Water. Desalination + reuse
Issue link: https://fhpublishing.uberflip.com/i/604344
| 24 | Desalination & Water Reuse | November-December 2015 RESEARCH WHEN RESEARCHERS AT Aston University in Birmingham, UK looked to design a desalination system with "energy efficiency approaching the theoretical thermodynamic limit," they had another, arguably more significant goal in mind. That goal was a contribution to improving access to drinking water for the 800 million people worldwide who, according to UN figures, lack that access. To that end they embarked on the design and construction of a desalination system that had the potential to operate with an energy requirement that was close to the minimum possible even at high recovery ratios – the ratio between freshwater produced and feed water processed. An ultra efficient, high freshwater-output system would address the challenge in preserving groundwater sources that is particularly prevalent at inland sites. To begin to fulfil this ambition, the team from the Sustainable Environment Research Group, led by Philip Davies, had to address inherent conflict in its aims to decrease energy demand and to reduce brine production through high recovery. The two measures conflict because in conventional reverse osmosis (RO) the energy needed to process a cubic metre of feed water – the specific energy consumption (SEC) – is determined not only by the osmotic pressure of the feed water – and, therefore, by its salinity – but also by the recovery ratio. This caps the achievable efficiency in conventional RO. RUNNING PROBLEM The continuous flow through the pressure vessel leads to an increasing energy demand gradient following the rising salinity along the length of the membrane module. This too creates conflicting requirements: • to overcome the osmotic pressure along the entirety of the module to maintain flux; and • to exceed the osmotic pressure only minimally to avoid wasting energy on unnecessary excess pressure. "However much effort is given to the perfection of pumps, membrane materials and energy recovery devices, this loss remains unless the system is reconfigured to overcome the conflicting requirements," says Davies. To get around the conflict, the options are: • to separate the system into a series of pressure vessels each fed by the reject from the previous one and operating at incrementally increasing pressure in line with the increasing salinity; or • a single stage, batch system where the pressure is varied with time to overcome the osmotic pressure increase as permeate passes through the membrane. The second of these options holds the potential to achieve ideal performance while for the first option; minimum energy demand is practically implausible as it calls for an impractical number of stages. BETTER BATCH Batch RO systems have been designed by other research teams says Davies: "But for the most part these systems have focused on a closed loop that is continuously topped up with incoming, pressured saline feed water, to compensate for water passed through the RO membrane." He says the mixing of the raw feed water with the more saline feed in the closed system robs energy from the system at the molecular level: "A closed loop will fail to achieve the theoretical minimum energy demand because the mixing of incoming saline water with more concentrated water in the loop is an irreversible process creating entropy of mixing and presenting a lost opportunity for energy recovery. "In any desalination system, we are putting in work to separate saline water into more and less saline streams and allowing two streams of different concentrations to mix without getting any work back is bad news. We avoid this by preventing the mixing," says Davies. His team of MEng students has constructed a prototype and preliminary results will be presented from tests using brackish water. He says the system should use only two thirds of the energy needed for closed- UK-based researchers have developed a reverse osmosis desalination system that they say promises to meet the challenges in providing freshwater to the millions worldwide who lack access to it. The research leader explains how his team has addressed the dilemma of cutting energy demand while upping recovery. _________ Trevor Loveday editor, Desalination and Water Reuse ___ The big push 0 1 2 3 4 5 6 7 8 9 10 0 0.2 0.4 0.6 0.8 1 Normalised SEC / P osm Recovery Specific Energy Consumption vs Recovery …especially at high recovery Continuous 1 stage + ERD 'Closed-circuit desalination' Continuous 2 stage + ERD Batch RO ERD: Energy Recovery Device Qiu and Davies: Water, 4, 690-706 (2012). Minimum Batch RO is more efficient than continuous flow – particularly at high recovery.