An existing AD plant with some associated infrastructure often can be used as is or with minor modifications to handle the codigestion substrate, thus minimizing capital expenditures. Codigestion systems can also function as regional digestion plants, helping to resolve waste issues for multiple generators. However, the benefits that can be realized from codigestion, as well as the potential pitfalls that can be encountered, need to be carefully evaluated. In the case of low-cost or free high-energy potential substrates, it pays to look the gift horse in the mouth. One reason for the increased interest in codigestion is the creation of numerous opportunities for the use of biodegradable wastes due to the tremendous number of AD plants online and currently being constructed in the United States. Research by Applied Technologies has estimated that there are over 600 operating systems in the industrial and agricultural fields in the U.S. handling various industrial and agricultural wastes. Figure 1 illustrates the number of constructed installations and growth in the anaerobic digestion field. Information published in February 2009 by AgStar, a program jointly sponsored by the U.S. Environmental Protection Agency, Department of Agriculture and Department of Energy estimated that there were 125 farm-scale digesters operating at commercial livestock farms handling manure in the United States.
In the public sector, many publicly owned treatment works (POTW) have for years incorporated AD processes into their overall wastewater treatment schemes to handle primarily biosolids (waste sludge from municipal wastewater treatment plants). A 2004 USEPA national survey of POTWs with a hydraulic capacity greater than 5 mgd estimated that there were nearly 1,100, of which nearly 550 utilized AD systems to handle biosolids. In terms of a single state, a Wisconsin Focus on Energy study completed in January 2006 identified 85 digesters in Wisconsin at POTWs that handle primarily biosolids, 16 of which are of 5 mgd capacity or greater. The latter two groups of AD systems, those handling manure and biosolids, probably number over 1,000 nationwide and are responsible for providing most of today’s codigestion opportunities.
Designers and operators of AD systems have a wide variety of potential substrates from which to choose when considering how to boost biogas production. In addition to the base substrate used, numerous wastes are available, many of which have been tested to benchmark fundamental characteristics. Table 1 provides a partial listing of substrates for which some biodegradability and/or biogas production data is available. Table 2 provides laboratory-scale methane yield data on a selection of substrates and the common base substrates of municipal biosolids and cattle manure. The information is provided in units of methane produced per unit dry mass (volatile solids, or VS) applied. It should be noted that data provided in the literature and from various system suppliers will be presented in many forms — biogas instead of methane, wet total solids (TS) instead of dry VS, applied versus removed substrate — requiring a great deal of evaluation before use in any comparative assessment. In addition, all substrates, e.g. food wastes, are not created equal; they possess different levels of protein, fat/oil/grease (FOG), and carbohydrates. Testing the specific waste stream is necessary to obtain a realistic idea of its biogas potential.
The variety of possible substrates and the variability in biogas potential can create some “selection” issues. However, quite often the overriding factor in the potential usefulness of many substrates is economic — the cost of obtaining, transporting and preprocessing the material to the point that it can be fed to an AD plant to obtain increased biogas production. Optimally, an AD facility should receive a tipping fee for handling the waste. Conversely, an AD facility may need to pay for and transport a codigestion substrate a long distance to the AD plant. These two scenarios bracket the range of possible economic outcomes for a codigestion application.
Unfortunately, when examining potential substrates for codigestion, most attention is paid to such characteristics as biodegradability (as measured by VS or COD destruction) and biogas production (as measured by cubic meters or cubic feet of biogas or methane per unit mass or volume applied). Other characteristics of critical importance are: Organic nitrogen; Presence of chemicals; Sulfur; Levels of K, N and other cations; pH and alkalinity; Phosphorus; Fat, oil and grease (FOG); and Gross solids.
These characteristics impact the operation and performance of an AD system. For example, knowing the level of organic nitrogen in a waste makes it possible to predict the amount of ammonia it will generate during anaerobic digestion, since nearly all organic nitrogen is converted to ammonia. One can then evaluate the impact of the ammonia on issues such as ultimate effluent discharge (e.g. discharge limits or surcharges), potential ammonia inhibition or toxicity to the anaerobic process, the economic viability of N recovery, etc.
Other characteristics can affect the choice of pretreatment process or AD technology. For example, the presence of straw, wood knots and plasticware might dictate the addition of a grinding or screening step upstream of the AD process. Similarly, high levels of FOG would favor use of an AD technology with good mixing versus those without (e.g., plug flow and anaerobic lagoon).
BLENDING AND FEEDING
Once the base substrate and supplemental codigestion substrates have been identified and are ready to be fed to the AD system, care must be exercised in blending and feeding them to minimize process upsets. For optimum AD process performance when running a codigestion system, it is advised to follow these guidelines:
- Test individual codigestate loads for characteristics of concern. At a minimum, test for COD, TS, VS, TKN and pH. If the quality of any load is suspect, segregate it and run a biomethane potential test on it. This is a laboratory test run in a sealed serum bottle with “standardized” anaerobic biomass and the substrate under investigation. The results provide an idea of the quantity and quality of biogas that can be produced.
- Store various codigestates in separate mixed and heated tanks or areas. This minimizes downstream preparation time and allows more precise blending and more careful control of organic loading.
- Develop a preferred feed ratio for the various substrates, making small changes to it as the availability of the various base substrates and/or codigestates changes. This reduces the chances of upsetting the anaerobic process through organic overloading.
- Collect full-scale system data and use it to adjust feed ratios and digester operating parameters.
East Bay Municipal Utility District in Oakland, California has been operating a program for industrial and commercial organic wastes, preprocessing and feeding them to existing biosolids anaerobic digesters to boost biogas production. The Inland Empire Utilities District in Chino, California handles dairy cattle manure and food processing wastes in a thermophilic digestion system designed and constructed specifically for codigestion. A number of POTWs in California manage programs that accept FOG-type wastes and feed them to existing biosolids digesters.
In Wisconsin, the availability of dairy production wastes from a number of small dairy operations has helped develop codigestion. For years, the Madison Metropolitan Sewerage District accepted cheese whey from a local dairy and fed it directly to existing biosolids digesters. Similarly, POTWs in Beaver Dam, Sheboygan, South Milwaukee and Waupun have accepted dairy and/or other wastes and used them in existing biosolids digesters to boost biogas production. In a unique codigestion application, the Milwaukee Metropolitan Sewerage District has been handling spent deicing fluid from Mitchell International Airport at its South Shore treatment plant since 2000, codigesting it with biosolids.
Companies employ codigestion in the private sector as well. Unilever in Maryland has codigested ice cream novelty production wastewater and waste product anaerobically since 1991. Microgy has three thermophilic digesters in Wisconsin, handling dairy manure along with alcohol production wastes, glycerin, FOG and other wastes. More recently, the Crave Brothers Farm in Waupun, Wisconsin doubled the size of its AD system to handle additional dairy manure, milking parlor wastewater, cheese production wastewater and cheese whey.
Rising interest in codigestion and an increasing number of operating plants are pushing the regulatory community with regards to permitting. POTWs that operate codigestion systems are perhaps best suited to deal with regulations, as they already have NPDES permits and most likely have dealt with air and solid waste permitting. Many POTWs are familiar with handling trucked-in wastes, such as septage, landfill leachate, grease-trap pumpout, etc., and have systems in place for record keeping, storage and handling. However, the potential impact of nutrients from codigestion of imported substrates may affect NPDES permit compliance efforts and needs to be carefully evaluated.
In the agricultural arena, CAFOs are strictly permitted and already have some authority to handle and store manure. However, the inclusion of nonmanure substrates can introduce solid waste and/or wastewater regulations. Codigestion of substrates with high nitrogen and/or phosphorus levels could impact comprehensive nutrient management plans. A presentation given by Joe Goicochea of the Ohio Environmental Protection Agency at the Biocycle 2008 Conference on Renewable Energy From Organics Recycling concluded that the classification and regulation of CAFO and on-farm codigestion systems varies significantly from state to state and can be influenced by numerous system operating variables. Multiple regulatory agencies could be involved in the permitting process, so it is advisable to begin permitting discussions early in the planning process to identify applicable permits and specific design and/or operational requirements.
Zoning issues also may enter the picture for a codigestion system, whether at a new or existing facility. For example, an on-farm digester that begins to receive shipments of glycerol or grease-trap pumpout may face the likelihood of a change in zoning classification from agricultural to industrial. As with permitting, it is recommended that zoning discussions be identified early in the planning process to identify potential issues that will need to be addressed.
In summary, the benefits of codigestion are numerous and the current availability and variety of possible substrates will generally improve the economic factors for an AD plant. Competition for the more common codigestates will increase, driving up prices and forcing facilities to consider nonstandard substrates. The search for and use of more unique substrates should be based on a careful assessment protocol to define biodegradability.
As noted, permitting and design issues are evolving as more codigestion systems are proposed and become operable. Employ an AD technology that is flexible in its ability to handle high TS/FOG substrates. If the amount of FOG codigested in an AD system exceeds 10 to 20 percent of the overall feedstock, plan on increased monitoring of system performance and higher maintenance costs. Once the AD plant is operating, collect as much data as possible, as it will be useful in making day-to-day adjustments, fine-tuning the system to achieve maximum efficiency (and return on investment) and troubleshooting problems.
Dennis Totzke, P.E., is a Vice-President at Applied Technologies, Inc. in Brookfield, Wisconsin, an engineering firm specializing in water and wastewater management.