A perfect storm
Water filtration is an ancient practice, but demand for clean water is at historic highs. Meanwhile, the increasing acknowledgement of the environmental impact of human activities – from the energy we consume to the refuse we discard – puts great scrutiny on efficiency.The result is a perfect storm: a convergence of water, energy and waste management in which water filtration technologies will be put to the test.
The first issue is supply. According to a joint paper by the United Nations World Health Organization and UNICEF, nearly 20 percent of the world's population lacks access to safe drinking water, and the quality of water worldwide is declining.
Even in nations with high-quality drinking water, agricultural and industrial supplies are facing competition from households that demand the water for domestic use. That creates social and political discord, and has been at the root of protests against the construction of industrial facilities around the world.
Competition for shrinking water supplies has led many industrial users to draw on lower-quality sources of water to meet their needs, whether it's turbid surface water or the product of brackish deep wells. Ironically, this scramble for lower-quality water has come at a time when industry demands higher-quality water than ever – whether it's to comply with the purity standards for ingredient water in food and beverages, or to conform to “clean room” manufacturing protocols for electronics, plastics or metals that require purity at the molecular level.
The scarcity of water combined with public pressure has also led to increased recycling in industrial and even municipal applications. In all of those cases, water filtration is a vital element. From large screens to protect against fish and gravel to membranes that squeeze out undesirable salts at the ionic level, water filtration holds the key to success.
Long history
Water filtration is an ancient science. Even before scientists understood the effects of germs on human health, people have strained sediments (and often coincidentally, pathogens) from their water for millennia.
Records in Greek and Sanskrit indicate that people passed turbid or sediment-laden water through sand or strainers in 2000 BC. Egyptians used alum to coagulate suspended sediments in water 3,500 years ago. Hippocrates, the father of medicine, developed a bag filter around 400 BC by creating a cone of linen to strain water and solutions for his patients.
The same basic methods have been at the forefront of water treatment ever since. Sir Francis Bacon used sand filtration to try to desalinate seawater in 1627. A century later, home water filters of fibres and charcoal appeared. In 1804, the first municipal-scale sand filtration installation protected drinking water for residents of the Scottish town of Paisley.
Fine-tuned technology
Sand media filtration is arguably the most common large-scale water filtration technology in use today. Water passes through large tanks containing sand or other media. As the water sinks through the media bed, contaminants get stuck between the sand particles. Clean water flows out the bottom. (Perhaps the most extreme examples of sand filters are the seafloor or beach wells from which many desalination plants draw their water, which essentially use natural deposits of sand as massive filters.) Filtration media have evolved over the years – today, beach sand stands beside an array of media that range from minerals to fossilised microscopic exoskeletons of plankton called diatomaceous earth to activated charcoal, whose adsorptive capabilities attracts many contaminants.
The strainers of ancient Greece have evolved into an array of screens that range from massive steel fish screens to cylindrical wedge wire “candles” and even more finely woven screens capable of capturing particles as small as 10 microns. Many screen filter units now also feature elegant self-cleaning systems.
Cloth filters used in ancient societies are common today, though the materials have changed to allow today's cartridges or bags to operate at down to filtration degrees of 1 or 2 microns. The latest generation of textile-based filtration uses microfiber threads wound around special spools to capture particles down to 2 microns in size.
For soluble contaminants, membrane technology uses sophisticated materials and high-pressure pumps to push water through microscopic pores. Membranes are classified according to their pore size, ranging from ultrafiltration (UF) to microfiltration (MF), nanofiltration (NF) and reverse osmosis (RO), the process of capturing salt molecules and other ions.
Some membranes can be augmented by specially selected bacteria placed on the membrane to biologically oxidise contaminants in pathogen-rich environments such as treated wastewater. On a macro scale, many communities are using plants and microbes for the bioremediation of treated wastewater, relying on plants for nutrient capture and microbes in the sediment to capture or oxidise nutrients and other contaminants.
Electricity is also playing an increasing role in water treatment. Ultraviolet light (UV) destroys pathogens in treated water. One Israeli technology called magnetic water treatment binds organic contaminants with magnetic charges, while another, called electro-flocculation, dispatches positively charged metal hydroxide particles to bind with negative foulants and pull them to the bottom of the treatment vessel.
Using those tools, water professionals can build filtration systems that range from tap water purifiers to filters at the point of entry to hospitals and high-rises to massive seawater desalination plants that can create irrigation or drinking water for entire regions.
New challenges
In general terms, the goal of filtration is the same as it has ever been – clean water. But today, some of the challenges are new and ever more difficult to tackle.
On a large scale, zebra mussels and their larger cousins, quagga mussels, invasive molluscs that have inhabited many of North America's key waterways, can bring waterworks to a grinding halt as they encrust gates, valves and pipes and choke membranes and other filters with their rapidly growing shells. Too fast-growing and numerous to scrape, zebra and quagga mussels must be effectively filtered out of water as microscopic larvae to avoid risking colonization.
In the pathogen arena, Cryptosporidium parvum was virtually unknown to most water treatment professionals before the pathogen spread through the water supply of Milwaukee, Wisconsin, USA in 1993. That outbreak sickened an estimated 403,000 people – nearly one-third of the population – and killed more than 100. After the Milwaukee incident and several other outbreaks, Crypto is viewed as a major problem in the American water industry.
On the molecular level, water utilities around the world are under increasing pressure to deal with trace levels of pharmaceuticals and other chemicals in drinking water supplies. Demand for treating that water will surely increase as more research is done on the topic – and as our ability to discover those compounds at ever-finer concentrations improves.
In addition to new contaminant challenges, water treatment professionals are challenged by new uses and standards for water. For instance, water in such finely honed manufacturing processes as high-tech ceramics or electronics must be pristine – there are no tolerances for stray metals or chemicals in high-performing materials.
There's also the increasing interest in tertiary treatment of wastewater. Singapore has returned treated wastewater to the drinking water supply through its NEWater program, and San Diego, California is working on a toilet-to-tap program. Such fine filtration opens up tremendous opportunities for water conservation, but demands well-designed, highly effective, multi-stage filtration to work effectively.
Nothing's perfect
The multi-stage filtration concept is based on the simple premise that no single filtration technology is perfect, so several technologies must be employed – selected to address the specific contaminants in that particular water supply, and arrayed in a logical sequence – to successively improve water quality.
Sand media filters are simple and effective, but back-flushing the systems requires a line shut-down or the maintenance of a second system to bring on-line while the first is being cleaned. The back-flushing process itself can use four times more water than some other processes, such as suction-scanning screens. Also, the process of stirring up the media can create media carryover – discharge of media downstream – and requires a post-flush period of settling down when the filters are not functioning optimally.
Screens are durable and efficient, but not all screens are created equal. Wedge-wire screens can be very tough, but have an Achilles' heel – their openings can be long and rectangular, so the nominal filtration degree (which measures the distance between two wires) is smaller than the long dimension of the opening. As a result, deformable materials or flat, disk-shaped particles larger than the nominal filtration size can slip through wedge wire screens if they are oriented parallel to the opening's long dimension.
Woven screens are much more effective and their nominal filtration measure works in both directions – their openings are square. However, they can only go as fine as about 10 microns.
Cartridges or bags – today's descendants of Hippocrates' sleeve – offer extremely fine filtration, but require that the filter medium be replaced when full. Cartridges can be very expensive (as can the labour to conduct the replacement and maintenance), and disposal can be a costly challenge. In addition, interrupting filtration to replace cartridges or bags creates the risk of introducing contaminants into the water when the system is open and vulnerable.
Membranes offer the highest degree of filtration available, but they are expensive, require a great deal of pressure to pump water through their fine pores and need chemical cleaning as part of their maintenance. The art of multi-stage filtration is to protect those membranes from foulants to optimize their operation, minimise the number of chemical cleanings that are needed, and reduce energy consumption before the membrane stage to lower the overall cost of water treatment in the system.
Environmental footprint
As water treatment professionals develop multi-stage filtration systems, they are increasingly aware of the environmental footprint of the operation. Environmental footprint goes beyond the carbon footprint concept that has captured headlines and prompted deep thought about the true costs of some technologies.
Environmental footprint includes:
- • water;
- • energy;
• consumables (cartridges, bags, etc.);
• chemicals;
• physical space and infrastructure.
As noted above, a well-designed, multi-stage filtration system can significantly reduce the environmental footprint of the water treatment process. Replacing water-intensive or chemically cleaned technologies with efficient, self-cleaning technologies can significantly reduce the discharges. Effective fine filtration upstream of bag or cartridge “polishers” reduces the frequency of cartridge replacement. Considering the head loss in a filter – or the entire filtration system – can lead to choices that minimise the need for energy-intensive pumps.
According to the Affordable Desalination Collaboration in California, USA, 9% of the energy budget of an optimised desalination plant is the energy cost of pre-filtration. Reducing that figure by choosing energy-efficient technologies – with little electrical demand, low-pressure operation, and minimal head loss – can have a significant impact on the overall cost of desalination. The same concept applies even to less intensive water treatment operations – in fact, the energy cost of pre-filtration in those systems may actually represent a higher proportion of the overall energy budget.
When Amiad introduced its Automatic Microfiber (AMF) filtration systems to the membrane market, we discovered that the self-cleaning thread filters could reduce the chemical cleaning requirement of RO filters by a factor of four compared to using standard cartridge filters before the membranes. In addition to the benefits of reduced chemical use and handler exposure, the RO plant functioned more effectively because cleaning downtime was quartered – as a result, hour by hour, recovery values were improved significantly. In addition, replacing a cartridge system eliminated the costs and labour involved in the removal, replacement, transport, and disposal of consumables. That single switch had a major impact on a number of key environmental footprint variables.
Physical footprint is an often overlooked element of environmental footprint. Every cubic metre and every kilogram of weight is of vital importance in developing a filtration system for, say, an offshore oil rig or a battleship. But even on dry land, infrastructure is increasingly expensive and space is at a premium. Good filtration system design takes that into account.
Teamwork is key
Multi-stage filtration systems require careful consideration. Developing a water treatment regimen for a particular water supply is a lot like a physician writing a prescription for a sick patient. The doctor must see the patient, assess the symptoms, understand the underlying causes, identify what aggravates the problem, then write the prescription.
Similarly, a filtration system designer needs to consider exactly what is in his or her water supply, and how much is present. The next step is to consider what the water quality goal is on the outlet end of the system. Is this drinking water, ultra-pure process water for making microchips, water for a cooling tower, or service water for industrial seals and pumps? Is the industrial plant making food or pig iron?
Finally, designers must look both upstream and downstream to determine how various technologies in their multi-stage filtration system will interact. Selecting optimal degrees of filtration, or determining how much pressure must be maintained or boosted at each step, is significantly impacted by where a particular filter lies within the system.
With all of those variables in mind, outstanding multi-stage filtration systems can be developed to efficiently, effectively produce the clean water we all need. With planning and good application of the existing technology, we can reduce the environmental footprint of our water treatment operations while achieving results that would make Hippocrates proud.
