Contaminant Movement in Ground Water
C0
No dispersion
1.0
granular
media
flow
Effect of
dispersion
0.5
C/C0
column
0.0
Time
With retardation
porous
plate
C
Model system
Vertical Section View
Point Source of Contaminant
Point Source of Contaminant
Contaminant plume
Lines of equal contaminant concentration
Plan View
NAPLs:
• DNAPLs-Dense Non-Aqueous Phase
Liquids
• LNAPLs-Light Non-Aqueous Phase
Liquids
Subsurface NAPL
Free-Phase
(Mobile)
Residual
(Trapped)
NAPLs:
Wastewater Collection
Systems
Outline
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•
•
•
•
Quantity & Characteristics of Wastewater
Combined Sewer Overflows
Sewer Basics
Sewage Pump Stations
Alternative Collection Systems
How much wastewater do we
produce each day?
Source
Domestic sewage
Shopping centers
Average Daily Flow
60-120 gal/capita
60-120 gal/1000 ft2 total floor
area
240-480 gal/bed
18-36 gal/student
Hospitals
Schools
Travel trailer parks
Without individual
90 gal/site
hookups
With individual
210 gal/site
hookups
Campgrounds
60-150 gal/campsite
Mobile home parks
265 gal/unit
Motels
40-53 gal/bed
Hotels
60 gal/bed
Industrial areas
Light industrial area 3750 gal/acre
Heavy industrial
5350 gal/acre
Source: Droste, R.L., 1997. Theory and Practice of
Water and Wastewater Treatment
These values are
rough estimates only
and vary greatly by
locale.
Wastewater Characteristics
Other Contributions to Wastewater Flows
• Infiltration
– Older sewer pipe did not have water-tight
joints
– Sewers follow topography, which means many
follow stream-beds or drainage swales where
groundwater is high
– Since sewers are not under pressure,
groundwater can enter in through joints (as
well as sewage leak out if ground water is
lower than pipe)
Other Contributions to Wastewater Flows
• Infiltration rates vary by depth of
groundwater, type of pipe joint, and pipe
diameter
• Infiltration can range from 1,000
gal/day/mile to 100,000 gal/day/mile
How do we quantify water pollution?
BOD - Biochemical Oxygen Demand
COD - Chemical Oxygen Demand
DO - Dissolved Oxygen Levels
TKN - Total Kjeldahl Nitrogen
Pathogen Levels (Coliforms)
NO3-N - Nitrate nitrogen
Suspended Solids
Aquatic Organisms or lack of (algae, fish, etc.)
Heavy Metals
Toxicity
Wastewater Characteristics
Dissolved Oxygen
Dissolved Oxygen (DO) is the amount of oxygen
dissolved in a liquid. It can be added to a liquid
by aeration or from the natural gas transfer
between the air (containing oxygen) and the
liquid surface.
The amount of DO in a liquid is dependent on
the liquid temperature and the salinity.
The maximum amount of DO that can be
present in a liquid is called the saturated
dissolved oxygen level.
The DO is used by aquatic organisms. If there
is no DO present, the water is considered to be
anaerobic.
Wastewater Characteristics
Biochemical Oxygen Demand
BOD - used to quantify the amount of oxygen
used by microorganisms to oxidize dissolved
organic and inorganic constituents in a water.
BOD5 - the amount of oxygen consumed (in
mg/L) over a 5 day period at 20oC (in the dark).
BOD5 is a measure of the bioavailability over a 5
day period under controlled conditions.
BODu - the maximum amount of oxygen usage
by microorganisms over a long period of time.
A good measure of maximum bioavailability.
Wastewater Characteristics
Biochemical Oxygen Demand
Ref: Davis, Cornwell, 1998, Introduction to Environmental Engineering
Wastewater Characteristics
Biochemical Oxygen Demand
Compound
BOD5, mg/L
Domestic Wastewater
~
200
Whole Milk
102,500
Skim Milk
73,000
Coke
67,400
Pepsi
79,500
Tom Collins
66,600
Ethylene glycol
400,000
Wastewater Characteristics
Biochemical Oxygen Demand
Carbonaceous BOD (CBOD) - used to quantify
the amount of oxygen used by microorganisms
to oxidize dissolved organic constituents in a
water.
Nitrogenous BOD (NBOD) - the amount of
oxygen used by microorganisms to oxidize
dissolved nitrogen in a water.
Wastewater Characteristics
Biochemical Oxygen Demand
Ref: Metcalf & Eddy, 1991, Wastewater Engineering Treatment,
Disposal and Reuse
Wastewater Characteristics
Nitrogen in Wastewater
Total Kjeldahl Nitrogen (TKN) - is the sum of organic
nitrogen and ammonia nitrogen. Expressed as mg/L
as Nitrogen
Organic Nitrogen - nitrogen that is complexed with
organic constituents (cell tissue, amino acids,
proteins, plant tissue, etc.)
Ammonia Nitrogen - nitrogen that is in the form of
ammonia. Expressed as mg/L NH3-N
Nitrite (NO2) nitrogen can be measured directly and is
typically expressed as mg/L as NO2-N
Nitrate (NO3) nitrogen can be measured directly and is
typically expressed as mg/L as NO3-N
Wastewater Characteristics
Nitrogen in Wastewater
Under aerobic (presence of oxygen), TKN will
oxidize to nitrite (NO2) and nitrate (NO3). The
oxidation of TKN to nitrate is called nitrification.
The reduction of oxidized nitrogen to nitrogen
gas (N2) is called denitrification. Denitrification
can occur under anaerobic (void of oxygen)
conditions.
Wastewater Characteristics
Other Oxygen Demands
Chemical Oxygen Demand (COD) is a measured
quantity of oxygen needed to completely
oxidize organic and inorganic substances that
are present in a water. The COD will oxidize
organics/inorganics that would not normally
oxidize under natural conditions. The COD is
not a measure of the bioavailability or biological
activity in a wastewater.
COD >> BODu  BOD5
Theoretical Oxygen Demand (ThOD) is the
theoretical amount of oxygen needed to
completely oxidize a substance.
Wastewater Characteristics
Solids in Wastewater
Total Suspended Solids (TSS) is a measure of
the mass of solids that are larger than ~ 1µm in
a liquid
Volatile Suspended Solids (VSS) is a measure
of the mass of TSS that can be burned at 550oC.
It is a good measure of biological mass in a
water.
Fixed Suspended Solids (FSS) is a measure of
the mass of TSS that is “inert”. FSS = TSS-VSS
Wastewater Characteristics
Other Wastewater Constituents of Concern
Phosphorus is a vital nutrient for aquatic plants
such as algae. Too much phosphorus may lead
to substantial growth of algae in receiving
streams and lakes. Aquatic plants get their
carbon from CO2 and HCO3 and their energy
from sunlight.
Heavy Metals: Zinc, Cadmium, Copper, Lead,
and etc. that are extremely toxic to aquatic life.
Others: pesticides, herbicides, chlorine, and
ammonia can be toxic to aquatic life in the
receiving stream. Volatile organic compounds
enter atmosphere during aeration.
Wastewater Characteristics
Ref: Reynolds, 1996, Unit Operations and Processes in Env. Engineering
Wastewater Characteristics
Older Systems Have/Had
Combined Sewers
• Sanitary sewer also collects storm water
runoff
• Quantity is highly variable and site specific
• CSO: Combined Sewer Overflow
– Wastewater flows greatly increase during a
storm
– If capacity of sewer or treatment systems are
exceeded, some of the combined waste is
discharged with minimal to no treatment
CSOs
Reducing CSOs
• Install separate storm and sanitary sewers
– Standard for all new construction
– Very expensive for existing systems
• Build pipes and treatment plants large enough to
handle all flows
– Very, very expensive – not feasible
• Store combined sewage, then pump to treatment
plant when storm ends and flows are back to
normal
– This option has been selected by many cities,
including Seattle and King County
Seattle’s CSO Projects include
tunnels and pump stations
Milwaukee Deep Tunnels can
store over 400 million gallons of
combined flows
Wastewater is pumped to treatment
plant when flows subside (no storm)
CSO LocationsKing County
Most Sewers Rely on Gravity Flow
• Most sewers are designed to flow by
gravity (water flows down-hill)
– Includes sewer pipe from home to septic tank
or to a municipal collector pipe
– Gravity sewers must follow the topography of
the land
– Where gravity flow is not possible, pumps are
used
• Individual unit pump
• Large municipal lift (pump) station
Hydraulics of Sewers
• Most sewers designed to flow at a velocity
of 2.0 ft/sec when flowing half-full
– Do not want pipe to flow full at peak flows
– Do not want pipe flow velocity too low—can’t
transport solids
• Most designs are complex, but use basic
hydraulic equations for computing
necessary size and slope
Manning Equation for Pipe Flow
• Manning Equation
V
1.486 2 / 3 1/ 2
R S
n
V = velocity (ft/sec)
n = coefficient of roughness (dependent upon pipe
material/condition)
R = hydraulic radius = area/wetted perimeter (ft)
S = hydraulic slope (assumed to be slope of pipe)
(ft/ft)
Basic Sewer Design
• Collector pipes (pipe in street) is minimum
8 inches diameter (to allow cleaning)
• Service pipes (home or building to
collector is 4 to 6 inches diameter
• Gravity sewer pipes have no bends,
manholes used to make transitions in
direction and pipe size
• Pipe sections between manholes are at a
constant grade or slope (S)
Typical Manhole
Fig 5.1, p 134
House or Building Service Connection
Pump Stations
• Pump (lift) sewage from low to higher
elevation, generally from end of one
gravity sewer section to another, higher
section
• Consist of a wet well and pumps
• Wet well forms a place for wastewater to
collect and be pumped from
Large Pump Station
Source: Metcalf & Eddy, Inc. Wastewater Engineering: Collection, Treatment and Disposal. McGraw-Hill:New
York, 1972.
Small Pump Station
Alternative Collection Systems
• Applications
– Small community with failing septic systems
– New, small developments
– Areas where gravity sewers are not feasible
•
•
•
•
•
•
•
•
Homes along edge of lake
Areas with unstable soil
Areas with flat terrain
Rolling land with many small elevation changes
High water table
Restricted construction conditions
Rock
Urban development in rural areas
• Types of Alternative Systems
– Pressure sewers
– Vacuum sewers
– Small diameter gravity sewers
Pressure Sewers
• Each
home/building has
individual pump
• Wastewater
pumped to a
central treatment
location
• Pumps “grind”
sewage solids
Pressure Systems Can Pump Wastewater Treated
by Septic Tank – Used for Homes Previously on
Septic Systems
Vacuum Sewers
• Wastewater flows by gravity to a central
collector well (up to four homes per well)
• When well fills a vacuum lines pulls
wastewater to a central vacuum tank
• Wastewater pumped from central vacuum
tank to treatment or a gravity sewer
Typical Vacuum Sewer Layout
Vacuum System Components
Could also come
from existing
septic tank
Small Diameter Gravity Sewer Systems
• Wastewater flows from home to interceptor
(septic) tank where settleable solids and
grease are removed
• Wastewater flows by gravity to central
collector pipe
• Central collector pipe can flow full in
certain areas
• Pipe sizes are typically 4 and 6-inch
diameter
Small Diameter Sewer Layout
Pipe will flow full, under pressure in these areas
Interceptor Tank (same as Septic Tank)
Carnation Treatment System
Vacuum collection system
Treated wastewater discharged to:
1) Uplands (infiltration to groundwater)
2) River discharge
3) Reuse (irrigation)
4) Wetland enhancement/treatment