On Site Sewage Facility
Pathogen Removal &
Disinfection Alternatives
Stephanie L Sturman, R. S., D. R., S. E.
Senior Inspector,
Watershed Protection Group,
Harris County Public Infrastructure
Department
What is the purpose of disinfecting
wastewater?
To destroy, de-activate or reduce to an
acceptable level, remaining pathogenic
organisms in wastewater to prevent the
transmission of infection, illness or disease.
What is a pathogenic
organism?
An infectious agent or organism that causes
or is capable of causing a disease.
Pathogen Survival
Even after proper treatment processes have
occurred, pathogens can survive for days in
the external environment, and most notably,
on plant life, (your fruit & vegetable gardens)!
Pathogen Survival (cont.)
Transmission of the pathogen, whether
directly or not, is most commonly through
ingestion.
The following pathogens are known to be
transmitted by the fecal to oral route.
Common Pathogenic Bacteria in
Wastewater
• Salmonella
– Typhoid Fever (general pain, fever, intestinal
bleeding) Foodborne Illness
• Shigella
– Shigellosis (Fever, Diarrhea [usually bloody]
& Stomach Cramps.)
• E. Coli (also, an indicator bacterium)
– Only a few strains cause infection
– Diarrhea, fever, abdominal cramps
Most Recent Bacterial Outbreak?
1. Listeria on Cantalopes
2. Salmonella on Spinach
Common Pathogenic Viruses in
Wastewater
• Rotovirus (stable in the environment)
– Fever, Vomiting, Diarrhea, Dehydration
• Hepatitus A (highly contagious)
– Fatigue, Nausea, Loss of appetite, Jaundice
• Norwalk Virus
– Vomiting, Diarrhea, Low-grade fever
(photo courtesy of the EPA)
Common Pathogenic Protozoa in
Wastewater
• Giardia Lambia
– Nausea, Bloating, Diarrhea, Anorexia
• Entamoeba histolytica
– Colitis, Dysentery, Diarrhea, Liver abscess
• Cryptosporidium parvum
– Mild to Chronic Diarrhea
So now we know what some of
the bugs in our wastewater are,
how do we eliminate pathogens
in wastewater?
Pathogen removal/ disinfection
• First: Primary Treatment (settling
and entrapment)
- Is septic tank large enough to handle the
organic load and to trap most pathogens in
the sludge or scum layer?
-Is it sized to prevent too much
“carryover”?
- Better effluent = Better Disinfection
Pathogen removal/ disinfection
(cont.)
• Second: Secondary Treatment
(Aerobic Treatment Systems, Filter Media
Systems)
Secondary treatment can remove up to
90% of organics and solids
Is the Treatment Unit sized properly?
Is it functioning properly?
Pathogen removal/ disinfection
(cont.)
• Third: Disposal or Disinfection &
disposal
–In subsoil disposal, disinfection occurs
in the soil itself
–In surface disposal, we need to
disinfect the waste water further.
Assuming the system is sized and
functioning well, what types of
disinfection are available to the On
Site Septic System owner?
• Chlorination
• Ultraviolet Light
• Ozone
Chlorination
• Tablet & Liquid Chlorination
The appropriate chlorine for disinfecting wastewater is
either Calcium Hypochlorite or Sodium Hypochlorite.
When diluted with the wastewater, Hypochlorous Acid
is formed, which actually attacks the pathogens.
Chlorine disinfects by attacking the pathogens’ cell wall &
interior structures, causing oxidation and death.
1.
2.
3.
4.
Very common disinfectant/ (disinfectant of choice).
Easy to use and monitor/measure
More effective on bacteria than on viruses or protozoa
May leave a chemical residue in the environment.
Examples of chlorinators
Ultraviolet Light
1. Ultraviolet Light kill pathogens by sending
electromagnetic energy from the lamp to the
cell’s genetic material, which kills or prevents
cell reproduction.
2. More effective at pathogenic virus and protozoa
removal than chlorination.
3. No chemical residue in the environment.
4. The intensity of the light is important for proper
disinfection (250 nm-270 nm is considered the
optimal range)
Ultraviolet Light (cont.)
4. Very important to have a fairly clear effluent so
the UV light can penetrate all pathogens passing
through.
5. Requires knowledge in installation, regular
maintenance, and cleaning.
6. Bulbs need to be changes about once a year.
Examples of UV Devices
Not really for Septic systems,
but the effect is similar- ZAP!
Ozone
• Utilizes pure oxygen or pure air and a high voltage
electrical discharge to produce Ozone.
• The Ozone is fed into a contact chamber with the
wastewater.
• Ozone disinfects basically by oxidizing the cell wall,
damage to interior cell components, rendering them
harmless.
Ozone
• Increases dissolved oxygen in effluent and any
receiving streams.
• More effective than UV & Chlorine against
virus, bacteria and protozoa.
• Any off-gases have to either recycled or
destroyed.
• Ozonation is not as readily available as other
technologies, as it fairly complex, expensive
and can be dangerous for improperly trained
persons working on these systems.
Conclusion
• It seems that the disinfection methods that
work best are also the most complicated.
• There is not a single method of disinfection
that is 100 % perfect for all systems.
• The designer, installer, maintenance provider
and property owner need to work together to
determine the most economic and effective
method of disinfection for the On- Site
Sewage Facility.