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Contaminant Movement in Ground Water

Model system

porousplate

groundwater flow direction

Lines of equal contaminant concentration

Plan View

Vertical Section View

Point Source of Contaminant

Contaminant plume

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

• Quantity & Characteristics of Wastewater

• Combined Sewer Overflows

• Sewer Basics

• Sewage Pump Stations

• Alternative Collection Systems

How much wastewater do we produce each day?

Wastewater Characteristics

Source Average Daily FlowDomestic sewage 60-120 gal/capitaShopping centers 60-120 gal/1000 ft2 total floor

areaHospitals 240-480 gal/bedSchools 18-36 gal/studentTravel trailer parks

Without individualhookups

90 gal/site

With individualhookups

210 gal/site

Campgrounds 60-150 gal/campsiteMobile home parks 265 gal/unitMotels 40-53 gal/bedHotels 60 gal/bedIndustrial areas

Light industrial area 3750 gal/acreHeavy industrial 5350 gal/acre

Source: Droste, R.L., 1997. Theory and Practice ofWater and Wastewater Treatment

These values are rough estimates only and vary greatly by locale.

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

ToxicityWastewater 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 CharacteristicsRef: Davis, Cornwell, 1998, Introduction to Environmental Engineering

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

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 CharacteristicsRef: Metcalf & Eddy, 1991, Wastewater Engineering Treatment, Disposal and Reuse

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-NWastewater 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.

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.

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

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 CharacteristicsRef: Reynolds, 1996, Unit Operations and Processes in Env. Engineering

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

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

Milwaukee Deep Tunnels canstore over 400 million gallons ofcombined flowsWastewater is pumped to treatmentplant when flows subside (no storm)

Seattle’s CSO Projects includetunnels and pump stations

CSO Locations-King 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 = 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)

2/13/2486.1SR

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

Source: Metcalf & Eddy, Inc. Wastewater Engineering: Collection, Treatment and Disposal. McGraw-Hill:New York, 1972.

Large Pump Station

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 comefrom existingseptic 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 LayoutPipe 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 discharge3) Reuse (irrigation)4) Wetland enhancement/treatment

C0C0 C granular media flow Time C/C 0 No dispersion Effect of dispersion With retardation column...

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