Dosing Aluminumsulphate?

[Chris73]

Hi Eng's;

One of "my" small WWTP is indicating very bad settling conditions in the secondary clarifier (package plant ~ 5 m3/day). The only substrate I have available right now is Al2(SO4)3. The clarifier possesses a volume of app. 2m3. What would be a proper dose to start with? A cup? In lack of any turbulent intake zones I will have to strew the granular Al2(SO4)3 right onto the clarifier's surface. Any suggestion? Probably neutralization necessary!?

 

[waterexpert]

Chris,

I'm not so sure that alum thrown on the clarifier surface is going to get you very far. Some of the possible causes of poor settling would be worsened by flocculant dosing. Efficiency of this technique of dosing is likely to be very low. Potential for acidification and possibly even high outlet aluminium concentrations is very high. Discharge of water with low pH and high aluminium concentrations to the environment is likely to be illegal.

Have you investigated why there is poor settling? What sort of poor settling is it? Is there foam on the clarifier surface? Can you see bubbles rising in the clarifier? is there a discrete settling layer visible?

 

[Chris73]

waterexpert;

I totally agree with all your concerns but I will have to do something. I couldn't observe a discrete sludge layer being created - actually the sludge was suspended over the full height of the clarifier. Obviously the sludge doesn't settle at all - another hint for that is that the sludge return is light - almost translucent clear. Small flocs are basically scattered up to the surface and of course pass the weir. That's why the TSS of the effluent is high. I thought creating bigger flocs and adding weight to the sludge would facilitate settling. For dosing of Alumn I would do bench scale tests before. No scum, no bubbles visible - DO between 4-5 mg/l - normal for the cold season.
I don't know what causes these problems - only residential sewage - no changes of operation, always worked fine - maybe the very low influent flow now in winter (tourism dependent)?

 

[waterexpert]

What is your MLSS concentration?

 

[Chris73]

I can't tell you. We usually do not check the MLSS in such small plants. I assume it is fairly low at the moment because there is hardly any sludge returned because of the bad settling. My estimate would be around 2-3 g/l but I could be off.

 

[waterexpert]

It would be interesting to know incoming flow, load and aeration tank MLSS. If it is an aerated tank, this might be the point to add your alum.

You mention a high tourist load. Are there any industrial feeds?

 

[Chris73]

No, there is industrial feed - only residential and one restaurant connected. It is an extented aeration system.

 

[Chris73]

Excuse me: it must read: there is no industrial feed!