From Brown to Green- Reducing Carbon Footprint via Biogas Cogeneration in a Phased Digestion Process Producing Class A Biosolids Sudhakar Viswanathan * Ky Dangtran - Degremont Technologies Sandra Diorka - Delhi Charter Township Thomas Grant - Hubbell, Roth & Clark, Inc Kevin S. Livingston - HESCO Sustainable Energy LLC * Degremont Technologies - Infilco, 8007 Discovery Drive Richmond, VA - 23229 U.S.A ABSTRACT Soaring energy costs have made it imperative to reduce consumption of traditional fossil fuels. 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 from anaerobic digesters to generate process heat and in some case electricity using cogeneration mechanisms have made anaerobic digestion an attractive treatment choice for many small to medium size facilities. Delhi Charter Township Wastewater Treatment Plant (WWTP) located in Ingham County, Michigan has established the states first integrated biomass-to-energy digester system. The plant upgrade included the implementation of a two-phase anaerobic digestion system with a fully integrated state-of-the-art microturbine based cogeneration system. The two-phase anaerobic digestion system is designed to treat primary solids and waste activated sludge to produce Class A biosolids and the microturbine system to capture, treat and utilize biogas to produce heat and electricity. This integrated biomass-to-energy digestion and cogeneration system was designed to help reduce energy consumption in two folds, first, by reducing the volume of sludge to be hauled to the final disposal location by minimizing fuel associated with the transporting of biosolids and second, by minimizing the use of natural gas and electricity usage by locally generating heat for process heating and electricity for electrical operation. At current capacity, the facility is able to reduce electricity consumption by over 40 percent and close to 100 percent of the process heat. This essentially sustains the operation of the solids handing system. The plant demonstrates that wastewater treatment facilities can be designed to significantly reduce consumption of traditional fuels and offset carbon footprint while producing numerous usable bi-products such as Class A biosolids, electricity and process heat. The WWTP is projected to be fully energy independent when the solids treatment facility reaches design capacity. KEYWORDS Sustainability, Anaerobic Digestion, Carbon Footprint, Cogeneration, Biosolids, Class A
INTRODUCTION Numerous 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 paper attempts to summarize efforts by one such facility that is utilizing a two-phase anaerobic digestion process to improve the quality of sludge to be certified as Class A, thereby reducing cost of transportation and simultaneously offsetting energy consumed by the treatment process by utilizing biogas cogeneration to produce process heat and electricity. BACKGROUND Delhi Charter Township Wastewater Treatment Plant (WWTP) is located just south of the City of Lansing in Ingham County, Michigan, a northern state surrounded on three sides by the Great Lakes. The WWTP was built in the year 1962; upgrades were made in 1980 and more recently in 2007. The plant as it is currently operated is capable of treating a wastewater flow of 9.5 million liters per day (2.5 million gallons per day) on average and 15 million liters per day (4 million gallons per day) during peak flow. Preliminary treatment is accomplished through shredding and grit removal. Primary treatment is accomplished in circular settling clarifiers. Secondary treatment consists of activated sludge process followed by secondary settling. The plant also has tertiary treatment that includes a nitrification tower and polishing lagoons. Disinfection is accomplished by chlorination of the nitrification tower effluent prior to passing through the polishing lagoon. Two additional lagoons are available for storage or additional detention during high flow storm conditions. Dechlorination occurs naturally or is enhanced with sodium bisulfite treatment. Prior to the recent upgrades, the solids handling system consisted of two trains of anaerobic digesters followed by gravity thickening, with a combined capacity to treat 45,425 liters per day (12,000 gallons per day) of sludge. It was estimated that the existing capacity at that time would be severely overloaded within 5 years. In 2005, the WWTP with the aid of two local engineering companies started investigating alternatives to increase the solids handling capacity to 113,562 liters per day (30,000 gallons per day). Construction started in 2007 and the contract was completed early 2009. PROJECT DESCRIPTION Objectives The objectives prior to the upgrade included the following: a. Increase solids treatment capacity from 45,425 liters per day (12,000 gallons per day) to 113,562 liters per day (30,000 gallons per day) b. Improve final product quality from Class B sludge to Class A and c. Reduce fossil fuel consumption in such as natural gas and gasoline During the course of the investigation a fourth objective was included that required the reduction of electricity usage within plant to the point of becoming self sustaining when the
anaerobic digestion system reached design capacity. Additionally, the plant designated the landscape around the facility as a no-mow habitat to further reduce fossil fuel usage. Treatment Options The initial investigation led to the following three possible treatment alternatives for the solids handling system: a. Do nothing, continue operating the plant with existing infrastructure b. Add two new conventional anaerobic mesophilic digesters to work in conjunction of the existing digesters; the new digesters will handle the increase in waste volume over the next 20 years and continue producing Class B sludge c. Add a new two-phase anaerobic digestion system independent of the existing digesters; the new two-phase sysem will treat all of the waste volume produced within the plant and will produce Class A biosolids Since doing nothing was not a viable sustainable option, it was dropped at the end of the first stage of evaluation. Of the remaining two treatment alternatives, the two-phase anaerobic digestion system offered numerous advantages over the conventional digestion system, more notably: a. Production of Class A biosolids, resulting in less distance travelled by end product for final disposal and unrestricted land application b. Increase volatile solids reduction, resulting in more biogas production for cogeneration c. Smaller footprint Finally, the net present cost of implementing the two-phase anaerobic digestion system with cogeneration trumped the conventional mesophilic treatment option. Upgrades Implemented Delhi Charter Township staff chose to upgrade the treatment system using an advanced, highrate, two-phase anaerobic digestion process. The facility upgrade included the implementation of two (2) parallel treatment trains of two-phased anaerobic digestion (2PAD) system by Degremont Technologies Infilco. These trains are capable of processing 113,562 liters per day (30,000 gallons per day) of sludge feed. The 2PAD system has been awarded a conditional National Process To Further Reduce Pathogens (PFRP) Equivalency by the United States Environment Protection Agency (US EPA). As a result the 2PAD system can produce Class A biosolids (Ferran B et al 2001). The system requires the design be per guidelines set by US EPA Pathogen Equivalency Committee (PEC). The conditional approval requires the monitoring of two parameters, namely Enteric Virus and Helminth Ova in addition to the parameters required for Class A biosolids certification. Since there are less restrictions as to where Class A sludge can be land applied, the solids generated from this plant can be trucked wet and delivered as an alternative to fertilizers, applied wet, offering the wastewater treatment facility an alternative source of income.
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 (2) 30 kwh turbines, with room for two (2) similar sized turbines to be added as the capacity of the plant increases. The multiple turbine system offers the following benefit over a single turbine system, such as: a. Flexibility in operation; control turbine usage to produce electricity predominately during the day to offset demand charges applied b. Optimize gas utilization; allowing the turbines to turn down or turn off during the night and when heating loads are low c. Flexibility in expansion; by addition of similar cassettes of microturbine skids The ultimate goal was to maximize heat and electricity recovery and minimize dependence on the grid and fossil fuel to power the facility. Process Description The two-phase anaerobic digestion process (figure 1) involves the treatment of sewage sludge in the absence of air in an acidogenic thermophilic reactor followed by a methanogenic mesophilic reactor. The mean cell residence time in the thermophilic reactor is 2 days followed by 10.5 days in the mesophilic reactor. Figure 1 - Schematic of two-phase anaerobic digestion system with cogeneration Combined Primary and WAS sludge with average total solids concentration of 4 percent is feed to each of the two thermophilic digesters in an intermittent draw and fill approach with a feed sequence occurring once every 8 hours (three cycles per day). The mesophilic reactor is fed from the acidogenic reactor (Farrell J.B., et al 1988) prior to feeding sludge from feed sequencing tank to the acidogenic reactor. This ensures no untreated sludge is discharged from the digestion system. There are time-temperature requirements that apply between two consecutive feedings. The alternative 6 classified time-temperature criteria
followed by this phased digestion process requires that the temperature inside the acidogenic thermophilic reactor is maintained at 55 o C (131 o F) for a duration of at least 3 hours within each feed sequence, in other words there needs to be provisions for a 3 hour window in every 8 hours batch where the temperature of the contents within the thermophilic digester is at or above 55 o C. Temperature inside the mesophilic reactor is maintained at 37 o C (98.6 o F) at all times. The 2PAD system uses a series of heat recovery, heat addition and supplemental cooling heat exchanger to recover, maintain and cool various sludge streams. The system design takes into consideration the interdependency of the various sludge streams for heating. Additionally the proprietary design has optimized hydraulic loading and feed sequencing that helps improve the overall thermodynamic efficiency of the system. Each digester is equipped with Cannon Mixer mixing system that will positively and continuously mix the entire contents of the digester. The variation in total solids throughout each digester is maintained at less than 10 percent standard deviation from the mean total solids for each digester, which is a PFRP requirement. Similarly, the sludge temperatures is not allowed to vary more than 0.5 o C (~1 o F) standard deviation from the mean digester temperature. The mixing system employed maintains an active digester volume in excess of 90 percent. This was demonstrated to be the case during performance testing undertaken at digester start up and included lithium chloride traces dispersion and temperature profile studies on one thermophilic and one mesophilic digester. The volatile solids loading into the two-phase digestion system is designed to be at levels greater than conventional anaerobic digesters. The thermophilic digester is operated at volatile solids loading rates greater than 12 kg/m 3.d (0.75 lbs/ft 3.d) with the overall volatile solids loading rate across the two-phase digester system maintained at levels greater than 2 kg/m 3.d (0.125 lbs/ft 3.d). The high volatile solid loadings and the high temperature in thermophilic digester leads to high rate acid formation suppressing ph within the digester; typically the thermophilic digester is operated at a ph of 5.5. The acids are consumed in the mesophilic digester where the sludge is restored to neutral ph. The separation of acid and gas phase of the digesters improves the volatile solids reduction (VSR) ensuring greater biogas production (Ghosh S. et al. 1997). The separation of each of the phases into its own reactor improves the efficiency of each respective reaction thus improving overall efficiency of the digestion process. The first phase acidogenic reactor is less sensitive to feed quality changes, allowing for the daily variation of sludge strength within the three batches. High volatile solids reduction across the 2PAD system at Delhi Charter Township results in high biogas yield. This integrated biomass-to-energy system (IBES) includes microturbines (figure 2) designed to operate as two independent units and when required in conjunction with one another. The biogas is captured and stored in the floating gasholder 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 remove impurities such as siloxane and corrosive agents such as hydrogen sulfide.
The microturbines are designed to operate with a turndown of up to 50 percent, which allows the two turbines to operate in the range of 25 to 100 percent of the combined operable capacity producing 15 to 60 kwh of electricity-on-demand. Since the electrical efficiency of microturbines of the scale used is around 30 percent, the majority of the energy generated from the cogeneration system is in the form of heat. The microturbine exhaust gas existing the system is at around 260 o C (500 o F), this 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 which is normally fired in the event the microturbines are not in operation and heat demand is sensed in the digesters. 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. Figure 2 - View of 2PAD-IBES plant with detail of Cogeneration System The electricity generated in used to operate pumps, compressors and other ancillaries related to the operation of the two-phase anaerobic digestion system. In addition to the upgrades to the solids handling system, the township has adopted an unconventional approach to manage the landscape around the treatment facility. The WWTP is maintaining the lawn in the facility by allow facility owned sheep and a lama to graze the fields. This switch to a green solution has significant reduced the carbon footprint of the facility by lowering fossil fuel usage in lawn mowers. RESULTS The two-phase anaerobic digestion system has been in continuous operation for a year now. The average sludge flow is at initial design conditions with average sludge flow of around 45,425 liters per day (12,000 gallons per day) fed into the system (figure 3) each day. Economic conditions have changed in Michigan since the time of design, which have slowed down commercial and residential development in Ingham County resulting in significantly lower amount of solids being fed to the digester. As such the plant is operating in a mode where the biogas is fed directly to a boiler to supply the heating needs of the digestion
system; excess biogas is then fed to the microturbine system where on average about 40 kwh are produced on a consistent basis. The main goals of the project of producing Class A biosolids are being met, but the electrical production from the microturbines have been lower than expected because of the reduced loading to the facility. Figure 3 - Volatile Solids Reduction within Individual 2PAD Trains for 1 st Quarter 2010 Delhi, MI - VSR: 2PAD Trains (3 Months) 80 35000 Volatile Solids Reduction (%) 70 60 50 40 30 20 10 0 Train 1 Train 2 Sludge Flow 30000 25000 20000 15000 10000 5000 0 01/01/10 01/08/10 01/15/10 01/22/10 01/29/10 02/05/10 02/12/10 02/19/10 02/26/10 03/05/10 03/12/10 03/19/10 03/26/10 Sludge Volume (Gallons) Date Although the volatile solids loaded into the system was considerably lower than estimated during design, the system has reliably reduced volatile solids on average to 62 percent with a combined solids retention time (SRT) in the thermophilic and mesophilic reactors of 13 days. In contrast, the conventional digester for the same period a year prior to the upgrade was on average achieving 55 percent VSR at SRT ranging from 24 to 29 days (figure 4). The resulting increase in biogas production (figure 5) has greatly helped in providing localized source of heat and electricity and offset consumption of electricity from the grid. Currently the microturbine processes only a small portion of available biogas. The two 30kWh turbines are being operated at 35 percent turndown generating a combined total of 40kWh. The electricity produced by the 2PAD-IBES process is currently capable of supporting the operation of only 40 percent of the wastewater treatment plant, but sufficient to power all of the solids handling system. The no mow strategy has also helped reduce the use of fossil fuel usage, resulting in addition cost savings and significant reduction of carbon footprint for the facility. The current method of final disposal is via land application. This is carried out by means of spreading of biosolids on farm lands and lawns within the service district of the wastewater treatment plant, further reducing fuel costs.
Figure 4 - Volatile Solids Reduction for 1 st Quarter 2009 and 1 st Quarter 2010 80 Delhi, MI - VSR: Conventional -vs- Phased Digesters (3 Months) 75 Volatile Solids Reduction (%) 70 65 60 55 50 45 40 Conventional 2PAD 1 8 15 22 29 4 11 18 25 3 10 17 24 31 Date Figure 5 Biogas Production for 1 st Quarter 2009 and 1 st Quarter 2010 Delhi, MI - Biogas: Conventional -vs- Phased Digesters (3 Months) 40 Biogas Production (cu. ft./lbs VSR) 35 30 25 20 15 10 5 0 Conventional 2PAD 1 8 15 22 29 4 11 18 25 3 10 17 24 31 Date
CONCLUSION The treatment facility demonstrates that waste treatment plants can significantly reduce consumption of traditional fuels and offset carbon footprint while producing numerous usable bi-products such as Class A biosolids, process heat and electricity in a sustainable fashion, all of which, results in an average annual cost savings of $75,000 USD for the WWTP using the 2PAD-IBES system. The reduction in fossil fuel consumption as well as the ability to produce energy from a renewable source not only offsets the net carbon footprint of the treatment facility but can also qualify for carbon credits if and when such provisions are available for this treatment district. The solids facility upgrade was recognized with a States' Clean Water Revolving Fund PISCES Performance & Innovation Award in 2008 by the US EPA. ACKNOWLEDGEMENT I would like to take this opportunity to sincerely thank my co-authors Sandra Diorka with Delhi Charter Township for her support with access to the facility; Thomas Grant with Hubbell, Roth & Clark, Inc.; Kevin S. Livingston with HESCO for providing information and access to the cogeneration system and Ky Dangtran with Degremont Technologies - Infilco for his guidance. Additionally, I would like to thank all the staff at the Delhi Township Wastewater Treatment Plant for their continued support and participation is helping make this project a great success. REFERENCES Farrell J.B., Erlap A.E., Rickabaugh J., Freedman D., Hayes S. (1988) Influence of feeding procedure on microbial reductions and performance of anaerobic digestion, J; water Pollut. Control Fed., Vol.60, 635-644 Ferran B, Huyard A. (2001) The Two-Phase Anaerobic Digestion: Application to the US- EPA/PEC for a National PFRP Equivalency. Report remitted to the US-EPA/PEC. Ghosh S., Buoy K. (1997) Two phase fermentation: an innovative approach to gasification of biosolids resources. Intern report at the University of Utah, Salt Lake city, UT. Han Y., Dague R.R. (1996) Laboratory Studies on the temperature-phased Anaerobic digestion of mixtures of primary and waste activated sludge. In Proc. 69th Annual Conference of the Water Environment Federation, Dallas, Texas. Huyard A., Ferran B., Audic J.M., Dieterlen J., Adamik J., Noel T. (1998) A challenge for the two-phase anaerobic digestion: to produce a class A biosolids and be approved as a PFRP process. In Proc. of WEFTEC 1998 conference in Orlando, Florida. Huyard A., Ferran B., Audic J.M., (1999) The two-phase anaerobic digestion process: sludge stabilization and pathogens reduction. In Proc. 1999 IAWQ specialized conference, disposal and utilization of sewage sludge. Lee K.M., Brunner C.A., Farrell J.B., Eralp A.E. (1989) Destruction of enteric bacteria and viruses during Two-phase Digestion, Journal WPCF, Vol 61, 1421-1429.