Water features15 October 2013
Existing and emerging water treatment technologies are changing the game for plants and factories in more ways than many may realise.
As we celebrate the centenary of water disinfection by chlorination – the first commercial gas metering equipment for public drinking water was installed in 1913 by Wallace & Tiernan, now part of Siemens Water Technologies – it's worth considering just how far treatment technologies have come. But, equally, with ageing plant, tighter regulations and the ongoing drive to cut costs, it's also high time for engineers and managers to review innovations that challenge conventional thinking around water treatment. And that's not only for waste water, but also everything from industrial effluent to boiler feedwater and potable supplies.
Take commercial effluent discharges, generally paid for by plants and factories according to the Mogden formula, which covers transportation and treatment costs, based on volume, COD (chemical oxygen demand) and TSS (total suspended solids). They're not cheap and prices can rise steeply – although many still don't know because they don't measure water usage. For example, a user discharging 73,000m3 effluent with a COD of 125 mg/l and 30 mg/l of suspended solids, requiring biological treatment, might expect a Mogden bill of some £64,000. But, if the same user's COD rises to 3,000, that charge slides up to £160,000.
Jason Robinson of Xylem Water Solutions is one among many now suggesting that this is ripe for change – with treatment of industrial effluent before discharge now entirely feasible and likely to save industry hundreds of thousands of pounds. That's not just due to Mogden charges being eliminated. It's also because suitably treated wastewater from one process can be re-used in another, so slashing the demand on potable sources and the volume of water eventually discharged to sewer.
"Crucially, water re-use technology is also included in the government's Water Technology List, which promotes water re-use, so is eligible for ECA (Enhanced Capital Allowance), meaning businesses can write-off the equipment cost against tax in the first year of purchase," explains Robinson. And he points to technologies including membrane filtration and aeration as being applicable.
"One example of the sort of savings that can be achieved involved a food processing plant in the Midlands," continues Robinson. "The company had an effluent volume of 40,000m3 along with a COD of 4,000 mg/l and TSS of 1,200 mg/l all of which resulted in a Mogden charge of more than £162,000. Our analysis demonstrated an opportunity to make a significant saving on annual effluent discharge bills and potable water consumption. With the effluent volume remaining the same, its COD could be reduced to 125 mg/l and TSS to 30 mg/l at a cost of £42,500 – meaning the plant could save £119,500."
And Robinson advises that those figures do not include other onsite effluent re-use options such as toilet flushing, boiler feed water and vehicle washing. "The payback varies depending on the site's choice of technology and on whether re-use is included following effluent treatment. However, typical payback is two to five years," he insists.
So far, so good. But we all know that water treatment plant and equipment is generally not cheap and does have operational implications. Or does it? Turns out that some water technologies are seeing more development than many may realise. Look at membrane systems, most of which use RO (reverse osmosis), harnessing high-pressure pumps to force water through semi-permeable membranes. Despite advances in RO membranes, these can be energy intensive and, certainly in desalination plants, have a reputation for fouling and a considerable chemical demand to maintain them.
But that's changing: Martin Currie, a member of the IChemE's Water Special Interest Group and a water quality and treatment consultant with Aqueum, points to what he describes as breakthrough technologies. "Researchers are currently working to scale-up biomimetic membrane processes employing aquaporin proteins [found in our kidneys] that let water through much more efficiently than conventional membranes.
"Also, UK universities and companies are at the forefront of FO [forward osmosis], a technology now in commercial operation that uses osmosis to suck water through the membrane rather than pumps to push it."
Both technologies promise "huge" energy savings and FO plants are already seeing "massive" reductions in the chemicals required to maintain the membranes, he says. The expectation is that they will transform the business case for, for example, municipal desalination plants, such as Thames Water's Beckton, East London site, which is capable of supplying 150 million litres of water per day, but currently only operates in times of drought.
Returning to the here and now, though, RO remains a profoundly underused technology that has applications beyond industrial and potable water treatment in, for example, steam plant. Indeed, according to Mike Griffin, steam system conditioning manager with Spirax Sarco, virtually all industrial steam users could benefit from RO, whether for tackling hard water in hospitals or fighting contamination in food factories.
"Used in combination with the right treatments, RO helps cut down on plant outages caused by scale and reduces corrosion in the condensate circuit," he says. And we might add that it is also safer (no need for regeneration using acid or caustic chemicals used in dealkalisation or demineralisation plants), while also offering opportunities for considerable energy and water demand reduction – meaning more cost savings.
Griffin makes the point that RO in this context is a robust, chemical-free technology capable of delivering purified water to boilers and clean steam generators. How does it help? "By stripping out almost 99% of the dissolved solids, RO can reduce boiler blowdown by an order of magnitude, resulting in significant savings in energy, water and treatment chemicals," he says.
And there's another consideration. Griffin advises that tighter controls on lead permitted in drinking water are heading our way later this year. "This is significant, because one of the ways water companies are responding is to add phosphate to water supplies as a corrosion inhibitor that prevents lead in old pipes from dissolving," he adds. Left untreated, however, phosphate can cause scale when it reacts with calcium or magnesium in boiler water which typically happens when boiler water becomes too hard.
"Avoiding this situation conventionally involves constant management of water alkalinity in the boiler, adding chemical dispersants and ensuring that dissolved solids are kept in check, through regular boiler blowdowns," states Griffin. That's time consuming, wastes energy and is expensive. But RO virtually eliminates the phosphate before it even reaches the boiler, with a pump forcing water through the membrane and separating it into two streams – reject (concentrate) and purified (permeate). "The permeate typically flows to a holding tank or hot well before being sent to the boiler or clean steam generator," he explains.
And if you're after proof, look no further than the Westons Cider plant in Much Marcle, Ledbury, which is reporting savings of £42,000 annually in fuel and water costs since installing Spirax Sarco RO plant. According to Westons' Jason Roberts, its new RO plant is compact, was quick and easy to install, and has reduced boiler blowdowns from 3% to less than 1%.
"Before we installed the RO system I could put my hand on the boiler blowdown pipe and it was warm, so there must have been hot water being discharged frequently," he says. "Now it's almost always cold, so we're definitely losing less water and energy."
The result: Fuel oil and water monitoring revealed a saving of 3,800 litres of fuel over a 15-day period. And further, as yet unquantified savings are likely, in terms of its Climate Change Levy, due to reduced CO2 emissions. What's more, Roberts reports a step-change in boiler control and efficiency, not least because the water is so clean that it has effectively descaled the boiler.
Xylem Water Solutions Ltd
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