Product category:
Hydrometry, Environment and Floods
News Release from: WRc | Subject: Sewage Flooding
Edited by the Processingtalk Editorial
Team on 26 July 2007
Assessing the infection risks from
sewage flooding
Flooding with sewage is a distressing and potentially expensive occurrence: even with short flooding events, the legacy of clean up and replacement of damaged goods can continue for many months
Those affected can also be concerned that a risk of illness remains from harmful microbes Remedial measures usually include mopping up, drying out, disposal of ruined furnishings, disinfection of surfaces and applying quarantine periods for outdoor areas that cannot be disinfected
This article was originally published on Processingtalk on 18 Nov 2003 at 8.00am (UK)
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In the past, these quarantine periods have been set on a fairly general basis that assumed the rate at which any harmful organisms would die or be washed away by rainfall.
Such uncertainties meant the water companies had to take an extremely precautionary approach to setting appropriate quarantine periods.
This prolonged the disruption and so increased the costs of compensation.
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With flooding on agricultural land, standing crops may have to be destroyed and, to avoid contamination of new crops, restrictions are imposed on when future planting can take place.
Again reliable predictions on the die off of pathogens in the soil are important if severe disruption of the farming schedule is to be avoided.
Recently, WRc has developed a management tool that allows water companies to be more certain about the health risks from specific flooding incidents, determine if quarantining is the best course of action and how long these quarantine periods should be to minimise health risks to the public.
Use of this tool has allowed water companies to identify priorities for action, the best clean up option to use and, in terms of public relations, provide them with scientifically based evidence to support their actions.
Water polluted with faecal material.
Water polluted with faecal material and organic deposits can remain on lawns and grassland after the main flood has subsided.
Does this present a risk and how long does the hazard remain?.
The Predictive Management Tool.
The tool is an interactive risk assessment model that can be installed on most personal computers.
It allows the user to input information about the specific sewage flooding incident in terms of its size, whether it was flowing or if it became "standing water", if it was neat sewage or had been diluted by rain water and the time that had elapsed since the flood "water" receded.
The model contains a database of the prevalence in the UK community of certain infections likely to be present in faeces and the numbers of these pathogens excreted by infected individuals.
Because the number of pathogens in sewage can be influenced by local factors, the model allows the user to enter specific local information where this is available or thought to be relevant.
Using these data inputs, the model indicates if risks to health exist in the flooded area.
Where disinfection is inappropriate, the "time since flood" function in the model allows adequate quarantine periods to be predicted.
Predictions for a Range of Situations.
The model predicts the risks for a number of indoor problems such as contaminated furnishings (carpets and curtains), hard floors and plaster walls.
It also looks at risks from flooded external surfaces such as lawns and patios.
Sewage flooding of agricultural land is also considered.
The model predicts the pathogen loading on root crops grown in soil that has been flooded by sewage and the number of pathogens likely to be ingested by animals grazing on contaminated pastures.
Presentation of the Risks.
The model presents its predictions in one of two formats.
In the first, the numbers of each pathogen predicted to be present, e.g per gram of soil or sq cm of surface.
In the second, the numbers are replaced by a red/yellow/green traffic light system to indicate severe risk, some risk or no risk based on the infectivity of each pathogen.
Practical Value of the Model.
Information from the agencies that deal with sewage flooding incidents indicates that the normal approach to flooding of the interior of properties is to recommend replacement of most of the articles affected.
In these situations the model merely acts to reinforce rather than guide the action taken since most soft furnishings will have been irreparably damaged by water.
For outdoor situations, disinfectants can be used to advantage in terms of removing offending odours and health risks.
The outputs from the model can be used to determine how effective such treatment will be if the efficacy of the disinfectants (i.e 99, 99.9 or 99.9% inactivation) is known.
Where disinfectants are unlikely to be an efficient means of clean up in outdoor situations, such as on soil and areas of lawn, the model can assist in making judgements regarding removal of the soil or lawn, burying the contamination by tilling or the use of quarantine periods if access can be restricted.
This may be particularly helpful for households where children use the lawn for play or indeed for larger playing fields where removal of the top surface would be a large and expensive task.
In the agricultural situation there would be little doubt that consideration should be given to destruction of any standing crop.
However, the model can be used to make a judgement on when the field may be used again and for what purpose.
Here comparison with current manure application or sewage biosolid applications guidelines will be of value in making assessments.
Although the model can be used in isolation, it is supported by a series of documents produced by WRc.
1) Factsheets for each of the pathogens of concern.
2) Data on suitable disinfectants.
3) Guidance on best practice for clean up procedures.
Practical Applications of the Model.
The model has now been in use by some water companies for over a year.
The security and usefulness of the information provided by the model has helped in the avoidance of unnecessary costs whilst at the same time not compromising public health.
To keep the data relevant in the light of newly published information, some of the agencies that have used the model are now part of a discussion group, lead by WRc, that reviews sewage flooding incidents, advises on what further information is required and suggests extensions to the scope of the model.
For further information please contact Robet Pitchers at WRc.
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