“Capacity” versus “Flow”
One question that is frequently asked by individuals reviewing domestic
wastewater treatment facility data is:
“What is the difference between “capacity” and “flow?”
This short article provides guidance on the significance and use of these
terms.
Flow
“Flow” is the easier of the two terms to grasp. It simply is the actual
amount of water flowing by a particular point over some specified time. In
most cases, we report flow in terms of millions of gallons per day. You will
see this unit of measurement abbreviated as “MGD.”
The operators of a domestic wastewater treatment facility maintain records
of the daily volumes of wastewater treated, the volumes of reclaimed water
sent to reuse systems, and the volumes of treated wastewater (effluent) that
may be disposed of (perhaps by discharge through an outfall pipe into a
surface water). The operators submit monthly reports to the DEP listing these
average flows in mgd on each day of the month. In addition, the daily flow
values are averaged to determine the average flows during the month (expressed
in MGD).
Capacity
“Capacity” represents the ability of facilities to move or process water.
Consider a pump. When the pump is turned on, it has the ability to move a
set amount of water within a particular time period. Again, we will use the
units millions of gallons per day (MGD). A pump having a capacity of 2 MGD is
capable of moving 2 million gallons of water during 24 hours (one day). If
that pump is turned on for only six hours (1/4 of a day) on a particular day,
the actual amount of water moved will be 1/4 of what the pump could have moved
over a full 24 hours. During the day that the pump ran for 6 hours, the pump
actually moved 0.5 million gallons of water, and the average flow rate over
the entire day (all 24 hours) was 0.5 MGD (0.5 million gallons divided by 1
day).
All permits issued by the DEP for domestic wastewater treatment facilities
identify the “permitted capacities” of the treatment facilities and any reuse
or effluent disposal facilities. Here the term “permitted capacity” represents
the maximum amount of water that the facility is allowed to treat or to direct
to a particular reuse or effluent disposal system. Consider a domestic
wastewater treatment plant having a permitted capacity of 3 MGD. The permit
may also identify a reuse system and a surface water discharge point. The
permit will specify the capacities of both the reuse system (assume that it
has a capacity of 2.5 MGD) and the surface water discharge (assume its
capacity is 2 MGD). The permitted capacities place limits on the operation of
these treatment, reuse, and disposal facilities. This treatment facility must
not process more than 3 MGD. No more than 2.5 MGD of reclaimed water may be
released to their reuse system. In a like manner, no more than 2 MGD may be
discharged to the surface water.
First, note that the capacities of the reuse and disposal systems need not
equal the capacity of the treatment facility – in our hypothetical example,
the treatment facility has a capacity of 3 MGD while the reuse and disposal
systems total 4.5 MGD. However, the capacities of the combined reuse and
disposal systems normally should equal or exceed the treatment plant capacity
to ensure that the wastewater being processed can go somewhere.
Our hypothetical treatment facility has a permitted capacity of 3 MGD. This
represents the maximum amount of wastewater that it may treat during a day. It
is quite possible that on some day, this treatment facility may only receive 2
MGD of untreated wastewater for treatment. The actual flow to be treated is 2
MGD.
Flows may be significantly less than the permitted capacity, particularly
shortly after a treatment facility is constructed. This results from the fact
that utilities will design their facilities looking 5, 10, or more years into
the future. It may take a number of years to plan, design, and build domestic
wastewater facilities. Hence, the wise utility builds today to serve future
population in their service area. When a new 3-MGD capacity treatment facility
is opened, the initial flows available for treatment may only be 1 MGD. As the
population in the facility’s service area grows, the flows coming into the
treatment facility will increase. By the end of the design period, flows being
treated will begin to approach the treatment facility’s permitted capacity. As
this occurs, it becomes time to plan for and construct expanded facilities,
such that the permitted capacity remains larger than the actual flow being
treated.
Summary
