ALTERNATIVE WASTE MANAGEMENT SYSTEM WITH METHANE COLLECTION
C.B. Fedler, PhD; Civil Engineering, Texas Tech University; Lubbock, TX 79409-1023
N.C. Parker, PhD; U.S. Geological Survey-Biological Resources Division, Texas Cooperative Fish and Wildlife Research Unit 1, Texas Tech University; Lubbock, TX 9409-21201jointly sponsored by Texas parks and Wildlife Department, Texas Tech University, The Wildlife Management Institute, and U.S. Geological Survey-Biological Resources Division.
Treatment of wastewater to a level suitable for reuse does not require the use of exclusively mechanical systems. Acceptable levels of treatment can be achieved using existing technologies related to natural wastewater treatment systems when appropriate design functions of several systems are integrated. Integrating the functions of an anaerobic pond into a facultative pond to form an integrated facultative pond can provide the level of waste stabilization required for land application while maintaining adequate control of odors. The effluent from the integrated facultative pond can go straight to land application or to an integrated aquaculture system where aquatic plants and fishes are produced, which provides additional revenue for the industry utilizing the technology. In the demonstration system presented, the potential revenue from the energy and nutrients available from the cattle feedlots located in Texas would be over $5 million. When the technology is applied to a land application system, additional revenue is possible from the crops that can be produced.
Key words: biogas, cattle, aquaculture, wetlands
Figure 1. Schematic of an integrated wastewaer treatment system for livestock waste with three options for the third stage of the process(not to scale).
|The Southern High Plains produces about 25%, or over 5 million head, of
the cattle slaughtered annually in the United States. Most of these cattle are processed
through approximately 200 feedlots, of which 87 have standing herds of over 5,000 head
(Sweeten et al. 1991). Considering an average gain of 600 lb/head while in the feedlots
and a feed conversion ratio of 9:1, then approximately 13.5 million tons of feed are moved
into these 200 feedlots annually. Manure produced in the feedlots at the annual rate of 9
tons (wet weight)/1000 Ibs of animal weight totals about 45 million tons and contains 0.31
million (0.57%) tons of total nitrogen (MWPS-18 1985). Nitrogen from the manure and urine
can be volatilized as ammonia gas, microbially converted into nitrites and nitrates as
potential water pollutants, removed from the feedlots as organic fertilizer or compost for
assimilation into plants, or further processed in an integrated aquaculture system (Figure
1). Sweeten and Wolfe (1993) twice sampled wells on 11 dairy farms and reported average
nitrate-nitrogen (NO3-N) concentrations of 1.2 +1.6
mg/L with a range of 0.0 to 4.35 mg/L -- well below the 10 mg/L nitrate-nitrogen level
permitted by the U.S. Environmental Protection Agency Standard for drinking wata. However,
feedlots are not the only source of nitrogen. For example, the annual nitrogen fertilizer
usage on U.S. farms increased from 14 million tons in 1950 to 145 million tons in 1990
The Southern High Plains is increasingly attractive for the intensive production of other animals such as swine. There are about 200,000 sows in the region producing over 4 million market size (250 lb each) pigs pa year (personal communication J. McG1One, Texas Tech University). With a feed conversion ratio of about 3.5 : 1 (3.5 lb of dry feed to 1 lb of gain in pigs), the approximately 500,000 tons of pigs produced annually would require 1.75 million tons of feed and produce about 175,000 tons (dry weight) of manure (1.75 million tons, wet weight).
The increasingly stringent county, state and federal regulations are forcing all industries to become better stewards of natural resources. The Agricultural Summit (Nelson and Jones 1994) held in College Station, Texas in October 1993 and the mini-Summit held at Texas Tech University on 16 May 1994 were attempts to develop agricultural strategies acceptable to consumers, environmentalists and regulators. As the human population has grown, ecologists have begun to develop global models to predict the carrying capacity of the world based on renewable resources (Perelman 1976, Roberts 1978, and Pestel, 1989). Computer models are available to simulate agriculture production, nutrient cycling and population growth and were instrumental in debates held during the 1993 Earth Summit in Rio. The parties that now express interest in agriculture and natural resources issues extend well beyond the traditional farm community (Speth 1992). In response to constituents, the 103rd U.S. Congress considered 571 bills in 27 broad topic areas affecting food, environment and renewable resources during the first session of 1993 alone. It is likely that this focus on agriculture by consumers, environmentalists and regulators will increase. Agriculturists will benefit by aggressively addressing the real and perceived problems associated with the production of food and fiber and develop environmentally sensitive methods of on-site reuse and recycling. Examples of some of the real and perceived problems are reflected in the study sites selected by the U.S. Geological Survey for the National Water-Quality Assessment Program (Hamilton and Shedlock 1992). Concern for agriculture systems and national water quality extends to the cattle feedlots in the Southern High Plains.
Wastewater Stabilization Ponds
Natural waste treatment systems have been in use in the United States since the late 1800's (Metcalf and Eddy 1991). These treatment systems were developed as an alternative to the high operational costs and the energy requirements associated with conventional treatment of wastewater by mechanical means. These natural systems are designed to obtain maximum benefit from the energy in wastewater, along with that provided by sunlight, wind, and gravity (Dinges 1982). Natural systems have been used for treating agricultural, municipal, and industrial waste liquids and sludges and are often combined with conventional treatment systems to improve effluent quality. The feasibility of a natural treatment system depends mostly on the climate, soil conditions, available land, and waste characteristics.
Stabilization ponds are used for treating a wide variety of wastes containing biodegradable organic matter from weak domestic wastewater to very strong industrial wastes. Stabilization ponds have been used for centuries with the first recorded constructed pond system in the United States in 1901 at San Antonio, Texas (Gloyna, 1971). The pond, known as Mitchell Lake, bad an area of 275 ha with an average depth of 1.4 m. However, many consulting engineers and water authorities, for many years, favored the more technologically advanced mechanical treatment systems and, therefore, dismissed the ponds as a "Third World Technology" (Mare et al. 1992). Another misconception about these stabilization ponds was that they were suitable only for hot climates. There are almost 2000 pond systems in France, 1000 in Germany, and one-third (over 7000) of all wastewata treatment plants in the U.S. are ponds used for treating municipal and industrial wastewater under a wide range of weather conditions (Mare et al., 1992; Reed et al., 1988). There is obviously interest in these systems because of the increased costs associated with the operation and maintenance of mechanical treatment.
Stabilization ponds are biological treatment systems designed to remove Biochemical Oxygen Demand (BOD) and to reduce the concentration of disease causing organisms (Gloyna, 1971). Organic wastes are decomposed in ponds by microorganisms, and the long detention times considerably reduce the concentration of the disease causing organisms (Watson et al. 1989). These systems are usually classified depending on the dominant type of biological process, duration and frequency of discharge, extent of pretreatment, and number and arrangement of cells. However, the most common method of classification is based on the dominant type of biological reaction in the pond and the ponds application: aerobic ponds, aerated ponds, anaerobic ponds, and facultative ponds.
Waste stabilization in all types of ponds is quite similar. The difference is the type of microorganisms that are performing the conversion. In the aerobic ponds, also known as high rate ponds, aerobic bacteria decompose organic matter. Algae, through photosynthetic reactions, provide sufficient oxygen for an aerobic environment. These ponds typically range in depth from 0.3 to 0.5 m, allowing the light to penetrate the entire depth of the pond (Martin and Martin, 1991). Sometimes these ponds are mixed to expose the algae to sunlight and prevent anaerobic conditions from occurring on the bottom surfaces. Detention times are usually shorter as compared to the other stabilization ponds mentioned. Aerobic ponds are usually of two types: (a) shallow ponds with limited depths of up to 0.5 m having high populations of algae; and (D) deep ponds of depths up to 1.5 m having high populations of bacteria. Bacteria use the oxygen released by the algae during photosynthesis for degrading organic matter, while nutrients and carbon dioxide released by the bacteria are in turn used by the algae. Aerated ponds are different only in that they are artificially oxygenated. Rates of mechanical oxygenation range from 1.2 W/m3 for simple aeration to 20 W/m3 for aeration and solid suspension.
Anaerobic ponds are commonly used for treating strong industrial and agricultural wastes. The principal biological reactions in anaerobic ponds are acid formation and methane fermentation. Usually designed with depths of 2.4 to 6 m, these ponds typically have retention times of 20 to 50 days and sometimes much longer. Facultative ponds are the most commonly used pond systems. The ponds are 1.0 to 2.5 m in depth and the wastewater is stratified into three zones. Oxygen in the top aerobic zone is provided by surface reaeration and is used by the aerobic bacteria to stabilize the waste in this region of the pond. The top aerobic layer also serves to reduce the odors while the wastes are being anaerobically stabilized in the bottom layer. The intermediate zone, which is partly aerobic and partly anaerobic, is where the facultative (aerobicanaerobic) bacteria degrade the suspended organic matter.
Facultative and Anaerobic Integrated Ponds
With of the pond types mentioned, there are a series of advantages and disadvantages that have prevented their widespread usage around the world, especially in terms of complete wastewater treatment. First, the aerobic and aerated ponds require the least amount of land area and short retention times, but they produce high concentrations of suspended solids in the effluent. In addition, the short retention times associated with treatment does not provide sufficient time for adequate coliform removal. The anaerobic and the facultative ponds typically produce odors due to the high organic loadings commonly used in these ponds. One possible solution to the inherent problems with the traditional ponds is to integrate the best functions of each into a single pond to allow related microorganisms to each perform their function.
The pond integration to be used at the Animal Science Fann of Texas Tech University consists of the anaerobic and facultative ponds. The first known integration of these two types of ponds was in Desert Lake Village near Boron, California in 1957 where a small, deeper sub-basin was placed within one corner of a much larger basin (Stone, 1960). Influent wastewater from the small community was transported into this sub-basin for the purpose of providing anaerobic treatment of the settled solids. The main pond depth was I In, while the sub-basin was at 2.1 m. The sub-basin measured about 0.25 ha, while the surrounding primary basin was 3 ha. Although no performance data were provided, some of the conclusions drawn from the operation of the pond system were that destruction of pathogenic organisms by natural causes was increased, greater algal and bacterial activity and thus treatment efficiency was maintained, and a higher level of control over insect breeding resulted. Effluent from the primary basin overflowed into a secondary and then tertiary pond for further treatment and chlorination These last two basins where about one-twentieth the size of the primary basin, thus providing only minimal treatment and the allowance of chlorination.
Another adaptation of the integrated pond system was that developed by Oswald (1968) and Oswald et al. (1969) in a complete wastewater treatment system, currently referred to as the Advanced Integrated Wastewater Ponding System (AIWPS). In this system, the influent wastewater flows into an anaerobic pond that is integrated into a facultative pond similar to that reported by Stone (1960), followed by three additional ponds for complete treatment. The second pond is a high rate pond used for the production of algae, while the third pond is a quiescent pond used to settle the algae prior to discharging into a large, final maturation pond. This four-stage pending system has provided excellent results in reducing wastewater strength and producing the equivalent of a secondary treated effluent.
Our objectives were 1) to determine the real or perceived problem for ground water contamination as caused by cattle feedlots in the Texas High Plains, 2) to identifier potential integrated wastewater treatment systems for on-site reuse and recycling of resources, and 3) to identify new potential products resulting from the recycling of current resources and their potential as revenue sources.
THE DEMONSTRATION UNIT
Figure 2. Approxiamte location of the ponding system in reference to the livestock operation at the Texas Tech University Animal Science Farm.
|A demonstration pilot plant (Figure 2) consisting of (1) an anaerobic pit
for digestion of came waste and generation of methane gas, (2) a facultative lagoon a
stratified digester with an aerobic surface and anaerobic bottom for first stage
production of microalgaeand the production of single-cell protein, (3) a well mixed
aerobic lagoon for production of microalgae and other aquatic plants to be harvested as
protein, and (4) an aerobic lagoon for culture of Snfish has been constructed. Water
passes sequentially from unit 1 through unit 4 and be recycled back to the farming
The demonstration project is an expansion of technology developed from an existing facility designed to culture the marine microalgae Spirulina using anaerobically digested biomass from cattle feedlots (Fedler et al., 1993; Parker et al., 1992). Cattle feedlots located in the Texas High Plains are typically located on sloping ground draining into plays lakes, which serve as major recharge sources for the underlying Ogallala Aquifer. Nutrient loading into playas threatens ground water quality (Fedler and Parker, 1994). An integrated treatment system to remove nutrients, produce on-site energy, improve water quality, and produce aquacultural byproducts is now possible using existing technology, but requires demonstration to promote acceptance.
This demonstration unit is located at the Texas Tech University Animal Science Farm that has a 1000-head cattle herd confined in either slotted-floor pens or in hard-surface pens (Figure 1). Runoff from these pens is currently captured and pumped to a settling basin before discharging into a plays lake. In addition, swine waste from a 180-sow farrow-to-finish operation is gassed over a solids separator prior to being pumped to the existing settling basins.
We designed the waste water treatment system by calculating soil to be moved, slope of beans, and placement of all components within the space and elevations available to us at the site. Approximately 3800 m3 (5000 yd3) of soil was moved to construct the ponds. A pump station was connected to the existing 0.61 m (2 ft) pipeline from which the collected waste water is pumped to the anaerobic pit. Water and waste pumped into the pond are measured by the pumping rate of the pump and the hours of operation. This treatment pond, an integrated facultative pond (IFP) (Figure 3), will discharge to two aquaculture ponds that can be operated in series or in parallel. The first aquaculture pond will produce aquatic plants, initially duckweed, for extraction of nutrients. The second pond will be used as a water source for production of fish principally tilapia. These two ponds will be covered with a greenhouse structure to maintain heat during the winter months and provide production throughout the year.
Biogas Collection And Use
Figure 3. The integrated facultative pond(IFP) showing the anaerobic pit and the biogas collection system.
|The IFP contains a plastic membrane, located only over the deep anaerobic
portion of the pond, for collection of biogas (Figure 3). This plastic membrane cover over
the anaerobic pit is designed to float and move vertically with changes in gas pressure.
The cover surrounds the vertical sidewalls directly above the pit in order to collect the
maximum quantity of biogas .
Space between cover and the wall allows waste waste to move from the anaerobic pit to the facultative pond. PVC pipe is connected to the center of the membrane cover for transfer of the biogas to the energy use building located next to the pond. Flotation support at the point of connection between the PVC pipe and membrane keeps the center of the membrane at the highest elevation during vertical movement.
Methane gas collected from the IFP will be used to fire wata heaters from which water will be pumped through a closed coil back into the anaerobic pit and returned to the heaters for recycling. The remaining gas will be used to generate steam and fed into a turbine coupled to an electrical generator. Heat from the combustion engine and exhaust gases are ducted through water jackets and delivered to the anaerobic pit. Electricity generated from this system will be used on-site to operate equipment or metered via a transfer switch into the lines of the local electric cooperative company.
The combined waste of the cattle and swine facilities is estimated to generate about 672 m3 (24,000 ft3) of biogas daily. Using conservative estimates, if all biogas was used to produce electricity, a 35-kW generator would be required and would provide 840 kWh of electricity per day. The exhaust gases and the heat from the turbine win be recycled to the IFP. Heating the IFP will increase the rate of anaerobic digestion, increase the rate of biogas production (Hashimoto, 1982) and improve effluent water quality.Subsurface Water Samples
The entire pond system has been constructed on sandy-clay-loam soil. One half of the floor of the 6 m (20 ft) deep anaerobic pit was covered with cattle manure. The other half of the pit was sealed only by the manure contained in the influent. Waste and nutrient movement from the ~ is monitored with a series of water sample collection devices (Figure 4) installed below the Boor of the pit and the main pond and compared to those water samples collected adjacent to the pond.
Figure 4. Layout of the integrated facultative pond used for treating livestock waste with locations of subsurface sample collection devices.
|These water samplers were installed below the floor of the anaerobic pit
in a transect running across the deepest portion of the pond (Figure 4). Collection
devices were installed adjacent to the pond, but outside of the basin. Water samples from
these collection devices were brought to the surface via two 0.3 cm (1/8 in) polyethylene
tubes running inside 7.6 cm (3 in) diameter polyvinylchloride (PVC) conduit. One tube was
charged with compressed air to force the water sample out through the other tube and to
the surface for collection. Water samples collected were analyzed for nutrients and other
typical waste water constituents.
The water samplers within the pit were placed in the ground in three matrices(1) sand, (2) soil excavated from the site and (3) sand that was covered with a sheet of plastic placed as an umbrella above the sampler. These three configurations were used to determine if water moves from the IFP vertically down to the sampler through the sand surrounding sampler site number 1, or if the movement was lateral to the sampler covered with the plastic umbrella as in site number 3. Samples at site number 2 provided a reference for the typical installation of sampling devices as currently used by others.Aquaculture Production
This highly integrated system will produce an effluent that is expected to be suitable for sustaining aquatic plants, especially duckweed. The duckweed will be harvested and fed to cattle to recycle nutrients, minimizing the waste water load flowing to the playa. Finfish to be cultured will include blue tilapia, Tilapia aurea, Nile tilapia, T. nilotica, and fathead minnow, Pimephales promelas. Broodfish will be stocked into the aquaculture pond and monitored for reproductive success. Water quality, specifically dissolved oxygen, pH, ammonia-nitrogen, nitrite-nitrogen, nitrate-nitrogen, conductivity, and temperature also will be monitored.
Confined animal feeding operations integrated with aquaculture and energy production has the potential to produce additional revenues valued at over S500 million in Texas. The total potential energy that can be produced from the manures produced by cattle and pigs raised in the Texas High Plains is calculated to be in excess of 4 million kWh/day. Waste heat from the generator system can be used to heat the pond contents, which increases the rate of fermentation and thus total biogas production. Aquatic plants and fishes can be recycled back to the animal production system or used as feed ingredients for other confined animal production operations to produce additional revenue. This integrated approach to treating confined animal waste is an alternative to conventional waste treatment and offers tremendous potential for producing new, marketable by-products. In addition, new businesses will be developed, some of which can serve as processors for the aquactic plants and animals produced.
Direct funding for this project was provided by the Department of Energy (Western Regional Biomass Energy Program), Environmental Protection Agency through the Texas State Soil and Water Conservation Board, Texas Higher Education Coordinating Board, and Texas Tech University College of Engineering and College of Agricultural Sciences and Natural Resources. Williams and Peters Construction Co., Inc., Lubbock, Texas, GSE Lining Technologies, Inc., and Environmental Spill Control, Inc., Hobbs, New Mexico donated equipment, Applies and operator time to complete the major construction. We thank Jeff Johnson, Jay Johnson, and John McGlone for review of this manuscript.
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