I have somehow missed this topic until now. There are some interesting contributions and here are a few more that may provide information on some of the points raised previously.

First, as stated by Canaday, biogas collection from Sulabh toilets is from their public toilets, which are not of the twin-pit type. I doubt very much if there would be enough biogas generated from individual household twin pit pour-flush toilets to make its collection a practical proposition.

Second, again in relation to twin pit toilets, I think that it is important to distinguish between what happens in theory and what happens in practice. Sulabh do claim that most of their twin pit toilets are working well but I am doubtful whether this is the case. A couple of years ago, I visited various services upgrading initiatives in low-income settlements in five towns in Bangladesh, all implemented under the UN/DFID-supported UPPR project. Most of these used twin pits. We found none that were being used alternately, as required by the design. In many cases, one of the pits had been built over so that it was now impossible to empty it. Few users knew how the system was meant to work. One of the factors here is that the design seems to be based on the assumption that households themselves will empty the pits but the reality is that even low-income households employ sweepers for this task and so there is little incentive for households to ensure that the contents are safe to handle when removed. I would not say that the UPPR twin pit toilets were a failure, the second pit provided additional capacity. However, given that they were not being used as intended, it would probably have been better to provide one larger pit.

There was a much earlier attempt to introduce twin vault toilets in Botswana - through a World Bank-funded project in the late 1970s. This used direct drop vaults rather than pour-flush but it was not a success. There are probably lessons to be learnt for other parts of Southern Africa. I do not have direct experience of this initiative but I get the impression that people wanted water-flushed sanitation. Official policy in Botswana is to provide sewers although I do not think that it makes a lot of sense for low-density settlements with limited access to water. When I was there about 3 years ago, some people argued that sewered sanitation was necessary because of the risk of nitrate contamination of groundwater but I would have thought that the health and environmental risks from potentially non-functioning sewers would be much higher.

Dave Still's figure of 23 litres of sludge accumulation per year is similar to figures obtained in Indonesia by Freya Mills, working with WSP. These gave median and mean accumulation rates of 13 and 25 litres per person per year, the difference explained by the fact that a relatively small number of pits had very high apparent accumulation rates. These figures, taken with some of the higher accumulation rates recorded in Southern Africa for direct drop pits suggest that the main need here is to keep extraneous solids out of the pit or tank. Failure to do this results in both an increase in the sludge accumulation rate and greatly increased difficulties in desludging pits. My view is that, where water is available, the benefits of providing a pour-flush pan (less odour, lower sludge accumulation and easier emptying) will compensate for the increased water need but this will obviously depend on local factors.

The toilets built in Zambia in the 1960s were indeed aqua privies rather than pour-flush toilets. They discharged to 100mm and 150mm diameter sewers. Marcus Vines and Bob Reed from WEDC reported on their performance in 1991. I have hard copies of some of the reports. At that time, around 30 years after they had been built, they were working well. They had never been desludged and this may have been due to a fairly high use of water for flushing (although this is conjecture I think). The report on Matero in Lusaka records many cases of sewer blockages, mainly due to pieces of cloth, plastic bags, hard paper and corn cobs. Presumably, the problems would have been reduced if pour-flush pans had been used rather than the direct drop aqua-privy arrangement.

I think that is all for now. Let me know if any of this needs further clarification


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Note by moderator: More information about Sulabh and biogas sanitation in China is available in this thread:
forum.susana.org/171-biogas-sanitation-s...-sanitation-in-china

In India Tamil Nadu they started to collect the urine to make Struvite, a useful fertilizer.
See www.thehindu.com/todays-paper/tp-nationa...u/article1467629.ece

It may be worthwhile to verify the results of that initiative.

Dear Arno and everyone,

I have found no cases in the internet of biogas being collected from TPPF (Twin-pit Pour-flush Toilets). Sulabh makes biogas from big community toilet blocks, not from dispersed TPPFs. (If it is possible to collect biogas from TPPF, please post links.)

Here is their discussión of TPPFs, with one of the main advantages being the safe, simple extraction of the decomposed feces after at least about 2 years:
www.sulabhinternational.org/two-pit-system/

I did, however, find another interesting option: the UDTPPF !
Dorothee Spuhler describes a Urine-diverting TPPF, with a divided squat pan
www.sswm.info/sites/default/files/ppts/S...%2520Twin-pits_1.ppt
Also www.sswm.info/content/pour-flush-toilet (I attach a photo from here.)

I would suggest that a UDTPPF could channel the urine directly into a 12-meter-long perforated hose that is buried 10 to 20 cm below the surface of the soil among fruit trees, banana plants, etc. This would:
-- more efficiently recycle the 90% of the nutrients that are found in the urine;
-- reduce by about half the amount of liquid that would carry the fecal contamination toward the groundwater and rivers; and
-- likely create better conditions for the decomposition of the feces.

We have not had trouble with these perforated hoses plugging, but, if that were to happen, there could be two hoses to be used alternately, thus giving time for the blockages to decompose ... a bit like the alternating pits.

Another interesting point that Sulabh mentions is that the soil absorbs the gases produced in the pits.

I think this sort of UDTPPF could have enormous application, where soils are absorbent enough and population density is low enough. It might also be feasible to do this with efficient UD sit-down flush toilets that really do not leak constantly (UDTPFT). A big advantage of Pour-flush is the ease of using greywater from bathing, laundry or dishwashing (preferably without chlorine and non-biodegradable detergents).

Best wishes,
Chris Canaday

Thank you Dave and Arno for you feedback. I'm wondering if there exists a viable market for a toilet/service where once a month (or so) a service provider visits the household and empties a 400 liter tank and all of those in the immediate vicinity. In some respects it would be similar to the Sanergy model but with less frequent emptying.

Thanks Sudhir for the technical reports dealing with the contents of pour flush pits and the impacts of dry pit latrines. The drainage of water is a key aspect to enhancing degradation rates. Too much water on one extreme and too little moisture on the other extreme will tend to restrict degradation, causing increased sludge accumulation.

Dave, the 23 liters of accumulated sludge per person per year for both VIPs and leach pits is an interesting result. That's about a 50% decrease in volume compared to annual production of faeces. I expect most of that decrease is due to compaction and there is very little degradation occurring. What about all that work on EM (effective microorganisms) that was borrowed from the compost people? What about the nitrate levels in these two systems? Keeping things wet and anaerobic could be a way to prevent production of nitrate.

Chris, re TPPF - Sulabh in India is of course the global success story when it comes to implementing this technology and reusing the products. They collect biogas and sludge fertiliser from these systems. Sending a person down the inactive pit to loosen up the digested sludge after a lengthy period of time has its risks if the digestion process has not been completed - there can remain methane and hydrogen sulfide gas. But I believe Sulabh has much experience with the digestion process rendering the inactive pits safe for disposal.

re the leach pits that don't fill up, this was a curious observation made while in China. We saw flush toilets leading to leach pits that had sealed concrete covers - never opened and in use for over 10 years. We asked to have one opened (using chisel and hammer) and observed there was fresh oxidized sludge in the pit about half a meter from the surface and about a half meter depth of sludge. These systems are free from toilet paper or solid waste. They have a gravel base and drain well. Presumably these would flood over if the soil contained clay or if in high ground water conditions. Question is whether they attain some sort of steady state in terms of sludge build up.

Andrew - what you are referring to are conservancy tanks that are sealed and yes this is a viable business if there are regulations that such systems are mandatory and people are willing to pay for the frequent emptying service. We have such systems in rural Sweden where septic tanks with leaching beds are not permitted. Eventually people change these for modern UDDTs to save on costs.

Regards

In the SA rural areas the TPPF is ofcourse possible. For saving water, the pour-flush toilet is much better than the water-closet, especially if grey water can be used as well. After having travelled in SA, I do not think that the double pit pour-flush is feasable in most townships. There is just not enough land available. Because of the large population growth, it is highly likely that soon housing in SA goes to double stories and higher.
In that context it is useful that toilets are constructed on the second floor (storey) with groundfloor sealed digestion and compost containers. By means of designed vehicles these can than be easily emptied. In locations where sewerage is available, the liquid only content can be discharged to the sewer, eventuall by vacuum system. Considering population numbers and densities, new sanitation concepts need to be developed in the townships.

Dear All,

I am surprised that no one is talking on this thread about Twin-pit Pour-flush (TPPF), since by alternating the pits fresh faecal sludge does not have to be dealt with, only relatively safe dry soil, if the pits are big enough and thus the detention time is long enough (maybe 2 years).

Arno, how could a pit be designed such that it never fills?

It was also interesting to see in the video that they made a very open S-curve for the water seal, presumably to allow newspaper to be flushed.

As mentioned in the video, I think it is key to promote the use of greywater (from bathing, dishwashing, etc.) for flushing (but I do not understand how the use of greywater affects the physical design). This reduces water consumption overall and provides treatment to the greywater.

Let us remember that TPPF is only feasible where the soil is absorbent, groundwater is deep, there is no flooding, and wells, streams and rivers are far enough away.

Best wishes,
Chris Canaday

Hi Andrew

We observed a median of about 23 litres of sludge accumulation per user per year with the pour flush toilets we were monitoring. Interestingly that is the same number (give or take) that Kitty Foxon came up with for VIPs once you take out the trash fraction.

But the leach pits were not sealed. If they were sealed then the accumulation rate would be several litres per user per day, so the 400 litres would not last long at all for a normal sized household.

Regards

Dave

Hi All,

I've just read through this topic, had a thought, and am now looking for some input. One of the early postings states that 392 liters of waste and flush water accumulate (worst case) in the storage tank of a closed, pour-flush toilet. I've been working on a manual pump that can pump 400 liters of sludge in less than 6 minutes (depending on shear strength and debris loading). Is the following concept feasible: Pour-flush toilet feeding a 400 liter, sealed tank that needs to be emptied annually. If the tank is properly designed, the pump could be attached, tank emptied, and pump detached, in 10 minutes. The real effort would be getting the extracted waste to an appropriate dump/treatment site. From an economic perspective, is there a realistic business model here?

Cheers,

AW

Hi Arno

Attached is the sludge characterisation study lesson. This was performed by UKZN. Some of your questions are covered in that lesson series. Although it does not have anaerobic / aerobic degrad. tests per se, there are parameters that can be used to determine the degradation and you find it inside the short doc. Enjoy the read.

Simon Lorentz and co also did the leaching study "Investigation into Pollution from On-Site Dry Sanitation Systems". I have also added that in attachment.

Dave Still has done the Sludge Accumulation Rates (SAR) for the toilets - its obviously much longer than VIP latrines. We have some of this data for homes and schools (institutions).

Hi Sudhir
Thanks for this new video about the developing popularity of pour flush systems in SA. If the leach pits are kept free from solid waste it would be useful to know what the rate of sludge build up is and whether they ever need to be emptied. A concern is the content of the leachate and the size and depth of the leaching zone. The latter will depend on the soil characteristics and rainfall plus ground water depth. How much microbiological degradation can take place in a stagnant leach pit? Can we use some of the experience with septic tank technology and dewats systems to improve the design of the leach pit in order to improve leachate quality?

I have seen leach pits in urban areas designed to never be emptied and rightly so the pits never fill up. One might ask are these systems sustainable in high density urban areas where leachate volumes per unit area would be considerable? For rural I think there is a good future as long as ground water wells are kept some distance away.
Regards

New video from Partners in Development:



Through partnership with the Dept of Science & Technology, over 250 units have been installed in Eastern Cape Province, South Africa.