To Flush or
Not to Flush
Pasadena Department of Water and Power
WHILE MANY COMMUNITY WATER SYSTEMS (CWS) have
converted to chloramines for control of disinfection by-products and
compliance with various regulations, nitrification can be a constant
struggle. Dead-ends and low-demand areas create areas where bacteria can
grow and produce water quality problems.
Challenges with limited water supply and the pressure to achieve increased
water conservation make flushing a challenge, not only because of the water loss,
but also the public’s perception that the water system is flushing water while
customers are being asked to conserve. This can be a difficult visual to justify.
Further, flood control districts are increasingly reluctant to allow CWSs
to discharge potable water into their stormwater facilities for fear of being
put out of compliance with the Federal Storm Water Rule as implemented
through individual National Pollutant Discharge Elimination System
(NPDES) stormwater permits. In some regions, Clean Water Act (CWA)
enforcement agencies have also placed discharge limitations on CWSs
directly through different NPDES permits. As a result of these factors, CWSs
face increasing hurdles to maintaining water quality.
Pasadena Water & Power (PWP) has historically discharged approximately
six million gallons of water a year for water quality purposes, in no small part for
nitrification control, but also for dead-end and unidirectional flushing. We also
discharge water for construction and maintenance purposes. We are currently
using a zero discharge flushing truck, the NO-DES truck. Also referred to as
a portable water treatment system, PWP’s vehicle includes a truck-mounted
pump, filter, and chlorination system. A fire hose is run out to a hydrant and a
second hose to a second hydrant. The water is circulated against the normal flow
at very high velocities, scouring the pipe surfaces. Solids, biofilms, and debris
are mobilized and trapped in a one-micron bag filter. An in-line turbidity meter
lets operators know when the scouring is being effective.
PWP operates part of its system with free chlorine; other parts are
chloraminated. Where free chlorine is used, the NO-DES truck can add
more chlorine when concentrations are low. Where chloramines have broken
down and free ammonia is present, fresh chlorine can be added to tie up
the free ammonia and prevent nitrification. In areas of severe nitrification, a
section of the distribution system can be broken over to free chlorine.
The decision to purchase the NO-DES truck was not initially obvious.
Among other considerations, the dollar value of the wasted water was small
10 SOURCE winter 2016
Are zero discharge flushing trucks an alternative
during drought and for NPDES permit compliance?
Two agencies describe their experiences . . .
compared to the substantial capital cost of purchasing the unit. The governor’s
2015 directive requiring California CWSs to achieve a fixed volume of water
savings under threat of a Notice of Violation was a turning point and provided a
substantive rationale. Not all the benefits were measureable in monetary terms.
Regulatory compliance with the CWA for ourselves, and assisting flood control
districts with their compliance, were important considerations. Finally, the
opportunity to take a leadership role and provide an example to our customers
and their elected representatives of how to address conservation was very
important to the decision-making process. We can “practice what we preach.”
Rather than flush the water and achieve limited benefits in maintaining
water quality while risking a negative public perception of the agency, we
now have a solution that recovers the water and achieves higher quality, and
helps us and local flood control authorities comply with NPDES permits.
—David Kimbrough, Water Quality Manager, Pasadena Water & Power
San Jose Water Company
SAN JOSE WATER COMPANY (SJWC) discontinued its flushing operations
in the summer of 2014 and opted to purchase a zero discharge flushing
truck. The truck pumps and filters water through one-micron filters from
one hydrant to another. The decision was made based on field results obtained
during a one-week demonstration of the truck in SJWC’s distribution system
in June 2013. The demonstration results indicated that the purchase of a zero
discharge flushing truck would address water discharge issues and yield better
control of flushing velocities, turbidity, and chlorine residuals.
The truck was delivered in April 2015 and was placed into service in May
2015. SJWC opted for a more sturdy truck chassis than is standard, and to
staff the truck with a Water Quality Worker and Crew Leader. In addition to
the standard pumping and filtering equipment, SJWC’s truck is outfitted with
a Segway and a portable pop-up tent with a camp toilet. These additional
features are designed to provide for increased mobility in the field for the
crew member operating the hydrants and valves and to allow workers to take
bathroom breaks without having to demobilize the truck.
SJWC primarily uses the truck to remove sediments and biofilm in mains
up to 12 inches. The truck can achieve velocities of up to five feet per second
in eight-inch pipes, but the practical maximum velocity for a 12-inch main
is three feet per second. The truck is also used to address nitrification issues.
It can add chlorine to the main, and it is possible to perform break point