1. Reducing Corrosion
in WWT Inlet Tanks by
Returning Mixed Liquor
Anna Mollergren

2. Presentation Outline
Current Situation/Problems
Theory
Experiments
Results and Conclusions

3. Corrosion Problems
Concrete & metal corrosion
Costly repairs, maintenance
and upgrades
Methods to
Reduce Corrosion:
Open-to-air tanks
Extraction of gases
Chemical dosing

4. What causes the Problem?
What is Hydrogen Sulphide?
 Chemical Formula H2S
 “rotten egg gas”
 colourless, very poisonous, flammable gas
Where does H2S come from?
Organic matter in sewage decaying without the
presence of an oxygen source.
What does H2S do?
H2S forms which corrodes
concrete and metallic structures in enclosed tanks

5. The Mixed Liquor Idea
What is Mixed Liquor?
Biological mass (activated sludge, “bugs”)
and sewage combined
Why and How does it work?
 Contains Nitrate (oxygen source)
 Contains certain bacteria that reverse the H2S production
 Dilution effect – dilution is the solution!
Why is it a ?
 Readily available – cheap and simple
 Nothing new is introduced to the process – no additional operational
hazards
 Plant Modifications simple – redirecting the ML to the inlet works

6. Basic Process Layout
H2S off gas
Effluent
Inlet Works
Raw Sewage Biological Activated
Sludge Tank
H2S in liquid (treatment)
WAS
Mixed Liquor

7. Trial Methodology
First Stage
Literature Review Would it Work and Why?
Second Stage
Bench Scale testing What and How do we prove it?
Third Stage
Full Scale Trials What, How and When do we do it?

8. Undertaking Full Scale Trial
Install ML pipes
Onsite Testing &Monitoring Reviewing data and dealing
with problems
 Wet Weather
 Diurnal Flow Pattern
 Sewage Quality
 Stability of H2S in liquid

9. Trial Results
Gaseous H2S average reduction
of 80%
Dissolved H2S average
reduction of 65%

10. Implications….
Increased life of assets
Reduced maintenance/replacement
Reduced odour emissions (happy neighbours)
Reduction in chemical usage
Increased understanding for design purposes

11. Thanks To…
My colleagues at Hunter Water Australia for
providing guidance and support
Hunter Water Australia for dedicating time
and resources
Hunter Water Corporation for funding a large
part of this project
Special Thanks to all of YOU!

12. Questions?