Steel slag filter as a potential solution for reducing dissolved phosphorus inputs into the Baltic Sea

Hankeaika: 1.6.2018 – 31.10.2019
Myönnetty rahoitus: Enintään 5 000 euroa
Lisätietoja: Aleksandar Klimeski, info@hydro-chemie.fi

Novel methods are needed to mitigate eutrophication due to losses of phosphorus (P) from agricultural fields and thereby P removal structures are attractive alternatives. Their maintenance and chemical costs are lower than, for example, those of chemical precipitation processes. Therefore, the application of solid P-retaining materials as filters for treating municipal wastewater as well as agricultural runoff has been investigated increasingly in the past decades. However, in Finland, rather limited knowledge about P removal structures exists.

The objective of the proposed project is to build a P-retaining filter, containing steel slag (cheap industrial by-product) as a reactive medium, in an agricultural ditch collecting runoff. The filter would possibly intercept the largest portion of the water flow in the ditch and would aim to remove a significant part of the dissolved P amount in the agricultural runoff. My previous laboratory experiments with steel slag showed that it removed 60% of the dissolved phosphorus (P) input while applying influent concentration of 50 mg P/l and retention time of about 1 minute. However, agricultural runoff contains significantly lower P concentrations, commonly below 0.5 mg P/l. The aim of the proposed project would be to treat the largest portion of the water flow in the ditch, potential peak flows during snowmelt could reduce the treated portion of the total flow. The steel slag filter would enable sufficient retention times and I estimate that it could achieve a reduction in the dissolved P concentration in the runoff by more than 30%.

In order to establish good water quality in the Archipelago and in the Baltic Sea, it is of uttermost importance to reduce phosphorus (P) inputs into the sea. Non-point sources of P (agricultural fields, animal feedlots) contribute about 50% of the total P input into the Baltic Sea (HELCOM, 2011). Pitkänen et al. (2007) showed with a model that reducing the total P inputs into the Gulf of Finland by 44% could, in 5 years, diminish the algal population by up to 35%. Therefore, in addition to other best management practices, building filters in agricultural areas that would remove a significant part of the dissolved P concentration in runoff (e.g. 30-40%) offers a vast potential to improve the quality of surface waters. Moreover, calcium-rich materials such as the steel slag offer the possibility to recycle the retained P. This filter material precipitates phosphates as calcium phosphates and these compounds could be brought back to agriculture as slow-release fertilizer.