Ferric sulphate is commonly used in waterworks and wastewater treatment plants to precipitate solids, organics and phosphorus. The same chemical is also useful in phosphorus stripping from runoff agricultural waters.
MTT (Agrifood Research Finland) presents in Jokioinen Elonkierto pilot area one possible solution for runoff treatment in a ditch. The dosers in runoff applications need to be simple and as maintenance-free as possible. This doser consists of a container with a piece of pipe led through its bottom, and a cone-shaped netting bag attached to the end of the pipe.
The container is filled with granular ferric sulphate (e.g. Ferix-3 manufactured by Kemira Chemicals Ltd.), that dissolves from the cone at a speed that depends on the surface area exposed to water (i.e., water level in front of the v-notch weir). The doser can be scaled up and down for different ditches by simply changing a different-sized pipe and netting cone, and fine-tuned by adjusting the angle of the v-notch weir.
According to MTT´s former experiences, a dose that drops pH of runoff water by 0.5 units is adequate for precipitating up to 95% of DRP in runoff. For flocking suspended material in runoff, a larger dose is needed (and a pH decline of >2 units). In higher doses, the flocks formed, however, are fluffy and take a large volume, and need a sedimentation basin after the doser to settle out.
The Ferix-doser is primarily meant for treating high-phosphorus waters, because the economy of chemical stripping is strongly dependant on the phosphorus concentration in runoff.
Sachtofer PR granules
Reactive permeable barrier is ideally combined with a wetland that, to some degree, even out flow peaks. In the site located at the Elonkierto area, MTT has utilized an old dam construction and attached a perforated PVC-pipe into a pre-existing flow regulation structure (inlet pipe).
The inflow into this prototype buffer is from below, through the granule mass, and out of the buffer via a v-notch weir over the lower dam. The 6.5-m area between the dam structures is filled with about 6-7 m3 (about 9 tn) of Sachtofer PR granules, and the theoretical phosphorus retention capacity of this granule volume exceeds 60 kg of phosphorus. If the granule buffer works as well as the laboratory tests suggest, this buffer would have an effective life-cycle of up to ten years.
Calcium-iron oxide buffer in Jokioinen, the first test site with boosted phosphate removal in the Active Wetlands project. Water from a small wetland is led through a granule mass that retains phosphate from the water. Kuva: MTT.
A Ferix-doser and a settling pond in Lake Rehtijärvi area, Jokioinen. At this site, ferric sulphate was dosed during spring flood, and application of 400 kg of Ferix-3 stripped 1 kg of P from water containing 60–140 µg DRP l-1 (i.e., typical field runoff situation); in hot-spot ditches, a higher efficiency is obtained. Kuva: MTT.
Until the end of the year 2011, the granule buffer has worked with a mean 30–45% efficiency in the removal of dissolved P (Figure 1). This is somewhat lower than in the laboratory (40–70% removal of dissolved P), but the efficiency is much higher than for the abatement methods for dissolved P that are currently used in the farms. The method also cuts the P transfer to watercourses immediately. For total P, the efficiency in abatement has been 5–20% because most of the P at this site is carried with eroded particles, and particulate matter largely passes through the buffer.
Figure 1. Removal efficiency (as %) for dissolved P by the Jokioinen/Ojainen Sachtofer PR granule buffer. Also shown is the flow (l/s) measured at the outlet from the buffer.