By Danette D. Sutton
Oct. 31, 2007 -- Your water storage tank is one of your most valuable assets at your facility. The American Water Works Association (AWWA) has guidelines in place to ensure your tank is keep at its most optimal level of operation. We are going to explore two very important issues, which during the rush of other daily tasks, are sometimes overlooked. One is routine inspections and the other is water circulation.
Let's start with inspections. According to AWWA M42 tanks need to be inspected by industry professionals on a regular basis to ensure structural and sanitary integrity. These inspections should also include washouts once every 3 years unless you are in an area prone to sediment problems than annual washouts are recommended. In addition to routine inspections, every tank repair job should be preceded by a detailed inspection of the structure and a report performed by a qualified inspector.
There are generally two options for inspection -- dry and ROV (Remote Operated Vehicle). Dry is just as it sounds. All water is removed from the tank and qualified inspection personnel enter tank to conduct the internal inspection in accordance with AWWA. Depending on the source of your water, this method will require the loss of water and may include an expense associated with re-filling the tank. Therefore an ROV option, which does not require the draining of the tank nor the lockout/tag-out procedures associated with divers, fully meets AWWA inspection requirements with a better solution. The ROV is controlled by qualified inspection personnel trained to use this equipment. By this method you or your representative can view, live, the inspection via monitor and speak directly with the inspection personnel. Just as with the dry inspection, all aspects of your tank is inspected -- structural, safety and coating condition. With either method, a written detail report of findings with photographs, corrective recommendations and cost estimates should be provided to you. The ROV method often includes a DVD.
Incidentally, you should make sure all submergible equipment is fully disinfected before being placed into the tank. Obviously, when it's time for the tank to be washed out, the draining of the tank will be required. However, a pressure relief valve or a temporary tank are options available to allow for bypass of the tank while still maintaining adequate water service to your customers.
Now concerning circulation. Circulation is the key to preventing stratification of the water within a tank. As a side note with proper circulation and water turnover, freezing of potable storage tanks should not occur. Typically, water is added and drawn from the bottom of the water container through the inlet and outlet piping. When the rate of fill is greater than the draw rate the water level rises. When the rate of draw is greater than the fill rate water level lowers. If the fill and draw rate remains consistently equal than the system is drawing the water just added. What results is the water at the top of the container sits with no circulation leading to stratified and stagnant water. Also the chlorine which remains in the stagnated water can become depleted, leading to microbial growth and the development of taste odors. A common and economical way to prevent this is to extend the inlet up to the mid-point capacity level. Thus, mixing occurs as water is put into the tank. With this approach no internal mechanical equipment is necessary.
We hope the above information assists you in making the best decisions for your tank.
Danette D. Sutton is in the Maintenance Sales Division of Pittsburg Tank & Tower Co. Inc., which is a well established company in the tank industry who provides services for fire protection and potable steel storage tanks in the public and private sector. Pittsburg Tank & Tower performs inspections using "Odyssey" 1 & 2 ROV units and circulation system installations. The company can perform one-time services or develop custom routine maintenance packages.