Weathering the storm

Storm tanks have long been an effective solution for the containment of untreated water. But how will they fair as the urban landscape continues to develop? ITT Flygt's market development manager, Tony Price, investigates

Storm tanks, or retention basins, are designed to store the flow of surplus polluted water caused by adverse weather conditions, releasing it back into the main sewage system for treatment once the water level has returned to a manageable level. Built environments like towns and cities create vast areas of impermeable surfaces meaning less rainwater is being naturally absorbed into the ground.

Combine this with the changing weather patterns in the UK and the result is excessive amounts of rainwater run-off entering the sewage system through the drains. During periods of heavy rain, excess water is held in storm tanks to enable the treatment plant to process the water in manageable quantities. But with more rainwater run-off than ever before, the mixing and flushing equipment inside the storm tanks are at risk of beginning to fail.

Many wastewater treatment plants are experiencing frequent equipment failure because the apparatus was originally designed to cope with lower volumes of water than is now being sent for storage in the storm tanks. In many instances there is an accumulation of solids at the bottom of the tank, which can cause serious bacterial and odour control issues. For many water companies, the only solution is to manually clean the storm tanks. But this is just part of the problem.

Frequent unplanned repair and maintenance bills plus equipment down time is costing many wastewaster treatment plants upwards of £50,000 each year, not to mention the sizable level of carbon wastage emitted from each storm tank.

Considerations
There are a number of key considerations when selecting the right combination of cleaning equipment for a storm tank. These include; the shape and size of the basin, position of the outlet and the size of the mixing and flushing ejector.

Installing a sufficiently sized ejector that has been developed for a specific tank shape is the most cost effective and energy efficient way of managing surplus untreated water.

It is important to remember that each ejector creates a different shaped mixing beam or flow path, and selecting the right one will ensure that all waste matter is brought into suspension. Providing this water flow is maintained whilst the tank is in use, all solid material will exit the basin through the drainage outlet with the rest of the untreated water, leaving the bottom of the tank free from deposits.

There are three effective ways of mixing and flushing the water in a storm tank. Each method is designed with a specific type of basin in mind and creates the ideal flow path and flushing pattern for that particular tank style.

Submerged fly jet ejectors
Using a primary flow of water, mixed with a secondary flow of air, submerged air/water fly jet ejectors produce a flow path ideal for flushing long narrow basins that have an outlet positioned along the wall. The elongated mixing beam easily reaches both of the downstream corners inside the tank, ensuring all rag material is fully suspended throughout the holding period. As the tank empties and the ejector is unsubmerged, it provides a powerful jet forcleaning the bottom of the basin.

Submersible hydroejector
In comparison to the fly jet's use of water and air, the submersible hydroejector combines a primary and secondary flow of water to deliver a shorter, broader mixing beam suitable for flushing shorter, wider basins. Following the same principle as the air/water unit, this water/ water hydroejector automatically switches to a flushing cycle to wash all remaining waste matter towards the drainage outlet.

Bulk flow principle
The bulk flow principle is used for mixing and flushing circular retention basins with a central outlet. This method uses a number of strategically placed mixers and hydroejectors to bring solids into suspension through the creation of a whirl pool style water flow. By penetrating the full volume of the tank, the agitated solids are dispersed evenly and can be drained off easily as the stored water passes through the central outlet.

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