The amount of coloured dissolved organic matter in peatland runoff can be reduced with the aid of filtering based on the use of bed sand from biopower plants. Using the sand on a large scale still requires further testing before any potential practical use.
Soil preparation and drainage repair in peatlands result in organic matter dissolving and getting into the waterways. Sediment basins and overland flow solutions, which capture suspended solids, are not useful for removing dissolved organic matter. Organic matter dissolved in runoff water raises the biological oxygen demand of water and turns it brown.
For a few years now, Metsä Group has been studying the use of materials generated in production side streams for purifying water extracted from the ditch network in peatlands. A total of 16 different side stream materials have been examined. The goal has been to find new uses for the materials and reduce the damage forestry causes to peatland.
“In laboratory tests, the brown water extracted from the peatland ditch network became clearer when it was fed through a filter made of discarded bed sand used in power plants’ fluidised bed boilers. The humic substances reacted with the filter, forming sediment. We also tested bed sand in small-scale field experiments,” says Director of Sustainability Vesa Junnikkala.
The exact reason for this remained unclear in Metsä Group’s own research, so the company asked VTT Technical Research Centre of Finland for help in determining the source of the purifying effect.
“Bed sand contains elements such as calcium, aluminium and iron, which are known to form sediments when they come into contact with humic substances. The bed sand treatment especially reduced the chemical oxygen demand of water and the quantity of coloured components,” explains Joni Lehto, Research Scientist at VTT.
“Recycled sand from fluidised bed boilers is itself a useful and inexpensive material for removing humic substances from acidic peatland drain water. However, further research is still required. As our tests were based on laboratory-scale experiments, the practical and large-scale use of sand calls for additional testing,” says Lehto.
The project’s results were published in Environmental Technology in the spring of 2021.
“By publishing our results, we want to ensure that our observations are freely available to all parties. We hope that these new results will inspire further development of bed sand use in the treatment of runoff water,” says Junnikkala.
For example, before bed sand treatment can be used more extensively, we need to know whether bed sand can be used to build point-like leaching beds into which humic waters can be fed, and how often the sand needs to be changed. It is also important to determine the kind of further processing required for the sand once it has lost its effect, and whether the sand can be left in place.
An example image is available here (only online resolution).