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| View of the entrance to the truck loading facility showing the truck entry doors with traffic lights indicating which bay (green) is full and ready to receive trucks. |
By Vincent Nazareth and Valera Saknenko, R.V. Anderson
Associates and
Kiyoshi Oka and John Bryson, City of Toronto
In spring 2001, the City of Toronto commissioned its new state-of-the-art truck loading and odour control facilities, the first of three design-build projects initiated by the City as part of its move to beneficial use of biosolids and termination of biosolids incineration.
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| View of the entrance to the truck loading facility showing the truck entry doors with traffic lights indicating which bay (green) is full and ready to receive trucks. |
Biosolids beneficial use program
The City of Toronto owns and operates the Ashbridges Bay Treatment Plant (ABTP), an 818,000 m3/day secondary treatment wastewater treatment facility bounded by residential, recreational and commercial areas and Lake Ontario. Primary and secondary sludges removed at the ABTP are anaerobically digested and dewatered using horizontal centrifuges, generating approximately 50,000 dry tonnes annually of dewatered biosolids, with a solids concentration in the range of 25-30 percent; most of this was incinerated.
In the Fall of 1998, the City decided that incineration was no longer an acceptable method of biosolids disposal, and embarked on a search for alternative disposal methods that could be classified as beneficial. Various biosolids beneficial use alternatives were investigated, including alkaline stabilization, direct application of dewatered product to agricultural land, composting, and pelletization. With the active participation of the public through a Biosolids Multi-Stakeholder Committee, two options were selected for implementation - application to agricultural land and pelletization.
These two options would provide the City of Toronto with a more diverse program, giving greater flexibility for biosolids beneficial use, and, consequently, an end to incineration at the ABTP. However, because the incinerators have been used as a heat source for the treatment plant and for foul air combustion, alternatives for heating and for treatment of odorous air streams from the existing and new biosolids management facilities would be required after shutting down the incinerators.
Implementation of the program would involve three projects having a total construction value of $85 million. These were a dryer/pelletizer facility, truck loading and odour control facilities, and a plant-wide heating system.
The City set an ambitious time limit to achieve the shut-down of incineration, allowing some 27 months for planning, bid document preparation, bid periods, evaluation, negotiation, award, detailed design, construction and commissioning. In view of this, a design-build project delivery method was chosen. Conceptual designs for the loading and heating projects were developed by the consulting team, led by R.V. Anderson Associates Limited, with Brown & Caldwell assisting.
New truck loading and odour control facilities
In many respects, the truck loading and odour control facilities project proved to be the most challenging of the three Toronto biosolids projects.
The objectives of the project were to provide the City of Toronto with:
Preparation of RFP documentation was required in twelve weeks and included reviews of various options, development of conceptual designs, outlining design criteria and performance requirements, process and air flow diagrams, site layout, building and major equipment and piping layouts, and development of component specifications. The preparation of the conceptual designs, and review of available options provided valuable opportunity for input from the City, allowing staff to define minimum quality requirements. These early considerations would allow the contractor selected to focus efforts on a scheme that had already been reviewed by the City.
The conceptual design phase addressed space constraints, the requirement to provide a minimum of four days of biosolids storage (approximately 2000 m3), biosolids conveyance, stringent odour control requirements and the need to minimize interruptions to plant operation. Many of the outwardly suitable options would have exceeded limitations in relation to the maximum distance for biosolids conveyance by pumping.
It was eventually decided to split the biosolids storage, providing intermediate 1200 m3 bulk storage in three biosolids storage silos, and 800 m3 of biosolids storage in four truck-loading hoppers located directly above the loading bays. After reviewing over a dozen possible options, the most favorable solution was to construct three of the new silos in the space used by the three existing incinerators. This resulted not only in the reuse of the existing building but also existing piled foundations, thereby saving construction time and money.
After considering the use of hydraulically driven reciprocating piston pumps (HDRPP), belt conveyors or screw conveyors, a system utilizing three HDRP pumps was selected as the best option for biosolids conveyance because of its ease of operation, maximum operational flexibility, reduced space requirements, and containment of odours. The pumps were located in the basement of the silo building directly beneath each silo. In order to reduce friction in the cake piping, a lubrication system utilizing water or a water-polymer mix was added to the conceptual design.
The new truck loading facility is designed to be an unmanned, fully automatic facility consisting of two truck bays, able to load trucks of any configuration, within a maximum period of 15 minutes from arrival at the facility. Each truck bay is equipped with two hoppers, and a weigh scale that allows weighing of the truck during the loading process. The loading facility, with a drive-through configuration, requires loading to be completed with truck-access doors closed, to prevent release of odours to the atmosphere.
Odour control of paramount importance
Since the incinerators were to be shut down, an alternative method of odour control was required to treat emissions from the existing and new sludge management facilities.
At the RFP preparation stage, foul air sources were sampled and analyzed for contaminants. Available technologies were evaluated and, based on the contaminant gases present, a conceptual odour control system was developed. The concept included three-stage treatment, consisting of two wet scrubbing stages (allowing use of different chemicals and redundancy), followed by biofiltration. Biofiltration was selected for treatment of the organic sulphides, based on lower life-cycle costs and successful history in treating sludge odours. The overall capacity of the odour control facility is 20,000 L/s.
Design, construction and commissioning
The consulting team developed a detailed evaluation protocol requiring four independent reviewers (two from the consultants and two from the City). Technical proposals were independently scored and subsequently price proposals were opened, scored and the combined scores calculated. ICON Systems Limited (now Earth Tech (Canada) Inc.) was the successful proponent.
ICON developed the design further, constructed and commissioned the facilities. Among the major changes to the conceptual design were the use of sliding frames for the biosolids storage silos and 'attaching' the odour control and truck loading buildings, which were adjacent structures.
The commissioning of the facilities was almost flawless due to the considerable effort of the entire project team during factory acceptance testing. During a 30-day performance testing period, the contractor's staff operated the facilities with no significant interruption. The project was turned over to the City of Toronto on April 20, 2001, some 27 months after the initiation of the conceptual design process.
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