WATER & WASTEWATER
When it comes to reverse osmosis (RO), two proper ties are typically of great interest and impor tance: energy consumption and brine discharge. By Nico-Ben Janse van Rensburg
Optimising permeate flow in RO plants
B
oth energy consumption and brine discharge have received great attention and innovation over the years, with the introduction of membranes operating at lower pressures while maintaining high rejection, energy recovery devices, high-efficiency pumps and motors, as well speciality membranes for high recovery. The distribution of permeate flow across the membranes in the pressure vessels is often overlooked. QFS believes that this is an important factor in RO design, which can result in a better return on investment (ROI) and lower energy costs on longevity of the lead membranes due to lower risk of exceeding the flux limit.
Problem statement The first element in a pressure vessel receives the lowest concentration of dissolved solids at the feed pressure of the system. A portion of the water passes through the membrane as permeate, leaving a more concentrated feed at a lower flow rate and slightly lower pressure to be fed to the second membrane element. The increase in dissolved solids increases the osmotic pressure, which in turn reduces permeate flow. The permeate flow is further decreased by the fact that the driving pressure is also lower. This leads to the formulation of the following hypothesis: optimal permeate flow distribution in RO is critical in the sustainable design and optimisation of an RO process.
operated in parallel with two similar plants drawing from the same seawater source. An audit was done on the existing two plants to ascertain their performance and compare them to the new plant, which has been optimally designed. The existing plants are labelled RO A and RO B respectively and the new plant designed for optimal permeate flow balancing by QFS is labelled RO C. Figure 1 shows the permeate flow rate of each membrane element in the
Method
Results
A plant was constructed and is FIGURE 1 Permeate flow per element Nico-Ben Janse van Rensburg, process engineer, QFS
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IMIESA October 2020
vessel from the raw water feed side to the brine side. RO A and RO B both show a sharp decline in permeate flow from one element to the next. This is a typical exponential decay where the permeate flow approaches zero from one element to the next. Production of membrane element six for both RO A and RO drops to about 0.1 m3/h, producing small volumes of product water compared to the first membrane element. RO C, which has been designed for optimal permeate flow balance, shows less rapid decay. The advantages to the design optimisation are not clear at this stage, and two questions need to be answered: • Does permeate flow balancing have any financial advantages? • Does permeate flow balancing have any operational advantages?
To investigate the advantage on capital expenses, the ROI for each individual membrane is investigated next to the ROI for
