RESOURCE MANAGEMENT
The P-saturated grains can then be used as P fertiliser. There is less post-processing and there is no addition of chlorides compared with other P-removal techniques. The technology can also be used to remove other pollutants. This technology is better suited to smaller nurseries or farms with limited amounts of wastewater and less capital for larger installations. Moving bed biofilm reactors works on a smaller footprint than an activated sludge process (wastewater treatment process using aeration, bacteria and protozoa to treat sewage or industrial wastewaters). Small plastic carriers each containing a biofilm are kept in motion in the water. As they move, the biofilms on the carriers pick up impurities from the water. The result is a high treatment capacity in a relatively small area. A 3m3 reactor with carriers would cost around £3800, and would be able to treat the wastewater of 2–3ha with a maximum flow rate of 13m3/day.
The prolonged hot and dry weather this year gives us a taste of what could be in store for the future Electrochemical phosphorus precipitation is another P-removal technology, but on a larger scale and is more expensive (about £180K). Wastewater runs into a tank containing a cathode and a ‘sacrificial anode’ made of magnesium. When an electric current is applied, the magnesium anode breaks down and reacts with the P and ammonium or potassium in the water, which then precipitate out of the water as P-salts. This process removes about 80% of the P, which can be reused as fertiliser. The technology can be expensive due to the amount of electricity it requires, but its modular nature means it can be scaled to fit the amount of wastewater produced, and no additional chemicals are needed. Finally, constructed wetlands are artificial wetlands designed to mimic natural wetland processes, such as acting as a biofilter, or removing sediments and pollutants such as heavy metals from the water. They work over a larger scale and require planning permission. The result though is a system that can process about 100-litres of drain water/m2 wetland each day. If the wetland were a large flow system, it would cost around £22/m2, whereas an aerated wetland can cost up to £890/m2. The system is relatively low maintenance and can last up to 15 years. A wetland is very efficient at removing nutrients in the first few years, and produces water with a consistent pH of 7–7.5. However, proper design and construction are essential, as is a source of carbon for the bacteria present in the system.
NEXT STEPS The Fertigation Bible can be downloaded free of charge at fertinnowa.com/the-fertigation-bible FERTINNOWA has information on technology that has come from other sectors or industries, which can be applied in horticulture. Find out more at fertinnowa.com/technologies-exchanged For more information about Nitrate Vulnerable Zones, visit: gov.uk/government/collections/nitratevulnerable-zones For advice on building reed beds, see: Building Research Establishment Good Building Guide No 42 (GG42) Reed Beds 2001. ISBN 1860814379
Dec/Jan 2019 23
