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Regular Tank Cleaning is Key to Water Quality


THERE'S SOMETHING in the water, and it's probably not what you expect. An amazing array of items can be found in water tanks, from beer bottles to vermin.

Sometimes what's left behind in a water tank is like a puzzle with missing pieces. Inspectors once discovered a small bicycle and a pool float inside a water tank located outside a prison complex. Was the bicycle stashed inside the tank by an accomplice as a method of transportation for an escaping inmate? Did teenagers use the water tank as some sort of playground, storing the bike and pool float inside the tank for their own amusement?

The reasons the bike and float were found in the tank didn't matter as much as determining whether their presence in the water posed a threat to the drinking water's integrity. Contaminated drinking water can trigger diarrhoea, vomiting, and nausea—symptoms that can crop up quickly after a person drinks tainted water. More serious long‐term issues also can arise, including cancer, developmental delays in children, and kidney disease, among others.

Simple practices can be used to help prevent storage tank damage and contamination, including installing perimeter fences to keep unauthorized people away from the tank, adding “No Trespassing” signs to perimeter fences, replacing damaged vents, and cleaning and disinfecting tank interiors.

Back to Basics

Although many components of a water system are buried and out of sight, water tanks stand tall on municipal landscapes across North America. However, like all parts of a water distribution system, storage tanks need to be properly maintained to help ensure safe, clean drinking water.

Stratification

Picture a lake: Sometimes the water looks clear, and you can see all the way to the bottom. Lakes aren't always transparent, however, because the water turns over, drawing debris‐laden murky water to the top. This happens to water in a storage tank, too, because of varying temperatures at different depths. If a tank's water temperature varies, it means stratification is occurring and turnover is inadequate. Mixing systems are frequently used to blend the new and old water, keeping a tank's water more potable overall.

Cleaning Frequency

Cleaning can help prolong a tank's life, particularly when it comes to its protective coating. Regular tank maintenance also keeps the tank in service longer than if no repairs were done. Tanks with cathodic protection should be cleaned and disinfected every five years, according to the National Fire Protection Association. However, if a tank doesn't have cathodic protection, cleaning and disinfection should occur every three years.

Galvanic protection and impressed current are two types of cathodic protection. Galvanic uses sacrificial anodes made from metals such as zinc and magnesium, which corrode faster than the steel tank the anodes protect. With the latter type, controlled amounts of direct current are impressed between an inert anode and the steel tank.

Disinfection Methods

Three is the magic number for tank disinfection. Liquid chlorine, sodium hypochlorite, and calcium hypochlorite are all forms of chlorine that can be used to disinfect water tanks and towers. There are also three chlorination methods recommended by AWWA. Operators can weigh the pros and cons of each method to determine which best suits their particular system.

1.In the first method, the water tank should be filled to the overflow level with potable water. No less than 10 mg/L of chlorine is then added to the water for a contact time of at least six hours.

2.The second method requires using 200 mg/L of chlorine solution in a portable sprayer and physically spraying the entire interior surface of the tank from the floor to the top of the overflow. The solution should stay on for at least a half hour. At the same time, the drainpipes should be filled with 10 mg/L of chlorine solution. Once the time has elapsed, the tank must be cleaned with potable water. The drainpipes should also be cleared.

3.The third method calls for at least 50 mg/L of chlorine to be added to the tank. This will fill approximately 5 percent of the total storage volume. The solution should sit no less than six hours. The tank should then be filled to overflow level by flowing drinking water into the highly chlorinated water. This should be left for no less than 24 hours. The tank is then drained.

Testing for Contaminants

Regardless of which method is used, bacteriological testing must be completed before the tank is returned to service. Any presence of coliform bacteria found in sample testing following disinfection indicates harmful contaminants may be present in the water.

Water analysis can reveal if any microbes and bacteria in a water sample may pose a threat to public health. By contrast, other contaminants are conspicuous from inside the tank. Dirt, mud, sand, rust, and other elements or particles can gather at the bottom of a tank, covering what would otherwise be a pristine white tank bottom with inches of sediment. Outlet pipes can also become clogged if too much sediment settles in the pipes.

The amount of accumulation varies from tank to tank, depending on the environment and other factors. Pathogenic microorganisms can thrive in such sedimentation. The risk for drinking water contamination and pathogenic microorganism growth increases the longer sediment stays in tanks. Follow AWWA disinfection standards and testing procedures during dry cleanouts.

TankCleaning Options

Several effective tank‐cleaning options are available.

Remotely Operated Devices

Remotely operated underwater vehicles (ROVs) can be used to clean most tanks. Controlled remotely by trained inspectors, these robotic devices are disinfected with a bleach and water solution before they're positioned in the tank. One advantage of using an ROV is that the tank doesn't have to be taken out of service during a robotic cleanout. Another benefit is that the ROV is equipped with a camera, which can document the inspection or cleanout with video for the inspectors and tank owners. ROVs can clean most storage tanks, regardless of tank material (steel, concrete, or fiberglass).

Divers

Trained diving companies that offer inspections are another acceptable option. Of course, divers must have all their gear and clothes thoroughly disinfected before they set foot in the tank. Although it takes longer for a diver to inspect a tank compared with an ROV, each approach takes about the same amount of time during the actual cleanout phase.

Trash Pumps

Trash pumps are used to extract dirt, sand, and other material out of the tank. It's not uncommon to find 1–4 in. of sediment at the bottom of a water tank. In some instances, trash pumps have been known to remove as much as a foot of sediment from a tank. Standard 3‐in. trash pumps can be purchased at home improvement retail stores. No permits are required.

Sediment is discharged based on directions given by the owner and regulations for the city, county, and state where the tank is located. California has some of the strictest discharge requirements. Some operators use filtration bags to gather the sediment so only filtrated water is discharged into the ground.

Dry Cleanout

Sometimes it's necessary to drain the tank because there's too much sediment to be removed by a diver or ROV. A tank that's in bad condition, such as one ravaged by a recent natural disaster, may also need to be drained so it can be thoroughly inspected to determine what's needed to restore its optimal efficiency. Owners tend to shy away from the dry‐cleanout option because it requires the tank to be removed from service, as the tank has to be empty for inspectors to complete the job. During dry cleanouts, AWWA disinfection standards and testing must be followed.

Steps to Prevent Contamination

Some of the best ways to prevent contamination are simple practices.

Deter Vandalism

Would‐be vandals can scale the tank and contaminate the water. Ladders should be 10–20 ft from the ground, making them difficult to climb. Ladder guards with locks should be installed with the ladder. Locking the roof hatch is another way to keep people out of the tank.

Secure fencing around the perimeter helps keep people away, particularly if the tank is topped by barbwire and the gate is locked. Signs also should be posted to keep out intruders. Investing in a security system with cameras is also a viable option. Effective security measures help to prevent contamination and keep civilians from accessing the tank, which would be a liability. If someone climbs the tank and falls off, that's an insurance claim waiting to happen.

This tank is secured behind fencing with barbed wire, but a “No Trespassing” sign should be posted. The tank could also use a paint job, as evidenced by the fading paint and rust streaks.

Prevent Animal Visitors

Sometimes trespassers are of the furry, feathery, or scaly variety. Microbes and bacteria can also hitch a ride on a bird, rat, snake, or other critter that finds its way into the tank through an opening. It's important to maintain tank water quality by securing vents, hatches, and overflow screens and to repair any holes or gaps that may be found during regular inspections.

Ensure Mixing Efficiency

Stagnating water is a breeding ground for contaminants because of stratification, which is when water is separated into layers arranged by temperature, pressure, and pH levels. Water that comes in through the piping stays toward the bottom. This water is also the first to be cycled back out unless the installation of an over‐the‐top fill has occurred. Tanks should be monitored regularly for mixing efficiency, especially those at the end of a water distribution system or with low‐filling cycles or high volumes. Installing a mixing system can help circulate the newer and older water to prevent stratification.

Inspect, Clean, and Maintain

AWWA offers a range of standards (www.awwa.org/standards) that can help tank owners identify the recommended methods most suitable for their tank cleaning and maintenance schedules based on many factors. The best way to prevent contamination is to have tanks inspected and cleaned regularly per code. Effective maintenance is key to identifying issues early and responding to adverse conditions promptly to ensure distribution system water quality and quantity.

Effective maintenance is key to identifying issues early and responding to adverse conditions promptly to ensure distribution system water quality and quantity.

© 2018 American Water Works Association