Hi Dean,

GREAT ! ! I am happy that you also see now the advantage in this design. I am aware of its limitations of use and we will only achieve the 5 year resting period IF the worms survive and contribute to the faecal reduction. But we will try our best to ensure this.

We can for example install an urinal to reduce flushing and we can install a low-flush device (ENVIROSAN, ZA claims to use only 2 L per flush) or we reuse hand-washing water for flushing which is also little.

Since I learned that the wall is relevant as infiltration area, not the floor, we can increase the height of the drainage layer to 50 cm for more security for the worms. Do you like that?

Thanks also for the explanations on the worms behaviour of feasting and resting.

You saved my day!
Ciao
Hajo

Dear Peter,

Thanks for your post and comments..

… and in Tanzania I have the feeling that not only officials want to ‘shine’ with promises of ‘sewers to come’ but people also prefer to believe it as it relieves them of own efforts… as long the situation is tolerable… otherwise they should object after 10 to 20 years of empty promises?

And there I see a difference between Namibia and Tanzania: the first President Nyerere pushed sanitation in Tanzania where every household was supposed to have a pit latrine. And today Tanzania has a coverage rate of almost 80 or 90% … although the condition, the looks and the smell of these latrines is often disgusting.

Another difference I see in the fact that our sanitation survey in Moshi town found that almost 50% of the households have pour flush toilets, some connected to septic tank and soakaway but most have spared the septic and feed directly into the soakaway which naturally fills up and eventually causes problems in emptying. This indicates to me that people are not necessarily after sewer but after flushing, and even seem to accept pour flush as a convenient solution. They only have to develop a better solution than the single soakpit which I see in the twin pit system with vermi-composting.

My concern with UDDTs is the huge amount of urine a service provider would have to collect, treat and dispose/reuse if there is not the chance of percolating it into the ground near the toilet. The collections of UDDT faeces I can imagine even at large scale and you seem to have developed a good system with the Otji Toilets in Namibia. Can you let me have a link or a drawing of your latest design?

Ciao
Hajo

Hi Hajo,

so considering that a simple 5 year resting period twin pit pour flush wet vermicomposting system has simplicity on its side and advantage in terms of 90%+ fecal reduction, then lets progress your concept.

I like your idea of doing away with concrete walls and soakaways, but in doing so I'm more comfortable with a conservative approach, given that flooding is death for worms. Infiltration will be the key. You would need to ensure no surface water gets into the pit and flushes are limited to less than the soakage capacity.

Would flushing be limited to poos or would pees get flushed also? If pees also, with 4 people per toilet and 4L/flush and each person flushing 5x per day, 4 x 4 x 5/d = 80 l/d. Of course low flush toilets would reduce this significantly.

A pit with 30cm drainage layer of cobbles in the bottom and diameter of 1.5m only has approx 1 square metre of infiltration area before flooding the worms above.

The bark acts as a kind of refuge aswell as a solids filter. The humus layer also becomes refuge but this builds up surprisingly slow. I've found that if the heap can spread sideways then it doesn't seem to build upwards, i.e. the surface area aids fast digestion. The worms are more active in the outside of the heap than in the middle, but as the edges are digested the middle "spreads" to the outside so only the freshest poo is in the middle. You don't really want the heap to build upwards too much as this would mean a deeper pit and less drainage. Wider and shallower is my recommendation.

cheers
Dean

Good morning Arndt.
As a manufacturer of UD pedestals in Australia I can also vouch for the preference being given to urine diversion against a mixed batch system. I can also vouch for the political intervention and preference given to "accepted" progress given to centralised sewer systems. In my home town we adopted a stance against the drinking water powered sewer but were unable to convince the politicians. In the end both a mains water supply and a sewer system were installed at a cost of $60,000,000, for a population of 1000 people. After installation it was discovered that to move the sewage, treated drinking water was used.

Dear Hajo,
I can confirm your observation that dry toilets are often associated with denied development. But we must have a closer look who is creating this perception and for what reason. In Namibia’s informal settlements of Otjiwarongo we have installed over 600 Otji Toilets (UDDT’s) which are well accepted by the population. In fact inhabitants constantly ask for more Otji Toilets as there are living over 20.000 people. But resistance comes mostly from officials, referring to future visions of sewer system. Only technicians manage to observe the lack of feasibility due to our water shortage. It seems that politicians like to “shine” with modern technologies even when it goes to the expense of the mass.

However, in Namibia more than half of the population does not have toilets at all. Therefore poor people are grateful once they get an Otji Toilet, because it works well. Liquid is separated and drained into the ground with a user friendly Urine-Diversion-Toilet-Bowl. This UDS-Bowl diverts regardless whether users are standing or sitting – no user instructions needed. Faeces are collected in 90 litre bins. There are two bins in the chamber, so the one which is full rotates with an empty one but remains inside the chamber. Once the second is full we empty the first one which is meanwhile dry and partly composted. This service is only twice a year needed, giving the bin a total time of one full year in the chamber before emptying. In the case of Otjiwarongo we observe that Otji Toilet owners voluntary share their toilet with neighbours since there are not enough toilets. Therefore the frequency for maintenance has become higher – about 3-4 times annually. However, the increased number of users (now 20 – 40 per toilet) does not harm the function. The maintenance is not left in the hand of the toilet users but is professionally organised by a competent service provider. But the annual costs for this service are still far cheaper than i.e. a sewage fee.

With this background of experience, I do not share your conclusion that UDDT’s would not be suitable for peri-urban area. In the contrary, I think they are ideal for high density area because here a coordinated maintenance can easily be organized as PPP. I also don’t think that quantity should be a concern. The bigger the number of toilets the more cost efficient is service provision. But also smaller numbers do create part-time business as we see in other towns of Namibia. In rural area maintenance is maybe more challenging due to distances. In Otjiwarongo we provide the service with a pick-up, transporting faeces to a huge basin within the Municipality sewage pond area. There it is stored over years on mountains. At this stage it is not used but maybe at a later stage. I do not know how fast it loses its value over the time? Any experience?

I agree with your idea to use a big tank for urine collection in areas where it cannot be filtrated into the ground. However, in Namibia this is not often the case.

Greetings,
Peter

Hi Dean,

Not that I try to prove you wrong but I want to avoid to build ‘your’ twin-chamber digester from concrete and blocks plus a soak-away if a twin pit system could also work which may be cheaper and easier to accept by the people.

I have again studied the EAWAG Compendium on PFTP (S6, page 68) and then the references given there. The document ‘The Design of Pour-Flush Latrines’ by D.D. Mara (1985) is very interesting and informative with regard to infiltration rates and area (see attached).

The Technical Guideline states:

1. Only walls not floor are to be considered for infiltration as the latter tends to clog (page 5;

2. Infiltration rates vary between 10 and 50 L/m2 for clay to sandy soils (page 5);

3. Infiltration area may be increased by excavating around the build pit and backfilling with gravel or cobbles (page 8 );

4. Pit dimensions for a family of 5 with pit in sandy loams are given as diameter = 900 mm and depth as 650 mm (page 24).

I would have no problem to make the pit 1.2 or 1.5 m diameter and between 1.0 and 1.5 m deep which provides infiltration area well above the requirements of this guideline to avoid flooding by waste water (groundwater and surface water to be ruled out anyway).

With the gravel layer, textile cloth, bark and aeration pipes we may then make life acceptable to our worms.

One more question to the behaviour of the worms because you mention the heap should be shallow: initially the worms move between the bark layer and the fresh faeces. Once they have worked the first layer of faeces into humus, will they then still move between bark and the now new layer of fresh faeces above the humus or will they move between humus and fresh faeces? If they always want to return to the bark, then of course the depth of the faeces heap must be limited, but if they accept the humus as ‘new home’ from where they ‘attack’ the fresh faeces, then the depth of heap is not necessarily limited.

Ciao
Hajo

I'm not convinced that drainage will be sufficient in a pit. The porous surface area is significantly less than a soakaway and although this would suffice for producing sludge, if the blackwater doesn't adequately drain the worms will die. We're talking 32+ litres per day of blackwater, so it might work in some cases with porous soil, but we do want to reliably reduce the solids bulk. Perhaps a wide shallow pit with a diameter of say 1.5m might improve the chances of success, both because of increased wall surface area for drainage but also for a wider shallower heap (again, for surface area as the worms work the outside of the heap).

cheers
Dean

Dear Dean,

I expressed in my previous post (11.4) the expectation that the vermi-composting would reduce the emptying cycle of the pour flush twin pits from 2 years to up to 5 years. So there we agree.

Soaking away urine, black water or grey water takes place with most of the on-site systems which are currently in use here: pit latrines soak away urine; (pour) flush toilets with septic tanks mostly have soakaways which soak away black or grey water; a new system has been 'developed' by people omitting the septic tank and feeding the flush from (pour) flush toilet directly into a soak-away which practically becomes a ‘soakpit’, storing decomposed faeces and soaking away black/grey water similar to a pit latrine.

In all cases ground water tables (as deep as possible below the floor of the respective pit) and soil conditions (no clay, no rock) should be observed. As it would be the case with a pour flush toilet with vermi-composting and soak-away. The effect on shallow water sources and the effect on soil conditions in areas with high infiltration quantities (high density areas with lots of on-site sanitation) must be monitored.

On a system similar to your twin-chamber vermi-digester I would definitely build a proper soakaway if ground/water conditions permit percolation into the ground.

If the twin pit system (as described in the EAWAG Compendium) could also work as a vermi-composting system with some improvements done (drainage layer, aeration system) I would avoid the extra soakaway as the pits work like soakaways as their walls and floor are open to the soil.

Your last recommendation (better UDDT than collecting blackwater) made me rethink what I disliked with the idea: it is actually the fact that UDDTs are currently always planned with 2x20L jerry cans for collecting the urine which have to be emptied almost on a weekly basis if not more often.
Thus if we cannot percolate the urine into the ground, why not dig a 1 m3 tank into the ground for the urine which is emptied with a tanker every 3-4 months. Since it is fluid it is easy to pump (unlike sludge from a septic) and can be collected by a small tank on a 3-wheeler or using a larger tanker with a long pump pipe. Comments?!?

I am ‘throwing’ all kinds of ideas ‘on the market’ hoping for critical and creative comments from the forum community…

Ciao
Hajo

Hajo, certainly worms will significantly reduce the bulk of the feces, which drying alone will not do. You'll definately use more water if vermicomposting the waste, so having a good soakaway is imperative. This means more care with water tables than if the liquid was only urine. That said, I doubt that percolating urine into the ground next to toilets, as practiced by some UDDT's, would work very well in the long run.

Good soakaways do cost money. But in my view the simplicity, reliability and easy maintenance of a pour flush VCT justifies constructing a proper soakaway. If you had to collect the blackwater because of a high water table, you'd be better to go UDDT.

cheers
Dean

Dear all,

To understand the following posting you must know that we already had an ‘off-line’ discussion about pros and cons and proper execution of vermi-composting. This discussion ignited EvM to emphasise (again ) her preference for UDDTs. She wants me to put my response on the forum for further discussion.


Dear Elisabeth,

I know you are fan of UDDT having even one in your villa in Germany. And I am sure it works well with people who know how to handle it and who can make use of dried faeces and urine in their garden!

But you may also know that the 100,000 UDDTs in eThekwini are not so well received/maintained by their users.

1. There is the feeling with people that they are denied development when any type of pit/bucket latrine (even if improved as UDDT or compost toilet) is recommended to them instead of sewers which all ‘developed’ people have;

2. (Therefore I try to make use of pour flush (which is considered a step of development) and connect it to a vermi-composting system);

3. For me UDDTs reach their limits when we want to install them in large numbers in peri-urban areas where people have 1) no space to make use of faeces and urine and 2) no interest to handle excreta by themselves;

4. At large numbers (possibly 10,000 to 20,000 in a town like Moshi) you have to introduce a collection service as part of the service chain;

5. And you also need large numbers to make the collection financially viable for the service provider and still affordable for the customer;

6. But at large numbers you really get a problem with the urine: 10,000 UDDTs, i.e. 40,000 users produce about 40-50m3 of urine every day which needs to be collected, transported, stored and reused, I doubt this can be done economically (unless you have huge flower farms next door which can make use of it);

7. So let us assume we can percolate the urine into the ground next to the toilets (as eThekwini does/did) and have only to collect the faeces;

8. Which amounts to 40,000 people x 0.3L/day = 12 m3/day whereby I assume that we use 50L containers which are filled/collected from a family of 4-5 at about monthly intervals;

9. At this service rate, I guess a ‘collection tariff’ can be worked out which is affordable for the customer and may be economical for the service provider for collection, transport, treatment and reuse especially if we can sell the product as soil conditioner/compost. This sustainability is one of the interesting aspects to be observed with Sanergy in Nairobi.

10. My concern is a bit, that we collect partially fresh excreta and the faeces are not yet treated at all when collected;

11. Therefore I see in pour flush with vermi-composting a number of advantages: 1) the desire of customers to have an ‘advanced’ flush toilet is partially fulfilled; 2) the ‘goose-neck’ of the pour-flush toilet prevents dumping of solid waste in the pit; 3) if successful the vermi-composter (pit or chamber) produces a compost which is hygienised, eventually even free of ascari eggs (if Dean is right) but definitely more pleasing to handle than the fresh faeces from UDDT containers; 4) pit or chamber needs to be emptied only every 2 to 5 years depending of use and success of the vermi-composting process.

12. I agree that the PFVCT (pour-flush cum vermi-composting toilet) is only that easy if we can percolate the black-water (or compost tea as Carl and Dean call it) into the underground without affecting the ground water. Otherwise we have a problem and need to collect the black-water. I would install a 2-3m3 underground tank from which black-water is collected at regular intervals and disposed of to the WTTP or treated for reuse if economical. With 4 people per toilet and 4L/flush, I expect an emptying interval of (4 x 4 x 2/d = 32 l/d à 3000 / 32 = 100 days) about 3 months. If all 40,000 users require that service, we have to collect 30 L/d x 10,000 = 300 m3/d but hopefully only 50% of customers need that service, still 25 trips for a 6m3 vacuum tanker per day.

13. You may be confused by all these ‘big’ numbers but I want to draw your attention to the fact that we are talking of serving XX,000 of people in one town, therefore we need efficient and working service chains and cannot depend on peoples’ willingness to empty and process their excreta by themselves.

14. Again: UDDT is a good concept but possibly more appropriate to improve sanitation in rural areas, maybe in rural villages, in my opinion definitely not in peri-urban areas of towns.

Looking forward to your comments !!
Ciao
Hajo