Biodigester

Introduction:
Perceptive studies of our environment conducted throughout the years have revealed many substances that were once believed to benefit natural resources as environmentally harmful. Farms in particular represent a major source of such substances, such as fertilizer runoff, machinery exhausts, even flatulent from cows; farmers may grow disgusted with such damage, yet they are forced to use such compounds due to their cheap costs. However, biodigesters may provide a natural fuel, both produced on the farm and producing energy for the farm, thus reducing costs and environmental detriment. Throughout the duration of ENSC 185, 5 students of the University of Vermont examined the possibility of incorporating biodigesters into the nearby Miller Farm, specifically a biodigester that produces enough renewable energy to offset the cost of heating and electricity. We eventually determined the most appropriate type of biodigester; however, such a conclusion requires many considerations regarding the particular factors of Miller Farm as well as the nature of the chemical processes. With this report, one may successfully generate a large percentage of a farm's energy from the processes of a biodigester, while also maintaining healthy crops from the byproducts of this new technology.

Anaerobic Digestion:
Before examining the construction of a particular biodigester, one must understand the various workings at play. The digestion process must only contain anaerobic bacteria, yet manure inherently contains oxygen. Before anaerobic digestion can begin, one must load the digester and allow for a sufficient period of time before anaerobic bacteria become the dominant culture and begin to digest the waste. However, one must never open the system during this period; despite the production of CO2 during aerobic digestion, a culture of anaerobic bacteria will eventually grow if the CO2 does not contact foreign compounds which may alter carbon dioxide into another oxygen-containing compound, delaying anaerobic digestion. Such chemical alterations would prevent the very production of methane. Anaerobic digestion begins with the first stage known as liquefaction, where digestible organic substances such as fat and proteins are simplified into volatile acids. These acids are derived from carbon dioxide, most importantly acetic acid which represents the most significant contribution to the production of CH4, methane. Yet, further consideration must be taken to assure that such chemical reaction continue completely; a slight change in temperature can result in drastic alterations.

Temperature:
Although anaerobic digestion can endure a wide range of temperatures, the bacteria are most productive at two sets of temperatures. Mesophiles consume at a rate which is most favorable to the chemical reaction at a temperature between 20-400 C while thermophiles flourish at temperatures of 40-600 C. Such high levels of kinetic energy hasten the conversion of acetic acid to methane; however the particles and electron transitions suddenly become quite fragile movements, making even the slightest additional rise in temperature a spark for a completely different product. Were one to conduct digestion at these high temperatures, one must make sure pressure the temperature and remain constant. Regions that endure high variability like Vermont can maintain constant temperature and pressure only with proper insulation, adequate protection from surroundings and durable material. Many projects have constructed digesters within an underground pit that contains a thin layer of cement around the walls, insulating and protecting the substrate from outside forces despite potential presence of cracks or openings around the digester wall. If the thermogenic chemical reaction continues completely without alterations, a higher percentage of pathogenic organisms can be eradicated, thus a conducive fertilizer produced as a byproduct. Such an attempt into self-sufficiency requires extensive efforts, yet such efforts can never begin without considering the size of the digester and loading rate required to fill whatever determined size.

Size:
While temperature and pressure regulate the amount of methane produced during digestion, the size of digester determines the maximum amount of biogas possible as well as the rate at which digestion occurs. Although Miller Farm appears as a small farm compared to most Vermont properties, the total livestock and multiple waste sources create the need for a relatively large volume. Yet, this does not suggest that vast amounts of methane may provide power in a short time; large amounts of waste must be divided into multiple loading periods which require long detention times to allow complete digestion and sterilization. When determining an appropriate size for a biodigester at Miller Farm, we researched the various sizes of biodigesters proven successful in multiple cases. Most were constructed for smaller farms with less livestock in tropical climates, thus permitting a digester with a maximum volume of only 200 cubic meters. Also, a tropical climate combined with livestock different than those in Miller Farm suggest the substrate contains different solids as well as different proportions of solids to liquids than the influent of Miller Farm. Determining an appropriate size of digester for Miller Farm requires a consideration of all such factors, leading our group to previous studies by UVM which analyzed Miller Farm. During studies conducted in 2009, the biodigester produced an annual volume of 2,650 cubic meters of waste which consisted of cow and horse manure, bedding, “post-consumer food waste” and yard waste. Such a diverse composition that accumulates into large volumes demands a tank that can digest a large combination of solids and liquids in one load, such as a Complete Mix Digester. Amongst the various forms of biodigesters, the “Complete Mix” digester exhibits the most potential for Miller Farm as the tank can decompose a wide range of wastes, thus acquiring the maximum amount of methane from each single load. However, the loading rate must extend further than other forms of digesters; with higher proportions of solids to liquids, the aerobic bacteria require longer durations to extract oxygen from stronger intermolecular bonds.

Loading Rate:
Previous studies conducted by the environmental engineering company, “Forcier, Aldrich & Associates” indicated minimum detention times of 40-50 days for digesters capable of dissolving such high proportions of solids. Therefore, each refill period performed every 40 days throughout a year would input a minimum of 290 cubic meters, assuming the total volume of Miller Farm waste remains relatively constant. Unfortunately, the total 500 cubic meter volume digester would not fill completely with this estimated volume, resulting in a level of methane much lower than the optimum capacity to supply sufficient levels of electricity and heat. Such efficiency may come to fruition with a digester that allows for longer detention times; with more time to permit digestion, the digester can contain greater volumes of waste, as well as greater percentages of solids. Previous studies conducted by “Forcier, Aldrich & Associates” analyzed this particular technology within a digester from MWK BIOGAS. With a detention time of 80-120 days, Miller Farm may accumulate a minimum of 580.8 cubic meters every 80 days, assuming the total volume of waste exhibits little inter-year and intra-year fluctuations. Furthermore, the MWK BIOGAS system converts a percentage of solids within influent from 12% to 4-6%, indicating efficient destruction of parasites and decomposition of acids which convert to methane. Yet, this superior efficiency does not conclude our search for the appropriate type of digester; with an insightful comprehension of digester processes, we may now determine which type of digester may exhibit most efficiency at Miller Farm.

Biodigester Classification:
A Plug flow digester is a long narrow insulated tank made of concrete, steel or fiberglass with a glass tight cover to collect biogas. Because of the absence of a mixing pit, it cannot include many solids such as bedding, and absolutely no sand. In fact, only 11-14% of the influent may contain solids, thus allowing for a short retention rate of only 15 to 20 days. This combination of few solids and absence of mixing pit allows one to move the substrate by simply adding more influent; the pressure from gradually adding new influent forces the old substrate further along through the digester, into the product storage tank. This process can operate at mesophilic or thermophilic temperatures. As explained previously, mesophilic temperatures maintain greater stability as the lower temperatures can endure minor surrounding influences without altering a reaction. Although the extreme temperature of thermophilic cultures destroy more pathogens – thus converting the product into potential fertilizer or bedding – Miller Farm does not produce enough waste to make such byproducts feasible. When the waste finally reaches the outlet, it discharges over a weir – or a small dam – that maintains the gas tight atmosphere but still lets effluent flow out. The biogas then created can be used for numerous functions such as heating nearby shelters, running the engine generator, heating the floor, heating water or steam production. This can offset the cost of purchased electricity, propane, natural gas or oil used on farm operations, however Miller Farm conveys factors inherent of a small-scale farm which diminish the net gain from a plug flow system.

This type of biodigester would not be the best choice for the Miller Farm. Plug flow systems are most successful on large dairy farms, where the manure is collected by scraping in order to obtain solid waste. The Miller farm is a small dairy facility with 34 milking cows. A plug flow system needs a large amount of input due to its short retention time and lack of mixing. With small plug flows the rate of input is higher however, the retention time is shorter. With a short retention time, the efficiency of gas production is lowered and therefore, limits energy potential. A plug flow system also requires time, which is valuable on the farm; system components must be checked, input and output data must be reviewed, along with temperature and pressure readings. Since the manure is not being mixed, the plug flow can easily clog or short circuit, especially in smaller versions where the rate of input is faster. This type of biodigester also requires a pre-heating tank because the tank is not insulated. The warmer the temperature the more biogas is created, consequently the more energy needed to heat the tanks, especially during the cold winters of Vermont. On a large dairy farm this biodigester would be ideal; however Miller Farm requires different specifications. Yet, there remains another type of biodigester which exhibits greater benefits: the Complete Mix system.

Complete Mix digesters are insulated and contained in constant elevated temperature in Mesophilic or Thermophilic range and contains a gas tight cover for biogas. This cover appears above or below ground depending on the consistency of feedstock. The contents of the digester are circulated through an external heat exchanger to maintain constant temperature; due to Vermont’s varying climate, such insulation would prove vital to the digestion process. It can be mixed with a motor driven mixer a liquid recirculation pump or compressed biogas. 3 to 10 percent of the manure is total solid and the system has a retention rate of 10 to 20 days. A dairy farm such as our studied property can use flush manure management, which separates the methane storage from the digesting influent. With a separate storage for methane, we can include a larger gas storage tank without decreasing the influent storage as they will not occupy the same particular area. Continuous flow is part of the complete mix digester family. A continuous flow unit adds and removes waster daily and is best suited for small-scale farms.

The continuous flow digester is another potential biodigester that could be used on the miller farm. This system adds and removes waste material on a regular or daily basis, and is best suited for small-scale purposes. The animal waste moves through the system either by force from the new feed or mechanically. The end products are constantly removed, preventing the addition of excess levels of influent which can alter the temperature, pressure and therefore, anaerobic cultures. As stated earlier, their digestion of substrate requires long periods of retention, usually about 8 weeks at warm temperatures. With a continuous flow digester, however, the gas production can accelerate due to continuous, daily additions of influent. However, there remains some concern with a continuous flow digester; the daily additions of influent not only suggest extensive care and supervision, but also decrease the volumes of added influent – so small that the methane produced may not reach levels required to produce sufficient power. With enough available space, the development of a second digester may solve this dilemma. Regardless, the fluid flow of substrate also establish this type of digester as a potential choice; by gradually adding new influent into the digester, the old substrate overflow at the top and into a second chamber where the digestion process continues to a better degree of completion. Such characteristics distinguish the continuous flow digester as an efficient option, however the amount of methane produced from each loading period may not satisfy Miller Farm’s demands for power within a given time. Therefore, we determined the complete mix system as the most efficient choice, specifically the MWK BIOGAS system studied in 2008.

The MWK-BIOGAS Complete Mix System is a two-stage, in-ground concrete system which uses two fermenters (primary and secondary) to break the substrate down into methane and effluent. This system functions on a long detention time, separate gas bladder storage, fully automated controls, and several other features. The data available on this system is broken up into costs, energy potential, and use of effluent, recommended inputs, manure collection, and crop and food waste.

The MWK-BIOGAS system has two separate costs for two different arrangements which yield different power outputs. The first is quoted at $400,000 with an estimated energy output of 40 kW. The second is $800,000 and 100 kW.

This energy output is possible because of several factors: The MWK-BIOGAS system features a series of two fermentation tanks, with contained complete mixers which allows for complete digestion of the substrate due to the frequent (multi-daily) agitation of the mixed waste and its essential bacteria. The system allows for the complete fermentation of the substrate by detaining it for a long period of time (80-120 days), which is made possible by use of multiple digestion tanks. As the waste is digested, the gas that is released is stored in a separate bladder, allowing for optimal conditions within the fermentation tanks while not wasting energy by burning excess gas. Once the gas is stored it is put through a combination heat exchanger and methane generator which captures both the thermal and electrical energy contained in the gas.

The MWK-BIOGAS system’s use of substrate is good, as indicated by its relatively low (4%-6%) solids in the effluent. This low percent of solids in the effluent is an indicator of the system’s efficiency because very little of the substrate is left in solid form after it is digested. Once the system is finished with a substrate, the effluent is stored in a separate concrete lagoon which prevents leaching to the surrounding environment. Biodigester effluent has been dehydrated (~30% solids) and used for bedding and fertilizer in the past, but in this case the effluent has such a low solid content that this utility of effluent is not economically practical. The types of inputs recommended for this system cover the board. Liquid manure is a possible substrate, and is most often used when the digester is on-site for manure collection, although some systems do exist where manure is used and a portion or all of it is shipped in via pump truck. Mixed crop substances can be used as a substrate or substrate additive and is often shipped in, although it is possible to use on-site crop waste if the digester is located on a combination animal and plant crop farm or farms. Pre and post-consumer food waste can also be used and is most often shipped in by a municipal or private company. The food waste that is digested may not have the need for pasteurization before external use of effluent, because of the extended detention time which results in pathogen removal.

This system can feature collection areas for all three types of substrates. Manure is collected in a manure receiving pit/wet well which features a grinder pump or gravity feed and a flow control valve. The crop waste is collected in a crop receiving pit or container and stored in bunker silos. This substrate requires screw pumps and augers to drive the material into the digestion tanks, and features fully automated controls. The food waste is received in a container of some kind, and needs to be screened and ground before it can be fed into the digester via screw pumps or screw conveyor. This also features automated controls. Once every characteristic was considered, we determined the MWK BIOGAS Complete Mix system as the most efficient digester, producing the most heat and methane that can offset Miller Farm’s electricity and heat costs by the greatest amounts.

Reference Page
Burns, R. T. Animal Waste: Anaerobic Digester Basics [PDF Document]. Retrieved from Iowa State University Agricultural and Biosystems Engineering Department: http://www.chpcentermw.org/presentations/041103-IA/burns.pdf

Darling, David. (5 July 2000). Anaerobic Digester. Retrieved from http://www.daviddarling.info/encyclopedia/A/AE_anaerobic_digester.html

Doran, Elizabeth. (7 June 2010). Biogas Production. Retrieved from http://www.habmigern2003.info/biogas/methane-digester.html

Forcier, Aldrich. (April 2009). Miller Farm Anaerobic Digestion Study Phase I, for 2009 Dairy Herd. Retrieved from: University of Vermont database

Forcier, Aldrich. (February 2010). Miller Farm Anaerobic Digestion Study Phase II, for Smaller 2010 herd plus Campus Organics. Retrieved from: University of Vermont database.

by: Evan Cuttitta, Caitlin Broman, Chloe Coggins, Marcus Wilkinson, Shannan Webb