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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