Dear Vishwanath,

I hope reply of Elizabeth is clear to you.
As per the Manual of septic tank by the CPHEEO, Ministry of Urban Development, Government of India, cleaning intervals of septic tank should be 1-2 years only.
A major concern in India is that most of the households construct septic tanks not as per the approved design of the CPHEEO. People construct two chambers of any capacity/ size with a baffle wall and name it septic tank. Size of the tanks depends on affordability of the households, not on the design. People feel more comfortable with larger size of tanks.
Secondly,it is never a felt need of quality of effluent coming out of the tank. Only after the tanks are completely filled with sludge and there is back flow, household realizes that the septic tank is not working and needs to be cleaned.In most of the cases function and mechanism of septic tank is completely misunderstood.I have seen several such septic tanks (connected with public toilets) with visible human excreta in effluent coming out of the tank, connected to a drain. But for the persons in charge of the public toilet,it was not a problem.
pawan

Dear Vishwanath,
Just about your septic tank question: Septic tanks seem to be generally little understood by those who own them even though they are so commonly used.

If a septic tank seems to not require fecal sludge removal for 10 years or more then it could have the following reasons:

1. It was designed very large for the amount of wastewater load (or the wastewater load much lower than expected).
2. The wastewater has very few solids in it.
3. The septic tank is not sealed at the bottom and sludge is somehow getting out (not so common (?)).
4. The septic tank should get emptied, and the effluent quality is deteriorating but nobody is noticing because solids are leaving with the effluent which is perhaps being infiltrated in a way that the solids "disappear" into the ground (?).

Have I got all the possible reasons together? I don't think aerobic degradation can be suspected as a reason as there is no aeration with a septic tank.

I think you might also enjoy this previous forum discussion on sludge quantities from septic tanks:
Quantities of faecal sludge from pits, tanks etc
forum.susana.org/forum/categories/53-fae...-from-pits-tanks-etc

And this one about septic tank emptying frequency in India:
Appropriate Septic tank Cleaning (emptying) Frequency
forum.susana.org/forum/categories/53-fae...tying-frequency#1916

There is also a Wikipedia article about septic tanks by the way: en.wikipedia.org/wiki/Septic_tank

It is not yet great; I have done some basic improvements on it (like added the schematic from the Sandec compendium, and added a link to the newly created Wikipedia page on groundwater pollution). But it would greatly benefit from more editing work if someone has the energy to do so.

Regards,
Elisabeth

P.S. Christoph: I think I should perhaps split off all the posts relating to Dale's design ideas into a new thread, wich a new precise title, right?

Note by moderator: this post was initially in this thread:
forum.susana.org/forum/categories/99-fae...water-ohio-usa#13278

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I do apologise for misrepresenting you!

And I am learning a lot, so thanks for that.

We have septic tanks in India that don't need to be cleaned for decades. How does that square with the impossibility of aeration? I am just asking. I really don't know ...