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Strathkelvin Instruments/About Respirometry

About Respirometry

Measuring the Oxygen uptake rate of aerobic organisms is the simplest way to determine the activity level of the sample. Oxygen consumption under controlled conditions is also fundamental measure of organism health and depending on the test methodology used can give insights into many different aspects of the processes underway. For Activated Sludge Bacteria a Resprirometer can be used to measure :

  • Respiration rate, which measures the rate of biodegradation by sludge bacteria
  • Toxicity of influents for both Carbonaceous and Nitrifying bacteria.
  • Respiration rate measurements are used for both toxicity management and process optimisation

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), autotrophic or 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 respiration may be 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. Prior to the development of the ASP-Con by Strathkelvin Instruments Ltd – online respirometers were prone to giving false readings and difficult to interpret data. The ASP-Con overcomes this by measuring other factors simultaneously such as pH, Ammonium, Dissolved Oxygen, MLSS and Settlement all of which are combined to determine an in-basin measure of toxicity. Even the ASP-Con is limited as it can only measure the toxicity affect in the basin and at a single concentration. The laboratory based Strathtox therefore complements the ASP-Con in providing a rapid and detailed analysis of influent streams off-line. 

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.

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Strathkelvin Instruments Limited,

Rowantree Avenue,

North Lanarkshire, ML1 5RX,


01698 730400
Fax: 01698 730401
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