Low cost secondary treatment of household wastewater using vermifiltration

  • goeco
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Low cost secondary treatment of household wastewater using vermifiltration

This system uses a vermidigester for primary treatment (removal & digestion of solids), then secondary treatment using vermifilters as described here. The system accepts all household waterwater including toilet, kitchen and bathroom. Treated wastewater is discharged to soil surface using dripper lines.

Not including labour the cost for the secondary system is between $200 and $300 depending on how much the barrels cost. This system is for a household of 3-5 people, and is scalable by adding more modules in series. The intention is to produce effluent of a quality suitable for food crops.

The primary vermi-digester uses two large fruit baskets but is not the focus of this topic:


This design is for a site where there is little or no fall and therefore this simple gravity system cannot be used:


Here it is:


and a diagram of the same design:


Please note that this is made entirely from reused 40 gallon plastic drums and other accessible materials that allow construction in any country. In this case because the house had electricity, an electric pump with float switch ($80) was installed in the pumpout drum. This discharges to drippers in the garden and where fall allows could be replaced with a dosage siphon (e.g. Flout ).

This system also uses 5 watt recirculation pumps ($10) that "feed" the vermifilters continuously. Where electricity is not available very small solar panels would drive the pumps. I have found these recirculation pumps to be very reliable:


The secondary vermifilters are constructed using plastic mesh, shadecloth, plastic pipe and cable ties:


I use bark or woodchip media, but other options with similar porosity are available. The drums have small holes drilled in them for ventilation.

Recirculated water is dropped on to a half-round "splasher" from about 30 cm above, which distributes the water evenly on the surface of the media:


This is a very simple and low cost system for treating domestic wastewater. Maintenance is minimal and treatment level can be adapted for the required application.

Dean

Dean Satchell, M For. Sc.
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  • JKMakowka
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Re: Low cost secondary treatment of household wastewater using vermifiltration

Looks interesting. But where do the worms come into play and is there still sufficient food for them after the primary vermifilter and the settling tanks? It seems to me that it might work just as well without them?

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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi Dean!

Thank you very much for sharing! This is such a great compact system!

Would be very much interested to see what is the effluent quality in the last removal tank compared with effluent from the primary digester. Could you test it somehow at least for total N?
Best wishes,
Bogdan

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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi Kris, the worms don't need solids in the wastewater at all, indeed their preference is to graze the biofilm (bacterial slime) that grows on the media. All that is required is water with a high BOD... which is what is provided. The (aerobic) media provides a huge surface area for the biofilm to grow on and the water is rich in dissolved and suspended organics, which is what I'm removing with the biofilm-coated media. The part that the worms play is in maintaining the biofilm on the media surface by grazing it, then converting it into stable (biologically inert) humus. The humus in turn, along with the worms, maintain the porosity of the media. So what I end up with is wastewater with a low BOD and low pathogen levels and low suspended solids, but that remains rich in plant nutrients.

Bogdan, thanks for your post and yes, you are right - I need to confirm that the nitrogen is retained through the process by testing the primary and secondary effluents. Unlike other sewage treatment processes that are designed to remove N into the atmosphere as ammonia gas (so the water can be discharged into water bodies... lakes, the sea etc), vermifiltration retains the N and is therefore my technology of choice for a sustainable future where crops can be irrigated with what can only be described as "liquid plant food". I haven't actually tested the water from this unit for plant nutrients because I've seen the impressive growth response in the plants, but I will do that and report the results!

cheers
Dean

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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi Dean!

Does it mean that vemifiltration reduces BOD mainly through suspended solids reduction while leaving total NPK levels still high enough?

In this case it looks more like a primary treatment stage for me even with pathogene reduction.

Best wishes,
Bogdan

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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi Bogdan,

What I am producing is a water quality suitable for irrigating productive plants, so I don't want to lose the nitrogen. I do want to reduce the BOD so the water is less biologically active and with low levels of pathogens. Even if you remove most of the suspended solids from wastewater, the BOD will still be high because of biodegradable dissolved organics such as sugars, urea and short-chain carbon molecules, along with ammonium etc. Vermifilters convert most of the dissolved ammonium to nitrate. The secondary treatment removes most of the suspended solids through settling and (vermi) filtration, so that helminth ova are removed from the effluent. Vermifilters reduce pathogen levels and BOD at the same time.

cheers
Dean

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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi Dean!
Ok, I got it. Nitrification is happening and you reduce dissolved organics.
Thanks,
Bogdan

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Re: Low cost secondary treatment of household wastewater using vermifiltration

Yep, nitrification consumes large quantities of oxygen, and this process takes place after the BOD levels are reduced... again requiring oxygen... so the more aerobic the conditions are in the vermifilter, the happier the worms and nitrifying bacteria are :)

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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi Dean,

how is the process working of reducing pathogens through a vermifiltration? What is the science behind that? Why are there less pathogens after the treatment? From my understanding in the past there were only the ways of a) 60+ C° over a sustained time, b) long-term storage of +2 years, or c) anaerobic treatment through fermentation.

Might be a very basic question, so if you can just forward a good source to explain it i´d be very happy :)

Best
Kevin

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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi Kevin,

I'm not a microbiologist, but aerobic secondary sewage treatment processes use microorganisms in a managed aerobic habitat to biochemically oxidise the pathogens. Wastewater treatment plants, whether domestic or municipal mostly use the same method whereby the primary treatment removes solids and the secondary treatment step involves aeration of the wastewater combined with a residence time to reduce biological oxygen demand (BOD), thus total bacteria/pathogen count. However, pumping air into water consumes large amounts of energy for the amount of oxygen dissolved into the water, so can be expensive.

What you describe as "anaerobic fermentation" is perhaps more accurately described as "anaerobic digestion". The DEWATS is an example of this treatment technology. This is a much slower treatment process and so a much larger capacity must be constructed for sufficient residence time, which can be more expensive.

So conventional aerobic treatment uses less capacity (tank volume) than anaerobic, but requires energy for aeration.

In my system, instead of pumping the air into the water, the water is pumped (and recirculated) through an aerated media, resulting in very little energy used to dissolve oxygen. The media also serves as a biological filter and medium for the microbial slime doing the oxidation. Because the process is so efficient (and self-sustaining because the worms maintain the media porosity and biofilm quality) the capacity can be quite small per person, significantly reducing costs.

You can find out more about vermifiltration on the Wikipedia page

cheers
Dean

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Re: Low cost secondary treatment of household wastewater using vermifiltration

To add to Dean's post:
Like most low-cost wastewater treatment system the primary focus of this is not and can't be pathogen reduction. Vermifiltration is primarily about a reducing organic load and secondarily about converting/oxidizing Nitrogen rich substances into plant available nutrients.

As part of this process you will see a reduction of pathogens as viruses will be consumed by other microorganisms, pathogenic bacterial will die off due to unsuitable environmental conditions and being out-competed by other bacteria, and the remaining pathogenic protozoa and worm eggs will mostly get stuck in the system being recirculated with the easy to settle out solids.

None of these is a perfect barrier and thus if pathogen reduction is your ultimate goal (or you serve a population with a high pathogen load) you will need an additional sterilization step before discharging the effluents. However as the water after such an multi-step vermifilter should be reasonably clear, this becomes much easier as you have to add much less chlorine / ozone etc. and UV irradiation might also work.

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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi Kris, thanks for your comments. Could you clarify what you are saying in your post please?

the primary focus of this is not and can't be pathogen reduction

Given that vermifiltration does achieve significant pathogen reduction, did you mean "pathogen elimination" rather than "pathogen reduction"?

if pathogen reduction is your ultimate goal ... you will need an additional sterilization step before discharging the effluents

Again, did you actually mean "pathogen elimination"?

The WHO guidelines describe an effluent quality of one or less helminth per litre of water and less than 1000 faecal coliforms per 100 ml as being suitable for irrigation of crops likely to be eaten uncooked.

So although I am not aiming for complete elimination of pathogens, which I agree would require additional sterilisation, the primary focus certainly is to reduce pathogens to levels that meet the WHO guidelines. Do you not consider such low cost technology suitable for this purpose? I note that the WHO guidelines suggest treatment using "A series of stabilization ponds designed to achieve the microbiological quality indicated, or equivalent treatment", which is certainly not a sterilisation step or pathogen elimination...
cheers
Dean

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