Hi Dean,

I come back to your last posting and will try to develop a VC system for septage from 500 users consisting of digester and filter considering the numerous conditions spelled out in your post.

First the digester:

Should be large in area; not building FS up high, if so, FS application needs to be directed to the next digester; aerated from bottom through false floor and media (first wood chips, later humus), I like your idea using paletts for the false floor and have traced them on the internet.

If I remember correctly you recommended 'your' fruit crates (1 x 1.2 x 0.9 m) for use as digesters, two of them to be used alternatively for a household of 4 people. That is 2 x 0.3m2/user. Can I use the figure for design of a larger system, i.e. 500 users require about 2 x 150 m2 used alternatively?

Then I would design a rectangular containment from blocks/bricks/concrete, 40m x 7.2m, walls about 1m high, floor falling from both long sides towards the middle, where a 200mm wide channel will collect effluent running down from the sides of the floor. On the slopping floor sit HDPE pallets (1 x 1.2 m, 3 Nos. each side of the channel, touching in the middle above the channel allowing the worms to migrate from one to the other side) with the textile and media on top. The outer walls have numerous holes (100mm PVC) at the pallet level where a) air can enter for aeration of the system from underneath and b) from where the floor can be flushed in case fine solids pass the filter and settle on the floor.

The digester is covered with a roof and the trusses of the roof carry two manifold pipes (DN100) with outlet pipes every 2m through which the septage is fed onto the media, alternating between the two 3.6m x 40m sections (halves) of the digester.

The vertical sides of the digester between roof and wall are closed with mosquito nets in frames which can be removed to allow the harvesting of the humus with long rakes from the middle to the walls once one side of the digester has built up enough humus. I assume the humus will only be harvested to such level that about 30 cm media remain as basis for the next processing cycle. We may have to consider that the full media must be removed occasionally when there are indications that the textile is torn or blocked, or?

Secondary/tertiary VC filter treatment:

VC filters should be high ensuring a long contact time of the effluent from the digester with the microbes and worms in the filter media. And they must be small in diameter to ensure good aeration of the filter media also in the centre.

You recommend two different systems, either round cartridges, about 600mm in diameter, some meters high or stacking ‘your’ fruit crates (4 Nos make about 3.6m height).

What do you recommend for use when you want to make the filter cartridge 3 to 4 m high? I cannot imagine any ready-made product on the market fulfilling our requirements, i.e. being sturdy to stand 4m high and having enough holes for aeration. Therefore, I like stacking of fruit crates better. Like the cartridges the stacks can also have single free-flowing entry points for the effluent. And the stacks can be kept apart like the cartridges ensuring good air access from all sides.

I understand that your aim is scaling down and if we succeed with our developments here in Rwanda this should also be possible in specific locations and environments. But please keep in mind, that we have also to adapt to existing conditions and cannot change these easily. In our towns exist hundreds of septic tanks which would best be replaced with VC digesters but as long as they exist, we have to provide an emptying and treatment service. For this treatment I want to recommend VC based FSTP, and not only to provide the necessary service but also to try/show that VC can also work on larger scale and could eventually even be used for decentralised (and centralised?) WWTP.

Another question: did you observe any specific problems of VC with rodents and pests (cockroaches) eventually being attracted by the smell, effluent or the worms where we must consider measures against?

Looking forward to your professional comments,
Ciao Hajo

Dean, just a big and quick 'Thank You' for your elaborate response...
.. will take some days to 'digest and filter' your information before I come back with questions and suggestions.

ciao
Hajo

Hi Hajo, 
With secondary treatment the level of aeration in the media impacts on efficiency and capacity of the system. The more aerated the media is, the more efficient treatment will be and therefore the smaller the reactor capacity required for the same level of treatment.

The video shows a secondary treatment vermifilter system. If you look carefully at the video, you'll see that the earthworms are all living near the surface. This is because that is the aerobic zone and this large population of worms are all breathing and competing for limited oxygen. You'll also see the pipes sticking out which provide ventilation to under the false floor, but is the full depth of media being aerated? I would suggest that there are anoxic zones that are "wasted" capacity. This design has been around a while, commercialised in South America and used around the world for everything from dairy effluent treatment to human sewage treatment. You'll see that the area is quite large and therefore lots of concrete and requiring a well constructed false floor to hold the weight of the media. 

What I've found with "cartridge" filters is that the treatment efficiency is vastly increased. The cartridge can even be several metres high and the worms live throughout the media because demand for oxygen never exceeds supply. The false wall provides ventilation right around the cartridge diameter and that diameter might only be 600-700mm. Because cartridges can be a combination of series and parallel, recirculation can be practiced, which further increases the effectiveness (both cost and treatment) of the installed media capacity. Another important benefit of cartridges is that each cartridge has a single free-flowing influent entry point. No nozzles are required because the surface area is so small. Sprinklers, although they distribute water over a large area, require pressure and are prone to clogging, so the influent would require high levels of pre-filtering or settling, which kind of defeats the purpose, which is to filter out the suspended solids in the vermifilter itself.

Keep in mind my focus has been on scaling down rather than up, because cost-effectively scaling down sewage treatment systems has always been the challenge. That said,  Lets consider secondary vermifilter "pods" of 36 big plastic crates, 4 layers high x 9 crates per layer (note these "forklift" crates are designed to stack and form a robust structure). My estimate is that two of those pods in series would treat 100,000 litres per day, suitable for irrigating a substantial banana plantation.

Next, on to primary vermi-digesters. Think about the surface area at the bottom of a wet pile of faeces or sludge. If the pile can spread out that surface area expands greatly. A circle with a diameter of 1m has four times the area of a circle with a diameter of 0.5m. The worms work the pile from underneath so can digest a low spread out pile way faster than a high pile constrained by walls, as a simple factor of surface area. So the aeration of the wet compost heap occurs only from underneath by the worms, which is why there needs to be ventilation under the false floor for fast digestion. Decomposition doesn't take place in the middle of a wet heap, so if the pile gets higher and higher then the worms aren't keeping up with the influent. The greater that surface area where the pile is sitting on the media, the faster the worms can digest the solids. If you are pumping septage onto a media bed with a false floor, be aware that surface area is what matters. But you wouldn't be using fruit crates, you would design and build a false floor that would handle the weight of the septage and allow the liquid to drain away. I'm suggesting plastic pallets covered by textile and a layer of media. You'd maintain the solids layer at about 30 cm high and fully utilise the surface area available. If the worms don't keep up then you just need more surface area (more pallets). The drained liquid, of course, would feed the secondary VC vermifilter. 

cheers
Dean

Hi Dean,
hi all,

how would you upscale a VC (vermi-culture) system of digester and filter to a community level? Let's say 2000 people in a small town are using septic tanks (the rest is on pit latrines or even OD). A vacuum truck is available to empty the septic tanks and should supply the FS to a central VC plant for treatment.

I can imagine that the vermi-filters for the second/third stage treatment can be designed as a battery of filter cartridges as I described already in an earlier mail.

But how can the digesters look like? I am asking because Dean always emphasis the need for good aeration of the digester, although I wonder how air can reach the core of the 1 x 1.2 x 0.9m fruit crates. Do we need a huge battery of fruit crates, one crate for 4 people, i.e. 500 crates?

But we have this video from Prof. Sinha where he shows a large bed of digester fed with sewage from sprinklers with no aeration from the sides. How can the worms survive?

Of course the septage pumped from septic tanks cannot be fed by sprinklers onto a digester like the sewage in the video. We probably need a manifold pipe with several outlets above the digester from which the FS is 'sprayed' onto the digester using a sludge pump (on the outlet of the vacuum truck?).

I am just throwing some ideas onto the paper and hope that we get into a creative discussion... I wonder whether such community digester for septage has been done before?

ciao
Hajo

dear all,

2). Evidence that the Worms by Disinfecting & Detoxifying the Sewage (or Sludge) do not become HAZARDOUS WASTE themselves?

DISINFECTION: Harmful Microbes (PATHOGENS) are killed selectively by the EARTHWORMS through PHYSIOLOGICAL, MICROBIAL & ENZYMATIC Actions. They release COELOMIC FLUIDS that have ANTI-BACTERIAL properties and destroy all PATHOGENS in the SEWAGE & SLUDGE or any WASTE BIOMASS. Earthworms can distinguish between HARMFUL & USEFUL MICROBES in the WASTE Materials & SOIL & selectively DEVOUR on the HARMFUL PROTOZOA, BACTERIA & FUNGUS as food leaving the USEFUL ones.

In the intestine of Earthworms some BACTERIA & FUNGUS (Pencillium & Aspergillus) have also been found. They produce ‘ANTIBIOTICS’ & kills the PATHOGENS in the SEWAGE & SLUDGE making it virtually STERILE ....

In his above posting (#27987 of 16.08.) Prof. Sinha says that worms can never become hazardous waste because they do not only store the toxins and heavy metals in their tissues but by all sorts of microbes process them and inactivate them. I like to believe it but that Sir Charles Darwin called worms ‘the creators of Mother Earth’ is not enough prove to me.

In the two papers which the Prof. attaches more arguments are brought up to support the evidence and the papers also include references to research done by others which the Prof. uses as support for his papers.

I am neither a researcher nor a chemist, most of the chemical description of compounds and/or processes, I do not understand. Do you have any idea who could be asked to verify whether the statement by the Prof. is supported also by other research literature to the extent that some/ most/ which toxins and/or heavy metals are turned inactive / harmless to humans and environment through the actions by the worms?

As indicated by Dean in another posting, the answer to the questions is not so relevant for the processing of household waste (i.e. vermi-composting of household waste water) as it will almost never contain hazardous waste (what about remains of medications?) and where it is processed at household level. More interesting it becomes already at a community level where FS from septic tanks and pit latrines is collected from various uncontrolled places and processed in a central FS treatment plant (now VFTP). Really relevant becomes the answer once we imagine treating sewage from a central town sewer system where also industrial/commercial plots are connected. Then the likelihood of toxins and heavy metals in the sewage becomes immanent.

Who understands whether the evidence provided by the Prof. supports the hypothesis that worms do not become hazardous waste in action or who knows someone who may know?

Thanks and ciao,
Hajo

Hi Dean,

thanks very much for the response, as usual very informative and detailed by you...

I will discuss your proposal for a 'neighbourhood' vermi-composter with my partner organisation.

From your explanations it looks that the vermi-filter using worm-cast/humus is almost self-maintaining (except for occasional scrapping of surplus humus): a perfect system.

I wonder how a vermi-filter will look like at a larger scale, i.e. beyond household vermiculture. The design parameters are the height of the filter which determines the filtration quality (either height or multiple filters in row) and the cross-sectional area not being too large ensuring ventilation and aerobic conditions at the centre of the filter. That probably requires building numerous filter 'cartridges' (preferably round) beside each other and being supplied with effluent from the digester in parallel (one manifold pipe supplied by a pump with effluent from the digester and with an outlet for each filter cartridge).

thanks and ciao
Hajo

Hi Hajo,
The large plastic fruit crates are convenient for a standard domestic installation because they are "self-contained" with walls and "false" floor and make the installation process easy. I would suggest that shared toilets with 15 users do require a larger surface area and this is easily achieved using other methods. Have you priced the plastic fruit crates and is the price reasonable? What about plastic pallets? Six plastic pallets would provide sufficient surface area for 15+ users. Here is a design for a simple twin-pallet domestic vermidigester but this could be scaled up easily:


The main issues to consider are that surface water and rainwater shouldn't get in, the pallet must be above any water table and the environment must be aerobic and ventilated - air needs to move underneath the digester. Also note how the "wet compost" heap should to be able to spread outwards rather than upwards, which gives the surface area underneath for the worms to work. A simple method can be devised to swap the ventilation hat around for the influent pipe to swap sides.

Regarding the filter media in a vermifilter, humus is the perfect medium. However, this takes some time to be generated which will be slower than the rate of decomposition for sawdust, wood shavings or woodchips. Therefore occasional topping up is required until sufficient humus is present such that rate of media decomposition doesn't exceed rate of humus generation. Then, at a certain point, once the media is all humus (which doesn't decompose), occasional removal of some media may be required, perhaps once a decade. So yes, with gravel it will not require topping up, and the humus will build up and require scraping off the top sooner. Keep in mind build up of humus is very slow in a secondary vermifilter compared with a primary vermidigester, and even slower in a tertiary vermifilter.

Dead worms decompose in the aerobic environment and although this adds to oxygen demand, this is negligible. The environment is rich in microorganisms and oxygen, so anything that can decompose will rapidly do so. It's a dynamic cycle where carbon provides energy for organisms, oxygen is used and CO₂ is generated. Microorganisms also die, just like worms, but there is always something else present to clean up.

cheers
Dean

Hi Dean,

I feel scaling the surface of the fruit crates according to number of users seems not practicable to me: the crates have a surface of 0.8m2 (used for 4 users). 15 users would require 4 x 0.8m2 = 3.2m2 which I hardly accommodate under a household toilet, and... I also need the second 'crate'. More practical seems to me a compromise: double the surface (1.6m2) and double the changeover frequency (from 1 in 4 years to 2 in 4 years). What you think?

Some questions on the filter media:

• why should the whole medium become humus if it does not need to be replaced?
• why do wood chips and saw dust require topping up if they are replaced by the worm cast humus anyway?
• if I use gravel will the voids not eventually be filled by worm cast humus because the gravel does not decompose (unlike wood chips)? And the production of vermi-cast as well as the growing worm population contribute to an increase in volume of the filter medium?
• are you actually saying that the vermi-filter does not require maintenance beyond scrapping off some vermi-cast as it builds up over time? what happens to dead worms as they do not live forever?

thanks and ciao
Hajo

Hi Hajo, yes 2 x 1m³ digesters are sufficient for 4 people. I'd suggest for 15 people the digesters would be a larger capacity rather than increasing the changeover frequency. Note that the digester is simply used until it fills and the toilet starts gurgling and letting you know one side is full. It's foolproof because eventually the flush will stop working unless you change to the other side. Only one side can be full at one time because the contents of the other side are fully digested by the time the other side fills (and the worms migrate between them). The surface area of the digester is what you scale according to number of people. You may find plastic pallets are available in Rwanda? Even broken pallets can be used for constructing simple primary vermi-digesters.

I prefer organic filter mediums for vermifilters so that the whole medium becomes humus. Wood chips and sawdust are fine but I'm just not sure what you have there in Rwanda that is suitable. Wood chips and sawdust do tend to decompose and require topping up. My favourite is pine bark because that is very slow to decompose. Vermicompost is good if you can get it, or gravel can be used. Reasonably porous is essential and soil doesn't tend to have sufficient porosity.

The filter medium doesn't need to be replaced, once the vermicast humus builds up this is a very stable media for the worms to live in, with just the right porosity. Essentially the woodchips are there as a temporary medium and eventually get replaced by the vermicast humus. Which is why I prefer organic mediums. None of the filter medium should get washed out, it is held in place with a geotextile cloth. The treated effluent is balanced in nutrients because it contains both the dissolved nutrients from urine but also the dissolved nutrients from digested faeces.
cheers
Dean

Hi Dean,

just another question to the vermifilter (2nd and 3rd stage of treatment):

the filter are also made up of medium (what material? also wood bark or gravel?) on which aerobic bacteria will settle helping to process the solids from the effluent (organic waste?) and on which the worms are also feeding (on the waste and on the bacteria?).

As the worms excrete, the filter media gets more and more blocked by the worm-cast/humus which improves the filter process but also reduces the filter velocity. At a certain point the filter needs to be 'cleaned' or 'replaced'. I can't remember having read about this stage of the process. Can you please explain it?

To a certain extend the humus will be washed into the effluent. This will possibly improve the value of the effluent as irrigation water carrying additional nutrients, or?

ciao
Hajo

Hi Dean,

how often you have to empty the digester will depend on its size and the number of users. I guess your emptying cycle is based on 1m3 (these great fruit crates) and a household of 4? We have to see what baskets we can get here, and in the African environment often 2 or more families share one toilet and the families are between 6 and 15 people. But that is not a problem, we can adjust the emptying accordingly as long as it is in ranges of years (or at least months).

New is for me the aspect that you actually change between the two baskets, until one is full and requires emptying. Which can become a problem if the baskets are not monitored closely and are both about full at the same time. But that is management and manageable.... and the worms will have to change 'home' several times...

What I like with vermi-digestion is the fact that we are quite flexible with the use of effluent: either percolate it into ground via soak-away or drain field, or put it through second or even tertiary filtration, at either household, neighbourhood or community level, and gravitate or pump it onto fields for irrigation. We can adjust to the conditions of the respective locations and can have different effluent systems even in the same community.

In our environment, I would possibly try avoiding continues pumping but provide some storage and a float switch controlled pump.

Coming week I'll have a meeting with the sanitation specialists from my local partner organisation and will hear what they think about application of vermi-culture... I am curious to hear...

ciao
Hajo

Hi Hajo,
A couple of points in response, firstly a domestic twin primary digester would only need humus removing every nine or twelve years, because a 3 year rotation could be done at least 3-4 times before the humus would build up sufficiently to require removal. Also, the effluent fields could be above the vermi-treatment plant by using a pressure pump, but this would need to be in operation at all times!
cheers
Dean