Thank you for the feedback and comments on our AD-Handbook.

Patrick, I’d like to refer to two textbooks, which provide more details on the fundamental principles of anaerobic treatment processes, incl. biochemistry:
- Mata-Alvarez, J. (2003): Biomethanization of the organic fraction of municipal solid wastes. IWA publishing. London
- Palmisano, A.C. & Barlaz, M.A. (1996): Microbiology of solid waste. CRC Press.

Kris, gastight sealing of the digester door was indeed a problem we couldn’t solve within the dry digestion project period in Kumasi, Ghana. The feedstock used was not mixed municipal waste from the landfill, but source separated pure organic solid waste (mainly originating from the market). The reason for low gas yield measured was primarily because of gas leakages rather than due to the waste source. One of the drawbacks of batch systems compared to continuous-fed systems is that several digesters have to run in sequence but partially overlapping in timing to ensure continuous waste treatment and stable gas production. Quite a challenging task in many low and middle income countries. Then the challenge with regular opening and closing of doors (which challenges gastightness) and the risk with gas-air mixtures when going in and out (even with flushing system). I agree that using leachate from digesters at different ‘ages’ in a centralized tank is the best way to have a balanced, high-quality inoculum circulating between the digesters.

Regards, Chris Zurbrugg and Christian (Riu) Lohri

Dear Kris,

I agree with you when you say:

I think the inter-linkages between (organic) solid waste management and a eco-san service chain are pretty obvious, but so far no-one has really taken this up on a larger scale. Municipalities are of course the natural partners, and I think their interest in sanitation would also increase if it was "sold" to them as a combined eco-san / solid waste recycling approach.

In terms of technology, it is in particular the composting or the anaerobic digestion systems where organic solid waste and excreta could be treated together.

As we discussed elsewhere on the forum, one limitation for co-composting is the pathogen content of the faeces which makes e.g. the operators of composting plants hesitant to take in that material as it would give them an added complication compared to just treating organic solid waste that has no faecal pathogens (e.g. see here: forum.susana.org/forum/categories/70-com...-sludgemobile-toilet and here: forum.susana.org/forum/categories/70-com...without-soil-contact).

But in theory I agree with you and wrote about it six years ago in this conference paper from 2008:

Rüd, S., von Münch, E. (2008). Ecological Sanitation Projects from around the World and their Links with the Solid Waste Sector. ORBIT 2008, 13th - 15th Oct. 2008, Wageningen, The Netherlands.
susana.org/lang-en/library?view=ccbktypeitem&type=2&id=904


When I did a quick Google search on this topic now, I came across this interesting factsheet by GIZ about a current project they have in Benin on this. Let's see if we can get the people involved (Alexandra Dubois, Edith Albert, Sofia Garcia-Cortes) to tell us more about this interesting project here on the forum?

(Edit on 21 Aug. 2014: See here for more information on this project:
forum.susana.org/forum/categories/70-com...-from-2012-2014#9566)

Solid Waste Management and Basic Sanitation

A pilot project for the improvement of hygienic conditions through integrated approaches for waste management and basic sanitation in Lokossa, Benin

Together with GIZ PEP (Water and Sanitation Programme, fr. Programme Eau potable et Assainissement) in Benin, the adviso-ry project decided to implement a pilot activity in Lokossa, a city with a population of around 77,000 inhabitants and located in south-western Benin.
The pilot project mainly includes the construction of several EcoSan latrines (ecological sanitation) for semi-communitarian use and a composting site, the collection and co-composting of organic solid waste from the central market and sanitary wastes from the latrines, and awareness raising campaigns.

Structures involved:
City council of Lokossa, Intercommunal group GI-Mono, NGO PROTOS, NGO DCAM Bethesda

Implementation:
September 2012 — August 2014

Specific objectives of the pilot project

• To contribute to the improvement of the waste management and basic sanitation of the City of Lokossa, through innova-tive, ecological and sustainable solutions
• To develop income-generating activities and boost the agri-cultural sector of the region on a long term, through the production of a natural soil conditioner (compost)

Long-term impacts
This project will help boost the agricultural sector while improving the hygiene of the city as well as health and sanitation situation of the population. The valorisation of biodegradable waste and excreta, replacing chemical fertilizers with alternative prod-ucts and reducing the practice of defecation in the open air will contribute to environmental protection and the preservation of public health.
The pilot project will contribute to improve the system of waste collection in the city and will also influence the socio-economic situation by allowing gardeners to improve the quality of their harvests and to sale the natural fertilizers produced at the com-posting site.

The factsheet is available here:
www.giz.de/expertise/downloads/giz2014-e...sanitation-benin.pdf

Is this a bit what you had in mind, Kris?

And do we have any readers from Benin, or working in Benin, on this forum?

Kind regards,
Elisabeth

Not too many responses I see

I think the inter-linkages between (organic) solid waste management and a eco-san service chain are pretty obvious, but so far no-one has really taken this up on a larger scale. Municipalities are of course the natural partners, and I think their interest in sanitation would also increase if it was "sold" to them as a combined eco-san / solid waste recycling approach.

Anaerobic dry-fermentation might play a pretty big role in it, especially if a cheap and easy to install digester like those mentioned above would be available. It's also pretty close to landfill-gas collection on a smaller scale.

Maybe as a general overview about Eco-san and dry-fermentation:
www2.gtz.de/Dokumente/oe44/ecosan/en-eco...ermentation-2002.pdf

Please also see the attached paper on process optimisation (like early flushing) of such dry-batch digesters, which should probably improve biogas production. Recirculating leachate between two or three digesters at different "ages" seems to be really promising and easy to do also.

Sorry for digging up this topic, but I would like to discuss the garage-type dry-batch digesters build out of an discarded shipping container that is mentioned in this book.

I have attached the most recent details I could find on that pilot in Ghana, but it seems that due to unsatisfactory results it has not been followed up on.

However due to the ease of construction/transport and the general availability of these containers (here in Uganda they are even used as part of houses) this is a really great idea and the sealing issues of the door don't seem like a insurmountable problem.

I also think that the choice of dry solid waste from a landfill really distorted the potential for biogas production, leading to the non-satisfactory low yields (although issues in operation and feed pre-treatment might have played a role also).

What I would be mainly interested in is the anaerobic "dry-batch" digestion (the term is a bit misleading as it just uses much less liquid) as a mean to set up a simple decentralized treatment centre for UDDT and latrine pit contents with smaller quantities of organic solid waste. Obviously with the aim to make it more attractive for a sanitation service provider to go and collect vault and pit contents.

Does anyone know of a study that evaluated biogas production from composting toilet/UDDT contents? I imagine that this mixed material digests quite well, and due to the drying process in UDDTs even older material should still have quite a bit of energy in it.

I think this book came in due time! The similar books that I have encountered have all been a bit old, so it is refreshing to see an update.

Some constructive criticism:

- I wish that the book elaborated a bit more on the biochemistry. I realize that the book has a practical focus, but I feel a bit more elaboration and an emphasis on the importance of the biochemistry would be in order. Especially more elaboration on the methanogenesis.

- I also wished there was more information (maybe even a standalone chapter) regarding suitable site-specific conditions. Such as; climatic conditions, hydro(geo)logical conditions, etc.

- I thoroughly enjoyed the case studies! They were very interesting to read.

- I also like that the book specifies that the value of the fertilizer is relative to the soil characteristics. This is in my opinion an often forgotten point.

All in all, I think it is a very good book!

New SANDEC/EAWAG Publication: Anaerobic Digestion of Biowaste in Developing Countries - Practical Information and Case Studies. Free download of online version. www.eawag.ch/forschung/sandec/publikationen/swm/dl/biowaste.pdf
This book compiles existing and recently generated knowledge on AD of urban biowaste
at small and medium scale with special consideration given to the conditions prevailing
in developing countries. Written for actors working in the waste and renewable
energy sector, the book is divided into two parts: Part 1 focuses on practical information
related to the AD supply chain (substrate-, process-, and product chain), and Part 2
presents selected case studies from around the world.