Regional Water Quality Control Plant

Raw sewage entering the treatment plant contains many kinds of impurities. It contains relatively large objects such as rags, plastic objects, pieces of food, sanitary waste, and occasionally large pieces of wood and rock as well as small particles and dissolved organic compounds. In the influent structure, the wastewater passes through bar screens that remove the larger objects from the waste stream.

Next the influent pumps lift the wastewater (which enters the plant about 20-feet below ground level) up to the level of the primary sedimentation tanks. As the waste water passes through the primary sedimentation tanks, the heavier solids settle to the bottom of the tank. These solids (also called primary sludge) are pumped to the solids processing system. Floating material which has passed through the bar screens, is collected from the top of the primary sedimentation tanks and also pumped to the solids processing system.

After passing through primary treatment, the wastewater still contains dissolved organic material and particles too light to settle. These must be removed by the downstream treatment processes. The first of these processes, secondary treatment, is described on the next page.

In the secondary treatment stage, wastewater is exposed to vast colonies of microorganisms. These microorganisms are specially bred to be consumers of specific waste components. The secondary treatment at the RWQCP provides two phases of such biological treatments.

First, hundreds of millions of these one-celled organisms neutralize sugars and fats in structures called fixed-film reactors. These reactors sustain the microorganisms on a pvc support, over which wastewater flows, preventing the microorganisms from migrating through with the wastewater.

The second phase of the biological treatment occurs right after the fixed film reactors, within structures called aeration basins. Here, without as much sugar and fat in the wastewater to consume, the microorganism colonies have no alternative but to forage for and digest ammonia, which is toxic to marine life, converting it to nontoxic nitrates.

The integrity of the plant's treatment process requires plant operators to carefully maintain optimal conditions and air for the microorganisms. Every factor must be monitored and controlled. Conditions allowing too many microorganisms will lead to a food shortage and microorganisms consuming themselves. Population imbalances can also allow unwanted and/or harmful bacteria to take over, leading to a lengthy process of reestablishing the beneficial colonies. Because the aeration basin microorganisms need to be free swimming, they flow to the clarifiers where they are recovered and routed back upstream.

The secondary treatment removes 99% of the ammonia, and over 90% of the organics and suspended solids.

The tertiary (final) treatment provides fine polishing of the wastewater through filtration prior to discharge to the Bay. This stage of the treatment has the most difficult task of capturing the finest substances and metals that cannot be removed by the primary and secondary treatments. Any microorganisms and bacteria remaining downstream of the clarifiers are removed through filtration and disinfection. At the end of this treatment, 99% of ammonia, organic, and solid pollutants are removed from the wastewater.