Environmentally benign waste disposal is becoming a more and more acute problem. A newly-developed process, supercritical water oxidation is demonstrating extremely efficient organic waste destruction (99.99 per cent) plus excellent environmental characteristics (none of the emissions associated with incineration). Steve Minett and Keld Fenwick report.
The supercritical water oxidation process is especially suitable for the treatment of wastewater and sludges from domestic sewage and the paper and pharmaceutical industries.
Water enters a special condition, or afourth phase', in addition to the familiar solid, liquid and gaseous phases, when its temperature and pressure are above 374oC and 221 bar. In this region, its properties change, density being less than that of the liquid, viscosity the same as the gas, and diffusivity about mid-way between the liquid and the gas. Most importantly, the solubility of gases and organic compounds are increased to almost 100 per cent, while inorganic compounds become almost insoluble.
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 is now known as supercritical water oxidation (SCWO). With organic molecules and the added oxygen fully dissolved, a uniform homogeneous mixture is created and reactions can proceed at the intrinsic rate, ie, the theoretically maximum rate for chemical reactions.
Consequently, residence time in the reactor is only about one minute. Despite this brief residence, the method achieves a 99.99 per cent destruction of organic contaminants.
Burning is also an oxidation process but suffers from several disadvantages, one of which is incomplete combustion and the consequent need to scrub the stack gases to rid them of environmentally dangerous compounds such as dioxins. SCWO results in complete destruction irrespective of the feed entering the process, even PCBs.
Due to the lower reaction temperatures, the harmful oxides of nitrogen are not formed; these are toxic and acidic and can cause eutrophication (algal blooms) in fresh water. Traces of nitrous oxide (N2O, laughing gas) may be formed if the feed contains a nitrogen source.
However, there is no requirement to monitor it, and it is easily separated into nitrogen and oxygen if required.
Benign output
Unlike incineration, SCWO is a totally enclosed process and the reaction products are discharged at very close to atmospheric pressures and temperatures.
Furthermore, they are benign, consisting mainly of CO2, water and nitrogen. These substances do not need expensive scrubbing to make them suitable for discharge to the environment. Organic and inorganic halogens are converted to the corresponding acids, and organic and inorganic sulphur are converted to sulphuric acid. These are far easier to deal with in liquid form than as gases such as sulphur dioxide which causes acid rain.
Heavy metals are oxidised to their highest oxidation state and are separated together with any inert materials as a fine, non-leachable ash which can be used much like power station ash for landscaping, aggregates and similar applications.
Heat economy
Both SCWO and incineration are auto-thermal, or self-sustaining, once the process is up to operating temperature. However, they both need sufficient organic material in the feed stock or external heating must be applied; incineration needs well in excess of 25 per cent before becoming auto thermal whereas SCWO can be auto thermal with only 3-4 per cent of organic material in the feed.
The SCWO process can therefore show considerable primary fuel savings when compared with incineration.
Modern plants of any kind, which have excess process heat, try and use this heat or sell it to defray operating costs and reduce dependence on fossil fuels. Most liquids have higher specific heats than gases so the heat transfer equipment on the SCWO plant is less bulky and expensive than that on incineration plant which gives up its excess heat from the stack gases.
Commercial beginnings
The Swedish-based company, Chematur Engineering AB, has been developing the SCWO process under licence from Eco Waste Technologies since 1995 and is marketing it under the name of Aqua Critox. Chematur acquired the worldwide rights to the technology in1999. (Chematur is a specialist in supercritical processes and also works with supercritical carbon dioxide).
The company has built a large-scale, pilot plant close to its head office in Karlskoga in central Sweden. Recently an even larger plant has been built at Chinko Pantek, near Kobe in the south of the main island of Japan (Honshu). This plant was commissioned by Chematur's licensee, Shinko Pantec. The company has been using these plants to trial the process for the treatment of different wastes for two years and encouraging results have been obtained.
Wet air comparison
The SCWO process has similarities to wet air oxidation which is in use in a number of plants. However, the wet air process operates at temperatures of up to 300oC and 200 bar (depending on the process).
This is below the critical point of water and typically achieves only 70 per cent destruction of organic carbon, even with residence times in the reactor of up to six hours. Such long residence times require much larger reactors, typically 100 times larger than those required for supercritical processing.
Safety issues
Nevertheless, Chematur's development director, Lars Stenmark, suggests that: "The wet air process may have paved the way for SCWO: people in industries that normally use unstressed equipment have a psychological barrier which makes them wary of high temperatures and pressures. In fact, compared with many modern pressure vessels, the SCWO process uses very modest pressures and temperatures the pressures are no higher than those in common gas bottles found in workshops worldwide, and the reactors are not much larger.“
Under normal operating conditions, both systems are completely safe. Aqua Critox plants have modern interlocks so that in the event of pressure drop, or other malfunction, the plant shuts down.
However, there can always be catastrophic accidents, such as a crane dropping a load or, in some parts of the world, terrorist attack. With a reactor burst under these circumstances, the smaller unit would cause less damage than the larger reactor, even though the contents flashed into steam, than the larger reactor. If toxic waste were being processed, the smaller vessel, containing only about five per cent toxic material, would contain less of it than the larger one.
The salt limitation
Other problems sometimes associated with SCWO are salt deposits and corrosion. Lars Stenmark claims that: "These are also more manageable than may be commonly thought“. Soluble salts in a few flows become insoluble under supercritical conditions, and this can lead to scaling of the heat exchanger walls.
Obviously, wherever possible it is advisable to avoid dealing with flows which contain high salt saturations. Meanwhile the possibility of pre-treating flows to remove salts is being investigated, as well as adapting the process so that higher salt concentrations are not a problem. In fact, scaling is minimal for slurries which contain inert materials because suspended particles act as nuclei for salt precipitation before the fluid reaches the walls of the heat exchanger.
Additionally, these inert materials can also scour the walls and pipe work, acting as an abrasive cleaner to remove salt deposits. One of the most important applications, domestic sewage, normally contains considerably less than one per cent salt and so does not cause a problem.
Stenmark points out that: "Difficulties with salt clogging have been exaggerated because of the small diameters of tubes used in the first laboratory-scale experimental plants.
"In the Japanese plant, which has a capacity of 1100 kg per hour, the smallest diameter pipe is more than one inch.“
Corrosive effects
Halogens and hydro chloric acids for example can be highly corrosive, especially at high temperatures. "The Forschungszentrum in Karlsruhe did studies on corrosion in super critical water during the 1990s' and discovered that no corrosion takes place at temperatures above 380oC because there is no dissociation above that temperature,“ said Stenmark.
The problem is therefore in the transitional stage from 300 to 370o approximately and therefore the vulnerable part of the process is the heat exchanger where the flow is raised to this temperature. There is no corrosion in the reactor itself. Chematur has developed proprietary methods to overcome this corrosion problem in the heat exchanger.
Sewage sludge is being brought from two communities near Karlskoga, where Chematur's plant is situated. These tests are being carried out in co-operation with IVL (the Swedish Environmental Research Institute in Stockholm).
Chematur is sufficiently confident with the test results to approach much bigger municipalities with proposals for treating their sludge. This project has established that organic reduction is complete and that all pathogens are destroyed, leaving a completely sterile residue.
In many countries, untreated sewage sludge is spread on the land, giving rise to fears of contamination if insufficient time is allowed before planting.
The inorganic solid residue from the Aqua Critox process is inert and can be used in the construction industry for roads and other projects without fear of leaching.
Associated costs
A considerable body of work has been carried out on the costs associated with SCWO and comparisons have been made with incineration. Regarding the treatment of sewage sludge, indications are that the process will emerge as more economical than incineration, with total treatment costs of about £20 per wet tonne as at the beginning of 2000. This £20 figure for SCWO is based on a throughput of six tonnes and is made up of a variable cost, plus a depreciation figure for the plant, minus income from the energy produced.
Chematur points out that the variable cost very much depends on how the labour cost is calculated, and there are bound to be wide variations in accountancy practices throughout the world.
Another component of the variable cost is the oxygen which is needed for the oxidation; the cost of this too varies in different locations.
Compressed air may be used instead, but the investment then increases by up to 20 per cent. The depreciation is based on 10 per cent over 10 years. This too may change according to accounting practices used by different authorities. The calculations are made using wet tonnes with 15 per cent dry matter; if the basis of the calculations are changed to using dry tonnes, treatment costs would be about £135 per tonne.
Stenmark said: "Those responsible for sewage sludge disposal often see incineration as the main option but planning can be a serious obstacle in the face of NIMBY (not in my back yard) protests, SCWO plants, on the other hand, have none of the emissions problems of incinerators and, provided suitable precautions are taken with the import of the sludge, they operate entirely without odour. They're also smaller and obviously have no chimney stack, so there's no reason why their presence should disturb surrounding communities.“
De-inking sludge
Work is also on-going to discover how the process performs on sludge produced in the de-inking process in the paper industry. This is likely to be an increasingly important application as paper recycling increases.
The Swedish-Finnish company, Stora Enso, is sending sludge from its plant in Hylte Bruk in southern Sweden for the trials. The company is currentlydewatering the sludge, incinerating it at the plant and then land filling the ash.
Filler recycling?
SCWO has a number of advantages over this routine; dewatering is much reduced or even eliminated completely. The inks, which mainly consist of carbon, and fibres are completely reduced to CO2 and water.
Many papers contain inert fillers, mainly china clay, which end up in the sludge, and these could be recycled after separation from the Aqua Critox super critical process.
Paper mills could satisfy a large part of their paper filler requirements from this source since the filler brightness is close to that of virgin material.
No landfilling is required which always involves the danger of contaminants leaching into the soil and environment. The filler is inert but it can discolour water courses. The heat value from the process may also be available at the site as a form of energy.
De-inking sludge, which was treated in the same quantity as sewage sludge, also showed a remarkably low treatment cost, £8 per tonne. This is due largely to income from the recycled paper filler.
Stenmark believes that the pharmaceutical industry could benefit widely from SCWO since it would provide a highly attractive environmental solution in the near future. "Because the process can destroy any organic compound, without leaving undesirable by-products, it could eliminate the risk of escape into the environment of very potent active ingredients. These have been known to kill fish and cause other environmental damage; waste products known as mother liquors for example, could be treated and broken down into harmless CO2 water and nitrogen.“
Chematur is planning to treat biological sludge from the pharmaceutical industry; if successful, the process could largely replace incineration which, with landfill, is currently the preferred method of disposal. As regards heavy metals: "They would be oxidised to their highest oxidation state which makes them less of an environmental threat; the same would be true for all other substances,“ said Stenmark.
Cutting oils
Cutting oils containing amines are used universally in manufacturing industries with metal cutting equipment. Chematur has carried out a number of tests on spent cutting fluids, varying the temperatures and varying the ratio of total organic carbon to total nitrogen by adding methanol. In all experiments the organics were destroyed resulting in a COD less than 30 ppm. A small quantity of nitrous oxide was formed, in the order of a few thousand ppm. This corresponds to about five per cent of the nitrogen in the influent.
Chematur Engineering AB is based in Karlskoga, Sweden. www.chematur.se
