Modification of PV Driven Single Stage RO to a Semi-Batch Closed Loop RO

Climate change is creating challenges for reliable water supply and management. The highly populated areas in the world either live in the water scarcity or approaching the water scarcity. The ground water in many of the arid regions has high salinity and not suitable for drinking or agriculture. The season also influences the salinity and the level of brackish ground water. The inland desalination can be a solution for potable water supply from the saline ground water resources. The main desalination technology used in these cases is reverse osmosis (RO). But the standard RO has its own limitations such as high energy demand, limited recovery rate in a single stage, sensitive to seasonal fluctuations of raw water quality, etc. Due to limitations in the recovery rate, large amount of brine is produced. The brine disposal is a big challenge especially for inland areas. If the brine is disposed untreated to the existing water bodies or to the environment it may worsen the situation and increase salinity of the water resources significantly in the long term. Also, if the energy source is fossil fuel, it increases the CO2 footprint of the system. The renewable energy driven RO can address this issue of carbon footprint, but not the issue of fluctuating raw water conditions. RO systems are not usually designed for transient operation. The closed loop reverse osmosis (CLRO), commercially known as CCRO, can address some of the disadvantages of standard RO, such as fluctuating feed concentration and limited recovery rate. The project ‘Enhancement of the recovery ratio in brackish water desalination systems for agricultural irrigation project – HighRec’ aims to develop technologically, environmentally and economically comprehensive concepts for high recovery brackish water desalination system with compliance to the irrigation water requirement. The focus in this project is on the CLRO desalination system driven by photovoltaics, as it can adapt to the energy supply and feed concentration. The CLRO process is a semi batch process and operates in two modes: plug flow and closed loop. The time of operation of closed loop and plug flow decides the overall recovery from the system. The project also includes building a small-scale demonstration system. in Qatar for water supply to hydroponic green houses. A small scale CLRO plant is modelled/sized in a CLRO tool developed at Fraunhofer ISE. The modelling or sizing tool is written in Python language. The tool is developed to overcome the limitations of commercially existing semi-batch sizing tool. The program allows the user to select from a vast range of membrane elements existing in the market and gives suggestions to choose a proper membrane based on the characteristics of feed water. While sizing the system, the program is set to consider the design guidelines from the membrane manufacturer without surpassing these limitations. The tool is modularized for simplification and for versatile use. The individual modules include Water and salt permeability coefficient, Osmotic pressure calculator, Membrane selection and design guidelines based on silt density index (SDI), System sizing for CLRO, Plug flow mode and Closed loop mode. The final output from the program contains element by element performance data and overall cycle performance data and the results are exported to an excel file. A commercially available, containerized, small RO system including pretreatment, dosing and cleaning units is selected as a pilot plant. The key parameters for the pilot system are 1.5 m³/h capacity, seawater as feed, 30% recovery rate, feed flow rate up to 5 m³/h. The containerized system offered by Boreal Light GmbH was selected as a base system as it fulfilled all the key parameters and was cost effective. The retrofitted system based on the data from the modelling tool is equipped with a feed and recirculation pump to realize closed loop and variable flow rates and pressures during the operation. The system is equipped with 21 kWp PV panels without battery backup and a grid connection for continuous operation. The system is currently under operation in Qatar and supplying water to the horticulture farm. The data access and monitoring of the system can be done remotely. In this contribution, the designed values obtained from the modelling tool and measurement data from the pilot plant are compared. In addition to that the comparison between the standard single stage RO system and retrofitted CLRO system in terms of energy consumption, water recovery and permeate production are given. The lessons learnt from this simple method of retrofitting are low effort in construction, reduction in cost intensive parts compared to systems with side conduit or energy recovery devices. On the other hand, it comes with its own disadvantages, such as frequent variation in the operating points of the pumps, realization with low automation, variable permeate flow rate and recovery.