Wastewater Treatment
Water Quality
T1 Introduction1. Surface Water Quality ManagementIn an
idealized setting, a community development will permit use of a
watershed for drinking water supply, for recreation and for the
discharge of treated wastes.T2 Watershed and beneficial uses [ask
class to identify locations for each]T3 Watershed and beneficial
uses best case solutionT4 Everyone lives downstreamT5 Everyone
lives downstream Sonora River2. River Water Quality
T6 Discharges to riversT7 Engineering design to protect
downstream stakeholdersHere we will consider the management of
river oxygen resources as impacted by the discharge of oxygen
demanding wastes, i.e. BOD. The task is best summarized by
considering the battle between oxygen consumption (BOD exertion)
and oxygen replenishment (reaeration).
River DO-BOD Overview
First consider the fate of BOD (which behaves as a first-order
decay) in a river after it exits the mixing basin.
T8 First order BOD decayNext consider the compensating effect of
reaeration, the rate of which increases as the oxygen concentration
decreases. The net effect is a profile with distance downstream in
the river termed the dissolved oxygen sag curve.
T9 The DO Mass Balance and the DO Sag Curve (the battle for
oxygen)
Deoxygenation: the BOD Portion of the DO Mass BalanceAerobic
organisms derive the energy required for maintenance of metabolic
function, growth, and reproduction through the process of
respiration. Chemoheterotrophs are organisms that utilize organic
matter (i.e., C(H2O)) as a carbon and energy source and, under
aerobic conditions, consume oxygen in obtaining that energy,
Chemoautotrophs are organisms that utilize CO2 as a carbon
source and inorganic matter as an energy source, and usually
consume oxygen. For example, in nitrification, the microbial
conversion of ammonia to nitrate, ammonia provides the energy and
bicarbonate ion contributes carbon,
Because oxygen is consumed in both of these microbially-mediated
reactions, biochemical oxygen demand (BOD) can be defined as the
amount of oxygen utilized in performing the oxidation. And it
follows from this that BOD is a measure of the strength of a water
or wastewater: the greater the concentration of ammonia-nitrogen or
degradable organic carbon, the higher the BOD. Note that chemical
strength (mg C(H2O)/L or mg NH3-N/L) is expressed here in terms of
its impact on the environment (oxygen consumed, mg BOD/L), i.e.
representation by effect.
The simple carbohydrates produced through photosynthesis are
used by plants and animals to synthesize more complex carbon-based
chemicals such as sugars and fats. These compounds are utilized by
organisms as an energy source, exerting a carbonaceous oxygen
demand (CBOD). In addition, plants utilize ammonia to produce
proteins, i.e. complex, carbon-based chemicals with amino groups
(-NH2) as part of their structure. Proteins are ultimately broken
down (proteolysis) to peptides and then amino acids. The process of
deamination then further breaks down the amino acids, yielding a
carbon skeleton (CBOD) and an amino group. Conversion of the amino
group to ammonia (ammonification) completes the degradation
process. The ammonia is then available to exert a nitrogenous
oxygen demand (NBOD) when utilized by microorganisms.T10 Sources of
BODThe oxygen requirement for complete oxidation of
ammonia-nitrogen to nitrate-nitrogen or a carbon-based compound to
carbon dioxide and water is termed the theoretical oxygen demand
(ThOD). For ammonia, the observed oxygen demand is equal to the
ThOD because all of the ammonia is oxidized. Carbonaceous
compounds, on the other hand, are not all easily or completely
oxidized by microorganisms (biodegradation) and the rate of that
oxidation may vary widely among different sources of organic
matter. For example, the carbon-containing compounds present in a
Styrofoam cup are not as biodegradable as those found in tree
leaves, and both are less biodegradable than the carbon compound
which constitutes sugar. Thus, while a mass of carbon present as
sugar or tree leaves or Styrofoam may have the same ThOD, their
actual oxygen demand may be substantially different. Further, most
wastes are a complex mixture of chemicals (e.g., Styrofoam + tree
leaves + sugar), present in differing amounts, each with a
different level of biodegradability. T11 BOD exerted and BOD
remaining Consider the oxidation of organic matter as a function of
time. In T11, yt is the CBOD exerted (oxygen consumed, mg O2/L) and
Lt is the CBOD remaining (potential to consume oxygen, mg O2/L) at
any time, t. At t = 0, no CBOD has been exerted (yt=0 = 0) and all
of the potential for oxygen consumption remains (Lt=0 = L0, the
ultimate CBOD). As the oxidation process begins, oxygen is consumed
(CBOD is exerted and yt increases) and the potential to consume
oxygen is reduced (CBOD remaining, Lt, decreases). The rate at
which CBOD is exerted is rapid at first, but later slows and
eventually approaches zero as all of the biodegradable organic
matter has been oxidized. The total amount of oxygen consumed in
oxidizing the waste is the ultimate CBOD (L0).
What might the ThOD of this sample be? Would it differ for
different compounds?
The exponential decline in CBOD remaining (L; potential to
consume oxygen) illustrated in T11 can be modeled as a first-order
decay:
where kL is the CBOD reaction rate coefficient (day-1). This
equation can be integrated to yield the analytical expression,
T12 Biodegradability and the nature of kL The reaction rate
coefficient (kL) utilized in CBOD calculations, is a measure of the
biodegradability of a waste. The relationship between kL and the
rates of CBOD exertion (dy/dt) and consumption (dL/dt) is
illustrated in T12. Typical ranges for this coefficient are
presented in the table below with reductions in the magnitude of kL
as one moves from untreated sewage treated sewage unpolluted river
water reflecting progressive reductions in labile (biodegradable)
organic carbon. As with other microbially mediated processes,
values for the CBOD and NBOD reaction rate coefficients vary with
temperature.Type of samplekL (day-1)
Untreated municipal0.35 0.70
Treated municipal wastewater0.10 0.35
Unpolluted river water< 0.05
Returning to the oxygen mass balance, we note that since BOD is
expressed in terms of oxygen consumed (representation by effect),
we can quantify the deoxygenation portion of the dissolved oxygen
mass balance using the first order BOD decay equation,
T13 The DO Mass Balance and the DO sag curveConsider the effects
of L, kL and temperature on the sag curve.
Laboratory determination of BODNote that the CBOD exerted at any
time t (yt) (T11) is given by the difference between the ultimate
CBOD and the CBOD remaining:
Substituting Equation 5-31 into Equation 5-32 yields:
and rearranging, yields an expression for the ultimate CBOD:
T14 Laboratory determination of BODReoxygenation: the Reaeration
Portion of the DO Mass Balance
The driving force for reaeration is the dissolved oxygen
deficit, D (mgO2/L), a representation of how hungry the water is
for oxygen. The deficit is defined as the departure from saturation
or the difference between the maximum amount of oxygen that the
water can hold and the actual oxygen concentration,
The maximum amount of oxygen that the water can hold is termed
the saturation concentration and varies with temperature according
to the Henrys Law,
The value for the Henrys Law constant for oxygen (KH) decreases
as temperature increases and thus the value of DOsat is lower at
higher temperatures.
T15 Oxygen saturation and temperatureReaeration is a first order
reaction based on the reaeration rate coefficient, Ka (d-1), and a
driving force as represented by the dissolved oxygen deficit,
The magnitude of the reaeration rate coefficient varies with
temperature (increases with increasing temperature) and the
turbulence of the stream (increases with increasing turbulence).
Consider the effects of temperature and turbulence on the DO sag
curve.
Quantifying the DO Mass BalanceWe have now quantified all of the
inputs to the DO mass balance,
In practice, the mass balance is written in terms of D because
it is really D not DO that drives the reaeration term,
In order to apply this equation in modeling the oxygen resources
of a river, the equation must be integrated,
Remembering that protection of the system at the critical point
protects the entire system,T16 The DO Sag Curve and the Critical
Pointwe would like to know at what t does the critical point
occur,
Applying the Mass BalanceWe seek to use the DO mass balance to
establish waste treatment objectives that will maintain a healthy
aquatic ecosystem, e.g. achieve a water quality standard for
dissolved oxygen.
T17 Organisms diversity and the DO Sag CurveT18 Organism
abundance and diversityLets examine a case where the mass balance
is applied.
T19 Applying the oxygen mass balance
3. Lake Water Quality
As with rivers, the management of water quality in lakes
requires an understanding of basic science, a quantitative analysis
of perturbation and response (engineering design) and the
application of technologies.
T20 Managing lake water qualityTemperature and the Physical
Environment in Lakes
Our starting point for a brief investigation of the science of
lakes (limnology) is the temperature density relationship for
water. Consider what would happen if this were a linear
relationship instead of having a maximum at ~4 C.
T21 Temperature density relationshipThe density function leads,
in lakes, to a phenomenon termed thermal stratification. Here, the
lake divides into a warm, less dense upper layer (the epilimnion or
upper lake), a cold, more dense lower layer (the hypolimnion or
lower lake) and a transition region (the metalimnion or changing
lake). Within the metalimnion, the depth where the change of
density (temperature) with depth is greatest is called the
thermocline and serves as the boundary when we seek to represent a
lake as a two-layered system.T22 Thermal stratificationThe driving
force for this layering effect is most clearly seen in examining
vertical temperature profiles for the spring (isothermal) and
summer (stratified) periods.
T23 Temperature layers and temperature profilesIn temperate
climates, lakes stratify in summer and winter (inverse
stratification), with intervening periods of circulation
(turnover).T24 Seasonal cycle of stratification and turnoverT25
Dollar Bay seasonal temperature cycleNutrients and Trophic
StructureStart Here4. Groundwater QualityWhat is groundwaterSources
and Types of Groundwater Contamination
For many years, the prevailing view was that contaminants
released into the subsurface would be cleansed by aquifer materials
or diluted in the aquifer water. However, a wide range of toxic
substances, including synthetic organic chemicals, trace metals,
and pathogenic microorganisms, have been detected at potentially
harmful levels in groundwater and concern over subsurface
contamination has grown.Groundwater contaminants include:
Pathogensfrom improperly designed or maintained septic tanks
Nutrientsfrom fertilizers or human waste, e.g. nitrogen and
phosphorus
Inorganic chemicalsfrom both natural (arsenic) and anthropogenic
(trace metals) sources
Organic chemicalsfrom pesticide application, spills, LUST and
improper disposal
Radionuclidesfrom the aquifer naturally and nuclear processing
facilities
The release of groundwater contaminants may exhibit different
areal and temporal characteristics. For example, pathogens released
from a septic tank may originate over a small area, while pesticide
applications may involve > 103 km2. Contaminant releases may
occur over a short time span, such as those associated with a
single, essentially instantaneous chemical spill, or over a longer
time span (e.g. decades), such as leakage from improperly stored
chemical wastes. Knowledge of the areal and temporal extent of a
contaminant release is required in the design of remediation plans
and preventative measures.
Pollutant Fate and Transport in GroundwaterTransport in
groundwater is mediated by advection and dispersion (diffusion plus
mechanical mixing). Since movement in the subsurface may occur in
all three dimensions simultaneously and can change in time and
space, prediction of advective transport can be quite complicated.
However, in a simple, one-dimensional system where groundwater flow
is constant in time and space, the travel time of a contaminant
over a given length can be estimated as:
where t is the travel time (d), L is length (m) and v (m/d) is
the pore (interstitial or seepage) velocity.The velocity is a
function of the driving force for transport (hydraulic gradient,
dh/dx, dimensionless) and the resistance to that transport
(hydraulic conductivity, K, m3m-2d-1). This relationship,
is based on the work of the French civil engineer Henri Darcy
and is thus termed the Darcy velocity.
Because the hydraulic conductivity is expressed in terms of the
cross-section of the entire aquifer and groundwater flows only
through pores, calculation of the true velocity requires
normalization for porosity (n, dimensionless),
Because , the true velocity will be always be greater than the
Darcy velocity.
EXAMPLE. CALCULATION OF GROUNDWATER TRAVEL TIME
A toxic contaminant is released 1 km upstream of a drinking
water supply well. The relevant aquifer properties are: hydraulic
conductivity, K = 10-5 m/s; porosity, n = 0.3; and, hydraulic
gradient, dh/dx = -10-2. Determine how long it will take for the
contaminant to reach the drinking water supply well.
Solution
The travel time is then determined from Equation 8-15:
The long travel time calculated here underscores the fact that
groundwater transport can be slow and may suggest that we do not
need to be concerned about the contamination for a long time
period. However, by the time the contamination might be detected at
the well, a large portion of the aquifer and a significant volume
of groundwater may be contaminated as the plume spreads.
Contaminants tend to spread out or mix (disperse) as they are
advected with the flowing groundwater, becoming more dilute and
occupying an increasingly larger volume. This second transport
process, dispersion results from the effects of two mechanisms:
diffusion and mechanical mixing. Diffusion is driven by
concentration gradients, while mechanical mixing results from
interactions with the aquifers solid matrix (a process analogous to
the eddy diffusion discussed in relation to air and surface water).
The contribution of mechanical mixing to dispersion dominates that
of diffusion except at low pore velocities (e.g. in aquifers with
low hydraulic conductivity). The travel time calculation presented
previously assumes that once the contaminant is released, the
contaminant molecules experience identical velocities as they
migrate through the aquifer. In fact, each molecule will be subject
to velocities that can range over many orders of magnitude due to
mixing phenomena occurring over a wide range of length scales.
Velocity differences at the pore scale result from friction with
the solid media, an effect manifested to a different degree at
different positions over the cross-section. Pore scale velocity
differences are also influenced by the flow paths that molecules
travel as they migrate through the aquifer medium. At the aquifer
scale, variations in velocity are caused by differences in
hydraulic conductivity resulting in contaminant arrival at a
particular location at different times.The practical significance
of this is that the travel time (for arrival of the front) is
shorter with dispersion than with advection alone. Referring again
to the travel time calculation presented previously, when the
effects of dispersion are included, the contaminant reaches the
drinking supply water well at potentially toxic levels orders of
magnitude more quickly than with transport only by advection.
Groundwater contaminants rarely behave conservatively, as they
may be degraded by microorganisms, react chemically or lost as a
result of physical processes (e.g. volatilization, sorption). In
many cases, contaminant reactions are beneficial, reducing
contaminant levels. However, in some cases (especially chlorinated
solvents), the reaction products can be at least as toxic as the
chemical originally released. Contaminant interactions with solid
materials in the aquifer through sorption are quantified through a
retardation coefficient and serve to slow the rate of chemical
migration.
Groundwater Remediation Strategies
Contaminated groundwater is expensive and time consuming to
clean up. This is because: 1) Pollution problems are often
discovered years after the initial contamination, offering the
chemical time to impact millions of liters of water and extend
kilometers from the source. 2) The subsurface is physically and
chemically heterogeneous. This heterogeneity leads to slow rates of
removal or in-place degradation of contaminants, even when
remediation is aggressive. 3) Little information is usually
available concerning the physical and chemical characteristics of
the contaminated aquifer system. The lack of information, coupled
with aquifer heterogeneity and the fact that the subsurface is
essentially invisible, means that the design and implementation of
economical and effective remediation systems presents a
challenge.
Groundwater remediation may be accomplished by physical,
biological, or chemical means. Physical remediation usually
involves pumping the contaminated groundwater out of the aquifer,
treating it above-ground, and disposing the treated groundwater
back to the impacted aquifer, to a surface water, or to a
wastewater collection system. Soil vapor extraction and air
sparging involve pumping and recovering air into and from the
vadose and saturated zones, respectively. Biological remediation
consists of injecting electron acceptors and nutrients. Chemical
remediation also involves adding chemicals, to either enhance
abiotic degradation or to immobilize contaminants through chemical
precipitation.
Go To Mass Balance and Kinetics
CMFR SS 1st Order Decay
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