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

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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

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  • hajo
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Re: Emptying of cesspit (holding tank)

dear Elisabeth,

thanks for your concern and commitment!

yes, it is a pit latrine with a fully lined holding tank. And at the moment it is planned only as household latrine, no thoughts yet spent on its usefulness for public toilet/ablution block.

No, we do not expect more stench than from the current pit latrines. The 'typical' toilet smell stems from the ammonia evaporated out of the urine within hours of its release into the toilet which happens to the same extent in an ‘ordinary’ pit latrine where the urine takes time to percolate through the FS heap and into the underground and also releases ammonia. I kindly ask experienced colleagues to correct this anticipation.

Additionally the majority of existing pit latrines has no vent pipe which will be provided on the new latrines on holding tanks and which will improve odour control in the toilet (at cost of the neighbourhood). And if this is not sufficient a SATO pan/pedestal can be installed on the drop hole which also helps controlling odour (and flies) and solid waste.

A holding tank will fill up faster than a pit latrine which is why we recommend only using it with dry toilets and without adding any greywater. At most the little flush (200 mL) of a SATO pan should be acceptable. We aim at 2m3 holding tanks for a household of 10 people. Considering 0.2 mL/cd of faeces and 1 L/cd of urine the containment requires emptying about every 3 months. If we can manage (and we work on it!) that no solid waste (SATO!) and no plastic bags go into the containment, the emptying should be possible and easy by any manual (Gulper), mechanical (eVac, VROM, Flexcravator) pump or even by a vacuum truck if road access is given. The new toilets on holding tanks should have easy access to the containment from the top and outside the latrine superstructure.

Any comments or corrections of our approaches are highly appreciated.

I want mention that there are ongoing discussions between the public stakeholders whether UDDTs shall be acceptable in the specific locations where no percolation into the underground should be permitted. Only holding tanks, UDDTs with urine tanks and eventually vermi-composting vaults with black-water tanks seem being possible solutions in the areas of Lusaka with extreme and high vulnerability of groundwater due to karstic rock, very high ground water table and barely any or very little topsoil cover in most places.

Ciao
Hajo

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Re: Emptying of cesspit (holding tank)

The toilet that you are describing sounds like an aqua privy to me (strange name, isn't it). They've become very rare and are not even included in the Eawag Compendium ( www.eawag.ch/en/department/sandec/publikationen/compendium/ ) - or actually they'd probably call it, like you, dry toilet connected to holding tank.

Wikipedia has this information about aqua privy:

en.wikipedia.org/wiki/Latrine#Aqua_privy

An aqua privy is essentially a small septic tank located directly below a dry toilet squatting pan or bowl which has a drop pipe extending below the liquid level in the tank to form a simple water seal to minimise odors.[5]


Are you planning to use such a long drop pipe? I think if you don't then the odor will be much worse than from a pit latrine (this is a guess, I don't have data on that available). The SaTo pan might help a fair bit with the odour except in that moment of flushing.

Also, having to empty the holding tank every 3 months is very often! Who would pay for the emptying? Would you subsidise this service? If the households have to pay for the emptying, they'll probably tell everyone in their family (in particular the males) to not use the toilet for urination but rather urinate next to bushes and trees in the gardens (which might be better environmentally anyhow...).

Are you an embedded expert with Lusaka City Council? How is their relationship now with Lusaka Water and Sewerage Corporation - who is responsible for the onsite sanitation systems with dry toilets? (could be the topic of a separate thread)

Elisabeth

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  • tgurksi
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Re: Emptying of cesspit (holding tank)

Hi Hajo,

The question of pumpability needs to be a bit more specific. There are countless different pumping technologies, some of which are suited to thick sludges and some of which are not. For example, a progressive cavity pump is perfectly capable of pumping even undiluted feces. But regardless of the technology, there is always the issue of suction. Unless the pump is submerged in the media, you always need suction to get the media to the pump, and the limiting factor there is atmospheric pressure. Even if you can draw a perfect vacuum, you only have about 100 kPa to actually motivate the sludge, which, for a thick sludge, might not be enough. The one exception to this rule is entrainment-based systems, where a large FLOW of air captures material and drags it along. These have their own issues, though, like power consumption and clogging of hoses.

If there is a particular pumping system you are considering, I can comment on its suitability to the types of latrines you are talking about.

And yes, trash is always a complicating factor!

Cheers,
Tom
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Re: Emptying of cesspit (holding tank)

Sorry, Elisabeth and Tom, for the VERY late response, but sometimes other matters pop up which re-direct my attention... but Elisabeth's reminder asking me for response has been in my intray since then and creating a bad conscience with me... ok, enough excuses

The toilet described by me is NOT an Aquaprivy, it is just an ordinary dry toilet with a drop hole (maybe SATO) on a watertight holding tank but NOT filled with water as the Aquaprivy is and as decribed in the Wikipedia definition.

The odour control in such holding tank needs to be investigated, we are there of opposite opinion, I do not expect the smell being worse than from a pit latrine without ventilation pipe.... although bad enough..

I do not consider an emptying cycle of 3 months being too often. Of course it has to be paid by the users, maybe tariffs can be kept low if we manage creating a useful product from the FS which can be sold (bio-fuel, bio-char, compost, ...). In Lusaka they pay currently about 100 USD for emptying 6m3 of FS, thus the holding tank of 2m3 will cost 33 USD, which is 11 USD/month, which is about 0.40 USD/day, which is about 0.04 USD/cd. A visit to a public toilet costs about 0.10 USD/visit.

My role in Lusaka City Council and their collaboration with Lusaka Water & Sewerage(sic) Company deserves really an extra thread - maybe a book... :(

Tom: also thanks for your contribution. A progressive cavity pump is a positive displacment pump? Something like the MONO pump? Those pumps have no suction, i.e. need to be stuck into the FS, or?

My question regarding pumpability was exactly aiming at the question whether faeces plus urine are wet/fluid enough to be sucked by a vacuum pump. As I said the matter should have a theoretical water content of about 95% but some may get lost by leakage or evaporation. Therefore my question to practioneers, whether anybody has practical experience with it. And I may have found someone: I learned that a project in Nakuru/Kenya has built 8,000 dry latrines with holding tank. They should have the experience by now. Once I receive a feedback from them, I will report here.

Entrainment-based systems are also called Reverse Circulation Air Lifting method? Where the principle is similar to the Jet Pumps? I have been in contact with KOERTING in Germany and they say that this type of pump will not be appropriate for FS.

Remains acc. to my knowledge only the use of vacuum pumps... if the FS in the holding tank is fluid enough for sucking... I am looking forward to a response from Nakuru...

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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  • tgurksi
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Re: Emptying of cesspit (holding tank)

Hi Hajo,

A progressive cavity pump is a positive displacement pump. They can develop significant suction so may not need to be to dropped into the tank. However, depending on the viscosity of the media and the depth of the tank they may not be able to prime or pump fast enough. They are also prone to fouling with stringy debris or things like bags (which are a nightmare for a lot of pumping systems).

As to pumpability with a vacuum pump, it will depend on the type and power of the pump. A typical vacuum truck with a healthy pump (usually rotary vane type) should be up to the task if the water content is in the 90-95% range, assuming it can get fairly close.

Regarding entrainment, I'm not familiar with reverse circulation air lift pumping, but based on what I saw from a web search I don't think that is particularly applicable to this situation. Entrainment simply means that you are sucking some fraction of air along with the media to create a fairly high speed flow rather than having the hose completely full of media. A household vacuum cleaner works via entrainment...you can suck up water but if the hose completely fills with water it will stall because more suction pressure than it can develop is required. With any vacuum system there is a tradeoff between how much suction pressure and how much volume flow you can get (the power required is essentially the product of the 2 factors, and some pumps develop more pressure and less flow and others vice versa).

A jet pump uses a high velocity flow of water injected by a nozzle to create suction. They can be very effective, but the two streams mix together so you would dramatically increase the volume of waste to be disposed, and of course you'd need a supply of clean-ish water. Not very practical for this application.
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