hi Dean,

for a possible design of a double-vault vermi-composting toilet, I try to do some calculations for the required size of the digester basket / toilet vault.

I have based the calculation on the build-up rates which Carl is using for the CompostEra which you find here www.compostera.eu/longterm.html

I attach an EXCEL where I have transformed the info from CompostEra into table of filling rates over 30 years, from which I have used the filling rate for 5 years (~150%) to calculate the required volume for the digester basket / toilet vault.

Can you please have a look at it and let me know how much you agree or where you would calculate differently?

Thanks and ciao
Hajo

Dear Peter,

One question for clarification: in the text you mention ‘urine diversion’ in the picture it is not shown for the pit latrine!?!

Your design is definitely an improvement over traditional pit latrines (I assume you also have a fly trap on it) with the lined pit and the proper access for emptying.

Nevertheless, I don’t like so much the idea that old and new faeces are not clearly separated with the old ones dried for longer period and I don’t like the labourer entering the pit.

My alternative proposal would somehow reflect the ‘double pit VIP (S4) in the EAWAG Compendium ( www.eawag.ch/en/department/sandec/publications/compendium ) : build two smaller pits (1.2m deep), but under one superstructure, the pedestal to be changed from one to the other pit (like on S7), with two emptying slabs at the back and provide the labourer with long handle rakes and shovels so that the pits can be emptied from above. Size of pits should be such that one fills in 2 years, then rests for 2 years before emptying.

Of course the system should only be applied in areas with low ground water table and without fear of flooding.

As I write this it comes to my mind that we could upgrade this system possibly to a vermi-composting system. Advantage being that the solid build-up is further reduced extending the emptying cycle considerably and the quality of the composted faeces is also improved. The moisture which the worms require can come from the hand-washing water. If you are interested in such an upgrade of your ‘Otji-Pit latrine’ let us know and we will discuss it with Dean..

Ciao
Hajo

Hi Dean,

I fully agree with you that our ultimate aim should be the re-use of ‘humanure’. But under the conditions we find in Africa (low priority of sanitation, low income, low appreciation of environmental protection, …) our first aim may be the provision of sustainable sanitation systems (as they did with 80,000 UDDTs in eThekwini, now they look into reuse). In doing so re-use does not disappear from the agenda. I am highly interested to implement pour-flush twin-pit vermi-composting (PFTPVC) systems as we discussed and would love if we can convince people of them at large scale.

But being practical may also mean, that we have to introduce sustainable service chains for pit latrines even if we don’t like so but we have to accept that we cannot replace thousands of pit latrines with PFTPVC systems in a short time, especially if the owners/users are asked to finance them: spending a lot of money on a sanitation system (!), a system they don’t know whether it works (!) and where they are sceptical about the aspect of reuse (!). These are many ‘obstacles’ to overcome, which takes time.

Can somebody from the forum comment on the assumption by Dean that vermi-compost after 3-5 years of resting is free of helminth eggs? I am a bit sceptical but would like if it is correct.

The re-use of the black-water is out if we use twin-pits because it percolates into underground (as from any pit latrine, soak-away, soak-pit). But of course if we replace the twin-pits with double-chamber digester (of concrete and blocks) we can recover the black-water for further treatment and re-use. How could secondary treatment possibly look like?

Ciao
Hajo

Dear Dean,

I agree with you, that reuse of human waste should not fall out of focus. But in the peri urban areas of Namibia it looks similar to Christoph’s picture from Lima - here grows nothing. It is the farmers which would be interested in fertilizer but they are far away. Therefore I was looking into options of concentrating N in order to bring transport costs down. Storing would also be needed. The fertilizer may should than again be diluted from the farmer onside before application.

What I worry now is your indication that N is volatile? Can you elaborate a bit more on this? I thought the UDDT’s would concentrate N (even too much) at the spot where infiltration take place? Methods of concentrating urine known to me are freezing or evaporating. Freezing would be too expensive in Namibia but evaporating could work. But both options require collecting urine before concentration can takes place. Here the costs occur even before processing has started.

Greetings,
Peter

Dear Hajo,

Thank you for racing the topic of pit latrines. Emptying pit latrines is in Namibia not yet a common practise. But I assume that this will come together with grows of our peri urban settlements. From the year 2020 it is expected that half of our population lives in towns.

A while ago we have designed a pit latrine mainly for rural area where professional maintenance of Otji Toilets is difficult to be organized due to transport costs. The super structure as well as the chamber is very similar to the Otji Toilet and this pit latrine is again produced as a prefabricated Self-Build-Kit. The differences to the Otji Toilet are:

1. The pit latrine doesn’t have collecting drums but have therefore a deeper chamber (1,5 or 1,9m deep = 2-3 m3).
2. The pit latrine has a second prefabricated floor slap outside the toilet hut covering the hole where normally the Otji Toilet’s lidbox is located. On top of this floor slap is a huge ventilation pipe installed (2.4m long and 220 mm wide).

The attached sketch illustrates the two systems with differences marked in red. Once the pit latrine is full it can be emptied relatively easy by removing the vent pipe and lifting up the second floor slap outside the hut. This provides an opening of 65cm x 77cm where a labourer can enter to shovel it out. You are right with your observation that the content various significant from pit latrine to pit latrine. But with urine diversion in place the content should not have too much moisture. It is also to note that the emptying process does not start from underneath the pedestal but from behind. Means the labourer has the chance to avoid fresh material by pushing it away and excavating only older material.

However, this design has not yet picked up on the marked since emptying pit latrines is here not popular yet. But whenever it will come to numbers than my company would be ready to offer this service of emptying those pits . Emptying the pit from a household toilet is maybe needed once in a decade.

Greetings,
Peter

Hi Hajo,

From this discussion it does appear that the focus on UDDT's is on sanitation rather than re-use of human waste. It doesn't impress me when I hear of solids being dried, collected and dumped. Neither does collecting urine or concentrating/depositing it in one place. Urine should never be concentrated because of the volatility of N, it should be dispersed. My contributions to this topic were initiated by a long term interest in sustainable cycling of nutrients. Vermicomposting reduces the solids volume multiple-fold and these can therefore be left to rest in-situ (in two chamber system) for as long as it takes to be free of helminth ova (I'm assuming 3-5 years). The product is then "ready to go" for use in crops. Because worms don't like pure urine, the input can be diluted by using handwash water for the next flush as per the GSAP model... wasn't it less than 500 ml per flush in their microflush... certainly no more than needed for handwash, and therefore no more than a UDDT if you take into account handwash water. The fact that the water goes through as blackwater means it is contaminated by pathogens, but surely a decent soak-away is affordable given that apparently people in both Tanzania and Namibia are happy to choose the (OMG!) soak-pit option (as per Hajo's definition above) which requires a large soakage area for blackwater plus costly extraction of sludge. If a vermicomposting chamber were lower cost than a septic tank (as per Hajo's proposal) then flush and reuse goes hand in hand with sanitation, neither needs to be given priority. If initial capital cost for vermicomposting digester/soak-away is higher than UDDT, I'd ask at what point in time would UDDT become more expensive given the additional costs of maintenance? Economics are an important consideration. Anyway, I'd much rather be talking about secondary treatment than soak-aways because if I can treat the blackwater so I can discharge to the soil surface then helminth-free nutrient-rich irrigation water should hold value in any community that values food, especially one with scarce water resources.

Dear all,

I started the thread with the intention to discuss possible sustainable on-site sanitation systems for use in peri-/urban areas in African towns.

We had so far intensive discussions about UDDTs in various forms. I think we agreed that the urban (large scale) application of UDDTs requires a collection service. I thought originally of a single container with a monthly collection cycle but the samples from Peru (Christoph) and Namibia (Peter) indicate that a 3-month or even 12-month cycle is possible with double containers and will surely be more economical. Urine can either be infiltrated in the soil near the UDDT or be collected at the same cycle as the faeces using larger tanks. We have not so much discussed secondary treatment and possible reuse of the outputs. But a sustainable sanitation service chain should be our first priority (as eThekwini did with their UDDTs) as long as we can safely dispose the outputs.

We have also discussed in length the use of pour flush in connection with twin pits possibly even as vermi-composting system. We are aware that this system asks for water availability in reasonable distance from the household. The infiltration of effluent (blackwater) requires low ground water table, soil permeability, and no flood prone area. Keeping the pit moist but not wet is especially important for the vermi-composting system. The pits need to be emptied only every 2 to 5 years but in the urban context again it should be done by a service provider not by the user.

Pit latrines (as improved or VIP) are quite common also in peri-urban areas. I like to start a discussion about their usefulness/sustainability in this context especially since we have a number of threads where we discuss how pit latrines can be emptied (gulper, rammer) and how pit emptiers can be protected.

I do not question the application of pit latrines in rural areas or rural villages where there is enough space to abandon a full pit latrine and dig a new pit. That is what pit latrines have been designed for!

But they have not been designed for emptying and that happens in peri-urban areas where there is not enough space to dig new pits when the old one is full. Problems faced when trying to empty a latrine pit are manifold:

1. A vacuum truck (honey sucker) cannot reach the pit due to congestion of buildings or poor road access;

2. The slab of a pit latrine has normally only one access, the squatting hole, which is too small for anything bigger than a suction pipe;

3. If the pit can be emptied there is always the danger of soil collapse as most pits are unlined;

4. The pit is often used as solid waste dump and all the debris hinders emptying in whatever form;

5. The sludge in the pit varies considerably in consistency from fluid to mud from top to bottom and from pit to pit depending on GW levels, surface water running in, use as bathroom. This inconsistency makes it impossible to have one emptying technique – at least we have not yet developed it;

6. Due to the technical problems in pit emptying no private enterprise has yet considered it as a viable business opportunity;

7. Public service providers (W&S utilities) have no obligation and no interest (except eThekwini in South Africa) to provide a service chain (emptying, transport, treatment, disposal/reuse) for pit latrines;

8. Thus pit latrine users employ informal service providers who have no proper equipment, have no protective clothing and are asked to provide the job as cheap as possible,

9. Which results in the FS being dumped somewhere in the environment, in gutters, streams as no authority is obliged to supervise the informal services.

Can we agree that pit latrines are not a sustainable sanitation solution for peri-urban areas where they need to be emptied? I know that we have thousands of pit latrines in African towns and they will be with us for the next 20-40 years. And we have somehow to empty them and we have to protect the people who do the emptying. So let us look into that we improve the service chains for those who still have to depend on pit latrines and let us also ensure that the FS is safely processed. But let us not recommend building more pit latrine as a sustainable solution in the peri-/urban context.

Ciao
Hajo

Dear Hajo,

Thank you for your feedback and the useful link . Very interesting the option to contain nitrate at that spot where it is filtrated into the ground. I had a similar idea - but yet not implemented. I was searching for an ideal material, to be deposited into the urine filtration hole which could be recovered later and used as fertilizer after absorbing a significant concentration of nitrate. I was hoping that such a recovery could be necessary not more often than once a year with an excavation of less than 0,1 m3 needed. This material should then be a concentrated fertilizer? Concentration should be the key to make reuse cost efficient.

Unfortunately we have not managed to conduct a research on nitrate contamination via UDDT. However, I think quantity of liquid plays a major role. Therefore I cannot see it as the same as for septic/soak-away or pit latrines. I think that contamination of ground with nitrate over an area of 1m2 or maybe 2m2 per household is acceptable even so, that at this very spot vegetation will be permitted. The main point is that this contamination is limited to a specific area and does not spread out due to lack of transportation. The less liquid the less risk.

There is also another research which I would find extremely interesting to be done (or was that maybe done somewhere?). I would like to see investigated the concentration of contamination of pedestals and toilet seats comparing UDDT’s with wet systems. My theory is that an UDDT pedestal is basically always dry due to ongoing ventilation and does therefore not provide living conditions for pathogens. In the contrary, the pedestal of a wet system is not ventilated but is flushed all the time, providing regularly the much needed moisture for pathogens. I even would assume that a not so well cleaned pedestal of an UDDT is maybe less contaminated then a clean appearing flush pedestal.

About the exchange of drums at Otji Toilets. Our chamber is big enough to host up to four drums (1,4m x 0.8m x 1,1m deep). But usually we have only two drums in. Yes, the empty drum is lifted over the full drum inside the chamber, as there is enough space. So we do not need to lift out the fresh full drum. At some toilets with many users we place three or four empty drums in the chamber, to reduce costs of maintenance. Once a drum is full we only need to exchange with another – sometimes the bar owner is doing this himself. Only once all drums are full we come with the pick-up to remove all of them – in those cases they are relatively fresh faeces. But please take note that this is the exception since 98% of the Otji Toilets are at individual households with the 12 month turns. School toilets are getting serviced in holidays, three time annually (trimester).

Yes, our “composting process” is indeed not really composting. It is whatever nature is doing with a huge mountain of mixture from faeces and soil (soil comes in when the bulldozers are moving it). However, I trust that nature will find its way to break it down over the time . The three years have been indicated earlier to me as maximum surviving period for pathogens and worms.

Greetings,
Peter

Dear Peter,

To put the price in perspective: the 700 TSH/m3 is the price in Moshi where most of the water is gravitated from springs and without much treatment (only chlorination) into town. In places where pumping and more treatment are required the price will be higher (the highest being DAR at about 1,200 TSH/m3). These prices mostly cover O&M only with minor contributions to depreciation in some towns.

With regard to your many technical questions, I want to emphasise that you have more practical experience than I do. I will try to answer some and hope colleagues will jump in and correct or amend where necessary.

Where a pour-/flush toilet is connected directly to a pit, I would like to call it ‘soak-pit’ (similar to a latrine pit receiving fluids and faeces) and different from a ‘soak-away’ which comes after a septic tank and only receives effluent/fluids (with minor sediments).

Yes, I think you are right that an increase of flush water increases the risk of ground water contamination but of course also the type of soil and the level of GW have a major bearing on the risk. We had a lengthy discussion on this but I cannot remember the thread, maybe Elisabeth can help .. .

I would like to have more information on the risk of nitrate contamination of GW by extensive urine infiltration. You call it ‘minimal’ but was there any ‘research’ done to justify the statement? Here ( forum.susana.org/forum/categories/106-us...or-south-africa#2808 ) you find a discussion on the topic which indicates that the risk (urine soaked away from UDDT) is not a specific risk of the technology but just the same as for septic/soak-away or pit latrine. What about, if we do not soak into the subsoil but flush (1:4, using handwashing water) into a banana plant for immediate reuse of the nutrients?

Quick question regarding the exchange of drums: a new empty drum can be ‘inserted’ under the pedestal without removing the other full drum which has just been moved ‘backwards’ (to the ‘resting’ place), i.e. the full drum has not to be lifted up and lifted down again before it is finally removed 6 months later?

The solid waste in the drums is surely a nuisance but once the stuff is ’composted’ should be easier to sieve out. I am not sure about the hygienic condition of the faeces after 3 years of resting. As far as I know ascari eggs may survive that easily since you seem not to have a proper composting process raising the temperature beyond 50°C (or whatever is required) for a specific period. I guess your ‘composting’ is a rather uncontrolled process of aerobic and anaerobic decomposition, isn’t it?

I am a bit sceptical that the ‘flood control’ of the drums may really work. But since you say that you ‘discovered an interesting side effect’ it must have worked, or? The drums were lifted by the users or by the service provider?

Ciao
Hajo

Dear Ross,

Thank you for sharing some photos of your UDS-Pedestal with us. Interesting is your manual device for preventing unpleasant view into the chamber. In Namibia it was also sometimes raised this question, on how to avoid visibility to faeces. What I did was reducing light incidence. In my own house I have an indoor Otji Toilet ventilated by an electrical fan. It works well, but the two bathroom windows have given that much light, so a full view into the chamber was provided (photo 1). Since both windows are relatively high placed I decided to install a small “roof” over the pedestal (photo 2). But the shading was not enough to reduce visibility. Finally I painted the area around the pedestal with dark colour (photo 3). This then did brought the impact I was looking for. The combination of shading and painting dark had reduced the light incidence sufficiently. Now when I am looking down I see just darkness. Off course, the depth of the chamber and the filling level of the drums are also playing a role.

Greetings,
Peter

Dear Hajo,

Thank you for your feedback. Wow - water in Namibia costs about 10 times more than in Tanzania! Nevertheless we meanwhile also get to know those makeshift full flush toilets feeding sewage straight into a soakaway. As this soakaways (for the reason of costs saving) are usually even too small designed they used to overflow quickly, creating beautiful playgrounds for the children of the neighbourhood. Still; our councillors promoting this system even so that our Municipality does not have a vacuum truck.

Technically I think a full flush toilet connected to a soakaway must be bearing the highest possible risk for ground water? This risk must be even higher than from pit-latrine or pour flush. As I understand (please correct me) that pathogens are mainly located in faeces while liquid serves for transportation – if available. The more liquid is available the more transportation is provided reaching deeper grounds. The less liquid is available the less risk for ground water contamination is given.
The Otji Toilet diverts 80% of the urine which is infiltrated into the ground. At the Otji Toilet is no water involved, apart from toilet cleaning; Namibians are wipers. The urine is acidly and does therefore not sustain significant amounts of pathogens. I am also not aware of a case where the little urine of one simple household Otji Toilet (5 – 10 persons) would have been enough to exceed the capacity of our soakaway; even so it is small designed (300 x 300 x 500mm deep). The input from individual household members is in fact minim. And that doesn’t change with increased number of units, as each unit have its own soakaway. However, we have to admit that the soil at this particular spot where the infiltration takes place is finally somehow over fertilized (max 1m3). But the risk of ground water contamination is still minim.

Problems with insufficient capacity of a soakaway for urine we have got with Toilets where bars are attached to. In some cases bars in our peri urban area having 200 customers almost every night, all these people using one or sometimes two Otji Toilets. Therefore for bars as well as for schools we recommend the installation of external urinals. There we use conventional urinals, but do not connect them to water. Those public facilities anyway must get cleaned and flushed manually once a day from a cleaner. That is enough to avoid production of ammonia out of urine since this would take two days. Also our soakaway for public facilities is of a bigger design. But even faeces are used to be more at those bars. At some bars we must change the drums on a monthly base, since the whole neighbourhood is using them due to lack of own toilets. However, in these cases the faeces did not have enough time to dry nor to compost. So we have to remove relatively fresh faeces. To avoid visibility of fresh faeces and to reduce odour we then cover the drums with a lid before further transportation. However, we are proud that an Otji Toilet can manage to serve daily 200 people over many years without failing in function also we do not promote public toilets.

Attached are three photos showing how we remove the drums.
1. A safety bolt must be removed with a spanner before the lid can be opened.
2. With two custom-made steel hooks we lift out the full drum which was resting under the hot lid. Weight depends on filling level and moisture level. However, two people can always manage. A team of four services about 40 Otji Toilets per day.
3. From there the drum comes on a pick-up for transportation towards the deposit. An empty drum replaces it inside the chamber, but rotated with the other full drum which was up to now under the pedestal. At the deposit bulldozers push huge quantities of faeces on mountains, leaving them resting for years. After three years (please correct me) no pathogens should remain.

A challenging aspect is the input of foreign objects i.e. household rubbish. On average about 20% of the drums content is plastic bags (used for wiping), bottles (plastic and glass), nappies but even shoes and t-shirts can be found. However, as simple as the Otji Toilet is, this doesn’t harm the functionality. But it difficult the potential later use of degraded faeces.

About your question of the remaining 20% of urine. This indeed falls into the drum together with the faeces. From there about half (10%) is evaporating due to ongoing ventilation. We use a 220mm vent pipe (rolled from 0,5mm galvanized metal sheet) which creates much more heat than the usual 110mm PVC pipe. The remaining 10% urine does indeed filtrates into the ground within the chamber. To facilitate filtration we have placed the drums on a perforated concrete plate 10cm above the ground. This liquid should be considered as problematic, as it has gone in contact with faeces and could therefore contain pathogens. However, the limited quantity can’t provide significant transportation to bring those pathogens far away from the toilet. For an individual Otji Toilet we can assume less than 1 litre/day infiltration of urine which has got in contact with faeces (given 5 users x 1,5 litre = 7.5 litre; 10% = 0.75 litre). When we compare this with Pit-latrines we have 10 times more problematic liquid released multiplying the radius of potential contamination. Pour flush connected to a soakaway provides maybe 50 times more liquid. However, a full flush toilet connected to a soakaway, releases about 200 times more problematic liquid then an Otji Toilet.

Recently we discovered an interesting side effect of collecting faeces in removable drums. Even so, that Namibia is a dry country, we have experienced several times flooding in the last years. If an Otji Toilet stands in an area which is about to be flooded, both drums can be removed before the water enters the chamber. Once the water is gone and the chamber has dried up the drums can be put in again and the toilets is ready for use. With Otji Toilets the risk of water contamination during flooding can be managed.

Greetings,
Peter

Dear Christoph,

I can understand your frustration but you must recall that such fundamental changes take time. I read on Wikipedia that it took 20 years to make the Germans accept the use of safety belts when driving – and they were only accepted after penalties were introduced.

Centralised sewer systems with WWTP have become state-of-the-art technology over 150 years. ‘Older’ W&S engineers have hardly learned anything else, only younger ones may know about alternative sanitation technologies if they became interested in them. Politicians, public service administrators and the general public in Europe and North America have little knowledge of any (alternative) sanitation. And what we use, serves as example for decision makers (and population) in the developing countries. All these factors produce a high moment of inertia against change. That is why I have those citations under my posts, always reminding us of our immense task.

Thank you for the link to your publication of 2007 with the detailed calculations for a large scale UDDT service chain for 10,000 households. Since that is the number I had in mind when thinking about on-site sanitation chains in Moshi, do you have an EXCEL worksheet which contains all the calculation which you present in the publication and would you let me have a copy as template?

I do not know Spanish thus do not understand the presentation of 2016 which you attached but the bench dbl vault UDDTs with the entire colourful tiles look nice. I am surprised by the very high collection costs of the single vault UDDT. Do you have an EXCEL sheet of the cost calculations which stand behind that graph and would let me have it?

It seems also to me that alternative sanitation (incl. UDDT) is judged more critical about its environmental and (public) health impacts with regard to ground water pollution, nitrate build up, ascari eggs, pharmaceutical residue, etc. If for some of the ‘criteria’ alternative sanitation can prove better results than conventional sanitation (sewer, septic) it can serve as a selling point.

But at least we should know what comes out of our systems. You did not mention ascari eggs, do we know what happens with them in UDDTs? When you recommend secondary treatment, is it just a safety precaution or you know it is indispensable? What is recommendable large scale secondary treatment for UDDT dried faeces?

Ciao
Hajo