Monovalent selective electrodialysis in greenhouses - An analysis of brackish groundwater and greenhouse wastewater treatment

Abstract

Water use in greenhouse can be optimized by switching to alternative sources of irrigation water and building zero-liquid discharge systems that recirculate greenhouse effluent. The use of brackish groundwater (BGW) has become a popular option for greenhouses that have adopted reverse osmosis (RO) to reduce the salinity of their source water. Similarly, RO has emerged as the most commonly used desalination technology for wastewater treatment. In either case, desalination treatment is necessary to remove monovalent sodium ions, which occur naturally in BGW and can build up in reuse loops when other ions and part of the water are consumed by the crops.
During RO treatment, ions are removed indiscriminately, including multivalent nutrients that support and encourage crop growth, leading to increased fertilizer use as the multivalent ions have to be reintroduced to the irrigation water. Unlike RO, monovalent selective electrodialysis (MSED) selectively removes monovalent sodium while retaining multivalent nutrients in the solution. This allows for the targeted removal of sodium ions without an inversely proportional increase in required fertilizer use. MSED can furthermore remove monovalent nitrate, a major environmental pollutant, from greenhouse wastewater, allowing for the safe discharge of treated effluent.
This thesis analyzes the experimentally determined monovalent selectivity of Fujifilm MSED membranes for 13 BGW compositions, as well as their ability to treat eight greenhouse effluent compositions with a focus on nitrate and sodium removal. In addition, the latter experiments were also conducted for a set of Neosepta MSED membranes. The Fujifilm membranes analyzed showed better BGW desalination performance (i.e. higher monovalent selectivity) than has been reported for Neosepta membranes. In the BGW experiments, solute ratio was found to influence selectivity, as the highest selectivity was observed at the lowest solute ratios. In the greenhouse effluent experiments, Neosepta membranes outperformed those manufactured by Fujifilm, as Neosepta membranes removed more nitrate and sodium.

Popular Abstract

Greenhouses can play an important role in ensuring food security in many areas with non-optimal conditions for conventional open field agriculture. One crucially important condition is access to high quality irrigation water. This thesis describes the analysis of a new treatment technology that can be used to improve the quality of water used in greenhouses.
While it may seem obvious that growers in areas with hot and dry climates will find it difficult to find water to irrigate their crops, this can also become a problem in places where water seems to be abundant because the groundwater that is often pumped into greenhouses to irrigate crops heavily varies in quality. Groundwater slowly moves through the layers of soil and rock before being pumped back to the surface. During this time, minerals, which are essential for crop growth, are dissolved from the surrounding underground environment. The problem is that plants are also very sensitive to certain minerals in groundwater. Increased concentrations of sodium ions, which we ingest every day as table salt (sodium chloride), are a common ingredient in groundwater that can cause lower crop yields or the death of plants. Greenhouses therefore often remove all minerals from their groundwater using a membrane technology known as reverse osmosis. All the beneficial minerals that plants need to grow are subsequently reintroduced to the treated groundwater as fertilizer. The technology discussed in this thesis, called monovalent selective electrodialysis or MSED, can selectively remove a small number of dissolved minerals including sodium from groundwater. This reduces the need to use a lot of fertilizer after treating the groundwater. The experiments conducted for this thesis show that MSED works and that the technology can be used as an alternative to conventional groundwater treatment.
After crop irrigation, the resulting greenhouse wastewater is either treated and discharged or recirculated and reused. Water reuse lowers groundwater needs, but it can also cause sodium to be trapped in the reuse loop as it is not utilized by the plants. Over time sodium concentrations increase until they exceed the threshold that crops can withstand and the reuse water has to be discharged to prevent crop loss. To avoid the discharge of reuse water, MSED can be used to continuously remove sodium from the recirculated wastewater.
In this thesis greenhouse wastewater was tested to see if MSED could remove sodium from such compositions. It was found that this is possible. Another aspect investigated was the removal of nitrate, which enters the greenhouse water cycle as added fertilizer, using MSED. While nitrate encourages crop growth, it is also considered an environmental pollutant that can pose health risks if it leaches into the drinking water supply. The experiments on greenhouse wastewater treatment using MSED showed that nitrate can also be removed by this technology.
Based on the results presented in this thesis, several avenues to MSED implementation in greenhouses are discussed. MSED groundwater treatment can be useful when old technologies that cannot selectively remove sodium are currently in use or when groundwater is currently not utilized for irrigation due to high treatment costs. Moreover, water reuse in greenhouses can be made more efficient using MSED. Greenhouses can even consider implementing this new technology to replace biological nitrate removal treatment steps, which use a lot of space and do not allow a high degree of process control. While more research into case-specific aspects of MSED use in greenhouses and economic feasibility have to be done prior to any major changes throughout the greenhouse sector, this work suggests that MSED could play an important role in optimizing future food production.

• author: Schücking, Georg
• supervisor: Åsa Davidsson
• organization: Chemical Engineering (M.Sc.Eng.)
• course: VVAM01 20201
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: desalination, water treatment, greenhouses, ED, MSED, monovalent selective electrodialysis, groundwater, sodium, nitrate, water engineering, environmental engineering, vattenförsörjningsteknik, avloppsteknik
• language: English
• id: 9032014
• date added to LUP: 2020-11-27 09:46:51
• date last changed: 2023-08-22 13:30:43