Sample Student Abstracts

Bacterial Culturing for Potential Pathogens

Rachel Cohn ’04 spent winter term of 2002 optimizing the membrane filter procedure to quantify the presence of bacterial pathogens in the Living Machine. She surveyed all tanks of the Living Machine for fecal coliform, the indicator organism utilized in the wastewater industry as an index of water quality with respect to pathogenic bacteria. Generally speaking, the presence of fecal coliforms in water indicates recent fecal pollution. As well as being in feces, fecal coliforms are also found naturally in the gut of warm-blooded animals, and are essential for the human body to function correctly. Specifically, fecal coliforms are a segment of coliforms that ferment lactose at temperatures above 35 C. The incubation temperature required is 44.5 C +/- 0.2 C. Because fecal coliforms possess the ability to grow at relatively high temperatures, they can be easily singled out and counted by experiment. The specific microorganism that is found using the membrane filter procedure with an M-FC medium is Escherichia coli (E. coli), the most common fecal coliform. Student operators now use this technique three times per week to verify the living machine system is meeting effluent standards.

Bioregulation of Water Flow Through a Constructed Gravel Marsh

James McConaghie ’03, Joel Musee ’04 and Mike Pennino ’05 are three of several students that have worked with Professor John Petersen to investigate the regulation of water flow through the Living Machine marsh. These studies examine how the introduction of emergent macrophytes to a sub-surface flow constructed wastewater treatment wetland might alter water flow. Flow patterns have been quantified using fluorescent dye and bromide ion. The effect of plant community development on nutrient removal and movement in the wetland have also been investigated through the use of tracers.

Nutrient Processing and Dynamics

Caroline Turner ’04 and Professor John Petersen investigated the efficiency and dynamics of nitrogen removal from wastewater by the AJLC Living Machine. The Living Machine is a recently developed greenhouse-wetland wastewater treatment system that relies on biological and physical processes to remove nutrients such as nitrogen and phosphorus from wastewater. They identified denitrification as the limiting factor for nitrogen removal in the AJLC living machine. Denitrification is limited in turn by carbon availability. The locus of denitrification intended by Living Machine designers was a subsurface flow gravel wetland. System operators added plants to the gravel wetland to provide carbon for denitrification. However, plant colonization remained sparse one year following introduction and no improvement in denitrification was yet observable. Turner and Petersen’s data indicate that most denitrification instead occurs when water is recycled back to anaerobic tanks at the beginning of the system where carbon is abundant. The author’s suggest this mechanism for nitrogen removal could be optimized in design of future greenhouse-wetland systems.

Automated Measuring of System Metabolism

In the Fall of 2000, an automated data collection system was installed in the Living Machine. Designed and implemented by Professor of Environmental Studies John Petersen and Alexander Maly ’02, the system consists of a series of dissolved oxygen probes in each aerated tank connected to a data collection and control system. The probes relay information to a Campbell datalogging computer which records real-time measurements of dissolved oxygen and water temperature every minute, 24 hours a day.

To measure rates of metabolism in the Living Machine, the air pumps to the tanks are turned off every 6 hours. The probes then measure how dissolved oxygen levels change as bacteria in the water use it up, and based on the time it takes for levels to drop by 1 mg/L, the computer calculates Total System Metabolism (TSM), an index of carbon content.

A Comparison of Nitrification and Carbon Metabolism Using the BOD5 Method

Timothy Haineswood ’03 and Naomi Morse ’03 examined the factors that limit denitrification in the gravel marsh that serves as the final stage of wastewater processing in Oberlin College’s Living Machine. Denitrifying bacteria, Pseudomonas denitrificans, are obligate anaerobes that use the reaction C6H12O6 + 4 NO3 --> 6 CO2 + 6H2O + 2N2 + energy, or variants with other sources of organic carbon, for respiration. Denitrification requires sufficient bacteria, nitrate, anoxic conditions and a source of organic carbon. They took samples from the marsh and added organic carbon in the form of sodium acetate to test the effect of carbon on denitrification. Tim and Naomi found that adding organic carbon significantly increases denitrification and that providing anoxic conditions without the addition of carbon does not. Furthermore, direct sampling of the marsh indicated that only a limited amount of denitrification occurred even though conditions were anoxic. They concluded that the only condition necessary for denitrification not being met in the Living Machine marsh is a lack of organic carbon, most likely because this marsh currently has few plants, while the marshes in comparable systems are densely planted with emergent vegetation.

The major goal of this project is to discern whether a relationship exists between BOD5 and the technique we’ve developed for measuring system metabolism. A positive correlation between BOD5 and TSM results would suggest that BOD5 provides a good measure of existing conditions in the tanks. It would also indicate that the relatively rapid TSM technique is a useful substitute for the labor-intensive and ex-situ BOD5 technique. A negative correlation would raise questions about how useful BOD is as a tool for managing wastewater treatment ecosystems.

Comparison of BOD5 and Total System Metabolism Techniques for Measuring Carbon in the System

One important indicator in an ecological system is the measure of its production and respiration over time. In an ecosystem, as in an organism, we can call this activity "metabolism". Measuring this metabolic activity gives us information on how the ecosystem functions. The Living Machine is specifically designed to maximize the removal of organic carbon from wastewater through the respiration from bacteria in the water. Because the water contains so much biological activity, the opportunity exists to compare two different methods for measuring system metabolism. Both methods assess metabolic activity by tracing changes in dissolved oxygen levels over time. By measuring the rate of decline in dissolved oxygen, we can gain an understanding of the amount of labile (easily broken down) carbon available to the biology in the system.