Making a critical assessment

Chris Webb looks at the use of supercritical oxidation to treat sludge

As the options for disposing of sewage sludge narrow, Yorkshire Water has become the latest UK water utility to consider using supercritical fluid technology. The company is amassing data on supercritical water oxidation (SCWO), a technique more commonly used in the disposal of chemical weapons, to deal with an increasingly taxing environmental problem.

The process subjects the sewage sludge to high temperatures and pressures to make it supercritical, at which stage it shows the characteristics of both a gas and a liquid. The supercritical state allows previously insoluble components of the sludge such as fat to be dissolved and complex molecules to be broken down.

Complex organic compounds such as carbohydrates and oil are broken down into simple inorganic molecules such as carbon dioxide and nitrogen. The same process applies to viruses and bacteria.

Once the breakdown of the sludge components is complete the pressure is released and gases vented off. The heat can be recovered to generate electricity. The remaining liquid consists of water and organic compounds and requires no further treatment.
By using this method the volume of sludge is reduced by as much as 80% and the residue does not contain any pathogens such as bacteria and viruses. The technology is much cleaner than many of the processes currently used in the UK and Europe, according to Yorkshire Water. The company is considering installing a SCWO plant in the future.

About half of the UK's sludge production is recycled to agriculture and the remainder is generally disposed of to landfill or, increasingly, incinerated. However, since heavy metals and toxic compounds may accumulate in the sludge, they pose a potential environmental hazard. Increasingly stringent regulatory standards and public opinion are also limiting the disposal options for sludges. Consequently, technologies that reduce both sludge mass and volumes, while producing a re-usable sludge product, are becoming increasingly favoured.

Moreover, incineration in the UK has traditionally been used as a last resort, chiefly because it is relatively expensive and energy intensive.

The search for suitable processes to treat sewage or drinking water sludge or waste streams containing valuable inorganic material has accelerated in recent years, particularly since the abolition of disposal at sea. Wet air oxidation (WAO) has been studied and commercially applied as sewage sludge treatment. A major disadvantage, however, is that organic matter is not fully destroyed, leaving large concentrations of residual low-molecular-weight volatile fatty acids (VFAs) in the effluent. Consequently, effluent from WAO processes always requires further polishing.

SCWO has none of these shortcomings. A major advantage of the process is that it is capable of completely destroying organic compounds in a totally enclosed facility without the production of harmful emissions.

SCWO uses water above its 'critical point' of 374°C and 22.1MPa. Above this point, both gas and liquid form a homogeneous single phase. In this way gaseous oxidants such as oxygen are completely miscible in supercritical water. Water density is also lower, thus diffusivity and ion mobility are higher. Organic compounds are also highly soluble in this environment. Combined with the inherent high temperatures, these properties result in rapid oxidation of organic compounds without interfacial mass transfer limitations or sparing availability of the oxidant.

In the Swedish plant reaction time for complete destruction is between 30 and 90 seconds, depending on the reaction
temperature. In complete destruction organic carbon is converted into carbon dioxide, organic and inorganic nitrogen into nitrogen gas, halogenated organics and inorganics into the corresponding acid, sulphonated organics and inorganics into sulphuric acid. Metals are converted to oxides of their highest valency and all volatile solids are destroyed.

Furthermore, SCWO is a radical oxidation reaction and exothermic. This means once the feed organic is concentrated enough, the reaction can produce enough energy to maintain the process temperature.

Hogan, her colleague Darrell Patterson and Lars Stenmark of Chematur Engineering recently described Chema-tur's proprietary Aqua Critox SCWO process. Developed by Eco Waste Technology, the process is at the heart of the trials in the 250kg/h demonstration unit at Karlskoga, Sweden. In operation since 1998, the unit complements the full-scale SCWO site in Kobe, Japan, commissioned in July 2000.

"Chematur is actively commercialising SCWO for waste destruction," said Stenmark. "One very interesting area is sludge destruction, both municipal and industrial. Because of its high performance, SCWO has proved to be an excellent tool for recovery of valuable inorganics from different waste materials.

"I certainly think that SCWO can be economical in large scale. We are looking into plant sizes of more then 100m3/h per line and we see that it is definitely economical, especially if there is an inorganic component in the waste that can be recycled after processing," he continued.

Earlier barriers to the commercial development of the SCWO technology included scale formation, caused by deposition in pipes, and severe corrosion in post-reactor heat exchangers and coolers. Scale formation had the effect of decreasing the heat transfer efficiency of the heat exchangers and ultimately led to plugging of the pipe, but the Aqua Critox process overcomes this debilitating shortfall. Corrosion, meanwhile, was aproblem caused by acids containing sulphur, halogen and phosphorus species formed during the reaction. Several construction and proprietary materials have been used to minimise this tendency.

But the Aqua Critox process uses a plug flow reactor concept which Chematur believes is the most effective from a process technology point of view. "Corrosion is a potential problem, but only in certain small areas of the plant and not in the supercritical state; only at hot subcritical conditions. This has very clearly been shown by the work carried out at Forschungszentrum Karlsruhe in Germany by Peter Kritzer and others. We have different ways of dealing with corrosion, mostly by using a high nickel alloy as most others are using," according to Stenmark.

During the process sewage sludges are fed into the process via a mixing tank containing a paddle mixer. The bottom outlet from the mixing tank is connected to a pump, providing recirculation flow. In the recirculation loop, a macerator is used to reduce the size of the effluent and improve homogenisation. If a large batch is being processed, the homogenised sludge is deposited into a holding tank. The sludge is then pumped into a stirred feed tank, after which a high-pressure feed pump raises the feed pressure to about 25MPa and pumps the sludge through the SCWO system. The sludge enters the tube side of a double pipe economiser where it is preheated by the reactor effluent. After leaving the economiser, the sludge enters the heater.

From the heater outlet, the hot sludge then enters the reactor. In the reactor, oxygen is injected to start the oxidation reaction. The oxidation reaction generates heat and, as a result, the reactor temperature increases. As the inlet sludge concentration is too high for complete oxidation to occur in one step, the sludge is oxidised in two stages. Therefore there is a second injection of oxygen further down the reactor. Quench water is also added with the oxygen to cool the effluent enough to allow the additional oxygen to continue the oxidation reaction without exceeding the temperature limit (600°C).

Aside from Yorkshire, Thames and Wessex are among other water utilities believed to be considering using the technology. It could prove to be the environmentally-sound process water companies need.


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