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Drying and Treating Bio-solids with Microwaves

Introduction

The costs of natural gas and liquefied petroleum are a concern to operators of municipal sewage plants. Aged drying facilities, coupled with escalating energy costs, capital equipment costs, refurbishment or replacement costs have made conventional drying concepts uneconomical. Efforts to substantially improve throughput efficiency or reduce energy costs have failed to meet economic or technical objectives.

Envirowave Energy provides a solution to the effective treatment of organic materials processed by municipal sewage plants, in a cost-effective manner. The EnviroWave process employs a combination of microwave and hot gas convection technologies to achieve these goals. This concept provides the most cost effective solution while achieving the total destruction of pathogens, resulting in a biosolid product which may be directly land-applied as an EPA-certified, Class A material.

Current treatment methods primarily expose municipal waste (biosolids) to a high pH-high temperature process to reduce pathogens, but do not necessarily achieve a Class A product. A solution must include (1) pathogen reduction to achieve a Class A product under U.S. Regulations, 40 CFR, Part 503, Alternative 6, (2) odor reduction and (3) economic viability.

Test results by an independent laboratory recommended by the U.S. EPA have confirmed the Envirowave microwave apparatus achieves total and complete destruction of all pathogens, without adhering to the previous requirement of time and temperature profiles. In addition, the dryness requirement is actually considerably less than previous EPA requirements. This contributes to less energy consumption, which further improves the economic factors.

Microwave Generator Design

The microwave generator serves as the source of microwaves for the applicator. To promote stability of operation for the microwave generator, and ultimately the microwave applicator, the power supply must be stable, as well as free of ripple voltage and harmonics on the output waveforms. Stability, noise-free outputs and power factor improvement are achieved by the EnviroWave system design.

Cooling water is provided to the magnetron from a reservoir of de-mineralized water. Cooling air is provided to the magnetron stem and dome by two separate radial fans.

Specific waveguide launcher design details are defined by the magnetron manufacturer and chosen for optimum magnetron performance. Optimum performance usually results in a compromise of efficiency, power output and stability when operating with mismatched loads. A programmable, stand alone, Programmable Logic Controller (PLC) controls each generator during startup, operation and shutdown. The PLC Program is designed to automatically start/stop the generators, automatically start/stop the applicator and automatically load/unload the generators to pre-defined operating points via the ramp up/down schedule.

For maximum protection of personnel and equipment, all shutdowns and emergency stop buttons are hardwired in series, external to the PLC program, for immediate, manual emergency shutdown, of all operating systems.

Microwave Applicator Design

Volumetric Heating is defined as the fixed continuous power dissipation by a volume of dielectric material, causing the average temperature of the material to rise linearly with time, as long as the power is applied, without consideration to the temperature of the applicator walls or air within the applicator.

Obviously, the effects of temperature, specific heat, humidity and circulating airflow within the applicator do contribute to changes in the surface temperature of the material, due to heat transfer characteristics of the surface material. The changes in the surface material of the dielectric load results in overall secondary heating or cooling effects. As the material is conveyed through the applicator, changes which occur in the material size/shape, temperature and moisture content directly affect the microwave absorption characteristics.

The separation and definition of material size and shape, directly affects hot gas flow between the particles of material on the belt, resulting in a changing a rate of evaporation. In bio-solids, the presence of lime and polymer contributes to changes in both microwave absorption and hot gas flow between the particles.

Microwave drying rapidly drives the moisture from inside the material to the surfaces of the material. Temperature gradients can form at the surface of the material due to the difference in internal and surface temperatures. At temperatures below the boiling point of water, the temperature differential assists in driving the moisture to the material surface. Through capillary action within the pores of the material, the microwave absorption and resulting temperature differential forces the water toward the material surfaces.

The design incorporates improved convection air movement, which subsequently increased material surface moisture evaporation rates, as well as reducing the rate of gas consumption. The higher velocity, hot-convection air moving parallel to the material flow, but opposite in direction, results in a higher rate of removal of surface moisture from the material.

The hot gas convection section of the combined microwave/convection applicator consists of burners, circulating fans, an insulated air plenum, and an exhaust blower. The convection air is drawn from the burners, across the applicator width (top) into radial inflow blowers, down an insulated plenum chamber covering the entire applicator length (side), where it is eventually drawn out, along with moisture-laden air through an exhaust blower.

For more information about the Envirowave Energy and commercial microwave technology please click on the Contact Us link on the top right of the web page.