Thermal Drying of Wastewater Solids : Thermal drying is one of the technologies available for processing of solids produced at municipal wastewater treatment plants (WWTPs). Thermal drying technology, although originally developed for material and chemical processing applications, has been successfully applied to WWTP solids and, thus, it is a viable and proven solids processing technology primarily aimed at producing a marketable product.

Thermal drying technology is based on removal of water from dewatered solids which accomplishes both volume and weight reduction. The added benefit of thermal drying is that it typically results in a product with significant nutrient value. Typically, dewatered solids (at approximately 18% to 35% dry solids content) are delivered to a thermal drying system, where most of the water is removed via evaporation resulting in a product containing approximately 90% solids. In the thermal drying system, the temperature of the wet solids mass is raised so that the water is driven off as a vapor. By removing most of the water from the solids, thermal drying results in a significant reduction in both volume and mass.

Significant thermal energy must be transferred to the solids to increase temperature in the drying process. This energy can be provided by combustion of a variety of fuels (natural gas, digester gas, heating oil, wood, etc.), by a reuse of waste heat, or by conversion of electrical power into thermal energy.

The high temperatures used in thermal drying assure that the US EPA time and temperature requirements for pathogen kill are met. Drying also meets the EPA vector attraction reduction standards by desiccating the wastewater solids to greater than 90% solids (or to greater than 75% solids if the solids have been previously stabilized). Although high temperatures are used in thermal drying, the temperatures are generally low enough to prevent oxidation (burning) of the organic matter. Thus, most of the organic matter is preserved in the dried material.

Material produced in the thermal drying process generally has a dry solids content of approximately 90% to 96% (or 10% to 4% water content). Thermal drying systems may produce a variety of forms of dry material, including fine dust, flakes, small pellets, or larger fragments, depending on the type of thermal drying system used, the characteristics of biosolids processed, and the use intended for the product.

Thermal drying typically must be preceded by, or done in conjunction with a dewatering process. Thermal drying is usually used as the last stage in processing of solids at municipal WWTPs. After the thermal drying process, dried material can be used for a variety of purposes.

Process Description : In the most general terms, thermal drying is the use of heat to evaporate water from wastewater residual solids. The reality is that the thermal drying process generally consists of materials handling and storage equipment, heat generation and transfer equipment, air movement and distribution equipment, air pollution control equipment, and ancillary systems. These equipment systems can take many forms, the details of which are beyond the scope of this paper. However, thermal drying systems are typically referred to in two primary categories, direct and indirect. This classification is based on the way that the thermal energy is transferred to the solids in the process.

In direct heat dryers, hot air/gas flows through a process vessel and comes into direct contact with particles of wet solids. The contact between the hot air and cold solids allows the transfer of thermal energy, which causes an increase in solids temperature and evaporation of water. The hot air/gas can be produced by almost any source of heat, but most often is produced by a gas or oil-fired furnace. The predominant method of heat transfer in direct drying systems is convection. Examples of direct drying equipment are rotary drum dryers, flash dryers, and belt dryers. A schematic diagram of a typical rotary drum drying system as shown Below

In general, the most common types of thermal drying systems include the following :