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OCR for page 108
lv
Biological Quality of
Water in the
Distribution System
One objective of the physical and chemical treatment of water intended
for public consumption is to reduce the levels of total coliforms to less
than 1 coliform/100 ml. Such treatment has proved to be an effective bar-
rier against transmission of infectious disease by water. Commonly used
processes such as coagulation, sedimentation, sand filtration, and disin-
fection yield a water essentially free of disease-causing viruses, bacterial
and protozoa.
The system of pipes, valves, and connections designed to distribute the
water to the consumer is disinfected but not sterilized during construc-
tion. Thus, drinking water is not sterile and may contain living material
that may influence water quality in the distribution network.
In a well-designed, -constructed, and -operated water distribution sys-
tem, changes in water quality will be minimal, and water provided to the
consumer will be similar to that leaving the treatment plant. In poorly de-
signed, constructed, and operated water systems, water quality will deteri-
orate in the distribution system and result in consumer complaints. In the
worst cases, outbreaks of disease may result.
BIOLOGICAL MATERIAL IN WATER DISTRIBUTION SYSTEMS
The nature of the biological material in water distribution systems results
from a complex series of physical, chemical, and biological reactions (Fig-
ure IV-11. Living organisms and nutrients may enter the distribution sys
108
OCR for page 109
Biological Quality of Water in the Distribution System 109
tern with the raw water, during the treatment processes, or from sources
such as leaks, cross-connections, back-siphonages, and open reservoirs.
Growth may occur at or near the pipe surface, the interface of suspended
particulates, and within the water itself. The chemical and biological
products leave the distribution system at the consumer's tap.
Biofilms
Fouling refers to the undesirable formation of inorganic and/or organic
deposits on pipe surfaces. These deposits can induce water quality
changes, increase the rate of corrosion at the surface, and increase the
fluid frictional resistance at the surface.
There are several types of fouling and combinations thereof: crystalline
or precipitation fouling, corrosion fouling, particulate fouling, chemical
reaction fouling, and biological fouling (biofouling). Biofouling can result
from development of an organic film (biofilm) consisting of microorgan-
isms and their products and from assorted detritus.
Development of an understanding of biofouling from field observations
has been limited because of the interaction of several contributing rate
processes. The mechanisms of fouling biofilm accumulation may be de-
scribed as the net result of the following:
· Transport and accumulation of materialfrom the bulk fluid to the
surface. Materials can be soluble (microbial nutrients and organic salts)
or particulate matter (viable microorganisms, their detritus, or inorganic
particles).
· Microbial growth within the film. Microbial growth in the biofilm
and extracellular polymers produced by the microorganisms contribute to
the biofilm deposit and promote adherence of inorganic suspended solids.
· Fluid shear stress at the surface of the film. Such forces can limit the
overall extent of the fouling deposit by reentraining attached material.
The reentrained material can result in significant water quality changes in
the pipe.
· Surface material and roughness of the pipe. Surface properties of the
pipe can influence micro-mixing near the pipe wall and corrosion pro-
cesses. Some metal surfaces may release toxic components into the biofilm
inhibiting growth and/or attachment. Some metals produce loosely held
oxide films under the biofilms. When the oxide film sloughs, the biofilm is
also removed.
· Fouling control procedures. Chlorine, the most commonly used disin-
fecting chemical, oxidizes biofilm polymers causing disruption and partial
removal in the shear stress field. Inactivation of a portion of the microbial
OCR for page 110
110
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OCR for page 112
112 DRINKING WATER AND HEALTH
population also occurs. Altered biofilm "roughness" and decreased viable
cell numbers will influence "regrowth" rates of the biofilm. Chlorine is
also capable of accelerating corrosion processes.
There can be no doubt that biofilms in water supply distribution sys-
tems alter water quality. The following changes, or their combination, can
occur in the distribution system as a result of biofilm processes:
· decrease in chlorine residual (Larson, 1966; O'Connor et al., 1975~;
· increased bacterial counts resulting from loss of chlorine residual
(Larson, 1966; O'Connor et al., 1975~;
· increased bacterial counts or "regrowth" in the distribution system
resulting from detachment of bacteria from the biofilm (Becker, 1975;
Russell, 1976~;
· reduction in dissolved oxygen content resulting from microbial activ-
ity in the biofilm (O'Connor et al., 1975~;
· taste and odor changes resulting from products of microbial metabo-
lism within the biofilm or their reaction with chlorine (O'Connor et al..
1975; Silvey et al., 19751;
· "red water" resulting from the activity of iron bacteria (Larson, 1966;
O'Connor et al., 1975; Russell, 1976~;
· "black water" resulting from the activity of sulfate-reducing bacteria
in anaerobic microenvironments within the conduits (Larson, 1966; Lee
and O'Connor, 1975~; and
· increased "hydraulic roughness," which increases turbulence in the
conduit. Increased turbulence increases mass transfer rates at the conduit
wall, influencing corrosion rates, leaching rates, and detachment of bacte-
ria from the biofilm (Picologlou et al., 1980~.
BlOFILM HORTON
face;
film.
Biofilm formation is the net result of several physical, chemical, and bio-
logical processes including the following:
· transport of solutes to the wetted surface;
· adsorption of solutes at the wetted surface;
· transport of particles (including microbial cells) to the wetted sur
· attachment of microorganisms to the surface; and
· metabolism and growth of microorganisms immobilized in the bio
OCR for page 113
Biological Quality of Water in the Distribution System 113
The rate of water flow and the concentration of growth-limiting nutri-
ents (i.e., substrate) are the primary environmental variables influencing
the rate and extent of biofilm formation. A general discussion of biofilm
formation follows, with special attention to problems encountered in
water supply distribution systems.
The formation of biofilm begins with transport to and adsorption of or-
ganic solutes at the pipe wall. The organic material may be of microbial
origin or a component of the raw water supply. Algae and bacteria in the
water treatment plant' on the open reservoirs, or within the conduits pro-
duce relatively high-molecular-weight organic compounds, primarily poly-
saccharide in nature. Raw waters frequently contain organic compounds
such as tannins or lignins. These polyelectrolytes are surface active and
adsorb readily, thereby conditioning the surface. Generally, the polyelec-
trolytes and the conduit surface are both negatively charged, and firm ad-
sorption of the polyelectrolyte to the surface is mediated by inorganic cat-
ions such as calcium, magnesium, and iron.
Microbial cells are transported to the conditioned surface where they at-
tach. Research by Marshall et al. (1971) and Zobell (1943) suggests the ex-
istence of a two-stage attachment process: reversible adhesion followed by
an irreversible attachment. Reversible adhesion refers to an initially weak
adhesion of a cell that can still exhibit Brownian motion. Conversely, irre-
versible attachment is a permanent bonding to the surface, usually aided
by the production of extracellular polymers. Marshall (1976) and Corpe
(1978) have implicated polysaccharides and glycoproteins in irreversible
attachment.
Attached microorganisms assimilate nutrients, synthesize biomass, and
produce extracellular products. In many cases, the rate of assimilation of
nutrients is limited by the diffusion of nutrients through the biofilm. As a
result, a significant concentration gradient exists between the bulk fluid
and the pipe wall. When nutrients cannot penetrate the lower layers of the
biofilm, the microorganisms in that region lyse. If dissolved oxygen be-
comes depleted in the biofilm, denitrification and sulfate reduction will
occur when nitrate and sulfate are present. The sulfate-reducing organ-
isms have been implicated in accelerated corrosion of pipelines.
At any point in its development, portions of biofilm peel away from the
pipe surface and are reentrained in the fluid flow. Detachment is a contin-
uous process of biofilm removal. It is highly dependent on hydrodynamic
conditions and is characterized by particle sizes similar to microbial cell
diameters. As the biofilm grows thicker, the fluid shear stress at the bio-
film interface generally increases and the potential for substrate oxygen or
nutrient limitation in the deeper portions is great. These limitations may
weaken the biofilm matrix and cause detachment. When the flow is turbu
OCR for page 114
114 DRINKING WATER AND HEALTH
lent, the rate at which the biofilm detaches increases as the thickness of
the biofilm increases (Trulear and Characklis, 19791. For a given biofilm
thickness' detachment rate increases with increasing fluid shear stress.
Sloughing, a random massive removal of biofilm, is generally attributed to
oxygen/nutrient depletion deep within the biofilms. Sloughing occurs
more frequently with thicker, dense films characteristic of laminar flow
systems and results in the reentrainment of large aggregates of biofilm.
The detached material consists of microbial cells. their extracellular
products, and any other adsorbed or entrapped material within the bio-
film.
The rate of water flow and the concentration of growth-limiting nutrient
(i.e., substrate) significantly influence the rate and extent of biofilm devel-
opment. For example,
· high rate of water flow and high concentrations increase transport
rate of soluble substrate to the pipe wall;
· high concentrations of substrate increase the rate of metabolic pro-
cesses within the biofilm;
· high rates of water flow and low concentrations of substrate result in
thinner biofilms; and
· high rates of water flow increase the detachment of material from the
biofilm.
PROPERTIES AND COMPOSITION
Microorganisms, primarily bacteria, adhere to surfaces ranging from the
human tooth and intestine to the metal surface of power plant condenser
tubes exposed to turbulent flows of water. The microorganisms "stick" by
means of extracellular polymer fibers, which are fabricated and oriented
by the cell and extend from the cell surface to form a tangled matrix
termed a "glycocalyx" (Costerton et al., 19781. The fibers may conserve
and concentrate extracellular enzymes that are necessary for preparing
substrate molecules for ingestion, especially high molecular weight or par-
ticulate substrate frequently found in natural waters.
The biofilm surface is highly adsorptive, especially because of its poly-
electrolyte nature, and can collect significant quantities of silt, clay, or
other detritus in natural waters. Biofilms can also accumulate large
amounts of heavy metals (Dugan and Pickrum, 1972~.
Physical, chemical, and biological properties of biofilms are dependent
on the environment to which the attachment surface is exposed. The phys-
ical and chemical microenvironments combine to select the prevalent mi-
croorganisms which, in turn, modify the microenvironment of the surface.
OCR for page 115
Biological Quality of Water in the Distribution System 115
As colonization proceeds and a biofilm develops, concentration gradients
develop within the biofilm and "average" biofilm properties change.
PHYSICAL PROPERTIES
The most fundamental biofilm properties are volume (thickness) and
mass. In turbulent flow systems. wet biofilm thickness seldom exceeds
1,000 mm (Picologlou et al., 1980~. In laminar flow, biofilms can be sev-
eral millimeters thick. The biofilm mass can be determined from the wet
biofilm thickness if the biofilm dry mass density is known. Biofilm dry
mass density reflects the attached dry mass per unit wet biofilm volume.
Measured mass values in turbulent flow systems range from 10 to 50 ma/
cm3. In laminar flow, higher values have been observed (Hoehn and Ray,
1973~. Biofilm density increases with increasing turbulence (Characklis
1980) and increasing substrate loading (Trulear and Characklis. 1979~.
The increase in biofilm density with increasing turbulence may be caused
by one of the following phenomena:
· selective attachment of only certain microbial species from the avail-
able population;
· microbial metabolic response to environmental stress; and
· "squeezing" of loosely bound water from the. biofilm by fluid pres-
sure forces.
The relatively low densities of biofilm mass compare well with observed
water content of biofilm (Characklis, 1973, 1980; Nimmons, 1979~.
The transport properties of biofilm are of critical importance in assess-
ing its effects on mass, heat, and transfer of momentum. Diffusion coeffi-
cients for various compounds through microbial aggregates have been re-
ported in the literature, mostly for microbial floe particles. Matson and
Characklis (1976) reported variation in the diffusion coefficient for glu-
cose and oxygen with growth rate and carbon-to-nitrogen ratio. The diffu-
sion coefficient of biofilms is most probably related to its density. In-situ
theological measurements indicate that the biofilm is viscoelastic with a
relatively high viscous modulus (Characklis, 1980~. The thermal conduc-
tivity of biofilm is not significantly different from that of water (Charack-
lis, 1980~.
CHEMICAL PROPERTIES
The inorganic composition of biofilms undoubtedly varies with the chemi-
cal composition of the bulk water and probably affects the physical and
OCR for page 116
116
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OCR for page 117
Biological Quality of Water in the Distribution System 117
biological structure of the film. Calcium, magnesium, and iron affect
intermolecular bonding of biofilm polymers, which are primarily responsi-
ble for the structural integrity of the deposit. Experimentally, ethylenedi-
aminetetraacetic acid (EDTA) is effective in detaching biofilm (Charack-
lis, 1980~. Table IV-1 shows the range of inorganic composition observed
in selected biofilms. Very little is known about the interaction between
biological fouling and inorganic scaling (primarily calcium precipitation
on the pipe wall). The combined process probably occurs in many water
distribution systems.
The organic composition of the biofilm is strongly related to the energy
and carbon sources available for metabolism. Herbert (1961) and Schaech-
ter et al. (1958) demonstrated the effect of environment and microbial
growth rate on the composition of the cells and their extracellular prod-
ucts. For example, nitrogen limitation can result in production of copious
quantities of microbial extracellular polysaccharides. Table IV-2 presents
data on the composition of biofilms developed in the field and in the labo-
ratory. Other chemical analyses of biofilm from laboratory systems indi-
cate relatively high levels of polysaccharide in the biofilm (Bryers and
Characklis, 19801.
BIOLOGICAL PROPERTIES
The organisms that colonize the attachment surface strongly influence the
rate at which biofilm develops and its chemical and physical properties.
However, organism-organism and organism-environment interactions un-
doubtedly shift population distributions during biofilm accumulation.
Several investigators have observed a succession of various microbial
species during biofouling (Corpe. 1978; Marshall, 1976~.
The first visible signs of microbial activity on a surface are usually small
"colonies" of cells distributed randomly on the surface. If biofilm devel-
opment continues, the colonies grow together forming a relatively uniform
biofilm. The viable cell numbers are relatively low in comparison to the
volume of biofilm. The cells occupy from 1% to 10% of the biofilm in di-
lute nutrient solutions (Characklis. 1980~. Jones et al. (1969) presented
photomicrographs to corroborate these data in natural and laboratory sys-
tems. Allen and Geldreich (1977) have measured bacterial populations ex-
ceeding 105 organisms per gram of deposit from a distribution pipeline.
These deposits consisted of sediment accumulations and "encrustations."
Figure IV-2 contains electron micrographs of encrustations collected from
water distribution systems across the United States (Allen et al., 1979,
1980).
A number of different genera of bacteria and fungi have been observed
OCR for page 118
118
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OCR for page 126
126 DRINKING WATER AND HEALTH
tion following construction and repair generally requires repeated high
doses of chlorine before the system can be placed into service.
Cross-Connections
Cross-connections and back-siphonages provide another opportunity for
large amounts of biological material to enter the distribution system.
These events generally result in noticable change in water quality, includ-
ing turbidity, increased content of solids, and undesirable tastes and
odors.
In many cases, cross-connections are not obvious and the resulting
changes in water quality are not detected by the consumer. Often, small
intermittent flows through cross-connections can back-siphon and be re-
sponsible for outbreaks of disease.
CONTROL OF BIOLOGICAL CONTAMINATION
Chlorination has been the predominant means of controlling fouling bio-
film formation in water distribution systems. Figure IV-3 illustrates the
effectiveness of these processes in a segment of a water distribution con-
duit. There are three basic steps:
· Chlorine species entering the pipe segment react with chlorine-de-
manding components (viable cells and chemical compounds) in the bulk
water.
· Chlorine species are transported through the bulk water to the water-
biofilm interface.
· Chlorine species diffuse and react within the biofilm releasing soluble
and particulate matter into the bulk water.
Since both "solid" and liquid phases are required, the chlorine-biofilm
reaction is a heterogeneous process. Therefore, physical transport of reac-
tants and products in each phase becomes important since transport limi-
tations can significantly affect the rate of the overall process.
Transport of Chlorine
WATER PHASE
The rate at which chlorine is transported to the biofilm depends on the
concentration of chlorine in the bulk water and the intensity of the turbu-
lence.
