VermiComposting (digesters and filters) in Kigali, Rwanda

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

Hi Hajo,

Interesting your observations about sewage (99.9% water content) on the PD.

No... to reach 1 egg per litre, 99.9% of helminth eggs need to be removed from sewage.

The solids component in piped sewage will be highly mixed and saturated with water, thus spreading as a thin layer over the whole surface of the PD as more sewage enters the digester. The water in this "homogenised" sewage will slowly drain, leaving a layer of fine solids on the surface ("mud"). Provided you have a good population of worms they will rapidly digest this layer from underneath, perhaps even in a couple of days.

You could install a distribution manifold and digester sections. The shelter cloth used to line the floor of the digester would go up the drainage walls and retain the solids. You might find that you can keep piping sewage into the digester once it pools. I've done that with my domestic digester, I pumped a homogenised thin sludge into the digester until the water level was 30cm above the media surface. This doesn't drown the worms because it remains aerobic below the layer of sludge, provided the digester is ventilated underneath (e.g. plastic pallet false floor). In my case it took a day or two to drain to a mud. So reciprocating between several digester sections sounds feasible, or even just two, but just remember that regardless of number of sections, total surface area is what matters the most, more is always better... and don't divide digester sections so worms can't migrate between them. Indeed, a thin spread out layer would be digested more efficiently than a mound of the same quantity.

If I understand you right, we have four layers in the PD from bottom to top: the coarse media of wood bark, the more permeable layer of humus made from worm casting, the less permeable layer of mud from suspended solids in the sewage, and on top the freshly applied sewage.

Mostly correct. Humus will be mixed through the coarse bark media but does tend to build up on top of the media. Humus is less permeable than coarse bark, but is still permeable. The layer of impermeable mud only accumulates if the solids in piped sewage are too thin to form a mound, so you will either have a mound with a perimeter, or a layer of mud... but not both, depending on the consistency of the influent. Neither are permeable.

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

Hi Dean,

As Elisabeth indicates the 2006 WHO guideline may also refer to the ‘1 helminth egg per litre’, I’ll search for it.

Good to hear that efficiency of VF builds up quickly by the slim development and that efficiency of PD can be influenced positively by increase of worm population in the first place.

Interesting your observations about sewage (99.9% water content) on the PD. First, I want to comment on the requirement that the sludge mound should be concentrated allowing water to drain away at the perimeter of the mound. As the water also carries suspended solids (especially with sewage, possibly less with freshly flushed solids), the perimeter will also get covered with ‘mud’ and slowly the perimeter will increase and increase eventually to the edge of the PD, or?

If I understand you right, we have four layers in the PD from bottom to top: the coarse media of wood bark, the more permeable layer of humus made from worm casting, the less permeable layer of mud from suspended solids in the sewage, and on top the freshly applied sewage.

Should a community-scale PD eventually be designed such that the PD is somehow divided into sections which are fed with fresh sewage one after the other? And the first section only be fed again when the layer of mud has been converted to humus and has become permeable? This will allow the water drain away faster than if the sewage was fed onto mud.

I had already before anticipated that I need to ‘move’ the inflow of sewage onto the PD and had developed an idea how to do it. Let us assume the PD has seven sections, each with its own inlet connected to a manifold. Monday to Sunday, each section gets its ‘shot’ of sewage. After seven days the circle returns to section 1. Because each section had seven days ‘resting’, the water of the sewage percolates quite quickly allowing the worms to feast on the suspended solids. Is one week enough time, for the worms to digest the suspended solids of a thin layer of sewage? If not, I must increase the number of sections or reduce the amount of sewage applied in each ‘shot’. Both will require more PD area. But somehow it should relate to the 0.2 m2/capita which we discussed earlier.

I know we do somehow research work but if I get to build such community-PD, I do not want to start with a size which is completely out of requirements. Therefore, these theoretical discussions.

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

hi Elisabeth,

thanks for the link to the WHO guideline 2006...  i have downloaded them...  700 pages!...  and partially not in searchable format...  I have transformed them to editable PDF...  and when I have time I'll search for ascaris and helminth...

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 Hajo,
The maximum 1 helminth egg per litre originally comes from 1989 WHO guidelines for irrigation of crops, pasture and trees. This is equivalent to a removal efficiency of up to 99.9% and also serves as an indicator for other pathogens such as protozoan cysts. The guidelines appear to have been designed for what is achievable using well designed waste stabilisation ponds. Apparently the WHO recommendations are not zero risk, but are more pragmatic.

The efficiency of the secondary vermifilter builds up quite quickly (a couple of weeks), this is achieved by the biological slime that builds up on the media rather than the presence of worms. The worms also digest the accumulated solids in the primary digester, so the sooner the population builds up the sooner digestion reaches an equilibrium with incoming solids. So yes, adding plenty of worms will get reproduction happening faster in the early stages.


The issue with solids in suspension is that the liquids are held for longer on top of the primary digester media. That is, it takes a few days for the water to drain out of sludge. It's difficult to form a mound with thin sludge because of the high water content in the solids, it spreads out on the surface as a mud layer and only drains slowly. Once there is a layer over the whole media, surface drainage is impeded. Contrast that to freshly flushed solids, which are sufficiently solid to form a low wide mound. The water runs down the side of the mound and drains through the media at the perimeter of the mound.


It's not about the coarseness of the media, but ensuring that the media is not completely coated with a layer of mud from accumulated suspended solids. Yes, the primary digester should use a fairly coarse media with good drainage, but worms add humus to this, which is finer... but importantly still drains. But add a layer of mud on the whole surface and you impede drainage. The primary digester should have a low, wide mound of solids in the middle, but the perimeter of this mound should never get to cover the whole surface area of the media.


For your community-scale primary digester the inlet must be positioned so the sludge doesn't spread to coat the whole surface area of the media, but the mound builds in a fashion that allows the water to drain. This might require no maintenance at all, like a domestic primary digester, but I don't have any experience with a community-scale digester. You might find the inlet needs to be moved at intervals.


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

Hi Hajo,
You asked about WHO Guidelines. Did you mean the ones from 2006 or something more recent?

The ones from 2006 are here:
WHO (2006). WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater (Volume IV: Excreta and greywater use in agriculture). World Health Organization (WHO), Geneva, Switzerland, ISBN: 92 4 154685 9
www.susana.org/en/knowledge-hub/resource...library/details/1004

The criterion of "less than 1 helminth ova per litre" was mentioned in there. But the guidelines were supposed to be adapted for each country.
Not sure if this helps.

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

hi Dean,

sorry, have been very busy on other things but VC is always at the back of my mind.

Where can I easily read up the WHO condition for the use of waste water for irrigation, either on trees or on uncooked edible veggies? Can you direct me to a relevant literature?

Why do you quote the '<1 helminth ova per litre'? Is it one of the WHO criteria?

The removal of suspended and dissolved removal of solids in sewage by PD and SV is clear. And that effluent from a drop toilet contains less suspended solids because of the lacking 'mixing action' in the sewer, is also understood.

We discussed earlier that PD (and SV) need some time to build up their efficiency, and even up to 6 to 12 months. This may be especially valid where we use PD for clearing sewage because of the high suspension and the subsequent low solid content. My interpretation: the wood bark is too coarse and it takes long that filter action by the PD is build up as pores in the wood bark close slowly with humus. What would you recommend speeding up the filter efficiency? Mixing of wood bark with saw dust? Adding more worms reducing their multiplication time?

Is it correct to assume that a PD under a household drop toilet will go into 'action' almost immediately because the faeces are (almost) not suspended? Yes, also the household PD will let through some more suspended solids in the beginning until the humus has built up to a reasonable filter, but much less suspended solids in the effluent than under a sewage PD in the beginning, isn't it?

Why I ask? Because I am not building a VC system for my own use where I probably will take interest and check daily how the worms and the VC are doing. I plan to build VC systems for others who only have the interest for a functioning toilet/treatment without paying attention to its proper operation. (No German household shows any interest in what happens after the flush!) And I cannot check the proper operation of multiple systems every day, I must rely that I can install the system such, that it requires checking only once a month (or once a week at most). Thus, an 'as good as possible' PD function from the start is paramount.

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

Hi Hajo,
the 20g/m3 comes from the NZ standard for surface irrigation. This seems to vary between countries, usually between 10 and 30 g/m3. However, this is NOT a standard for irrigating food crops. It certainly would be suitable for irrigating tree crops or banana palms, but not lettuces! I'm not sure what level of suspended solids equates to <1 helminth ova per litre, but the evidence is that secondary vermifiltration is very efficient at removing helminths. I assume that is because helminth ova are sticky and attach to the biofilm. I tend to use the "simple clarity test" for suspended solids... if the treated wastewater is clear then the suspended solids have been removed. The SV removes both suspended (filter action) and dissolved solids (biological action), whereas the PD doesn't. It just removes most of the solids. But my point is that if the PD is straight underneath the flush toilet, only a small proportion of the solids become suspended. Mix everything through sewers and I assume the suspended solids percentage will become higher.

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.
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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, 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: 

cgi.tu-harburg.de/~awwweb/wbt/emwater/le...n_a1/lm_pg_1066.html
cgi.tu-harburg.de/~awwweb/wbt/emwater/le...n_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
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,
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,
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.
E.F. Schumacher
Everything should be made as simple as possible, but not simpler. :-)
Albert Einstein
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