Nutrients and water from drain water in agriculture
Currently, the horticultural sector still faces major challenges in the processing of drain water, the excess nutrient water enriched with plant fertilisers that growers can no longer reuse.
Due to the presence of nutrients and plant protection products (gbm's) in that water, the Manure Decree (MAP5) obliges growers to process it in an environmentally responsible way. This means that they can choose to either sell the flow on grassland, but there are important restrictions and conditions for this, or have it collected by an accredited processor, which is very expensive.
Simple techniques to remove these gbm's or nutrients are already known, but they have never all been applied together for the total purification of sludge stream.
Verhoeve Milieu & Water, Proefstation voor de Groenteteelt and the Belgische Plantenkwekerij joined forces to find a solution to this problem and came up with the ZuNuRec concept. In that project, they tried to recover the nutrients from the discharge water and purify it of crop protection chemicals. These recovered nutrients are then reused as fertiliser in a so-called recirculation system.
They came to the conclusion that the combination of new techniques and the optimisation of existing processes offers new and promising possibilities for the treatment of drain water.
In general, nutrients can be effectively removed (via resins) from the spray stream and recovered for reuse. At the moment, this only applies to the nutrient nitrogen, phosphorus we still have to investigate further.
Fertilisation trials with tomato seedlings and cucumbers show no significant difference between the standard fertiliser and ZuNuRec fertiliser as far as plant development, production and fruit quality. However, the concentration of fertiliser is very low, so that it has to be dosed rather continuously.
The costprice of nutrient recovery and removal is estimated at about 40 euros/m³. That would be about €15/m³ cheaper than what growers pay on average for the removal and external processing of the water.
We had assumed that after treatment the nutrients from the spray stream would have a concentrated volume that was 10 times smaller (a 90% thickening). In practice, we achieved only 55%.
MOST IMPORTANT LESSONS LEARNED
The psychological aspect is another important barrier. The concept requires a complete change of mind on the part of the growers. At present, they are used to disposing of their drainage water externally in order to use fresh water afterwards, supplemented with nutrients to the desired concentrations. Various information sessions and trade fairs have already proved their worth, but additional awareness campaigns are necessary.
The fact that three different parties, each looking at the problem from a different angle, worked together on this project is a real added value. Each time a problem arose, we looked for solutions and explanations from different points of view, but together and constructively.
4Growing cycles of tomato seedlings in fertilisation trial
60Cucumber plants in fertiliser trial
15euro/m2 sewage cheaper
10+events and information sessions for dissemination
WHAT DOES THE FUTURE HOLD?
The combination of new techniques and optimisation of existing processes offers new and promising possibilities for the treatment of waste water. The ZuNuRec project has certainly opened the door to alternative processing of the sluice stream, but does not yet offer a fully-fledged solution.
In the near future, we want to further analyse how we can increase the current thickening of the nutrient flow, mainly by researching the efficiency of the resins that help remove the nutrients. In addition, we will examine whether it is not more interesting to apply a combined or alternative technique, such as electrodialysis. We can also investigate the use of other types of resin.
Furthermore, we are conducting research into the removal and recovery of phosphorus from the spray stream. At the end of September 2019, new resins were delivered and we started new lab tests.
In addition, we are still focusing on dissemination. Verhoeve and Proefstation are members of the steering committee S.O.Spuistroom. This group aims at informing growers about the wide range of purification technologies for the removal of phosphorus, nitrogen and GMOs. The steering group examines and evaluates various techniques from different angles (technical, economic, etc.), organises awarenesscampaigns and also provides advice with concrete solutions for the short and long term. In this way, Verhoeve and the Research Station constructively contribute to finding even better solutions and keep abreast of what is happening in the sector.