A greener and cheaper revolutionary solution

For many years the safe disposal of drinking water sludge has been regarded as a costly and controversial issue. John O'Regan, the chief executive of the Irish firm SCFI, explains a sustainable alternative

The UK currently generates hundreds of thousands of tonnes of sludge each year from the treatment of drinking water using alum (aluminium sulphate) or ferric sulphate. The disposal of this waste is a mammoth, costly task for the industry, often close to or exceeding £100 per tonne.

This is because it cannot be processed through traditional methods of sewage sludge disposal (such as land spreading or incineration) and therefore, hazardous landfill is considered the only safe option. The shortage of landfill sites and the enforcement of EU directives means this disposal method is becoming an increasingly challenging and costly affair.

Landfill taxes are on the up and drinking water treatment plants are faced with heavy cost burdens to collect, transport and dispose of the waste, not to mention the loss of valuable coagulant (alum).

However, a new process has been developed that is set to turn what is a significant environmental and financial issue for water treatment plants into a sustainable and economically sound solution.

The newly commercialised AquaCritox technology is the first of its kind to use super critical water oxidation (SCWO) to fully recycle the coagulant in a sustainable way and provides complete organic waste destruction in a fast, safe and odourless process - eliminating the need for landfill.

Sustainable destruction
Following more than 15 years of development and investment in excess of £24.7M, SCFI has successfully developed a technology that harnesses the SCWO process into a revolutionary solution for the sustainable destruction of wet organic matter. The technology is especially suitable for the
treatment of drinking water sludge, sewage sludge and industrial organic aqueous waste streams.

The patented AquaCritox technology (see WWT Ireland, February) is proven to deliver 99.99%+ destruction of wet waste in an economical, efficient and environmentally friendly way. Further, the technology generates renewable energy in the process.

An attractive alternative to existing alum sludge disposal methods, AquaCritox can also recover used coagulants such as aluminium or iron hydroxide, which can then be reconstituted to generate new coagulant for reuse. This negates the huge wastage levels associated with disposing of used coagulants, and also avoids the socio-political issues that surround sludge disposal, as all organic materials are converted into safe, inert gases and clean water.

Waste To gold
Currently, companies and utilities that ship their waste to landfill are paying approximately £75-£110 per tonne to dispose of alum sludge waste and this cost is steadily rising. In addition, plants are effectively throwing away coagulants for each treatment cycle and therefore incurring the cost of replacing the flocculent.

By completely destroying the wet waste in a sustainable process and recycling the coagulant, AquaCritox eliminates both these costs and delivers considerable savings to water treatment companies.

Beyond these significant savings, the technology also eliminates the need for expensive dewatering equipment and generates enough heat for the process to be autothermal in reaching and maintaining the supercritical temperature.

The nature of AquaCritox as a single-step process with a rapid reaction time also makes for units that are small in actual size. There are three variants available for drinking water sludge applications, all equipped to provide a smaller plant footprint.

Under pressure
By using SCWO, which is achieved when water temperature exceeds 374°C and is pressurised to 221 bar, AquaCritox turns water into a universal solvent for gases and organic compounds, even those that are normally insoluble in water. It then adds an oxygen supply to create a very rapid and complete
oxidation reaction.

This not only generates thermal energy, but also converts all organic materials in sludge into CO2, nitrogen and clean water, leaving the aluminium/iron hydroxide as a water-insoluble precipitate. The nitrogen can be safely released into the atmosphere and the CO2 (which is a short cycle carbon rather than greenhouse gas), may be released or sold on for industrial applications or dry ice production.

The resulting pure precipitated coagulant hydroxide takes the form of a gel. It is readily dewatered and the water is clean and safe to discharge. It can be reacted with sulphuric acid to form fresh aluminium or iron sulphate. This process can take place on-site or in another location.

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