VermiComposting (digesters and filters) in Kigali, Rwanda

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  • goeco
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Hajo,
This is a reasonable model :-)
The only variable we don't yet know is digester surface area, so the assumptions you have used are all good for an unknown surface area... except that the volume of the solids is not halved by worm digestion. The solids are 9/10 converted to humus... i.e. 1/10th of the original volume. Adjust your model and repost those graphs :-)
cheers
Dean
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

hi Dean,

I thought I had calculated the surface area and I repeat it here. Did you overlook it or is it not correct/sufficient?

1 person produces 300ml faeces plus100ml toilet paper                = 0.4L/cd solids
                 = 400 cm3/cd : 1 cm/d
                 = 400 cm2/c = 0.4 m2/c
i.e. required surface area is 0.4 m2/c for ONE digester (we build two of them).

Are you sure that volume of solids reduces to 10% by worm digestion? To be on the safe side, I have assumed 20% for this model.

I forgot to consider that ' they should only be added once some biological slime has built up on the media,resulting from the high BOD wastewater' . You do not specify how long that takes. I assume 'one month' and therefore in this model FS builds up without reduction by worm digestion for the first month. Now the model comprises:
  1.   1st month only built-up of FS to 150 mm total;
  2.   2nd month: 1mm/d FS converted to 0.2mm/d humus, total at end of month 276 mm;
  3.   3rd month: 2mm/d FS converted to 0.4mm/d humus, total 378 mm;
  4.   4th month: 5mm/d FS converted to 1 mm/d humus, total 408 mm;
  5.   for 2 month 12 days: still inflow 5mm/d, reduction of heap by conversion of 10mm/d FS to 2mm humus, total 192 mm;
  6.   from now inflow 10mm/d only in one digester, conversion 10mm/d FS to 2mm/d humus, humus heap rises by 2 mm/d.


What worries me is your remark from an earlier posting: There shouldn't be much smell but I'm sure there will be some.  In my first envisaged application the treatment is right between houses (30 m to the nearest house). So far, I believed VC is a solely aerobic process and as such smell-free. I only learned through the latest discussion that the excreta first build up for 6 months before the system is balanced. Do you expect more smell during the 6 months only or also after when the digestion is almost immediate?

ciao
Hajo
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Hajo, what I am saying is that the assumption that the worms digest 1cm of solids per day from underneath is (at this point) arbitrary. Because we don't know digestion rate we don't know corresponding application  rate (which depends on surface area to equal digestion rate). Therefore, for a "balanced" system (application = digestion), application rate is the response variable, digestion rate is the explanatory variable. You need digestion rate to determine application rate (and therefore surface area).

However, at the small scale I have found a 1m2 surface area to be balanced for 4-5 people. The second digester (another 1m2) provides a buffer while worm population builds up (and also allows digested solids to rest).

I have my household primary digester directly under my front deck and if it smelled this would be embarrassing for me every time someone came to my front door. My concern is that at a larger scale, what smell there is would be amplified by the large surface area of solids. This does smell (not like rotten eggs... it is a kind of sweet pooey smell when I open up the digester to take a look... it isn't a "bad smell"). 

Another concern I have is that through a sewage line there will be significant mixing of solids and liquid. This might be slower to filter through the media than something immediately flushed. What is the consistency of the treatment plant influent? You should set up a pilot beside one of your failing treatment plants and test this. Infiltration rate could be an issue if the solids are mixed and suspended in the liquid.

Volume of solids does reduce to 10% by worm digestion. This is actually conservative, I'd go with that in your model.

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

Hi Dean,

I must admit that I took your 1 cm/d digestion by the worms as too certain, but I also think it is not a too bad assumption considering your small-scale experience that 1m2 is in balance with 4-5 people. If the small-scale needs1 m2 for 5 people, that is 0.2 m2/c.

If my assumption of 0.4 L/cd solids is somewhat correct, the application rate (= digestion because in balance) is 400 cm3/cd : 2000 cm2/c = 0.2 cm/d. Ok, it is still out, but only 😊  by a factor of 5.
But somewhere I must start, even for a first trial I need to know whether I design the digester with 0.2 m2/c or 2 m2/c or between. Thus, I think your experience with 1 m2 for 5 people is a reasonable start. My first envisaged project has about 125 users, i.e. one digester would be 25 m2 (8 x 3 m2).

Your information on the smell at your doorstep 😊 (or rather not) sounds reassuring. Of course, with larger quantity the smell will amplify. But the next nose is 30 m away, not 2 m. I can design the application mechanism (of sewage onto the digester) in two ways: either putting it more confined or more dispersed over the area. With regard to smell what would you recommend or are there other aspects more relevant. I thought, spreading it, will make it easier for the worms to work on.

I expect the solids being mixed and suspended in the flush water as it is usual with sewage having a solid content of maximal 1%. I anticipate a problem in the beginning as the wood bark alone is too porous until the pores clog up, slim builds and worms create humus. Maybe the wood bark should be covered with saw dust for the start to increase filtration resistance? Eventually the saw dust will be replaced with humus.

Volume of solids does reduce to 10% by worm digestion. This is actually conservative. Where do you get this information from, your own experience or literature?

Ciao
Hajo
We can't solve problems by using the same kind of thinking we used when we created them.
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

hi Dean,
hi all,

when we were discussing this morning at the breakfast table(! :-) ) how to design/improve a VC system for treatment of sewage, we had the following idea: is it possible that the sewage has the consistency of the effluent from a digester which had been fed with FS or septage, i.e. with higher percentage of solids? This effluent then goes into VC filter (high, small diameter) for polishing.

Why not consider dropping the digester and feed the sewage (maximum 1% solids) straight onto a battery of VC filters, i.e. high and small in diameter as discussed before? We may need a lot of them and two or three stages to achieve WHO standard quality for surface irrigation.

But it saves the problem how to design/develop the VC digester for sewage which seems being problematic.

ciao
Hajo
We can't solve problems by using the same kind of thinking we used when we created them.
Albert Einstein
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Albert Einstein
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Dear Hajo, dear Dean

thanks very much for sharing all the details with us. I read everything but up to now I can't add any wise statement ;-) But if I  need help one day, I know who to ask!

Have a nice day,

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

Hi Hajo,

the whole concept of the Primary Digester is that the solids are intercepted and dealt with there. The shallow media ensures it remains aerobic at all times, so the sump is the liquids minus the solids, which is suitable for the Secondary Vermifilter. You don't want a layer of solids building up in the SV, it isn't designed to be as strong as the PD, which is built with plastic pallets on a concrete pad and can take the weight of accumulated solids. Also, the SV "columns" are small in surface area and deep, the opposite of the PD. Buildup of solids on the surface of a SV will block infiltration of the liquids.

This is why I design as two stages, PD and SV. The outlet of the PD can feed straight into the SV though... provided you have fall.

What I am saying is that by mixing your solids with liquids, your 1% solids sewage "brew" should simply be tested for rate of infiltration. As an example, lets use the extreme - sludge. Sewage sludge is really really slow to lose its liquid. In one of my systems I use a tank to settle out the solids after the PV. Every few years I pump the accumulated sludge back into the PD. Because the solids and liquids are really well mixed, this sludge takes a week or so to drain its liquids and become a solid mass. Contrast this to fresh poo and toilet paper flushed straight onto the pile in the domestic PD, this drains almost immediately.

I don't think your sewage will be problematic, but when designing your PD surface area you should match infiltration rate to the influent volume. The surface area might need to be wider than a domestic PD because the solids will get mixed into the liquids more by travelling through sewage pipes.

You could put some sawdust down on top of the bark, but this might not be necessary, the solids can penetrate the surface and you will have textile (shade cloth) at the bottom on top of the pallets. Perhaps start with a thin layer of sawdust in the middle of the PD and apply on top of this to create a mound in the middle on which you continue to apply sewage. You'll always want a low wide pile sloping upward into the middle, so water does not pool (which just happens in a domestic PD!).

Hope this clarifies things. Oh, yes, 10% is the magic number for residual solids (humus). I've read that in the literature and also found this myself.

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

Hi Dean,
Dear all, 

The design and general concept of PD and SV are clear to me. And I can imagine how they work with household effluent (like at your home), with FS collected from latrine pits and septage from septic tanks, where in all cases the solids and fluids are somehow at larger percentages and somehow well defined. And where the PD is meant to collect and digest the solids while the SV will remove contamination and BOD from the effluent.  

But what is the consistency of the effluent from the PD? What does it contain? What is the difference between the effluent from the PD and sewage (minus the solid waste in it!). I need to understand the difference so that I understand why a SV cannot treat sewage without PD before. Sorry, if I am pesky ….

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: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Hajo,
the effluent from the PD is classed as "primary treated", so would still have some suspended solids, along with a very high BOD and lots of pathogens. A bit like what comes out the end of a septic tank (in terms of suspended solids and BOD) , but there will be differences. I haven't attempted to measure these but the process is very different. The PD process takes minutes whereas a septic tank involves a long slow anaerobic process, which probably removes some of the nitrogen and definitely removes all of the dissolved oxygen, but not the BOD. Not that these differences should matter as far as the SV process goes, but I'm assuming the PD effluent, with plenty of dissolved oxygen and aerobic organisms intact, would be better than septic tank effluent, as influent into the SV.

One important consideration with respect to PD effluent, is that "worm juice" is added to the wastewater flow through the digester. By digesting the solids, the worms produce liquid "effluent" themselves. So although the wastewater retains some dissolved oxygen through the PD process, the BOD will increase (so it is different to just sewage with the solids removed). 

It simplifies the design of the SV if the solids are removed from the flow. You don't want too many solids accumulating on the surface because they'd divert the influent flow to the edges. The SV is designed to remove almost all of the suspended solids and the worms digest these while maintaining the porosity of the media. It just works and is maintenance free. But if solids build up on the surface the flow would be diverted to the edges of the reactor and flow down through only some of the media. What we want is a maintenance-free reactor; that uses the whole media volume effectively (in contrast to the PD which doesn't need to treat the liquid).

An important design consideration for the SV is that the influent needs to be dispersed evenly across the media surface, but nozzles inevitably block from accumulating biological slime. A free-flowing inlet and "splasher" overcome this, but if you had solids coming in also, how would you manage to evenly disperse the influent on the media and how would you continue to do so over time?

I hope I am answering your questions!
cheers
Dean
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Dean, hi all, 

I think the major aspect at this point of discussion are the solids, not BOD, not pathogens. A better understanding of the properties of the solids in sewage, I found through the following pages: 

https://cgi.tu-harburg.de/~awwweb/wbt/emwater/lessons/lesson_a1/lm_pg_1066.html
https://cgi.tu-harburg.de/~awwweb/wbt/emwater/lessons/lesson_a1/lm_pg_1079.html
 


And out of this, I developed the following graph, where I try to compile the properties which are relevant for the separation of the solids from the sewage:



As you see solids constitute only 0.1% of sewage. Of these 70% are dissolved and can only be extracted by chemical (mostly in WWTP) and biological processes (as in the SV).

The suspended solids (of which 90% are organic, 10% mineral) must be filtered out and digested by the worms. As we discussed previously the digestion and application rate is in balance at a PD area of about 0.2 m2/c. This area is not provided by an SV and inevitably solids will build up on the filter. Therefore, we require the larger area of a PD (at 0.2 m2/c) for the filtration and digestions of the suspended, organic solids. Of course, there are the other ‘side effects’ of the PD which you mentioned: dissolved oxygen, aerobic organism, worm juice, BOD increase, …

For those, who are not yet so familiar with the VC processes: by long or multiple SV runs, the effluent can be polished to WHO standard for surface irrigation water. Thus, effluent will NOT go to surface waters but onto fields and a higher nutrient content is therefore appreciated.
Ciao
Hajo
We can't solve problems by using the same kind of thinking we used when we created them.
Albert Einstein
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Everything should be made as simple as possible, but not simpler. :-)
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Re: VermiComposting (digesters and filters) in Kigali, Rwanda

Hi Hajo,

Not sure if your better understanding is of benefit. It's easy to get confused by solids species because these are arbitrary. The difference between "dissolved solids" and "suspended solids" is decided by the 1.5 micron filter, but many particles smaller than 1.5 microns in size are definitely not dissolved. If the PD catches 95% of the solids it doesn't matter whether those solids are volatile or fixed, nor that they are suspendible or dissolvable. What gets through the PD is suspended in the wastewater and the SV removes it. The system needs to reduce the solids down to low levels.

The level of suspended solids (not including dissolved solids) in untreated wastewater is around 200 g/m3, which needs to be reduced to something like 20-30 g/m3 via secondary treatment, for surface discharge to land. "Suspended solids" probably represents helminth count fairly well because the less suspended solids, the less helminth eggs there will be.

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

 Hi Dean,

I think better understanding is always of benefit. And if my classification helped me to understand better which is at least of benefit to me. I wanted to share, as I thought, it also may help others. At least, my understanding helped me to see the need for the digester before the VC filter!

You are more conversant with physical and chemical properties of the stuff we talk about. And that the 1.5 micron filter decides between ‘dissolved’ and ‘suspended’ where the smaller particles are not necessarily dissolved is also of benefit for my understanding.

Your figure of 200 g/m3 of suspended solids in untreated sewage compares fairly well with the figures in my graph above where total solids are 0.1% (i.e. 1000 g/m3) and of these 30% are suspended solids (i.e. 300 g/m3). When you say it needs to be reduced to 20-30 g/m3 via  secondary’ treatment, you probably mean ‘primary’ (which takes the most) and ‘secondary’, or? Where secondary treatment cares more for ‘dissolved’ solids, or?

And don’t we have ‘settlers’ below the primary and secondary treatment, so that almost no suspended solids remain in the effluent? I would wish/expect that there are virtually no suspended solids remaining in the effluent after the secondary filter, so that we can be sure that no helminth eggs remain in the irrigation water. Otherwise its use cannot be recommended for surface irrigation in accordance with WHO standards, or?

Where does the requirement of 20-30 g/m3 comes from? Is it to avoid clogging of sprinklers or drain pipes?

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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