LIQUID PROCESSES

PRIMARY TREATMENT

Plant Influent
 
Influent flows to the Southeast Plant consist of gravity flow and pressurized flow. Analyses of the amount of solids (TSS), the concentration of the organics (BOD) and the chlorides are performed on both these flows to allow the plant to analyze the basic characteristics of the incoming flows and to provide a baseline for future performance evaluation.

Screening

The influent first passes through coarse bar screens which remove any large objects such as rags, plastic, and construction debris before they can become lodged in the main lift pumps. Then the influent passes through fine bar screens to remove smaller rags and garbage which would otherwise end up in pump suction lines or volutes or at the bottom of settling tanks or digesters.

Grit Removal

The second pretreatment process is 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 15,000 lbs. of grit and screenings are hauled from Bldg. 011 every day, and trucked to landfill for disposal.

Primary Clarification

Primary sedimentation is a physical process in wastewater treatment which removes settled or floating solids. After grit removal, the 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 solids blend tank. The floatable materials such as oil and grease that rise to the surface of the tanks are skimmed and sent to the blend tank along with the primary sludge.

Aeration

Secondary treatment is a biological process – so-called activated sludge. 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 microorganisms 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.

Secondary Clarification



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.

TERTIARY TREATMENT 

Disinfection

The final step in liquid treatment is chlorination and dechlorination. Wastewater effluents are chlorinated to destroy bacteria viruses which may affect the public health. Viruses such as hepatitis and cholera are not destroyed by either primary or secondary treatment and must be disinfected directly.

Disinfection takes place in the chlorine contact channels. Long, narrow channels insure that the wastewater will be in contact with the chlorine for at least 30 minutes. Once the chlorine has had sufficient contact time, it is neutralized with sodium bisulfite so that it does not harm marine life after discharge.

Plant Effluent

Once the wastewater has been chlorinated and dechlorinated it is discharged into the Bay. The Plant must meet and report strict discharge requirements on our Final Effluent. In addition, the biologists perform bioassays to demonstrate that our final effluent is able to sustain marine life.

Plant Recycle

The digester underflow, the centrate as well as other flows resulting from cleaning parts of the plant are collected in a structure and are returned to either the sewer leading back to the plant or 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 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.

Blend Tank

After the waste activated sludge is thickened to approximately the same percentage as the primary sludge, it flows to the blend tank where it gets mixed with the primary sludge and scum.

Anaerobic Digestion

Anaerobic digesters are large fermentation tanks provided with mechanical mixing, heating, gas collection, sludge addition and withdrawal ports, and supernatant outlets. Sludge digestion and settling occur simultaneously in the tank. Sludge stratifies and forms the following layers from the bottom to the tip of the tank: digested sludge, actively digested sludge, supernatant, a scum layer and gas.

Anaerobic digestion consists of a series of microbiological processes that convert organic compounds to methane. In a digester two main different groups of microorganisms coexist. First there is a group of facultative anaerobic bacteria, which convert organic solids to liquid organic acids by means of enzyme secretion. Complex materials such as fats, proteins, and carbohydrates are hydrolyzed, fermented, and biologically converted to simple organic materials. Then there are the methane-forming bacteria that are strictly anaerobic and even small quantities of oxygen are harmful to them. These bacteria convert the liquid organic acids to methane (CHB4B) and carbon dioxide (COB2B). Of the two populations, the methane-formers are the more sensitive group. These bacteria require a relatively neutral or low acid environment, as well as temperatures between 85 and 95F. They cannot tolerate high concentrations of volatile acids, even though they rely on these same acids for their food supply. If the proportion of acid-forming bacteria increases relative to the methane-forming bacteria, the pH will drop due to unused volatile acids, and the activity of the methane population will decrease.

Once the methane gas has been collected from the reactor, it is cleaned and separated from other biogas constituents such as carbon dioxide, hydrogen sulfide, and excess moisture. After cleaning and purifying the methane gas, it is stored for later use or used immediately. Digester gas is used as fuel for hot water boilers, water pump engines, blowers, and electric generators. It can also be used to fire incinerators or burned to heat the influent sludge during pretreatment.

Thus for municipal wastewater treatment plants it is most cost effective and environmentally sound to use anaerobic digestion in the stabilization of sewage sludge. Not only does this greatly reduce the final volume of waste to be disposed, it also creates enough energy, in the form of methane gas, to fuel parts or the whole plant.

Cogeneration

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 Southeast Plant, the cogeneration facility consists of one Waukesha Engine-Generator set, with the capability of producing 2110 kilowatts of electricity and input of 7,000,000 BTU/hour into the hot water heating system. The electricity feeds power back to the Primary Switching Station. All electricity produced by the Engine-Generator is utilized within the Southeast 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 fueled by digester gas.

A similar process takes place at the Oceanside Water Pollution Control Plant. At the Oceanside Plant there are 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 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.

Centrifugation

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. In our Southeast plant the chemically conditioned sludge enters through a feed tube and ports in the conveyor. The conveyor turns inside the bowl at several rpm faster than the bowl. Centrifugal force causes solids to be flung against the inside walls of the bowl. The conveyor scrapes the solids off of the outside walls and moves it toward the discharge at the opposite end.

Many newer treatment plants like our Ocean Side Plant are designed with filter belt presses for 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.

Biosolids Disposal

After dewatering, the biosolids from the two plants is transported by the City-contracted hauler, Sunset Scavengers. From April 15th to October 15th, a portion of the biosolids from both OSP and SEP is transported to Solano County for Land Application by Synagro Technologies in fields between Fairfield and Rio Vista. Starting in 2004, Synagro is also land applying a small amount biosolids from both plants in rural areas of Sonoma County near Highway 37. We ensure that the biosolids meet all the criteria that ensure that they are stabilized and safe for the public and supply this data on a regular basis to the vendor. SFPUC personnel inspect field operations weekly to ensure compliance.

Figure 1: A truck is spreading biosolids from our plants in a field in Solano County

Figure 2: SFPUC personnel is visiting the fields

The remainder of the biosolids during dry-weather are 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 Solano County. After the biosolids leave our plants, they are placed in a lined storage unit for the winter. During dry weather the biosolids are removed from the “pond” and the biosolids undergo a two stage drying process and when acceptable moisture levels have been obtained, the biosolids are mixed soil and the mixture is used as a an on-site building material to help reduce the on-site’s soil deficit. The landfills have specific Waste Discharge Requirements and we supply them with all relevant analytical data.

Figure 3: A truck is disposing of waste at the Hay Road landfill

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.