StoPWa

Results

The StoPWa project explored how construction and demolition waste (CDW) can be reused as filter materials
for stormwater treatment, addressing both water pollution and the large volumes of underutilised waste
generated in the construction sector. The work combined laboratory experiments, pilot-scale testing in three
countries, and environmental and economic assessments.

Two main filter materials were investigated. Crushed CDW concrete proved to be technically feasible but not
so effective in water treatment. Laboratory and field results showed a limited performance in removing key
pollutants such as suspended solids, organic carbon, nutrients and some heavy metals from stormwater, with
the 16–32 mm fraction being the most suitable for practical use. The material’s natural alkalinity initially
increases pH, but overall treatment performance is promising and relatively stable. In contrast, agglomerated
mixed CDW, produced by binding fine waste fractions with recycled plastics, showed mixed and largely
negative results. Although laboratory tests indicated significant nutrient removal and microbial activity, pilotscale
studies revealed that the material can also leach nutrients, metals and potentially microplastics into
water. This means it may worsen water quality rather than improve it. As a result, the report concludes that
mixed CDW and plastic-bound agglomerates are currently not advisable for stormwater filtration without
further development.

Three stormwater field pilot sites were designed and constructed in the project in Finland, Estonia and Latvia.
The sites had variable designs, dimensions, filter applications and filter materials to provide a broader
comparison and to enable the use of local CDW materials. Preliminary stormwater monitoring results showed
a reduction in the suspended solids and nutrient levels, but variable results in the heavy metal levels.
Life Cycle Assessment (LCA) showed that stormwater treatment systems provide clear environmental
benefits during operation by reducing nutrient loads (nitrogen and phosphorus) and toxic metals entering
water bodies. However, these benefits can be offset by environmental burdens especially from construction
but also from transport, maintenance and end-of-life handling. Using CDW can support circular economy
goals and reduce the need for virgin materials, but sustainability depends strongly on factors such as
transport distance, durability and potential leaching. From an economic perspective, CDW-based filters are
competitive due to low material cost, local availability and reduced processing requirements compared to
conventional materials like expanded clay or biochar. Overall, CDW-based solutions offer strong potential for
cost-effective and circular stormwater management.

Several knowledge gaps remain. The long-term performance and durability of CDW filters in real conditions
are still uncertain, especially regarding clogging, ageing and pollutant breakthrough. The risks of contaminant
leaching—particularly from mixed CDW and agglomerates—require further investigation. In addition,
questions remain on microbial processes within filters, optimisation of material processing methods, and the
possible leaching of microplastics. Finally, scaling up production and ensuring consistent material quality
remain key challenges for wider implementation of CDW. Further pilot studies and standardisation are
necessary before large-scale adoption.