VermiComposting (digesters and filters) in Kigali, Rwanda

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  • hajo
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VermiComposting (digesters and filters) in Kigali, Rwanda

dear all,

I first had this question under an existing thread (see here: forum.susana.org/205-vermifilters-or-ver...-produce-clean-water ) but was prompted to open a new thread for it attracting new attention. My aim is finding design criteria for an up-scaled use of vermi-composting (digester plus filter).

We have in Kigali housing estates of up to 1000 people connected to a small sewer system leading to a small WWTP. They are called 'semi-centralised sewer system' by the authorities. The WWTPs are state of the art and operators have problems keeping them running due to financial and management constraints. Why not consider replacing them with VCTP (vermi-composting treatment plants)?

The design idea: after a trash rack removing solid waste, a vermi-digester follows which separates smaller solids from fluids and provides the habitat for the worms doing the digestion of the solids. The effluent goes through vermi-filters (a series of them if necessary) to bring the water to WHO standard for surface irrigation. The housing estates always lie along slopes (unavoidable in Rwanda :-)) and between them and the small stream in the valley (which currently takes the outflow from the WWTP) is always some small scale farming land which can make use of the effluent from the VCTP.

Questions:
  1. Digester: what 'filter' area is required to separate solids and fluids without drowning the worms? Is the design figure for rapid water treatment sand filters (4 - 12 m3/h/m2) a useful guide? Who has more information?

  2. Filter: in one of the first posts in this thread (Dean, 10 Feb2017), he proposes a 10 m3 volume for a 75 m3/day flow. Is that figure still valid or who has newer information?

Looking forward receiving some useful figures (or related suggestions),
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. :-)
Albert Einstein
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Re: Vermi-Trickling Filters (or vermifilters) for Sewage Treatment (looking for help to design)

Dear Hajo.
The thread on the solutions for your apt waste management has been on from start on 24.02.2018 Sr No. 24178,
10 reviews till 02.12.2018 Sr No. 26637.

Now this one the latest by you raised on 14.03.2020 Sr. No. 29275.
It shows that a lot of inputs have gone into this prospect - project .

At the start you carry out a detailed waste audit for all wastes in all streams / path.

Title : Waste Mangement for Kigali HH
200 HH, 1000 Persons.

1.Waste water: 150 l p dy = 150 kl p day
( quantify paths from all uses )
2.Solid waste: 1 -2 -3 kg p dy = pl (quantify wastes 3 paths= 1. bio, 2.non-bio, 3. sanitary)
3.Toilets and flush water = quantify..

Pl try as far as possible, not to mix all the above.
This could be a draft starter document
Pl give the assessed inputs for your solutions.
Well wishes.
Prof. Ajit Seshadri, Faculty in Marine Engg. Deptt. Vels University, and
Head-Environment , VigyanVijay Foundation, Consultant (Water shed Mngmnt, WWT, WASH, others)Located at present at Chennai, India
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Re: Vermi-Trickling Filters (or vermifilters) for Sewage Treatment (looking for help to design)

Dear Hajo
I'm working with industrial wastewater treatment with vermifilters, is a small project (1.5 m3/day) and have also post for coments about my work.
Respect to filter area most work references give a 1:1 for domestic wastewater, in case of industrial wastewater the ratio waste water: filter area can be 0.5:1. Also you have to consider the substrate permeability, depp of substrate, organic and hydraulic load, etc.
In my particular case I'm using a ratio 0.5:1 wastewater: vermifliter surface area. That is I have an hydraulic load of 0.5 m3/day per 1m2 vermifilter surface area and a organic load initially about 700 mg/l BOD.
The worms are very strong they can stand BOD loads arond 1500mg/l or much more, in my case I use cosmetic wastewater , result are quite good 75-90% BOD removal. Actually I'm trying to be more efficient with the substrate material, that is traying to find the right one to fin an equilibrium between retention time and BOD removal. I use biological substrate at top with graveldifferent sizes at the botom.
In previous post Dean have mentioned very usefull and interesting comments and information about vermifiltration, as well as his position related to shre experiences about this fascinating work of sustainable tecnologies.
Although my work is not related with domestic o grey wastewater, hope my experiences can help with some ideas about vermifiltration capabilities.
Regards
Arturo
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Dear Arthur, dear all,

for the further discussion on this topic, I like to get some clarification.
I called the topic 'VermiComposting (digesters and filters)' because I want to distinguish clearly between the two processes:

Digester:
the first step of the vermicomposting is the vermi-digester where the sewage, septage or FS is added onto a 'digester' (of wood-bark and worms) where the solids of the 'waste water' are separated from the fluids. The solids are retained and digested (-> digester) by the worms and turned into worm cast (humus) while the fluid is seeping through and led to the next step of treatment, the vermi-filter (see below).
The design criteria for the vermi-digester are area and depth. The depth should be as small as possible (30-50cm?), so that the 'digester' receives air from top and bottom and the likelihood of anaerobic condition in the middle layer of the digester is minimised. VermiComposting is an aerobic process throughout.
The question is about the required area so that the waste water is not building up on the digester and killing the worms by drowning them. The fluids must seep through without much hindrance. What is the seepage rate of first wood bark and worms and later of humus (worm cast) and worms? My question was whether the design figure for rapid water treatment sand filters (4 - 12 m3/h/m2) is a useful guide?
Please take note that I tried avoiding the word 'filter' for the digester although it is filtration (separating solids and fluids) before the digestion by worms takes place. But the digestion is the major action in this step!

Filtration:
The effluent from the digester is lead (by gravity or pumping) to the vermi-filter. Also this is first a habitat of wood bark and worms before it turns into humus and worms. The wood bark decomposes and is slowly replaced by the humus (worm cast). The humus actually being a better filter and habitat for the worms and improving the quality of the effluent.
Again the design criteria are area and height, but this time the other way around: height should be as much as possible increasing the contact time of the effluent with the filter media. The more contact, the better the quality. Contact time can be increased by extending the filter, duplication of filters or re-circulating the effluent through the same filter.
Filter area (diameter) must be as small as possible because now the air has to enter from the sides of the filter and again a core of anaerobic conditions must be avoided by all means. A maximum distance from outside to centre of max 30 to 50 cm sounds reasonable (Dean, please confirm). In one of the first posts in the previous thread (Dean, 10 Feb2017), Dean proposes a 10 m3 volume for a 75 m3/day flow. Is that figure still valid or who has newer information?

Can we agree on this distinction between 'digester' and 'filter' in vermicomposting? And again: what are reasonable design criteria?

Thanks for help coming,
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. :-)
Albert Einstein
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Hajo,
could you elaborate on why the "state of the art" WWTPs are failing ("financial and management constraints")?
Perhaps a compelling reason for not replacing them is that simply operating them correctly would resolve the issue?
More capital would need to be invested replacing them with "vermi-composting" treatment plants ...and would the community trust that it will be done right this time?

There might be some lessons there for the sanitation community... I believe in simplicity. Are these WWTP's too complicated for the local community to maintain? Do they "break down" or are they high maintenance? Has the designer walked away?

From what you have described of the situation, in my view these communities should use simplified sewers (existing?) with a primary digester at each home. The toilets and household wastewater would enter the domestic primary digester and the filtrate would exit into the simplified sewer and drain into a community vermifilter. I recommend twin household digesters each of 1m2 for the average-sized household.

The community vermifilter "secondary treatment plant" would need to treat the wastewater to a high level if applied to the soil surface and especially food crops. Tree crops such as bananas are generally considered safe for application of secondary treated wastewater. Are we designing this for 1000 people?

Hajo... I'm not sure if the term "vermicomposting" is appropriate for a vermidigester? This has been debated in the forum and the "compost" purists insist that only hot composting produces a product one can call "compost". Thus I tend to avoid the controversial term "compost" and use the term "digester" for the slow "wet composting" process that "compost purists" do not understand. I didn't understand it myself until I studied it. The wet solids accumulate on the surface of the digester and the pile is not aerobic. The media is sufficiently coarse to remain aerobic. The worms reside in the aerobic media and "work" the wet pile from underneath. Lets use an example where the solids spread out on the surface an impede water infiltration. So the fluids cannot seep through the surface and water will build up. Will the worms die? Yes they will IF there is no ventilation underneath, because no air can get in from the surface! BUT, if the digester is ventilated from underneath, the media remains aerobic and water that seeps through the media will drip out the bottom into the ventilation cavity then sump. The only bad thing that will happen is that the water might keep building up and overflow the top of the digester. By using good design the system becomes resilient and the worms won't die even if the surface is not porous. Of course a conservatively large surface area means there will always be a good "seepage rate".

cheers
Dean
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Dean,

From information I collected, I understand there are different reasons for the failure:

• Consumers do not pay (enough) for the service, thus not sufficient funds for O&M;
• No clear allocation of responsibility for O&M, because the housing estates are first built by one company, then houses sold to individuals without organising O&M of public services;
• Nobody properly trained and employed to operate the plants;
• The designer was probably a foreign consultant ... as the rehabilitation of some of the plants is now designed by a foreign consultant.

Yes, if properly maintained and operated, the existing plants could probably do the job, I assume the designer at least knew his job.

My intention is not, to replace all (40+) semi-centralised WWTP with VC. I want first to test and prove it on one or two plants that the technology works in such application. I see the advantage that I have not to install or operate anything else than the VC plant. The sewer system exists and hopefully functions. And the VC system will be cheaper and simpler in O&M. And I intend also to organise the O&M and its financing, not just the construction. I am destined to prove that VC works and works at that scale.

Your idea of on-site digesters, simplified sewers for the effluent and semi-centralised vermi-filters for secondary treatment, I also developed already in my mind for new housing estates, I wouldn’t consider revamping existing estates to that level. The conventional sewers are already there. The twin-digesters for households: do they have 1m2 in total, or 1m2 for each of the twins?

Composting : digester. I used the word ‘vermicomposting’ for the two-stage process of ‘vermi-digester’ and ‘vermi-filter’ as described in my previous post. I know about people insisting on that only ‘thermophilic composting’ can be called composting. But that is why I call it ‘vermi-composting’, it is the decomposition of matter not by heat but by worms. I want to reserve the expression ‘digester’ for the first stage of vermi-composting. If you find two other distinct words to distinguish the first and second stage of vermi-composting, I don’t mind getting used to them.

Very interesting how you describe what happens on the digester, with the pile being worked on by the worms from underneath. Which raises the question how high the media can be to allow aeration up to the top of the media (and the underside of the pile)? Surely depends on the media material, i.e. is it still wood bark or already humus (worm cast). When you call it ‘sufficiently coarse’, you do not talk of gravel, or? And when you talk of ‘ventilation’, you do not mean mechanically but just by enough openings allowing natural air flow, or? And lastly the question, what is a ‘conservatively large surface area’ which ensures a good "seepage rate" and avoids overflow? Any guess?

Thank you very much for your always motivating contributions,
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. :-)
Albert Einstein
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Hajo,
Is the service too expensive to justify consumers "chipping in" for O&M? Is the system sustainable if O&M were set up?
Even with an improved model (lower costs, simpler to maintain), allocation of responsibility for O&M would be required... there is no such thing as maintenance-free as far as I am aware...

One "complication" with setting up a centralised VF digester (in my mind at least) would be abdicating of responsibility for flushing unwanted waste down the toilet. If the consumer has their own digester they would own that responsibility. So with a community system you'd get all sorts of undigestible waste disposed of down the toilet and received by the central digester. I don't like that... it goes against my design philosophy. In my view you should set up a new system from scratch for the next "village" using low cost simplified sewers (small pipe diameters because the solids have been removed at source) and a nice simple low maintenance community vermifilter. I am afraid that a community digester would not receive the attention it requires, whereas I'd be much more confident with setting up a community-scale secondary vermifilter.

At whatever scale, the primary digester would require some maintenance and an understanding of its limitations. Lets use the domestic-scale model as an example, twin digesters (1m2 surface area each, for 3-6 users). Yes, the media would be coarse and does not need to be deep. It might be 20-30cm deep coarse pine bark sitting on porous textile (e.g. shade cloth), with passive ventilation underneath and to the sides. The media remains aerated because the humus builds up on the surface and retains the solids on top. For the first year or two (as the worm population builds) the influent might need to reciprocate to the other twin digester in 6 months or a year. This is because during the initial stage as the worms increase in numbers they won't keep up with the solids. Not a problem for the domestic system because if the toilet refuses to flush (backs up) the user knows that the digester is full and they need to change over the outlet to the other digester. There is no option, it needs to be done, thus the system is failsafe. After a year or two, once the worm population keeps up with the influent, it might be three or four years before the user needs to switch to the other side when the digester is full. With a larger capacity or a smaller number of users it may be ten or more years before the first digester fills, provided the system is working well.

This works well in the domestic situation, because it is user maintained by necessity.

The problem I see is that when a community is involved, nobody will be responsible for monitoring the system, because O&M is irregular and infrequent. The result from a whole sewer network backing up and overflowing is unthinkable and only avoidable by appropriate pre-emptive maintenance. Which history tells us will likely not happen.

One way of avoiding this is to ensure capacity is significantly oversized. But that costs money. We have discussed larger scale vermidigesters in another post . Lets assume that one thousand users will require a surface area of 200 m2 x 2. However, unlike the domestic digester, you will now need a method of distributing the wastewater evenly over the media surface, so the solids do not build up in one place (inefficient use of area). I would suggest that this is achievable but outside my area of expertise... but clearly requiring monitoring and active maintenance. You could also set up alarms in case your influent volumes exceed the hydraulic conductivity. Then, in addition to active maintenance of the influent, you'd require community-scale removal of humus from the rested digester at intervals.

Again, I'd suggest that the domestic scale is more appropriate for such works.

In contrast, secondary vermifilters at the community scale are "almost" maintenance-free, and users of the irrigation water would have an active interest in its quality.

cheers
Dean
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Dean,

I agree that with individual digesters, customers can be taught not to flush unwanted waste down the toilets. The question is whether not a trash rack before a central digester can also do the trick. It would be easier to install on the existing system than 200 household digesters (on a 1000 pop estate). Remember it is only household wastewater.

As said, I also had the idea with household digesters, simplified sewer and central vermi-filter. But it will be more difficult to convince a financier trying a completely new technology which hasn’t been proven. You and I believe it will work, but not the decision takers, not the local engineers, not the administrators, not the financiers.

It may be rather possible convincing them to revamp an existing, not working WWTP with a vermi-digester and vermi-filter and test/demonstrate that the technology works. I fully agree there is nothing like maintenance-free and I am willing not only to install such system but also care about the maintenance and make sure it works. I want to test and to prove that VC works, that is the aim. I will ensure regular O&M, and monitor the system like you monitor your household system. That is part of a development project: not only building new technology but also develop its maintenance. After, others will dare trying it on different settings.

I also see no problem providing 2 x 0.2m2/person, 20-30 cm high with passive, underneath aeration for the digester and installing a system which applies the incoming sewage evenly over the digester. And I will ensure monitoring and active maintenance for this process. Also, the removal of humus from the rested digester we discussed already in the previous posting which you also quote.

What is new to me (or I may not have paid attention to it) is the changing over between the digesters in the first year allowing the worm population to build up. What weight of worms would you recommend applying in the first instance, how many kg/m2 or how do you measure it?

… and even when it goes against your design philosophy, I still depend on your advice…

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. :-)
Albert Einstein
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Hajo, that would be great if you could do this... the only way knowledge and wisdom accumulates is by innovating and trying something different with an understanding that there is risk involved. I'm thinking that used condoms, tampons, disposable nappies, wipes and other things that do not decompose will get flushed down the toilet and they will remain undecomposed in the humus. At the household scale people would learn not to do that.

As we have discussed before, probably the cheapest way to construct a community scale primary digester would be using plastic pallets for the false floor to allow ventilation underneath. You'd also need a way of keeping insects, vermin and dogs etc out. Scavenging of human waste wouldn't be a good look. There shouldn't be much smell but I'm sure there will be some. You'd need a roof to keep out the rain. Essentially two 200 m2 buildings with roof and ventilated walls, a concrete drain/subfloor with plastic pallets and 20-30cm of media substrate.

I don't ever worry about the weight of the worms initially, because with the right conditions they build up quickly. The main thing is to provide enough capacity for the solids pile to grow because not much digesting will take place at first... and be prepared to reciprocate the inlet from one digester to the other regularly at first, so the layer of solids doesn't get too thick. This only until the worm population grows and digests at a similar rate to input volume. I don't think it matters how many worms you add initially, they won't keep up with the influent volumes for a period, so you design your system to allow for that. Sure, If you can get 50kg or 100 kg of worms you're better off than with 10 kg... but don't get stressed about quantities, and make sure that you don't add the worms until the system is operating for a few weeks.

cheers
Dean
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Dean,

I will try getting involved in O&M (and finances, and training, and reuse, and governance, and …). I have been a development worker my whole career, even if my employers and my government did not agree to this. 😊

I am not sure that household digester will teach people not to flush nappies and condoms. The user is not maintaining the toilet (unlike you 😊) and will never see what’s in the digester. S/He has a service provider for this work.

The design of the community digester we have discussed in the previous thread, I have an idea what it will look like.

Controlling the solid layer on the media: how much is ‘too thick’? What would you say how to judge it?

I asked for the kg/m2 of worms because I thought if I put more, the digestion starts earlier leveling the inflow earlier?

Why adding the worms only later? Do I put them on top of the solids? I thought they are first put in the wood bark before the sewage arrives, so they are covered and work from underneath? Obviously small but relevant steps to consider, I am lucky you have that experience.

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. :-)
Albert Einstein
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Hajo,
think of it like this... if you are adding 2cm of solids per day and the worms digest 1cm of solids from underneath per day, you are accumulating 1cm per day. It doesn't matter whether you have accumulated 10cm or 100 cm, apart from the weight factor. The worms will still only digest 1cm per day. 
But while the worms are breeding up they might only digest 1mm per day for the first month, 2mm per day for the second month, 5mm per day for the third month and by the 4th month have reached their capacity of digesting 10mm per day. Double the surface area and you double the overall capacity. It just takes time to reach the capacity so the idea is to use both digesters so you don't get too much accumulation at first. But one digester should meet your required capacity (the worms should digest the solids at the same rate  they are coming in). There should be a low mound of solids rising towards the centre and the mound should not go all the way to the edge, so that the liquid can easily drain away. Clearly for your prototype you should conservatively design for plenty of surface area.
Obviously you should add plenty of worms to get it going (and breed these in advance of setting it up) but yes they should only be added once some biological slime has built up on the media, resulting from the high BOD wastewater. They should be added to the edge of the pile, they will move into the media and to where they want to be.
cheers
Dean 
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Dean,

This is what I love with my job: I have to learn and understand such variety of different things. Now I learn about rearing worms in shit… 😊..  And I like your practical explanations of problems and solutions, thank you!

I understand how to get the thing going and what to observe. Remain two questions where I don’t know whether you just ‘assumed’ a number to explain the sample or whether they are numbers from your experience:
  •         It takes 3 months for the system to be in balance (if properly designed and started)?
  •         In a balanced system the worms digest about 1 cm of solids in a day?
I am asking because I would use the figures to design the size of the digester: 

1 person produces 300ml faeces plus 100ml toilet paper   =  0.4L/cd solids
              =  400 cm3/cd : 1 cm/d
              =  400 cm2/c = 0.4 m2/c 
Any comments by someone on above assumptions?

Build up during development phase in digesters:
  • Both digesters are used parallel,
  • Therefore, inflow is only 5 mm/d in each digester,
  • In the first month 1 mm/d is digested, i.e. 1mm FS is converted into 0.5mm humus, at the end of month total height is 135 mm of which 15 mm is humus and 120 mm FS. I assumed that the volume of the solids is halved by worm digestion.
  • In the second month 2 mm/d are digested, i.e. 2 mm FS  ->  1 mm humus, total 255mm, 45 mm humus, 210 mm FS.
  • In the third month 5 mm/d are digested (= inflow), 5 mm FS  ->  5 mm humus, end of month total is 330 mm of which 120 mm is humus, 210 mm FS.
  • In the 4th month the worms reach 10 mm/d digestion. If we maintain inflow at 5 mm/d (still using both digesters parallel) the worms after 4.5 months (day 132) have converted all build-up FS into humus.
  • Now we can switch all inflow (10 mm/d) to one digester as digestion by worms (10 mm/d) is in balance with inflow.
I am quite aware that all these are very theoretical calculations, but I like to have something which can be manipulated to reflect eventually the reality once we have more reliable input data.The EXCEL containing the calculation for the graph is attached. Have fun ...

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. :-)
Albert Einstein
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