It is becoming increasingly common to use combined domestic and sprinkler water storage cisterns instead of using separate cisterns for sprinkler systems and domestic use. The Association of Tank and Cistern Manufacturers (ATCM) believes that combined water storage systems are not only impractical, but also raise some very serious issues in terms of water quality.
Possible risks
The general reason given for combining the cisterns is space, when in reality it is cost driven. With modern manufacturing and installation techniques it is possible to put two cisterns against each other without taking up extra valuable space.
It’s essential to maintain a minimum volume of water to be able to supply the sprinkler system in the event of a fire and any domestic water needed would need to be stored in addition to this.
Most domestic water storage cisterns are oversized, which means more water is stored than is required. Whilst a delayed action valve can be used, there will remain issues with turning the water over, which should happen every 24 hours. If the water is not turned over at this frequency then an environment is created that is ideal for bacteria to thrive.
HSE Legionnaires Disease HSG 274 part 2 states that the volume of stored cold water should be minimised and should not normally exceed that required for one day’s water use.
Legal requirements
It is important to note that to meet the legal requirements of the Health & Safety at Work Act, water storage cisterns for domestic use must be inspected, cleaned and chlorinated as part of a legionella risk assessment – this is not a discretionary undertaking.
The risk assessment would normally dictate that the cistern would have to be cleaned and chlorinated usually as a minimum of once a year. For combined cisterns, due to the overcapacity, this may have to be more frequent, based on that assessment.
As the cistern is also part of the dedicated fire fighting for the building, any cleaning and chlorination would have to be undertaken when the building is not in use and the building may be un-insured unless additional fire protection provision is put in place.
The logical argument to this is to use a cistern with a division so one side can be kept in use, while the other is in service. However, when considering a combined cistern, it would be advisable to contact the local water company early on as there may be requirements to meet.
A divided cistern, however, would mean having over double the amount of storage as each compartment would have to have to be able to cope with 100% of the demand. Again, a delayed action valve could be used to aid turnover, but both valves would need to come on at the same time to prevent stratification or a biofilm potentially forming on one side. This is no easy task because as soon as one valve comes on it back fills the other side of the tank, preventing the other valve from dropping – something we are seeing more of in the industry. This can be addressed using specialist valve arrangements, but care needs to be taken with the design.
Water regulations
As well as your HSE requirement, there are also issues with meeting the Water Regulations 1999, which classes stored water for fire fighting to be fluid category 5 due to stagnation and potential microbial colonisation, and therefore requires an AB air gap as an appropriate solution.
The problem with this is that although the weir slot should be screened, and should have a light cowl on the inside, it still allows for the ingress of dust, potentially affecting the quality of water in the tank, which is used for drinking.
It also raises the chances of the water in the cistern becoming warm in the summer, as it is more open to the atmospheric temperature of the plant room. Therefore, an AB should not be used, which begs the question – is a combined tank water regulation-compliant?
A lot of the issues with having a combined cistern may not come to light when the building is handed over and may only be highlighted when regular temperature and sampling of the cistern is carried out. At this point, to rectify the system would not only be extremely costly, but very difficult to overcome from a design and installation point of view.
Given the possible dangers, an ever-increasing insurance backed industry and litigious society, using a combined cistern is high risk.
For further information contact the ATCM.
