Blackburn Wastewater Treatment Plant Upgrade Study

Section 2.0 - UI Mitigation Measures
Infiltration can enter a sewer system through deficiencies or cracks in 3 categories of
the sewer line: connections, in-street pipes including joints, and manholes. Infiltration
that can be expected to result from each of the 3 categories is calculated on a mmdiameter-m
basis. Other assumptions involved in the proportioning of infiltration
amongst pipes, manholes, and connections is the lower construction standards of
connection installment (45 percent), and the 7 mt10 m split of municipaVprivate
connection length.
Table 2.2a calculates that of the 33 percent of III that is infiltration, approximately 21
percent is from pipes, 3 percent from manholes, 4 percent from municipal connections,
and 5 percent from private connections.
Table 2.2b and Table 2.2~ calculate the percent infiltration reduction required from
pipes including joints, manholes, and connections in order to reach the required yearly
reduction as previously described in Table 2.1. The amount of physicaf rehabilitation
is based on the assumption that grouting a section of pipe will only reduce its
contribution to infiltration by a maximum of 75 percent. Therefore, the percent of
rehabilitation must be higher in order to achieve the required percent of infiltration
reduction. For example, during the first year (2000) of the 5 year plan, about 6 percent
of the infiltration in pipes must be reduced. This means that approximately 8 percent
ofpipes must be grouted to achieve the required reductions in infiltration.
Table 2.3a and Table 2.3b estimates the cost of rehabilitation based on the grouting of
each sewer element. The cost of grouting is assumed as follows:
Pipes, $/m
Manholes, $/unit
Connections, $/m
The total cost for the 5 year and 10 year rehabilitation plans are $1,580,000 and
$1,120,000 respectively. The Net Present Value (NPV) is calculatd in Table 2.4.
Based on an annual discount rate of 6 percent, the NPV for the 5 and 10 year plans are
$1,319,000 and 5815,000 respectively.
The 10 year plan requires less work to be done as it is based on the percentage of flow
reduction each year to the previous year. Therefore, the overall capital cost is
$460,000 less than the 5 year plan. However, it should be noted, that by committing
to a 10 year plan, the City of Prince George is not meeting the 2 x ADWF criteria for
the Blackbum sewerage area for an additional 5 years. This makes the 5 year plan a
flow based benefit plan, and the 10 year plan a cost based benefit plan.