Twin pit pour flush latrines in India - is it timely to call into question the design and operating principles?

  • goeco
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Twin pit pour flush latrines in India - is it timely to call into question the design and operating principles?

Note by moderator: this post used to be in this thread about the "New SATO Connection System for Twin-Pit Pour Flush Latrines"
forum.susana.org/sato-pan-latrine-with-c...-pour-flush-latrines


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I'd like to ask about the specific purpose of these twin chamber pits used in India (see image below). It looks like wet composting whereby the intent is for lack of soakage to be overcome by rotating when buildup of liquid/sludge fills one side? Can the design be improved?

My initial observation is that capacity comes at a cost and for sanitary "compost" the resting period needs to be sufficient for destruction of pathogens, in particular helminths. The faster the pit fills the greater the required capacity for a sufficient resting period, which increases cost.

How can this be improved? Enhancing decomposition with earthworms would mean the pits fill slower, increasing the rotation period. However, any excess of liquids would be unfavourable for digestion - adequate drainage being the key to rapid digestion. Seems to me that if those pits were wider and shallower, then you'd have the same capacity but an increased soil surface area and therefore improved percolation... to directly improve digestion and extend rotation length or lower cost. If the environment were optimised for worms, the solids would get digested rapidly, allowing rotation every 3-5 years instead of 1-2 years, therefore producing a sanitary compost.

Also, by lining your pits with shadecloth the soil surface would not become "sealed" with sludge because the bricks would isolate the solids from the soil and only liquid would filter through to the wall soil surface for soakage. What was the space between the bricks and clay wall filled with? I guess I'm just wondering if there is active work on optimising this system for rapid digestion?

The SATO diverter is a fantastic innovation for twin pits, I'd like to see systems in place that really take advantage of it so sludge becomes a thing of the past. Now that would be more than just a fantastic innovation, that would be nothing short of revolutionary.

cheers
Dean

Dean Satchell, M For. Sc.
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  • mchalej
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Re: New SATO Connection System for Twin-Pit Pour Flush Latrines

Dear Dean,

Twin Pit Pour Flush Latrines have been utilized in India for at least 40 years. Roughly 50,000 per day are currently being constructed under the Swachh Bharat Mission. Our intention was to develop something that would eliminate some common breakdowns and enable them to be implemented and operated more effectively, not to change the composting mechanism or basic operating principle itself.

For background information on twin pit latrine construction and composting process please see reports from World Bank and India Ministry of Drinking Water and Sanitation below.

World Bank Report
India MDWS

Thanks,

Jim
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Re: New SATO Connection System for Twin-Pit Pour Flush Latrines

Given the number of these being constructed, seems timely to call into question the design and operating principles. To me they look less than adequate.

The diverter itself is great... but it leaves the rest of the technology dragging behind. I see no improvement in the 2012 manual to the 1984 twin pit design... Perhaps because the design is well proven? Nevertheless I'll call into question aspects of its operation.

The design is based on some very simple rules of thumb for filling each pit with sludge within 3 years:
  1. Minimum pit capacity of 250 litres per person;
  2. 30 litres infiltration per day per cubic metre of pit.

Noting again that each pit is designed for a maximum of 3 years buildup of solids, the operation manual states that:

"after 2 years pathogen free manure (degraded sludge) is dug out and used for horticulture and agriculture purposes"


I am yet to be convinced that degraded sludge is pathogen free (as claimed) after two years. Helminths take a long time to die off to levels safe for agricultural use and this varies according to medium (e.g. very slow in soil), so until solid data is available for degraded sludge, in my view conservative design should aim for 5 year rotations as being "safe".

So... what if the same rules of thumb were applied as above... but a 5 or more year rotation could be achieved?

One of the key flaws of the MDWS twin-pit system is that drainage is unnecessarily impeded. This is because:
  1. As sludge levels rise, more brick honeycomb would become blocked, proportionally reducing percolation surface area. A half-filled pit has half the surface area for percolation. Not good design.
  2. The cavity between the brick honeycomb and soil wall is back-filled with soil. Thus, if the 1m deep hole has a diameter of 1.4m and the brick honeycomb wall has a diameter of 1.0m, if the cavity between them is backfilled with soil this gives a wall soil surface area available for percolation of 1.25 m2 (assuming bricks take up 60% of the surface area). If instead the 0.2 m gap were backfilled with coarse drainage material such as rubble, the soil surface area available for percolation would increase to 4.4 m2. Small change, big difference.

Add handwash water to the mix and soakage becomes even more important...

I wonder, have these design guidelines not been questioned in 30 years? Sure, one does not really have to worry about impeded drainage if the rotation length is accepted as 2 years. However, capacity comes at a cost and improvements to the design by creating an environment for worms to thrive would overcome the slow decomposition of sludge.

I'd note that the tiger toilet people have become active in India because they know decomposition is vastly improved compared with pits, by simply providing an environment suitable for worms to digest solids. However, they haven't achieved sanitary production of manure either because their product uses a primitive single chamber design.

Simple adjustments to pit design would render a rapid digestion aerobic environment. These would include:
  • a vent;
  • wider more shallow pits;
  • Coarse drainage media between the brick honeycomb structure and soil wall.


The result would be the responsible step forward in sustainable sanitation, to ensure cost is minimised and safety is assured.

cheers

Dean

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  • alexandra85
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Re: New SATO Connection System for Twin-Pit Pour Flush Latrines

Dear colleagues

For those who are not familiar with the Indian context (including myself), can someone explain in few words what is the principle of a twin pit pour flush latrine?
Thanks

Alexandra Dubois

Technical Advisor
Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH
Nairobi, Kenya
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Re: Twin pit pour flush latrines in India - is it timely to call into question the design and operating principles?

Hi Alexandra,

the principle is very simple... Two pits. The toilet flushes into one pit until it fills with sludge, then the diverter rotates the flush into the other pit. By the time that side is full, the other side has reduced to manure and is ready to be dug out. Same principles as twin vermicomposting, just slower. The point I am making is that the process is very slow, being mostly anaerobic. This means increased capacity and/or reduced rotation length. Sludge decomposes very very slowly in a pit, although perhaps faster once influent flow is diverted away and pit water table drops, allowing earthworms and other organisms to digest the contents.

cheers
Dean

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Re: Twin pit pour flush latrines in India - is it timely to call into question the design and operating principles?

In my role as moderator:

Hi Dean,

Just letting you know that I have branched off this thread into a new thread about twin pit pour flush latrines in India, as it was deviating from the original topic of SaTo pans. I have made links in both directions, i.e. from the end of the other thread to here, and from the start of this thread to the other thread so that people can find their way around.

You've probably seen it but in case not: the topic of twin pit pour flush latrines in India and South Africa has been actively (and heatedly) discussed in a couple of previous threads on the forum, see here:
"Do pour flush toilets produce a pathogen free sludge (twin pit alternating)?"
forum.susana.org/251-pit-latrines-e-g-si...twin-pit-alternating

And here:
"Are pour flush toilets a good idea for (South) Africa?"
forum.susana.org/comparisons-of-various-...dea-for-south-africa

You're very welcome to bring those older threads back to the forefront if there are aspects that should still be added, questioned or commented on, or even just repeated (as we always get new people on the forum, or even forget ourselves).

It's certainly a "hot", controversial topic, especially in India but I assume also in e.g. Bangladesh.

Regards,
Elisabeth

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  • JKMakowka
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Re: Twin pit pour flush latrines in India - is it timely to call into question the design and operating principles?

As much as I agree that the design could be improved significantly, it is ultimately all about scale. This design is well known by many local masons and easy to implement.

However besides the somewhat questionable claims about "safeness" of desludging, the big elephant in the room is that they will probably never get desludged anyways due to caste issues, as outlined in this recent interview: www.firstpost.com/india/casteism-is-the-...-coffey-3925663.html

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Re: Twin pit pour flush latrines in India - is it timely to call into question the design and operating principles?

Dear Alexandra and everyone,

I include a summary of Twin-pit Pour-flush (TPPF) toilets in my Suggestions for Sustainable Sanitation, which is attached (and includes various other beneficial systems that can be applied in many places, including Kenya, where you are working).

One improvement on this design that I mention in that document and that seems to almost never be applied is urine diversion. This would allow for:
-- more efficient recycling of the abundant nutrients found in the urine via the biologically active topsoil and the quick assimilation by plants;
-- less water pushing fecal pathogens toward the groundwater;
-- more aerobic conditions for the decomposition of feces;
-- less ammonia that may otherwise inhibit to some degree the organisms that decompose the feces (including worms).

Given the added volume of flush water, this is all the more reason to distribute the urine via perforated hoses buried 10 cm under the surface of the soil among fruit trees, etc., such that it disappears immediately, automatically and productively, without anyone having to deal with it. This dilution would also prevent overdosing with nitrogen and productively take advantage of this water for irrigation. Optimally, greywater from bathing or dishwashing can be used for flushing.

There is more info on TPPF here:
www.sswm.info/content/twin-pits-pour-flush

Best wishes,
Chris Canaday

Conservation Biologist and EcoSan Promoter
Omaere Ethnobotanical Park
Puyo, Pastaza, Ecuador, South America
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  • pkjha
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Re: New SATO Connection System for Twin-Pit Pour Flush Latrines

Dear Dean and Everyone
I would like to add a few clarifications.
Depth and dia of a twin pit for 5 users of toilet and for the soil having percolation rate of 30l/m2 , is 1 m each. Even a pit is filled by half, there is adequate space for percolation on incoming liquid. Simple calculation is as under.
For 5 users of toilet having Indian pan with 20 mm water- seal, 10-15 liters of water is required for flushing human wastes. However for safer side we can take 25 liters, as some people might use more water for cleaning.
For 25 litres water, area required for leaching would be 20 / 30 = 0.66 m2 (infiltration rate 30l / m2 per day is taken on a safer side). ( In case of 15 litres such area would be 0.5 m2 only.
Infiltration depth = Area required / circumference of pit
= 0.66 / 3.14 = 0.21 m
In addition, there is filtration through the bottom of the pit. Area of the bottom of the pit of 1 m dia would be 0.78 m2.
Depth of 0.21 m is added in total depth of a pit, in addition to total sludge accumulated in 3 years of retention time of a pit. Therefore, volume and circumference of a pit is sufficient for 5 users of toilets.
The problem is, people use same design for pit even for high water table areas and low percolation rate. In such case there is chance of filling pits earlier and survival of pathogens. In fact, such toilet is not suitable for high water table areas, there is chance of contamination of ground water.
There is no aerobic condition inside pit. It is completely anaerobic.
More design criteria of a twin pit toilet is available in the book Technical options for on-site sanitation, released by the Ministry of Drinking water and Sanitation, Government of India in 2012. The updated version of the draft version of the book is available on the site ( www.mdws.gov.in ) of the book. Both the books were written by me while working with the Ministry.
Manure is completely free from bacterial and helminthes pathogen after taking it out after 3 years. It has been tested through a NABL accredited laboratory in Delhi.
For the growth of Earthworms, it is quite difficult to maintain suitable environmental condition inside twin pit, as there is complete anaerobic condition.
To changeover the twin pit when one is filled, requires manual handling to block one incoming pipe and open another. SaTo design is very suitable as it does not require such manual handling block/ unblock pipe.
Regards
Pawan

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Re: New SATO Connection System for Twin-Pit Pour Flush Latrines

Hello Everyone,

I've been following the forum for a while, but since it is my first post here I will first say hello.

I wanted to add to the points raised by Chris and Dean as we have recently done a Life Cycle Assessment study on twin pit latrines in India. Twin pit latrines are sometimes referred to as "sustainable" decentralised sanitation solutions. However, we found that if every household in India who currently does not have access to improved sanitation would be provided with twin pit latrine, this could result in a significant increase of greenhouse gas emissions in India. This is mainly due to methane emissions. Introducing urine diversion like Chris mentions would significantly reduce negative environmental impacts while providing quite a significant portion of nutrients for Indian agriculture

washdev.iwaponline.com/content/early/201.../22/washdev.2017.005

The challenge would be to convince people to use these urine diversion systems as it would require some behaviour change.

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Re: New SATO Connection System for Twin-Pit Pour Flush Latrines

Dear Michal
Emission of methane is from all toilets based on anaerobic condition like Septic tanks, single or two pit toilet, Shankar Balram toilet, ABR, Anaerobic filter, and Ecosan Toilet. It is not only Twin pit toilet where methane is formed.
In case of twin pit toilet, there is no vent pipe and produced gases are not directly emitted in atmosphere. Such gases are diffused in soil through honeycombs.
Urine diversion toilet has definite advantages as urine is used as source of plant macro-nutrients for agriculture. However, It is not clear how it significantly reduces negative environmental impacts? What are such impacts?

pawan

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Re: New SATO Connection System for Twin-Pit Pour Flush Latrines

Hello Pawan,

It is absolutely true. In our study, we looked at the twin pit system as a case study but this concerns all simple anaerobic toilet systems, including single pit latrine. I am not sure methane can be sequestered in the ground by the lack of ventilating pipe. My worry is that it is escaping somewhere. It would be good to verify the estimates with direct measurements.

As for why urine separation reduces methane emissions: urine contains about 40% of the total carbon in our excreta, diverting it reduces the load of carbon going into the pit and being converted into methane through anaerobic digestion. Urine also contains the great majority of nitrogen in the excreta which can be used in agriculture replacing fertilisers.

For details on data and assumptions please have a look into our paper.

Best wishes,
Michal

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