Hi Hajo,

The rubble layer does not need to be deep, perhaps only 10-15 cm thick. Width, however, is determined using simple calculations of soakage area to ensure that the 30 litres per day soaks into the soil rather than pools. This method avoids the "short circuiting" whereby the deeper the soakage is (how deep are your pits?) the more likely the wastewater directly enters the water table. Keeping soakage only in the topsoil allows for good evaporation at the soil surface and transpiration by plants, to keep the water removal efficiency high. The soil also acts as a filter, so clearly the nearer the discharge is to the surface the better.

An impervious polythene will be difficult to install without perforations during construction

The impervious layer is not actually necessary, all this is for is to reduce rainwater from percolating through the soil and entering the soakage area. The polythene is pierced for planting the bananas and it certainly doesn't need to be perforation-free, but could even be made from recycled plastic packaging etc.

The rubble or gravel will require a soil protection of the polythene first

The soil surface below the rubble and the wall of the shallow "pit" soaks the wastewater. Above the rubble layer is an impervious cloth that prevents soil from falling into the rubble and clogging lateral flow. The rubble layer will be colonised by plant roots but because there are so many air cavities (being rubble) the wastewater can still distribute throughout the layer and soak into the soil over the whole soil surface area of the pit. Provided the diamater of the pit is determined according to the soakage capacity of the soil, this will not become a wastewater "swamp". Soakage capacity of soils is determined by digging a small hole and filling it with water, then measuring the rate at which it soaks into the soil. A large area will not be required for 30-50 litres per day. The bottom of the pit does need to be exactly level though.

What about putting a vermi-filter as secondary treatment after the digester and let that effluent just soak into the underground?

There is no need for secondary treatment if you soak underground. Indeed you could discharge to soil surface if you raised the toilet up on a platform. The primary vermi-digester and secondary vermifilter will need up to 2m between the ground and floor of your WC (depending on level of treatment required) to eliminate the need for a solar pump. When I talk about "secondary treatment" I mean treated to a high enough level to safely discharge to soil surface.

How are your septic tank soakage fields constructed in Lusaka? I'm a bit anti- anaerobic digesters unless they capture the biogas. A primary vermi-digester is cheaper to construct and has less O&M issues and costs, and can replace a septic tank while using exactly the same soakage field.

cheers

Dean

Hi Dean,

Yes, we need solutions for different conditions of: geology incl. groundwater, geography, socio-economic, existing piped water, existing sewer, … I work with the Lusaka City Council on a sanitation catalogue of possible on-site sanitation (OSS) solutions… we hope to ‘sell’ it to the Lusaka Water and Sewerage Company (LWSC, the local utility) and to the Lusaka Sanitation Project (LSP). LWSC is not very fond of getting involved with too much OSS, but the LSP has in its 270,000,000 USD budget (most of it for sewers) also the construction of 12,000 OSS facilities. LCC aim is that we come up with some solutions which are acceptable for LWSC and LSP for the 12,000 budgeted toilets. And we intend to build demonstrations of the selected solutions for the people to see, smell and try.

We have discussed so far that the LCC ‘OSS ladder’ will have at the top the septic tank with soak-away. At the lower end, we imagine to equip existing pit latrines with a SATO pan. The main purpose of the SATO is preventing too much solid waste and plastic bags going into the pits. We had an emptying test with the Gulper recently and it showed that the Gulper works well unless it is choked by the plastic bags. In the long run (10-20 years?!), we want to replace the pit latrines with something ‘better’ especially in view of the environmental (groundwater) and public health (spillage during emptying, unsafe disposal) threats (as you describe them).

For this we need something in between: up the ladder from the pit latrine, more sustainable, safer, affordable but not yet septic with soak-away (too expensive in construction and O&M). Possible solutions are UDDT and vermi-composting and cesspits. The envisaged problems with UDDT I have described in an earlier post. For vermi-composting I am discussing with you solutions for the effluent. Cesspits (only for excreta and VERY little flush water, i.e. SATO) would be the simplest solution, but a 2m3 pit still needs to be emptied every 3 months.

With regard to your sketch of a vermi-composting plant without pumps, I have the following questions/concerns:
1. An impervious polythene will be difficult to install without perforations during construction.
2. The rubble or gravel will require a soil protection of the polythene first.
3. The rubble layer will prevent the banana roots from being drowned by waste water but
4. Will the deep rubble layer not also prevent the bananas from accessing the waste water?
5. And will we eventually have a ‘swamp’ of waste water in the rubble layer (remember: water always runs down and will rather accumulate at the bottom of the rubble, eventually too far for the bananas)?

What about putting a vermi-filter as secondary treatment after the digester and let that effluent just soak into the underground? What is the quality of the effluent coming from the vermi-filter: BOD, COD, pathogens, ascaris? May be it will be permissible due to good quality.

Or, coming back to the solar pump: take the effluent from the vermi-filter and pump it sub-soil to the bananas which are not sitting on a gravel layer and impervious polythene: risk is that some effluent escapes the bananas and goes into groundwater (but maybe permissible as above).

Thanks for your valuable discussion,
Ciao
Hajo

Hi Hajo,

It seems to me that it will need to be horses for courses, with a range of solutions, each depending on the exact circumstances (level of poverty, land available for soakage, water table... etc...). $20 per person to set up vermifiltration might be affordable for some and seems to me worthwhile to set up a demonstration unit to test functional challenges.

I'm now wondering if the problem isn't because you have 100,000 drop holes, but the method used for constructing them. The deeper and narrower the pit the more likely that groundwater becomes contaminated... it is like a direct route to the water table. Here is a picture showing the opposite extreme, for improved surface soakage. The deluxe model would include a 0.5 l flush sato pan and twin vault vermi-digester, with bananas planted in the soakage area. This solution should be no more expensive than a standard pit, but at only 20-30 litres per day liquid effluent, should eliminate groundwater contamination in all but swampy ground.



What your group could do is select a range of solutions to trial in real households, that form the basis for decisions on resolving the issues.

cheers

Dean

Hi Dean,

If I understand correctly, you propose in your first paragraph using UDDT: ‘If there is no space to irrigate then one should not have drop holes either’. We have about 100,000 drop holes into latrine pits, which need to be replaced as their operation and emptying is a constant threat to environment and public health, but no space for irrigation.

Therefore UDDT could be a solution, but I have 3 major concerns:
1. Pilots of few hundred UDDT in Lusaka have failed. The authorities claim they are not ‘socially acceptable’.( I think that they also had no reliable service chain provided for collection of faeces and urine.)
2. UDDT are critical where users of one toilet are not restricted to one family who have been ‘alerted/trained’ on its proper use, thus there is always a high possibility of urine ending in the faeces hole in the high density compounds.
3. With 1,000,000 people as target group we produce at least 400 m3/DAY of urine which has to be collected, transported, treated/stored, reused at a price which justifies the service chain. (yes, if we had already a process which converts the urine to dry, packed fertiliser within few hours..?!?)

200 USD for the solar pump sounds little but for the people in the compounds it is probably 2 to 3 months of salary. But I am more surprised that I need 6 -10 bananas and 500 litres of vermi-filter to get rid of the 60 litres effluent. But because of the problems 1-3 (quoted above) with the UDDT, at the moment I rather tend to vermi-composting. But as I said previously the worms require a bit of moisture, isn’t it? Thus, if we install a drop hole with SATO which prevents solid waste from going into the vermi-digester, it requires only 0.5L per flush which cleans the SATO and helps the worms survive. Then the total effluent expected will be at most 20 L/day for 10 people (1.2 L urine, 0.5 L/flush). That will require 200 litres of vermi-filter and 2-3 bananas. For that we may have the space.

The point with the solar pumps is that the solar panels will be stolen or spoilt. Even the pump itself, once it starts leaking, falls down, gets stolen, … maintenance becomes a problem… the challenge will be that the users appreciate its function taking ownership of the idea and the infrastructure… how many bananas can I get from 3 plants in a year, what's their value compared to the price of the pump? I will have to find out...

Not so easy finding sustainable solutions, especially when sanitation does not have a high priority…

Ciao
Hajo

Hi Hajo,

if you use urine diversion there would still be considerably less liquid to deal with than if you use micro flush. With a drop hole you can't control what happens to the liquid whereas with urine diversion the urine can be collected in a tank and removed. If there is no space to irrigate then one should not have drop holes either, the mind boggles at the consequences!

The big breakthrough with vermifiltration is in cost. For secondary treatment you're looking at US$200, including the solar panel, pump and tank. For 60 litres per day surface discharge you'd need maybe 6-10 banana plants and a vermifilter capacity of 500 litres.

In the environment we work even small solar pump will not last long

Could you explain please? Certainly in terms of expected service life, brushless pumps and solar panels should last for many years regardless of environment.

Hi Dean,

'horses for causes'... I like that... even if I do not flush at all but just use a drop hole (what you call a 'dry' toilet, I presume), I still have the (contaminated) urine running through which I need to get rid of... OR?

For the 40-60 L/day, how big must be a vermi-filter for the effluent? sqm or litres? I could imagine putting a drum of required size behind the toilet for vermi-filtration... Problem is, how do I get the water out of the ground? In the environment we work even small solar pump will not last long, And what to do with the 'irrigation' water, there is no space to irrigate?

If I was sure about the quality, I can use it as hand-washing water... that would be cool (like the Blue Diversion by EAWAG, only adjusted to African standard) .... and very difficult to sell.... and just imagine our worms 'have been on leave' or 'travelled'... disaster....

ciao
Hajo

Hi Hajo,
the thing is that irrespective of your primary treatment method (vermifilter or septic tank) disposal of effluent, even for micro flush, will involve subsurface discharge to land. You could use wetlands (or vermifiltration) as your secondary treatment method, but unless you also evaporate the liquid you will still need to discharge to land. With 1000 mm rainfall, evaporation + transpiration from a wetland using reeds will not always exceed the inflow and this will require space just for secondary treatment.

If you don't have land to discharge to then you will need to use a storage tank (if you flush), or (in my view) the better option of dry toilet. It's horses for courses... flush toilets are for those who can easily deal with the wastewater.

Indeed banana's do require drainage, which is why I offered subsurface discharge method to grow them instead of a liner, which inherently impedes drainage. The risk with subsurface discharge leaching into water tables is essentially one of scale. More land area for soakage means less point discharge and lowered risk of groundwater contamination. However, once you can discharge to surface the area of land required is vastly smaller because the risk is minimised. Do you have enough land to grow bananas? Secondary treated effluent will ensure they are very productive.

cheers
Dean

Hi Dean,

thank you for your valuable thoughts. I agree that secondary vermi-filtration and surface irrigation would be a nicer solution and I will come back to it once we start discussing about de-centralised treatment plants, i.e. for institutions or confined residential areas.

But in the environment we plan currently there is no space for irrigation neither surface, nor sub-soil. We just have to get rid of the little effluent from a vermi-digester as described earlier. I am looking for a solution like we have it with the 'vertical gardens' for the uptake of grey-water from kitchen and bath: as small as possible size, we are in high density areas, the plots are very small.

I learned already from an Australian website that bananas may be not the appropriate plants for this application as their roots are vulnerable to stagnant water, but may be the reeds which are used on constructed wetlands are the right ones.

I think I have to get hold of Heike and Christoph, they are experts in constructed wetlands as far as I know, may be they have a good advice.

thanks and ciao
Hajo

Hi Hajo,

So if your only option is discharging the primary treated blackwater to subsurface, (assuming the land is flat or you can work around the contour), carefully designed soakage trenches could ensure that surface runoff is controlled, with subsurface flow distributed evenly throughout the whole soakage field. The distribution pipes would need to be exactly level, with no fall whatsoever or unevenness, because water always goes to the easiest and lowest point. The perforated pipes would be bedded in a thick layer of gravel and depending on soakage you might need 60 m of distribution pipes for 60 litres per day. You can do percolation tests to calculate the required length, the longer it is the less risk of groundwater contamination (i.e. better dispersal). You could mound soil over the pipes with depressions between the mounds to drain surface rainwater away. Shadecloth over the gravel and soil over that. The issue with plants like bananas is the risk of roots clogging the distribution pipes. That is why surface irrigation wins every time in my mind. Surface irrigation is cheaper and can even be moved around if required. Of course that would require secondary treatment which would require a solar pump if the land is flat.

Other forum users might be able to advise better on design of subsurface soakage fields in Africa, my interest and experience is with secondary treatment using vermifiltration and surface irrigation, my preference over subsurface discharge every time...

cheers

Dean

Hi Dean, hi all,

I agree, it sounds a bit odd to collect black-water in a high density poor neighbourhood for carrying away… and I would prefer soaking it away… but let me explain...

I don’t think there are issues with dry toilets. Pit latrines are basically dry toilets unless people use them for bathing as well but no flushing. Vermi-composting may raise eyebrows but we don’t know yet as it hasn’t been discussed with the communities. We are discussing to construct demos, therefore these questions.

And there is no intention of having full flush toilets as we understand them. But we should not have plain drop holes either because they ‘misguide’ users dropping solid waste and plastic bags into the pit: neither a pit latrine nor a vermi-digester likes that. To prevent that behaviour we discuss to have either SATO pans or small flush (2 L) squat pans with goose neck installed. Both require a little bit of water.

And if I understand our worms correctly, they like a bit of moisture anyway. Also it is helpful if the urine is a bit diluted. Thus a bit of ‘flush’ water is actually in favour of the vermi system. But as it rinses through the worm habitat and drops down, I have to deal with it.

In Lusaka we have three different cases of water percolation problems in different areas: there are areas with high ground water tables (probably due to impermeable layers underneath), we have areas with fissured or karstic rock just under the topsoil with cracks connecting to the groundwater and we have areas which are easily flooded by surface run-off.

All three cases could be solved with a confined constructed wetland as described before: a waterproof ‘basin’ filled with soil and bananas and the black-water lead in by subsoil pipes from under the vermi-digester. It has to deal with about 60 L black-water in a day thus needs not be large… but how large? I want to design it as a ‘water proof’ basin (like HCW are constructed with an impermeable membrane) so we can claim that we do not percolate black-water into the groundwater or into the karstic rock and the basin is also slightly raised so it cannot be flooded and black-water be washed out. Does this make sense? I am grateful for any advice.

Sorry, I forgot: where grey-water does not go into the pit, it is just splashed onto the surface and easily evaporates or is taken up by the minimal topsoil layer. People do not have plenty water to waste around.

Ciao
Hajo

Hi Hajo,
I'm wondering if the cost of installing and maintaining a blackwater tank justifies having a flush toilet in a poor urban neighbourhood. Are there any cultural issues with dry toilets? I'd suggest that the convenience of a flush toilet might only be justified if soakage is available or the products have value (e.g. irrigation of crops).

Is the high water table a direct result of the impenetrable layer, or is the high water table a separate issue to the impenetrable layer and due to rainfall/drainage issues? Where the water table is a result of the land being low lying ("swampy"), soakage will not work, whereas if the topsoil is dry and there is an impenetrable layer underneath, there is hope.

What is done with the greywater if there is no soakage?

cheers
Dean

Hi Dean, hi all,

we discuss using a double vault vermi-digesters for single households (max 10 people) in high density areas of Lusaka: thus little space, little, money, little water and ... the subsoil does not allow percolation of the effluent into the ground due to either high ground water or rocky/karstic underground. We assume small black-water production by either SATO-pan (0.5L/'flush'), low flush system (2L/flush) or pour flush system (4L/flush) which adds up to about 60 L/day (including urine) assuming the 10 people.

Because we will not be allowed to percolate the effluent into the ground, there seem to be two solutions: either have a black-water underground tank which has to be pumped out regularly (2m3/month for 10 people) by a vacuum truck or having a small 'constructed wetland' made of a waterproof pond, filled with soil, planted with bananas and fed with the black-water sub surface. Would that work? Has anybody tried it and can provide us with some necessary design criteria: size, soil type, other plants?

Our limitations are: we cannot use pumps for circulation, there will be no to little maintenance of the 'CW', it just has to 'function' by itself.

Or somebody has another idea what to do with the effluent? I assume that we get the service chain for the vermi-digester working (exchanging a full basket of vermi-humus against an empty one every 3 years and kick-start with wood-bark and worms).

ciao
Hajo