Water optimization programs increasingly are focusing on zero liquid discharge (ZLD). Approaching or achieving that milestone may involve many different technologies. No single strategy suits every wastewater, note vendors such as Dow Water Solutions, SUEZ Water Technologies & Solutions, and Veolia.
“Water scarcity and stricter discharge limits are really putting industrial customers in a tough situation. Depending on the severity of the two sides of this equation, customers are being pushed to achieve minimal liquid discharge (MLD) or zero liquid discharge (ZLD) — and we are seeing a trend towards more and more ZLD,” says Tina Arrowood, lead research scientist, Dow Water Solutions, Edina, Minn.
The company offers a broad set of technologies to help reduce the cost and energy requirements of treating wastewater. These include ultrafiltration (UF), reverse osmosis (RO), nanofiltration (NF) and ion exchange (IX). The latest additions are the Fortilife CR100, XC70 and XC80 RO elements, which specifically tackle biological and organic fouling, and the XC-N ion-separation membrane, which enables recovery of a large fraction of salts.
“We’ve really focused product development on technologies that convert large quantities of wastewater into purified, reusable water. RO elements are a fantastic technology for this and are much cheaper than using thermal distillation technologies. Our new filter elements are key to achieving high water recovery from high total dissolved solids (TDS) wastewater, leaving a high-quality permeate that is good enough for re-use,” she stresses.
Fouling resistance was a key theme in the development of the new elements; each is designed for the fouling potentials expected within the successive water recovery processes. In addition, the elements must be durable because these harsher, more-fouling environments require more cleaning (Figure 1). Both the membranes and the spiral wound module itself must withstand exposure to extreme pHs, too. Also important is using the elements at a point in the concentration process where membrane separation efficiency is maximized.
“The bottom line is: ‘Can we concentrate and maximize water removal from the wastewater stream and reduce the amount of water going either to downstream processes such as thermal treatment, or to discharge?’” she explains.
The biggest push towards MLD and ZLD is coming from chemical manufacturers in Northern China and Indian textile producers, two regions posing the twin challenges of water scarcity and discharge restrictions, according to Arrowood. Meanwhile, the power industry increasingly is embracing the approach because of its fluegas desulfurization (FGD) processes. Other pockets also exist, particularly in North America and Europe; there, regulations and corporate sustainability goals, either singly or in tandem, are driving demand in the chemical, and petrochemical sectors as well as among food and beverage makers.
As an example, she cites a project in the coal-to-chemical industry in Northern China, where a large chemical complex typically generates more than 1,500 m3/h of wastewater with a total dissolved solids (TDS) level ranging from 1,000–2,000 mg/L. The plant was recovering and recycling over 65% of this water using conventional fouling-resistant RO products. However, it wanted to reclaim an additional 90% of purified water from the concentrate of the primary wastewater recovery system where TDS exceeds 5,000 mg/L. Deploying Dow Filmtec RO elements enabled the desired water volume reduction, significantly decreasing the cost of the downstream thermal dewatering process for achieving ZLD.
However, capital and operating costs for running a ZLD system represent only one challenge for a treatment plant of this size. Another is dealing with the about 40 t/d of final solid waste salts produced. The high TDS wastewater contains an unusable mixture of sodium sulfate, sodium chloride and a number of other trace contaminants. So, the plant installed a system with Fortilife XC-N elements to treat this complex salt mixture after being concentrated to above 50,000 mg/L using RO elements.
“More than 60% of this water was converted to a purified sodium chloride solution for reuse applications. Moreover, the 40 t/d of waste salts going to the landfill could be reduced by nearly half,” notes Arrowood.
Another example involves an industrial park in Asia with a common wastewater treatment facility that long has used RO to reduce the amount of water discharged to the environment in line with the facility’s strict discharge permit.