Optimising permeate flow in RO plants

When it comes to reverse osmosis (RO), two properties are typically of great interest and importance: energy consumption and brine discharge. By Nico-Ben Janse van Rensburg

Both 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.

Method

A plant was constructed and is 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 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 m 3 /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?

Results

To investigate the advantage on capital expenses, the ROI for each individual membrane is investigated next to the ROI for the entire pressure vessel.

The following assumptions are made in the calculation:

• Replacement cost of a membrane is R11 475, including transport and installation on-site.

• Water is sold at R20 – this is a hypothetical number and may vary greatly; the trends remain the same.

• Membrane life span is five years, which can be significantly extended through good pre-treatment.

• Inflation of 4% per annum, both for membrane replacement costs and water sale costs.

• The plant operates 24 hours per day, with a three-hour shutdown for cleaning in place every 28 days.

• ROI is based on turnover and operating costs for the rest of the plant are not considered.

FIGURE 2 ROI on membrane elements

Figure 2 shows the individual membrane element ROI in the top row. Both RO A and RO B have negligible ROI for elements five and six. The fact that RO A is pushed much harder than RO B only affects elements one, two and three. RO C shows a smaller spread of ROI from elements one to six, and a lower yield in the first membrane element, yet the cumulative production seen in the bottom row of plots in Figure 2 is significantly greater. As this is the same feedwater, it is clear from the plots that RO C operates at a lower feed pressure than RO A judging by the lower permeate flow of the first membrane element – yet the total production is significantly more.

RO C is therefore more energy efficient and has the potential for a higher recovery than RO A. Pushing RO A further for better recovery will only exceed the operational limits of the first three membranes, while no improvement is seen on the last three membranes.

A further advantage with optimal flow balancing is that RO C has seven membrane elements and the seventh element provides a significant contribution. The cumulative ROI plot for RO C shows the ROI for all seven elements, but for fair comparison, only the first six elements are considered and are shown with the red line in Figure 2.

Conclusions

There is a great improvement in ROI for RO C with optimised flow balancing. Table 1 not only shows a significant improvement in ROI for the whole pressure vessel, but an even greater improvement in ROI for elements four to six.

Although the discussions and numbers are centred around ROI, which is critical when budgeting for long-term maintenance, there are some other benefits that are also important in terms of sustainable design:

• The disposal of used membranes remains an environmental concern. A membrane that hardly produces water is unnecessary wastage.

• Lower pressure results in better energy efficiency and therefore reduced operational costs and carbon dioxide emissions.

• Improved ROI can be the source of funds for further optimisation, such as upgrading energy recovery devices and motors for more efficient operation.

Although this study was done on a seawater plant with a single stage, the same principles would apply to any RO skid with any number of stages. Flow balancing has both financial and environmental advantages.