4 minute read
What is backflow and how must we protect against it?
The movement of any substance in any direction is called flow. So why the term backflow when water moves from an installation towards the municipal potable supply? Firstly, it is assumed that the water in any installation originates from the municipal supply and so the water that flows back towards there is viewed as flowing back and thus called “Backflow”.
In most instances this would be the case, so why would we need to guard against it? What has happened to the water that makes it unsafe to return it from where it comes from? Firstly, we need to take cognisance of the fact that once the water passes past the water meter it now becomes the responsibility of the homeowner and the municipality no longer has control over what happens to it. Once they have lost control over what happens to the water, they do not want to let it back into the municipal system. We could let the water back into the municipal system if we had it tested and certified as SANS 241 quality compliant, but we know that that is not practical.
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What do we do to the water that could be viewed as contaminating it?
• We often have water softeners on the supply to the home and this is done by adding chemicals to the water. • The water is often heated in an aged Hot Water Cylinder that will add limescale to the potable water in a backflow situation. • When summer comes and we are filling the pool and fishponds with a hose this could also be a cause of contamination. So, if we look at it from their view the probability of getting contaminants from domestic installations is very high and we have not even ventured into the water from alternative sources such as boreholes and rainwater harvesting. Thus, it is no wonder that SANS10252/1 of 2018 says in D.1.1 Design, installation and maintenance. All water supply systems shall be designed, installed, and maintained to prevent contaminants from being introduced into the potable water supply system. The big question now is not the quality of our water but how to prevent backflow from happening. The regulations are very clear on what is expected from us and how to achieve zero backflows from entering the potable system. In terms of the National Regulations, SANS10252 -1 (water supply installations for buildings) refer to 7.4 of SANS 10252-1 ‘Preservation of Water and Water Quality. Reduced Pressure Zone Back Flow Preventers must be installed in any installation where there is a risk of contaminated water or harmful substances being back syphoned or flowing back into a potable water supply line. The regulations state that Back-flow Preventers shall be installed in certain types of installations and buildings, for example:
Medical treatment of people and animals, pharmaceutical or chemical research and manufacturing, dairies, nurseries, stadiums, abattoirs, combined fire systems etc. Most of the municipal bylaws underpin this by stating that it is the responsibility of the Homeowner (Not the tenant) to have this maintained once a year. A record must be kept of when this maintenance was done and by whom. The unit must be overhauled or replaced every five years and a record of this must also be kept. A national regulation requires that any Backflow Preventer discharges where it is visible so that any malfunction of the system can be readily detected and attended to. This means that we cannot hide this valve somewhere in a duct or a manhole, it must be above ground level where the discharge is visible. If theft is a factor the valve may be closed with a surround, but the discharge must still be where it is visible. Below is a drawing of a typical installation.
How does the valve work?
There is a mechanical check valve at the inlet and outlet of the valve. In the middle, there is an area (Zone) that is open to the atmosphere. As soon as water pressure enters the valve a diaphragm is pushed down, and the relief port is closed. Once this area is now filled with water the second check valve is opened and the downstream plumbing system can be filled with water. If for any reason the upstream pressure drops below the downstream pressure, the check valves will close, and the Reduced Pressure Zone will open to discharge any water in that area. If any of the check valves are faulty and are causing water to enter the Operating Chamber, a discharge of water will be visible from the Discharge Port to alert any passers-by of the system failure. It would be prudent to have a contact number near the valve so that anybody could contact the responsible person for this. A full range of spares is available from the manufacturer. Contact your local valve supplier for availability.
