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

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

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

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

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 

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

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

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 4^th 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

        
 

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 

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

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

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