By G. Palmateer, M.Sc.,
GAP EnviroMicrobial Services
During the 1990s, the major thrust in the management of waste treatment plants in Ontario has been towards the optimization of their performance. Based on a need to produce higher quality effluents in terms of reduced biochemical oxygen demand (BOD5) and suspended solids (SS), as well as the necessity to eliminate effluent toxicity, operators and engineers responsible for the waste treatment processes have had to seek new and innovative methods and technologies.
As communities grow, the demand on their pollution control plants to treat elevated loadings of domestic waste increases. Since funding for expansion is often limited, the need to optimize the treatment performance of the existing facilities is of the utmost importance.
In addition, as a result of industries that have been established in many communities, it is not uncommon for pollution control plants, that were originally designed to treat primarily domestic sewage, to now have to deal with more complex wastes. In some instances, specific organic or inorganic chemicals inherent to the industrial waste stream may be inhibitory to the biomass comprising the activated sludge. In other circumstances, some of the chemicals may be biodegradable; however, they are frequently mixed with biocides to enhance their shelf life. This also creates a detrimental impact on the biomass of the activated sludge.
A series of studies, conducted from September 1993 to November 1994, showed that the two plants at the Ingersoll Pollution Control Centre periodically suffered severe inhibitory distress from the industrial wastes that are received through the Ingersoll Sanitary Sewage Collection System (ISSCS).
One of the results of these studies was the establishment of new limits on the amount of each industrial waste that can be safely introduced to each plant without causing an unacceptable inhibitory impact on their biomass.
Respirometry
In general, respirometric devices are used to measure the total biochemical oxygen uptake (TBOU) and biochemical oxygen uptake rate (BOUR) exerted by a microbial population. Under aerobic conditions, these microorganisms consume oxygen proportional to the amount of organic carbon and biomass in the reactor. The expression:
Organic Carbon + Biomass + Oxygen =
Carbon Dioxide +
Water + New Biomass, describes this relationship.
As the reaction proceeds in the respirometer reactor, the carbon dioxide by-product leaves the mixed solution and collects in the reactor head space. As the produced carbon dioxide is proportional to the oxygen consumed, no change in the head space pressure occurs. However, by placing a strong alkali (KOH) in the head space of the reactor, the carbon dioxide evolved during microbial respiration is adsorbed, creating a pressure differential (negative) relative to the reference pressure set at the start of the test.
This change in pressure is detected by the system and an equal amount of oxygen is delivered to the reactor, bringing it back to equilibrium. The oxygen supplied is computed as a function of time and stored for future analysis.
(Based on a presentation to Regional Directors Meeting, MOE, S.W. Regional Laboratory, London, Ont.)
This article has been abridged.