Jessup-Bould Limited
A S Jessup-Bould: Sludge Drying

Sewage Sludge Drying

MCUA; New Jersey, USA

2000 / 2001	Services provided to CPL Biomass for the front end process design for of a facility for the partial drying of sludge.
2004 / 2006	Services provided to R^3m inc for detailed design review and technical support for during installation.
Project Summary In 2000 CPL Biomass were contracted by R^3m Limited to design a 5 stream sewage sludge partial drying plant as part of MCUA’s (Middlesex County Utilities Authority) modifications to its existing land-based residuals management facilities at the Edward J Patten Water Reclamation Facility in Sayerville, New Jersey, USA.		Courtesy MCUA
The purpose of the drying process is to condition the sludge for a new pasteurization plant which replaces a pre-existing curing process. Following a disruption to the contract, the sister company R^3M inc finished building the plant in 2006. In July 2004, as part of this work, Jessup-Bould Limited was engaged by R^3m to provide engineering and consultancy services.
Courtesy R^3m		During this contract JBL undertook the following tasks: The production of a set of final design calculations Generated a design basis report for the thin film dryers and support services. Provided equipment and process design reviews for the drying plant and its peripherals. Where variations from the original conceptual design or other concerns were found they were raised and discussed with R^3m. Advised Client of recommendations and modification requirements; for example; valve fail positions, locking of valves, pump impeller de-rating, operational techniques to minimise fouling, personnel training, dryer blade configuration record keeping (for maintenance)
Process Description
The sludge treatment plant dries raw primary liquid sludge to produce a dried cake product suitable for mixing with lime for use as a composting additive. It is designed to operate under automatic control 24 hours per day, 8000 hours per year treating raw undigested sewage sludge from Middlesex County Utilities Authority sewage treatment works. There are five drying streams, 3 of which operate for one half the year and 4 for the other half. A fifth stream is a standby. Sludge cake from the dewatering building is fed to five wet cake silos via duty / standby drag link type conveyors. Each silo serves a dedicated drying stream as follows: A hydraulically driven discharge device that moves across the flat floor of the silo pushing sludge into the single discharge screw at the base of the silo. The variable speed discharge screw transports the sludge cake via a chute into the feed hopper of the progressive cavity wet cake pump. Sludge cake is fed at a set rate, to the thin film dryer. The dryer is a thermal fluid jacketed, horizontally mounted, cylindrical vessel within which there is an unheated rotor with rows of stainless steel blades. Turning of the rotor spreads the incoming sludge cake in a thin film over the heating surface and transport the material through the unit while maintaining it in contact with the heating surfaces. The retention time of the sludge in the thin film dryer is sufficient to achieve a solids dryness of 55-60% and provide good pathogen kill. Dried sludge falls from the end of the thin film dryer on to one of two enclosed belt type conveyors for transfer to the Pasteurization section of the plant. Odorous air from the Pasteurizer air manifold enters the dried product discharge end of the dryer. The air and water vapour driven from the sludge leaves the vapour hood located at the wet sludge inlet end of the dryer. This provides counter-current flow for optimum drying and capture, by the wet solids of dust from the odorous air. In the event of an emergency shut-down, a pressurised water spray system is activated to cool and rewet the sludge thus generating steam to provide an inertising atmosphere inside the dryer to minimise the risk of dust explosion.

The air and water vapours from the dryer are drawn into a vapour condenser by the vapour fan, which automatically maintains a slight vacuum condition within the thin film dryer and thus slightly enhances the evaporation rate of water from the sludge and avoids leakage of dust. Recirculating condensate cooled in a duty / standby plate heat exchanger is used as the coolant in the vapour condenser. Filtered final effluent is the cooling medium in the plate heat exchangers. The non-condensable vapours drawn from the top of the vapour condenser by the vapour fan, are sent to a common thermal oxidiser. A droplet separator is installed prior to the fan. In order to prevent excessive cooling of the condensate in the plate heat exchangers and hence minimise the possibility of fouling by fatty deposits, the return temperature of the condensate is controlled by automatically adjusting the flow rate of final effluent through the plate heat exchangers. Excess condensate is pumped to a common condensate blow-down tank.

Courtesy MCUA

Each dryer has a dedicated thermal fluid heating and distribution system to provide the heat for sludge drying. The thermal fluid heater has a fully modulating burner suitable for burning landfill gas, natural gas or gas oil. A thermal fluid cooler is provided for emergency shutdown use to prevent the sludge in the dryer from being over-heated or over-dried. Odorous air discharged from the five drying streams is treated by a thermal oxidizer, along with odors from the wet cake silos.

Specific Responsibilities

• Process Design Engineer at conceptual design stage (Drying, Vapour Condensation, Thermal Oil, Thermal Oxidizer).
• Consultant Engineer for final design and installation.

Project Value (2002) US$ 35 million Materials of Construction Mainly stainless steel. Equipment Silos Progressive cavity pumps Thin film dryers Direct contact vapour condensers Thermal fluid heating system Regenerative thermal oxidizer Plate heat exchangers, shell and tube heat exchangers Centrifugal pumps, fans

Hazards Potentially explosive dust. Pathogenic material. Hot thermal fluid. Liaison Client and plant Owner. Equipment vendors. JBL Engineers. Documents Produced Basis of design. Material & energy balances. P&IDs. Process data sheets. Calculations: pressure drops, heat transfer. Design basis report. Process and equipment design reviews

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