Primary, secondary, advanced ? what the treatment level on a wastewater plant record actually tells you about discharge quality and environmental impact.
Every plant on UtilityRadar carries a treatment level field: Primary, Secondary, Advanced, or Not Reported. Here is what each value actually means for the water that ends up in your river.
Primary treatment is mechanical. Incoming sewage passes through screens, grit chambers, and settling tanks that separate out solids and greases. Roughly 25-40% of suspended solids and 25-35% of biochemical oxygen demand (BOD) are removed at this stage.
A plant that discharges after primary treatment only is releasing water that is still heavily loaded with dissolved organics, nutrients, and pathogens. In most of Europe and North America, primary-only discharge is illegal for anything larger than a village.
Secondary treatment is biological. After settling, wastewater is aerated in tanks where microbes consume the dissolved organic matter. The resulting sludge is settled out and the water clarified.
Secondary treatment removes 85-95% of BOD and suspended solids. It is the EU Urban Waste Water Treatment Directive's minimum standard for most agglomerations, and the baseline for US EPA Clean Water Act compliance.
The 18,000+ secondary plants in our directory are the workhorses of modern wastewater infrastructure.
Advanced treatment targets what secondary leaves behind: nitrogen, phosphorus, pharmaceuticals, microplastics, and remaining pathogens. Techniques vary: activated carbon, ozone, UV, membrane filtration, biological nutrient removal.
Advanced plants are required in sensitive catchments: drinking-water reservoirs, eutrophic lakes, bathing waters, and coastal zones with limited flushing. About 8,300 plants in our directory report advanced-level treatment, concentrated in Germany, Scandinavia, and coastal Europe.
About half of the records in our directory list treatment level as Not Reported. This does not mean the plant has no treatment. It means the underlying dataset did not publish that field. Most of these are in countries where the national regulator does not release plant-level data.
Where you see Not Reported, cross-reference with population served and discharge volume. A 200,000-person plant with a quoted discharge of 40,000 m³/day is almost certainly operating at secondary or better, regardless of whether the database spells it out.