Water. Desalination + reuse
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| 24 | Desalination & Water Reuse | February-March 2016 Projects technologies to manage water supply and discharges more thoughtfully," he says. "Regulation is a "top driver" for adoption of advanced technology such as ZLD in China and increasingly in India where Matheson sees significant opportunities for Oasys in 2016. "India has a similar dynamic to China. It's about a decade or two behind China in the modernization of its power grid. They have only about 10% of China's generation assets so there will be a significant build out of generation," he adds. Matheson foresees opportunities for Oasys in the Arabian Gulf territories and in the US – again fuelled by environmental regulation and awareness of the economic gains from more efficient discharge management and reuse. Beyond this year and the next, Matheson says there are openings too in Latin America and Africa where mining is a promising target sector. Trading ZLD solids is part of the Changxing deal and there are "very interesting" possibilities to turn solid waste streams into value streams," particularly in mining businesses says Matheson. While the gains from discharge management are more apparent in the developing and recently developed nations, they will, Matheson asserts, in time become integral to businesses globally: "You have to find a balance between supporting economic and industrial growth and remaining sustainable. And water is going to be a very big part of that equation," he says. Important design decisions included: complete softening of the FGD wastewater, maximum MBC process flexibility, and the maximum turndown ratio. softening The high concentration factor required for ZLD meant complete softening of the FGD wastewater was essential to minimize opportunities for scaling in the pre-concentrating RO system and for premature saturation of minerals in the crystallizer. Stoichiometric softening in a contact clarifier was combined with weak acid cation ion exchange polishing. Flexibility MBC process flexibility was maximized to manage the wide range of feed flow and water quality. To that end the team defined four design cases for flow and incoming TDS. The RO and FO components of the MBC were then designed to produce stable brine TDS, allowing flow and overall recovery to float as necessary. turndown ratio To maximize the turndown ratio the FO component of the MBC was split into three parallel trains. This configuration made it possible to operate the MBC at flows from 60% to 110% of design maximum. The FO section of the MBC has a shared draw solution recovery system and any configuration of its three trains can be in service at one time. The FO MBC concentrates wastewater dissolved solids from some 60,000 mg/l in the RO concentrate to 220,000 mg/l or higher. It does the brine concentration work of a thermal or mechanical evaporator with the simplicity and modularity of a membrane system. The FO trains are driven by an osmotic pressure gradient across a semi-permeable membrane to achieve spontaneous and preferential diffusion of water molecules from a saline feed into a proprietary draw solution. As the draw solution exits the FO array it has extracted the prescribed amount of clean water from the wastewater stream. The draw solution is then reconcentrated to separate the clean water stream for reuse. recoNceNtrAtIoN To reconcentrate the draw solution it must be heated to between 45°C and 65°C above the inlet dilute draw solution temperature, to volatilize the ammonium carbonate and subsequently recondense and absorb ammonia and carbon dioxide. The Changxing system consumes energy at 90 kWht per m 3 of wastewater processed while the draw solution recovery was designed to use steam as the energy source for volatilization and water cooling for condensation. The fresh water stream exiting the MBC draw solution recovery loop combines with the pretreated wastewater stream and passes through the RO system. It leaves the system in the second pass RO permeate stream with TDS at less than 100 mg/l. It is stored for reuse in the power plant's boiler makeup water system. The MBC system is designed to recover up to 23 m 3 /hr, or up to 87% recovery. Concentrated brine exiting the MBC system feeds a two-stage crystallizer at a target TDS concentration of 220,000 mg/l or higher. Currently, the crystallizer processes 2.5 m 3 /hr of brine, operates at pH 8.0 – 8.5 and concentrates the brine to saturation. A pusher centrifuge and hot-air dryer remove remaining water to less than 0.5% and produce mixed salt crystals of greater than 95% sodium chloride and sodium sulphate which are packaged for sale to chemical manufacturers in the region. DeBUt PerForMANce Oasys' patented technology is desalinating wastewater from Changxing's FGD for reuse in the boiler feedwater loop at some 630 m 3 /day to reduce intake of local surface water and outflow of industrial wastewater. This project has enabled Oasys to introduce an innovative, FO-based brine concentration and water reuse process to treat FGD wastewater while serving as a reference for other power producers in China where water and environmental regulations continue to grow in number and stringency. Changxing includes the world's first commercial FO-based ZLD.