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ECO-SOLIDS PROCESS - TECHNICAL INFORMATION
A schematic of the Eco-Solids Process is shown in the figure below. The process requires sludge, which can have water content anywhere between 50 and 93 percent (50% - 7% dry solids (DS)). The basic process comprises of mixing the dewatered sludge and calcium oxide (quicklime) in a hopper which is then transferred into a continuous plug-flow recycle reactor. The amount of calcium oxide required in the process can vary between 15-65% of the dry solids, depending upon various factors; including the temperature of the raw sludge, the recycle ratio and the requirements of the final product. A very small amount of TNS69, a pozzolanic type material, can also be added (optional) to the hopper to improve the final texture of the treated material and to make it easier to transport.  As the materials mix, the calcium oxide quickly reacts with the dewatered sludge, raising both the temperature and the pH of the material. The temperature of the sludge is raised to between 60 ºC and 80 ºC, and the pH of the material is raised to between 11 and 12.Inside the reactor the sludge is pressurised to between 200-500kPa. Since the reactor is pressurised all the heat generated by the chemical reactions is used to elevate the temperature of the biosolid, i.e. no heat is lost due to vaporisation. This means that the process is more thermally efficient than lime stabilisation processes that operate at atmospheric pressure. At a pH of 10.5 and the elevated temperature, ammonia (contained within the biosolid) is released from the biosolid. The release of ammonia raises the pH further, to a pH of 13.3, and is mixed thoroughly throughout the material to further enhance the pathogen destruction.The reactor itself is custom designed to ensure that temperatures and minimum residence times required by the appropriate regulations are exceeded. The combination of high temperature, high pH and the biocide effects of the ammonia ensures that the biosolids are pathogen free in accordance with the most stringent enhanced treatment (Class A/EQ) requirements.One of the key advantages of the Eco-Solids system is that a proportion of the treated material is recycled back into the hopper along with the other raw materials at the start of the process. By recycling a proportion of the treated material, the temperature of the raw material is increased and this can significantly reduce the amount of calcium oxide required in the process. By reducing the amount of calcium oxide required, the overall operating costs of the process are reduced.The remaining treated material is discharged out of the main reactor through a restricting orifice and passed into a chamber having a lower or reduced pressure. As the pressure is released steam, ammonia, amines and volatile solids are vaporised and recovered in a barometric water condenser, where they are stripped off into a water solution. This liquid is an aqueous ammonia nitrogen product, which can either be sold as a nitrogen source (called Eco-Liquid) or recycled to the treatment plant's headworks. This controlled removal of ammonia from the biosolid results in an overall reduction in the nitrogen content of the biosolid, which in-turn allows a higher application rate when the biosolid is applied to land.When the pressure is released at the end of the reactor and certain elements are vaporised, the pH of the biosolid drops to about a pH of 11-12. The pH level of the end product can be reduced by blending it with sulphates or any material, including organic material, having a pH <5.This all leaves a non-malodorous enhanced treated (Class A/EQ) biosolid, called Eco-Solid, which is placed into a container for disposal off-site. Being highly stable and low in odour, the Eco-Solid product does not require silo storage, which results in further CAPEX savings for the overall scheme. | |
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