Emptying of cesspit (holding tank) - Is the faecal sludge pumpable? (question from Zambia)

  • hajo
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Emptying of cesspit (holding tank)

dear all,

first for clarification of definition: I used to call a completly sealed pit which does not leak any fluids a 'cesspit' but just noted that for Wikipedia and the Compendium, a cesspit can be either 'an underground holding tank (sealed also at the bottom) or a soak pit (not sealed at the bottom)'. I may get used calling it 'holding tank' although I like 'cesspit' better. What is your opinion?

If I use a holding tank to receive excreta from a dry toilet (i.e. no flush water, not even low or pour flush) what is the condition of the FS in the pit? Is it pumpable? Considering 1.2 L/cd of urine and 0.3 L/cd faeces, the original water ratio should be about 93%. There may be fluid losses by leakages and evaporation. Therefore the question to practioners: do you have experience with pumping/sucking such FS from 'holding tanks'? Does it work without adding water for fluidising? Because if we need to add water, we may as well connect a pour flush toilet. My concern is only that then the water becomes so much that the tank has to be emptied too often which will be costly for the user.

Any other aspects regarding the use of holding tanks/cesspits are also welcome.

ciao
Hajo

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  • muench
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Re: Emptying of cesspit (holding tank)

Dear Hajo,

I think the term "holding tank" is much better because cesspit has different meanings in different countries as you rightly pointed out (see Wikipedia: en.wikipedia.org/wiki/Cesspit ). Also why use a somewhat old-fashioned sounding name than a more "technical" term? And the word "pit" for me means hole in the ground, not sealed, perhaps just some lining on the sides. That's why a septic tank is called "tank" not "septic pit" as it's fully sealed at the bottom.
But that's just my opinion. :)

For your second question I have some questions back: how does the material get into your holding tank in the first place? Are barrels full of urine and faeces manually emptied into it? Or are vacuum tankers discharging into it? Where is the material coming from? If it's public toilets then maybe there is far more urine than faeces in which case it could be pumpable?
Also perhaps there are pumps available that can pump a very thick slurry? What's the thickest that your locally available pumps can pump? Could the excrevator work? ( www.susana.org/en/knowledge-hub/projects/database/details/85 , oh wait, its new name is flexcrevator, they got new funding from the Gates Foundation: www.susana.org/en/knowledge-hub/projects/database/details/542 )

Regards,
Elisabeth

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  • hajo
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Re: Emptying of cesspit (holding tank)

dear Elisabeth,

it's kind of you digging out old posts nobody had responded to. I had even forgotten that I had posted this question...B)

I think I can get used to use 'holding tank' especially since I learned from Wikipedia and EAWAG Compendium that 'cesspit' has the double meaning of either 'holding tank' or 'pit with open floor'. I have made my colleagues in Lusaka City Council aware of it.

In our planning in Lusaka the 'holding tank' is the containment under a dry drop toilet without any flush water being used reducing the need for numerous emptying. Therefore my assumption in my previous post that we have a theoretical moisture content of 93% in the holding tank.

Therfore my questions to possible practiconeers whether the moisture content stays high enough in spite of leakages of the tank and evaporation allowing pumping without further fluidising. Is there anyone on the forum who has practical experience or observations of emptying holding tanks under dry toilets???? Once we know what the possible moisture content and viscosity of tank contents is, we can look for appropriate pumps.

The Flexcravator is a vacuum pump which has an extra device (a spiral) which should prevent solid waste (which unfortunately is in the pit/tank) from entering the suction pipe. We tested the Flexcravator last year in pits in Lusaka and it had the same problems as any other pump dealing with plastic bags in the pits (which get there either as wrapping of solid waste or as alternative toilet paper). Otherwise it is as practical for pumping FS from pits and tanks as any other vacuum pump.

ciao
Hajo

We can't solve problems by using the same kind of thinking we used when we created them.
Albert Einstein
Any intelligent fool can make things bigger and more complex... It takes a touch of a genius - and a lot of courage to move in the opposite direction.
E.F. Schumacher
Everything should be made as simple as possible, but not simpler. :-)
Albert Einstein
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  • muench
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Re: Emptying of cesspit (holding tank)

Dear Hajo,
Yes, that's my job as moderator (and I enjoy doing it) - every post that deserves an answer or a reaction should get one! Often, once a post has received its first reply then it will get more replies. And your posts are always worth replying to! B)

I still can't picture what type of toilet we are talking about: Is it like a pit latrine, except that the pit is not a pit but a fully lined holding tank? If that's the case wouldn't it be terribly smelly and fill up very quickly (compared to a pit where the urine infiltrates into the ground).

If that's the case then the next question is, is it a public toilet or a household toilet? If a public one then perhaps there is less fecal matter than expected (if people are more likely to use it for urination only; or maybe men would not pay for a public toilet if it's only for urination but use the bush instead?). In either case, I would expect the collected material to be pumpable because the volume of urine is so much larger than the volume of feces (which are 80% water themselves).

Thanks for the info about the Flexcravator. I thought it was more powerful than ordinary pumps. (reminder for others: detailed discussion about the Flexcravator was here: forum.susana.org/99-faecal-sludge-transp...hnology-flexcrevator )

Elisabeth

Community manager and chief moderator of this forum via SEI project ( www.susana.org/en/resources/projects/details/127 )

Dr. Elisabeth von Muench
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This email address is being protected from spambots. You need JavaScript enabled to view it. Twitter: @EvMuench
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