Filtration Systems Enable Industrial Wastewater Reuse

The wastewater reuse and recycling market is one of the highest-growth markets in the water industry, increasing at cumulative annual growth rate of nearly 20 percent,

Ultrafiltration (UF) is a pressure-driven membrane separation process that separates particulate matter from soluble components in water. Image courtesy of Dow Water & Process Solutions

according to Global Information Inc. However, many factors affect a decision to reuse water, specifically when it pertains to industrial processing.

With local water stress (availability and access to a clean water source), price, and wastewater treatment driving water reuse, manufacturers are considering cost-effective water filtration systems to reduce their water footprint, waste, and chemical consumption.

Industrial wastewater reuse poses challenges because initial feed water characteristics can differ from facility to facility.

For example, in the oil and gas industry, water discharge may contain hundreds of parts per million (ppm) of oil and total dissolved solids levels over 200 ppm, whereas discharge water in the pulp and paper industry may contain more than 1,000 ppm of fibrous solids. In addition, the required water purity may vary. Users may need the water discharge to be slightly upgraded (25 to 50 percent of the solids removed), upgraded to potable (drinkable) standards, or brought to ultrapure water quality.

Water filtration systems differ because of the incoming water conditions and the purity requirement for end use. Although wastewater reuse systems are specific to each plant, two types of water treatment systems can provide effective water processing to increase plant efficiency and lessen strain on local water resources—particle filtration and membrane filtration.

With an outside-in flow configuration the membrane is less susceptible to plugging. Image courtesy of Dow Water & Process Solutions.

Particle Filtration

Particle filtration is a physical or mechanical process that separates solids from fluids. Particle filtration typically is defined as the filtration of particles larger than 1 micron and is used as one of the first filtration steps in industrial wastewater treatment. Common technologies that are used for fine-particle filtration include bag filters, cartridge filters, multimedia filters, and self-cleaning filters.

Because industrial wastewater characteristics can vary so dramatically, depending on the industry, it is common to use multiple technologies to process the water. The properties of the solids in the water, including particle size, shape, density, stickiness, and quantity, as well as other materials in the water such as oil, determine which filter technologies are used.

The water quality is described in terms of total suspended solids (TSS) in units of milligrams per liter (mg/l). The TSS level helps to determine which filtration technology to use. Additional information such as the particle size distribution and particle shape can help to identify the appropriate technology to use and the correct filter cutoff. For example, if the particle sizes range from 20 to 80 microns, then a 20 micron or lower filter cutoff would remove 100 percent of the particles.

Figure 1 highlights some common particle filtration technologies used for industrial wastewater treatment and key performance specifications. The table illustrates that with relatively clean water having feed water TSS levels of ~10 mg/l, a cartridge filter or multimedia filter may be a good choice. However, if the feed water contains TSS levels that are hundreds or thousands of mg/l, then a continuously cleaning technology may be more desirable.

Industrial wastewater reuse is driven by the economics, regulations of the recycled water, and sustainability priorities compared to the cost of treating other water sources such as groundwater and surface water. In addition, it is not uncommon for municipal waste treatment plants in the U.S. to charge manufacturing companies a surcharge on their wastewater if the TSS levels are above a certain threshold (i.e., 200 to 300mg/l). The magnitude of that surcharge may determine whether an industrial company treats the water before discharging it and/or reuses it.

Figure 1: By examining these common particle filter technologies and their typical performance characteristics, and different feed water types, manufacturers can determine which is most suitable for their applications.

 Membrane Filtration

When fine-particle filtration alone does not provide sufficient water reuse quality, membrane filtration can be used to purify the wastewater further. The required treatment steps are dictated by the water’s final end use as discharge, in wash stations, cooling towers, and processes; as high-pressure boiler makeup water; or for semiconductors. The cleaner the water must be, the more sequential technologies must be used.

Membrane technologies and integrated membrane systems have become the industry standard for water reuse in which the highest water quality is required.

Reverse osmosis (RO) membranes are needed to reduce contaminants such as dissolved solids and small organic molecules to an acceptable level.

Micro- or ultrafiltration (MF/UF) methods often are used as pretreatment for reverse osmosis. Concurrently, double membrane filtration provides a multi-barrier solution for bacteria or pathogen removal to help facilitate access to safe water.

As reuse technologies have progressed and become more common, membrane filtration systems also have evolved to reduce consumables and energy consumption. Although membranes require certain features that help them operate successfully, recent membrane technology developments have improved fouling resistance, productivity, longevity, and increased solute rejection. These have reduced the overall cost of water treatment. With today’s technologies, products can be tailored to handle harsher waters. Considerable effort has been expended to develop ultrafiltration fiber and reverse osmosis membrane chemistry, module and element configuration, and operational modes to reduce capital and operation costs of the water treatment plants, so as to lower the total cost of water.

Ultrafiltration is a barrier technology with a pore size small enough—5 to 100 nm—to remove water pollutants such as silt, colloids, pathogens (including viruses), and medium- to high-molecular-weight organics. While other configurations are possible, ultrafiltration membranes commonly are constructed as hollow fibers bundled together in a pressure housing. Filtration is performed by applying relatively low pressure (up to a few bars) that forces water through the outside walls of the hollow fibers to their lumen. Hollow-fiber ultrafiltration for industrial water reuse is dead-end filtration, but it is interrupted periodically (up to three times per hour) by a brief backwash of ultrafiltration permeate to dislodge the accumulated foulants from the membrane surface and flush them out of the ultrafiltration module.

Low concentrations of chemicals, such as sodium hypochlorite (NaOCl), can be used during the backwash to aid the cleaning impact. During a normal backwash, chemicals are in contact with the fiber for a short period of time. On a less frequent basis (about once every 0.5 to 7 days), a chemically enhanced cleaning with longer contact time often is recommended. This involves adding a chemical cleaner, such as an acid, to remove colloids and inorganic salts from the membrane pores or a mixture of caustics and chlorine to remove organics or biofoulants. Even less frequently (approximately once every 1 to 3 months), ultrafiltration modules can be cleaned more thoroughly by an offline clean-in-place program. By combining these cleaning methods, an ultrafiltration module performance typically can be maintained for 5 to10 years.

Ultrafiltration serves as a pretreatment to reverse osmosis, facilitating a longer life for the reverse osmosis membranes to enable fouling resistance. When compared to conventional, non-membrane-based pre-treatments, ultrafiltration offers higher removal of suspended solids, microorganisms and colloidal matter—all of which cause operational challenges in reverse osmosis systems.

Reverse osmosis membranes offer the finest level of filtration known, helping to remove dissolved solids and other soluble contaminants not removed by other filtration techniques. As a pressure-driven technology, the amount of pressure required to drive through the membrane barrier depends upon the amount of dissolved solids in the wastewater.

Because wastewater has a higher potential to foul the membrane compared to other types of feed water, the spiral-wound design of the elements use cross-flow filtration so that the feed water passes across the surface of the membrane while the remainder of the water flushes across the surface, sweeping away rejected solids. This aids in controlling the amount of foulants that build up on the surface. In addition, both the polyamide membrane chemistry and other materials of construction can be modified to handle high fouling streams better.

For more information , read the case example, “Meat processing plant reuses wastewater using continuously cleaning filtration system.”