The quest to optimize water re-use and anticipate regulatory pressures is driving many plants to give increasing attention to their wastewater. To help meet the challenges, wastewater treatment specialists such as Dow Water & Process Solutions and Microdyn-Nadir are introducing new technologies. Meanwhile, researchers at universities in Germany, Spain and Singapore are working on potential future options, scaling up separation techniques based on ionic solutions and biomimetic membranes.
“As part of Dow’s reduce, reuse and reclaim circular economy approach we are really focused on the finite water supply. So we want to squeeze as much as possible from all the water used by industry, including the chemical industry,” says George Barclay, global R&D director at Dow Water & Process Solutions, Edina, Minn.
To do this, the company has developed a broad portfolio of treatment technologies ranging from mechanical filtration for suspended solids, pressurized ultrafiltration (UF) for macromolecules and bacteria, nanofiltration (NF) for separation of mono- and di-valent ions and very small molecules to the more rigorous separations of reverse osmosis (RO) and ion exchange (IX) resins.
“However, there is no one size fits all. You have to design solutions for different wastewaters: different contaminants need different suites of technologies. The challenge for the chemical industry is to increase water recovery and ensure treatment reliability,” he adds.
One way that Dow is helping here is with its minimal liquid discharge approach, which focuses on reducing the cost of brine concentration and maximizing water recovery. This is especially important in China, where much of the chemical industry operates in the more arid north and pressure to use water as sparingly as possible is great. So there, brine concentration before crystallization of the salt commonly relies on mechanical vapor recompression and evaporators.
“The addition of ultra-high-pressure (UHP) RO to the membrane filtration process can reduce the water going to the evaporation step by 50%. So that saves on energy use and we can squeeze another 5% of water out of the process,” explains Barclay.
Another example is a new UF module for particle removal (IntegraFlux SFP-2880XP) that is based on novel hollow-fiber technology made by a diffusion-induced phase separation process. That process enables accurately controlling the pore structure of the fiber down to 30 nm, allowing for consistent removal of bacteria, viruses and turbidity. Moreover, increased flux, achieved by modifying the internal structures of the fibers, decreases pressure and associated energy costs.
To cut water hardness and chemical oxygen demand (COD), plants typically turn to IX resins. Here, Dow has used new monomers to develop a resin (Amberlite IRC) that it says has both a higher throughput (48%) and greater stability than existing IX options. “As a result, there are fewer regeneration cycles, which saves energy and reduces chemical use, plus you can handle tougher waters,” notes Barclay.
The company also offers new ion separation membranes (Filmtec Fortilife XC-N) for NF. These convert a large fraction of RO concentrate waste into purified salt solutions that are easier to crystallize or re-use. The membranes reportedly boast high permeability and ion selectivity; they provide high sodium chloride passage and very high di-valents and COD rejection. The salt then can be re-used.
To illustrate this in action, Barclay cites a chemical plant in China that produces ferric sulfate as a raw material for the lithium ion battery industry. The production process generates a large amount of low pH wastewater containing high concentrations of both ammonium nitrate and ammonium sulfate — neither of which are allowed to be discharged. “Here we have developed an innovative process based on RO technologies, including UHP RO, to reclaim and concentrate the ammonium compounds from production wastewater as valuable chemical fertilizer, and simultaneously re-use the purified water as process water.”