Wastewater Technology Provides Electricity, Fuel
By Michele Braas, Project Engineer, RETTEW; Holly White, Communications Specialist, RETTEW; and Tom Darby, Executive Director, Hermitage Municipal Authority
Most of the technology used for wastewater treatment is not new, but the application of these existing technologies combined with out-of-the-box thinking, can provide a valuable source of renewable energy.
In Hermitage, PA, near the state’s border with Ohio, municipal leadership have been working with environmental engineers to accomplish just that. What began in 2005 as a mandate to resolve inflow and infiltration issues transformed into a wastewater treatment plant revitalization. The project included installing a new biosolids recycling system and generating enough electricity to sustain the entire facility. Today, the leadership is working on a process to use the system to provide fuel for the municipality’s fleet as well.
The History
The Hermitage Municipal Authority owns a sewage treatment facility within the City of Hermitage, in Mercer County, Pennsylvania. City personnel operate the treatment site, which serves Clark Borough, the City of Hermitage, Jefferson Township, Shenango Township, South Pymatuning Township, and Wheatland Borough. The facility operates with about 5 million gallons per day (MGD), serving a customer base which includes several industrial users. The plant discharges all treated wastewater to the Shenango River.
Many municipalities experience inflow and infiltration from aged collection and conveyance systems, and unfortunately, Hermitage was no stranger to these challenges. After experiencing several sanitary overflows, Hermitage entered into a Consent Order and Decree with the Pennsylvania Department of Environmental Protection (PA DEP). The order required facility upgrades to eliminate those overflows.
While workingto meet the consent order, Hermitage elected to use this opportunity as to refocus the municipal leaders into a comprehensive plan for their wastewater treatment needs in the future. The leadership split its upgrades into two phases. Phase I would construct new tankage and headworks facilities to reduce overflows, while Phase II would convert the new tankage into treatment facilities and upgrade the remainder of the facility.
These upgrades would total $40 million and provide enhanced wastewater treatment capacity as well as Net Zero renewable results. They were to include two equalization basins, new headworks, and a new ultraviolet disinfection system. The design would also include an advanced anaerobic digestion complex to stabilize sludge and produce biogas. Such a process would reuse the biogas as energy, contributing to significantly reduced operating costs. The upgrades resulted in lifting the PA DEP’s consent order, as well as preparing the treatment plant for the next 40 years of use.
Wastewater Treatment Center Improvements
The Authority and City first undertook design and construction of the equalization tanks. Both came online in 2008, designed for conversion in Phase II into sequencing batch reactors.
At that time, Hermitage also updated the headworks facilities. Today, following improvements from both phases, the facilities now include a course bar screen, followed by a fine screen at the headworks. Next, the wastewater is exposed to grit removal, and afterwards is processed through three sequencing batch reactors. Ultraviolet light disinfects the effluent, which is then discharged to the Shenango River. The facility is permitted for a hydraulic loading of 7.7 MGD, but can process up to 42 MGD during intense wet weather situations.
Net Zero Biosolids
Along with its wastewater treatment, Hermitage also now provides treatment of waste-activated sludge (WAS) that’s generated within its process. The site also processes waste delivered by third-party users, including food waste, such as expired dairy products and liquid residual wastes.
The system treats all wastes via a temperature-phased anaerobic digestion process that allows for the destruction of volatile solids as well as the production and capture of biogas. The Authority uses the biogas for energy generation, while the volume of sludge not converted to biogas is treated to the level of an Environmental Protection Agency 503 Class A Biosolid. The biosolids are then used by local companies and agricultural businesses as fertilizer.
Sludge to Biosolids Processing: Preliminary Treatment
The WAS first is stored, then transferred to a thickening and dewatering building. Two gravity-belt thickeners provide mechanical sludge thickening to reduce the volume of sludge — by about 84 percent — fed to the anaerobic digestion process. Through the belt thickeners, the total solids are increased to between five and six percent. The thickened WAS is then ready to be mixed with the third-party food and liquid wastes.
A gravity belt thickener reduces the volume of sludge.
Tanker trucks deliver liquid residual waste on a daily basis, which is then unloaded into the thickening and dewatering building. The wastes pass through a flow meter, moving into the food waste hydrolysis tank. Trucks also arrive regularly with packaged food waste, such as expired dairy and drink products, still in the original packaging. The Authority maintains equipment to remove the cardboard and plastic packaging, with the contents then conveyed to the hydrolysis tank. All cardboard and plastic materials are then recycled at another location. The combined food and liquid waste then passes through a screen, flow meter, and gravity thickener.
Sludge to Biosolids Processing: Anaerobic Digestion
After preliminary treatment, WAS and combined residual wastes are mixed and sent to a temperature-phased anaerobic digestion process. The digestion process first allows for material hydrolysis, or decomposition, to occur. Then volatile acid fermentation, also called acidogenesis and acetogenesis, takes place, which is the transformation of sugars, fatty acids, and amino acids into carbonic acids, alcohols, hydrogen, carbon dioxide, and ammonia. Finally, the substance goes through methane formation, also known as methanogenesis.
These steps transpire by first conveying the waste to two sequencing feed tanks. As more sludge arrives in the feed tanks, the waste is regularly transferred to the thermophilic digester, and then later to one of three mesophilic digesters.
The digesters are part of an Infilco Degremont Inc. 2PAD system for the co-digestion of the combined sludge. The Infilco process uses a temperature-phased approach combining thermophilic and mesophilic digestion. This phased approach separates process stages, making it more efficient and effective at volatile solids destruction as well as resulting in biogas generation. At Hermitage, the sludge resides in the single thermophilic digester for about two days and can stay in one of the mesophilic digesters for about 10 days.
The piping transferring the sludge from the sequencing tanks to the thermophilic digester is adjacent to and aligns with the piping used to transfer between the thermophilic digester and mesophilic digesters. During simultaneous batch transfers from the sequencing tanks to the thermophilic digester and the thermophilic to the mesophilic digester, heat is transferred into the piping. That heat is then used to preheat the raw sludge arriving in the thermophilic digester.
Also, as the sludge passes in adjacent pipes, heat transfers from the higher-temperature waste, which is leaving the thermophilic digester to the lower-temperature waste, which is arriving in the thermophilic digester. This results in a decreased temperature of the waste arriving at the mesophilic digester, moving from approximately 135 degrees Fahrenheit to 95 degrees Fahrenheit.
The thermophilic digester operates in the temperature range of 122 to 150 degrees Fahrenheit. This range shortens process time by aiding hydrolysis and acidogenesis, as the fats and sugars break down and become other elements. Because of its high temperature, the digester promotes pathogen destruction, a requirement in producing Class A Biosolids. Thermophilic digestion also enables enhanced volatile solids treatment by creating an environment for certain types of bacteria to live, which in turn contributes to a higher level of gas production in the next phase of the process.
After residing in the thermophilic digester for 48 hours, the sludge is then transferred to one of three mesophilic digesters. Temperatures in the mesophilic digesters range between 85 to 100 degrees Fahrenheit, providing a more suitable environment for the slower-reacting acetogenic and methanogenic microorganisms. From this process comes biogas, in quantities large enough to help supply the facility’s energy needs. Hermitage collects the biogas from the mesophilic digester for use.
One of the three Infilco mesophilic digesters
Biosolids Reuse
Not all of the solids are converted to biogas. After the digestion process stabilizes the solids, a belt filter press dewaters the substance to separate the liquids from the solids. The liquid, also called filtrate, cycles back to the start of the wastewater treatment plant. The City then sends the solids, now qualified as Class A Biosolids, off site for reuse. Area businesses and residents use the recycled waste as a soil amendment, by applying it to agricultural land, or in landscaping work.
Biogas Treatment and Use
Hermitage sends the biogas collected from the mesophilic digester to be cleaned for electric generation. Any excess gas is burned off via flare in a specially designated area.
The biogas to be used in energy generation requires a multi-step purification process. First, the gas passes through an iron sponge system to remove hydrogen sulfide with a proprietary media. Then the gas undergoes a process to remove moisture. Lastly, a set of carbon filter beds remove siloxanes from the biogas supply. The treated biogas is then sent to a combined heat and power (CHP), or cogeneration, internal combustion engine and generator. This equipment produces electricity onsite and contributes to heating the facilities. When combined with natural gas, the CHP unit provides 100 percent of the heat for the digestion process and the entire facility’s electrical power needs.
The facility’s internal combustion engine and generator.
Net Zero Wastewater
Through the recent upgrades, Hermitage has converted its wastewater treatment facilities from a system that experienced overflows to a system that is environmentally responsible. The current self-sustaining operations provide its own power while meeting a community need for beneficial reuse of residual wasted and expired food products.
More to Come
The City and Authority have made tremendous strides in energy and solids management, and are continuing in their commitment to being environmentally responsible by initiating a Phase III for their wastewater treatment facilities.
For the next phase, Hermitage is investigating additional uses for its generated biogas. The agencies contracted with RETTEW to provide a preliminary evaluation on the viability of converting biogas to compressed natural gas (CNG). The CNG would then be used as fuel to power CNG vehicles for the municipality’s use. Preliminary results demonstrated that Hermitage could generate substantial savings — in excess of $100,000 annually — by switching to a CNG-derived form of biogas.
Phase III of the upgrades will provide equipment to convert the biogas to CNG, as well as alter the City and Authority’s fleets to CNG vehicles. The biogas currently treated and sent to the CHP will be compressed and sent through a pressure swing absorption vessel for carbon dioxide removal. Hermitage will then compress the gas again, and keep it in a cascade storage system. After being odorized, the final product will be dispensed through a fast-fill dual dispenser. CNG dispensers operate like a typical gasoline dispenser, with the ability to track volume and cost and a keypad for user operation.
The agencies are currently pursuing funding opportunities to finance the biogas conversion to CNG and subsequent use. The project analysis, design, and permitting are slated to begin in 2015.
The Story
When the personnel at The City of Hermitage and The Hermitage Authority began to address an inflow and infiltration issue years ago, they began the journey to renewable energy. Now, the agencies are looking to generate their entire electrical power needs as well as fuel for their fleet vehicles, all while continuing to meet the needs of local residents and businesses. Hermitage is an example of how far a commitment to advanced water treatment technology can go.