Cost Versus Benefits of Reducing Inflow & Infiltration
Discussion Paper Summary
Inflow and Infiltration refers to rainwater and groundwater that enters the sanitary sewer collection system. A certain amount of I&I is unavoidable and is accounted for in routine sewer design. However, when I&I exceeds design allowances, sewer capacity is consumed and usually results in overflows and increased conveyance costs or a reduction in the future population service capacity. Experience has shown that the I&I allowance used in the original design of older systems is significantly below the wet-weather flows these systems now experience.
It is not uncommon for wet-weather peak flows to be much larger than the average daily flow of wastewater. Such large peak flows are primarily due to the numerous defects in the collection system, caused by system deterioration and illegal connections over the years.
CRD Goals
Reduction of I&I flows is a long term project, one that the CRD has committed to addressing over the next 50 years. The goal of the CRD and its municipal partners is to:
- Reduce inflow and infiltration
- Minimize total conveyance, treatment and disposal system costs
Cost
Cost of reducing I&I is determined by the areas in need of sewer rehabilitation and the desired levels to which I&I is reduced. Areas that contain the most antiquated sewer equipment are also the areas with most need of I&I reduction.
A number of benefits, including cost savings, come from reducing I&I flow. These include reduced conveyance costs (pumping), reduced treatment costs (through the elimination of wet weather plants), hydraulic benefits (plants will last longer and cost less to maintain) and environmental and social improvements, through reduction of sewer overflows and their associated health risks.
It is unlikely, however, that reduced I&I flows will result in making the new treatment plants smaller or less expensive. This is due to the fact that the actual flows (as measured at Clover and Macaulay Point pump stations) are below 2xADWF (Average Dry Weather Flow) 95-99% of the time. The only real potential cost saving would be to reduce the flow down to a maximum of 2xADWF so that the wet weather, primary treatment facilities would not be required.
The table below shows potential costs and benefits to reduce inflow and infiltration to 4 times and 2 times average dry weather flow. Savings are also indicated.
Cost vs. Savings |
Reduce I&I to 4xADWF |
Reduce I&I to 2xADWF (million) |
|
Cost to Reduce I&I |
$205.48 |
$421.14 |
|
Savings from Reduced I&I 1 |
($40.20) |
($230.79) | |
Net Cost Difference |
$165.28 |
$190.35 |
|
Annual cost over 100 years 2 |
$2.05 |
$4.21 |
|
Other Benefits |
|
|
|
Reduction in Annual Overflows |
|
|
|
Current average number of overflows per year |
60 |
5 |
0 |
Reduction in Average Age |
|
|
|
Current average age |
75 |
50 |
40 |
© Image courtesy of Evan Leeson