Biosolids Management

Dewatered biosolids are pumped to storage via the existing dewatered sludge-pumping system. It is then pumped from either of the two biosolid storage silos into the mixer and mixed with recycled dry product to create a pellet for drying. The pellets are dropped into the inlet of the drum dryer, between the furnace and the drum. Recirculated gas and combustion air are mixed and heated in the furnace using natural gas. The evaporation of water from the preformed pellets takes place in the rotating drum. High-speed gas, along with drum rotation, conveys the pellets through the drum until they are dry enough, and thus light enough, to be conveyed out of the drum. At the exit of the drum, the process air carries evaporated moisture and dried particles via a ceramic-lined duct to the two stage separation system, which consists of a pre-separator and a polyclone, where the solids are separated from the process air stream.

The dried material exiting the equipment that separates the process air from the solids consists of dry particles of many sizes that drop onto a screen and are then classified by size. Trash is screened first, and this drops to a trash bin. Coarse and oversize pellets are dropped into a roll crusher and sent to the recycle bin along with the fine particles. The remaining material (approximately 0.5 mm to 4 mm in size) is conveyed to a pellet cooler and then pneumatically conveyed to pellet storage. The product is loaded into trucks via a screw conveyor with three dustless load-out chutes. The load-out system is designed such that pellet oil may be added to minimize dust generation during transport and use.

The hot process air exiting the separation equipment carries the evaporated water vapour and some very fine particulate. An impingement tray condenser/sub-cooler cools the process air, causing the evaporated moisture to condense and drop out while at the same time removing the particulates and condensable volatile organic compounds (VOCs). Most of the process air is recirculated back to the furnace and remains in the process loop, thus minimizing air emissions. The remainder passes through a Venturi scrubber and a three-chamber regenerative thermal oxidizer (‘RTO’) before exiting the system and entering the atmosphere through a stack.

The drying system incorporates process dust–collection systems using fabric filters to remove dust from a number of enclosed sources, including the conveyors, screen, pellet cooler, product silo, and load-out chutes. The collected dust is recycled into the process or removed as trash. The dust collectors are ducted to the odour-control system for treatment. Dryer process safety systems include emergency quench, spark quench, carbon monoxide and oxygen monitoring, temperature monitoring, and other safety mechanisms as required by NFPA 68, 69, and 820. There is also a nitrogen-blanketing system for the product silo and recycle bin.

The dryer utilizes mineral oil or a similar non-hazardous product as a dust suppressant for the beneficial product. The oil is stored in a double-walled tank with secondary containment. The required utilities include electricity, natural gas, effluent water, and potable water. Process wastewater discharges will be from the tray and Venturi scrubbers.

The project allows the City for the processing and marketing of biosolids in the most efficient manner, providing fixed known cost over a 30-year period that was validated through a predetermined affordability threshold. The public-private-partnership was supported through funding from Infrastructure Canada under the P3 Canada Fund, and the construction and continued operation and maintenance of the new facility will be led by Harbour City Solutions.

Immediate Benefits

• Minimal dust
• Lower odour
• Greater marketable opportunities as a pathogen-free pellet product
• Less truck traffic (from 20/week to 6/week)
• Less waste to landfills
• Greater cost certainty over the 30-year term