Step 1 - Pumping

The first step of secondary treatment, primary effluent is pumped to the top of large concrete tanks called trickling filters.

Step 2 - Trickling Filters

In the trickling filters, bacteria from the effluent cling to plastic filter material and consume organic matter as the effluent tricles past. As layers of bacteria become thick and heavy, they fall off the filter surface and are replaced by a new layer. Meanwhile, the old bacteria are carried away to the solids contact tanks. This process is ongoing.

Step 3 - Solids Contact Tanks

The effluent then flows into solids contact tanks. Here, air is pumped into the effluent, causing the suspended solid material to stick together. The air also helps growth of another kind of bacteria which feed on the remaining organic material. This further reduces the Biochemical Oxygen Demand (BOD). Trickling filters and solids contact tanks reduce the BOD by an additional 35% over primary treatment levels.

Step 4 - Secondary Clarifiers

In the clarifier tanks, the solid organic material (sludge) formed in the contact tanks settles out from the effuent. Just like the sludge from primary treatment, secondary sludge is thickened and pumped to digesters for further processing. At this point, the remaining clean water, now called secondary effluent, is ready for the final step.

Step 5 - Killing Bacteria During the May 1 to September 30 swimming season, secondary effluent is disinfected in exactly the same way as primary effluent.