Preventing contamination in water storage tanks

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By Erika Henderson

Despite efforts to protect public water supplies through testing, treatment and regulation, water contamination still occasionally occurs. The type and source of contamination can vary greatly, so on-site water source monitoring is recommended to help prevent contamination outbreaks.

Water is treated and tested for micro-organisms during the water treatment process, but if it is stored in a contaminated water tank, then it becomes contaminated again. Not only does the water in a storage tank need to be tested and treated regularly, the storage tank itself must also be inspected and cleaned.

Unauthorized Access

A damaged vent is one of many ways in which animals can enter a water tank.
A damaged overflow screen is one of many ways in which animals can enter a water tank.

Trespassers can cause great damage to a water tank and its contents. Therefore, it should be protected from unauthorized access. Trespassing signs, surveillance cameras, lights and a fenced in area with locks, should be installed around the perimeter of the tank. Exterior ladders should terminate 2.5 metres above the ground, with a locking ladder guard.

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Other trespassers such as pathogenic micro-organisms can enter the water supply through a host. Hosts may include aquatic organisms, insects, birds and rodents, and may access the tank through openings insufficiently covered. These can

As well, a damaged vent screen is another point of entry.
As well, a damaged vent screen is another point of entry.

include: damaged vents and overflow screens, holes or gaps in the roof and shell, floor or roof

hatches that have not been properly sealed or welded. Aquatic organisms can also gain access through inlet/outlet pipes, depending on where the water comes from.

Stagnation

All tanks should be regularly monitored for mixing efficiency. However, tanks at the end of a water system, or with low filling cycles or high volumes, should be monitored more often due to their susceptibility to developing stratified or stagnated water. Stagnation occurs when water is separated into layers arranged by density from temperature, pressure and pH. The incoming water stays near the bottom and is first to exit if an over-the-top fill has not been installed. Stagnant water lures potential hosts such as flies, mosquitoes, water fleas and other insects and crustaceans, attracted to the bacterium.

Mixing systems are often installed to help prevent stratification issues by taking denser, newer water from the bottom, and mixing it with less dense, warmer surface water. Mixing systems can also help lower the carcinogenic disinfectant byproducts trihalomethanes and haloacetic acids. These byproducts are present in almost all chlorinated water supplies, but the key is to keep these levels as low as possible.

Sedimentation

water tank sediment
Pathogenic micro-organisms can find shelter and food in sediment.

Water with excess sedimentation will result in a higher turbidity reading or muddiness, and it will become cloudy when shaken or disturbed. Sedimentation can include anything from dirt particles to rust and interior coating particles. Sedimentation often accumulates on the bottom, or in the bowl of the tank. It can also accumulate inside the outlet pipe, causing obstructions and a greater rate of deterioration. Accumulation rates vary by tank, but it is important to understand that pathogenic micro-organisms find shelter and food in this excess sedimentation. The longer sediment remains in a tank, the greater the risk for contamination and pathogenic micro-organism growth.

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