Water storage tanks are an important, visible component of OCWA’s regional water distribution system. Operation, maintenance and rehabilitation of our water storage tanks is critical to our ability to safely, reliably and affordably provide water to our 500,000+ consumers. These facilities not only help ensure that we provide adequate water pressure throughout our system, but they also provide critical back-up supplies in the event of main breaks or fires.
This article explains the types of tanks OCWA utilizes, their locations and sizing, how we operate and monitor water quality within them, and our process for inspecting, maintaining, painting and rehabilitating our tanks.
Water storage tanks are generally designated as either elevated tanks or ground tanks (including underground tanks). Elevated tanks are used when the natural topography of the land requires some elevation to enable water from the tanks to reach their entire service area. Ground tanks are used when natural elevation is sufficient to allow the tanks to cover the service area.
Each category of tank has multiple different designs. OCWA operates 19 elevated tanks in its water distribution system broken down by the following types:
- Hydrosphere (6) – Welded carbon-steel spherical storage tank supported by a single circular carbon-steel support pedestal with a flared conical base.
- Fluted column (5) – Welded carbon-steel water storage tank supported by a single large diameter corrugated carbon-steel support column.
- Multi-column (4) – Welded carbon-steel water storage tank supported by a series of carbon-steel supporting columns and cross braces.
- Composite (3) – Welded carbon-steel water storage tank supported by a large diameter steel-reinforced concrete support tower.
- Glass-lined (1) – Glass-fused-to-steel technology encapsulates the steel and avoids the need to ever painting the tank.
OCWA operates 36 ground storage tanks encompassing the following types:
- Above-grade concrete (17) – Prestressed concrete tanks constructed in-place utilizing a cylindrical concrete wall placed in permanent compression; includes horizontal strand prestressing and vertical post-tensioning, which is the strongest use of concrete.
- Steel (standpipes) (15) – Flat-bottom cylindrical steel tanks with a shell height greater than its diameter.
- Glass-lined (2) – Glass-fused-to-steel technology encapsulates the steel and avoids the need to ever painting the tank.
- Below-grade concrete (2) – Prestressed concrete tanks constructed in-place utilizing a cylindrical concrete wall placed in permanent compression; includes horizontal strand prestressing and vertical post-tensioning.
The Authority also operates six (6) ground storage tanks as part of its water treatment process at its Otisco Lake and Lake Ontario Water Treatment Plants.
Locations & Sizing
OCWA’s 55 distribution system tanks hold a combined 159.6 million gallons of water. Ideally, all tanks would be located on the top of a hill in the center of their service territory. That is not always possible given limitations on land availability and the logistics of building in particular locations. Therefore, we locate our storage tanks in the most elevated and central locations as possible, always ensuring that the locations selected provide adequate flow and storage for the areas they serve.
The sizes of our tanks range from the smallest steel standpipe holding 50,000 gallons to the largest above-ground concrete tank holding 30 million gallons. The size of each tank is determined by conducting a thorough analysis of the amount of fire flow needed for a particular area, the size of the service area and the expected growth in the area.
Operations and Monitoring
Like other components of OCWA’s water distribution system, our storage facilities are operated remotely using a Supervisor Control and Data Acquisition (SCADA) system. The system continuously monitors each tank’s water level and when it hits a pre-determined low point, automatically refills the tank to just below the overflow level. This process not only ensures that we have adequate water storage in our system, but also acts to “cycle” the water to ensure it stays fresh and safe to drink.
During the winter, we refill our tanks more frequently to not only ensure adequate water quality, but also keep ice levels down by mixing in warmer water. The SCADA system automatically sends alarms to our operators when water levels become too low or too high. SCADA is also used to continuously monitor various water quality parameters, including chlorine.
OCWA has a dedicated maintenance program for our water storage facilities which includes conducting detailed interior inspections of all our water storage tanks at least every five (5) years. These inspections typically include draining each tank to allow for a dry inspection followed by a thorough cleaning. We will then make any spot maintenance treatments using epoxy, as necessary. In the rare instance that we are unable to take a particular tank out of service, we conduct our interior inspections using remotely operated vehicles (ROVs).
The maintenance program also includes regular exterior inspections. OCWA owns a small hand-held drone which allows us to conduct annual visual inspections of our storage tanks. While our concrete tanks virtually never require exterior maintenance, our steel tanks typically require repainting both inside and out every 10 to 15 years. We hire an outside contractor who specializes in this work, and the entire process can take up to 16 weeks per tank.
Finally, OCWA performs major rehabilitation or replacement of its water storage tanks whenever an inspection reveals structural deficiencies.