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In: Proceedings of the North Carolina State University Animal Waste Management Symposium, Raleigh, North Carolina, January 27-28, 1999.

Evaluation System For Swine WasteTreatment And Energy Recovery

Jiayang Cheng¹, Kurt F. Roos², and Leland M. Saele^{3 1}Assistant Professor, Department of Biological and Agricultural Engineering, North Carolina State University, Raleigh, NC 27695-7625;²Director, AgSTAR Program, U.S. Environmental Protection Agency, 501 3rd Street, NW, Washington, DC 20001; ³Environmental Engineer, Natural Resources Conservation Service, 4405 Bland Road, Raleigh, NC 27609.

Introduction

Barham farm is a farrow-to-wean swine farm with 4,000 sows in two farrowing houses and four gestation houses. Pit-recharge system is used for collecting manure from the houses. There are eight pits in each house. The full pit volume for each pit is 5,000 gallons in the gestation houses and 7,800 gallons in the farrowing houses. Two lagoons in series are used for waste management: a dedicated volume covered anaerobic lagoon for primary waste treatment and a variable volume storage lagoon. One pit is discharged to the covered anaerobic lagoon and recharged with water from the storage lagoon every day in each house. The covered anaerobic lagoon has a surface area of 265 ft x 265 ft and a depth of 20 ft with a wall slope of 3:1. The storage lagoon has a surface area of 240 ft x 1,070 ft and a water level of about 8 ft. The designed hydraulic retention time in the covered anaerobic lagoon is 65 days and a loading rate of 9 lbs. VS / 1000 ft3-day meeting design criteria established under NRCS Interim Standard No. 360. The system was started in December 1996 with a high density polyethylene factory fabricated modular cover. Under designed operation, an electric generator is in operation with the combustion of biogas produced from the covered anaerobic lagoon and waste heat collected from the engine exhaust and radiator to heat a 10,000 gallon water tank providing heat to the farrowing houses. The cover collected up to 1,200 ft3/hr of biogas to the generator or a boiler until fabrication and material problems resulted in air infiltration. The cover was replaced under manufacturer warranty in November 1997. The new cover has experienced the same problems. Due to these problems, a new design of bank-to-bank cover with high density polyethylene (HDPE) material was installed in July 1998. The covered anaerobic lagoon system has performed well since then with the added benefit of eliminating rainwater from the primary treatment lagoon.

Objectives

The goal of this project is to evaluate a covered anaerobic lagoon system as an alternative technology for treating swine waste on a commercial scale and to develop a financial assessment of the system. The specific objectives of the project are to:

1. Investigate the waste treatment efficiency (COD removal and volatile solids destruction) and the quality of the effluent from the system

2. Monitor biogas production from the covered anaerobic lagoon as related to volatile solids destruction and temperature, and evaluate the effectiveness of methane collection.

3. Develop a farm energy profile comparing total energy requirements (electricity and heat) to energy available from biogas.

4. Compile electrical and heat generation data for cost-benefit analysis on the system.

Methodology

The performance of the covered anaerobic lagoon system at Barham farm has been monitored by measuring organic degradation, volatile solids destruction, nutrient conversion, and biogas production. Samples are taken from the gestating and the farrowing sow wastewater, effluent of the covered anaerobic lagoon, and storage lagoon water. The items analyzed include Chemical Oxygen Demand (COD), Total Organic Carbon (TOC), Total Kjeldahl Nitrogen (TKN), NH₃-N, NO₃-N, NO₂-N, Total Phosphorus (TP), o-PO₄-P, Total Solids (TS), Volatile Solids (VS), pH, and alkalinity. All the analysis has been performed in the Environmental Analysis Laboratory in Biological and Agricultural Engineering Department of North Carolina State University. Measuring biogas production and methane content in the biogas was not possible until the bank-to-bank cover was installed in July 1998. The flow rate of the effluent from the covered anaerobic lagoon was measured with a ISCO 4150 area/velocity flow meter. Since the lagoon is completely covered from bank to bank, the total flow rate of the raw swine wastewater should be the same as the flow rate of the effluent from the covered lagoon. The efficiencies of organic degradation, volatile solids destruction, and biogas production have been determined through mass balances.

The average flow rate was higher before the bank-to-bank cover was installed at the end of July 1998 because the lagoon was also collecting the rain water when a modular cover was used. After the installation of the bank-to-bank cover, the average flow rate of the effluent or the raw swine wastewater was about 29 gpm (gallons per minute). Based on the total volumes of the pits in both farrowing and gestation houses, the ratio of the wastewater flow from the gestation houses to that from the farrowing houses is estimated at 1:0.78. Therefore, the wastewater flow rates from the gestation houses and farrowing houses are estimated at 16.3 and 12.7 gpm, respectively. The average analytical results for raw wastewater from farrowing and gestation houses, covered anaerobic lagoon effluent, and storage lagoon water are shown in Table 1.

Table 1. The characteristics of raw wastewater from farrowing and gestation houses, covered anaerobic lagoon effluent, and storage lagoon water.

The loadings of organic, volatile solids, and nutrients in the raw wastewater, the covered lagoon effluent, and the storage lagoon water were calculated and are shown in Table 2.

Table 2. The loadings of organic, volatile solids, and nutrients in raw wastewater from farrowing and gestation houses, covered anaerobic lagoon effluent, and storage lagoon water.

The removal efficiencies of COD, TOC, TS, VS, TKN, NH₃-N, Total-P, and o-PO₄-P were estimated by conducting mass balances on the covered anaerobic lagoon. The following equation was used for the calculation: Removal Efficiency = (Mass Input, lbs. - Mass Output, lbs.)/Mass Input, lbs. H 100% (1) The estimated removal efficiencies in the covered anaerobic lagoon are listed in Table 3. High removal efficiencies for COD, TOC, and VS were achieved in the covered anaerobic lagoon. Nitrogen (TKN and NH₃-N) and phosphorus (TP and o-PO₄-P) were also removed by over 50% in the covered lagoon. The removal of N and P is probably due to the formation of precipitation in the covered lagoon. TABLE 3. The efficiencies of organic degradation, volatile solids destruction, and nutrient removal in the covered anaerobic lagoon.

Biogas production from the covered anaerobic lagoon and methane content has been stable since the bank-to-bank cover was installed. The average biogas production rate and the methane content in the biogas from August 1 to October 31, 1998 are 895 + 157 ft3/hour and 71.42% + 0.33%, respectively. Monthly variability is due to temperature effects and gas yield consistent with observed rates (VS destroyed/ ft3 biogas) in other covered lagoon systems. The relationship between methane production and volatile solids destruction can be expressed with the following equation:

CH₄ Production (m³) = f • VS Destruction (kg) (2) where f is a coefficient. In this case f is estimated as 0.52.

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