Swedish report on WWTP and microplastics
The report shows that waste water treatment plants (WWTP) remove 99% of microplastics from waste water, only 40% of these microplastics end up in sludge, and there are existing microplastics in arable land, whether fertilised with sludge or mineral fertiliser. Microplastics is a widespread problem, and the water sector is handling its part very well.
Of society's total emissions of microplastics, only a few percent end up in WWTP plants. Most of these microplastics come from road traffic, artificial turf and plastic waste, with very little actually coming from homes. WWTP are very good at their role, and remove the vast majority of microplastic particles.
Microplastics are found in agricultural land - whether it is fertilised with sludge or not
Microplastics may end up in agricultural land via wind, through littering, wear n tear, and through fertilisation.
The study compared land fertilised with sludge, with land fertilised with only mineral fertiliser. The report shows that, although sludge has been used as a fertiliser for over 35 years, with about 30% more sludge than in a normal dose (1 ton of dry sludge per hectare and year) - the occurrence of microplastics in the soil in Petersborg outside Malmö did not increase.
Compared with the amount of microplastics that theoretically should be in the field, the farmland fertilised with normal sludge for 35 years contains about ten times fewer microplastic concentrations than could be expected.
The results indicate that over time there is a continuous reduction in the amount of microplastics in soil fertilised with sludge. This may be due to ground degradation or fragmentation of the microplastics to below the detection limit of 0.010 mm.
WWTP remove almost all microplastics
The report shows that Sjölunda WWTP in Malmö, removes around 99% of microplastics* from waste water, but only around 40% of the removed microplastics are found in sludge.
"We are not fully sure why such a small proportion of microplastics are found in sludge. They could get trapped in sand or grease at the WWTP, or are further broken down or fragmented. We need to look deeper at the analyses" said Anders Finnson, Senior Environmental Advisor at Svenskt Vatten”.
We are at the beginning of new knowledge - there are still uncertainties
There are several surprising results in this study. “It is important to keep in mind that the study gives us an overview of the level of microplastics. It is vital that we keep developing standard methods for analysing microplastics in sewage, sludge, arable land and biogas. We need to know more before we can draw clear conclusions about microplastics” Finnson concluded.
* Comparison was made between incoming sewage, after passing a three mm cleaning grate, and outgoing sewage.
Microplastics are small plastic particles, less than 5mm in size; microplastics of between 0.01-5 mm were analysed for this study. Performance reporting focuses on particles of size 0.01-0.50 mm.