Reducing Corrosion in WWT Inlet Tanks by

Returning Mixed Liquor

Anna Mollergren

Presentation Outline

Current Situation/Problems

Theory

Experiments

Results and Conclusions

Corrosion Problems

Concrete & metal corrosion

Costly repairs, maintenance

and upgrades

Methods to

Reduce Corrosion:

Open-to-air tanks

Extraction of gases

Chemical dosing

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

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

Basic Process Layout

Biological Activated

Sludge Tank

(treatment)

Raw Sewage

WA

S

Effluent

Inlet Works

H2S off gas

H2S in liquid

Mixed Liquor

Trial Methodology

Third Stage

Full Scale Trials What, How and When do we do it?

Second Stage

Bench Scale testing What and How do we prove it?

First Stage

Literature Review Would it Work and Why?

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

Trial Results

Gaseous H2S average reduction

of 80%

Dissolved H2S average

reduction of 65%

Implications….

Increased life of assets

Reduced maintenance/replacement

Reduced odour emissions (happy neighbours)

Reduction in chemical usage

Increased understanding for design purposes

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!

Questions?