NBS Case Study
Biofilter in Hiedanranta
In autumn 2018, a biofilter (area 100 m2) was built to purify nutrient-rich leachate. Water is conveyed into the biofiltering layers with an underdrain on about 1 m depth. The soil layer below the underdrain is around 400 mm deep peat-based growth medium that contains spruce biochar. In the drainage layer of the system, light gravel aggregates are used to improve pollutant and odour removal. The drainage layer also includes an underdrain pipeline that finally conveys treated water to nearby lake Näsijärvi. The topmost layer of the biofilter is a common growth layer enabling several plant species to grow. The species include flowering plants that offer a good living environment for pollinators. Activated charcoal is used for neutralising the unpleasant odours. The performance of the biofilter is monitored through two automatic stations and water samples.
One of UNaLab's demonstration areas in Tampere is Hiedanranta, which is a former pulp mill area 4 km from city centre. It is going to be developed into a dense urban area of 25 000 residents and 10 000 jobs. In the current urban planning phase, the City of Tampere has opened Hiedanranta to be an innovation platform where new ideas, pilots and urban culture flourish. Due to the area's industrial history, soil is contaminated and there is an old industrial landfill that contains mostly fibre waste.
To treat nutrient-rich seepage waters from the old landfill and to build a purification system that can be utilised in Finland and abroad. Neutralising the unpleasant odours caused by the water, binding carbon and increasing the number of pollinating insects.
Initial results show that the biofilter removes at least nitrogen, phosphorus and solids from the landfill leachate. At best, the nitrogen levels can be decreased by almost 80% and the phosphorus levels by 90%. The amount of solids can be decreased by 90% as well. The solution has also proven to work well in winter conditions. On the other hand, the large amount of meltwater has affected the biofilter’s performance when the filter capacity has been exceeded. The vegetation to be planted may improve the situation as the roots of the plants increase the absorption of water into the filter materials. Studies will be carried out concerning how the biofilter binds carbon and increases the number of pollinating insects.
A well-designed and properly built biofilter with a suitable capacity can effectively remove solids and total nutrients. Biofiltration is also effective in removing bacteria and phosphates. It should be noted that the biofilter can only handle relatively small amounts of water and, in the case of overflow, nutrients and other contaminants may be leached out of the structure. The advantage of biofilters is that they can be built into a small space.
Biofilter can only handle relatively small amounts of water. With a large drainage basin and therefore large amounts of water, the capacity of the biofilter needs to be sufficient or, alternatively, multiple solutions need to be used.