Bioremediation of Wastes in the Agricultural and Food Industries. Crystal A Markley

Transcription

1 Bioremediation of Wastes in the Agricultural and Food Industries Crystal A Markley

2 Abstract The agricultural industry has a large waste stream. Animal manure and food wastes contain nutrients that are more valuable to reuse than to landfill. Anaerobic digestion and composting are two bioremediation methods that stabilize the waste product, kill pathogens, and produce a nutrient-rich usable product. Each method has its set of concerns and regulations imposed on operating facilities. Although the technology is moving in the right direction the real successes are realized on an individual level. I. Introduction Waste is a necessary by-product of virtually any industry. The agricultural industry produces a large amount of waste every year. This includes animal waste as well as food residual waste. Animal wastes from farms may include bedding material and also wash down water. The food harvesting and production industry generates crop residuals, as well as pre- and post- consumer food wastes. There is a need to find ways to deal with these wastes other than by sending them to landfills. A. Methods for processing wastes There are two main methods of bioremediating agricultural waste. Both methods rely on the degradation of organic matter by microorganisms to reduce volume, moisture, and odor from volatile organics. The first method is anaerobic digestion. Digester systems consist of a vessel in which the waste is heated to a certain temperature, to allow the microorganisms to consume at an efficient rate. The other method is composting, an aerobic process where microorganisms break down the organic material in the waste when air and water are added to the waste product. The method in which this is done varies widely, from using simple composting piles to composting vessels. Yet another option is to combine one or 84

3 more composting methods, incorporating the beneficial aspects of each, to produce the desired composition of compost. B. Reasons for processing organic wastes There are several reasons for processing these wastes through bioremediation. One of the biggest reasons is the reduction in waste volume that can be accomplished through this method. Since transportation and disposal costs are based either on weight or volume, there is a need to minimize these parameters, especially to eliminate weight due to water content. Processing these wastes can reduce unpleasant odors associated with these wastes. Bioremediation also kills pathogens and conserves valuable nutrients in the waste product. These nutrients can then be used as soil amendments, rather than wasting valuable nutrients by placing them in landfills. II. Anaerobic digestion of wastes Anaerobic digestion of manure was widely investigated in the 1970 s. There was a great deal of research money given to explore this technology, however, the projects receiving funding were not well conceived, and most failed to produce any relevant results. l,2 After a twenty-year hiatus, the technology is making a comeback. New research in the area is promising, yet the industry is slow to accept it. There is still a negative feeling about digester technology left over from the funding mishap. With carefully designed studies and on-farm site demonstrations, the benefits will become visibly apparent. In April 1996, anaerobic digestion was approved as a nutrient management technology for the first time.* 85

4 A. How anaerobic digestion works Anaerobic digestion is an efficient method of breaking down the organic components of animal waste and stabilizing the non-organic nutrients. It is a system of processes that transforms an unwanted waste product into a usable source of nutrients. The process has several parts, or stages, each serving their own purpose. As animal wastes and bedding material are collected at the facility, the first stage is in the digester chamber. Here is where the greatest transformation takes place. Bacteria and microorganisms consume the organic components of the waste stream. First, acid bacteria turn the waste into organic acids; then methane bacteria produce methane from the acids. The environment must be kept at a constant 105 degrees Fahrenheit for the microorganisms to function optimally. The vessel must also be continuously stirred, to ensure proper interaction between the waste food and the bacteria. The amount of time the manure remains in the digester is termed hydraulic retention time (HRT). The HRT is a vital variable since the duration of time that the manure is allowed to react with the bacteria is critical. Once the bacteria have had ample opportunity to consume the organic portions of the waste stream, the resulting product is pumped into a clarifier chamber, where solid particles settle and are collected. Small portions of the collected matter are recycled to the digestion chamber to help initiate the digestion process. The rest of the solids stream is dried and readied for use as a nutrient supplement. Lastly, the liquid supematant is extracted from the top of the clarifier and is used to produce biogas. When biogas production is not completely efficient, the remaining supernatant becomes liquid fertilizer. The usable solids have a defined composition of nitrogen, phosphorus and potassium (N, P, K) For every ton of manure produced, 10 pounds of nitrogen, 4 pounds of phosphorus, and 8 pounds of potassium are produced. 3 The majority of weight is due to moisture, with small portions of volatile and non-volatile materials. Farmers can utilize the anaerobically digested sludge as fertilizer on 86

5 their fields. In addition, any excess nutrients that farmers can not use, can be sold for the market price of each nutrient. Nitrogen has a market value of $0.24 per pound, phosphorus has a market value of $0.22 per pound, and potassium has a market value of $0.13 per pound. With approximately 32,000 pounds of manure produced per day on an average small dairy farm (330 animals), it is clear to see that the sale of these nutrients can be a very profitable venture for farmers. Biogas is a valuable product of the digestion process. The gas that is collected from the clarification chamber is a combination of methane, carbon dioxide and water vapor. Once the carbon dioxide and water vapor are removed, the methane can be burnt and the energy used for heating as well as to power other farm operations. Thus, methane is a valuable by-product of the anaerobic digestion process. B. Issues of concern with anaerobic digestion The performance of an anaerobic digester system is related to the volume of flow through the system, and larger operations may have more success than the smaller operations. Due to the relatively high costs of constructing a complete anaerobic digester system, many average and small farmers do not have such systems on their farms. Maintenance is also an issue of concern, as this type of system requires much more labor to maintain, compared to just storing manure in a pit or manure lagoon. There are also many regulations that must be met now for installing new systems. One method of dealing with these issues can be found in communities where many farms using digesters exist in the same area. Farmers hire someone to service their digesters for a monthly fee. Several farmers in the area hire the same service person to monitor biological production and mechanical reliability of the system. This system has worked quite well for farmers in some areas. 2 Incorporating a digester system into the energy management of a farm is sometimes challenging. With this new source of energy, it is necessary to explore other methods of using the energy produced from the biogas. When a cogenerator 87

6 is used, the electricity generated can be used as a power source elsewhere on the farm. If all energy needs on the farm are met, the electricity can be sold back to the electric company for a profit. 3 Heat produced in the digestion process can be used to heat the digestion chamber itself, or to heat water used for flushing out the milk house. The final form of the nutrients is sometimes not easy to handle. The biggest problem is that no nutrient separation occurs. If the nutrients are applied to a field, the crop receives the ratio of nitrogen, phosphorus and potassium that is in the digested manure. Farmers usually apply this type of nutrient product on a nitrogen basis, as the nitrogen demand is highest for most crops. This leads to an over application of phosphorus and potassium. 4 Nutrients that cannot be used directly on the farm are transported and sold elsewhere, as far as several countries away from the point of origin. 5 Optimizing a digester system is crucial for achieving the most benefits from the technology. Methane production can be very variable if the system is not optimized for the number of animals on the farm. An optimal HRT will allow methane production to occur at the maximum capacity for the system. Because of this, anaerobic digestion systems must be optimized on an individual level for each farm system and operation. When this is done, a system can be designed to allow variability in those areas that make sense for the farmer. For example, a farmer may know ahead of time that herd size will increase in the next few years. By knowing parameters ahead of time; the system design can be enhanced. Optimization of the system would then provide for a higher tolerance in the number of animals for the system. III. Composting Composting is a technology that has been utilized for a long time by residential gardeners. Composting grass clippings and leaves is done by many homeowners and municipalities, as a method of reducing the volume of waste that is sent to landfills. This is a low technology process and is very easy to use. 88

7 A. How the composting process works Heat and moisture assist naturally occurring microorganisms in the breakdown and stabilization of organic materials. The optimal amount of moisture and heat allows this process to occur faster, but the end product is of relatively the same quality; whether the material is simply allowed to sit in a pile, or composting is assisted by the addition of heat and moisture. The energy consumed varies considerably with each composting method. There are several parameters that effect the composting process. The moisture and temperature have already been mentioned. The carbon to nitrogen ratio (C:N) is also important. Composting is much slower for materials that are either mostly carbonaceous or mostly nitrogenous. Optimum C:N ratios for composting lie between 20: 1 and 30: 1. Leaves and sawdust are good carbon sources, while food residuals are high in nitrogen. Mixing household food wastes with small amounts of leaves and grass clippings is a good combination. For one company composting fish and paper pulp residuals, a C:N ratio of 3: 1 worked we11. 6 The amount of aeration, and the amount of manipulation given to the composting material are also factors. Some systems have artificial aeration and manipulation to speed up the process. The resulting compost material is rich in nutrients and can be used in several ways. The most common uses are as soil amendments and mulch. Compost and food residuals are also being explored as animal feed amendments. B. Types of composting systems There are many different ways composting is accomplished on a large scale. Some of these methods are simply larger versions of backyard technology, while other methods have been developed for use with much larger amounts of materials. 89

8 Windrows are simply long strips of piles of composting material left to compost without much intervention. Higher energy intensive variations include forced aeration, and periodic turning of the pile. One operation in Lodi, California reports a sizable land savings and increased process efficiency by closing up space between the windrows. In-vessel systems can take many different forms. Some may be tank reactors, similar to reactors in the anaerobic digester systems. Others may be semi-formed compartments, a cube with one or more sides missing, or something similar to a row of animal stalls. Material to be composted can be treated by the batch; when one batch is done, a new batch is added to the vessel. Another method is to have a plug flow system, where material is added at one end, it travels through the vessel as it is processed, and emerges at the other end when the composting process is complete. Piles used for composting may be static piles or aerated, and may be the simplest form of composting organic materials. There is no equipment needed except for a non-permeable surface to place the pile on to reduce pollution due to runoff. In many areas regulations require these surfaces, usually made of concrete, to protect local streams from over nutrification. Some systems that use aerated static piles enclose them in buildings to control odors. Bins provide another simple option, as the size and the shape of the bins can vary considerably. Bins are treated as batch reactors in which one batch of material is composted at a time. Vermicomposting is very different from other composting methods. Vermicomposting makes use of worms to process the waste, producing compost as well as worm castings, which are an excellent soil amendment. Success stories of vermicomposting operations are numerous. Some facilities are selling worms as starters for new systems throughout the world. The variation in composting methods used can be seen in Table 1. This represents a National survey of composting facilities done in The two most popular methods are In-vessel and windrow, both in existing facilities and those in the planning stages. 90

9 Operational, Pilot/Demonstration Number of Facilities Windrow 113 In-vessel 22 Aerated static piles 14 Static piles 11 Aerated Windrow 9 Vermicomposting 8 Bins 4 Enclosed ASP 2 Combination of methods 15 Construction, Planning, Permitting Number of Facilities In-vessel 8 Windrow 4 Enclosed ASP 2 Aerated static pile 1 Static pile 1 Table 1. Composting Methods (1997 National Biocycle Survey Adapted from reference 9) C. Issues with composting operations As composting moves from a small-scale backyard operation, to a profitable large scale business, there are several issues of concern. The biggest issue is the uniformity of quality. Quality of the end-product can vary between methods and also between materials being composted. As the composting industry expands to an industrial scale, composting facilities are receiving material from the industrial food industry, including food growers and processors, as well as restaurants. There is a problem with contamination of non-organic material in food streams, including plastic packaging and cutlery coming from post-consumer food waste streams. Finding reliable markets for composted material is also a concern. When special niche markets for selling compost are lost or severely reduced, the default seems to be bagging the material for sale in small retail stores. 91

10 Industry networking is an important aspect of the composting business. It is helpful for compost facility operators to have a variety of contacts to supply them with organic material, as sometimes, residual food streams are seasonal. The seafood industry is just one example. Certain types of seafood are not harvested all the year, and so facilities receiving seafood waste must have other supplies to stay in business continuously. As the composting industry expands, there is evidence of facilities being custom designed to fit the needs of the community and the types of materials that are delivered for composting. Food residual composting projects have experienced a 280 percent growth in the last two years. One facility that began composting crab chum learned that the standard recipe was causing the compost to heat up too fast. Cooling techniques were employed to regulate the composting process. 11 There are other lessons like this concerning new ways to handle unique waste streams. Iv. Regulations and start-up programs in the composting industry There are many new regulations facing the composting industry. As the industry continues to grow, more regulations on how these businesses must operate will come into existence. There are a few concerns which regulators have in mind when passing these laws. The main concern is controlling pathogens that may appear in decomposing organic matter, and preventing the pollution of the local environment. In some states, there are several levels of compliance for composting facilities. Facilities at institutions or farms often are only required to meet minimum environmental management requirements, such as managing surface runoff and groundwater contamination. 9 Commercial facilities have other requirements to meet, such as total annual quantity limit. 9 Sometimes a clay liner is required under the composting surface, to help protect against groundwater contamination. Where odors are a problem, enclosed buildings are prescribed. Also, the minimum time between receiving material and incorporating it into the 92

11 pile is regulated by many states as 24 hours. 9 This helps keep putrification and vectors to a minimum. There are many subsidy programs available to help communities start their own cornposting programs. There is a maximum startup period during which a facility can claim subsidy money, after which it must be on its own. This transition usually means that the facility must start charging a tipping fee for haulers to dump their organic waste and food residuals. These tipping fees are regulated in most areas as well. The average per ton tipping fee in the Untied States in 1996 was thirty-two dollars. In 1995, the average fee was thirty-four dollars. 9 The state of New Jersey has some of the highest tipping fees, ranging from seventy-five to one hundred dollars per ton. Tipping fees are not well received by everyone, as some people feel that they should not have to pay to dump food residuals if the compost facility is turning around and making a profit on the compost product. The average per ton tipping fees can be seen in Figure 1 from a national survey of cornposting facilities. Per Ton Food Residual Tipping Fees 93

12 IV.. Conclusions Bioremediation of agricultural wastes is growing rapidly, as demonstrated by the growing number of new facilities in use. Anaerobic digestion is gaining popularity with new advances in optimizing each system to meet the needs of the individual farm. Cornposting has become a growing business in the past few years, as more people are making decisions to use organic wastes for their nutrient values, not just to waste them in landfills. The technology is moving in. the right direction. There is a common goal of wanting to better use the energy and nutrient value of the generated organic wastes. The real issues are being addressed; sustainability, networking, and monitoring through regulatory agencies. The real success of these ventures is measured on a case by case basis. Individual farms and individual industries are realizing large savings in disposal costs and gaining valuable nutrient sources at growing rate. As these systems are fine-tuned and optimized for the individual client, they will become even more efficient. References 1. Goldstein, N., D, Block, Nationwide Inventory of Food Residuals Composting, Biocycle Journal of Composting and Recycling, August 1997, pp Glenn, J. 300,000,000 Tons of Manure, Biocycle Journal of Composting and Recycling, January 1998, pp Riggle, D. Anaerobic Digestion Gets New Life on Farms, Biocycle Journal of Cornposting and Recycling, January 1997, pp Koelsch, R.K., et al. Anaerobic Digesters for Dairy Farms. Agricultural and Biological Engineering, Extension Bulletin, Cornell University. 94

13 5. Wagner, David, PE. Assistant Professor, Extension Specialist Precision Agriculture. Personal Communication, Department of Agricultural and Biological Engineering, Pennsylvania State University. May Glenn, J, Putting Manure to Work... Profitably, Biocycle Journal of Composting and Recycling, February 1998, pp , Ashley-Marx, A. Cornposting Fish and Paper Pulp Residuals, Biocycle Journal of Cornposting and Recycling, September 1997, pp Kayhanian, M, B. Tresan and G. Eowan Compressed Windrow Cornposting in California, Biocycle Journal of Composting and Recycling, November 1996, pp Jensen, J. Worm Farm Takes on New Challenges, Biocycle Journal of Cornposting and Recycling, January 1998, pp Glenn, J. Year End Review of Recycling and Cornposting, Biocycle Journal of Composting and Recycling, December 1997, pp Block, D, Crab Cornposter gets Claws on New Feedstocks, Biocycle Journal of Composting and Recycling, August 1997, pp