Respirometry

Recent developments in biotechnology and the availability of reliable respirometric systems provide engineers with a powerful tool to evaluate and design bioremediation projects. This article describes the basic concept of respirometric studies and presents results of their application.

Within the last decade, the biological treatment of wastewater, wastes and contaminated soil has become one of the major technologies used in environmental engineering. The basis for such an interest in biological processes is that they are less expensive, compared to the other technologies, and more environmentally friendly. Bioremediation destroys most organic wastes, eliminating health and ecological effects as well as future environmental liabilities.

But while bioremediation is less costly than other technologies it is scientifically intense. The optimization and control of microbial transformations of organic contaminants requires the integration of many scientific and engineering disciplines. It also requires laboratory techniques that enable us to study and model biological processes in various environmental conditions.

With any biological treatment system it is particularly important to determine the biokinetics. In the past, this process was laborious, error-prone, and time consuming. Now, the task has been greatly facilitated by aerobic and anaerobic respirometry.

What is respirometry?

Respirometry is the measurement and interpretation of the rate at which a biological system consumes oxygen (aerobic respirometry) or generates biogas (anaerobic respirometry) under well-defined experimental conditions.

Aerobic respirometry allows the simultaneous monitoring of biochemical oxygen demand in several reactors at a precisely controlled temperature. The reactors are fed with the same amount of biomass but different concentrations of the investigated wastes. Its biological activity can then be determined in relation to the concentration of the wastewater.

On the basis of the biological oxygen uptake data and results of chemical analyses of the reactor contents before and after the tests, monitors can calculate the biokinetic constants, namely maximum growth rate ( max), sludge yield (Y), half rate concentration (Ks) and inhibition constant (Ki). They can then use the biokinetic constants to calculate the optimum design and operational parameters of the biological treatment system.

Anaerobic respirometry allows the simultaneous monitoring of biogas generation in several reactors at a precisely controlled temperature. Again, the reactors are fed with the same amount of biomass but different concentrations of the investigated wastes or the particular substance. Based on the generation of biogas and chemical analyses of the contents of the reactor before and after the tests, monitors can calculate the biokinetic constants. The respirometric system also enables the monitors to determine how much methane and carbon dioxide are being generated, and thus establish conditions for maximizing the production of methane.

Sewage and contaminated soil

CRA supported a number of studies at its analytical laboratory in Waterloo using respirometry. They were done on municipal sewage, leachate from the landfill, industrial wastewaters and soil contaminated with various oils. The purpose was to determine the effect of leachate on the performance of treatment systems.

The main components of the respirometric system used in the studies were:

Computox-OX, N-CON (Larchmont, NY) 12 channel respirometer equipped with aerobic and anaerobic set up modes)

12 reactors, each 1-litre volume equipped with injection/sampling port

Water bath with precision temperature control; range: 5-40C

PC equipped with CTOX computer program for data collection and instrument control.

CRA’s laboratory analyzed the basic water parameters: chemical oxygen demand (COD), biological oxygen demand (BOD), alkalinity, total suspended solids (TSS), and volatile suspended solids (VSS), as well as specific parameters, i.e. major cations and anions and heavy metals (ICP technique).

The respirometric reactors were fed with various mixtures of municipal sewage with leachate and returned activated sludge (RAS) from the existing treatment plant.

The respirometric plots indicated that the leachate had a strong inhibitory effect on the biological system. Calculations of biokinetic constants revealed, however, that the addition of leachate — up to 10% — should not substantially change the efficiency of the biological treatment. Applying this finding to the existing system confirmed the respirometric study results.

Industrial waste

Respirometric studies on industrial wastewaters from the automotive industry were conducted to determine the feasibility of biological treatment on various industrial waste streams. The first and most important part of these studies was to measure the acclimatization of returned activated sludge (RAS) from the existing treatment plant to the investigated wastes.

The effect of various doses of wastes on the biomass was monitored using microscopic analyses. Respirometric reactors were fed with the mixture of wastes and the acclimatized returned activated sludge. Several respirometric runs indicated that the mixture of wastes could be treated biologically as long as individual streams were maintained in specific ratios. The research also determined that the overall flow of industrial wastes into the municipal treatment plant should not exceed 15%.

The main contaminants of investigated wastewater from the chemical industry were formaldehyde (up to 8000 mg/L), urea (up to 850 mg/L) and ammonia (up to 80 mg/L). The purpose of the respirometric studies was to determine whether an aerobic or anaerobic system would be more suitable to treat this wastewater stream.

Aerobic respirometry demonstrated that the waste had a very strong inhibitory effect on the biological system. Based on the data, it was calculated that effective aerobic treatment of the waste would be possible only when it was diluted by 10%. Anaerobic respirometry indicated that anaerobic treatment of this waste would be successful with 25% dilution. Using these results, the research team designed a pilot treatment system that confirmed anaerobic treatment was applicable for this wastewater.

With soil contaminated with oil, studies were done to determine the rate of oil biodegradation and what factors affected this process. The respirometric reactors were fed with the same amount of soil slurry and various amounts of nutrient solutions were added to each reactor. By analyzing the content of the soil before and after the test and recording the biological oxygen demand from each reactor, the researchers determined the rate of oil biodegradation in soil and the optimum dose of nutrient.

In conclusion, respirometric techniques allow researchers to evaluate the biokinetic constants of any aerobic and anaerobic system in a relatively short time using small amounts of investigated wastes. Respirometric tests can identify and evaluate the inhibitory effect of various wastewater streams or specific substances on biological systems, and the data can be used to optimize existing or new wastewater treatment plants.

Jan Kochany, P.Eng. is a project manager and head of a treatability laboratory of Conestoga-Rovers Associates in Mississauga, Ontario. E-mail: Jkochany@CRAworld.com.