How OWTSs Using
Aerobic Treatment Work
Texas AgriLife Extension Sevice
Overview
 Treatment
Processes
 Components of an OWTS using Aerobic
Treatment, Disinfection and Spray
Dispersal
 What is a Flow Equalization Tank?
Aerobic Treatment Unit System
Wastewater Treatment
Processes
 Oxygen
state
 Aerobic vs. Anaerobic
 Processes
Physical
 Chemical
 Biological

Aerobic vs. Anaerobic Processes
(???)
 Aerobic



Aerobic bacteria require O2 to live and grow
Aerobic treatment processes require O2 to proceed
Common condition in soil treatment, media filters,
MATUs
 Anaerobic



Anaerobic bacteria grow in absence of free oxygen,O2
Anaerobic treatment processes do not use oxygen, but
consumption of items, breaks oxygen bonds Ex. SO4,
NO3
Common condition in septic tanks, processing tanks,
and usually any saturated environment
Juvenile Rotifers Feeding on Rod-Shaped Bacteria pH 7.04 DO 1.2 ppm
Large Rotifers Attached to Aerobic Treatment Media Feeding on Bacteria
& Organic Nutrients pH 6.94, DO 4.54 ppm
Mite on Aerobic Treatment Media pH 6.94, DO 4.54
Aerobic Treatment Unit System
for a Residence
ATU, Disinfection and Spray
Distribution System Components
 Septic/Trash
tank
 Aerobic Treatment
 Air Supply
 Clarification
 Sludge Return
 Disinfection
 Pump tank with pump and controls
 Spray field
Septic / Trash Tank
Septic/Trash Tanks
 Most





common treatment system:
Water tight tanks.
Anaerobic Treatment.
Most common treatment device.
Septic tank: Minimum of two
compartments
Trash tank: Typically smaller with one
compartment
 Some
tank.
systems do not have a trash
Two-Compartment Septic Tank
Septic Tank Treatment Process

Physical separation processes –





Settling of heavy materials – sludge layer
Floating of lighter materials – scum layer
Clear zone (clear layer)
Effluent screen to assist in trapping solids (septic tank)
Time is the main requirement to allow separation
to occur

Detention time:
• Septic tank - 2-3 days
• Trash tank – 1 day


Distance between inlet/outlet baffles is critical
Calm conditions to prevent mixing in the tank
 Anaerobic digestion and storage of materials until
the tank is pumped
Aerobic Treatment
Aeration Chamber
Aerobic Treatment Process
 Aerobic
microbes
digest solids

Organic matter + O2 = CO2 +
H2O + new cells
 Sludge
accumulates in
tank
 Extended aeration


High DO, Long detention, Low
Food/Microorganisms ratio
Organic matter to microbes,
microbes eat each other, result
in sludge
Suspended Growth

Aerobic microbes free
swimming in the
aeration chamber
 Mixing in the chamber
mixes the microbes
and the wastewater
contaminants
 Extended aeration to
limit biomass wasting
Submerged Attached Growth/Fixed
Film Media

Media is submerged in
the aeration chamber
 Microbes are attached to
the media
 Effluent is circulated
through the media thus
passing contaminants by
the microbes
 Extended aeration to
limit biomass wasting
Suspended Growth
Configurations

Should be three
chambers/tanks.
 Single tank with three
compartments.
 Multiple tanks.
 May combine aeration
and clarification
chamber with divider
Attached Growth Configurations
 Most
have three
chambers
 May be a separate
media filled insert
 May have media
placed in the aeration
chamber
 Check manufacturer
literature
Air Supply
Aeration of Wastewater
 Air
is 21% oxygen
 Oxygen must transfer
to wastewater for DO
 Quantity of oxygen
transferred directly
related to BOD
removal
 Mixing of sewage
Dissolved
Oxygen
O2
Less Transfer
O2
More Transfer
Oxygen Transfer
Aspirator/Aerator

Vacuum pulls air into
the water
 Spinning shaft or
impeller causes the
vacuum
 Limited resistance
Compressors
 Greater
pressure
 Lower air flow
 Two distinct types
of compressors


Rotary
Rotary
Linear
 Air
inlet
 Housing - cover
Linear
Compressor Air Flow
Rotary

Rotary compressor Relatively straight
increase in flow with a
decrease in pressure
 Linear compressor –
rapid decrease with
increasing pressure
Linear Compressor Curve
4

Flow drops with
resistance to air flow
Must have the correct
compressor to have the
correct air flow
3.5
3
2.5
CFM

Linear
Model 5078S
2
1.5
1
0.5
0
0
1
2
3
4
PSI
5
6
7
8
Blowers







Greater air flow
Lower pressure
Air inlet screens/filters
Air flow discharge
from unit
Larger pipe
Air flow stops sharply
Cannot operate under
higher pressure
Air Distribution Lines

Minimize friction
loss in the
distribution lines:
 Proper pipe
diameter
 Reasonable
distance between
aerator and
aeration chamber
 Minimize number of
fittings (changes in
direction) in the line.
Air Diffusion
Perforated Pipe
Porous Stone Diffuser
Clarification
Clarification

Process occurs in a
“clarifier”
 Clarification is the
process were the
microbes, cell waste
and biomass settle
out of the water.
 Sludge blanket in the
bottom and a clear
zone below the
discharge point
Vertical Settling Chamber

Calm Environment
 Particles of Sufficient
size and mass to
settle
 Flow dependent
 Upward flow rate
must be less than the
settling rate
Configurations






Separate chamber with no extra
filtration
Cone inserted into aeration
chamber
Many have sloped walls
Added tertiary filter or screen in
clarifier
Filtration through socks placed
in the aeration chamber
Added filtration improves quality
but increases maintenance
Flow Equalization Tank
Controlling Flow from a Residence
 Owner
controls water
usage to match
clarifier capacity
 No large water using
devices
 500 gpd ATU has a
max of 42 gph without
flow equalization
 Or Install larger
capacity system
Flow Equalization/Surge Tank
Concepts

Moderate Flow



Daily fluctuations
Weekly fluctuations
Down stream components


Function more effectively
May allow decrease size
 Timer
controls dosing
to ATU
Flow Equalization/Surge Tank
Incorporating Flow Equalization

Modify the treatment train by adding
components
 Add an additional tank between the trash tank
and aeration chamber
 Upsize trash tank to full size septic tank, add
pump with timer controls to dose ATU
Sludge Return
Sludge return

Settled solids passing
into a previous
treatment chamber
 Passive system



Settled solids passing
through the bottom
opening
Most common method
Active system

Settled solids blanket
below the outlet baffle
Disinfection
Disinfection,
not Sterilization
The goal of disinfection
is to rid the wastewater stream
of organisms capable of causing infection
Sterilization is freeing
the wastewater stream of ALL LIFE.
Chlorine
Destroys target organisms by chemical
oxidation of cellular material.
Some organisms are resistant to low
doses of chlorine
Oocysts of Chrytosporidium parvum
Cysts of Endamoeba histolytica
Cysts of Giardia lamblia
Eggs of parasitic worms
Chlorination Considerations
 Chlorine
is an oxidizer.
 Need clean water (low BOD, low TSS):
chlorine is not “stolen” to oxidize organic
matter.
 Chlorine reacts with ammonia to form
chloramines.
 Chloramines are not as effective as
hypochlorous acid and hypochlorite ion for
disinfection.
 Interferences: BOD, TSS, Humic Materials,
Nitrite, pH, Iron, manganese and hydrogen
sulfide
Dosing
Dose = Concentration x Time
 Increasing either dosage or contact time,
while decreasing the other, can achieve
the same degree of disinfection.
 Breakpoint- the process where sufficient
chlorine is added to the system to obtain a
free chlorine residual
Caution

People should be
trained to properly
handle and work with
chlorine products
 High risk



Exposure
Handling
Corrosive
 Unstable
 Be careful where
stored!!!!!!
Tablet Chlorination

Tablet chlorinators
generally have four
components:




Chlorine Tablets.
A tube that holds the
tablets.
A contact device, which
puts the chlorine tablets
into contact with the
wastewater.
A storage reservoir,
usually a pump tank
where the water is stored
before it is distributed.
Typical Chlorine Tablets

Wastewater tabletsCalcium Hypochlorite


Swimming pool –
Trichlorocyanuric acid


Basic compound –
high pH
Acidic compound –
low pH
Hazard to mix acids
and bases
 Must use products in
accordance with label
Residential Liquid Bleach
Chlorinator

Reservoir
 Delivery of
chlorine



Vacuum/suction
Pump
Dose volume
 Mixing with
effluent
Residential Liquid Chlorination

Flow passes through
aspirator developing a
vacuum or through a
pump to draw a dose
of chlorine.
 Tubing delivers
chlorine dose to
aspirator or pump
 Control volume of
chlorine
 Mixing of chlorine with
effluent in pump tank
Ultraviolet Light Disinfection Units

UV light destroys
microorganisms by
altering their genetic
material and / or
retarding their ability
to reproduce
Ultraviolet Light Disinfection Units

Electromagnetic
energy (UV light)
from source lamp
is emitted into a
chamber through
which effluent
passes
Other Interferences to Effective
UV Disinfection
 Dissolved
organic compounds (BOD,
TOC, COD)
 Organic iron compounds
 Humic compounds (tannins)
 Inorganic coatings on protective tubes
Pump Tank with Pumps
and Controls
Pump Tank & Pump
 Stores
water until time
for dosing effluent
 Acts as storage for
disinfection
 Storage capacity when
system breaks
 Pump



Type: effluent or sewage
Not a grinder or sump
Specific flow & pressure
Panel & Electrical Control Circuits







Panel is a housing for
components needed to
control a system.
Record valuable
operational information
Provide a means to collect
operational data on the
system
Timer for night spray
Timer for flow equalization
Audio and visual alarms
Power must be ON!
Spray Field
Spray Field
 Lawn
Sprinkler systems? --- NO!!!
 Spray treated wastewater over the surface
of a yard
 Requires greatest level of treatment
 High risk system!!!!
Water Quality






High potential for human
contact with water
Secondary- Quality Effluent
Remove 85-98% of solids and
organic matter
Remove pathogens?
Nutrients (nitrogen and
phosphorus) are not required to
be removed
Soil for Final Treatment
Role of vegetative cover in
treatment system
A
healthy cover crop is essential for the
system to function properly. Plants will:




Take up nutrients
Take up water
Stabilize the soil and prevent erosion
Provide food and habitat for beneficial soil
organisms
 Soil

microbes are the final treatment!!!!
This is effluent – NOT DRINKING WATER!!!!
Connecting Distribution Heads to
Laterals
Summary
 Treatment
requirements
 Components of an OWTS using Aerobic
Treatment, Disinfection and Spray
Dispersal
 What is a Flow Equalization Tank?