Proton compares closed-circuit versus conventional RO

Proton anti-scaling software from Amercian Water Chemicals was first introduced at the AMTA Conference in 2014. The platform’s innovative approach to modelling membrane treatment systems which, unlike other anti-scalant predictors, accounts for ion complexes and ion activity in water, and differentiates between nano-filtration and reverse osmosis membranes, enabled it to improve on the accuracy of predicting water quality and chemical dosing requirements.

Four years on from its introduction, the product has taken another step forward by modelling Desalitech’s Closed Circuit Reverse Osmosis (CCRO) system design, enabling plant designers and operators to compare the potential recovery rates and dosing requirements for varying feed waters and membranes in a conventional reverse osmosis (RO) system design versus a CCRO.

“For our rejection co-efficient to be accurate, we accounted for the system design, because things like flux rates and concentration polarisation can impact on membrane scaling,” explains Mo Malki, AWC chief executive and technical director. “So when Desalitech started to use Proton and tried running projections, a lot of times the software would behave as if the recovery rate was impossible, even though Desalitech knew for a fact that it was possible.”

AWC then became convinced that it should model the CCRO desalination system design within Proton, a challenge that took about six months in all to complete. Now in the Proton platform, conventional RO system designs run in parallel with the CCRO model, enabling users to switch from one to the other to make comparisons. “You log into the software and enter feed water chemistry — calcium, magnesium, sodium, sulphate, and so forth. Then the next step is to design a system or enter an existing design, and for different recoveries you can see the scale formation you may have. Now, you can click the Desalitech mode, and it will show you the highest recovery you can get, relevant to the water chemistry and hydraulics,” says Malki.

Hydraulic challenge

The reason why AWC wanted to model the Desalitech system has to do with hydraulics. A conventional system running at 90 per cent recovery might comprise three stages, each with seven membrane elements. In such a system, the required feed pressure would be so high that the first-in-line elements would desalinate the majority of the water, and the final elements would run dry. “The biggest problem was when the feed water has total dissolved solids (TDS) of several thousand parts per million. Then, even using boost pressures and permeate back-pressure adjustments, it didn’t work hydraulically with a six or seven element-long system,” says Malki. “Of course, one could build a system with booster pumps every four elements, but that adds capital cost and would take up a large footprint.”

By comparison, the CCRO has a single stage of five or six elements, and the feed pump increases its pressure with every cycle as the osmotic pressure increases. “This means that you can never run into hydraulics issues where the pressure required in the tail element causes the feed elements to make too much water. These scenarios are a great fit for Desalitech,” Malki adds.

Chemistry benefit

In modelling the CCRO, one big difference in the water chemistry between a closed-circuit and a conventional RO system that emerged was pH, with implications for membrane scaling. “In a conventional RO, the pH rises between the feed and the concentrate, and the concentrate pH is always significantly higher than the feed pH. If you’re running at 90 per cent recovery, and your feed pH is 8, the concentrate pH may be 8.8 or 8.9. With CCRO, 100 per cent of the concentrate is blending back into the feed water. Concentrate makes up half of the water re-entering the membranes with every cycle, therefore the concentrate pH is getting pulled back down so overall, the concentrate pH that affects scaling ends up being significantly lower. The CCRO has a chemistry benefit that helps to reduce the scaling,” explains Malki. “As well, when the concentration gets very high it gets purged, and exits over a short period — there are lots of types of scaling that are slow to form, they may take several minutes, and the CCRO flushes the water containing the forming scales before the crystals can embed into the membranes.”

System comparison

The application of Proton software can be at the start of a project as part of understanding aspects of its feasibility, or to help optimise or replace an existing treatment system. In one example, a pulp and paper manufacturer that wanted to double its production of paper was barred by the local municipality from increasing its discharge to the sewer system. The company was therefore driven to innovate and to find ways to produce more water for use in the paper-making process without increasing its volume of wastewater.

“The customer lighted upon Desalitech, and in that instance could go to Proton, input its existing water chemistry and conventional RO system design, and confirm ‘yes, we are currently seeing that recovery,’ and then switch to the Desalitech CCRO mode and evaluate on that same water quality,” explains Matt Jones, Desalitech regional director. “In that particular case they were able to go from a 75 per cent recovery rate with a traditional RO system, to 92 per cent recovery with the CCRO.”

Jones adds: “The best thing is that it’s a one-stop-shop software where you run the anti-scaling projections, model various designs, compare traditional RO with CCRO. That comparison is powerful for designers and engineers out there who want to see truly the differences between the two processes, from a chemistry and hydraulics perspective, and to compare different membranes. You can do it all in one place in real-time, change pH and temperature, and see the effect on recovery.”