It is a challenging time to be in the water and wastewater industry. Much of the water and wastewater infrastructure in the U.S. is reaching or exceeding its design life, and municipalities are stretching their budgets to replace outdated equipment. Concerns over nitrogen and phosphorus discharges have also driven new regulations that require more robust treatment. If these two factors were not enough to challenge any budget, the price to operate these facilities has increased significantly as energy prices increase across the board.
In the past, the cost of operating aerators, pumps and digesters was acceptable. Energy was relatively inexpensive, and the focus of engineers and operators alike was to meet their permit requirements. The permit is still the primary focus today, but municipalities are now taking a more stringent look at their energy costs. Energy audits are performed at facilities to improve pump efficiency, minimize waste and improve the bottom line. In addition to improving operating efficiency, some wastewater treatment plants have a potential source of energy offset that they may currently be wasting.
It was reported by the U.S. Environmental Protection Agency in the December 2006 report titled “Opportunities for and Benefits of Combined Heat and Power at Wastewater Treatment Facilities” that approximately 20% of wastewater treatment plants with a flow rate greater than or equal to 5 mgd with anaerobic digesters utilize gas for energy or combined heat. Faced with challenging electrical costs and budget requirements, potential exists to see this percentage increase over the next two to four years. In the past, methane gas produced from digesters was viewed as a nuisance or necessary byproduct of sludge digestion. This attitude may change with the opportunity to offset energy costs by 40% to 70%.
Biosolids-to-energy is a topic being discussed with increasing interest. Wastewater treatment facilities with anaerobic digestion have the opportunity to convert waste gas to electricity and combined heat for use on their own plant. The application of gas-reciprocating engines to offset energy costs can be beneficial for plant owners and represent a paradigm shift from the previous use of digester gas. Gas-reciprocating engines currently provide the best combination of electrical and thermal efficiency in combination with lower capital costs for installation.
The capital cost per kilowatt produced by reciprocating engines combined with the typical operation and maintenance costs allows for reasonable project payback periods without dependence on extensive incentives or financial assistance. One of the primary factors used to evaluate project economic feasibility is the cost of electricity for the treatment plant.
Additionally, in determining the feasibility of gas-reciprocating engines, several factors should be considered. First, the gas quantity—or flowrate—will be proportional to the energy produced from the engines. Realistically, a wastewater flow rate of 8.5 mgd or greater with typical gas production is required for a project to be economically and technically feasible. The determining factor will be the volumetric flow (scfm) of gas produced by the digester. Second, the quality of the gas can determine the fate of the biosolids-to-energy project. Digester gas consists of approximately two-thirds methane and one-third carbon dioxide, with the presence of some constituents in trace concentrations. Digester gas will typically have a low heating value in the range of 600 to 650 Btu/scf.
Gas constituents can be determined from a detailed gas analysis. Two constituents of concern—hydrogen sulfides and siloxanes—should also be provided on the analysis. Elevated presence of these contaminants will force designers and owners to choose between increased gas pretreatment or increased maintenance intervals for the engine. One may choose to install more robust gas cleaning equipment, which will increase initial capital costs but reduce maintenance costs for the life of the engine. On the other hand, one may choose to minimize gas cleaning and experience lower capital costs, but experience shorter maintenance intervals and increased maintenance costs.
Because gas-reciprocating engines are combustion driven, air contaminants such as NOx and CO will be produced and discharged to the environment. The installation of an engine requires an air permit to be installed. Obtaining an air permit should be completed before selection of an engine. Gas reciprocating engines may easily achieve 1.1 g NOx/bhp. The specialized lean engines may obtain levels as low as 0.6 g NOx/bhp or lower.
Modern reciprocating gas engines have electrical efficiencies from 35% to 41% running on digester gas. This electrical efficiency determines the amount of energy costs offset at a treatment facility. Additionally, the heat from these engines can be collected, allowing the units to achieve combined electrical and thermal efficiencies in the range of 80% to 88%. The heat can then be used to heat the anaerobic digesters and additional plant processes. Compared to other common biosolids-to-energy applications, reciprocating engine efficiencies are higher and suffer less derate when operating at less than a full load or higher ambient temperatures. Gas engines are robust and can handle dirty gases with wide fluctuations in heating values. Engine availability through a calendar year can be as high as 95%. Some engines also allow mixing of natural gas “on the fly” to supplement gas flows during low-volume or low-heat-value gas flows.
Gas engines come in a variety of sizes based on electrical production. The selection of an engine is based on determining the engine size that will operate at full load during average gas flow. The designer is looking to maximize the power and combined heat from the engine, so a wide range of engine sizes is desirable to accommodate gas volume.
Engines are available as stand-alone units or containerized units. Stand-alone models are required to be installed in an enclosed, engineered structure. Container models are plug-and-play, with all of the controls, ducts, engine and generator in one box. Container units only require a slab to be installed. Containers can be sited outside and require only a gas line into the unit and a line taking electricity away from the unit.
The solution to rising operating costs at wastewater treatment facilities will not be provided by one action or one piece of equipment. Improving energy efficiency will be a process of many actions within the plant; however, offsetting energy costs from a range of 40% to 70% is a large contribution to establishing energy efficiency. The application of gas-reciprocating engines can provide long-term savings with a relative short-term payback.