Zero liquid discharge for greenhouse horticulture (phase 2)

The objective of this project is the development of a concept to achieve zero liquid discharge (ZLD) in greenhouse horticulture. In light of the fact that discharge standards and environmental requirements will become steadily stricter into the future, it is important that a concept be developed for the reuse and/or neutralisation of residual streams. This would involve making residual streams more ‘liquid’, so that they can be reused or discharged without problem. In this study the ZLD principle is given broader scope and with zero pollution discharge is the ultimate aim.

Technology

The concept to be developed is directed at the treatment of discharge water and the reuse of the resulting residual streams. The concept consists of several treatment techniques. To determine the right concept, the techniques are sub-divided into pre-treatment and desalination groups. The goal of pre-treatment is (I) the removal of plant protection products (PPPs), growth regulators and pathogens, and (II) the protection of the desalination processes through the removal of suspended solids and organic matter. The goals of the desalination step are (I) the highest possible concentration level for a maximisation of water reuse, (II) an effective nitrate removal from the discharge water, and (III) the generation of an ultimate residual stream (concentrate) that can be used as a nutrient stream. High recoveries (>96%) in membrane installations are attainable through the combination of nanofiltration (NF) softening and reverse osmosis (RO).

Challenge

The objective is to develop a jointly-conceived integrated ZLD concept for greenhouse horticulture, which is potentially viable and demonstrable (phase 1); and subsequently (phase 2), to conduct a proof-in-principle of the concept.

Solution

A theoretical ZLD concept for the specific situation has been developed, based on membrane technology. It draws in part on two workshops held with the project partners and branch stakeholders in 2013. In order to underpin the theoretical concept, a number of bench-scale experiments were carried out with water-practice water using three NF membranes with different permeabilities. The results showed that the retention properties of these membranes in the treatment of discharge water as a result of the specific matrix (including high NO₃) differ from the retentions in the treatment of ‘natural’ water; this led to an adjustment of the original concept. The modified concept represents an innovative new principle which makes optimal use of the properties of the carefully selected membranes, in combination with a smart, variable operational management of the NF and RO. This led to a separation of strongly concentrated components in different water streams. The concept is the basis of a proof-of-principle research project with an 8-inch pilot installation (capacity about 1m³/h). The results of this research will be incorporated into the definitive treatment concept for possible use in a projected demo installation of 5 m³/h (not part of the present TKI project).