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Reducing carbon footprint with two-phased digestionNumerous wastewater treatment and recovery plants are adopting novel technologies to help reduce dependence on fossil fuel and provide an off-the-grid option to power their waste treatment facilities. This has resulted in demand for alternative sources of energy from sustainable, localized sources that are considered renewable in nature. Increasingly, the use of biogas to generate process heat and electricity using cogeneration mechanisms has made anaerobic digestion an attractive treatment choice for many small to medium-size facilities.
Delhi Charter Township Wastewater Treatment Plant (WWTP) is located just south of the city of Lansing in Ingham County, Mich. The WWTP was built in 1962; upgrades were made in 1980 and in 2007. The plant is capable of treating a wastewater flow of 2.5 million gal per day on average and 4 mgd during peak flow.
Preliminary treatment is accomplished through shredding and grit removal. Primary treatment takes place in circular settling clarifiers. Secondary treatment consists of an activated sludge process followed by secondary settling. The plant also has tertiary treatment that includes a nitrification tower and polishing lagoons. Disinfection is achieved by chlorination of the nitrification tower effluent prior to passing through the polishing lagoon.
Prior to the 2007 upgrades, the solids handling system consisted of two trains of anaerobic digesters followed by gravity thickening, with a combined capacity to treat 12,000 gal per day (gpd) of sludge. It was estimated that the capacity prior to upgrade would be overloaded severely within five years.
Addressing Capacity Needs
In 2005, with the aid of two local engineering companies, WWTP staff started investigating alternatives to increase the solids handling capacity to 30,000 gpd.
Delhi Charter Township staff chose to upgrade the treatment system using an advanced, high-rate, two-phase anaerobic digestion process. The facility upgrade included the implementation of two parallel treatment trains of two-phased anaerobic digestion (2PAD) system by Degremont Technologies – Infilco, a Suez Environment Co. (see Figure 1).
The 2PAD system has been awarded a conditional “National Process To Further Reduce Pathogens Equivalency” by the U.S. Environmental Protection Agency (EPA). As a result the 2PAD system can produce Class A biosolids. The system requires the design meet guidelines set by the EPA Pathogen Equivalency Committee. The conditional approval requires the monitoring of two parameters, enteric virus and helminth ova, in addition to the parameters required for Class A biosolids certification.
Because there are fewer restrictions as to where Class A biosolids can be land-applied, the solids generated from this plant can be trucked wet and delivered as an alternative to fertilizers, offering the WWTP an alternative source of income.
Offsetting Carbon Footprint
Additionally, a state-of-the-art microturbine system capable of cogenerating process heat and electricity was integrated with the two-phase anaerobic digestion system. The microturbine system consisted of two 30-kWh turbines, with room for two similarly sized turbines to be added as the capacity of the plant increases. In doing so, Delhi’s WWTP has established the state’s first integrated biomass-to-energy system. The multiple-turbine system offers numerous benefits over a single-turbine system, most notably the flexibility in operation, strategic gas utilization and future expansion.
High volatile solids reduction across the 2PAD system results in high biogas yield. The biogas is captured and stored in the floating gas-holder covers on the mesophilic digesters. When heat or electricity demand is sensed, the stored biogas is treated through a biogas conditioning skid prior to usage in the microturbines. The biogas conditioning skid consisted of a series of heat exchangers to remove condensation and media filters to address impurities such as siloxane and corrosive agents such as hydrogen sulfide.
The integrated biomass-to-energy system (see Figure 2) includes microturbines designed to operate with a turndown of up to 50%, which allows the two turbines to operate in the range of 25% to 100% of the combined operable capacity, producing 15 to 60 kWh of electricity on demand.
Because the electrical efficiency of these microturbines is around 30%, the majority of the energy generated is in the form of heat. The microturbine exhaust gas exiting the system is at around 500°F and is captured using a tube-in-shell, gas-to-liquid heat exchanger. The hot water from the heat exchanger is cycled through a biogas boiler. The hot water, in turn, blends with return water from heat-addition heat exchangers for the thermophilic digester sludge heating and heat-addition heating jackets on the internal mixing system within the mesophilic digesters.
At current capacity, the facility is able to reduce electricity consumption by more than 40% and eliminate almost all of the process heat; this sustains the operation of the solids handing system. The treatment facility demonstrates that waste treatment plants can reduce consumption of traditional fuels significantly and offset carbon footprint while producing numerous usable byproducts (e.g., Class A biosolids, process heat and electricity) in a sustainable fashion. The result is an average annual cost savings of $75,000 for the WWTP using the 2PAD-IBES system. The solids facility upgrade was recognized with a States’ Clean Water Revolving Fund PISCES Performance & Innovation Award in 2008 by the EPA.