Discussion about pros and cons of vermicomposting digesters, including groundwater pollution aspects

  • goeco
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Re: Questions Experts Ask vs Questions PRISTO Customers Ask (about Biofil Toilet Systems)

Hi Hajo,

thanks for your excellent questions. Here in New Zealand most systems used where there is no reticulated sewerage are septic tanks (primary treatment) or aerated domestic wastewater treatment units (secondary treatment). Septic tanks are cheaper and only installed where soil conditions meet requirements for underground effluent trenches (good soil drainage and plenty of space).

With secondary treatment the only requirements to meet for putting liquid effluent through drippers are 5 day biological oxygen demand (BOD) of 20g/m3 and suspended solids (TSS) of 30 g/m3. For primary treatment there is no limit, but an engineer needs to design the soakage fields to rigorous standards. Nobody here uses low flush or cares about water use, domestic systems are designed to meet these needs. In developing countries where vermicomposting is apparently catching on, they are using low-flush outhouse toilets with direct soakaway. I'm interested in what research is guiding the design for each installation.

Vermicomposting digesters are accepted here as primary treatment, thus standard practice is to connect to underground effluent trenches just as with septic tanks. I've been working on the next step, which is to produce secondary treated effluent at low cost using aeration, which has been surprisingly easy because of the effluent properties. I only have prototype installations, but there is a commercial vermicomposting system available in NZ using reed beds for secondary treatment.

Nobody here has ever questioned sanitary handling of digested solids and seem to accept claims of goodness-for-purpose. There are conflicting references in the literature regarding helminth levels and I can only assume that increased resting time will lower levels of pathogens. Thus in my view the only safe option is the twin chamber design with a year of resting before removal.

There are no problems with discharging kitchen sink water into the digester. Worms are surprisingly resilient to such things as soap and fat, even some disinfectant. However, the bulk of the greywater should be diverted from going through the digester. Bath and washing machine water doesn't have enough solids to justify putting through the digester and might go straight to the effluent field or secondary treatment process.

The thing is, once getting into household wastewater, each design will vary according to soil conditions, effluent volume and quality... therefore method of application to soil, all made much easier if secondary treated.

Vermicomposters have not become standard in New Zealand's domestic sanitation market. The market is currently dominated by expensive secondary treatment systems, all using essentially the same principle. Vermicompost digesters can only claim primary treatment status and only compete with septic tanks, which have gone out of favour. The opportunity I'm pursuing is to lower the cost of secondary treatment, which currently uses multiple stages - starting with a septic tank (anaerobic), then sedimentation with sludge recirculation, then aeration then filtration, then pumping to pressure drippers ...and costing NZ $10,000 -$20,000.

Happy to PM you a couple of NZ suppliers of vermicomposting digester systems.

Dean Satchell, M For. Sc.
Go-Eco Sustainable Solutions
www.go-eco.co.nz
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Re: Questions Experts Ask vs Questions PRISTO Customers Ask (about Biofil Toilet Systems)

hi Dean,

thanks so much for the plentiful information and the websites of the commerical composters: a lot for me to read and to digest. I will keep you ( and others) informed of further developments and findings in Moshi.

Just one remark on your statement re helminths: 'There are conflicting references in the literature regarding helminth levels and I can only assume that increased resting time will lower levels of pathogens.' As far as I know from the SuSanA discussions, resting time may reduce other pathogen levels BUT not helminth eggs. They survive 3 years concreted in! You only kill them by heat, thus thermo-composting may do it (like the Higgin's Hot Box), the LADEPA does it by infrared.

The question could be whether the tiger worms eat and digest (all?) the helminth eggs and in such way remove them?

looking forward to further collaboration,
ciao Hajo

We can't solve problems by using the same kind of thinking we used when we created them.
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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. :-)
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Re: The Biofil toilet technology for onsite sanitation in poor urban communities (Ghana, Senegal, Bangladesh)

Dear Andrea,

thank you for your information and advice. I am quite aware that the information provided is not sufficient for you to make any statement how safe GW could be in Moshi. And I was not expecting such judgement.

What I want to know is the following. The factsheet of WG11 "How to keep your groundwater drinkable: Safer siting of sanitation systems" describes how sewage sources under certain geological (soil) and geographical (vertical and horizontal distances) can or may not affect GW.

In combination with the research findings by Adane Molla (attached by Dean in one of his postings), I derived that loamy, sandy and red laterite soils have such good filter and treatment properties that under specific soil conditions and distances, soils can protect GW from sewerage contamination. The German 50-day-travel rule may also be an indicator for such behaviour.

Besides the confirmation that sufficient distance of the right soil will protect the GW, I also want to hear from you whether research has shown that this 'treatment' property of the soil remains over time independent of sewage loads, no matter how much and long sewage effluents are infiltrated. Someone may think that this is not possible, but I recall oil spills (where oil tankers ran on ground) and many years later nature had handled the oil. Maybe similar developments have been found in soils which have been loaded with sewage effluents for long time?

Why I ask this? It is not a problem for Moshi town as the main water sources are springs (85%) and boreholes (15%) above and out of the urban settlement. And in the lower lying high density peri-urban areas water wells exist but need to be forbidden for potable water as seldom the 100m (or 30m rule as it is in Tanzania) can be maintained. My concern are the villages outside Moshi which draw GW out of boreholes which could be contaminated by the massive sewage infiltration by pit latrines (65% of Moshi population) and septic tanks (15%).

Is that possibly prevented by the soils if they are of the right geology? Of course we have to investigate the geology and the GW aquifers, of course we need then 'hydrogeological maps, geological maps, soil maps or even ideally a vulnerability maps of the groundwater' as you rightly pointed out.

I hope my questions have become a bit clearer,
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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  • andreanick
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Re: The Biofil toilet technology for onsite sanitation in poor urban communities (Ghana, Senegal, Bangladesh)

Dear Hajo,

thanks for the clarifications, I think you already have quite a good understanding of the hydrogeological processes. My main advise is that you put a monitoring routine in place and check the wells/boreholes in the surrounding villages in regular frequencies.

I always find it helpful to see these things in pictures, but I don't have one ready to upload, so just search for "groundwater contamination plume" and you'll see a great variety of illustrations available in the www, a lot of them referring to oil spills, hydrocarbon plumes which behave a bit different from sewage plumes, but in any case: the spreading of pollution definitely depends on the volume as well (see the last criterium in the checklist). From a groundwater perspective the contamination only starts once the pollutants have passed the soil (also called the unsaturated zone in comparison to the groundwater which is the saturated zone, because the pores are filled with water to the saturation point) and entered the groundwater. Depending on the type of soil this compartiment is extremely efficient in cleaning up, due to the microorganisms that live there in myriads, but also due to the long time it takes water to pass through because unsaturated flow is slower than saturated flow. And the effluents you introduce are not something poisonous to the life in the soil, it's natural, so its capacity to deal with it (treat it) will be high. But it depends on the time the microbes have to eat the nurtrients and fecal bacteria (etc), the longer the better, that's why too much volume might reduce the treatment capacity of the soil.

The treatment properties of the soil would not change unless the conditions change. Say you infiltrate so much urine over time that the pH (the acidity of the groundwater) increases (or better: decreases, the lower the pH the more acid the solution is), than you can even bring other contaminants into solution that were formerly no problem (because they were fixed to the rocks), for example see the Kabul study here ( www.susana.org/en/resources/library/details/1437 ).

Another study you might want to look at is from Yaounde: www.bgr.bund.de/EN/Themen/Wasser/Projekt...erun/kamerun_en.html (has also been posted in the forum previously).

So regarding the threat your sanitation option might pose on the surrounding villages, you'll need to check a) what direction your groundwater is flowing in, b) what is the status of the groundwater now in those villages which would be affected by the outflow of your toilets ("Background value") and c) how does the quality of the groundwater there change over time (monitoring).

Hope this has answered some of your questions?!

All the best,
Andrea
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Re: The Biofil toilet technology for onsite sanitation in poor urban communities (Ghana, Senegal, Bangladesh)

Dear Andrea,

thank you also for the plenty information. As I said to Dean, 'a lot to read and to digest'.

I like especially your statement 'Depending on the type of soil this compartiment is extremely efficient in cleaning up, due to the microorganisms that live there in myriads, but also due to the long time it takes water to pass through because unsaturated flow is slower than saturated flow. And the effluents you introduce are not something poisonous to the life in the soil, it's natural, so its capacity to deal with it (treat it) will be high.'

It seems that subject to various condition being fulfilled which we have named and discussed before, soils can be an efficient protection for the GW.

We are at the very beginning of our sanitation activities in Moshi and once we have more information (and questions) I will come back to you.

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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  • goeco
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Re: The Biofil toilet technology for onsite sanitation in poor urban communities (Ghana, Senegal, Bangladesh)

I would add that dilution is always better than concentration. The more soil volume the effluent has to filter through, the better. Constantly inundating a small volume or surface area of soil (e.g. latrines) is asking for trouble. Soil particles attract nutrients, but once saturated cannot do so any more. Also, anaerobic processes are much slower than aerobic processes, therefore anaerobic soil does not provide conditions conducive for breakdown of pathogens. Biological oxygen demand of the effluent is important, because if high the potential for effluent to cause anaerobic soil conditions is increased. The golden egg is secondary treatment because drippers can disperse nutrients etc over a wide area and BOD is reduced.

cheers
Dean

Dean Satchell, M For. Sc.
Go-Eco Sustainable Solutions
www.go-eco.co.nz
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