Effluent filtration is a tertiary treatment system that removes fine suspended organic solids from the treated sewage. We use sand filters to provide tertiary treatment.

This sand filter cell is one of eight sand filter cells we have on the site. This photo was taken when the cell was down for maintenance. The grid at the bottom is part of the support structure for the sand media. About 12 inches of sand is placed on top of the grid. The trough that extends along the center of the cell helps distribute liquid from the secondary clarifiers to the sand along the length of the sand filter cell. When the sand surface eventually clogs with solids, the same trough is used to carry backwash water with the accumulated solids to a backwash pump station.









Liquid from the secondary clarifiers enters the center trough and eventually flows over both sides of the trough to flow through the sand.

As solids accumulate on the surface of the sand, it becomes harder and harder for the water to saturate through the sand, and the water gets deeper and deeper over the sand. When the water gets deep enough, a valve stop water from coming into the trough, and another valve opens to force backwash water to enter the tank from underneath the cell and reverse the flow of water in the cell. This fluidizes the sand, and carrys away the solids (which are lighter than the sand particles) backwards into the trough and eventually out of the tank to a backwash pump station.

The backwash pump station tranports the flow to an equalization tank for temporary holding. From the equalization tank, the backwash water is fed, at a uniform rate, back into the plant for treatment.

From the sand filters, the liquid flow travels to a chlorine contact tank where sodium hypochlorite, bleach, is injected into the flow, on a seasonal basis, to kill pathogenic bacteria that may remain in the effluent. Sodium hypochlorite is toxic to biological life in our receiving stream, Spring Brook. So, immediately before we discharge the fully treated liquid flow into Spring Brook, sulfur dioxide is injected to reduce residual chlorine to a level that is non-toxic and won't affect biological life in Spring Brook.

1. Preliminary Treatment
2. Primary Treatment
3. Roughing Filters
4. Activated Sludge
5. Effluent Filtration
6. Effluent Disinfection
7. Sludge Digestion
8. Sludge Dewatering
9. Sludge Storage
10. Other Unique Features