Evaluation of Woodchip Bioreactors Dodge and Rice County

Five woodchip bioreactor systems (WBS) were examined and two met the criteria for design, associated land management, nutrient application, and access to the monitoring equipment. Two key water quality issues emerged: at one location (Dodge County) nitrate losses through pattern tile while, at the other location (Rice County), bacteria losses after fall manure application and associated nitrate losses (Figure 1-1). With nitrate contamination, drainage water from agricultural field will be treated via biological reactions; by using a source of carbon like woodchip, facultative anaerobes reduce nitrate to nitrogen gas under low oxygen conditions, less than 5 mg/L.
Nitrate is a pollutant in drainage water, flowing through waterways discharging into mixed salt-freshwater ecosystems and hypoxic zones near the mouth of the Mississippi River in the Gulf of Mexico. The woodchip bioreactor system has been tried in several locations in the Midwest and has yielded reasonable reduction of nitrate concentrations. The system is based on using woodchips as a carbon source (electron donor), the ability of certain bacteria (facultative denitrifiers) to use nitrate (electron acceptor) for respiration under anaerobic conditions. Under these anaerobic conditions, oxygen is quickly consumed first, and then microbes move on to use up nitrate. Depending on the local conditions, nitrate can be transformed to nitrogen gas, which escapes later to the atmosphere.
Bacterial contamination is also studied in this investigation. Pathogen contamination in surface water constitutes a significant component of the TMDL program in Minnesota. Bacterial population count is also expected to be reduced via several mechanisms under anaerobic conditions. Mechanisms for bacterial reduction may include the effects of temperature, sedimentation, absorption, filtration, and the fact that an anaerobic environment is hostile for growth of microorganisms considered in this case (Khatiwada and Polprasert, 1999).
The two sites chosen for this project are located in Dundas (Rice County) and north of Claremont (Dodge County). The Rice County site contains two fields with one as a controlled drainage system and the other as free drainage system. Controlled drainage system covers an area of 6.6 acres while the free drainage system has 15.6 acres. The length of bioreactor is close to 90 feet with a width of 3 feet and a depth of 6 feet (2 ft of top soil and 4ft of woodchip). The Dodge County site has a contributing area of 26 acres with a bioreactor of 240 feet long, 6 feet deep, and 3.5 to 4 feet wide. The bioreactor depth contains the same proportion of materials as described at the Rice County site. Other details regarding field operations and chemical applications are given in more details in the appropriate paragraphs.
Woodchip bioreactors in Minnesota consist of a rectangular-shaped trench in the ground, and range from a length of 50 to 250 ft, a depth of 4 to 6 ft (top soil 1-2.0 ft), a width of 2.5 ft to 4.0 ft filled with woodchips. The water level in the trench is controlled to cover the entire woodchip depth, thereby maintaining anaerobic conditions. The trench is covered with a layer of soil to allow regular field operations and crop growth. Hydraulic retention time in the system is important as it influences the efficiency of nitrate removal. On small-scale systems, retention times range from 1.5 hrs to 24 hrs. Parameters, like pH and temperature, need to be monitored as they limit nitrite reduction rates and leave nitrate removal incomplete (Smith et al., 2005). This concern regarding pH arises with basic substrate in dolomitic or limestone regions. Low temperature conditions during early spring can retard denitrification; relatively cold conditions will slow biological reactions thereby decreasing WBS performance.
The woodchips used in this experiment consist of maple and red oak. Red oak makes up 12.5% of woodchips by weight. Its physical characteristics such as porosity, grain size, and moisture content of air-dried material, were tested at the University of Minnesota laboratory. Maple woodchip is almost square with a side length of 1.0 inch. Red oak woodchip is of rectangular-shape with length between 0.5 to 3 1/8 inches and a thickness of ¼ to 1/8 inches. Granulometry is given by Table 1-1 and Figure 1-2. Moisture content of woodchip samples was 7.3% by weight. Average porosity of red oak and maple woodchips is 0.58. Average water absorption of the material reaches 15.4% of dry weight over three days.