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In industrial and municipal wastewater treatment, abnormal color in effluent often means problems in the system. Recently, an engineer asked:
“The effluent from the secondary clarifier looks clear, but it’s always yellow. What’s the cause?”
This issue is not rare. It often appears in wastewater from livestock farms, dyeing factories, or during rainy seasons.
This article explains the main reasons why secondary tank effluent turns yellow. It helps operators quickly find problems and fix them.
External Factors: Raw Water and Chemical Matter
- High color in incoming water
Wastewater from livestock or dyeing plants often contains natural or dye pigments. If the pretreatment or biological process cannot remove these fully, the effluent will stay yellowish.
- Too much iron
If the raw water or coagulants contain Fe²⁺ or Fe³⁺, the color can turn yellow. Especially ferric iron (Fe³⁺), when oxidized, gives the water a strong yellow color.
- Overdosing PAC
Polyaluminum chloride (PAC) is yellow or light yellow. If overdosed, it may not only cause turbidity but also leave color in the effluent, raising the water’s color level.
Biological Process Problems: Sludge Affects Water Color
- Sludge aging and over-aeration
Old sludge can break apart, releasing fine particles into the water. This often happens when DO is too high or the system runs too long. Check DO settings and sludge discharge time.
- Bulking sludge (filamentous or non-filamentous)
If the sludge expands, it won’t settle well. Flocs may float, making the effluent look yellow and increasing suspended solids.
- High flow shocks or fast flow
During heavy rain or peak flow, sludge may not have enough time to settle. Fine sludge flows out with water, making it appear yellow.
Sedimentation Tank Operation: Don’t Ignore the Equipment
- Too much sludge, slow sludge removal
If sludge age is too long and the tank isn’t cleaned in time, settled sludge may float up and go with the effluent, turning it yellow.
- Surface loading too high
If the tank is too small or flow is too fast, sedimentation gets worse. This makes the water look yellow. In this case, try adding PAC or PAM to help floc formation and improve settling.
Conclusion and Suggestions
Yellow water from the secondary tank is not just a color issue. It may come from the raw water, chemicals, system operation, or poor equipment management.
You should check each part one by one. Solve the problem from the source to the final step.
If the problem can’t be fixed quickly, try using polymer flocculants. This can improve sludge settling as a short-term solution.
