A Baffling Solution to Sewage Conveyance In York Region, Ontario

The Regional Municipality of York, in Ontario, Canada, is currently implementing its new Southeast Collector (SEC) sanitary trunk sewer, the latest stage in the twinning of the York Durham Sewage System (YDSS) in York and Durham Regions, which will extend from 9th Line and 14th Ave in York Region to Valley Farm and Finch in Durham Region, a total length of 15 km. The new system is being constructed by tunnelling. The finished diameter of the tunnel will be 3.0 m, and the maximum design flow rate for the tunnel is 16.2 cms.The profile of the SEC includes two significant vertical drops, one approximately 19.5 m deep and the other approximately 21 m deep. The design of the two drop shafts needed to consider a number of key factors, including the type of drop shaft; key components of the drop shafts and associated appurtenances; hydraulic capacity; flow variability; energy dissipation; air entrainment and ventilation; odor and corrosion control; subsurface conditions and constructability; operations and maintenance requirements/concerns; environmental impacts; and life-cycle costs (including initial capital and long-term operations and maintenance costs).Common sewage drop shaft configurations include plunge, vortex and baffle type drop structures. Plunge type drop structures are simple to design and construct, but the turbulence created in these structures may cause excessive air entrainment leading to increased odor and corrosion in the drop structure and in the downstream sewer system, unless sufficient measures are employed to reduce excessive turbulence.Vortex type drop structures cause the sewage flow to spiral down and cling to the walls of the vertical shaft, minimizing air entrainment and potential odor emissions. The vortex action may be induced by a specially designed inlet structure installed on the end of the incoming sewer at the top of the shaft, or by a special insert installed inside the shaft itself. Different types of vortex inlets include circular, scroll, spiral, helical and tangential configurations.Cascade or baffle type drop structures employ a number of horizontal baffles installed at a predetermined spacing within the vertical drop shaft to continually dissipate the energy of the falling sewage as it travels down the shaft, instead of a single plunge pool or impact cup at the bottom of the shaft. Baffle type drop structures have been around for decades - Cleveland employed a baffle configuration for an extremely deep drop structure as early as 1914 - but have not been widely employed recently in wastewater collection systems. The baffle drop shaft design does not require a de-aeration chamber at the bottom of the shaft, which can significantly reduce construction costs and risks and shorten the implementation schedule, especially where the bottom of the shafts are to be located soft ground, as is the case for the Southeast Collector. Recently, baffle drop shafts with improved design features, including separate vertical portions, one for the sewage flow, and another for air flow and to facilitate drop shaft and tunnel inspection and maintenance activities, have been successfully implemented in New Zealand, Britain and the United States.Some other advantages of this type of baffle drop structure include the following:• A single shaft accommodates sewage and air flow, and provides access for personnel and equipment to carry out drop shaft and tunnel inspection and maintenance activities.• Hydraulically, the baffle drop structure maintains atmospheric conditions throughout, and thus avoids cavitation issues common in other types of drop structures.• Construction does not involve complex or expensive materials or methods. Both precast concrete and cast-in-place concrete structures have been successfully implemented.• The structure is flexible, accommodating a wide range of flows, virtually any depth of drop, multiple incoming pipes with different flow rates, different approach angles, and different invert elevations.The employment of baffle type drop shafts was considered by the Region of York for the SEC and their evaluation and design was completed with the help of hydraulic modeling, using a 1:19.7 physical scale model of the proposed baffle drop structures, constructed and tested at the Iowa Institute of Hydraulic Research (IIHR) at the University of Iowa. The scope of work for the physical model testing included:• Testing of 5 different baffle spacings to identify the optimum vertical spacing, considering sewage conveyance capacity, air ventilation, construction cost, and ease of maintenance.• Testing of 3 different locations for vertical dividing wall to identify its optimum location, considering sewage conveyance capacity, air ventilation, construction cost, and ease of maintenance.• Testing of 2 different locations for maintenance inspection ports beneath each baffle, including locations near outer circumference of shaft and closer to centre of shaft.• Testing of different orifice configurations at bottom of drop shaft to throttle flow leaving the structure to further limit entrainment of air into sewage entering downstream tunnel.• Testing of different plunge drop shaft configurations, using a separate scale model, to provide side-by-side comparison of plunge and baffle type sewage drop structures.• The above tests were completed for a variety of SEC flow rates, including Qmax = 16.2 cms, Qavg = 8.1 cms and Qmin = 4 cms.Based on the results of the physical model testing, the Region decided to proceed with implementation of two baffle drop structures for the SEC. This paper presents the methodology and findings of the physical model testing of the potential baffle and plunge type sewage drop shafts, including the preferred drop shaft type and recommended design parameters for the two proposed sewage drop structures, and discusses the final design of the two baffle drop structures.