Horizontal vs Vertical Flow Constructed Wetland

  • alexandra85
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Horizontal vs Vertical Flow Constructed Wetland

Dear DTF community

As we are trying to solve the siphon system issue (see previous post ), our team is questioning the suitability of the Vertical Flow Constructed Wetland (VFCW) in our DTF design (for more information on the whole project, click here ). Some members of the team claim that Horizontal Flow Constructed Wetland (HFCW) does not require discontinuous flow discharge. Apparently, replacing the VFCW by a HFCW would enable us to get rid of the siphon. Do you have any experience with HFCW and if yes, what is its advantages (and/or disadvantages) over VFCW.

In attachment you will find the structural and architectural drawings of our VFCW.

Thank you in advance

Kind regards
Alexandra

Alexandra Dubois

Technical Advisor
Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH
Nairobi, Kenya

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  • JKMakowka
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Re: Horizontal vs Vertical Flow Constructed Wetland

VFCW usually require less total surface area as the processes are more aerobic, loading them intermittently improves this even further. As some aeration takes place when loading the bed from the top, it is likely true that a VFCW is working better than a HFCW with continuous flow (Edit: but HFCW are usually designed so large that continuous flow will also work).
Otherwise VFCW are also easier to maintain (Edit: see message below, i.e. this seems to be not always true) and repair but also more easily damaged by invading animals for example.

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  • Marijn Zandee
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Re: Horizontal vs Vertical Flow Constructed Wetland

Dear Alexandra,

I am a not an expert, but I did try to read up on the subject recently. Here is my take:

Vertical Flow Constructed Wetlands (VFCW) usually have rough sand as the filter medium. They need intermittent loading (with pump or Siphon) to work well. Partly this is to make sure that the aerobic and anaerobic stages of the treatment both happen. However, there is one more reason why the intermittent loading is important. If the sand is continuously wet, biofilms will grow in the filter medium and it will block. (This is one reason why I am personally not 100% convinced that VFCWs will always work in regions with heavy monsoon rains.)

The main advantages that VFCWs have over Horizontal Flow Constructed Wetlands (HFCW), are:

1. much less space required. (However on the drawings you attached, the wetland design seems relatively shallow, and thus still requiring a lot of land.)

2. Much better nutrient removal (especially Nitrogen). Thus better quality effluent.

From my own (non systematic-non significant) look at existing wetlands in Nepal. My impression is that HFCWs are less often blocked, and thus seem to be able to stay functional under "sub-obtimal" O&M better.

I hope that some of the real experts, like Christoph can weigh in on this thread also.

For further reading, please do some searches on the forum here, as there are quite some old discussions on wetlands.

Finally, as an introduction there is a manual from UN Habitat ( unhabitat.org/books/constructed-wetlands-manual/ ) which covers some of the basics.

Regards

Marijn

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  • Florian
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Re: Horizontal vs Vertical Flow Constructed Wetland

As I understand, vertical flow wetlands need intermittent feeding (via syphons or pumps) more than horizontal ones, because you want to distribute the water evenly on the entire surface, and you want some oxygenation between the flushes.
In a horizontal flow wetland, you need to have uniform distribution of water only along one side of the filter, and this is more easy to achieve with low continous flows than the uniform distribution on the entire surface. But also for horizontal beds, intermittend feeding may be a good idea, epsecially if you have times with low flows.

Advantages of VF vs. horizontal flow have been mentionned. VF and HF wetlands can also be used combined in first and second stage. I think often it is just a question of choice, or the question with what type your engineers/companies are more familiar with...

GIZ has also produced a good summary on wetlands, you'll find it in the Susanna library:
www.susana.org/en/resources/library/details/930

Best, Florian

PS: what is a DTF?


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  • alexandra85
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Re: Horizontal vs Vertical Flow Constructed Wetland

Dear all

First of all thank you for your replies. It was very useful and I realised very quickly that my question was very confusing as I mixed up the terms Vertical and Horizontal Flow Constructed Wetland. Let me blame it on the siphon issue which is giving me sleepless nights… I have now corrected my initial post. Thanks to the literature from UN Habitat and GIZ that you suggested, I have now a better understanding of each system, which will help us taking some decisions regarding the up-scaling of our Decentralised Treatment Facility-DTF concept.

This is the summary of my findings:
HFCW
Pros:
• low maintenance requirements
• no need for intermittent flow therefore no need for electricity (pump) and good option in case of low gradient
• efficient removal of organic pollutants (TSS, BOD5 and COD) and nitrates.
Cons:
• large surface area required
• limited oxygen transfer therefore limited removal of nutrients (especially nitrogen)
VFCW
Pros:
• much greater oxygen transfer capacity resulting in good nitrification
• less surface area required
• efficient removal of BOD5, COD and pathogens
Cons:
• intermittent short-term loading intervals (4 to 12 doses per day) therefore need of electricity (for pump) or a sufficient gradient for a siphon pulse loading system
• less efficient in solid removal : can become clogged if the media selection is not correct

Alexandra Dubois

Technical Advisor
Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH
Nairobi, Kenya
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  • Florian
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Re: Horizontal vs Vertical Flow Constructed Wetland

• less efficient in solid removal : can become clogged if the media selection is not correct


Not sure if this horizontal beds are more robust in this sense? Correct selection of gravel/sand is important in both types I think.

I'd like to add, that vertical flow CWs can be built in a way to recieve raw wastewater, with sludge accumulating and mineralisation on the top (this is often called the "French System"). In my opinion, avoiding pre-treatment (settlers) is a very important advantage in many situations, as very often, sludge is not removed in sufficient frequency from pre-treatment facilities and then solids are carried over to the wetlands, where they clogg the filters and reduce treatment efficency.

Best regards,
Florian


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Re: Horizontal vs Vertical Flow Constructed Wetland

I think part of the confusion comes from the fact that HFCW and VFCW are really meant for two different use-cases.

To my understanding HFCW are for small scale systems with efficient pretreatment and low quantities of low strength waste water. Under these conditions it is a more or less maintenance free and maybe more importantly nuisance (e.g. smell) free/low system.
A typical case would be one (or a small number of) rural households with a well working 2-3 chamber septic tank or a small ABR with a HFCW as a mostly worry free secondary treatment. But if something goes wrong with that system and the HFCW cloggs with solids, a costly repair becomes necessary... almost a rebuild.

VFCW on the other hand are the more "professional" systems for larger quantities of higher strength waste water, and as Florian mentioned sometimes even non-pretreated waste water or even septic sludge. If well managed they can endure much more and are usually also easier to get working again after a serious problem. They are also definitely not nuisance free for nearby inhabitants (but with pre-treated waste water it isn't that bad).

Basically HFCW are for households, VFCW are for small waste water treatment plants... maybe more comparable to treatment lagoons.

Last but not least, for both the type of plants used have a significant effect on the functionality. My guess would be that using semi-aquatic plants that can transfer oxygen to their root zones is even more important for HFCW, while VFCW will probably work with a wider variety of plants and thus are also more robust in that regard.

P.S.: I still think someone should try to build a pilot "upflow micro-aerobic constructed wet/peatland" as conceptualized a while ago on this forum ;)

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Re: Horizontal vs Vertical Flow Constructed Wetland

Waoo, i particularly find this discussion very interesting and useful. i am very grateful to you all for the responses. i have registered for a PHD and my research work will be trying to build a constructed wetland system for treatment of domestic greywater for either a single house or a cluster of them. this discussion therefore has given me some insight as well while i think through and read i around. i may be posting my own questions once i kick start the study.

Bismark D-A
Dept. of Environmental Health and Sanitation.
College of Agriculture Education
University of Education, Winneba
Asante-Mampong, Ghana
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  • AquaVerde
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Re: Horizontal vs Vertical Flow Constructed Wetland

Dear Alexandra,

You may read pro & cons on horizontal CW / vertical CW by Dr CM Weedon at: http://watercoursesystems.co.uk/constructed-wetlands/

Additional the upcoming new guidelines for CW's in Germany DWA A 262 do not recommend anymore horizontal CW as the main treatment step after septic tank, in future only as a second stage after a vertical CW.

To keep vertical CW's in longer term free of clogging I made good experiences with a "pre-filter" ( effluent filter ) within septic tank outlet, to keep all particles within the tank. I do not recommend brushes!

If you have any slope you may go for simple siphon distributions and avoid pumps where ever possible.

As Florian pointed already out, the 2-stage CW (" French System ") by Dr. Käthe Seidel is maybe the best choice as you can avoid septic tank and "do" sludge humification on top of first stage immediately. Bud you need a level difference from inlet to outlet at least 3,60 - 4 m.


Good Luck
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Re: Horizontal vs Vertical Flow Constructed Wetland

Dear Detlef, and others,

Thanks for the link above, and the news that new German regulations do not allow only a horizontal CW after the ABR/septic tank anymore.

Between the lines of the article Detlef posted one can read the main reason for preferring vertical CWs over horizontal flow ones. This is that an ABR-HFCW system consists of two anaerobic treatment steps after each other. (There usually is a semi-aerobic zone in the top layer of a HFCW, but it is small) In my interpretation, this is also part of the argument Christoph made once: forum.susana.org/forum/categories/229-de...sed-treatment-plants . Just to be complete, it is the aerobic phase in the VFCW that helps the removal of nitrogen compounds.

A quick response to Kris’ post above.

I think part of the confusion comes from the fact that HFCW and VFCW are really meant for two different use-cases. ... Basically HFCW are for households, VFCW are for small waste water treatment plants... maybe more comparable to treatment lagoons.


I don't think that is quite true. There are probably "pressures" that create a tendency in that direction. For example, the extra land requirement of a HFCW over a VFCW would make it less and less attractive as scale increases. But, I would not agree with the idea that the HFCW is intended (and always more suitable) for household scale, and that the VWCW is intended (and always more suitable) for larger scale operations.

Anyway, it seems that internationally there is a move away from from HFCWs towards VFCWs, as the latter offer better treatment performance. At least, this is the case in developed (and more developed) countries. For me this leaves the question: "What would be better in a least developed country (LDC)?"

As I have said before, from non-systematic field visits, I have the feeling that HFCWs are less likely to develop problems when O&M is bad. As I have seen a lot of ponding (blockages) in VFCWs. Also, I would be worried about the impact of heavy monsoon rains on the ability of the sand in VFCWs to dry out sufficiently between charges of waste water. Further, I am not convinced that one can always find sand of sufficiently big grain size.

Does anyone have an article with empirical data (field studies) of how the reliability of both types of wetlands compares in the developing world?

Regards

Marijn

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Re: Horizontal vs Vertical Flow Constructed Wetland

French system is just incredible. I fell in love with it when went to France to see it working. My best thanks to Dirk Esser who made it possible. All this "shit fountains" every 15 min are so nice -- it looks like a water show!
It doesn't smell even 1 meter from a bed. I witness. Maybe it was wrong time (November) but this what I experienced.
I think the head can be even less -something like 1,5 metters for the syphon.

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Re: Horizontal vs Vertical Flow Constructed Wetland

Hallo Bogdan,

You are right this nice generic system (invented by Dr. Käthe SEIDEL,* 1907 in Frankenstein, Erzgebirge, Sachsen; † 1990 in Krefeld, I regret not by a French lady ;-) )from Interval-reservoir (siphon) to top of bed needs maybe 1,5m, even less is possible just a slope 1:100. But from Inlet wwtp to outlet at least 3,60 - 4 m, as you have two filter passages.

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