How does the Activated Sludge Water Recycling Plant work?

Wastewater, from the collection system, is pumped to a mechanical bar screen to remove material that could plug or harm downstream treatment equipment. The wastewater then enters aeration chamber #1 where it is mixed with activated sludge recirculated from the floor of the clarifier-settling basin. Activated sludge is a "soup" of microorganisms that use the organics in the wastewater as food. This dynamic population of microbes removes the soluble organics from the wastewater. The microbes are not added to the wastewater, but are grown from those already present in some numbers. They are those typically found in rich garden compost. The ones that proliferate are the particular microbes that are best suited to grow on the materials in the wastewater. They then become dominant in the activated sludge population. This is classic Darwinian natural selection of the best organisms to fill a niche. What we have to do is provide the proper environment rich in oxygen and the organic substrate "food". Then a natural wet compost process develops. Microbes that do not find suitable conditions for growth just die. Those that do not proliferate may survive in low numbers until they are killed or removed as described below. Human pathogens do not fare well in the activated sludge process.

The microbe and wastewater mixture passes to aeration chamber #2 and #3, and then transfers to the clarifier. In each chamber, there is less "food" available as the microbes convert the organics to maintain respiration and growth. The clarifier uses gravity to separate the microbes as well as other solids from the water. This leaves only clear, clean water at the surface. The clarified water flows over peripheral weir troughs and then enters a chamber where chlorine is added to kill any remaining microbes before it flows to the storage pond for reuse. Chlorine is the same disinfectant we use in our drinking water. It is very effective at killing any pathogens present.

Unlike anaerobic, or septic system microbes, aerobic organisms do not produce unpleasant odors. Activated sludge microbes are aerobic organisms and therefore require oxygen to thrive. High volume compressors provide this oxygen transfer from air, by bubbling it through the water. The rising air bubbles also mix the aeration basins to bring the microbes in contact with the organics in the wastewater. The hydraulic contact time for the wastewater with the microbes is about 12 hours while the microbe system residence time varies between 8 to 12 days. This long microbe residence time is achieved through continual recirculation of the activated sludge.

The conversion of organic material to microbes produces clean water but also produces more activated sludge microbes. This excess sludge is removed from the clarifier and transferred to the sludge digester. The controlled rate of removal determines the average age of the microbes and is the primary mechanism to control activated sludge quality. The excess sludge is thickened and aerated in either of two aerobic digesters, which allow the microorganisms to metabolize remaining organics and then stabilize. Water separated from the sludge in the thickener returns to the beginning of the process for re-treatment. Sludge stabilization is achieved after an additional 10 to 15 days of sludge retention in the digesters. Stabilized sludge is no longer active, and will not produce odors, even if not aerated. The belt filter press dewaters the stabilized sludge to a Play-Doh consistency, which is then delivered to a landfill. Belt press filtrate and belt wash water also return to the beginning of the process for re-treatment. The site irrigation and in-plant uses are all recycled water.