The system includes five main groups of equipment:

Mixing: Wet and recycled dry sludge are mixed to produce a 1-4 millimeter sphere that has a solid dry core surrounded by a thin wet film of sludge.
 
Drying: The semi-dry mixed sludge is introduced into a stream of hot air produced by the furnace and pneumatically conveyed and dried through a rotary drum dryer constructed of three interconnected concentric cylinders. 
 
Air/Solids Separation: The saturated air and dry product produced by the drum are pneumatically conveyed to a preseparator to separate out the gross solids and then to a polycyclone to remove dust and fines from process air stream.

Air Treatment: The saturated air is conveyed to a venturi scrubber and then to a combined wet scrubber and saturator to cool and condense the moisture out of the air.  The air is then conveyed to a centrifugal separator to remove any additional moisture.  Approximately 90% of the dry air is directed back to the burner and used as process air.  The remaining 10% is treated with a Regenerative Thermal Oxidizer and then discharged through a stack to the atmosphere.

Solids Classification: The solids are classified into fine, product size, and large size pellets using a vibrating screen.  A trash screen is also incorporated to remove very large inorganic products. The product size (1-4 mm) pellets are pneumatically conveyed to a pellet cooler and final product storage bin.  The large pellets are crushed and conveyed along with the fines to the recycle bin.   The trash is removed and conveyed to a bin for disposal. 

The following process flow diagram shows how each major equipment system interacts within the overall CTD drying system.

Sub-System 1:  Mixing

As shown in the Mixing System Schematic, wet product (15% solids – 70 lb/cu ft) is combined with dry recycled product in a plow-share mixer.  During the mixing process, the wet product attaches to outside of the dry recycled product forming a thin layer of wet material over a dry core.  The mixed product that is formed has a total solids concentration of about 70% and a bulk density of about 55 lb/cu. ft.

Sub-System 2:  Drying

As shown in the Drying System Schematic below, digester or natural gas is fed to the furnace that combusts the gas and generates hot air.  A blower conveys the hot air to the drum dryer where the mixed sludge product is introduced.  The hot dry air comes in direct contact with the mixed sludge in the triple pass drum and evaporates off the moisture.  By the end of the drying process, the water evaporated in the process is contained in the now saturated air while the mixed sludge has essentially been converted to a 95 percent dry sludge product.  The saturated air and dry sludge mixture are conveyed to a preseparator for gross separation and then a polycyclone to separate out the fines.

Furnace
The direct, horizontal end-fired furnace is equipped with a combustion air blower and forced draft burner that provides heat input into the drying system by discharging combusted methane or natural gas from the burner into the furnace chamber and mixing it with secondary air before entering the rotary drum dryer.  An annular ring is provided in the furnace to allow preheating of the secondary air before mixing with the burner flue gas in the interior combustion chamber.  The rotary drum is coupled to the furnace through a rotating seal. 

Drum
The drying drum is a triple pass direct heat rotary drum with a horizontal concurrent residuals and gas stream flow pattern.  The rotary drum is designed to meet specified evaporation rates while maintaining a drum outlet temperature of 190°F or less under all conditions. The dried solids emerging from the drum consist primarily of 1-4 millimeter, 95% dry spherical shaped pellets.

Sub-System 3:  Air/Solids Separation

The preseparator is essentially a baffle that changes the direction of the combined air and solids stream and allows the heavier solids to fall out by gravity.  The polycyclone further separates the solids from the air through a tube type settler.  Solids and air enter the side of the tube and are directed into a downward spiral flow.  Centrifugal action forces the solids to collide with the walls of the tube where they slide down and are collected in a closed hopper.  The clean air is directed up the tube and conveyed to the air process train for further treatment.

Sub-System 4:  Air Treatment

The Air Treatment System is comprised of the following equipment:

Venturi Scrubber
The Venturi Scrubber further removes particulate matter from the process air stream by increasing the velocity of the air and forcing collisions with water droplets.  This is accomplished by routing the air through a narrow throat and introducing a fine spray of water.  The small dust particles attach to the water droplets and fall by gravity to a collection well at the base of the scrubber unit.  The particulate laden water is routed back to the wastewater treatment plant for processing and the relatively clean air is routed to the scrubber saturator for further cleaning and condensing.

Scrubber/Saturator
The scrubber/saturator removes additional particulate from the air stream and cools and condenses the air prior to recycling it back to the burner.  This is accomplished by routing the air to an expansion chamber cooled by up to three water sprays.  The drop in pressure combined with the cold water condenses the steam in the air and converts it to water.  The spray also helps remove fine particulate matter from the air stream.  The majority of the clean air (about 90 percent by volume) is recycled back to burner and used as process air.  The remaining 10 percent of air is routed to a centrifugal separator for final moisture removal before the Volatile Organic Compound destruction and final discharge into atmosphere.

Centrifugal Separator
The centrifugal separator removes water and fine dust from the process air stream coming out of the scrubber.  The dust laden moist air is first mixed with a fine water spray, which captures the small airborne particles.  The water and dust mixture then collides with rotating blades that uses centrifugal force to direct the mixture to a collection cone.  The water and dust mixture is allowed to drain by gravity to a discharge chute.  The clean dry air is routed to an expansion chamber and discharged to a Regenerative Thermal Oxidizer for VOC destruction prior to being discharged to the atmosphere.

Regenerative Thermal Oxidizer
The Regenerative Thermal Oxidizer combusts the VOC’s in the air through thermal oxidation.  The RTO consists of three ceramic chambers heated by a fuel-fired burner to combustion temperatures.  During the operation, one of the three chambers acts as a preheater, the second chamber is used strictly for combustion, and the third chamber is on standby.   After several minutes, the preheat chamber becomes depleted of heat while the combustion chamber becomes saturated with heat.  At this point the flow of air through the chambers are reversed and the preheat chamber becomes the combustion chamber while the combustion chamber becomes the preheated chamber.  After combustion, the VOC free air is discharged through a stack into the atmosphere.

A schematic of the Air Treatment System is shown below:

Sub-System 5:  Solids Classification

As shown below, the dry product removed in the preseparator and polycyclone, is conveyed to a vibrating screen for classification.  Three screens classify the dry pellets into trash, large, medium, and fine sizes.  The trash is conveyed to a bin and disposed of.  The large material is transferred to a roller mill for size reduction and conveyed back to the recycled bin along with the fines.  The medium (1-4 mm) or final product is cooled in a pellet cooler and conveyed pneumatically to the dry product/truck loading bin.