Demand growth worldwide for water is placing a burden on available resources and creating competition among industrial, agricultural and municipal users. At the same time, environmental regulations continue to impact water intake and effluent purity. These pressures are motivating an interest in filtration technology to process wastewater so that it can be reused rather than discharged into the environment.
Tapping the unused potential of seawater by removing the salt and various impurities is one means for dealing with water shortages. The technology for desalination has been available for decades, but population pressures and the demand for fresh water are driving the push for innovation. Because of continuing advances, one type of filtration—reverse osmosis—is likely to dominate this process, according to the IHS report, Reverse Osmosis for Seawater Desalination.
IHS analyst Ron Smith points out that 99.3% of Earth's water is unfit for human consumption or agricultural use and that 97.5% of the total amount is salt water. Desalinated seawater is, therefore, likely to grow as a major potable source for areas of the globe with access to seacoasts. He cites the Arabian Gulf area, southern Spain, North Africa, the California coast and China as likely areas, based on demand and supply.
A recent report on worldwide trends for membrane separation emphasizes both increasing demand for water and stricter government regulations for protecting the environment as forces leading to market expansion. The global market grew from $19 billion in 2010 to $21.2 billion in 2013 and is expected to increase at a compound annual growth rate of 11.1% between 2013–2019.
The chemicals, pharmaceuticals, semiconductor, food and beverage industries consume large amounts of highly purified water. These industries use filtration to purify their process water and in other aspects of production. Increasingly, they also require finer porosities to increase efficiency by using smaller particle sizes, and higher operating temperatures to handle hot liquid suspensions and exhaust gases, according to filtration consultant Ken Sutherland writing in the April 2013 issue of Filtration + Separation magazine.
Membrane filtration is becoming the technology of choice for these applications. It is based on semipermeable membranes designed to selectively separate unwanted particles or molecules from an incoming stream. The substances that are retained are called the retentate and those that pass through are known as the permeate.
A differential pressure applied across the membrane barrier drives the process. Membrane media are split into four categories based on the sizes of the substances that can pass through them. In decreasing size order they are microfiltration, ultrafiltration, nanofiltration and reverse osmosis.
Microfiltration and ultrafiltration operate on the sieve principle, which holds that only some particles are small enough to fit through the membrane’s pores. They differ from traditional filters, however, in the extremely small sizes of their pores, the largest of which is 10 micrometers.
Nanofiltration and reverse osmosis do not physically block particles. Instead, they operate on the principle of diffusion through the molecule-sized spaces in the membrane material. An important difference is that microfiltration and ultrafiltration require relatively low pressure (1–30psi) to drive the process, whereas nanofiltration and reverse osmosis require up to 1,200 psi. Reverse osmosis and nanofiltration, therefore, use significantly more energy.
Reverse osmosis is unique among the filters in that it is based upon the natural principle of osmosis. This holds that if two fluids containing different concentrations of dissolved solids are separated by a semi-permeable membrane, the fluid containing the lower concentration will move through the membrane and into the fluid containing the higher concentration of dissolved solids.
However, by applying a pressure that exceeds the osmotic pressure, the reverse effect occurs. In this case, fluids are pressed back through the membrane while dissolved solids stay behind, thereby reversing the natural process. In order to desalinate seawater, the applied pressure must exceed the natural osmotic pressure of seawater, which is about 400 psi.
Food, beverage, and agriculture producers are investing in anaerobic digestion to generate income while treating their organic waste streams. Most adopters have achieved payback in less than two years, says Global Water Engineering CEO Jean Pierre Ombregt.
The American Biogas Council defines anaerobic digestion as a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen.
The process is used at wastewater treatment plants where suspended organic material in the form of sludge can generate biogas, which is mainly methane and carbon dioxide. Since the carbon in biogas comes from organic matter that recently captured it from atmospheric CO2, biogas is considered to be carbon neutral.
The methane can be used as fuel to generate electricity or burned to produce heat or steam. Any excess can be added to the existing gas grid to increase the overall fuel supply, while it reduces the level of pollutants discharged into the environment. Research also has shown that ammonia and phosphates, two valuable chemical products, can be harvested from the biogas process. In addition to being fed by municipal and industrial waste, anaerobic digesters can use agriculture waste products or purposely cultivated feedstock crops.
Membrane bioreactors (MBRs) are modifications of conventional wastewater treatment plants, which rely on microorganisms submerged for an extended period of time until the effluent is pure enough to be released into the environment. The addition of membrane filtration, in the range of 1um, to remove the clear liquids from the sludge offers a number of potential benefits. The bioreactor can operate with approximately four times the solids concentration of a conventional system and its footprint can be cut in half since it does not require a secondary clarifier and sand filter.
According to the U.S. Environmental Protection Agency publication Wastewater Management Fact Sheet — Membrane Bioreactors, MBRs also produce a higher quality effluent, which means that it can be discharged to local bodies of water or sold for uses such as irrigation. Because it requires shorter retention time, it also is possible to automate the process.
One possible configuration places the filter in an external vessel. A second configuration submerges the filter in the bioreactor itself. Both require more energy to operate than conventional systems because of the pressure needed to drive the process and the need to clean he filter to reduce fouling. Since the submerged system requires less operating pressure, it is generally preferred.