More nitrogen and phosphorus removed during sewage treatment

In 2022, the Netherlands’ 313 sewage treatment plants treated nearly 8 percent less sewage water than they did in 2021. This drop was mainly due to the prevailing dry weather conditions in 2022. There was 11 percent less precipitation, on average, resulting in less rainwater being discharged into sewers.

Improved treatment of nitrogen and phosphorus compounds

Sewage treatment plants try to remove as much nitrogen and phosphorus from wastewater as possible in order to prevent the eutrophication of surface water. In 2022, 86 percent of nitrogen compounds (92 million kilograms) were removed from treated surface water, an increase of 1.4 percent point over 2021. In addition, 88 percent of phosphorus compounds (12.9 million kilograms) were treated from wastewater, which was slightly more than in 2021.

The efficiency of water treatment has never been this high. This is partly due to the ongoing improvements of the water treatment process, but also due to the lower volume of waste water treated, which meant that sewage treatment plants were able to process it more efficiently.

Reduced residual discharges of nitrogen and phosphorus

Residual discharge of zinc and copper is more stable

In 2022, the residual discharge of zinc (69 thousand kilogrammes) was 2 percent lower than in 2021, and for copper (9 thousand kilogrammes) it was 10 percent lower. The treatment efficiency for zinc has fluctuated at around 83 percent in recent years, while for copper it was around 93 percent.

Zinc and copper are classified as heavy metals. These metals can be harmful to aquatic life in surface water.

The contribution of treated sewage to water pollution remains substantial

Total nitrogen pollution in surface water from all sectors combined was 75 million kilogrammes in 2022. Seventeen percent of this was attributable to residual discharges from sewage treatment plants. That was more than in the previous year, when the share was 15 percent.

Total phosphorus pollution in surface water was 6.2 million kilogrammes, 26 percent of which came from sewage treatment plants. That was higher than in 2021, when 22 percent came from water discharged from sewage treatment plants.

Agriculture, meanwhile, contributed 43 percent of nitrogen and 47 percent of phosphorus, mainly through run-off from arable land due to the use of mineral fertilisers.

Share of residual copper and zinc discharge from treatment plants remained stable

In 2022, the total amounts of copper and zinc polluting surface water were 109 thousand kilogrammes and 355 thousand kilogrammes, respectively. Of this total, residual discharges from sewage treatment contributed 8 percent for copper and 20 percent for zinc, almost unchanged from 2021.

Treated sewage is virtually the only source of pharmaceutical residues

Due to population growth and the ageing population, the use of medicines and the consequent discharge of pharmaceutical residues are slowly increasing. Around 1.9 million kilogrammes of active substances were prescribed by pharmacies alone in 2018. The use of medicines results in pharmaceutical residues, which ultimately enter the sewage system. In 2022, these residues amounted to approximately 1.4 million kilogrammes per year, half of which consisted of laxatives such as Macrogol and Lactulose. Metformin, an anti-diabetes drug, was the next most common residue at nearly 320 thousand kilogrammes.

In the Netherlands’ pollutant release and transfer register, the quantities discharged into surface water after treatment are estimated for approximately 30 substances. Some of these residues, such as paracetamol, can be removed for the most part. Nevertheless, an average of 40 percent of the active substance remains in residual discharges from sewage treatment plants.

Residual discharges of almost all substances are increasing by an average of 1 to 1.5 percent per year, which is in line with the ageing population and population growth.