Sewage Treatment Plant - Filter Feed Pumps

 FILTER FEED PUMPS

FILTER FEED PUMPS are used to take the water from the clarified water sump and pass it through the pressure sand filter and activated carbon filter installed in series.

PRESSURE SAND FILTER (PSF)

The upper layers of the sand perform the actual filtration function. The gravel layers merely provide physical support to the upper sand layers.

The sand used in the PSF is not ordinary construction sand: It has particle size in a specific range, and is specially sieved for this purpose.

The solid particles in the water get entrapped and enmeshed in the spaces between the sand particles. Gradually, the space between sand particle gets filled with incoming solids. 

This blocks the passage of water. As a result, the pressure at the outlet drops rapidly and wastes the pumping power, and reduces the throughput of the filter.

When the pressure drops beyond a limit, the sand is cleaned by backwashing of the filter with water, in which water is passed in the reverse direction. 

This process agitates, fluidizes and expands the sand bed. The backwash water carries away the lighter pollutant solid particles as backwash waste.

Operation And Maintenance Considerations

• The operations essentially consist of a long filtration run, followed by a short backwash
• sequence.
• The filter needs backwash when the pressure drop across the filter 0.5 kg/cm2.
• However, it is a good practice to backwash once in a shift, irrespective of the actual amount of pressure loss. 
• A five to ten minute backwash will typically rid the filter of all accumulated muck.

ACTIVATED CARBON FILTER (ACF)

An activated carbon filter receives the water that is already filtered by the Pressure Sand Filter and improves multiple quality parameters of the water: BOD, COD, clarity (turbidity), colour and odour.

This filter uses the adsorption action of activated carbon. Activated carbon is typically manufactured from coconut shell or charcoal.

The water filtered by the Pressure Sand Filter enters the Activated Carbon Filter. Unlike in the case of the sand filter, trapped molecules in the carbon cannot be backwashed and got rid of. Hence, activated carbon in the filter has a finite capacity to adsorb and hold the pollutants, after which the carbon is said to be exhausted. The exhausted material is removed from the filter and disposed off: Fresh activated carbon is charged in the filter.

Operation And Maintenance Considerations

• Just as the PSF, the ACF also needs to be backwashed, albeit at a lesser frequency to dislodge any solid particles trapped by simple filtration action. 
• 
  
• When the carbon gets exhausted (indicated by no improvement in water quality across the ACF), fresh carbon needs to be filled into the filter.