In 1997, the Bluestem Solid Waste Agency decided to examine the effectiveness of bioreactor technology in cold climates, such as Iowa's.

By Curtis L. Hartog, Marten Cieslik, and Timothy J. Hall

A bioreactor in Iowa will provide valuable study data on the use of bioreactors in highly variable climates. Iowa, specifically Cedar Rapids, can experience temperatures higher than 100°F in the summer and as low as -20°F in the winter.

The Bluestem Solid Waste Agency, with assistance from the consulting and engineering firm of Foth & Van Dyke, received a grant from the Iowa Department of Natural Resources under the Landfill Alternative Grant Program. The grant application identified the primary study objectives as:

• bioreactor technology as a method of landfill optimization through reclamation and landfill cell reuse using bioreactors,
• development of a low-cost composting process suitable for smaller landfills.

To provide additional information to evaluate the primary study objectives, half of the bioreactor includes wastewater treatment plant sludges to determine if they can enhance the biological process. At the end of this seven-year study, the bioreactor will be excavated and waste material screened to determine levels of decomposition.

The bioreactor was constructed with a Subtitle D composite liner in 1998. Because of high groundwater levels, the bioreactor construction required the design and installation of a groundwater underdrain system to lower natural groundwater levels to the required 5-ft. separation between groundwater and compacted clay liner. To determine if wastewater treatment plant sludges would be beneficial, the bioreactor was constructed to have two cells, each with its own leachate collection and recirculation system. When bioreactor basal liner construction was completed, samples of geotextiles and geocomposites were placed above the gravel leachate collection layer to evaluate clogging and survivability of these materials in bioreactor operations.

Waste placement in the bioreactor began in December 1998 and was completed in August 1999. Filling was conducted in a manner typical for MSW landfills. No preseparation of waste was conducted except to prohibit construction-and-demolition waste. Both cells in the bioreactor received approximately 6,000 tons of MSW. Additionally, Cell B received approximately 6,000 gal. of liquid sludges and 70 tons of cake sludges from the Cedar Rapids Waste Water Treatment Plant. As they were being filled, a vertical soil berm was constructed to separate the two cells, two temperature probes were placed in each cell, and a horizontal landfill gas (LFG) collection system was installed in each cell and connected to a common blower.

In September 1999, a 2-ft. soil cap was placed on the bioreactor to provide both erosion protection and a working surface for installation of leachate recirculation lines and removable capsules for evaluation of decomposition through the process. Leachate recirculation lines and recirculation pumps were designed to permit targeted recirculation. Four pumps, which remove liquid from the leachate storage pond, are each connected to two leachate recirculation lines that are installed in each cell at a specific level to target recirculation as needed. Removable capsules were constructed of plastic netting and filled with MSW. Each capsule was buried in the waste and will be excavated throughout the study period. Capsules were buried in both cells to aid in the evaluation of sludge-addition effects on bioreactor performance. Final closure of the bioreactor occurred on November 5, 2000, with the installation of a tan reinforced polypropylene cap. The cap will be exposed throughout the study period so that its survivability through various conditions can be evaluated. As part of the closure, an operations plan was developed to aid Bluestem in monitoring bioreactor parameters.

The operations plans recommend that the following parameters be monitored at regular intervals: leachate head on bottom geomembrane, temperature inside the bioreactor, subsidence, recirculation pump operation, leachate recirculation line valving, leachate analytical testing, and LFG parameters.

The operations plan also details recirculation strategies and winter operations when recirculation is stopped and the leachate collection system is closed to prevent freeze damage and overfill of the leachate storage pond.

During the first year of full bioreactor operation, the sludge-amended side of the bioreactor (Cell B) received an additional 72,000 gal. of liquid sludge and 98,000 gal. of water. The nonsludge side (Cell A) received 86,000 gal. of water. The total amount of liquids added to the bioreactor in 2001 was approximately 35,000 ft.³

The addition of liquids to the bioreactor produced leachate, which was analyzed weekly for pH, conductivity, and temperature. During 2001, pH was recorded in the 6.8—7.0 range and conductivity stabilized at approximately 12 milliseconds/cm. Leachate temperature exhibited some variation; high temperatures around 78°F were recorded in July, and low temperatures of 62°F were recorded in November. In 2001, leachate samples were taken in August and October to monitor specific constituents. Results are provided in Table 1. Of interest is the reduction of volatile acids. Increasing volatile acids can affect waste decomposition. Furthermore, the increase in chemical oxygen demand (COD) can be attributed to the addition of sludges to Cell B throughout 2001.

LFG was also analyzed in 2001; data on LFG temperature indicate a steady decline in both cells. This might be a result of the temperature probe location's being subject ambient conditions. LFG constitutes were also analyzed in 2001. Table 2 provides the results from a composite (both cells) LFG sample. Continued monitoring of LFG constituents is scheduled through the life of the bioreactor.

Another critical reading to evaluate bioreactor performance is waste temperature. Temperature probes are located at two levels on each side of the bioreactor. Temperature readings in 2001 initially were well in the optimum zone for a bioreactor. By August, temperature readings began to stabilize throughout the bioreactor to around 80°F (Figure 1). Further evaluation of temperature data will determine when the bioreactor is at optimum biodegradation temperature.

During 2001, the Bluestem bioreactors performed well. Indicator parameters confirm that considerable decomposition of waste is occurring. For 2002, the following goals were established to further the study of bioreactor technology in the Midwest:

• Evaluate alternative temperature sensors to verify existing sensors are functioning and accurate.
• Add approximately 60,000 gal. of liquid sludge to Cell B and 50,000 gal. of water to Cell A.
• Install flow meters on recirculation lines to better quantify recirculation.
• Continue to conduct analytical testing on leachate and LFG.
• Exhume one capsule from Cell A to determine decomposition rate.

With additional data from 2002, bioreactor performance can be optimized, speeding decomposition and furthering the study objectives.

Curtis L. Hartog, P.E., is head environmental engineer and Marten Cieslik, P.E., is senior project manager with Foth & Van Dyke, headquartered in Green Bay, WI. Timothy J. Hall, P.E., is with the Iowa Department of Natural Resources.

MSW - November/December 2002