Core Concepts for the Civil PE Exam Water Resources and Environmental Depth
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Civil PE Exam Water Resources and Environmental Depth

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Click below to expand each topic. All of this is included in the paperback version of the Water Resources Depth
Analysis and Design
Mass Balance
Mass balance refers to the conservation of mass as it enters and exits a system. This concept is essential to the Water Resources and Environmental exam as it is a common method in solving for unknowns in many types of problems. This principle simply put is that mass is always conserved in fluid systems regardless of the properties of that system. Therefore what enters the system must also exit the system. The equation can be represented as mass:
M1 = M2
M1 = Mass entering the system
M2 = Mass exiting the system
The equation can also be represented by equating flow rates. In this case the equation is commonly known as the continuity equation:
Q1 = Q2
Q1 = Flow rate into system
Q2 = Flow rate out of a system
Given that the flow rate is equal to the area multiplied by the velocity, the equation can be used to equate these variables as well:
A1V1 = A2V2
Hydraulic Loading
Hydraulic loading refers to the flows in MGD (Million Gallons per Day) or cu. Ft./day to a treatment plant or treatment process. The equation is as follows:
HLR=QA
HLR = Hydraulic loading rate
Q = Flow rate
A = Surface area of the wet basin
Detention time is the amount of time it takes a given volume of wastewater to pass through the clarifier:
td=VQ
V = Volume of clarifier
Solids Loading
Solids loading similarly to hydraulic loading is the amount of suspended solids in a substance as it flows to the treatment facility. Solids loading is expressed as the following:
Hydraulic Flow Measurement
Many flow devices are available to measure either the flow rate or velocity of a given system. These methods are often used in conjunction with the laws of energy and mass conservation to be able to analyze a system. Here are a few of the more prevalent ones for the purposes of the PE exam:
 Pitot Static Tubes
g = Force effect due to gravity (32.2 ft/s2)
h = Difference in elevations of the fluid columns (ft)
ρ = Density of water (62.4 lb/ft3)
ρm = Density of manometer fluid (lb/ft3)
v = Velocity (ft/s)
Cf = Flow coefficient
Ao = Orifice area
A1 = Pipe area
p = Pressure
ρ = Density
g = Force due to gravity
Cd = Discharge coefficient
Cc = Coefficient of contraction
Fva =Velocity of approach factor
HydraulicsClosed Conduit
Bernoulli Continuity Equation
The Bernoulli equation for the conservation of energy states that the total energy is equal to the sum of the pressure + kinetic energy + potential energy of a system and is conserved at any point in the system. Therefore:
Pressure Conduit
Pressure conduits refer to closed cross sections that are not open to the atmosphere such as pipes:
The Darcy Equation is used for fully turbulent flow to find the head loss due to friction. The equation is:
hf = Head Loss due to friction (ft)
f = Darcy friction factor
L = Length of pipe (ft)
v = Velocity of flow (ft/sec)
D = Diameter of pipe (ft)
g = Acceleration due to gravity, (Use 32.2 ft/sec2)
The HazenWilliams equation is also used to determine head loss due to friction. Be aware of units as this equation may be presented in different forms. The most common is the following:
hf = Head Loss due to F\friction (ft)
L = Length (ft)
V = Velocity (gallons per minute)
C = Roughness coefficient
d = Diameter (ft)
In addition to these losses, there is also what is called minor losses of energy due to friction
Minor Losses – Friction losses due to geometric changes such as fittings in the line, changes in the dimensions of the pipe, or changes in direction:
 Minor losses can be calculated as per the Method of Loss coefficients.
 Each change in the flow of a pipe is assigned a loss coefficient, K
 Loss coefficients for fittings are most often determined and provided by the manufacturer
Loss coefficients for sudden changes in area can be determined:
For Sudden Expansions:
D1=Smaller diamter pipe
D2=Larger diamter pipe
Loss coefficients are then multiplied by the kinetic energy to determine the loss:
Pump Application and Analysis
A pump is a machine which adds energy to the flow of water or other fluids. A pump is often used to oppose the effects of gravity to transport a fluid to a position up grade.
The head added by a pump can be determined from the following equation as a function of the total energy:
Pipe Network Analysis
A system of pipes can be arranged in different configurations to be able to appropriately transport water. There are a few types of common arrangements that can be used. Each has certain principles to follow when determining the flow through the system. It is important to remember the conservation of mass or flow principle when analyzing these systems:
Series Pipe System: Pipes of different areas connected along the same line.
There are three concepts which are important to keep in mind during the analysis of parallel pipes:
 The head loss in parallel pipes is equal
 The head loss between the inlet and outlet is equal to that of each pipe individually
 The flow rate at the outlet is equal to the sum of the flow rates from the parallel pipes
Pipe Networks: These are more complicated systems of pipes which have flow breaking off in multiple directions.
Often pipe networks are very complicated and left to iterative analysis on computers. It is important to note the two concepts which govern the analysis however:
 The flow entering the system is equal to the flow leaving the system (conservation of flow)
 The sum of head losses in any closed loop is equal to zero
HydraulicsOpen Channel
Open Channel Flow
For open channel flow use the ChezyManning equation:
Q = (1.49/n)AR2/3S1/2
Q = Flow Rate (cfs)
n = Roughness Coefficient
A = Area of Water (ft2)
R = Hydraulic Radius (ft)
S = Slope (decimal form)
The hydraulic radius is the area of water divided by the wetted perimeter which is the perimeter of the sides of the channel which are in contact with water.
Hydraulic Energy Dissipation
A weir is a low dam used to control the flow of water. Weirs have shaped outlets notched into the top of the dam to allow water to flow out. The most common shapes are triangular and trapezoidal:
Triangular Weir
H = Height of water (ft)
θ = Weir angle
Trapezoidal Weir
b = Width of base (ft)
Broad Crested Weirs (Spillways)
Spillways are used to control the flow of excess water from a dam structure. Essentially they are large weirs and therefore can be called broad crested weirs. The calculation of discharge for spillways is taken as:
However, often in a dam situation the approach velocity can be taken as zero since it is so small and the equation becomes:
Cs = Spillway coefficient
Stormwater Collection and Drainage
There are many components used in the collection of stormwater. Some examples include:
Culverts: A pipe carrying water under or through a feature. Culverts often carry brooks or creeks under roadways. Culverts must be designed for large intensity storm events.
Stormwater Inlets: Roadside storm drains which collect water from gutter flow or roadside swales.
Gutter/Street flow: Flow which travels along the length of the street. Gutter flow can be approximated often by an adaptation of the Manning Equation:
K = Gutter flow constant = 0.56 ft3/(s*ft)
z = Inverse of the cross slope of the gutter (Decimal)
n = Roughness coefficient
s = Slope of the gutter (Decimal)
y = water depth at the curb (ft)
Storm Sewer Pipes: Pipes installed under the road which carry the water from inlets to a suitable outlet.
Sub and Supercritical Flow
Sub and super critical flows are relative to the critical flow depth. This depth is defined as that which minimizes the energy of the flow of water for a given channel cross section and slope. The critical flow depth is important because since the energy is minimized the flow rate is maximized.
So when the flow depth is greater than the critical depth, the flow is subcritical and the velocity is less than the critical velocity. When the depth is less than the critical depth, the flow is supercritical and the velocity is faster than the critical.
For a rectangular channel, the following equations can be used to determine the critical depth and critical velocity:
dc = Critical depth of flow (ft)
Q = Flow rate (ft3/s)
w = Width (ft)
The Froude number is used to qualify a flow channel and can be used to determine if it is sub or supercritical. The number is dimensionless:
v = Velocity (ft/s)
L = Characteristic length and is determined based on the channel geometry.
For a rectangular section L = d
For a circular section flowing half full L = πD/8
For trapezoidal and semicircle sections L = the area of flow/top width of channel
If the Froude number is less than 1, the flow is subcritical
If the Froude number is greater than 1, the flow is supercritical
A hydraulic jump is a rise in water elevation due to a supercritical flow abruptly meeting a subcritical flow. The following equations can determine the heights and velocities in a hydraulic jump for rectangular sections:
d1=depth of supercritical flow (ft)
d2=depth of subcritical flow (ft)
Fr = Froude Number for associated depth
v = Velocity at associated depth (ft/s)
g = Force effect due to gravity (32.2 ft/s2)
Hydrology
Storm Characteristics
A design storm must be specified when performing any calculations. The design storm is defined by its recurrence interval which is the given amount of time it is likely to see a storm of a certain intensity. Design storms are often 10, 20, 50, or 100year storms meaning a storm of a certain intensity would only occur once within the given duration.
Runoff Analysis
The rational method can be used to determine the flow rate from runoff of a drainage area. The equation is:
Q = ACi
Q = Flow Rate (cfs)
A = Drainage Area (Acres)
C = Runoff Coefficient
i = Rainfall Intensity (in/hr)
NRCS/SCS Runoff Method
This is an alternative method for determining runoff:
S = Storage Capacity of Soil (in.)
CN = NRCS Curve Number
Q = Runoff (in.)
Pg = Gross Rain Fall (in.)
Hydrographs
Hyetographs – Graphical representation of rainfall distribution over time
Hydrograph – Graphical representation of rate of flow vs time past a given point often in a river, channel, or conduit. The area under the hydrograph curve is the volume for a given time period
Parts of a Hydrograph are shown graphically:
Unit Hydrographs can be determined by dividing the points on the typical hydrograph by the average excess precipitation.
Synthetic Hydrographs are created if there is insufficient data for a watershed. This method uses the NRCS curve number and is a function of the storage capacity.
To develop the synthetic hydrograph, you must calculate the time to peak flow:
tR = Storm duration (time)
Lo = Length overland (ft)
SPercentage = Slope of land
The equation for peak discharge from a synthetic hydrograph then is:
Rainfall
Storm characteristics include duration, total volume, and intensity
Duration: The length of time of a storm. Often measured in days and hours.
Total Volume: The entire amount of precipitation throughout the duration of the storm in a defined area.
Storm Intensity: Total volume of the storm divided by the duration of the storm event. Intensities can be averaged over the entire storm or at shorter intervals to provide instantaneous high intensity portions of the storm. Hyetographs are bar graphs used to measure instantaneous rainfall intensities over time and are covered below.
Stormwater Management
Detention and retention ponds are often used to collect water for flood control and stormwater runoff treatment.
Detention Ponds: Also known as dry ponds. These are ponds which are often kept dry except during flood events. The pond will fill up during increased precipitation to control the flow intensity. This is common in dry, arid, or urban areas to prevent excessive flooding. The ponds typically will be designed to hold water for about 24 hours. Detention ponds also control the amount of sediment since it is captured in the pond and then typically becomes accessible after the pond has dried.
Retention Ponds: Also called wet ponds since they contain a volume of water at all times. The elevation of the water will vary depending on precipitation but will always maintain a permanent amount of water based on low flow conditions. This allows sediment control since the deposits will settle to the bottom and allow for collection.
Infiltration is the rate of which water seeps into the ground. The Horton equation can be used to approximate this rate. This assumes that the water supply is infinite and the ground is saturated:
Time of Concentration
Depletions
The change in storage for a body of water can be approximated from the following equation:
ΔS=P+R+GIGOETO
S = Storage
P = Precipitation
R = Runoff
GI = Groundwater inflow
GO = Groundwater outflow
E = Evaporation
T = Transpiration
O = Surface water release
Stream Gauging
Stream gauging is the measurement of a stream channel to determine the discharge by obtaining the depth and velocity of the channel over time. The channel can be approximated by areas created by connected the dots of the measured depths. The discharge can be calculated by the following:
w = Width of cross section (ft)
y = Height of cross section (ft)
v = Velocity at indicated cross section (ft/s)
Groundwater and Wells
Bernoulli Continuity Equation
The Bernoulli equation for the conservation of energy states that the total energy is equal to the sum of the pressure + kinetic energy + potential energy of a system and is conserved at any point in the system. Therefore:
Pressure Conduit
Pressure conduits refer to closed cross sections that are not open to the atmosphere such as pipes:
The Darcy Equation is used for fully turbulent flow to find the head loss due to friction. The equation is:
hf = Head Loss due to friction (ft)
f = Darcy friction factor
L = Length of pipe (ft)
v = Velocity of flow (ft/sec)
D = Diameter of pipe (ft)
g = Acceleration due to gravity, (Use 32.2 ft/sec2)
The HazenWilliams equation is also used to determine head loss due to friction. Be aware of units as this equation may be presented in different forms. The most common is the following:
hf = Head Loss due to F\friction (ft)
L = Length (ft)
V = Velocity (gallons per minute)
C = Roughness coefficient
d = Diameter (ft)
In addition to these losses, there is also what is called minor losses of energy due to friction
Minor Losses – Friction losses due to geometric changes such as fittings in the line, changes in the dimensions of the pipe, or changes in direction:
 Minor losses can be calculated as per the Method of Loss coefficients.
 Each change in the flow of a pipe is assigned a loss coefficient, K
 Loss coefficients for fittings are most often determined and provided by the manufacturer
Loss coefficients for sudden changes in area can be determined:
For Sudden Expansions:
D1=Smaller diamter pipe
D2=Larger diamter pipe
Loss coefficients are then multiplied by the kinetic energy to determine the loss:
Pump Application and Analysis
A pump is a machine which adds energy to the flow of water or other fluids. A pump is often used to oppose the effects of gravity to transport a fluid to a position up grade.
The head added by a pump can be determined from the following equation as a function of the total energy:
Pipe Network Analysis
A system of pipes can be arranged in different configurations to be able to appropriately transport water. There are a few types of common arrangements that can be used. Each has certain principles to follow when determining the flow through the system. It is important to remember the conservation of mass or flow principle when analyzing these systems:
Series Pipe System: Pipes of different areas connected along the same line.
There are three concepts which are important to keep in mind during the analysis of parallel pipes:
 The head loss in parallel pipes is equal
 The head loss between the inlet and outlet is equal to that of each pipe individually
 The flow rate at the outlet is equal to the sum of the flow rates from the parallel pipes
Pipe Networks: These are more complicated systems of pipes which have flow breaking off in multiple directions.
Often pipe networks are very complicated and left to iterative analysis on computers. It is important to note the two concepts which govern the analysis however:
 The flow entering the system is equal to the flow leaving the system (conservation of flow)
 The sum of head losses in any closed loop is equal to zero
Wastewater Collection and Treatment
Wastewater Collection Systems
Wastewater is collected by a network of pipes known as sanitary sewer. Some residents may not be connected to the sewer line and may resort to septic tanks for their wastewater disposal. Wastewater in sewer networks is transported to the wastewater treatment plants to be treated. Lift stations are pump stations which can be used to facilitate the transportation of the waste water to the treatment plant.
Smoke testing is a method of determining if there are leaks in a wastewater system. Smoke is pumped into a pipe and will seep through any cracks which can then be identified.
Infiltration is water which enters the system due to imperfections in the system such as cracks in the line or improperly constructed portions.
Inflow is water that enters the system from unanticipated or illegal means.
Wastewater Treatment Process
Wastewater treatment processes are the procedures for treating wastewater so that it may be used again. This process will remove sediments, sludge, taste, odors, and any other undesirable characteristics of the water. The process can be divided into preliminary, primary, and secondary treatment which will be discussed further below.
Wastewater Flow Rates
The quantity of wastewater from a municipal needs to be determined to properly design the treatment system. This is based on anticipated discharge from residential, commercial and other buildings. In addition, the system must account for infiltration and inflow as defined previously.
Flow rate can be approximated as the average flow or the peak flow. The peak flow is the highest daily flow rate. The average and peak flow are related by the peaking factor:
Preliminary Treatment
Preliminary treatment is the first step in the wastewater treatment process. This portion of the process is mostly the mechanical removal of debris and other large objects which may be caught in the flow. Heavy chemicals and large amounts of oil are also removed during this process. In general, anything that can be identified with the naked eye and easily screened will be removed during the preliminary treatment process. This process is often performed with large mechanical screens or filters. These large obstructions must also be removed so that they do not damage or impede the subsequent processes.
Solids Treatment
Mixed Liquor Suspended Solids (MLSS) is the concentration of bacteria, solids, and any other undesirable material in sludge. To remove sludge, the MLSS is considered food for the activated microorganisms in the aeration process. It is often important to determine the food to microorganism ratio from the equation below:
Phosphorous Removal
Phosphorous removal can be separated into two different types. A small percentage is insoluble and can be removed during primary settling. The remaining amount is soluble and must be chemically converted to an insoluble material for removal.
Often aluminum sulfate, ferric sulfide, and lime is used to complete this process so that the phosphorous can precipitate and settle for removal. The most common is aluminum sulfate. The Chemical equations for removal are:
Nitrification and Dentrification
Nitrification is the use of oxygen by autotrophic bacteria. In this process the bacteria oxidizes ammonia to nitrites and nitrates. This process is important to understand as it relates to determining the Biochemical Oxygen Demand (BOD) of a particular sample and more specifically, the Ultimate Biochemical Oxygen Demand (BODu). To test for BOD, samples are diluted and dissolved oxygen is measured initially and typically after a 5day period. The following equation is used to determine the Biochemical Oxygen Demand after that 5day period (BOD5):
DOi = Initial Dissolved Oxygen content
DOf = Final Dissolved Oxygen content
V = Volume
The process of nitrification causes a deviation from the trajectory of the carbonaceous process of oxygen demand as it relates to time. This must be accounted for when determining the Ultimate BOD. The BOD at any time t is called the BOD exertion and is related to the ultimate by the following equation:
It is important to note that at initially in a sample there is only a small amount of autotrophic bacteria present and the process of nitrification is delayed from having a significant effect on the BOD process. For reference the chemical equation for nitrification is:
Denitrification is the removal or loss of nitrogen by the means of bacteria. The chemical equation is:
Secondary Treatment
The most intensive of the levels of wastewater treatment is the secondary treatment. This may involve biological treatment in tickling filters and sludge treatment. The most amount of BOD will be removed in this stage.
Primary Treatment
Primary treatment is the second level in wastewater treatment. In this portion the wastewater is allowed to settle to remove any remaining oils and any solids which are able to separate. Typically about half of the solids will be removed during this portion of the process. It is also expected that this level of the process will remove 25%35% of the Biochemical Oxygen Demand (BOD) in the wastewater.
Digestion
Digestion is a process of treating sludge that is too thick or bulky to be easily worked with for disposal. In other words if the sludge is too thick it can be further broken down by digestion so that it can be moved more easily. There are 2 processes of digestion: aerobic and anerobic.
Aerobic digestion is putting the sludge in a large open holding tank for a period of time. In this tank the sludge is stirred and left open to air. This allows bacteria to consume the sludge reducing the solids. Often, up to 70% of the solids can be removed through this process.
Anaerobic digestion as the name suggests occurs without the use of oxygen. This process is more delicate in nature and proper care must be taken during as to not upset the desired result. However, it is often a more economical solution. Bacteria which does not require oxygen is introduced to the system. These bacteria, in a threestage process, convert the sludge to gases which can then be released.
Disenfection
Disinfectants are chemicals which are used to kill bacteria that is present in water. In general when disinfectants are discussed, the chemical referred to is chlorine. Chlorine is easily the most widely used mainly because of the cost comparative to other types of disinfectants. Chlorine however is a toxic substance and can be extremely dangerous to public health. It must be handled safely and properly.
In wastewater chlorine can be used to destroy common bacteria such as coliform. This is the presence of fecal matter in water supply.
Advanced Treatment
Advanced treatment also known as tertiary treatment and is a final level of the wastewater treatment process. This phase handles any remaining pollutants that are still above allowable levels that have not been removed during the previous stages. Here are some of the pollutants that may be removed during this level.
Suspended Solids – At this point any solids remaining are very small in size and would need to be removed by more advanced techniques. This may involve microstrainers or filter beds which are able to remove very high gradation solids.
Phosphorous – This stage may require the removal of phosphorous. This is done through the use of chemical precipitation. This process utilizes aluminum, iron, and lime coagulates.
Ammonia – There are many processes for use of removal of ammonia to acceptable levels. These may include stripping, biological denitrification, breakpoint chlorination, anion exchange, and algae ponds.
Water Quality
Open Channel Flow
For open channel flow use the ChezyManning equation:
Q = (1.49/n)AR2/3S1/2
Q = Flow Rate (cfs)
n = Roughness Coefficient
A = Area of Water (ft2)
R = Hydraulic Radius (ft)
S = Slope (decimal form)
The hydraulic radius is the area of water divided by the wetted perimeter which is the perimeter of the sides of the channel which are in contact with water.
Hydraulic Energy Dissipation
A weir is a low dam used to control the flow of water. Weirs have shaped outlets notched into the top of the dam to allow water to flow out. The most common shapes are triangular and trapezoidal:
Triangular Weir
H = Height of water (ft)
θ = Weir angle
Trapezoidal Weir
b = Width of base (ft)
Broad Crested Weirs (Spillways)
Spillways are used to control the flow of excess water from a dam structure. Essentially they are large weirs and therefore can be called broad crested weirs. The calculation of discharge for spillways is taken as:
However, often in a dam situation the approach velocity can be taken as zero since it is so small and the equation becomes:
Cs = Spillway coefficient
Stormwater Collection and Drainage
There are many components used in the collection of stormwater. Some examples include:
Culverts: A pipe carrying water under or through a feature. Culverts often carry brooks or creeks under roadways. Culverts must be designed for large intensity storm events.
Stormwater Inlets: Roadside storm drains which collect water from gutter flow or roadside swales.
Gutter/Street flow: Flow which travels along the length of the street. Gutter flow can be approximated often by an adaptation of the Manning Equation:
K = Gutter flow constant = 0.56 ft3/(s*ft)
z = Inverse of the cross slope of the gutter (Decimal)
n = Roughness coefficient
s = Slope of the gutter (Decimal)
y = water depth at the curb (ft)
Storm Sewer Pipes: Pipes installed under the road which carry the water from inlets to a suitable outlet.
Sub and Supercritical Flow
Sub and super critical flows are relative to the critical flow depth. This depth is defined as that which minimizes the energy of the flow of water for a given channel cross section and slope. The critical flow depth is important because since the energy is minimized the flow rate is maximized.
So when the flow depth is greater than the critical depth, the flow is subcritical and the velocity is less than the critical velocity. When the depth is less than the critical depth, the flow is supercritical and the velocity is faster than the critical.
For a rectangular channel, the following equations can be used to determine the critical depth and critical velocity:
dc = Critical depth of flow (ft)
Q = Flow rate (ft3/s)
w = Width (ft)
The Froude number is used to qualify a flow channel and can be used to determine if it is sub or supercritical. The number is dimensionless:
v = Velocity (ft/s)
L = Characteristic length and is determined based on the channel geometry.
For a rectangular section L = d
For a circular section flowing half full L = πD/8
For trapezoidal and semicircle sections L = the area of flow/top width of channel
If the Froude number is less than 1, the flow is subcritical
If the Froude number is greater than 1, the flow is supercritical
A hydraulic jump is a rise in water elevation due to a supercritical flow abruptly meeting a subcritical flow. The following equations can determine the heights and velocities in a hydraulic jump for rectangular sections:
d1=depth of supercritical flow (ft)
d2=depth of subcritical flow (ft)
Fr = Froude Number for associated depth
v = Velocity at associated depth (ft/s)
g = Force effect due to gravity (32.2 ft/s2)
Drinking Water Distribution and Treatment
Wastewater Collection Systems
Wastewater is collected by a network of pipes known as sanitary sewer. Some residents may not be connected to the sewer line and may resort to septic tanks for their wastewater disposal. Wastewater in sewer networks is transported to the wastewater treatment plants to be treated. Lift stations are pump stations which can be used to facilitate the transportation of the waste water to the treatment plant.
Smoke testing is a method of determining if there are leaks in a wastewater system. Smoke is pumped into a pipe and will seep through any cracks which can then be identified.
Infiltration is water which enters the system due to imperfections in the system such as cracks in the line or improperly constructed portions.
Inflow is water that enters the system from unanticipated or illegal means.
Wastewater Treatment Process
Wastewater treatment processes are the procedures for treating wastewater so that it may be used again. This process will remove sediments, sludge, taste, odors, and any other undesirable characteristics of the water. The process can be divided into preliminary, primary, and secondary treatment which will be discussed further below.
Wastewater Flow Rates
The quantity of wastewater from a municipal needs to be determined to properly design the treatment system. This is based on anticipated discharge from residential, commercial and other buildings. In addition, the system must account for infiltration and inflow as defined previously.
Flow rate can be approximated as the average flow or the peak flow. The peak flow is the highest daily flow rate. The average and peak flow are related by the peaking factor:
Preliminary Treatment
Preliminary treatment is the first step in the wastewater treatment process. This portion of the process is mostly the mechanical removal of debris and other large objects which may be caught in the flow. Heavy chemicals and large amounts of oil are also removed during this process. In general, anything that can be identified with the naked eye and easily screened will be removed during the preliminary treatment process. This process is often performed with large mechanical screens or filters. These large obstructions must also be removed so that they do not damage or impede the subsequent processes.
Solids Treatment
Mixed Liquor Suspended Solids (MLSS) is the concentration of bacteria, solids, and any other undesirable material in sludge. To remove sludge, the MLSS is considered food for the activated microorganisms in the aeration process. It is often important to determine the food to microorganism ratio from the equation below:
Phosphorous Removal
Phosphorous removal can be separated into two different types. A small percentage is insoluble and can be removed during primary settling. The remaining amount is soluble and must be chemically converted to an insoluble material for removal.
Often aluminum sulfate, ferric sulfide, and lime is used to complete this process so that the phosphorous can precipitate and settle for removal. The most common is aluminum sulfate. The Chemical equations for removal are:
Nitrification and Dentrification
Nitrification is the use of oxygen by autotrophic bacteria. In this process the bacteria oxidizes ammonia to nitrites and nitrates. This process is important to understand as it relates to determining the Biochemical Oxygen Demand (BOD) of a particular sample and more specifically, the Ultimate Biochemical Oxygen Demand (BODu). To test for BOD, samples are diluted and dissolved oxygen is measured initially and typically after a 5day period. The following equation is used to determine the Biochemical Oxygen Demand after that 5day period (BOD5):
DOi = Initial Dissolved Oxygen content
DOf = Final Dissolved Oxygen content
V = Volume
The process of nitrification causes a deviation from the trajectory of the carbonaceous process of oxygen demand as it relates to time. This must be accounted for when determining the Ultimate BOD. The BOD at any time t is called the BOD exertion and is related to the ultimate by the following equation:
It is important to note that at initially in a sample there is only a small amount of autotrophic bacteria present and the process of nitrification is delayed from having a significant effect on the BOD process. For reference the chemical equation for nitrification is:
Denitrification is the removal or loss of nitrogen by the means of bacteria. The chemical equation is:
Secondary Treatment
The most intensive of the levels of wastewater treatment is the secondary treatment. This may involve biological treatment in tickling filters and sludge treatment. The most amount of BOD will be removed in this stage.
Primary Treatment
Primary treatment is the second level in wastewater treatment. In this portion the wastewater is allowed to settle to remove any remaining oils and any solids which are able to separate. Typically about half of the solids will be removed during this portion of the process. It is also expected that this level of the process will remove 25%35% of the Biochemical Oxygen Demand (BOD) in the wastewater.
Digestion
Digestion is a process of treating sludge that is too thick or bulky to be easily worked with for disposal. In other words if the sludge is too thick it can be further broken down by digestion so that it can be moved more easily. There are 2 processes of digestion: aerobic and anerobic.
Aerobic digestion is putting the sludge in a large open holding tank for a period of time. In this tank the sludge is stirred and left open to air. This allows bacteria to consume the sludge reducing the solids. Often, up to 70% of the solids can be removed through this process.
Anaerobic digestion as the name suggests occurs without the use of oxygen. This process is more delicate in nature and proper care must be taken during as to not upset the desired result. However, it is often a more economical solution. Bacteria which does not require oxygen is introduced to the system. These bacteria, in a threestage process, convert the sludge to gases which can then be released.
Disenfection
Disinfectants are chemicals which are used to kill bacteria that is present in water. In general when disinfectants are discussed, the chemical referred to is chlorine. Chlorine is easily the most widely used mainly because of the cost comparative to other types of disinfectants. Chlorine however is a toxic substance and can be extremely dangerous to public health. It must be handled safely and properly.
In wastewater chlorine can be used to destroy common bacteria such as coliform. This is the presence of fecal matter in water supply.
Advanced Treatment
Advanced treatment also known as tertiary treatment and is a final level of the wastewater treatment process. This phase handles any remaining pollutants that are still above allowable levels that have not been removed during the previous stages. Here are some of the pollutants that may be removed during this level.
Suspended Solids – At this point any solids remaining are very small in size and would need to be removed by more advanced techniques. This may involve microstrainers or filter beds which are able to remove very high gradation solids.
Phosphorous – This stage may require the removal of phosphorous. This is done through the use of chemical precipitation. This process utilizes aluminum, iron, and lime coagulates.
Ammonia – There are many processes for use of removal of ammonia to acceptable levels. These may include stripping, biological denitrification, breakpoint chlorination, anion exchange, and algae ponds.
Engineering Economics
Wastewater Collection Systems
Wastewater is collected by a network of pipes known as sanitary sewer. Some residents may not be connected to the sewer line and may resort to septic tanks for their wastewater disposal. Wastewater in sewer networks is transported to the wastewater treatment plants to be treated. Lift stations are pump stations which can be used to facilitate the transportation of the waste water to the treatment plant.
Smoke testing is a method of determining if there are leaks in a wastewater system. Smoke is pumped into a pipe and will seep through any cracks which can then be identified.
Infiltration is water which enters the system due to imperfections in the system such as cracks in the line or improperly constructed portions.
Inflow is water that enters the system from unanticipated or illegal means.
Wastewater Treatment Process
Wastewater treatment processes are the procedures for treating wastewater so that it may be used again. This process will remove sediments, sludge, taste, odors, and any other undesirable characteristics of the water. The process can be divided into preliminary, primary, and secondary treatment which will be discussed further below.
Wastewater Flow Rates
The quantity of wastewater from a municipal needs to be determined to properly design the treatment system. This is based on anticipated discharge from residential, commercial and other buildings. In addition, the system must account for infiltration and inflow as defined previously.
Flow rate can be approximated as the average flow or the peak flow. The peak flow is the highest daily flow rate. The average and peak flow are related by the peaking factor:
Preliminary Treatment
Preliminary treatment is the first step in the wastewater treatment process. This portion of the process is mostly the mechanical removal of debris and other large objects which may be caught in the flow. Heavy chemicals and large amounts of oil are also removed during this process. In general, anything that can be identified with the naked eye and easily screened will be removed during the preliminary treatment process. This process is often performed with large mechanical screens or filters. These large obstructions must also be removed so that they do not damage or impede the subsequent processes.
Solids Treatment
Mixed Liquor Suspended Solids (MLSS) is the concentration of bacteria, solids, and any other undesirable material in sludge. To remove sludge, the MLSS is considered food for the activated microorganisms in the aeration process. It is often important to determine the food to microorganism ratio from the equation below:
Phosphorous Removal
Phosphorous removal can be separated into two different types. A small percentage is insoluble and can be removed during primary settling. The remaining amount is soluble and must be chemically converted to an insoluble material for removal.
Often aluminum sulfate, ferric sulfide, and lime is used to complete this process so that the phosphorous can precipitate and settle for removal. The most common is aluminum sulfate. The Chemical equations for removal are:
Nitrification and Dentrification
Nitrification is the use of oxygen by autotrophic bacteria. In this process the bacteria oxidizes ammonia to nitrites and nitrates. This process is important to understand as it relates to determining the Biochemical Oxygen Demand (BOD) of a particular sample and more specifically, the Ultimate Biochemical Oxygen Demand (BODu). To test for BOD, samples are diluted and dissolved oxygen is measured initially and typically after a 5day period. The following equation is used to determine the Biochemical Oxygen Demand after that 5day period (BOD5):
DOi = Initial Dissolved Oxygen content
DOf = Final Dissolved Oxygen content
V = Volume
The process of nitrification causes a deviation from the trajectory of the carbonaceous process of oxygen demand as it relates to time. This must be accounted for when determining the Ultimate BOD. The BOD at any time t is called the BOD exertion and is related to the ultimate by the following equation:
It is important to note that at initially in a sample there is only a small amount of autotrophic bacteria present and the process of nitrification is delayed from having a significant effect on the BOD process. For reference the chemical equation for nitrification is:
Denitrification is the removal or loss of nitrogen by the means of bacteria. The chemical equation is:
Secondary Treatment
The most intensive of the levels of wastewater treatment is the secondary treatment. This may involve biological treatment in tickling filters and sludge treatment. The most amount of BOD will be removed in this stage.
Primary Treatment
Primary treatment is the second level in wastewater treatment. In this portion the wastewater is allowed to settle to remove any remaining oils and any solids which are able to separate. Typically about half of the solids will be removed during this portion of the process. It is also expected that this level of the process will remove 25%35% of the Biochemical Oxygen Demand (BOD) in the wastewater.
Digestion
Digestion is a process of treating sludge that is too thick or bulky to be easily worked with for disposal. In other words if the sludge is too thick it can be further broken down by digestion so that it can be moved more easily. There are 2 processes of digestion: aerobic and anerobic.
Aerobic digestion is putting the sludge in a large open holding tank for a period of time. In this tank the sludge is stirred and left open to air. This allows bacteria to consume the sludge reducing the solids. Often, up to 70% of the solids can be removed through this process.
Anaerobic digestion as the name suggests occurs without the use of oxygen. This process is more delicate in nature and proper care must be taken during as to not upset the desired result. However, it is often a more economical solution. Bacteria which does not require oxygen is introduced to the system. These bacteria, in a threestage process, convert the sludge to gases which can then be released.
Disenfection
Disinfectants are chemicals which are used to kill bacteria that is present in water. In general when disinfectants are discussed, the chemical referred to is chlorine. Chlorine is easily the most widely used mainly because of the cost comparative to other types of disinfectants. Chlorine however is a toxic substance and can be extremely dangerous to public health. It must be handled safely and properly.
In wastewater chlorine can be used to destroy common bacteria such as coliform. This is the presence of fecal matter in water supply.
Advanced Treatment
Advanced treatment also known as tertiary treatment and is a final level of the wastewater treatment process. This phase handles any remaining pollutants that are still above allowable levels that have not been removed during the previous stages. Here are some of the pollutants that may be removed during this level.
Suspended Solids – At this point any solids remaining are very small in size and would need to be removed by more advanced techniques. This may involve microstrainers or filter beds which are able to remove very high gradation solids.
Phosphorous – This stage may require the removal of phosphorous. This is done through the use of chemical precipitation. This process utilizes aluminum, iron, and lime coagulates.
Ammonia – There are many processes for use of removal of ammonia to acceptable levels. These may include stripping, biological denitrification, breakpoint chlorination, anion exchange, and algae ponds.