WATER POWER:A new system can deal with waste water cleanly by using just water and oxygen, writes BARRY McCALL
IT SOUNDS LIKE the stuff of science fiction: destroying organic waste using just water and oxygen. But that’s precisely the technology being promoted by Cork firm Super Critical Fluids International (SCFI). According to the company, the highly innovative technology has the potential to offer an environmentally benign option for waste disposal.
The process, known as super critical water oxidation offers efficient organic waste destruction (99.99 per cent) as well as environmental characteristics with almost none of the emissions associated with incineration. It is seen as especially suitable for the treatment of waste water and sludges from domestic sewage and the paper, pharmaceutical and biopharmaceutical industries.
Waste treatment and recycling firm Eras Eco has commissioned the first commercial scale system from SCFI for its new €10 million treatment facility in Youghal. Eras Eco specialises in the treatment of waste water treatment plant sludges and materials recycling and it plans to use the SCFI system for the destruction of aqueous organic waste.
SCFI is also involved in an InterTradeIreland Innova programme project with Northern Ireland partners Cleanfields and Williams Industrial Services to demonstrate the viability of the technology for use on a commercial scale by water utilities.
The technology is based on special properties water acquires when it enters what is known as a supercritical condition or fourth phase. This occurs when its temperature and pressure are above 374 degrees and 221 bar respectively. The phase is additional to the more familiar solid, liquid and gaseous phases.
Under these conditions, the physical properties of water change: it becomes less dense than in its liquid state, it has the same viscosity as in its gaseous state and, most importantly, the solubility of gases and organic compounds are increased to almost 100 per cent, while inorganic compounds become largely insoluble.
“Every molecule has a supercritical point but supercritical water is particularly useful,” says SCFI chief operating officer David Kerr. “At the supercritical point a substance takes on new properties. These can often be quite useless, but in the case of water they are particularly useful.”
These special properties have been known theoretically for a long time and in the 1970s and 1980s work began on exploiting them for practical applications. An oxygen supply was introduced and the process became known as supercritical water oxidation (SCWO).
When a stream containing organic material is added to a reactor containing supercritical water, and oxygen is introduced, a very rapid and complete oxidation reaction takes place – having the same effect as incineration but with few, if any, of the undesirable by-products. Unlike incineration, the only gaseous emissions from the process are carbon dioxide (CO2) and nitrogen (N2).
The technology has already been tested on sewage sludge by Swedish company Chematur Engineering AB. The test results to date confirm 99.99 per cent destruction of the organic material and the generation of a small amount of inorganic inert residue or ash. SCFI has performed its own extensive tests which bear out the Swedish results.
“The process is well proven now,” says Kerr. “It will work in any area where there is organic waste containing carbon and produces no hazardous waste. It is also extremely environmentally friendly as the process actually produces more energy than it uses. The oxidation process gives off heat and the water temperature increases to between 380 and 550 degrees at 221 bar by the end of the process. We are able to put this water through a heat exchanger to create steam to drive a turbine to generate electricity. The CO2 can be sold back to the gas supplier who’ll give credit for it as they can use it for dry ice manufacturing or other industrial applications. The nitrogen can be safely released into the atmosphere.”
Of particular interest in Ireland is the use of the technology for the treatment of sewage sludge. The disposal of sludge has become a major headache for local authorities in recent years and the SCWO process has the potential to offer complete destruction of the organic portion of the sludge, converting it into carbon dioxide, nitrogen and sterile water. Furthermore, according to Kerr, the residual inorganic fraction from the process can be subjected to a process to recover phosphorous and other materials for resale.
According to Patrick Phibbs, operations manager with leading water company Aecom Ireland, the technology may have potential. “Should the process prove to be commercially viable, it may have a role to play in the water industry as an alternative to drying and composting of sewage sludge and possibly alum sludges,” he says.
“As with all new potential better technologies, the industry will be watching the current efforts to have this process prove its worth to the Irish water industry. The process itself is well known but the issue is, how do you make it work across wide-scale applications? That means it must be proven to be robust, efficient and easy to operate at a price that fits below the whole-of-life costs of existing solutions available in the market, of which there are many.”
With Ireland’s first commercial SCWO plant due to come online in Youghal shortly, and the process being mid-way through the InterTradeIreland Innova project to demonstrate its viability for local authority use, it may not be too long until Ireland has a viable alternative to incineration for the destruction of organic waste streams.
