Selection of a Sewer Design Basis using Life Cycle Costing and Triple Bottom Line Criteria

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Abstract:

Introduction: Gravity sanitary sewers should be designed to achieve self-cleansing, thereby reducing the potential for blockages. The traditional approach to self-cleansing is based on achieving a minimum flushing velocity in the sewer pipes. This design criterion is typically implemented as a table of minimum slopes for the range of available pipe diameters. The Manning's equation is used to determine these minimum slopes under idealized conditions. Problems arise when these idealized conditions are not observed within the collection system. The current study, undertaken by The City of Calgary, investigated the implications of the minimum slope design basis. Life cycle costing and Triple Bottom Line frameworks were applied to quantify the impacts of alternative design approaches.

Methodology: Three alternatives to sanitary sewer design were investigated. Each alternative was defined in terms of minimum flushing velocity, Manning's n, and whether the design basis is minimum slope or minimum flushing velocity. The City of Calgary currently requires sewer pipes that can achieve a minimum flushing velocity of 0.76 m/s under half-full or full pipe flow conditions with Manning's n of 0.011 for PVC and 0.013 for concrete. These criteria are translated into a table of minimum slopes for the range of pipe diameters. This design approach represents the base case for the study. Alternatives to this base case were created by applying a flushing velocity of 0.6 m/s (commonly reported in the literature) and a common Manning's n of 0.013 for both pipe materials. An alternative was also created by using a velocity design basis, whereby the sewer must be able to achieve the minimum flushing velocity under anticipated flow conditions rather than assuming half-full or full pipe flow conditions. A representative community was redesigned using the design alternatives. Each alternative was evaluated using life cycle costing and Triple Bottom Line criteria. Maintenance data were used to establish the probability of blockage as a function of pipe diameter, pipe material, pipe slope and expected flow conditions. The life cycle costs of each design were determined, including the expected maintenance costs for pipe blockages. A Triple Bottom Line analysis, whereby social, environmental and economic impacts are quantified, was also undertaken for each alternative.

Conclusions: The life cycle costing and Triple Bottom Line analyses indicate that a minimum slope design basis poses a greater risk of blockage failure, and higher life cycle operations and maintenance (O&M) costs and Triple Bottom Line (TBL) consequences. In contrast, the design alternative utilizing the minimum velocity design basis provides a favourable benefit/cost ratio against the current design practices that rely on minimum slope tables. The increased capital costs are mitigated by lower O&M costs and lower TBL consequences. The lower O&M costs can be attributed to reduced risks of blockage failure. The results will inform and guide the design practices for sanitary collection systems at The City of Calgary.
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