Low cost secondary treatment of household wastewater using vermifiltration

  • JKMakowka
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Re: Low cost secondary treatment of household wastewater using vermifiltration

I dislike talking about pathogen elimination, as nearly no treatment other that prolonged heat pasteurization will achieve that.

So yeah, it's a bit of semantics, as obviously vermifiltration also reduces pathogens as a side effect of the treatment as outlined in my previous post. I believe it can even do so quite significantly if everything runs optimally.

However contrary to treatment technology that has as the primary purpose pathogen reduction, vermifiltration does not seem to have sufficient process security (multiple barriers, redundancy etc.) that I would consider it safe to use lets say during a Cholera epidemic (or in a hospital) unless further treatment steps are added.

The same could be said about most low-cost waste water treatment, and stabilization ponds also do not have high process security, especially if force aerated.
Of course the WHO is likely correct in their overall risk assessment that these options typically offer a good enough pathogen reduction to be used for irrigation purposes. But if someone specifically asks for a technology that safely reduces pathogens, it has to be acknowledged that these technologies have a different primary purpose and were not specifically designed for pathogen reduction (or elimination if you prefer that).

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WASH news aggregator at: news.watsan.eu
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  • goeco
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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi Kris,

Kevin originally asked about the process of reducing pathogens through vermifiltration and the science behind it. He also raised heat treatment and long term storage as known methods for reducing (well, actually eliminating) pathogens. Semantics aside, those "further treatment steps", along with tertiary sterilisation technology such as chlorination, UV and ozone, can by design "eliminate" pathogens... but at a cost. So does vermifiltration as a "low cost technology" require "further treatment steps" to "safely remove pathogens" to a level suitable for irrigating food crops? I don't believe so and I don't agree with your contention that vermifiltration does not have pathogen reduction as its primary purpose.

Compared with traditional sewage treatment processes, vermifiltration is a relatively new technology and so the science and our understanding of the process is by no means mature. However, the evidence available to date shows that the mechanism for pathogen removal in vermifiltration extends beyond just using microorganisms to remove dissolved organic matter, along with time outside their host environment for pathogen death. Earthworms also apparently promote the development of bacteria and fungi species capable of producing antibiotics that kill pathogenic organisms in wastewater, thereby actually disinfected the effluent. The literature is abundant on this topic... as an example see Pathogen removal during wastewater treatment by vermifiltration
"vermifiltration technology can be considered as a low-cost, effective and sustainable option for wastewater treatment, which results in a clear pathogen-free effluent" (Arora et al. 2014).

The whole point therefore is to:
  1. Avoid discharging to water bodies to remove the risk of cholera and other diseases being transmitted in drinking water, noting that Cholera survival time on crops is usually less than two days, five times less than in water.
  2. Remove the risk for pathogenic transmission, to be able to safely apply treated effluent to food crops. The process does have as its primary purpose removal of pathogenic organisms, keeping in mind that additional factors (barriers) controlling transmission of disease are agronomic, such as the crop grown, the irrigation method used to apply the wastewater, and the cultural and harvesting practices used.
  3. Retain the plant nutrients for an efficient nutrient cycle.

The consensus of health experts leading to the WHO guidelines was that the actual risk associated with irrigation with treated wastewater is low with respect to bacterial pathogens, whereas the risk is high for parasitic diseases both for individual and overall public health where associated with the use of insufficiently treated wastewater in agriculture. Note that helminths may survive for many months if applied to crops, so this design is for efficient removal of helminths.

Process security? How important is redundancy and multiple barriers at the domestic scale?

So can this method consistently meet the present WHO Guidelines? I would say yes, because it is modular and scaleable to both treatment level and number of users. This isn't rocket science, but a simple, low cost, reliable and effective method that uses vermifiltration technology to treat wastewater to a safe level for applying to crops.

Finally, let's not forget that we live in a world where open defecation is rife and raw sewage continues to be applied unabated to food crops in many countries. The key to breaking those pathogen cycles is cost-effective treatment solutions. Anaerobic systems like DEWATS are expensive and require further process steps to deliver water suitable for irrigation. They also produce sludge, another problem. Conventional domestic-scale aerobic systems are even more expensive. Bill Gates and others are investing millions of dollars on service providers who don't seem to have yet delivered a simple, cost-effective solution for the household. Is this just that?

cheers
Dean

Dean Satchell, M For. Sc.
Go-Eco Sustainable Solutions
www.go-eco.co.nz
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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi! I'm an architecture student and I'm doing a sustainable island tourism development for our thesis. We would like to incorporate vermifiltration as our solution for sewage treatment. I'm kinda lost with all these stuff about it even though I've read a couple of articles. Most examples I find online are either vague or jus too hard to undersand for me considering I have no idea about vermifiltration. In regards to this, I'd like to ask the following questions:

1. Is a primary digester necessary? Is it the same as a vermifilter tank?
2. If the sewage would be coming from multiple buildings is it best to have the sewage stored in a a tank first then will just pump it to vemifilter tanks?
3. From the diagrams I saw online, it's always just sewage to first vermifilter tank up to third vermifilter tank. Then, what does the settling drums and pumpouts do?
4. If sewage is being treated, the end products would be liquid and solid. I am so lost with this one, can the solid product be considered as a compost alredy? or does it need to be composted yet. And how do you remove the solid product from the tank? Manually?
5. Lastly, can vermifilter tanks be continuously fed? Or is there a specific period of time wherein they need to process the sewage first before it can accommodate more?

Regards,
Camille
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  • stevensugden
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Re: Low cost secondary treatment of household wastewater using vermifiltration

I think Dean is right in saying that the design has to be "modular and scaleable to both treatment level and number of users". We need such flexibility so a treatment plant can adapt and grow as usage grows. Can anybody share a design of such a modular vermi-filter?
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  • goeco
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Re: Low cost secondary treatment of household wastewater using vermifiltration

Hi Camille,

Lets start with the solids. The primary vermi-digester is actually a vermifilter that separates and captures the solids and allows the liquids to filter through. In the primary digester the worms digest the solids and turn these into humus. Because of seasonal and load variations, the primary digester requires a large capacity because the worms may not keep up with the incoming solids at all times. This capacity allows a pile to build up, but because the worms are digesting the solids at the same time, the humus only builds up very slowly, taking many years for the humus to build up into a big pile. By having twin digesters, the influent is diverted every 5 years or so and the pile is allowed to rest. By resting for 5 years there are no helminth eggs or other pathogens left in the humus, so it is safely dug out and used for soil conditioning before diverting the influent back to that side.

You can have multiple dwellings feeding the digester, but capacity needs to be calculated accordingly. Ask yourself why would you centralise? Do you have one area you want to irrigate? You might want a primary digester at each dwelling and small pipes (simplified sanitary sewers) from each primary digester to a central secondary vermifilter treatment plant.

A "vermifilter tank" is probably someones idea of a primitive vermifilter? Properly constructed vermifilters have air flow around the outside of the basket and also underneath. Look at the baskets above and figure out how you can achieve that in your diagrams, which is quite different than just a tank filled with media. Good ventilation is critical to optimise operation.

The primary digester can also act as a vermifilter that treats the liquids, essentially it is depth of media that provides treatment, by providing surface area and retention time for the microbes. In my designs above I have separated the primary digester because there are two that feed a single secondary vermifilter. I haven't used much depth of media in my primary digester because I am really only capturing the solids in this stage. Later I recirculate or gravity feed more vermifilters to treat the wastewater. Also where the land doesn't have sufficient fall, it may be necessary to pump the water into the top of the recirculation vermifilter, shown in my other diagram. The advantage with this is that I can recirculate through the vermifilter rather than be limited to one pass. This provides more consistent treatment and potentially to a greater level. The settling drums are to remove helminths, but mostly to clarify the water sufficiently to ensure reliable operation of the recirculation pumps, which are very small and can otherwise clog.

The pumpout drum uses a cheap pump with float switch. When the level rises enough, the pump switches on and pumps the treated water out to the drippers. Once most of the water is pumped out the float switch turns the pump off and the drum slowly fills again with treated influent. A pump makes it much easier to dose the dripper lines because the pressure makes the drippers work evenly.

If you have sufficient fall then design for a simple gravity system, including gravity dosing of the dripper lines.

Also check out the wikipedia page

cheers

Dean

Dean Satchell, M For. Sc.
Go-Eco Sustainable Solutions
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