The process defines the depth of the gravity pressure vessel.

Wet-Air Oxidation of Sludge
Wet-air oxidization of sludge should be carried out at a depth of 6,000 feet. Sludge at 3 percent to 6 percent dry solids passes down the outer annulus and oxygen is injected near the bottom. Oxidation is rapid, raising the temperature to 600 degrees Fahrenheit. The treated material rises through the updraft to the outlet for final treatment, degassing and heat capture. As it rises, it passes heat through the updraft to pre-heat the descending sludge prior to oxidation. The process achieves more than 95 percent destruction of biological/chemical oxygen demand and neutralizes all inorganic material.

Since sludge can be processed as a liquid, it can be taken directly from sewage treatment works. There is no need for expensive drying as required for other processes such as incineration. The process is self-sufficient in energy and even generates a surplus, which can be converted into electricity.

Dilute-Acid Hydrolysis of Biomass
Dilute-acid hydrolysis of cellulose to sugars requires a 1,600- to 2,000-foot deep gravity pressure vessel. The biomass mash containing 8 percent to 12 percent dry solids flows down the outer annulus and steam is injected at the bottom to initiate a temperature rise. Oxygen is added at the entry to the updraft to burn off dissolved lignin. Acid is then added. As the cellulose disassociates to saccharides (sugars), the temperature rises to 460 degrees Fahrenheit. An alkali is injected, immediately neutralizing the acid.

Once autogenic thermal balance is established, the steam supply is cut. Heat from the rising saccharide solution passes through the updraft to pre-heat the cellulose mixture that is descending in the outer annulus. Using the gravity pressure vessel increases the efficiency of converting biomass to sugars by two- to three-fold, greatly enhancing the potential of producing ethanol for biofuels and other applications.

Most ethanol today is made from crops rich in sugar and starch, raising concerns about elevated food prices and fuels inflation. Using a gravity pressure vessel in subcritical water to convert non-food biomass to ethanol is an important part of the solution.

Ethanol can be made profitably from a wide range of biomass sources including non-food crops. Using municipal solid waste as a raw material has the added advantage of being a steady source of biomass throughout the year, unaffected by seasons, climate, disease or international pricing cartels.

The gravity pressure vessel process can assist the household waste industry because it changes a waste material that currently incurs a cost to treat to a raw material that can create an income from treatment. As biomass represents approximately 60 percent of municipal solid waste in the United States (66 percent in the European Union and 87 percent and more in Asia), it is profitable to convert to ethanol. Sewage sludge, which contains approximately 30 percent biomass, can also be treated and converted in the plant.

Municipal solid waste-to-ethanol facilities work in three identifiable stages. The first is preparing the biomass by shredding, settlement in water to remove inert materials, maceration and thickening. The second is to treat the biomass with supercritical water, and passing it through a settlement tank and molecular sieves to clean it. In a third stage, the contained saccharides are converted to ethanol. The process plant and equipment used are standard to the wastewater industry and are enclosed and covered. There are no
airborne emissions from the treatment of waste. Dioxins cannot be produced since the working temperature is low. Smells and particulates are avoided.

Water from the process is recycled and any residual will be treated for discharge to inland waterways. The carbon dioxide produced can be used as the acid in the hydrolysis reaction with the rest available for sale or sequestration. Using municipal solid waste to make ethanol betters all existing and projected environmental targets for treatment. It eliminates landfilling and cuts out the greenhouse gases that would otherwise be emitted from landfill or from the treatment process. The process is entirely carbon negative and qualifies for
carbon credits. Ethanol made from municipal solid waste offers major benefits toward biofuels substitution targets in any country without affecting the food economy.

A municipal solid waste-to-ethanol plant is affordable. Its capital cost can be significantly less than 40 percent of an equivalent incineration plant, and is simple and more economical to operate and maintain. The income from municipal solid waste tipping fees and/or the sale of ethanol can finance the design, construction, operation and maintenance of a plant within a few years without fees increasing above current landfill charges.

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