Published By: Wastewater Enterprise
LIQUID PROCESSES
Plant Influent
Influent flow is pumped to the Oceanside Plant head works via a 48-inch force main from the Westside Pump Station. Analyses of the concentration of solids (TSS), organic compounds (COD), and chlorides are performed to allow the plant to analyze the basic characteristics of the incoming flows and to provide a baseline for future performance evaluation.
After the wastewater passes through the bar screen, it enters the vortex grit chamber for grit removal. Grit includes sand, gravel and other heavy particulate matter, which is removed from the wastewater flow by rapid settling. Grit removal protects pumps from abrasion and prevents pipes from clogging. Grit accumulation at the bottom of settling tanks or digesters reduces their effective volume, thus reducing their performance capabilities.
In a combined sewer system (includes storm sewers) much of the grit sits in the sewers until the first big storm of the year, and comes into the plant all at once.
Sampling and analysis of grits is not easy. There is usually so little of it coming into the plant that you would need many gallons of influent to get a few grams of grit. It is easier to monitor the total solids and volatile solids in the primary sludge. If the fraction of inert solids in the sludge increases during storm flows, grit and silt are getting through the grit tanks and into the primary tanks.
Screening and grit removal have a big impact on the operation and maintenance of the rest of the plant. An average of
Primary Clarification
Primary clarification or sedimentation is a physical process in wastewater treatment which removes settled or floating solids. After grit removal, wastewater flows into the primary settling tanks where the suspended solids are allowed to settle. The removal rate for suspended solids (TSS) is 40 – 60 % and 20 – 30% for the organics (BOD). The settled material, referred to as primary sludge is then pumped to the thickening process. The floatable materials, such as oil and grease that rise to the surface of the tanks are skimmed and send to the scum concentrator and through the blend tank to the anaerobic digesters.
SECONDARY TREATMENT
Aeration
Secondary treatment is a biological process – so-called activated sludge process. A population of microorganisms (a mixture of bacteria and small protozoas) is maintained in the aeration basin. Like all aerobic organisms, these microorganisms need a source of energy, carbon for the synthesis of new cells, and nutrients. Using oxygen as the energy source and the wastewater as a source of carbon and nutrients, the microoganisms utilize organic material in the wastewater and produce new cells, carbon dioxide and ammonia, thereby purifying the effluent.
The aeration process in our plants is pure-oxygen activated sludge. To control odors and to ensure an oxygen-rich environment, the aeration basin is covered and supplied with high purity oxygen in order to increase the transfer efficiency and therefore decrease the dimensions of the aeration tanks. To produce the high purity oxygen, the air is first compressed and cooled until it liquefies. This way the oxygen is separated from nitrogen and other gases in the air by the different vaporization points while waste nitrogen gas is bled off in distillation columns.
After a specified period of time in the aeration basin, wastewater is passed into the settling tanks. Because the cell tissue has a specific gravity slightly greater than water, the cells grown in the aeration basin can be removed from the treated liquid by gravity settling in the clarifiers. A portion of the settled cells is recycled to maintain the desired concentration of organisms in the aeration basin, and a portion is wasted. The level at which the biological mass in the reactor should be kept depends on the wastewater characteristics, the system capacity, the desired treatment efficiency and other considerations related to growth kinetics.
fINAL Treatment
Disinfection
Because of the remarkable Southwest Ocean Outfall line (SWOO), the OSP does not have to disinfect its final effluent flow. However, disinfection tanks were constructed to prepare for future regulatory changes.
Effluent flow is discharged
Plant Recycle
The filtrate from thickening process and from dewatering process as well as other flows resulting from cleaning parts of the plant are collected in a structure and are returned to combine with primary influent to the primary clarification tanks.
SOLID PROCESSES
Sludge Thickening – Gravity Belt Thickener
Secondary sludge, primary sludge and secondary scum are combined and pumped to the three gravity belt thickeners. Before entering the thickener, the sludge is conditioned by blending with polymer. The thickened primary and waste activated sludge (TPAS) discharges from the gravity belt thickeners into a common TPAS tank. This blending and thickening reduces the water loading to the anaerobic digesters. This step reduces the cost of heating sludge and increases the sludge’s detention time in the digesters.
Digester feed Tank
After the waste activated sludge is thickened to approximately 6-7% of total solids, the thickened sludge is pumped by two progressive cavity pumps to the digester feed tank and is blended with primary scum. The blended sludge is withdrawn from the bottom of the feed tank and is pumped to the four egg-shaped anaerobic digesters.
Anaerobic Digestion
Anaerobic digestion consists of a series of microbiological processes. It utilizes anaerobic bacteria and facultative bacteria to consume thickened sludge as food thereby stabilizing the sludge. The anaerobic process must be controlled to maintain the types of bacterial cultures that foster efficient anaerobic digestion, which is a natural fermentation process. Process bacterial actions and cultures are controlled by maintaining a specific range of temperatures, feed ratios and residence time.
OSP has four egg-shaped anaerobic digesters. The unique shape of these units lessens the time and expense of regularly required emptying and cleaning of the units and greatly promotes mixing of contents. Mixing of the bacterial cultures in the digesters with the thickened feed sludge (food) ensures thorough break down and consumption of much of the organic solids sent to the digesters. Once sludge has been processed for at least 15 days in the digesters, the resulting matter is much changed in physical and chemical structure and is called biosolids. We send our biosolids to landfills and also to local farmland for beneficial reuse. Each of the four anaerobic digesters can hold up to
Cogeneration is the process of converting one form of energy into other forms of energy. In the case of a wastewater treatment plant, a cogeneration facility converts methane gas (a byproduct of the digestion process) into electricity and hot water. There are cogeneration facilities at both the Southeast Water Pollution Control Plant and the Oceanside Water Pollution Control Plant. At the Oceanside Plant there are two Waukesha Engine-Generator sets. Each set produces 550 kilowatts for a total of 1100 kilowatts. The hot water is also utilized to heat the digesters. The electricity feeds power back to the Primary Switching Station. All electricity produced by the Engine-Generator is utilized within the Oceanside Plant. Heat is added to the hot water return system via heat exchangers in the engine cooling loop. The hot water return water is then fed into the boilers that provide hot water to the digesters. The boilers are fuelled by digester gas.
The cogeneration facilities allow methane that is normally flared off to be used for the production of useful forms of energy (electricity and hot water). This decreases our reliance on purchased electricity and improves the overall efficiency of the hot water heating system. Cost savings are generated by reducing the cost of electricity purchased.
Once the sludge has been digested, it is dewatered. Dewatering simplifies sludge handling by decreasing volume, which reduces transportation and disposal costs. As new EPA standards reflect the increasing shortage of landfill space, federal regulations will require drier and denser cake. Dryer cake is also safer, and easier to dispose of. The drier the sludge, the less water there will be at the disposal site to possibly leach into the local water supply.
Dewatering is primarily a physical/mechanical process. Chemical aids such as polymer and ferric chloride are used to condition the sludge, so that the water and solids separate more easily. At the OceanSide Plant, belt presses take place in sludge dewatering. The belt press squeezes the sludge between two belts, using rollers to apply increasing amounts of pressure. As the belts separate, the cake is scraped off with a doctor blade. The belts are continuously washed by spray nozzles. The belts can also be replaced fairly simply when they become worn.
After dewatering, the biosolids from the Southeast and the
Figure 1: A truck is spreading biosolids from our plants in a field in
Figure 2: SFPUC personnel is visiting the fields
The remainder of the biosolids during dry-weather is transported to several local landfills where the material is amended and beneficially reused as Alternative Daily Cover. From October 15th until April 15th, the biosolids from both facilities are beneficially reused at the Hay Road Landfill in
Figure 3: A truck is disposing of waste at the
Figure 4: A pile of biosolids mixed with wood waste to be used as ADC
We are required by contract to supply both the landfills and the land application sites with biosolids that meet Class B criteria.
