Quantifying the waste in waste water
When effluent is treated at a water treatment plant it goes though a number of stages. One of these stages – ‘secondary treatment’ – is where the effluent is placed in to large tanks and air is pumped through. This allows aerobic bacteria and protozoa to digest much of the organic material in the effluent. ‘Aerobic’ describes microorganisms that require oxygen, so an indirect measure of how much organic matter is in effluent is the amount of oxygen required to remediate it. Extra processing costs time and money so if you’re producing effluent with high BOD and COD you are likely to have to pay a surcharge.
Biological Oxygen Demand (BOD)
BOD is a measure of the biodegradability of the effluent. The higher the value, the more oxygen needed to break this down.
Chemical Oxygen Demand (COD)
COD is a measure of total strength of the effluent. This includes chemicals, biological compounds, organics and inorganic chemicals and suspended solids. COD is less specific than BOD since it measures everything that can be chemically oxidized, rather than just levels of biodegradable organic matter.
Link Between BOD and COD
This depends on the industry but as a rule COD is 3 – 4x BOD. The introduction of a biological system will initially increase BOD but that will then decrease, bringing the COD down with it. A biological dosing system can reduce the COD by up to 60 %.
BOD and COD values are most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20 °C. The higher the value the more waste treatment costs.
Total Suspended Solids (TSS)
TSS is the dry weight of suspended particles remaining undissolved in a sample of water that can be trapped by a filter that is subsequently analysed. It is another water quality parameter used to assess the quality of a specimen of water. It lacks the specificity of BOD and COD because will always be variables such as type of filter paper used but it is still a useful measure when properly controlled.
