Keeping Stored Water Safe in Buildings

Part 5 of 12 in Cobler's Water Fundamentals course. New here? See the course page.

Fill a glass from a tap on level 15 of the office tower. That water left the treatment plant clean and safe to drink. But think about the trip it's had since. It passed through the bulk meter and sat in the suction tank. Then the transfer pumps from Part 4 lifted it to a roof tank, where it waited before running down to the floor.

So is it still as safe as when it left the plant? That's the question of stored water safety in buildings. We'll look at how stored water can become unsafe on that trip, and how the plumbing is designed to stop that. Then we'll see who's responsible for each piece.

Drinking and non-drinking water

Let's start with what's in the building's pipes. Water that's safe to drink is what we call potable water. In most Malaysian buildings it's the treated water from the public supply, which Air Selangor provides in Selangor, Kuala Lumpur and Putrajaya. Any water that isn't meant for drinking is non-potable water, and a building can hold several kinds:

• Water in the fire-fighting system, which can sit in the pipes for months.
• Water circulating in cooling towers, which is warm and full of treatment chemicals (Part 8 covers towers).
• Rainwater collected from the roof for flushing toilets or watering plants.
• Water in air-conditioning pipework, heating systems or industrial processes.

The basic rule is that none of these systems may ever join the drinking-water pipes. The national technical guidelines from SPAN (Suruhanjaya Perkhidmatan Air Negara, the National Water Services Commission) say this clearly. Piping for domestic water, air conditioning, fire fighting and non-potable water "shall be separated from each other", with no interconnections (SPAN, 2018). From here on we'll call that document, the Uniform Technical Guidelines for Water Reticulation and Plumbing, "the SPAN guidelines".

For rainwater and other non-potable supplies, the same guidelines add some practical safeguards:

• Potable and non-potable water are stored in separate tanks that aren't connected to each other.
• Pipes carrying non-potable water are painted green along their whole length.
• Taps and outlets for non-potable water are labelled "Not for Drinking Purposes".

A joint between a drinking-water pipe and any other system is what we call a cross-connection. Cross-connections often get added later, during a renovation or a quick repair. So a walk round the building looking for them is a useful first check anywhere.

Backflow and air gaps

But wait, if the systems are kept apart, how could dirty water ever get into the clean pipes? Water normally flows one way, from the supply, through the building and out of the taps. If the pressure in a supply pipe drops, though, water can flow backwards, and we call that backflow. It can happen when a main bursts, when a large pump starts and draws hard on a pipe, or when a supply is shut off. Now imagine the end of a hose lying in a bucket of cleaning chemicals at that moment. The falling pressure can siphon those chemicals back into the pipe.

The plumbing has two defences against backflow. The first is an air gap. That's a clear vertical space between the end of the pipe that delivers water and the highest level the water below can reach. That water might be in a tank, a sink or a basin. Water can't jump upwards across open air, so no matter how low the supply pressure falls, nothing can be drawn back into the pipe. The SPAN guidelines set minimum gaps:

• At least 75 mm between the delivery point and the water line of roof tanks, suction tanks, baths, sinks and basins.
• At least 150 mm between a kitchen tap fed directly from the supply pipe and the top edge of the sink.
• At least 225 mm above the overflow level where drinking water tops up a rainwater tank, or feeds the balancing tank of a swimming pool.

The second defence is a backflow preventer, a valve that lets water pass one way only. The SPAN guidelines ask for a dual check valve, which is two one-way valves in series, after the meter for higher-risk premises. Examples are hospitals, mortuaries, veterinary clinics, automated car washes and factories using toxic chemicals. They also ask for a single check valve before a garden tap fed from the supply pipe. Valves can wear or jam, so they need testing and maintenance, while an air gap has no moving parts at all.

A mains-fed roof tank, like the shoplot's, protects drinking water in several ways. There's the air gap at the inlet, a covered and mosquito-proof opening, visible overflow and warning pipes, and a scour pipe for draining and cleaning.

Now here's a nice surprise. The building's storage tank is itself one big air gap. Because the float valve discharges above the water surface, water in the tank can't flow back into the public main. The SPAN guidelines list this as one of the reasons a building has a storage tank at all. And they say equipment fed from an approved storage tank doesn't need a separate backflow preventer.

But the air gap only works if it stays open. A float valve set too high, or an inlet pipe extended down into the water during a repair, quietly removes it.

Water age

So the plumbing keeps dirty water out. But stored water has a second problem, and this one comes from inside. Treated water leaves the plant with a small amount of disinfectant still in it, usually chlorine. It keeps killing bacteria in the pipes on the way to the tap. We call this leftover disinfectant the residual.

The catch is that it fades with time. So the longer water sits in a tank or a pipe, the less protection is left. The time water spends in the system is what we call its water age. For a tank, a rough estimate is:

water age in days ≈ volume of water stored ÷ volume used per day

Let's try it on the office tower. It uses about 3,000 m³ of water a month, mostly on its 22 working days. And all of it passes through the suction tank, including the cooling tower make-up. That's about 3,000 ÷ 22, or about 136 m³ on a working day. So its 300 m³ suction tank holds a little over two working days of water (300 ÷ 136 ≈ 2.2).

That's the low end of the two to three days we saw in Part 1. And over a weekend, when use falls, the water sits longer, and the water in the roof tanks and pipes adds more time on top.

Warm water and Legionella

So when does old water become a real risk? The US Centers for Disease Control and Prevention (CDC) names four factors that control the growth of Legionella, the bacterium that causes Legionnaires' disease. They are sediment and biofilm, temperature, water age and disinfectant residual (CDC, Controlling Legionella in Potable Water Systems). Biofilm is a slimy layer that grows on wet surfaces. Stored water becomes a risk when these factors combine:

• Water age is high. Common causes are a tank much larger than the building now needs, a vacant floor, or a pipe branch that no longer serves anything. We call that last one a dead leg.
• The water is warm. CDC advises keeping stored cold water below 25 °C, because Legionella grows best between 25 °C and 45 °C. In a hot country, a roof tank in the sun can sit inside that range. So measuring the temperature of stored water tells us something useful.
• Sediment has built up. Sand and rust settle at the bottom of a tank and give bacteria a place to grow.

The same CDC guidance recommends flushing little-used pipe runs and dead legs at least weekly. It also says to check that the disinfectant residual can still be detected all through the system. We can apply both to a vacant floor, by running its taps and flushing its toilets regularly until tenants move in.

Cooling towers are a separate Legionella risk, because they spray warm water into the air as fine droplets. Part 8 covers tower water, and Legionella in cooling towers covers that risk and how it's managed.

The tank itself

So far we've talked about the water. But most problems with stored water start with the tank itself. The SPAN guidelines describe how a tank should be built and fitted, and we can use the same points as an inspection checklist:

1. Cover. The tank has a chlorine-resistant cover that keeps out dust and mosquitoes. It has an access opening of at least 300 mm diameter above the float valve.
2. Security. A shared tank, such as the office tower's roof tanks, is kept locked. Anyone entering the tank area is registered in a booklet that the management keeps, with the reason recorded.
3. Overflow and warning pipes. These discharge where people can see them, so an overflow gets noticed. Part 3 explains how they work with the level controls.
4. Scour pipe. A drain pipe at the lowest point of the tank lets the tank be emptied fully for cleaning.
5. Access. There's enough clear space around and above the tank to inspect and clean it.
6. Inside the tank. Look for sediment, rust, algae, damaged lining, insects or anything that has fallen in.

So how often should a tank be cleaned? You'd expect a fixed interval, but the SPAN guidelines don't give one for commercial buildings. They say owners "should take initiatives to inspect, clean and disinfect" tanks at regular intervals, on a voluntary basis. For shared tanks in high-rise residential buildings and gated communities, they refer to separate rules on tank maintenance. Those rules were still only proposed when the guidelines were published, so check with SPAN whether they're now in force. For your own building, agree an inspection interval with a tank-cleaning specialist, and shorten it if inspections keep finding sediment.

Keep a simple record for every tank. Each entry should say:

• The date, and who inspected or cleaned the tank.
• What they found, such as sediment, damage, and the condition of the cover and the float valve.
• What was done, including how the tank was disinfected and flushed afterwards.
• Any water test results, and when the tank went back into service.

There's one more rule after any plumbing change. In buildings with shared water distribution, a permit holder must disinfect new or altered pipes and fittings before anyone uses them.

Who is responsible

So whose job is all this? Responsibility for the water changes hands at the meter.

• The water operator supplies treated water up to and including the meter, and in our examples that's Air Selangor. Air Selangor states that the property owner is responsible for leaks within the property. That includes supply pipes, internal pipes, fittings and appliances (Air Selangor, Leakages FAQ).
• The building owner or manager is responsible for everything after the meter, meaning tanks, pumps, pipes and taps. In a strata building, Air Selangor says the internal plumbing, pumps and water tanks belong to the management corporation (MC) or joint management body (JMB).
• SPAN regulates the water services industry in Peninsular Malaysia and the Federal Territories of Kuala Lumpur, Putrajaya and Labuan. It publishes the technical guidelines, registers the pipes, valves and tanks that may be used, and issues permits to plumbers. Only a holder of a SPAN Type A permit may do plumbing works.

The water operator is responsible up to the meter. After the meter, the building owner or management is responsible, following SPAN's rules.

For the office tower, that means the facilities team owns the water quality risk from the bulk meter to every tap. For the shoplot, the owner owns the pipe from its meter to the roof tank, and every fitting inside.

When to call a specialist

A facilities team can check covers, locks, overflows, temperatures and records itself. But some situations need a specialist:

• Discoloured, cloudy or smelly water, or complaints of illness. Stop using the affected supply for drinking, and call a tank-cleaning and water-testing specialist. If the water is already bad at the meter, report it to your water operator.
• Anything found inside a tank, such as a dead animal, heavy sediment or a broken cover.
• After a long shutdown or a flood, before the system goes back into use.
• Any plumbing change. A SPAN-permitted plumber must do the work, and the new work must then be disinfected.
• Suspected cross-connections or failed backflow valves, which a registered plumber should test and correct.

Optional detail: where the SPAN rules are. All of these clauses are in the Uniform Technical Guidelines for Water Reticulation and Plumbing (2018):

• Separate piping systems with no interconnections: C.3.1(g).
• Rainwater and non-potable safeguards: C.5.2.
• Air gaps of 75 mm at tanks, baths, sinks and basins: C.4.1(a). 150 mm at a directly fed kitchen tap: C.3.1(e). 225 mm at rainwater tanks: C.5.2(f), and at pool balancing tanks: C.3.7(c).
• Dual and single check valves: C.4.1. No separate backflow preventer after an approved storage tank: C.3.3 and C.4.1(d).
• Tank construction and fittings: C.3.3. Voluntary inspection and cleaning: C.4.2(b).
• Disinfection of new work: C.4.3. The SPAN Type A permit for plumbing works: C.3.1(a) and (c).

The meter at the boundary

So the water stays safe if we keep the systems apart, keep the air gaps open, keep the water moving and look after the tanks. And we've drawn one line through the whole building, at the meter. On the street side, the water is the operator's. On our side, everything is ours, including the bill.

But that meter does more than mark a boundary. It decides what we pay. So how does a meter turn moving water into the one number on the bill? And why can a slow leak pass right through it without being recorded? That's Part 6: How Water Meters Work in Buildings.

Check your understanding

1. A contractor replaces the float valve in the shoplot's roof tank. The new valve's outlet sits below the water surface. What protection has been lost?

2. Level 8 of the office tower is vacant between tenants and may stay empty for three months. What happens to the water in that floor's pipes, and what should the facilities team do?

Recap

• Drinking water and non-drinking water must stay in separate pipes and tanks, with no cross-connections.
• Backflow can pull dirty water into clean pipes when pressure drops. Air gaps, at least 75 mm at tanks and basins, and backflow preventers stop it.
• Stored water ages and loses its disinfectant residual. Large tanks, vacant floors, dead legs, warm water and sediment all raise the risk.
• Inspect tanks against a checklist, and keep records.
• The water operator is responsible up to the meter, and the building owner or management after it. SPAN sets the rules and permits plumbers.

This is Part 5 of 12 in Cobler's Water Fundamentals course. Previous: Water Pumps in Buildings Explained. Next: How Water Meters Work in Buildings.

Cobler builds CobiNeural, a platform that shows a facility team its building's energy, water and indoor air data as live numbers across the whole site. To see how your building performs, talk to us.