"RE-CYCLE": AN ENVIRONMENTALLY SOUND SYSTEM FOR HOG WASTE MANAGEMENT J.B. Koger, G.A. Wossink[1], B.A. Kaspers, and T. van Kempen Introduction Animal production provides significant agricultural receipts for many state economies, but the environmental impact of animal industries is coming under increasing scrutiny. In North Carolina, hog production accounted for $1.3 billion in cash receipts or nearly 20% of the 1999 agricultural receipts. However, the nearly 10 million hogs in this state annually produce 65,000 metric tons of nitrogen, 20,000 of phosphorus, and 26,000 of potassium. The current waste management strategy of flushing waste from houses, storing it in lagoons, and ultimately applying it to dedicated spray fields has led to public outcry concerning environmental eutrophication, potential pathogen spread, and emissions of odor and ammonia. A new strategy must be developed which addresses these concerns if the industry is to remain a viable, strong contributor to the economy. “REcycle” is a closed system of integrated technologies that addresses all these identified concerns. The first component of this system is a belt-based waste harvesting system that separates the liquid from the solid waste and partially dries the fecal portion with normal ventilation air. Feces are trucked to a centralized gasification/steam reforming facility for thermal decomposition to a medium Btu gas and a sterile mineral ash. The product gas, or "syngas" as it is called, has many potential end-uses giving the entire system additional flexibility for adjusting to changing power markets. It can be converted to liquid fuels such as ethanol, by a process known as catalytic liquefaction, or it can be used to generate electricity, produce steam, or synthesize chemicals. The ash, a by-product, can be used to produce fertilizer pellets or it may possibly be processed directly into animal feed. The liquid waste, on the other hand, is then directed to an enclosed vessel for processing in a sequencing batch reactor (SBR). The SBR returns the nitrogen to the atmosphere as dinitrogen gas, which normally constitutes 80% of our atmosphere. The treated effluent can then be used as irrigation water. These technologies together recover nutrients for use in the production cycle, yet have no open storage of waste streams or over-application of nutrients to a limited land area. Thus, the "REcycle" system eliminates lagoons, provides "green" or renewable energy, recycles minerals, and avoids environmental eutrophication. The belt-based manure separation system is the first step for the “REcycle” program. Shown schematically in Fig. 1, the system is adapted to the partially slatted pen design used by some producers so that retrofitting existing structures is possible. The belt runs beneath the slatted portion of the pen, the usual flush area, where the animals naturally dung away from the feeding and sleeping quarters in the solid floored portion of the pen. The slanted belt allows urine to drain into a long, sloped gutter that directs it into a covered storage container. Feces are dried to 50-80% dry matter (DM) during their two to three day residence time on the belt. A demonstration, belt-based housing unit for 100 animals has been installed and is the subject of another report (Kaspers et al., 2001). Figure 1. Schematic of the belt-based hog pen design. The belt-based collection system offers many advantages over the flush method. With over 50% DM, rather than the 1% typical of lagoons, transportation is no longer cost-prohibitive. Moreover, this type of waste harvesting is expected to reduce odor and ammonia emissions by approximately 80% (Aarnink, 2000). Microbial metabolism, the source of much of the odor, is inhibited by the drier condition of the waste. The separation of the urine from the feces reduces ammonia formation since the fecal microbes are not allowed to convert urea to ammonia and carbon dioxide. Ammonia emissions are further reduced by the fact that the urine is immediately directed to a closed container thereby greatly reducing its atmospheric contact. Roughly 70% of excreted nitrogen is in the urine and the fecal nitrogen compounds are slow to release ammonia, so curtailing ammonia emissions from the liquid waste stream dramatically reduces the overall ammonia release. Sequencing Batch Reactors Sequencing batch reactors are currently in use and continue to be investigated by other research groups. The characteristics of the liquid stream: 15 g/L BOD, 42 g/L COD, and 5% total suspended solids suggest that nitrification/denitrification should occur with a shorter residence time than for whole waste. The nitrogen nutrient load of this waste stream can thereby be converted to dinitrogen gas and released to the atmosphere. Use of this technology with the liquid waste eliminates its polluting potential without the sludge build-up that can occur when SBR is used with the whole waste stream. It is noteworthy, from the environmental standpoint, that the liquid waste can be successfully managed, but SBR research is not being done in this laboratory and will not be discussed further. Steam Reforming Gasification Steam reforming gasificationis the technology of choice for recovering the energy and mineral nutrients of fecal waste. Gasification is a clean technology yielding only energy and mineral ash while completely disposing of the waste with no dioxin formation and little or no odor production. A gasifier schematic, Figure 2, helps to explain the process. The feedstock is metered into the heated gasifier chamber (600-800°C) and, in the case of a fluidized bed gasifier, is mixed with a sand-like bed material that assists in heat transfer and mixing. Super heated steam helps to fluidize the bed and to thermally "crack" or decompose the organic compounds into low molecular weight gases as shown in equation 1 (Reed and Gaur, 1999): (1) CH1.4O0.6 + 0.35 O2à 0.4 CO + 0.6 H2 + 0.4 CO2 + 0.1 H2O + 0.2 C Gases and fine particulates percolate through the fluidized bed and are partially separated from one another in the disengagement zone and the subsequent cyclone (not shown). Particulates may be returned to the gasifier for further cracking. The gases are collected, cooled, and scrubbed of any entrained particulates, hydrogen sulfide, or ammonia that may be in the gas stream. The extreme temperatures guarantee the destruction of any pathogens, microbial populations, and bioactive molecules such as hormones. Figure 2. Schematic of a steam reforming gasifier (Reed and Gaur, 1999). As a thermal cracking process, gasification is distinguished from combustion and pyrolysis by the reaction temperature and the amount of oxygen available. In combustion, hydrocarbons are converted principally to carbon dioxide and water. Temperatures are in excess of 1500°C and oxygen is equal to or greater than the stoichiometric amount required for complete oxidation. Pyrolysis, by contrast, occurs at around 500°C in the absence of oxygen beyond what is available in the feedstock. It results in the formation of gases and charcoal. Gasification is intermediate between these first two processes. Biomass gasification is conducted at 600- 800°C in a low oxygen environment in order to maximize carbon conversion without fouling the gasifier. As a result of the low oxygen concentration, dioxin formation does not occur. When fluidized with super-heated steam, as opposed to air, the resulting product gas has a high hydrogen content, is not diluted with nitrogen, and thus has an energy value of 350-500 BTU per standard cubic foot (a medium energy gas, similar to natural gas). These characteristics offer advantages for flexibility in end-use. Hog waste offers many advantages as a source of renewable energy. In the first place, it is continuously available rather than being seasonally limited, as is the case with crop sources. Being a waste material, it is a low to zero cost feedstock. The proximate and ultimate analyses of the fecal waste are shown in Table 1. The energy content of 14.3 BTU/g (6,500 BTU/lb) and the 45% carbon content suggest an excellent feedstock for thermal cracking processes. Moreover, hog fecal waste has a low amount of nitrogen and sulfur, minimizing concerns with NOx and SOx emissions. The low chlorine value is also important for avoiding agglomeration within the gasifier. Indeed, analysis of the bed material at the end of each of four gasification trials showed no formation of agglomerates. Table 1. Proximate and Ultimate Analyses of Hog Feces Weight Per Cent As Received Dry Basis Proximate Analysis Moisture 23.2 ------- Ash 9.4 12.2 Volatile Matter 57.4 74.8 Fixed Carbon 10.0 13.0 HHV (Btu/g) 14.3 18.6 Ultimate Analysis Moisture 23.2 ------- Carbon 34.6 45.0 Hydrogen 5.3 6.9 Nitrogen 3.1 4.0 Sulfur 0.3 0.4 Ash 9.4 12.2 Chlorine 0.2 0.3 Oxygen (by difference) 24.0 31.2 Mineral analysis of swine feces by inductively coupled plasma spectroscopy (ICP) suggests that the ash by-product of gasification could be useful as a feed additive. Results of the analysis are shown in Table 2. The calcium to phosphorus ratio of 1.5 is adequate when the phosphorus is highly bioavailable, as is expected to be the case in an ash material. This ratio promotes more efficient utilization of phosphorus (National Research Council, 1998). Ash from the trials conducted to date fails to demonstrate such a favorable composition. Contamination of the ash, presumably by the magnesium oxide bed material and the non-refractory lined test unit, results in an ash containing more magnesium than was supplied in the feedstock. The trials were run, however, in a very small test unit (0.5 lb/h) with no cyclone to remove particulates from the ash.