Solids Treatment Process
Primary Sludge and Scum Straining
The primary sludge and scum from the primary clarifiers is pumped through a fine strainer, known as a strain-press, to remove any residual debris. The solids have small amounts of residual trash, hair, plastics, and other inert debris that pass through the openings between bars of the influent bar screens in the headworks. It is important to remove these inert materials so they do not end up in the downstream digestion process and in the treated biosolids leaving the facility. The strain-presses have perforations which allow the organic solids to pass while retaining the larger inert trash and debris. The inert material is compacted and sent to the landfill and the organic solids are sent to a downstream blending process (the Primary and Secondary Sludge Mixing Tank) and then on to the anaerobic digesters.
Waste Activated Sludge Thickening (Schematic #11)
Waste Activated Sludge (WAS) comes from the secondary clarifiers. WAS is typically around one percent solids by weight which means it has a high water content. It is too thin to send directly to the digesters (because this would require a lot of heating), so the WAS is thickened using a machine called a gravity belt thickener to remove water. First polymer is added to coagulate the solid particles and allow them to easily separate from the water. Then, the mixture flows on the top of a moving fabric belt where the water drains through while the coagulated solids remain on top of the belt. The thickened solids are now six percent solids by weight and fall from the end of the belt into a tank from where they are pumped to the Primary and Secondary Sludge Mixing Tank.
Primary and Secondary Sludge Mixing
There are two tanks involved with primary and secondary sludge mixing. The first tank is called the Mixing Tank. This is a small tank (20,000 gallons) with high energy mixing. The two sludges are rapidly mixed under turbulent conditions for about an hour, resulting in a consistent blended feedstock for the anaerobic digesters.
From the Mix Tank, the blended sludge is pumped to the Equalization Tank. The Equalization Tank is a large, gently mixed tank (350,000 gallons) that can hold the sludge flow for one to two days. Since the sludge flow rate varies diurnally based on the wastewater flow rate into the treatment plant, the Equalization Tank is used to equalize the sludge flow rate to the anaerobic digesters. Using both the Mix and Equalization tank provides both a consistent feed concentration (5 to 6 percent solids) and feed flowrate to the digesters and allows the digestion process to operate under steady-state conditions.
Anaerobic Digestion (Schematic #12)
Sludge in the Equalization Tank is pumped to the anaerobic digestion process. This is a process in which a series of biological processes take place in which microorganisms break down biodegradable material in the absence of oxygen. The sludge is held in large, closed tanks that are gently mixed for many days.
Carbon containing materials are first converted to volatile fatty acids such as acetic and propionic acid, nitrogen compounds are reduced to ammonia, sulfur compounds are reduced to hydrogen sulfide, and oxygen is converted to carbon dioxide. Water is also produced in the digestion process. The volatile fatty acids are then consumed by a second group of bacteria in the digester and are converted to methane gas also known as biogas. This process reduces the volatile or carbon containing solids by about 65 percent and the total solids content of the sludge by about 50 percent, making the residual solids into a more stabilized material. The residual solids are called biosolids and can be safely used directly for soil amendments on agricultural land or further processed into compost for use in landscaping and gardening. The methane gas is used for electrical energy and heat production.
The CVWRF uses a two stage mesophilic (at 98 degrees Fahrenheit) digestion process. The first stage digesters are two 1.65 million gallon egg-shaped digesters. The egg-shaped digesters look like large eggs sitting on end and are shaped to optimize mixing efficiency and reduce the deposition of grit on the bottom and foam on the top. The solids remain in the egg-shaped digesters for 15 to 20 days.
After leaving the egg-shaped digesters, the solids flow to four, 1.0 million gallon, traditional circular digesters where they remain for 20 or more days. About 90 percent of the volatile solids reduction and methane production occur in the egg-shaped digesters. The conventional digesters mainly provide additional odor reduction, sludge stability, and pathogen reduction while only producing a small amount of methane gas. The solids content of the sludge leaving the anaerobic digestion process has been reduced to around 2.5 to 3 percent from a feed sludge concentration of 5 to 6 percent.
Biosolids Dewatering (Schematic #15)
The anaerobic digestion process produces digested biosolids that are 97% water and 3% solids. The solids must be separated from the water so that they can be properly disposed of. Organic polymers are added to the biosolids slurry to help coagulate the solid particles and allow separation of the water from the solid material. The coagulated solids mixture is then applied to a belt filter press dewatering unit to separate the water and produce a solid material. The dewatered biosolids material referred to as “cake”. Belt filters are considered simple and reliable, with low staffing, easy maintenance, and a long life.
A belt filter dewaters by applying pressure to the biosolids to squeeze out the water. Biosolids are sandwiched between two tensioned porous belts that are passed over and under rollers of various diameters. The initial rollers are large diameter and the roller diameter decreases through the filter press. Increasing pressure through the press is created as the belt passes over smaller and smaller diameter rollers. The general mechanical components of a belt filter press include dewatering belts, rollers and bearings, belt tracking and tensioning systems, controls, motorized gear drives, and a belt washing system. The dewatering objectives include reducing the volume of biosolids to reduce the transport costs, producing appropriate quality material to mix with wood chips for composting, producing material that is spreadable using a manure spreader when used for land application
Biosolids Land Application
In the United States, the Code of Federal Regulations (CFR), Title 40, Part 503 governs the management of biosolids. Within that federal regulation, biosolids are classified differently depending upon the quantity of pollutants they contain and the level of treatment they have been subjected to (the latter of which determines both the level of vector attraction reduction and the level of pathogen reduction). These factors affect how biosolids may be disseminated and the level of monitoring oversight required which, in turn determines where and in what quantity they may be applied.
The stabilized solids cake from the anaerobic digestion and dewatering processes is mostly comprised of highly degraded organic matter and is rich in essential plant nutrients such as phosphorus and nitrogen. These biosolids can be returned in dewatered form directly to the environment by applying them on agricultural land as a Class B product. Class B Biosolids is a designation for treated sewage sludge that meets certain U.S. EPA guidelines and is suitable for land application as fertilizer. Approximately 70 percent of the dewatered biosolids from the CVWRF are trucked to farm lands for agricultural land application to supply nutrients and organic carbon for beneficial agricultural use.
Composting and Sales (Schematic #16)
About 30 percent of the dewatered biosolids produced at CVWRF are mixed with wood chips and composted to make an Exceptional Quality Class A compost that is sold to the public for landscaping and horticultural use. The CVWRF uses anaerobic digestion and in-vessel aerated static pile (IASP) composting to ensure the composted biosolid have the Class A – Exceptional Quality rating, EPA’s highest rating for biosolids products. Class A Biosolids is a designation for dewatered and heated sewage sludge that meets U.S. EPA guidelines for land application with no restrictions. The Class A EQ (Exceptional Quality) rating describe a biosolids product that not only meet, but exceed, all Class A requirements.
The ground wood chips are produced from waste wood, branches and logs generated from the local tree trimming and service industry. The wood waste is processed through a large industrial chipper/shredder and screening system to produce the optimum particle size mixture for compost production. The CVWRF’s compost products are marketed under the label Oquirrh Mountain Compost Products (www.OMCompost.com).
Energy Recovery (Schematic #13 and 14)
The CVWRF’s cogeneration system is a critical element in the plant’s daily operation and success. It serves as the primary and standby source of power and heat for all the plant’s treatment units and process buildings. The CVWRF produces an abundant supply of digester gas, an important renewable energy source, through the anaerobic digestion of its sludges. Through the cogeneration system, the digester gas provides power and heat for the entire plant, enabling the plant to be self-sufficient and environmentally sustainable.
The CVWRF has successfully operated a cogeneration facility since it was initially constructed in the 1980’s. Based on the approximate 3,300 kW power demand of the treatment facility, the CVWRF currently relies on Rocky Mountain Power to supply approximately 15 percent of the electrical energy used onsite per month. The cogeneration system supplies the remaining 85 percent of the power used by the treatment plant. A mixture of digester gas and natural gas fuels the engine-generators of the cogeneration system. Biogas produced by the digesters accounts for one half of the gas consumed while natural gas makes up the remainder of the fuel. Natural gas used at the CVWRF is supplied by British Petroleum (BP) via Dominion Gas’ pipeline system.
In addition, the engine heat from the cogeneration engine-generators is harnessed by a heat recovery system which allows this heat to be productively used within the treatment plant for sludge digester heating and for building and tunnel space heating. Engine jacket water heat, and exhaust heat from the cogeneration engines is captured via a heat exchangers and distributed around the plant in a hot water loop. Excess engine heat that is not needed for heating facilities is wasted to the wastewater flowing through the treatment plant.