the system will depend on topography and whether the system is open to surface water intrusion (for example through gully pots), misconnections with water drains or leaky covers and lids. ● In most cases flow will be low or stagnant and there will be an accumu- lation of silt. Following the large-scale flooding events in Sheffield in ● The health risk presented by surface water contaminated with sewage will be low as there will be considerable dilution of any potentially harmful organisms in the flood water. ● Floods can bring rodents into much closer contact with people and increase the risk of serious illness from leptospirosis. ● There may be localised increased risk from farm wastes, but again dilution would be expected to be substantial. ● Continued provision of safe drinking water and applying normal hygiene after contact with flood waters will be an important part of minimising risks from disease. Managing intrusion Contamination risk

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