July 1, 2004 | General


BioCycle July 2004, Vol. 45, No. 7, p. 55
A design course at Villanova University gives undergraduates opportunity to build a digester to serve special manure management needs of “client farm.”
Metin Duran and Ronald A. Chadderton

ENGINEERING PROGRAMS at universities nationwide are offering senior level “capstone” design courses in their curricula. Students in these courses incorporate technical knowledge into “real world” design problems. At Villanova University near Philadelphia, Pennsylvania, our Civil and Environmental Engineering course is focusing on anaerobic digester design for manure management.
The project chosen to fulfill course objectives was designing a digester for managing manure on a a poultry and dairy farm. Students worked with a 100-acre family owned farm in Chester County, Pennsylvania with 250,000 layers and 40-cow dairy herd. A field trip gave students a chance to meet their “client,” ask about specific objectives, observe current manure management methods, and determine site specifications for locating the digester. They also learned about regulatory pressures forcing farmers to adopt better practices and obtained manure samples for analysis and anaerobic treatability studies.
Samples were analyzed in the laboratory to determine key characteristics of manure, namely total nitrogen and phosphorous content, total and volatile solids, and organic matter content through Chemical Oxygen Demand (COD) measurement. Students also conducted Biochemical Methane Potential (BMP) tests by incubating various concentrations of poultry and dairy manure separately to determine digestion rate, minimum required hydraulic retention time, and methane potential. The experiments were carried out in triplicates and compared to published values to determine the accuracy. Table 1 summarizes the results that this year’s design team obtained. As indicated by low standard deviations given in parenthesis in Table 1 (except for total nitrogen measurements), tests were highly reproducible and close to published values.
The BMP tests provided students with two essential criteria for their design. First, the linear portion of cumulative methane production as a function of time indicated the maximum volumetric loading rate that could be applied. Second, the time it took for methane generation to cease indicated the minimum required hydraulic retention time. The students used these two design criteria to determine the volume of the digester. Another important piece of information derived from BMP tests was the CH4 generation potential per unit mass of manure applied which was subsequently used in annual energy budget estimations and cost calculations.
The laboratory component of the course was designed to have students improve their laboratory skills by doing the experiments themselves (each student was required to carry out at least one test) which helped them build their confidence working in the laboratory. Students learned to understand the importance of waste characterization and treatability work and value of the experimental results in designing biological treatment processes. Another important experience was to recognize the uniqueness of different wastes and to deal with those differences to design a process that will digest mixed wastes.
High solids content of manure was presented to the students as a design constraint. As indicated in Table 1, poultry manure is nearly 30 percent solids by weight and dairy manure is nearly 20 percent. Thus, the solids content of the manure was higher than the U.S. Department of Agriculture/Natural Resources Conservation Services interim standards, which recommend influent TS concentration between 2.5 percent and 10 percent for complete mix digester and between 11 percent and 14 percent for the plug flow digester. One intention of this kind of design constraint was to teach students to conduct a literature search to identify the latest technologies to deal with such challenging projects that will be a part of their future careers.
One group designed a two-phase digestion process with supernatant recycle from the second phase to the first phase. This approach has gained popularity in recent years for digestion of high solids-containing wastes such as the organic fraction of MSW and manure. The advantage of the two-phase systems is that high solids-containing waste is solubilized in the first phase and the soluble organic material is then transferred to the second phase reactor by recycling the supernatant from the second phase reactor. In this design, the second phase reactor receives only soluble organic material and influent solids concentrations are very low.
An important component of the design was to size mechanical equipment for heating (a boiler or a heat exchanger depending on heat source the students chose), mixing (a motor and an impeller for mechanical mixer; or a compressor and a bubble generating tube for mixing by biogas recycle), and pumping (for influent, effluent, recycling, and excess sludge withdrawal). If there was surplus biogas generation, students also sized a biogas run generator to convert excess methane into usable electricity. This part of the design process taught students not only how to size mechanical equipment, but also how to compare available products and make technically sound and economically rational choices. With hundreds of companies providing thousands of selections on each line of equipment, students conducted extensive research, talked to sales representatives, and critically evaluated an extensive body of information they collected.
A cost analysis including capital and operating and maintenance costs was included as part of the course requirement. Table 2 summarizes the itemized total cost for a 478 m3 (17,000 cf) second phase digester following a 25 m3 (885 cf) first phase reactor. The students chose to bury their tank to provide insulation; this approach increased the total cost significantly.
Since generation of a marketable product, CH4, is one of the advantages of anaerobic digestion, students calculated expected gas generation based on the BMP results and incorporated this potential as revenue in the cost analysis. To calculate surplus methane potential, students had to develop an energy balance for the CAFO. Energy input requirements for heating, mixing, and pumping were calculated as energy demand and were compared to the energy potential from the expected methane generation. The question of whether biogas generation can be sufficient to subsidize the initial investment depends on a number of factors such as manure characteristics, size of operation, and dynamics in the energy market. In most cases, anaerobic digesters are at least self-sufficient as far as the energy requirements are concerned. In the case of the farm the students worked with, there is surplus biogas generation with total energy content of approximately 1.7 billion BTUs (498,000 kWh) per year.
The final requirement in the course was building a pilot-scale digester, which did not have to be a prototype of the students’ design. This requirement was intended to develop students’ understanding by connecting all the design components into a physical model. The pilot-scale digester was constructed using a 55-gallon HDPE tank and other material readily available at a hardware store. Students were asked to design and construct the influent and effluent structures, heating coils, mixing equipment, and finally insulation.
Probably the best learning experience was the construction of the pilot-scale digester. Restricting students to use only the readily available material at the laboratory or at a hardware store forced them to be creative. For example, insulation was provided by a water heater blanket bought at the local home improvement store. Gas tightness of the effluent structure was provided by a “goose-neck” piping that the students designed. To operate the digester in batch mode, students installed three withdrawal ports for variable volume operations. Constructing the digester from scratch enabled them to develop a better understanding of the mechanics of a digester.
In conclusion, the capstone course involving anaerobic digester design has been a valuable learning experience for Villanova students for the last two years. An open ended, real life project involving an actual farm, hands-on laboratory analyses, and construction of a pilot scale digester were main factors in the success of the course. Consequently, two of the student designs have been invited to compete in the national ASCE/EWRI Parsons Brinkerhoff Capstone Design Contest, one last year and another this year.
Metin Duran and Ronald Chadderton are in the Department of Civil and Environmental Engineering at Villanova University.

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