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Energy-Rich Sludge Successfully Recycled

Oil from sludge (OFS) technology is one process that provides an environmentally sound, cost-effective sludge-recycling system. It is a system that Enersludge Australasia has used effectively in a series of trials at Sydney's Malabar sewagetreatment plant
THE disposal of sludge, the residual semi-solid matter generated by sewage treatment, is a universal problem which has been tackled with less than satisfactory solutions.
In the past, sludge has been used as landfill, discharged to the ocean or incinerated without any attempt at resource recovery. However, sludge, like treated waste water, is a resource that can and should be recycled.
Sludge is rich in nutrients and energy, though ft contains also most of the undesirable components that arc discharged to the sewer, such as heavy metals, industrial organic chemicals, pesticides, herbicides and insecticides.
Such material is not destroyed in the sewage-treatment process and accumulates In the sludge, as do pathogenic organisms. An effective sludge-recycling process must be able to handle these undesirable components in an environmentally acceptable manner.
Oil from sludge (OFS) technology is one process that provides an environmentally sound, cost-effective sludge-recycling system. It is a system that Enersludgc Australasia has used effectively in a scries of trials at Sydney's Malabar sewage-treatment plant.
The technology has been under development and demonstration for the past 10 years in both Australia and Canada and has been shortlisted by the Sydney Water Board • for Management of Sydney s sludge.
In essence, the technology mimics nature in making oil. The starting material (sludge) is a microbial and organic-rich substrate, just like the substrate used by nature to make oil. Unlike nature, however the technology does not need high pressures to convert the sludge to oil and the process operates at only moderate temperature.
in the first stage of the OFS converter dried sludge Is heated to 450 degrees celslus in the absence of oxygen at atmospheric pressure. Under these conditions, roughly half the sludge solids arc vapourised to complex long-chain organic compounds, predominantly carboxylic acids and proteins.
In the second stage these vapours come into contact with the solid residue (char) from the first stage, where vapour-phase catalysed reactions convert the complex molecules to saturated hydrocarbons.
Finally, these vapours are condensed to oil and water, which are separated in a centrifuge. Some of the organics fight fractions) remain as a gas, called non-condensable
gas or NCG.
The oil product contains about half the energy originally available in the sludge. This oil is very similar to a medium fuel oil as far as energy density and other parametres are concerned. Extensive testing has shown it can be used as a fuel for industrial kilns, boilers or furnaces. From a combustion viewpoint, it can be used in purpose-designed engines (diesel) to generated electricity
Charcoal has an energy content similar to lignite. The remaining two products, gas and
water, also have fuel value. The gas contains carbon monoxide and hydrogen as combustible constituents and the water contains low-molecular-weight carboxylic acids.
In essence, the OFS converter recovers all of the sludge energy in the four products. This high-energy efficiency is one of the principal advantages of the OFS technology.
The energy efficiency is further enhanced by use of the conversion by-products (char, gas and water) to provide the energy required to dry the sludge. These by-products are fed to a hot gas generator (HOC) which produces the hot air needed for sludge drying.
The HGG is a fluid-bed furnace designed to meet the needs of the OFS process In an environmentally sensitive manner. The gas finally is cleaned using modern air-pollutioncontrol equipment, before being discharged to the atmosphere.
Two end-products from the process arc oil and ash. The ash Is an Ideal substitute for sand and can replace the fine aggregate In most concrete mixes. No waste products are produced by the process.
An Integrated OFS plant comprises sludge drying, conversion energy recovery and gas cleaning.
Sludge from the treatment plant is first mechanically dried using centrifuges or filters to produce a "cake" containing roughly 35 per cent solids and 65 per cent water.
The sludge produced at the treatment plant normally contains about 95 per cent water.
The sludge cake (a shovellable solid) is then thermally dried to produce a granular or pelletised product of at least 95 per cent solids. This dried sludge is then fed to the OFS
secticides and industrial orga-nochlorine compounds such as PCBs arid hexachlorobenzene (HCB) that accumulate tn sludge are destroyed in the OFS converter.
Under the highly reducing conditions in the converter (hydrogen-rich atmosphere) these organochlorine compounds are catalytically de-
converter as described above.
The OFS technology not only offers total sludge recy cling. but does so with numerous environmental benefits, including complete destruction of pathogens, eliminating any health concerns. The heavy metals in the sludge are captured in the ash product.
Furthermore, these metals are converted to oxides and silicates in the HGG and. therefore, are no longer teach able for transport up the food chain.
Pesticides, herbicides. In-
chlorinated. For example, HCB is converted to benzene and hydrochloric acid. This Is a very significant advantage of OFS technology compared with thermal sludge processes such as Incineration
Of major Importance is the fact that since no organochlo-rines are fed to the HGG. this system can operate at a lower temperature to minimise heavy-metal emissions.
By contrast, an incinerator must provide conditions to both destroy organochlortnes (high temperature) and minimise heavy-metal emissions (low temperatures), clearly conflicting requirements.
Since the OFS technology captures half the sludge carbon in the oil product, which is used to replace indigenous fossil fuels, carbon dioxide emissions from an OFS plant are only half that of other thermal processes, such as incineration.
The OFS technology, providing 100 per cent recycling of sludge produces no waste products that require landfill disposal.
Australian Science and Technology Newsletter