New Process Speeds Biosolids Breakdown

A new process that uses steam and sudden decompression to break down the cellular structure of municipal sludge may give cities another option in dealing with the tons of biosolids generated every day by sewage treatment plants.

A new process that uses steam and sudden decompression to break down the cellular structure of municipal sludge may give cities another option in dealing with the tons of biosolids generated every day by sewage treatment plants.

The process was developed at the U.S. Department of Energys (DOE) National Renewable Energy Laboratory (NREL) and recently licensed for commercial development by Peak Treatment Systems Inc. of Golden, Colo.

The NREL process is said to destroy all pathogens in municipal sewage sludge, allowing the treated biosolids to be considered for beneficial uses. The sanitized waste could even be converted to high-protein feedstock or used in high-efficiency waste-to-energy digesters producing methane-rich gas, which could in turn be used for heat or to run a turbine to generate electricity.

The process that Peak plans to market is a continuous two-stage operation. The initial stage uses thermal explosive decompression forces to inactivate or kill all microbial cells and viruses. This initial stage also ruptures a small portion of the microbial cells, thereby increasing the biosolids’ fluidity. In addition, the remaining intact cells are made more susceptible to shear. In the second stage, high rate shear is imparted to the biosolids which completely ruptures the remaining intact cells. This shear step also breaks up any undigested organic solids, increasing their surface area for subsequent processing.

The Peak process incorporates a high-pressure, high-solids pump to deliver the biosolids to a steam mixing valve or “hydro heater.” The biosolids are exposed to high temperature and pressure for a very short period of time before being released to atmospheric pressure in a flash tank.

This rapid decompression at high temperature, also known as steam explosion, acts to rupture a small portion of the microbial biomass and softens the remaining intact microbial biomass. This step also increases the overall fluidity of the biosolids.

The steam which is flashed off carries with it valuable volatile components, most notably ammonia. The components can be condensed and captured as a value-added fertilizer co-product.

The next step uses another solids pump, capable of operating at high temperatures, to deliver the partially treated biosolids to a sheer-imparting device. A modified in-line, cage mill grinder, commonly used in sewage treatment works, provides the shear required to rupture the remaining intact microbial cells. This step increases the fluidity of the biosolids further while releasing the valuable nutrients contained in the biomass. An additional advantage of the shear step is that any undigested organics contained in the biosolids are also sheared, thereby increasing surface area and enhancing the rate of subsequent bioconversion processes.

After passing through the pretreatment process, the sludge would be further processed in a standard high-solids anaerobic digester. From there it could be used as feedstock for secondary anaerobic digestion producing fuel gas and Class A compost.

Peak plans to offer a turnkey system with a small footprint using industry standard equipment.
According to figures compiled by the company, more than 290 million tons of biosolids are generated annually in the U.S. as a byproduct of municipal wastewater treatment. A similar quantity of sludge and biosolids is produced by industrial sources.

Waste sanitized with this process could be converted to high-protein feedstock or used in high-efficiency waste-to-energy digesters producing methane-rich gas, which could in turn be used for heat or to run a turbine to generate electricity.

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