NTPlus trial turns water treatment 'waste' into liquid fertiliser

Chemical fertilisers help farmers increase yields, but they are taking a toll on the environment. This project looks to address the challenges while retaining the positives.

The NTPlus project, funded through Innovate UK’s Sustainable Innovation Fund and led by Agua DB Ltd in partnership with Carbon Data Resources and CHAP, ran from January to March 2021. It focused on feasibility testing a transformational process that removed nitrate from groundwater. The process produced a solution that could be used as liquid fertiliser in agriculture, turning something hitherto discarded as waste into a valuable resource.

Challenge

The introduction of chemical fertilisers – nitrogen and sulphate – helped farmers increase yields in the 20th Century, but dependence on these is taking its toll on the environment.

Nitrogen deposits in agricultural land reduce natural resistance to climatic and pest threats and can leach out into nearby streams and rivers, killing any life in them. Nitrous oxide emissions are also a main cause of greenhouse gas – more than carbon – so reducing them would help to achieve Net Zero.

Nitrates in surface and groundwater (agriculture is responsible for about 60% of these), affect drinking water quality, and potentially impact human health. In the UK, treatment of groundwater to remove nitrates and meet drinking water standards is increasing, especially in the south and east.

The same areas are also seeing increased need for crop irrigation so there is an opportunity to kill two birds with one stone, so to speak: can we re-use the resulting ‘brine nitrate’ waste as a smart, variable-rate liquid fertiliser and irrigation (fertigation) product?

Methodology

Nitrates are removed during the water treatment process using a sodium chloride solution, with about 10 tonnes of sodium chloride salt needed to remove 1 tonne of nitrate. At the same time, the farm next door to the treatment works buys potash (of which potassium chloride is the most commonly used source) fertiliser.

By harnessing Agua DB’s patented ion-exchange process technology to build a prototype treatment system at Affinity Water’s Chipping plant, and by optimising the treatment process using potassium chloride in place of sodium chloride, NTPlus was able to:

  • Increase the process efficiency of nitrate treatment from 10% to more than 90%, considerably reducing the amount of salt needed.
  • Demonstrate effective and repeatable nitrate removal to concentrations below the drinking water standard and in line with water company standards; critically proving the feasibility of the process for water company adoption.
  • Produce a soft-water solution fertiliser product – in place of waste – that contains all four principal fertiliser nutrients (Nitrogen, Potassium, Phosphate and Sulphur) and has a low chloride content.

Given the short project timescale and winter season, CHAP worked with partner Stockbridge Technology Centre to test this product as a hydroponic feed in a vertical farming setup, rather than conducting broad-acre field trials. The 21-day trial successfully grew Russian Red Kale to micro-green size and assessed the growth, root structure and plant biomass, comparing these results with a crop grown using a conventional liquid feed.

The NTPlus crop showed good germination, root structure and overall morphology, a comparable fresh weight versus the control, and a representative nutritional profile with no deficiencies.

Results

The crop trial validated the NTPlus nutrient solution as a hydroponic fertiliser, proving its utility within Controlled Environment Agriculture. It could equally find a route-to-market as a liquid fertiliser or smart, variable-rate fertigation product within field agriculture, particularly in row or bed crops. Smart irrigation and variable-rate fertilisation are recognized as best practice and can deliver yield improvements of about 10% yield.

Agua DB Ltd is actively engaging with a number of water companies to assess the opportunities and challenges for modifying nitrate treatment plants to use the NTPlus process. Further crop trials are needed to assess the NTPlus nutrient product in representative field agricultural contexts, and the NTPlus consortium hopes to engage farmers in overcoming any other barriers to adoption.

Potential Impact – Net Zero

Carbon Data Resources Ltd worked with Agua DB and CHAP to translate results into a carbon footprint, comparing this with the status quo. Calculations based on the trial operations at Affinity Water’s Chipping plant, indicated that the conventional nitrate treatment process had a carbon footprint of approximately 23.2 tonnes of CO2 equivalent per year. The NTPlus treatment process cut this to about 12.5 tonnes, with the majority of that saving coming from material and transport reductions.

With the abstracted groundwater composition used within the NTPlus project, an overall ‘54% recycled nutrient’ contribution to a fully balanced liquid hydroponic feed (including 58% of the nitrate) was achieved.

While it is difficult to translate this fully for field agriculture (as fertiliser use is farm-, crop-, water- and soil-dependent) when used as a smart irrigation product, which itself has the potential to reduce the amount of nitrate fertiliser required by up to 60%, there is a significant sustainability value to recirculating more than half of the required nutrients back into the system.

Based on a moderately-sized nitrate treatment plant, estimates show there would be sufficient NTPlus nutrient product for 200 hectares of field fertigation. Similarly, a business-to-business relationship with one or more of the principal liquid fertiliser manufacturers in the UK could be mutually beneficial.

For more information on the CHAP facility involved in this trial go to Vertical Farming Development Centre  capability page or visit Stockbridge Technology Centre. For information about the funding body, go to Innovate UK.

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