The present invention relates generally to production of biogas from wastewater treatment, and more specifically to multistage anaerobic digesters to improve production of biogas as a fuel source for combined heat and power (CHP) cogeneration.
Cities and municipalities are continually overburdened by energy, environmental, and water processing challenges. The US Department of Energy (DOE) has determined that 30% more electricity will be needed by 2025; thus, another challenge to become energy independent.
In the United States, our current power system is burdened with an increasing demand for more electricity. Moreover, the Electric Power Research Institute (EPRI) has projected in their 2003 Electricity Technology Roadmap that 7,000 GW of additional electric generation will be needed by the year 2050. The U.S. is also confronted with the ongoing conundrum of how to produce additional electricity without increasing the demand for more water, and without further contributing to greenhouse-gas emissions.
In April 2005, a Lawrence Berkeley National Laboratory Study (E.O. Lawrence Berkeley National Laboratory Study, April 2005, LBNL-57451, expressly incorporated by reference for all purposes) estimated the electricity potential from methane produced by the anaerobic digestion of wastewater biosolids from Industrial, Agriculture, and Municipal facilities. In Table 1, a segment of their Summary of Electricity Production and Emissions Reductions are shown; if the electricity were generated from fossil fueled power plants on the electricity grid. From the facilities in this segment, the Study calculated a total annual production potential of 8,900 GWH of electricity; more than the 2005 production of Hoover Dam, Glen Canyon Dam, and Shasta Dam, combined; with 3,233, 3,209, and 1,806 GWH respectively. Most importantly, this energy is readily available, without building new coal fired power plants or adding to the electricity grid infrastructure; saving untold billions of dollars.
TABLE 1Summary of Clean Energy Technologies Potential(NOTE: CO2 @ million metric tons)Electricity ProductionEmissions Reduction (metric ton)Technology(GWH/year)CO2NOxSOxHgIndustrial 3000.161996950.00WastewaterAgriculture 1,4000.829933,4780.02WastewaterMunicipal 7,2004.205,09117,8350.09WastewaterTOTAL:8,9005.186,28322,0080.11
Over a 10-year period, the above Clean Energy Technologies Potential is equivalent to removing 57,100,000 ton of CO2 from the environment and a reduction of 163,170,000 barrels of imported oil, thereby reducing foreign payments by $9,790,200,000-@ $60 per barrel.
The treatment and production of sewage sludge is the most energy intensive component in Wastewater Treatment (WWT), consuming up to 60% of the total energy requirements of a municipal WWT plant. In the United States, this equates to an annual consumption of 12.6 billion kilowatt hours of electricity, while simultaneously producing more than 10 million tons of sewage sludge. Conspicuously, the production of this sewage sludge has created a massive waste disposal, environmental, and sustainability problem.
Prior to the mid-1940's sewage sludge was neither a consideration nor an environmental problem because untreated wastewater was simply discharged directly into local waterways, carrying a heavy load of bacteria and other unwanted organisms along with it. After the mid-1940's, the WWT plants that were constructed had the ability to process, treat, and separate the sludge from raw sewage. Thus, began the era of the energy intensive production of sewage sludge and its inherent disposal, environmental, and sustainability issues.
Subsequently, rather than address the disposal problems associated with sewage sludge, many municipalities began constructing new WWT systems that employed the same old technology, rather than encourage the development of new techniques. This shortsightedness was primarily due to the availability of massive federal funding, promulgated by the 1972 Water Pollution Control Act, whose treatment infrastructure lessened the need to search for the most cost effective solution.
Recent advances have introduced newer treatment techniques: such as large-scale activated sludge systems, advanced anaerobic digestion processes that significantly enhance the breakdown of organic materials, and single-stage and multi-stage anaerobic digestion (AD) with biogas utilization for the production of combined heat and power (CHP). In spite of the incremental advances that have been made with these similar sludge treatment processes, the production of sewage sludge still remains energy intensive and the massive disposal, environmental, and sustainability problems still persists.
The CHP recovery potential at WWT plants can represent an important policy lever for sustainability. The Water Environment Research Foundation (WERF) has stated that sewage contains 10 times the energy needed to treat it. Dr. Mark Shannon, University of Illinois at Urbana-Champaign, addressing Chicago's WWT issues, has stated that harvesting methane from Chicago's sludge could yield a potential 5 mega-joules of energy from each cubic meter of wastewater treated (5,385 kilowatt hours per million gallons treated). This sludge potential has more than 12 times the energy produced with current AD processes. Accepting these authoritative energy potentials, and aware of the inherent limitations, it is unlikely that the current AD technologies will ever approach these projections without the achievement of a major breakthrough.
In California, there are 293 cities and towns with wastewater flow rates in the range of 0.1 to 5 MGD. The EPA has evaluated the current AD technologies and has established that flow rates of 5 MGD or less to be the lower economical limit for co-generation, also known as Combined Heat and Power. By transforming outdated, energy intensive wastewater treatment plants into energy producing Resource Recovery Plants, in these small cities and towns, the annualized excess electricity production could be greater than 78,000 megawatt hours. This excess electricity would be fed directly into the local grids. The overall net-energy advantage could exceed 646,000 megawatt hours of electricity. This is an unlimited, renewable energy source that equates to 80% of the U.S. 2009 net-electricity generation from Solar Thermal/PV, without adding to the electricity grid infrastructure. The net-energy advantage is also equivalent to removing more than 519,000 tons of carbon dioxide from the environment.
An energy producing Resource Recovery Plant should:                1. Reduce by more than 50% the cost to upgrade and the cost to build new WWT facilities.        2. Reduce the operational footprint by 80% (50′×50′ per MGD), and recover unused land.        3. Operate 24/7/365 indoors and provide redundancy, with modular scalability for the future.        4. Avoid sewage sludge and related costs.        5. Reduce operation and maintenance costs by 25%.        6. Eliminate current electricity costs: 2,500 kWh/MG. (kilowatt-hours per million gallons processed)        7. Produce electricity @: 1,400 kWh/MG.        8. Consume electricity @: −750 kWh/MG.        9. Sell excess electricity to local grid: 650 kWh/MG.        10. Attain a net energy advantage: 3,150 kWh/MG.                    Example: Any City, U.S.A, (˜1,000 population) processing 0.1 MGD, will realize a net energy advantage of 115,000 kWh annually.            Any City, U.S.A, (˜10,000 population) processing 1 MGD, will realize a net energy advantage of 1.15 million kWh annually.            Any City, U.S.A, (˜50,000 population) processing 5 MGD, will realize a net energy advantage of 5.75 million kWh annually.                        11. Give 15,610 WWT facilities, with flow rates of 5 MGD or less, the option to become energy positive.        12. Qualify for EPA's ENERGY STAR label for Superior Energy Efficiency.        13. Qualify for State and Federal rebates and carbon and energy credits.        
What is needed is a synergistic Resource Recovery Plant Concept designed to: 1) Filter wastewater to EPA standards; 2) Quantitatively recover the energy latent organics from the wastewater; 3) Transfer those organics to a 4-Stage Anaerobic Digester; 4) Produce and generate a maximum amount of methane and electricity—all occurring within minutes, instead of days; 5) Avoid sewage sludge; and 6) Reduce the operational footprint by 80%.