Wastewater Treatment Plant
The City of Brentwood's Wastewater Treatment Plant is a 5.0 million gallon per day (MGD) extended aeration/activated sludge facility. The treatment plant discharges tertiary treated effluent into Marsh Creek. This effluent meets or exceeds CA Title 22 drinking water standards. The treatment plant was designed and built to facilitate an expansion to an average dry weather flow of 10 MGD.
The beginning part of the treatment process, raw wastewater is collected from the entire City is pumped to the highest elevated portion of the treatment process. Here it passes through mechanical barscreens that remove objects larger than 1 inch in diameter. The screened wastewater then flows through a grit removal system that removes inorganic material such as sand and gravel.
is the process of removing nitrogen from the wastewater. The City’s permit to operate specifies a certain amount of nitrogen that must be removed from the water prior to discharge. Following the headworks, the raw wastewater is mixed with the Return Activated Sludge (RAS) and the mixed liquor where it flows into an anoxic zone. In the anoxic zone a particular species of bacteria is cultivated. This bacteria organically converts nitrate into nitrogen gas, which is insoluble in water and is discharged into the atmosphere.
3. Oxidation Ditches
After removing nitrogen, the wastewater is circulated in large oval tanks called oxidation ditches, while air is introduced into the water. By dissolving a specified percentage of oxygen in the water, the process is able to selectively cultivate another specific type of bacteria that converts naturally occurring ammonia to a less toxic chemical called nitrate. A computer system controls a mechanical aerator that speeds up and slows down as appropriate to maintain the desired level of oxygen in the water. Upon completion of this oxidation process, a portion of this nitrate rich water is recirculated back to the denitrification process. The remainder of this water flows to the secondary clarifiers.
4. Secondary Clarifiers
Following oxidation the bacteria/water mixture flows into one of two circular tanks called secondary clarifiers. In these tanks gravity is used to separate the solids (bacteria) from the liquid. The solids (bacteria) settle out to the bottom of the tank where they are collected and pumped back to the denitrification tanks where they begin the entire process all over again. The clean water flows over a weir at the top of the clarifier and goes to a tertiary filter.
5. Tertiary Filters
Water from the secondary clarifiers (called secondary effluent) is pumped into one of four tertiary filters. These filters consist of a layer of sand approximately 6 feet deep. As the water flows down through the sand, fine particles that were too light to settle out in the secondary clarifier are trapped in the sand. This process mimics what happens in nature when rainfall is absorbed into the earth and naturally filters out particulates as water flows into underground aquifers. The quality of the water leaving the filters is monitored and controlled by a computer system connected to chemical analyzers. In this way, if the water ever doesn’t meet the permit requirements, the computer system will automatically divert the water to a holding pond until the quality improves enough to satisfy the permit requirements.
6. Chlorine Contact
After filtration the clean water flows into chlorine contact tanks. A concentrated solution of chlorine bleach is added to the filtered water. The tanks are sized so that at normal flows it takes about 2 hours for the filtered water to pass through. This 2 hour time period allows the chlorine sufficient time to disinfect (destroy) any harmful microscopic organisms that remain in the water. At the end of the disinfection process, sodium bisulfite is added to the water to neutralize the remaining chlorine prior to the water being discharged to Marsh Creek. By disinfecting the water it makes it safe to recycle and use to irrigate landscaping medians, parks, and golf courses.
7. Cascade Aerator
Before discharging into Marsh Creek, the final effluent cascades down a man-made rock structure that increases the dissolved oxygen content of the water. This helps ensure the Marsh Creek is a healthy environment for aquatic species.
8. Belt Filter Press
As bacteria grow and reproduce in the denitrification and oxidation processes, it becomes necessary to periodically remove some of these bacteria from the system to maintain an appropriate balance. A belt filter press is a mechanical process and squeezes the solids (bacteria) between two porous belts which removes the water and concentrates the solids to approximately 15% solids. These concentrated solids are then hauled to a landfill for disposal.
Definition of Wastewater Terms
Anoxic Zone: an area that lacks sufficient oxygen to sustain normal aerobic bacterial respiration: the treatment process that speeds up the decomposition of wastes in the wastewater.
Denitrification: an anoxic process that occurs when nitrite or nitrate ions are reduced to nitrogen gas.
Dissolved Oxygen: atmospheric oxygen that is dissolved into the wastewater.
Mixed Liquor: a mixture of activated sludge and raw sewage.
Oxidation: the addition of oxygen to support organic matter being oxidized to a more stable substance.
Return Activated Sludge (RAS): the solids that are removed from the bottom of the secondary clarifier.
Secondary Clarifier: a large tank that allows the heavier particles to separate from the wastewater.
Turbidity: the appearance of the treated wastewater of appearing to be cloudy or muddy.
Title 22: the State of California's code for health and safety standards for drinking water.