Oxygen demand level
The oxygen demand level summarizes the residual pollution remaining in the sewage treatment plant with regard to the three most important parameters for oxygen-consuming pollution. The examination results of the parameters are summarized in five stages
- biochemical oxygen demand (BOD 5 ) - records the biodegradable pollution
- chemical oxygen demand (COD) - also records the pollution that is no longer biodegradable
- Ammonium nitrogen (NH 4 -N), the oxidation of which initially requires 4.6 times more oxygen than the BOD 5
An overall level is calculated from the three measured values, which quickly shows how well a sewage treatment plant is working and how much it pollutes the water.
In Germany, the legal requirements for sewage treatment plant drains are set out in Appendix 1 of the Waste Water Ordinance (AbwV). Sewage treatment plants meet these requirements with regard to the three parameters mentioned as soon as they reach oxygen demand level 3 (moderate residual pollution). Sewage treatment plants in SB levels 1 (very low residual pollution) and 2 (low residual pollution) exceed the minimum requirements.
Distribution of the oxygen demand levels in the DWA performance comparison of municipal sewage treatment plants 2006: level 1: 64%, level 2: 33%, level 3: 3%, level 4: 0.3%, level 5: 0.1% (based on population values; KA 10/2007)