Membrane processes like nanofiltration and reverse osmosisare the more suitable options. Due to the small pore sizes the membranes can remove biological contaminations such as multiresistant pathogens as well as residues of anthropogenic trace elements like microcontaminants (pharmaceuticals, micro-plastics) as well as colloids and ions.
Several operation modes of reverse osmosis plant for reduction of nitrate levels in drinking water are introduced below.
The filtration of the total flow volume using one or more reverse osmosis plants of TDL Energie is the simplest solution in terms of technology. However, due to the removal of almost all drinking water components such as minerals, the water no longer conforms to requirements of the drinking water ordinance and thus needs to be re-treated. In addition the concentrate – the waste of the plant – is about 10 % of the inlet flow. The concentrate can be further treated in a municipal wastewater plant.
To simplify the post-treatment of low-mineral drinking water, a partial flow of the drinking water can be separated upstream of the reverse osmosis plant and later mixed with the RO permeate. The blending reduces the nitrate level at the outlet so much that it definitely falls below the limit. In addition the remaining salts are sufficient to make post-treatment unnecessary.
An elegant method to combine the advantages of both concepts is the combination of nanofiltration and reverse osmosis. This requires the selection of an NF membrane with as little retention for nitrate as possible. Divalent ions are removed and accumulate in the concentrate while the nitrate remains in the permeate. The concentrate is later mixed with the NF-permeate cleaned by reverse osmosis. This way the amount of minerals remaining in the drinking water is high and no post-treatment is necessary. The concentrate volume is cut almost in half compared to that of a mere reverse osmosis plant without Nanofiltration installed upstream as “nitrate filter”. The added expense can be justified by the fact that the pre-filtration of dissolved and suspended particles reduces the operational pressure needed for the RO plant, therefore making the use of smaller, more energy-efficient pump systems possible. High capacity RO plants in particular can cut cost this way in comparison to the bypass solution. As an alternative the water yield can be increased so that the concentrate volume can be reduced to less than 5 % by adjustment of the pump pressures.
As a plant manufacturer and operator of water technological plants TDL Energie will gladly provide you with further information. We will be pleased to make you an individual offer.