lauren@kelman.ca

Construction of I+I Tanks
available to ensure safe and effective
tank cleaning. Control methods (either
manual or through SCADA) may be
needed for boosting of the effluent
water pressure during tank cleaning.
In addition, sufficient volume should
be available within any pressure tank
that’s connected to the effluent water
network to prevent excessive hose surges
and pump cycling when tank cleaning
occurs. Additional methods to facilitate
cleaning include tank sloping to
accommodate the free drainage of debris
and partition walls to allow progressive
filling and overflowing of successive
tank cells to match the I&I event size.
I&I wet well, sump and pump
station design: A means to flush or
clean out I&I wet wells or sumps that
remain unused for extended periods of
time should be considered. This can be
accommodated via selectively located
hose connections, concrete benching to
promote removal of solids or proprietary
pre-rotation pumps, which maximize
achievable sewage drawdown.
Control, automation
and flow measurement
Instrumentation associated with
storage tanks is fairly limited, however,
the process control logic for operating
automated stormwater detention
facilities can be more involved than
conventional pumping stations. It is
important to build in control logic for
unmanned detention tanks in order to:
• prevent repetitive tank flushing after
or during a storm event;
• prevent premature emptying of the
tank into a surcharging downstream
sewer; and
• protect against small volumes of
stagnant sewage accumulating in
the tank.
Communication with downstream
pumping stations may be needed to
ensure tanks are not emptied too early
and may need a modulated release. For
control of the tank flushing sequence, a
series of conditions (permissives) can be
built into the pump control sequence to
measure if the storm event is truly over
prior to starting a flushing sequence.
For example, the system could verify
that the running average water level in
the tank has not changed by a pre-set
percentage over a one-hour period.
A separate pump control sequence is
then required to differentiate between
incoming storm flow into the tank and
water accumulating in the sump pit
after a flushing sequence.
Measuring the volume of overflows or
bypasses will be required by the federal
Wastewater Systems Effluent Regulations
(WSER) (and generally the Ministry of
the Environment and Climate Change);
therefore, any overflow relief points in
the tank should be monitored. It is not
always practical to install conventional
magnetic flow meters or parshall flumes
at these inaccessible locations, so creative
volume measurements may be required.
The current WSER calls for a minimum
15% flow measurement accuracy at
the WWTP effluent. No accuracy
requirements were specifically mentioned
for CSO points, although it may be
specified through the Environmental
Compliance Approval (ECA). Some nonconventional
techniques used for bypass
volume measurement have included:
• measuring pressure at the pump
discharge to relate it to the pump’s
certified test curve to establish an
instantaneous flow rate through the
pump discharge;
• using level transmitters to take a
‘time stamp’ level measurement prior
to emptying the tank to establish the
volume stored.
• for adjustable overflow levels, using
a spot plate to measure the relative
position of a weir to the liquid level
via a second level ultrasonic.
Construction and structural issues
When constructing long, narrow,
sanitary detention tanks that are
confined by existing utilities or rightsof-way
(ROWs), consideration should
be given to using one-sided formwork,
placed against any shoring system.
This method maximizes the usable
width of the tank within the available
site. Always consider the minimum
width required by an excavator to
efficiently complete the works.
During construction, proper
concrete curing techniques must be
followed to mitigate against shrinkage
cracking in long concrete walls and
expansion joints used for large tanks.
Controlled permeability formwork
(CPF) liner on the face of water
retaining concrete surfaces can be
specified to improve the concrete finish
and freeze thaw resistance.
As storage tanks remain mostly
empty, they are relatively light structures
and consideration must be given to
control buoyancy to avoid structural
failure due to the base slab ‘floating’
if high groundwater levels occur.
For enclosed tanks, the weight of the
tank can be increased by burying the
top roof slab. For large area open tanks,
an extensive subdrain network may be
required and consideration is also needed
for protection against frost heave.
Inflow and infiltration into a sewage
collection and treatment system can have
significant negative outcomes, including
treatment system overload, insufficient
treatment, and basement flooding.
Addressing I&I can be done either at
the source or through expanding the
capacity of the system. If expansion
is deemed necessary, the hydraulic
capacity of the conveyance system
must be assessed to determine and
eliminate bottlenecks before a detention
facility can be designed. From that
point, considerations need to be given
to facility type, tank access, cleaning
methods, control and automation, and
construction constraints in order
to generate a viable design basis.
The proper combination of features
can yield an I&I detention system than
will be able to service a community for
many years into the future.
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INFLUENTS
Fall 2015
61