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to the bacteria which are growing on the media. It is this splashing and turbulence which gives the odorous compounds a chance to escape to the atmosphere before the bacteria can adsorb and oxidize them. This problem is made worse when the anaerobic side stream from the digester is returned to the main flow. That particular plant has a characteristic "rotten egg" smell . That smell is hydrogen sulfide, a compound the human nose is sensitive to at extremely low concentration levels. It is produced in anaerobic conditions! While the process used at this plant is not very good some of the odors that are perceived from the highway may be due to the adjacent industry(which is located upwind) . They are doing waste treatment of a more rudimentary type which can also easily produce odors. i The plant to the north of Lathrop uses a combination of an activated sludge treatment process and "maturation ponds" to stabilize their waste stream. Besides having anaerobic processes the maturation ponds produce odors. The odor which is prevalent at that plant is an algae odor. To most people it is a slightly disturbing odor that is described as a "low tide" , "rotten grass" , or "swampy" odor. The ponds are termed "facultative ponds" , this means that they are aerobic in the upper layers and anaerobic in the lower layers . Because of the nutrients contained in the sewage and the abundant sunlight algae grow profusely during most of the year. The oxygen produced by the algae helps to treat the wastewater. The heavy materials sink to the bottom and are decomposed. The nutrients are then "naturally" recycled and more algae grow. Eventually there is enough decomposing algae for the odors to escape to the atmosphere as gas bubbles become large enough to rise all the way to the surface, hence the "swampy odor" . In the spring as the temperature increase the ponds can "Turn" , which means that the bottom and top waters change locations. At this time the odors can be extremely strong. The Lathrop plant uses an extended aeration activated sludge process coupled with a nitrification/denitrification process(which removes the nitrogen content) . The sludge produced by the process is aerobically stabilized in the process and is rapidly dewatered so that there is no potential for odors. Odors which might develop in the collection system are controlled by having the head works enclosed, and by having the air drawn out of the building and used to aerate the activated sludge. Because of the depth of the diffusers ( 14 feet of water) and the diffuse manner in which the air is added there is adequate time for the potential odor causing materials to be adsorbed and oxidized. The wastewater is disposed of to percolation ponds. The ponds will typically only have 6 to 12 inchs of water in them at a time, and then only for a week or so, before they are allowed to dry. In this way the problem of algae growth is avoided(and the potential for odors from that source) , In addition the treated effluent from this process will have less then half the strength(BOD and S.S) then the other two plants, not to mention the nutrient control .