Backwashing

Backwashing is the process of reversing water flow in the sand filters, flushing trapped particles and material (debris) to waste (sanitary sewer).

When to Backwash

Per PHTA guidance, backwashing the sand filters is necessary when the gauge pressure increases 8 to 10 PSI above the starting pressure (typically about 9 to 10 PSI).

In other words, when the tank gauge shows 17 to 20 PSI, backwashing is necessary. That said, depending on other parameters (such as flow rate), backwashing before this threshold may be necessary. To maintain proper filtration, the system must have sufficient flow to fully turn over every 6 hours (4 times daily).

Excessive Backwashing

Excessive backwashing will result in an inability to remove smaller particles. Sand filters are most effective at trapping small particles when they are at the latter (higher pressure) part of the cycle. Therefore, premature backwashing may result in lower water clarity.

Backwashing also dumps a tremendous amount of water into the sanitary sewer, often leading to a necessary fill post-backwash. Reducing the frequency of backwashing (to only when it's deemed necessary versus on a too-frequent interval) conserves water and can improve overall water clarity.

Procedure

Preparation

1. Disable the call for heat by lowering the temperature setpoint on the temperature controller to a value several degrees below the sensed temperature (in the future, a switch may be added to do this).
2. If the boiler was operating in Step #1, wait for the boiler circulation pump to stop. This may take several minutes. Observe that during this period, the manifold temperature will drop, approaching the boiler inlet (sensed) temperature. It is important to allow the boiler to complete a proper cool-down prior to cutting water circulation. Failure to do so could result in an over-temperature trip, or damage to the boiler itself.
3. Stop the main pump motor by pressing the Stop button on the motor starter.

NOTE: These steps are nearly identical to the first three followed when cleaning the strainer basket.

Set Valves to Backwash

The pump must be stopped prior to adjusting the valve positions.

1. Open Valve #1
2. Close Valve #2
3. Open Valve #3
4. Close Valve #4

A valve is open when the lever arm is parallel (in line) with the pipe. A valve is closed when the lever arm is perpendicular (turned 90 degrees with respect to) the pipe.

NOTE: Improved marking and documentation of these valves and positions to follow.

Backwash

1. Start the main pump motor by pressing the Start button on the motor starter.
2. Verify flow is present at the outflow pipe on the north side of the pump house.
3. Operate until the water flows clear or 10 minutes has elapsed, which ever occurs first. An excessively long backwash is not recommended as it may result in flushing filter sand into the sanitary sewer.
4. Stop the main pump motor by pressing the Stop button on the motor starter.

The stack light indicator should flash an alternating red-green pattern while backwash is active.

Do not turn on pool fill during backwash. It is desired to send as much pool water (versus fresh/city water) to the sanitary sewer as possible.

Set Valves to Normal Operation

The pump must be stopped prior to adjusting the valve positions.

1. Close Valve #1
2. Open Valve #2
3. Close Valve #3
4. Open Valve #4

Start Normal Operation

1. Start the main pump motor by pressing the Start button on the motor starter.
2. Return the temperature controller setpoint to the nominal value (i.e. 82 °F) if adjusted earlier.
3. Bleed air from the filter tanks after several minutes of operation.
4. Adjust the output flow of the pump to be within 600 to 650 GPM. The flow rate must exceed the required minimum (570 GPM with some margin) but not grossly exceed the combined maximum flow rate of the sand filters (560 GPM). Note that 560 GPM is not a typo; refer to Sand Filters for discussion on this discrepancy.