Author(s): Vallo Korgmaa; Salme Ruberg

Linked Author(s): Vallo Kõrgmaa

Keywords: In-stream bioreactor; Nitrate pollution; Urban river

Abstract: Clean water is essentially needed for all people around the world. This paper focuses on the wickedness of water management issues with an aid of a pilot study. Soolikaoja (Linnaoja) is located in Lääne-Virumaa, Estonia. For the most part, Soolikaoja runs through the town of Rakvere, and serves as a recipient for several wastewater treatment plants as well as stormwater outlets. The stream is a 7.5 km long heavily modified waterbody in public use with catchment area 122.1 km2. The ecological status of Soolikaoja is bad according to the WFD due to high content of nutrients (nitrates) and eutrophication. Although Soolikaoja is one stream, it can be divided into three totally different ecosystems. The upper part of the stream receives water from nitrate-sensitive area with intensive agricultural production. There are several dams within the city creating extensive network of small lakes that are eutrophic due to the nitrate polluted groundwater. The middle part is canalized and flows through pipes. The lower part is an open water channel. Due to the connection with Selja River, there is a population of trout living there. The stream is located in Nitrate Sensitive Area and fed mainly by groundwater. 48.8% of the land use of the catchment area is related to agriculture. It was estimated that 69% of the nitrogen and 44% of the phosphorus load comes from anthropogenic non-point sources (including agriculture and the non-sewered population). Taming non-point source pollution in urban area is by itself a wicked issue and limited by non-existing possibilities for land use. In order to reduce nitrate concentrations in the surface water a pilot scale in-stream woodchip reactor was installed in June 2021. For this purpose, 7.5 m3 of wood chips packed in 40 liter bags were installed at the bottom of the canal (14 x 3 m) between two small dam lakes in Rakvere. The bags were attached to each other with cable ties so that the entire bottom of the channel was evenly covered. Reinforcing nets were placed on top of the bags to secure and immerse the wood chips, at the ends of which hollow blocks were placed as an additional weight. Preliminary results show that an average NO3-N removal rate varies between 39.5 and 84.0 gN m-3 d-1 and an average removal efficiency is between 32.7 and 68.0 %. The nitrate removal efficiency is strongly (Pearson's P = -0.74, p-value < 0.01) dependent on the dissolved oxygen concentration and is limited by photosynthesis and eutrophication. Using in-stream wood chip reactors create a new possibility for taming non-point source pollution, but have to be designed with special care in order to avoid harmful effect on the water ecosystem.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221307

Year: 2022