Using a double vault vermi-digesters for single households (max 10 people) in high density areas of Lusaka?

  • hajo
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Re: using a double vault vermi-digesters for single households (max 10 people) in high density areas of Lusaka?

Hi Dean,

Sorry, I misunderstood your sketch in the previous posting - caused also by my colour-disturbance.

I thought the impervious layer is UNDER the rubble and the pervious layer above the rubble. The latter is correct but the impervious layer is under the topsoil and under the rubble there is nothing. I was also mislead by my understanding that a constructed wetland is isolated from the underground by an impervious layer (either clay or polythene). I thought you had somehow reflected this in your design. My misunderstanding makes all of my questions in my last email irrelevant, sorry for wasting your time.

But with the design having NO isolation between rubble and underground, it is accepted that (a major part of) the effluent percolates into the underground because water always flows downwards and the roots of the bananas entering the rubble from top will only soak and eva-transpirate a smaller part of it. Correct? If yes, this may not be a solution in areas where any percolation will be disallowed due to high groundwater or fissured/karstic rock underground. It will be a solution where the underground has a low infiltration rate and the effluent needs to be distributed over a larger area by the rubble layer (like with a leach field).

In areas where percolation into underground is forbidden, we may have to come back to a solution with secondary treatment with a vermi-filter and distributing the such treated effluent to the surface around the bananas using a solar pump (2m gravity flow is too high). You say ‘treated to a high enough level to safely discharge to soil surface’. Do you have test data of the properties of this effluent, i.e. BOD, COD, pathogens, ascaris? This will be necessary to convince ZEMA (our local environmental protection agency) allowing the distribution of the effluent to the surface.

I will have to discuss with our partners whether they can imagine replacing septic tanks with vermi-digesters. I also see the advantages of aerobic digestion, easy reusable output product, possibly longer emptying cycles and safer service chain management.

We are not only discussing the use of vermi-composting for on-site sanitation facilities but also as a solution for large scale treatment of sewage and faecal sludge collected at a central treatment plant. Have you heard of Prof. Dr. Rajiv Sinha who has developed such treatment plants at Griffith University and says having implemented them commercially in India through TRANSPEK? If you haven’t, I can send you a number of research papers which he has given me. I like to have your opinion on his research and commercial application. He also claims ‘his’ treatment eliminates helminths (which is why I contacted him in the first place).

Ciao
Hajo

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Re: using a double vault vermi-digesters for single households (max 10 people) in high density areas of Lusaka?

Hi Hajo,

But with the design having NO isolation between rubble and underground, it is accepted that (a major part of) the effluent percolates into the underground because water always flows downwards and the roots of the bananas entering the rubble from top will only soak and eva-transpirate a smaller part of it. Correct?


No, not quite. Percolating into the undergound water table happens in pits because you dig down below the topsoil where there are fissures etc that drain the septage directly into the water table. That is the short circuit we are trying to avoid.

By making your pit say 10 cm deep and 3 m diameter, you are discharging onto a surface of about 7+ m2 of topsoil. It might soak in 5 cm on average. If there was heavy use maybe even 7 or 8 cm, this is just the buffer, the sponge that avoids pooling. The topsoil is both the filter and sponge that avoids a short circuit... water does not flow straight down through a sponge, but soaks sideways (unless there is too much water, which is a surface area issue). The banana roots aren't just in the rubble layer, they penetrate into the soil below, as do the worms. This is a dynamic biological buffer, a vermi- integrated macrophyte filter. The bananas do evapo-transpire that water.

How much would it cost to build one of these as a demonstration?

Keep in mind that topsoil is heavily weathered. Very rarely would you get a short circuit from the first 30 cm of soil into the water table. In the rare cases where it is, then a central sewer is the only option. I assume LSP understand this and are focussing on building sewers and centralised treatment where percolation is disallowed?

This is not about soil filtration rates and "percolation". Topsoils with good filtration rates don't necessarily leach into the water table and topsoils with poor percolation don't necessarily cause septic runoff, its about customising the solution to the wastewater volume, soil type, space and aspect. Thinking septic tank and soakage field is a backward step in this century.

cheers
Dean

Dean Satchell, M For. Sc.
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Re: using a double vault vermi-digesters for single households (max 10 people) in high density areas of Lusaka?

Dear Dean,

Thanks for your mind twisting contributions… I always find myself understanding a new aspect and having to think in a different direction… and I hope you also benefit a bit learning about our working conditions in Africa.

I understand your proposal as a sort of ‘leach field’ but the effluent should not percolate into the subsoil but be taken up in the topsoil by the (banana) plants and be evapo-transpired if all is calculated correctly and behavior follows the design.

Unfortunately we cannot check and guarantee that the users will pour only urine and minimal cleaning water (SATO, 0.5L/flush) into the vermi-digester. Therefore the authorities will insist on using the system only in areas where there is sufficient distance to the ground water level. Where it remains to be defined how much is ‘sufficient’. But for these areas it is definitely a sustainable solution as we have not to transport black-water, empty the vermi-digester only occasionally and harvest bananas and valuable humus.

Referring to your remark that the authorities should build sewers where the effluent could short-circuit into the groundwater, I need to point out that this is not possible for extended areas and for hundred thousands of people due to various reasons: not sufficient funds, not sufficient flush water: either potential consumers have no house connection or water supply is erratic, many potential customers lack ability to pay for sewer connection, sewer charges and (flush) water charges. Further these areas are unplanned and the settlement pattern congested with no clear servitudes for sewer lines.

Currently I see only UDDTs and cesspits as possible alternatives. If we can keep solar pumps working, vermi-digester and vermi-filter may be possible. Any other idea?

Ciao
Hajo

We can't solve problems by using the same kind of thinking we used when we created them.
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Any intelligent fool can make things bigger and more complex... It takes a touch of a genius - and a lot of courage to move in the opposite direction.
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Everything should be made as simple as possible, but not simpler. :-)
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Re: using a double vault vermi-digesters for single households (max 10 people) in high density areas of Lusaka?

Hi Hajo,

there is an interesting solution here that is fully lined, cheap and even produces biogas. Sounds kind of experimental but possibly worth setting up as a demonstration?

cheers
Dean

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Re: using a double vault vermi-digesters for single households (max 10 people) in high density areas of Lusaka?

Hi Dean,

your reference to this 'technology' knocked me for a loop... I wasn't sure you want to take me for a ride, because this technology includes almost everything you normally dislike: anaerobic digestion, bio-gas, no recovery and reuse of resources. If these are not you concerns, then they are mine...:(

We are currently compiling an OSS catalogue for peri-urban areas in Lusaka, that is why I have been quite active on the forum recently discussing solutions. Of course vermi-composting is also in the catalogue and during the discussions the following question cropped up.

You remember we have been talking about the effluent from a two-vault vermi-digester being pumped into a vermi-filter after which the effluent is good enough for surface irrigation.

Question: If we dig the vermi-filter into the ground, we avoid the pump and can gravitate the black-water into the vermi-filter. The outlet of the vermi-filter is then about 50-80cm below ground. Can we put a soak-away/rubble layer at that outlet and percolate that effluent into the ground? Do you have any 'water' laboratory tests to that extent? What are the bio-/chemical and physical properties of the effluent from the vermi-filter? How much soil cover between outlet vermi-filter and top of GW would you eventually recommend to protect the GW from contamination?

ciao
Hajo

We can't solve problems by using the same kind of thinking we used when we created them.
Albert Einstein
Any intelligent fool can make things bigger and more complex... It takes a touch of a genius - and a lot of courage to move in the opposite direction.
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Everything should be made as simple as possible, but not simpler. :-)
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Re: using a double vault vermi-digesters for single households (max 10 people) in high density areas of Lusaka?

Hi Hajo,
heh heh, well no... I have nothing against anaerobic digestion PROVIDED the biogas is recovered (rules out septic tanks) and there is no groundwater contamination (rules out septic tanks). Anaerobic digestion is slower and therefore requires a larger capacity digester, which means more cost. That solution linked to above does tick some boxes if it works... just not recovery of solids. It's a use-till-it-doesn't-work-any-more-then-discard solution, so although low cost in the short term, perhaps it is more of a stop-gap measure. This might suit poor peri-urban areas requiring flush and forget?

What you are suggesting is that groundwater contamination with secondary treated sewerage might be okay? The more you lower the outlet of the secondary vermifilter into the ground, the more likely this will happen in the situations you have outlined above. That is why I'd start with surface discharge and go up. For secondary treatment this means the floor of your latrine is 1.6 m above ground level if you use two large fruit crates stacked on top of each other.


Large fruit crate, 1.2m x 1m x 800mm high.

The top crate is the primary digester and the bottom crate the secondary vermifilter. It seems that it might be easier to build steps up to a platform rather than use solar pumps to raise the water? The advantage with plastic crates is that you can use these for the latrine structure because they are very robust and provide bracing... walls can even be screwed directly to them. These crates might cost US$70 each and should be available in Zambia.

The effectiveness of the secondary vermifilter depends on the retention time, which for a low flush toilet should be high. That is, because there is not a large volume of water going through it, you would use a fine medium that slows the through-flow of water. Think sawdust rather than woodchips.

What level of treatment is required for surface discharge in Zambia? Vermifiltration significantly reduces BOD, suspended solids and pathogens, but not nitrates (so good for plants). You need to build a pilot, the cost is so small and the potential so great.

cheers
Dean

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Re: using a double vault vermi-digesters for single households (max 10 people) in high density areas of Lusaka?

Hi Dean,

What you are suggesting is that groundwater contamination with secondary treated sewerage might be okay?


Not really: I want to use a soil layer between floor of soak-away (after vermi-digester and vermi-filter) as 'tertiary' treatment before the effluent reaches the groundwater. Which is of course only possible where groundwater is deep enough for the soil to provide such filtration. Whether the soil can provide that treatment is also dependant on the characteristic and quality of the effluent from the vermi-filter. That is why I asked whether you have such data, do you?

I like the fruit crates, they look very sturdy. On the other hand you cannot move them when full. And I expect that if this technology is accepted in Lusaka at all, it will require a collection service which is easier to provide, if the full crates can be moved out of the vault and be replaced with empty ones. The full one will go to a central treatment place for emptying and further processing. I am considering crates of max. 120 L which can be lifted by two people onto a tricycle or pick-up. Of course they must be collected more regular than bigger ones.

ciao
Hajo

We can't solve problems by using the same kind of thinking we used when we created them.
Albert Einstein
Any intelligent fool can make things bigger and more complex... It takes a touch of a genius - and a lot of courage to move in the opposite direction.
E.F. Schumacher
Everything should be made as simple as possible, but not simpler. :-)
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Re: using a double vault vermi-digesters for single households (max 10 people) in high density areas of Lusaka?

Hi Hajo,
I'm not comfortable even with tertiary treated sewage being directly discharged into the water table. There will always be variables such as number of users per day that will influence effluent quality, so you'd need a larger "redundancy" built into your system. That is why I have raised the crates above the ground - the only really reliable way of avoiding a short circuit is to use the topsoil as your buffer... an insurance policy for your low cost system. In my mind this is the only way, rather than trying to provide tertiary treatment in your soakaway, because with surface discharge you are evapo-transpiring most of the water (and also directly feeding plants) and because the topsoil also provides excellent filtration, there is no short circuit to water table.

The only negative is that the user has to walk up steps. The design and construction is very simple.

I can't provide you effluent quality data for ten users per day, because some days there might be twenty. You might have to fine-tune the level of built-in redundancy according to number of users and total water added. I can tell you that helminths will be completely eliminated from the effluent, although BOD, suspended solids, turbidity, coliforms etc, they will depend on volume of the vermifilter and effluent quantity (number of users and how much water they use).

The full crates are really easy to move with a forklift, these crates are specifically designed for this. A removal service would have a forklift available to move crates on to the pick-up, or at worst a manual hydraulic forklift

.
However, you also need to keep in mind that the bottom crate (the vermifilter) never needs to be emptied or taken away - as the sawdust decomposes more might need to be added, say every 5-10 years.... although after a long time as it builds up some humus might need to be skimmed off the top. The top crate (the vermidigester) might fill in 5-7 years. So either that crate is exchanged and taken away, or there are two stacks with two crates each (i.e. 4 crates) over the same discharge area, so that the latrine alternates between stacks every 5 years and pathogen-free humus is dug out from the alternating crate by hand.

In my country we have discharge limits for surface irrigation, 20 g/m3 BOD and 30 g/m3 suspended solids. The WHO guidelines also give recommendations for surface discharge, depending on whether this is feeding food crops or not. Do you have standards or regulations in Zambia that set limits for surface discharge of secondary-treated sewage? If you set up a prototype you'd be able to demonstrate meeting those requirements. Surface discharge is the holy grail, historically only achieved using expensive methods. Now you have vermifiltration.

cheers
Dean

Dean Satchell, M For. Sc.
Go-Eco Sustainable Solutions
www.go-eco.co.nz
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