Phosphorus precipitation at a small hotspot ditch
Two small sized phosphorus precipitation devices were installed in nutrient load hot spots of Aurajoki river catchment. The device uses ferric sulfate (Kemira Ferrix-3) to convert dissolved phosphorus into particulate form, sparsely available to algae. Granular precipitant is dosed directly into stream water with dispenser unit originally developed by Aaro Närvänen at MTT. The dispenser devices used at the pilot sites are a small size adaptation of the original dispenser unit.
Components installed in the solution
The dispenser unit consists of a cone shaped polyester dosing headpiece, chemical container, dosing pipe and mounting part. Precipitant is dissolved into stream water from dosing headpiece that is partly submerged in water. At the demonstration sites, V-shaped pipe outflow (demonstration site 1) and V-notch weir (demonstration site 2) were used to change the water level and volume of submerged headpiece in relation to water flow in the ditch. As the flow increases, also the contact with precipitant increases, and more ferric sulfate is dispersed into the stream.
Both demonstration sites were established at small sized agricultural ditches at the municipality of Lieto. These ditches are located in Savijoki catchment area, which is one of the biggest sub catchments of river Aurajoki. The demonstration sites were chosen for pilot studies due to high dissolved phosphorus concentrations, verified by water samples taken from potential nutrient hot spots.
Field ditch at demonstration site 1 collects water from a small catchment of under two hectares in size. Catchment area includes horse stable, horse pens, agricultural field and housing. Pilot site 2 is located in a small ditch, downstream from cattle pasture areas of organic cattle farm. Total area of the catchment is 19,4 hectares, including horse stable and horse pens, agricultural field and housing.
According to the water samples taken, ferric sulphate precipitation has reduced significantly the concentrations of dissolved phosphorus in the pilot ditches. Dissolved phosphorus reduction at demonstration site 1 was 85-99 % (average 96 %) and in demonstration site 2 11-98 % (average 66 %).
It is considered that higher velocity of the flow, in closed pipe structure, caused increased friction to the chemical dispenser, leading to over dosing of the chemical. In demonstration site 2, notable difference in pH did not occur between the sampling points. Changes varied from 0 to 0,3 units (average 0,06).
The method was proven to have high reduction rate for dissolved P, when operational and can be recommended for high dissolved P load hotspots. Dissolved phosphorus reduction percentage on both demonstration sites was considerable.
Because of the hygroscopic properties of ferric sulphate granules, the precipitation chemical clogged several times in dispenser unit dozing pipe and headpiece. Clogging was partly prevented by improving the waterproofness of the chemical container and dozing pipe. Dispenser unit functioning was also limited during winter time due to subzero temperatures and icy conditions.