United States
Environmental Protection
Agency
Risk Reduction Engineering
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-90/007 August 1990
4>EPA Project Summary
Separated Flow Conditions at
Pipe Walls of Water Distribution
Mains
Lawrence S. Galowin
The objectives of this research
project were to develop and evaluate
a method for determining residence
times for separated recirculation
cavity flow conditions, and to
determine the rate of growth and
surface ramp contours developed
from particulate deposits at
obstacles that induce separation and
eddy formations in water mains used
to distribute drinking water. Resulting
biofilm formations contribute to
accelerated corrosion rates,
increased flow resistance, and the
formation of encrustations and
colonization that may lead to water
quality deterioration. The depend-
ency of conditions at the pipe wall on
viscous flow parameters was
identified from experiments and
analysis of simulated biological
growth and decay rates.
This Project Summary was
developed by EPA's Risk Reduction
Engineering Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Background
An increasing number of drinking water
utilities in the United States are reporting
persistent violations of the safe drinking
water coliform maximum contaminant
level (MCL). When the treated water
leaving the plant is in compliance, the
implication of such violations is that the
water quality is deteriorating within the
distribution system. Some of these
utilities have also reported physical
conditions that are consistent with biofilm
formation on pipe walls—one possible
reason for these occurrences. Biofilm
formation also has some significant,
negative impacts on the costs associated
with distribution system operation. Biofilm
layers contribute to increased corrosion
rates, increased flow resistance, and
formation of encrustations and
colonization, anyone of which may lead
to water quality deterioration. As a result,
the Drinking Water Research Division of
the U.S. Environmental Protection
Agency's Risk Reduction Engineering
Laboratory in cooperation with the
National Bureau of Standards initiated a
study to examine the hydrodynamic
conditions that may affect the formation
of biofilm layers on distribution system
pipe walls. Research to evaluate the
dependency of conditions at the pipe wall
on viscous flow parameters was identified
from experiments and analysis of
biological growth and decay rates.
Past research recommended fluid
dynamic investigations of those flow
properties and transport coefficients in
the water flow layers adjacent to the pipe
wall that contribute to biological and
chemical formations. Current physical
and analytical models do not take into
account the simultaneous processes of
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momentum exchange, diffusion mecha-
nisms, distribution of chemical species
concentrations and reactions, and the
thormochemical state conditions that
govern the corrosion and biological
phenomena. Resistance to disinfection at
pipe wall surfaces and in dead-end mains
can exist even with the residual chlorine
concentration levels (based on bulk flow
conditions) normally applied to preserve
water quality in the pipes. For EPA to
recommend procedures for control and
prevention of "biofouling," it is necessary
to understand the wall phenomena and
the flow parameters that govern the local
pipe environment, which ultimately
contributes to bioorganism colonization.
Study Objectives and Scope
Fluid flow conditions influence, and
may govern, the growth of pipe wall
formations, colonization, propagation, and
downstream reattachment/deposition
locations of bioforms. The transport and
distribution profiles of constituents and
chemical concentrations or nutrients
affecting growths are strongly coupled to
the hydraulics. Extended time periods for
bioform cell reproduction and life cycles
result from separated flows with
recirculation eddies or stagnant regions
at the pipe walls, branch pipe fittings,
dead-end branches, and flow obstacles.
The scope of this investigation included
the following tasks. The experimental task
was to determine upstream and
downstream solid formations developed
from small solids streaming toward and
about obstacles at the pipe wall and the
nearby recirculation patterns caused by
the captured solids over a range-of-flow
Reynolds numbers. The obstacle
simulations represented partially open
valves or reductions in pipe cross-
sections. Solids captured from the flow at
a junction fitting from a main to a dead-
end branch were investigated to
determine the extent of a separated
recirculation flow condition at the dead-
end branch inlet. The analytical task was
to determine separation and recirculation
flow regions at wall cavities and at steps
in order to define the flow streamline
paths, the motion of small particulates
detained within the region, and extended
residence times. The Navier-Stokes
equations for steady flow in the viscous
layer near the pipe wall in the vicinity of
the disturbance were solved numerically
through a finite difference method
computer program.
Water Main Distribution
Systems
Flow conditions in water main
distribution systems are highly variable.
Design requirements for sizing pipe
diameters are usually based on flow rates
for fire fighting. As a result, the normal,
potable-water flow rate conditions are
below the design values. When high
demand occurs, loosely bound
sedimentary deposited materials, biofilm
materials, and other aggregated
substances are shed from the pipe wall
and carried downstream.
To provide remedial procedures for
control and prevention of corro-
sion/fouling, the complex mechanisms
that cause these problems must be
understood.- Simple teehniques,-sueh~as~
mechanical pipe wall cleaning, are
unsatisfactory as a long-term solution for
elimination of deposits and corrosion.
Recurrence of corrosion and aggregate
formations, containing bioorganisms at the
pipe wall indicates that the fundamental
causes must be determined to effectively
and permanently treat distribution mains
to preserve water quality.
Modeling Methods
Currently available biofilm models do
not consider the simultaneous local
processes and mechanisms of
momentum exchange, chemical species
distribution, and thermodynamic state.
Global procedures lack accurate details
of flow interactions in the biochemical/
physical structure formations dependent
on local conditions that control the
processes. Corrosion and biofilm
development depend on the kinetics and
reaction rates among local constituents in
the bulk flow and with the pipe wall as a
source of solute materials. Locally, pH
and chemical, .concentrations may differ
from those in the bulk flow.
Flow velocity, chemical species
concentrations, energy, nutrients, and
biomass contributors that are close to the
wall have profile distributions with large
gradients. The local properties depend on
the mass transfer exchange and
mechanisms of mixing and chemical
reactions in the region. Models to
improve the determination of influencing
factors near the pipe wall require
consideration of the extent of coupling
among: the fluid convective transport
properties (turbulent energy fluctuations);
diffusion (shear layer or laminar
conditions modified with turbulent, bulk-
flow eddy effects); chemical reactions
and species concentration distributions
(variations dependent on the bulk-flow
viscous layer); chemical reaction rates in
the solution involving the ionized states
(as well as corrosion due to
oxidation/reduction reactions coupled
with electropotential differences of pipe
wall material impurities/constituents);
nutrients available with extended
residence times (for enhanced
bioorganism reproduction)
Pipe Configuration Conditions
Stagnation, separation regions, and
flow recirculation or eddies in pipe flow
can occur at obstacles (fittings, bends,
branch junctions, flanged connectors,
valves) and at wall roughness sites
(welds, pitting of surfaces, corrosion/
encrustation locations) on irregularly
-eoated^ manufactured-pipe-—surfaces^—|
Chemical reaction equilibrium conditions
depend on both the pipe wall materials
and state as well as on local flow
conditions. Precipitate wall attachment
and entrapment can also result in
microscopic aggregation clusters that
may unite or expand into larger
formations on the wall. The constituent
wall conditions can be significantly
different from the bulk-flow properties
because of foreign materials coming from
biological cells.
When pipe flow velocities are very low,
the sweeping velocities of the water can
be insufficient to remove particulates
from the pipe wall surface so that
deposits formed on the wall can result in
sheltered regions that promote bioform
growths. Without the bulk-flow
disinfectant penetration to the interior of
wall formations, uninhibited growth can
continue. Also, at all bulk-flow pipe
velocities, the viscous wall effects result
in vanishing velocities near the wall.
Consequently, even under ideal
COTdjtiojis, the region at the walL.is
essentially* slow moving, and transport
phenomena considerations must include
the diffusive transport mechanisms.
Conclusions
Calculations illustrate that properties of
separated flow conditions in cavities
along the walls of water pipe systems can
be determined from the governing
equations of fluid dynamics. Capture of
buoyant, neutral, and nonbuoyant
particles, over a range of sizes, can occur
in the circulation field of the separated
flow. Particulate depositions at cavity wall
corners are also possible. Extended
residence-time periods can result in
nucleation or incubation sites within local
regions. Changes in the pipe flow
conditions can purge the cavities and
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thereby become a method of propagating
newly developed bioform colonies to
other downstream locations along the
pipe wall.
Modeling showed the self-purging of
cavity particle constituents depend on
diameter, specific gravity, and initial
locations in the flow field. Also, it was
shown that buoyant and nonbuoyant
particle motions depend on the flow
conditions. Therefore, the assumption
that particle falling or rising conditions
exist only because of specific gravity can
be erroneous. The controlling processes
for biocolonization, reproduction, and
adhesion mechanisms govern the size of
formations and their density. Changed
mass groupings result in new particle
..irscJ^jDaths^that^increasingly depart from
streamline motion as a function of the
fluid forces and inertial properties.
Experimental results showed flow
separation upstream and downstream of
pipe wall obstructions that resulted from
the formation of recirculating eddies
within separated regions. As the velocity
of the flow is reduced, the recirculating
eddies have insufficient velocity to keep
solids contained in the circulating motion
and the larger solids aggregate at the
pipe invert. The accumulation of solids
become more extensive as the flow
velocity is further reduced. The pattern
of solids deposition depends on the
strength and the position of the
recirculating eddies.
In a dead-end branch pipe from a main,
separated flow regions were created in
the entry vicinity of the branch fitting,
with recirculating eddies within the
branch itself; these were noted at all pipe
flow velocities. The eddies rotated in both
horizontal and vertical planes. In the
regions of solids deposition, it would be
expected that conditions for the
bioorganism colonization would be
enhanced because of increased
residence times. At higher flow velocities,
the scale of containment of solids within
the separated flow regions showed
extended residence times and increased
nutrient concentrations from sedimentary
materials; contrary to the expectation that
greater scouring would occur.
Recommendations
The determination of the chemical
species concentration distribution profiles
in the vicinity of the pipe wall is
necessary. The lack of free chlorine, as a
disinfectant to attack colonies, at the pipe
wall requires explanation. Research on
the dependency of biofilm formations,
limitations on growth depths because of
the shearing action in the stream, and
attachment mechanisms require further
study with the use of experimental
methods and extension of analytical
techniques. The modeling methods of
this study should be enhanced to include
porous fiber structures of film layers on
the pipe wall so that an accurate shearing
flow interface model by a porous surface
layer simulation can be established.
Experimental measurements need to
be made for very fine particulates and
sedimentary materials that appear on
pipe walls and at obstacles. Since the
shear and drag forces for creeping flow
along the wall are generally linearly
dependent on flow velocity, any
differences in deposition formations from
the flow wall would be shown. Chemical
species concentration profiles need to be
measured with various pipe "wall
materials, such as, inert glass, concrete,
iron, and copper. These measurements
would provide data to evaluate the
interactions of different wall materials with
flow constituents for the range of pH
levels representative of water utility
distribution systems.
The full report was submitted in
fulfillment of Interagency Agreement No.
DW13931205 between the U.S.
Department of Commerce and the U.S.
Environmental Protection Agency.
U. S. GOVERNMENT PRINTING OFFICE: 1990/748-012/20062
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Lawrence S. Galowin is with the U.S. Department of Commerce, National Institute
of Standards and Technology, Gaithersburg, MD 28099 .
Richard G. Ellers is the EPA Project Officer (see below)
The complete report, entitled "Separated Flow Conditions at Pipe Walls of Water
Distribution Mains," (Order No. PB90-188 897/AS; Cost: $23.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-90/007
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