Project shows nitrate ‘waste’ can be repurposed as fertiliser

A recent project has refined a process that allows nitrate ‘waste’ from groundwater treatment to be captured for use in agriculture as a liquid fertiliser.

The NTPlus project, financed by Innovate UK’s Sustainable Innovation Fund, was led by Agua DB Ltd, in partnership with Carbon Data Resources Ltd and CHAP. It focuses on how nitrates can be efficiently removed from groundwater for use as a resource, rather than disposal as a waste.

In doing so, this can decrease agriculture’s dependence on the carbon-intensive Haber-Bosch and Mannheim fertiliser manufacturing processes, assisting the industry in its drive towards net zero.

CHAP Innovation Network Lead, Dr Harry Langford, said: “Treatment of abstracted groundwater to remove excess nitrate in order to meet important drinking water standards is on the increase. Unfortunately, this treatment process produces a ‘brine nitrate’ waste stream, often tankered to treatment works, bringing a further carbon footprint cost.

“At the same time, we are seeing an increase in the need for crop irrigation, which is how the opportunity for this project came about – can we re-use nutrients from diffuse pollution as a smart, variable-rate liquid fertiliser and irrigation product, that also helps to reduce the greenhouse gas emissions associated with managing agricultural soils?”

The existing method for extraction uses ion exchange to remove nitrate, using a sodium chloride solution as a liquid wash during the process. This requires 10 tonnes of sodium chloride salt to remove 1 tonne of nitrate.

By using Agua DB’s patented processing technologies, the project has refined nitrate extraction to enable the use of potassium chloride salt as the wash, increasing efficiency from 10% to >90%. Not only is the amount of salt reduced, but so is the carbon footprint and logistics costs.

The end product is a soft-water solution fertiliser product containing all four principal nutrients – nitrogen, potassium, phosphate and sulphur, with a low chloride content.

The efficacy of this has been tested during a validation trial of Red Russian Kale at CHAP’s partner, Stockbridge Technology Centre, to explore the use of the solution as a hydroponic feed within the Vertical Farming Development Centre.

Dr Langford added: “Compared to the control, which was treated with conventional liquid feed, the NTPlus crop showed good germination, root structure and overall form. The nutritional profile was also favourable.

“Although this was demonstrated within a hydroponic-based controlled environment agriculture setting, the fertiliser could be trialed further for use as a variable-rate product within field agriculture, as it’s particularly suited to row crops.

“Smart irrigation is a sector forecast to grow by 17% per annum by 2025, and variable rate applications are recognised as best practice offering considerable yield benefits.

“All in all, the fact this is recycled from a by-product, improves the processing methods used, and could reduce agriculture’s reliance on carbon-intensive fertiliser production, makes for a very promising development for our industry.”

Agua DB is now in discussions with water companies to assess opportunities to modify their nitrate treatment plants in line with the outcomes of the NTPlus project. It hopes that additional field trials will enable relevant exemption certifications for its use to be obtained.

For more information on the project go to NTPlus. To find out about the CHAP facility involved in this trial go to Vertical Farming Development Centre.