The backwash tank is not designed to be a watertight holding tank, but merely an observation point. It is frequently built from a concrete block (that is concrete-filled) with an epoxy-coated interior. A concrete cap is preferred but not mandatory. The backwash tank could also be constructed of reinforced concrete with continuous water stops. The construction of backwash tanks is a structural issue.
The sewer must have the capacity for a 5-minute backwash cycle of the largest filter cell at a backwash rate of 15 GPM/SF. If sewer capacity is not available or unknown, the backwash holding tank should be sized as follows:
Filters should be backwashed when a pressure differential between 10 PSI and 15 PSI is reached.
Large pool (50-meter, stretch or dive pool) – Backwash basin designs set at 4’-0” by 4’-0” by 5’-0” allow a liquid capacity of approximately 479 gallons at a 1-foot freeboard. The bottom is normally at the deck/floor level. The large pool must have backup sewer capacity but is not recommended to meet required sewer capacity. The backwash tank’s primary function is as an observation point.
Other Pools – Backwash basin designs set at 3’-0” by 3’-0” by 4’-0” allow a capacity of about 202 gallons at 1-foot freeboard. The bottom is normally at deck/floor level. The large pool must have backup sewer capacity but is not recommended to meet the required sewer capacity. The backwash tank’s primary function is as an observation point.
Sand in the backwash tank indicates a distribution problem in the filters. If trash or other debris is found, there is a problem with the hair and lint strainer basket, or it might be missing.
If mud is seen, a chemical problem might exist. The presence of mud may indicate that the filters have not been cleaned from the previous backwash cycles.
During a normal backwash cycle, the operator should note the water level in the backwash tank. If the water level in the backwash tank increases, it is an indication of sewer blockage or a change in the backwash rate. A sewer backup that floods the pump pit will damage the pump motors and electrical accessories.
If the backwash of the largest filter exceeds the sewer capacity, a holding tank is required.
Backwashing must be done at the design flow, usually 15 GPM for each square foot of filter surface area. A slower rate for inadequate sewer lines is certain to allow conglomerated sand to develop. If the backwash velocities are insufficient to break up and wash the clumps to waste, they will work their way deeper into the filter bed, helping to create “channels”. Ultimately such passages will permit unfiltered water to take the path of least resistance through the medium and the systems will fail to function.
Project managers should be careful in designing the flow rate. If the flow rate is too high for sand filters, the supporting gravel layers may become dislodged or the sand washed out of the tank.
Backwash Funnels: CH standard is not to use backwash funnels except as a cost saving measure on outdoor pools only. Watch the sewer capacity closely. Flooded pump pits could be costly. Any overflow must drain outside and not to the pump pit. Funnels are not to be considered for high-end projects.
It is CH standard to call out the size of the reducer, splash collar, and funnel. The sewer line should not be called out since this is typically by Civil or Plumbing and they can confirm what the actual sewer line size or the reducer size would be.
Confirm linkage for twin cell has not been installed when only one cell backwashing is the design intent. Confirmation should be made during CA site visit.
Never design a backwash tank that depends upon valving and safety features.
In cases where the backwash pit is located below the filter vessels, the backwash pit should be considered the primary discharge for the air vent lines. This could eliminate any nuisance water from the air vent lines running across the floor.
If the pit is to be recessed into the filter room floor, the architect should indicate a removable grating cover for the opening.
The interior of backwash tank should be waterproofed.
The backwash pit may also be a good location to discharge the deck drain if the code requires an air gap.
In some jurisdictions, the EPA has approved discharging backwash and pool drain water to waterways. In other areas, discharge to storm sewers has been rejected. While pool chemicals are not usually considered beneficial to water treatment plants, particularly in small cities, this is a highly local issue that must be verified with the local building or public works department.
For draining pools to streams or storm sewer, sodium thiosulfate is often used to rid the pool water of chlorine before prior to discharge.
Pools can backwash manually, semi-automatically, or automatically. Semi-automatic backwash is CH standard. It is typically used for pools with multiple filter tanks. Automatic backwashes will backwash a filter on its own with no interaction from the pool operator. A semi-automatic backwash requires only a push of a button by the pool operator to initiate backwashing. A manual backwashing system is performed entirely by the pool operator.
For outdoor heavy demand pools, automatic backwashing systems should be recommended (Pirate’s Cove).
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