Septic tank and package treatment plants: liquid effluent pollutants and typical concentrations
Updated 13 January 2017
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Effluent analysis
You can expect a typical septic tank or package treatment plant to have a typical range of pollutants. Where this isn’t appropriate, or if you discharge over 50 cubic metres of effluent a day, you’ll need an analysis of the effluent as part of any quantified risk assessment.
When no other information is available, you can use these values when you carry out a risk assessment for an infiltration system. However, site-specific data is always preferred.
| Contaminant | Sewage (mg per litre) | Septic tank discharge (mg per litre) | Package treatment plant (mg per litre) | Domestic waste water from single property (mg per litre) |
|---|---|---|---|---|
| BOD (5 day ATU) | about 380 (residential properties) | 368 | 20 - 55 | 300 |
| COD | 677 | 210 | 400 | |
| Ammonium (as NH4+) - see further information for more details | about 50 (residential properties) | 104 | 89 | 50 |
| Chloride | 68.6 | 88.1 | ||
| Phosphorus | 15.8 | 10.5 | 10 | |
| Boron | 0.84 | 0.37 | ||
| Lead | less than 0.005 | |||
| Zinc | 0.03 | 0.01 | ||
| Copper | 0.015 | 0.007 | ||
| Iron | 0.13 | 0.04 | ||
| Cadmium | less than 0.002 | less than 0.002 | ||
| Arsenic | less than 0.015 | less than 0.015 | ||
| Pharmaceuticals | No information | No information | ||
| Other organic products | Sporadic: mineral oil, quintozene, methoxychlor, permethrin, phenol, 4-methyl-phenol, toluene, phthalates | Sporadic: mineral oil, quintozene, 4‑methyl-phenol, toluene, phthalates |
Further information
For ammonium (as NH4+), ammonium may be transformed to nitrate in the drainage blanket or unsaturated zone and your risk assessment may need to consider the impact of nitrate on groundwater quality. In theory, 50mg per litre of ammonium (NH4+) could be converted to 172mg per litre of nitrate (NO3-) if there were no losses of nitrogen.
Data sources
You can get further information on the data sources for:
- sewage (mg per litre) from British Water
- septic tank discharge and package treatment plant from the Environment Agency - be aware that the effluent quality of package treatment plants will depend on the type of plant and treatment process
- domestic waste water from single property (mg per litre) from the Environmental Protection Agency