Vermi-Trickling Filters (or vermifilters) for Sewage Treatment (looking for help to design)

  • AditiOS1
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Vermi-Trickling Filters (or vermifilters) for Sewage Treatment (looking for help to design)

Dear All,

Looking for help to design and setup a trickling bio-filter for an upcoming project for treating sewage - 75,000 litres per day. The incoming BOD would be in the range of 80-100 mg/l as primary settling and treatment would take place before the trickling filter.

What would be the HRT, HLR, area of the filter. What media would be required and how many earthworms would be required to carry out successful treatment of mixed waste (Blackwater + Greywater). What are the outlet parameters that one can expect?

Thanks in advance.

Regards,
Aditi.A
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  • goeco
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Re: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

Hi Aditi,
You should be aware that with such "new" technology to some degree you'd be pioneering the perfection of a system for your needs. I have certainly been having very good success on a domestic scale. There has been plenty of research and lots of research data available for vermifilters on google. The key interactions are between inflow volume, retention time and reactor capacity. This is as much an art as a science.

Could i ask why you need to further treat effluent that has 80-100 mg/l BOD? What happens to the treated effluent?

I've been using recirculation through a vermifilter with low retention (i.e. course media). This is because with recirculation the wastewater is not limited to one pass and it is easy to adjust the flow for optimum level of treatment. Thus aeration is achieved even if filtration is not 100% per pass, making for a reliable system that can be "tuned". Recirculating through coarse media is reducing BOD via both dissolving oxygen and removal of organics, whereas a finer media would have a more limited inflow volume and longer retention time, with a greater removal of BOD from one pass.

I'm not about to put together a table referencing flows, capacities and retention times/media volumes... this would require a considerable research effort to produce, albeit very worthwhile if resourced. However, this is also very much about common sense, primarily ensuring that hydraulic retention is not too high, i.e. drainage must exceed inflow. A simple rule of thumb I've been using for calculating capacity is 200 litres for 1 litre per minute. That is a 10 m3 reactor for your volume, or five 2m3 parallel reactors each with 15,000 litres flow per day. Taller reactors are more effective because the wastewater has a longer contact time with the media, but the tradeoff is between volume and head per watt of energy (if the system isn't passive). A wide reactor would need drippers/tricklers spread around to deliver the water to its surface area. There might be more risk of anaerobic conditions in the "core" with larger volume reactors. I use mesh to create a cavity between the media and the reactor walls, to provide ventilation around the media. This is to ensure the media remains as aerobic as possible. The reactor requires good ventilation - the microorganisms and worms are working hard and will be respiring.

You don't need lots of worms to get the system going, they soon breed and achieve a stable population regulated by food source. What I've found happens is that in the initial period of operation removal of BOD is not very high because the biofilm has not yet built up on the media. This might take a few weeks. Add the worms after a couple of weeks of operation (say 200g per m3 of reactor volume), once there is enough biofilm for them to feed on. You might also find the biofilter works better over time because of increased media surface area as the humus builds up. I use pine bark for my media. Here in NZ this comes in various grades, You just need to make sure there is sufficient porosity for the inflow volume. Test your media in a drum or even bucket and ensure that you achieve the right balance between retention and flow... I prefer organic media because it doesn't matter if this mixes with the humus on removal (i.e. gravel mixed with humus is not as nice for spreading under trees...). Woodchips should work well but they decompose faster than pine bark so would occasionally need "topping up". Sawdust or peat should work okay also, but would have a higher hydraulic retention time than pine bark. Materials can be layered, depending on required retention/filtration. What organic materials are readily available?

I'm not familiar with the construction materials you have access to. Here is an example of a 250 litre domestic scale trickling vermifilter, I used plastic mesh, shadecloth and vertical strips of plastic pipe which are readily available here to create a cavity between the media and reactor walls. I then drilled lots of small holes in the reactor walls to increase the ventilation. There needs to be top access for humus removal. Hope this helps

Dean Satchell, M For. Sc.
Go-Eco Sustainable Solutions
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Re: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

Thank you. This information was very helpful.
Harry
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  • AditiOS1
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Re: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

Dear Dean,

Thank you so much, this information is highly helpful. We have been trying to employ natural treatment systems to treat waste water especially in rural and peri urban areas where continuous electricity consumption is a challenge.

As to your question regarding requirement for BOD reduction post primary treatment, it is required as norms in India are becoming more and more stringent in India under the new government and BOD levels ranging from <10-30 mg/l is accepted for reuse or discharge.

We are looking at incorporating vermi-filtration post primary and secondary modules. We have done some pilot runs for 1000-2000 litres. However, we found that garden soil gets choked/clogged fairly quickly. Should we be looking at putting only humus in the top layer on top of coarse media (gravel size of 50 mm above?)

In certain areas we are looking at making a series of step filters to minimize the energy consumption, but some areas don't have the required space, so I guess we will have to strike the right balance.

As far as construction material is concerned, we are looking at making brick structures (open to air) with proper drainage at the bottom. For media, all the mentioned materials are available but wood chips would be costlier as compared to gravel/pebbles. I will have to check on the availability of pine bark here.

Thank you once again for your time and effort, it definitely gives us stepping material and the confidence that the methods we are employing are in the right direction, just needs more fine tuning in terms of media etc.

Regards,
Aditi.A
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  • goeco
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Re: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

Hi Aditi,

Thats amazing that India now require similar levels of BOD to New Zealand for surface application/discharge.

Indeed if using garden soil as your filtration medium you would need infiltration rates that can be maintained over time. One important factor to consider when using soil is that it needs to be at the right moisture content when packing the filter. Too dry and the clods don't break up sufficiently, too wet and you get what is called "lubrication" where you risk making mud. Once soil has become mud it is useless. High moisture content + movement = mud. Therefore the filter needs to be packed with really well broken up soil at exactly the right moisture content, and packed to exactly the right level of compaction, that is not too much to impede drainage but sufficient to ensure the soil doesn't move or further settle once wet.

I don't use soil at all, and consider it too unreliable over time. For passive systems (step filters) use sawdust. This will be cheap and available, but make sure that retention time never gets less than conservative loading rate. You'll need larger reactors than with an active (recirculating) systems.

I use bark or woodchips when recirculating because of higher loadings and lower retention times. In your case the outlet of the secondary module would recirculate through the vermifilter into the inlet of the secondary module, aerating the wastewater. That is, improved treatment also takes place in your secondary module (if it was previously anaerobic), not just the vermifilter.

Make sure that a layer of pervious cloth (shadecloth) is placed over the drainage layer before adding the sawdust.

The drainage layer (e.g. large gravel) requires some form of additional ventilation if scaling up width. Narrow and tall with side wall ventilation is great, but scaling capacity is most cost effectively achieved by wide reactors. These may require vertical pipes through the sawdust and set into the drainage layer, say one 100mm pipe per 3m2 of surface area.

cheers
Dean

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  • guilherme
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Re: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

Dear Dean, thanks for your posts. I´ve noted you have a very small diameter pipe feeding your vermifilter. Can I ask you what sort of water you´re treating, and what are the previous treatment stages you have before the vermifilter?

I have some experience working with raw wastewater with vermifilter as a primary treatment, as well as small septic tanks as primary treatment and vermifilters as a secondary stage, always on gravity flow. Woodchips have been used as the filtration media. I never even considered recirculating, but I found it a very interesting idea indeed, where context allows.

Cheers!

Best from Brazil

--
Guilherme Castagna
Fluxus Design Ecológico
São Paulo/SP - Brazil

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Re: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

Hi guilherme,

I have built a series of prototypes over many years for my family-sized domestic wastewater treatment, with the intention of refining to be low cost and capable of producing effluent quality suitable for safe surface irrigation.

Most domestic vermifilters I hear of either use a septic tank followed by secondary treatment with the vermifilter, or alternatively they're just primary treatment vermifilters followed by disposal to subsurface effluent fields.

I've put the primary and secondary vermifilters in series. Because the primary treatment vermifilter is aerobic, the oxygen demand of the primary treated effluent is much less than with using a septic tank. It is also cheaper.

For secondary treatment I use a series of sedimentation drums, the purpose of these is to settle out residual suspended solids rather than filter these. This is because I believe that it is much easier to settle helminth ova out rather than try and remove with filters. Helminths being the key parasite to remove if using for irrigating food crops. Sedimentation tanks are very simple to build, with inflow going to near the bottom and outflow from near the top, in series.

My trickling vermifilter takes wastewater from the final settling tank and pumps this back through the secondary treatment vermifilter, which discharges to the first settling tank. Thus the settling tanks act as aerobic treatment reactors even though they are not ventilated to the external environment and oxygen. This is because of the large media surface area and trickling action of the secondary treatment vermifilter dissolving oxygen continuously into the wastewater.

My prototype domestic system recirculation pump is 3 watts which flows about 1 litre per minute at a 1.5 m head. The 1.5 m head gives me good depth of media in my vermifilter so I only have one pipe feeding the centre of the the vermifilter media surface. The water quickly spreads through the media as it travels downwards and the 15mm pipe is big enough to never clog. One litre per minute is 1.4 m2 per day cycling through the reactors.

The settling tanks therefore have both continuous (recirculation) and intermittent flow (when wastewater enters the system) with periods of quiescent operation resulting in good effluent clarity without having a filter to maintain.



One key advantage with the system is that I can easily pump sludge from the settling tanks back into the primary treatment vermifilter where this is rapidly converted into humus. Because so little sludge is generated this is rarely required.

The system is not designed to remove plant nutrients (NPK etc) because I want to retain these for fertigation. Provided pathogen removal is sufficient, I see value in the treated effluent. I don't like the idea of using plants within the treatment process, but prefer treating wastewater to then use on plants productively.

cheers
Dean

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  • AjitSeshadri
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Re: Reply: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

I have been doing Dewats mode waste water treatment.

Hydraulic Retention Time HRT in all phases of the treatment is important.

All together try to have minm. 8 to 10 days. Then the treatment works fine. Inlet BOD could be say 400-600 ppm and outlet would be less than 50 ppm.

Ensure to carry out regular operation & maintainance - O&M practices .This would call for efforts on manual labour. etc.

When it is completed then quantify sludge/ bio residues . And take them and co compost them with bio wastes or dairy sewage wastes.

All these make excellent manure.

With well wishes

Ajit Seshadri
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Re: Reply: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

Dear All

I have just come across this thread. One question I have is what are the perceived advantages of a vermifilter over an old-fashioned trickling filter: The trickling filter is not a filter but a form of biological treatment - very simple and would seem to have a much shorter retention time than some of the technologies described in previous posts.

Trickling filters do need follow up secondary sedimentation but for small plants that could probably be done in a small pond. The other challenge is to get even distribution of wastewater across the filter surface - the traditional approach to this was to build circular beds with distributor arms driven by the pressure of the water emerging from them - but something simpler would be required for a small installation. The other point is that the flow should be discontinuous - this can be achieved with some form of dosing syphon. There was some interesting discussion on this on another thread on the forum.

Awaiting replies with interest
Kevin

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Re: Reply: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

Hi Kevin,
I see the vermifilter as the next step in the evolution of the trickling filter. It's still an aerobic packed bed reactor with a high specific surface area. By incorporating worms it becomes self maintaining - because the worms graze on the biofilm and convert this to humus, the microbial layer does not slough off and therefore a post treatment clarifier/sedimentation tank is not required to settle this as sludge. The humus itself improves the filtration because it increases the media surface area while remaining porous.

From my understanding of trickling filters, the dosing is to ensure a sufficiently aerobic environment is maintained on the media surface for optimum biofilm growth (air - water -air - water). From my experience at a domestic scale a rotating biological contactor or dosing system can be replaced by continuous tricking flow because the worms maintain an aerobic environment with good drainage, meaning oxygen dissolves into the wastewater as it trickles through the media, therefore not limiting microorganism growth. Would be interesting to see whether efficiency could be improved with dosing on a municipal scale, but in a passive domestic vermifilter system this would happen every time the toilet is flushed and in an active recirculating system the reactor capacity and flow rate are matched for greatest effectiveness and lowest cost.

cheers
Dean

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Re: Reply: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

Hi Dean!

Thank you very much for sharing very interesting info on vermifilters! It would be a challenge to build these in cold climate but believe this could be overcomed possibly utilizing the residual heat of wastewater and having the reactors insulated or installed within glasshouses.
I support your approach of using moderately pumps and other etechnical devices as soon as they low energy consuming
What do you think about enhancing air supply through the fans in the reactor lid or aerating the substrate actively with air pumps like it is done in aerarated constructed wetlands recently?

Best wishes
Bogdan

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Re: Reply: Vermi-Trickling Filters for Sewage Treatment (looking for help to design)

Hi Bogdan,

I believe a Chilean company has built a working vermifilter in Antarctica for processing sewage! The worms would only die if the reactor froze, so insulating this well is probably all that is required...

In my view small reactors should have sufficient ventilation without additional air supply (especially if there is an air cavity between the bottom/side walls and the media). However, once reactor volume increases to community or municipal scale I'd imagine it might be more difficult to provide sufficient oxygen passively into the core. What I like is how small scale one can go at low cost, so haven't really taken an interest in scaling up.

Yes, fans use very little electricity and would improve ventilation where air movement is insufficient. My experience has been that air pumps use more electricity to aerate water compared to using water pumps with cascading flow through porous media, but I'm not familiar with aerated constructed wetlands. Were you thinking of small diaphragm air pumps blowing air into the core of the reactor?

cheers
Dean

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