A cold lime softening unit used in a zero liquid discharge process at an ethanol facility. (Photo courtesy of U.S. Water Services)
As the scarcity of water becomes a more pressing issue worldwide, the drive for industry to reclaim and reuse wastewater grows more important. Moreover, industry faces increasingly stringent state and federal requirements regulating contaminant limits for wastewater discharge. As such, conventional industrial wastewater treatment technologies alone—such as precipitation, filtration, and ion exchange—are unable to meet new and emerging discharge limits.
“The real impact of regulations has been the result of lower nutrient limits, such as phosphate,” says Gary Engstrom, technology manager for U.S. Water Services. “To meet the less than 1.0 PPM or 0.3 PPM phosphorous limit requires chemical precipitation in most instances. These levels can only be achieved if carryover and fine particulate matter can be completely eliminated.” This puts pressure on the final filtration steps, which require better technology, operation and control. The tighter nutrient and total dissolved solids have also driven greater water reuse in the industrial sector, which requires better filtration and higher quality industrial wastewater treatment streams.
Industrial Wastewater Treatment Technology & Application Trends
Over the past decade or so, industrial wastewater treatment has experienced a transition from conventional media filtration to high-efficiency centrifugal filters to cloth media disk filters and, finally, to micro-, ultra-, and nano-membrane filtration systems. Engstrom says the evolution of technology has not only ensured high-quality effluent for discharge, but also enabled the trend toward reuse.
“Many times, the reclaim approach requires salt removal, which is most economically achieved by the use of reverse osmosis,” says Stanley Karrs, technical director for Power & Mining at Evoqua Water Technologies. “This has resulted in the need for filtration products to properly condition the wastewater to provide high-quality feed water to the reverse-osmosis units.”
In addition to removing foulants, such as turbidity, organic matter, and total suspended solids (TSS) to meet the optimum SDI (silt density index) for feeding a reverse-osmosis unit, Karrs says removing silica and/or softening the feed water is often important for industrial wastewater treatment.
There has been an increase in demand for tubular microfiltration systems to replace solids contact clarifiers and multimedia filters for chemical softening to remove calcium, magnesium and silica for SDI feed to a reverse-osmosis system. Similarly, Karrs says municipal applications are employing hollow-fiber membrane filters on the discharge to remove turbidity, organic matter, and TSS to provide the low SDI needed for reverse-osmosis reclaim of sanitary plant discharges.
From an application perspective, there is a growing trend toward tertiary industrial wastewater treatment to facilitate water reuse in industrial facilities.
“A number of plants are now employing ultra-filtration after clarification with a portion of the stream going to reverse-osmosis units to permit application as cooling and boiler water make-up,” says Engstrom. “This cuts across many industrial market segments, including food & beverage, fuel ethanol, and power to name a few.” Further, he says, several plants have actually converted to zero liquid discharge, which requires exceptional filtration systems to be successful.
Meanwhile, the use of hollow-fiber membranes for filtration prior to reverse-osmosis of municipal plants has moved to the industrial wastewater treatment market and allowed significant modifications to activated sludge-type systems.
“Membranes have been installed in the activated sludge basins and allow higher TSS levels in the process, which reduces the size of the reactors,” says Karrs. “This membrane biological reactor (MBR) is becoming a trend, especially for larger flow systems, because of the reduction in system footprint and construction cost.”
Karrs also cites zero liquid discharge as a promising application, but says the energy requirement will play a part in future growth.
Industrial Wastewater Treatment Keys to Success
As the use of MBR systems grows and reaches out into different industries, the membranes are exposed to a wide variety of organic compounds, some of which may have an adverse impact on the membranes. When using a membrane-filtration approach in a new application, it is important to test the application to ensure there are no fouling or chemical degradation impacts.
“The economics behind water recycle/reuse solutions have to be thoroughly examined before choosing a certain treatment approach,” says Karrs.
Likewise, Engstrom says the most common mistake end-users make when designing their industrial wastewater treatment solution is not investing the time and energy to get enough and the right data to address the significant variability that is usually encountered in wastewater treatment systems. “The particle size distribution and total suspended solids loading can vary dramatically depending on seasonal, production schedule, or product mix factors—not to mention upset conditions that may occur from time to time in the production facility,” says Engstrom. “Plants are too frequently designed for averages instead of peak conditions or are too optimistic in terms of performance or redundancy.”
The Future of Industrial Wastewater Treatment
As water becomes more scarce and expensive in some geographical areas, membrane technologies will continue to be developed for water recycle and reuse applications. Also, more stringent effluent requirements will be the norm, resulting in high rejection rates of chemical species for membrane technologies.
“With membrane technologies becoming a key player in the future, energy-efficient products will need to be developed,” says Karrs. “Higher membrane surface area in smaller packages will also be an important design consideration.”
Engstrom says the membrane journey will continue with better materials of construction and system configurations that will tolerate broader operating condition windows for industrial wastewater treatment. Membrane surfaces will have higher or more consistent flux capacities with better utilization of available surface area.
“The membranes may also be less prone to fouling or resistant to more aggressive cleaning regimes,” says Engstrom. “As total dissolved solids and salty discharges become more tightly regulated, additional wastewater ROs will appear.”