About Respirometry
- Respiration rate measures the rate of biodegradation by sludge bacteria
- Toxic wastes inhibit both respiration and biodegradation
- Respiration rate measurements are used for both toxicity management and process optimization
Respiration basics
In the aerobic tanks of a biological treatment plant, the complex molecules of the wastewater are broken down by the combined activities of heterotrophic bacteria (that break down organic carbon compounds), nitrifying bacteria (that break down ammonia) and other micro organisms. In terms of process control, there are three major processes at work:Biodegradation (the breakdown and removal of organic carbon and ammonia), Growth (growth and multiplication of the bacteria, which results in new biomass) and Respiration (oxygen consumption that provides the energy that the bacteria require for growth).
The three processes are interdependent. As a result, by measuring the rate of respiration, a measure of the rate of biodegradation can be obtained. For this reason, a respirometer maybe used as the control variable in process optimisation. If toxicity in the influent wastewater inhibits the respiration rate of the activated sludge, it follows that there will be an associated reduction in the rate of biodegradation.
A simple explanation of the relationship between biodegradation, growth of biomass and respiration is provided in a paper by Davies and Murdoch (2002).
On-line v laboratory respirometers
On-line and laboratory respirometers have complementary roles in toxicity management. Laboratory respirometers have an additional role to play in process control and management.On - line respirometers and other early warning devices. A review of available early warning devices and their efficiency has been published recently in the USA by a WERF working party, together with recommendation for further developments. At the time of installation of on-line monitors, storage facilities need to be built, in order to contain the diverted toxic waste. Such waste is normally treatable. However, the extent of the toxicity has to be determined, using a laboratory respirometer, in order to calculate the rate at which it should be fed into the treatment tanks, to achieve a dilution at which it will no longer cause a toxic effect. Currently, as shown by the WERF report, on-line systems are prone to generating false results, and they require some care in their operation.
Laboratory respirometers. The Strathkelvin respirometer was developed by technology transfer from the biomedical division of the company. It is a closed chamber respirometer requiring only small (20ml) samples of activated sludge. By using multiple electrodes and dedicated software, respiration rates can be measured in only 5-10 mins. All of the time-consuming aspects of respirometry have been removed. Wastewater treatment plant staff can be readily trained to operate the instrument, for a wide range of applications in both process control and toxicity management.