Any idea why sand is transferred from sand filter to clarifier tank during the backwash?

The following are the few reasons for sand transfer.

1. Due to more than the design flow.

2. fautly strainers.

3. More pressure due to fauly pressure gauges.

4. Extened backwash timing.

5. Prolonged air score timing.

Due to high backwash water flow

due exessive air blowing (agitaion )and backwash flow rate is higher than the design requirement. and also if the backwash continues for longer time even the sand filter is cleaned, hence there is no resistance to flow upward which will result in  carry sand over  

Sand filter is only over size of sand only filtered,so for carry over less thn over size of sand transfer o clarifier tank,To avoid Multigrade sand filter.

Sand escaping from sand filter during backwash is due to overflow during backwash and rinsing.

the backpressure during backwash and rinsing is less than 10m (1barg) and this backpressure meets more than design flow for backwashing filters, espically when the duty and standby pumps are working togather. To solve this problem a restriction orifice plate shall be provided at the backwash and rinse outlet.

Due to strainer damage or due to the friction caused due circulation of the water between the sand particles the difference in the size can be possible reason and over pressure also effects if the quality of the strainer is poor.

It depends on the health of the nozzles.

If the openings of the nozzles are open or broken, then the sand will pass.

Also, the nozzle selection should be based on the size of the sand grains.

athis step is done to free the sand from suspended matters which has been accumulated between sand particles 

A simple thumb rule will suggest us to check the valves are closed perfectly to clarifier, generally during backwash the inflow to filter is stopped which mean no water from clarifies.

Check also if there is any malfunctioning of the valves, I used to have such problems in early days.

For Effective awash and not loss the sand. 

For an against-washing to be effective it will have to last at least three minutes, until the water runs clear at its output.     -Turn Off the pump;    -Turn The sand filter valve position 'Backwash' or 'washing';     -Restart Pump, washing begins;     -Leave Turn it until the water passing through the control becomes completely translucent;     -Stoppez The pump;     -Turn The filter valve on the 'Rinse' or 'Rinse';     -Restart Again the pump and let it run for a minute;     -Stop The pump;     Now -Positionnez the valve on the 'Filter' or 'filtration';     Restart the pump.