Alternative to gravel stones for filter media of Constructed Wetland

  • alexandra85
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Alternative to gravel stones for filter media of Constructed Wetland

Dear SuSanA Community

Here in Kenya, the contractors in charge of the construction of the decentralized treatment facilities are facing some challenges in sourcing round shaped gravel stones for the filter media of our Vertical Flow Constructed Wetland (as well as the Sludge Drying Reed Beds). Has anyone experienced such situation and if yes, what alternative solution was chosen? In our case we are very tempted to replace the gravel by ballast of the same size (8-16 mm) and apply a very strong PE liner (1 to 1.5 mm thick) on top of a 100 mm of a compacted clay layer to avoid punctures and therefore seepages. What do you think?





Thank you and best wishes to all

Alexandra

Alexandra Dubois

Technical Advisor
Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH
Nairobi, Kenya
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  • Florian
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Re: Alternative to gravel stones for filter media of Constructed Wetland

Hi Alexandra,

that's not the first time I heard of this, sourcing the right gravel in the right size, and sufficiently clean, seems to be surpisingly difficult in some places.

I'm not exactly an expert in these details, so you may want to wait for better advise. But I think building in the wrong materials is one of the most common mistakes done in construction, which is impacting on performance, so I think its worth to invest some energy into finding the right material, or advise contractors how to produce (sieve and wash) it from available materials (e.g. sediment dug from a nearby river)

I'm also not sure why you mention the PE liner together with the gravel? Do you assume the gravel should be rounded to avoid puncture of the liner? I don't think that is a problem. At my knowledge, the rounded shape has more to do with hydraulic properties and for providing sufficient pore space.

Best wishes,
Florian


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  • JFranciscoDeLeon
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Re: Alternative to gravel stones for filter media of Constructed Wetland

Hi Alexandra My Name is Juan Francisco de León and i work by BORDA Las Américas on México City i have a lot of experience with Vertical Constructed Wetlands (12 years) and i can tell u that there isnt a real problem when the form from the gravel is more important which stone is and the granulometry..

Dont forget that the wastewater treatment will be in the sand layer, the 2 layers gravel that i saw in your drawing is only for drainage pouurposes

cheers and best wishes

Juan Francisco
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  • alexandra85
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Re: Alternative to gravel stones for filter media of Constructed Wetland

Dear Florian and J. Francisco

Thank you for your answers. Yes, this is what I read in literature, the filtration process occurs only in the sand layer, therefore it is really important to get a good quality sand (test requested in attachment). What I understood is that the two gravel layers have the purpose of protecting the feeding pipes on top and the drainage pipes at the bottom and they should allow sufficient drainage. We have chosen a gravel size of 8 to 16 mm (allowing 10% out of this range). Literature is recommending round shape in order to avoid puncture of the plastic liner at the base of the basin. But I guess if we invest in a thick resistant plastic liner on top of a well compacted clay layer, we are minimizing the risks of leakages...

Alexandra Dubois

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

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  • JKMakowka
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Re: Alternative to gravel stones for filter media of Constructed Wetland

I guess the main problem with regular crushed aggregates would be that over time they could break down further and thus you would have some settling of your bed and would loose some of the under-drain functionality due to loss of pore-space.

I have used vulcanic rocks before, which was working quite ok, and here in the Philippines we were thinking about using waste mussel-shells that are quite abundantly available in the fisher communities. Broken down corals might also work?

Otherwise you can probably just lay more drainage pipes into the sand with a proper slotting or a textile filter to avoid washing out the sand.

Edit: we discussed similar topics before, and if you also have problems with getting good sand you could try a murram (laterite) trench design like described in the document I linked here: forum.susana.org/forum/categories?func=v...tid=229&id=6407#6463

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  • JFranciscoDeLeon
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Re: Alternative to gravel stones for filter media of Constructed Wetland

Hi Alexandra.. i wannt to ask you something! How big is your filter bed?!
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  • alexandra85
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Re: Alternative to gravel stones for filter media of Constructed Wetland

Hello J. Francisco

The Vertical Flow Constructed Wetland consists of 2 filter beds of L 14.5 m x W 10 m x H 0.85 m (with 0.15 free board on top). So, 290 m2 surface area and 246.5 m3 volume

Alexandra Dubois

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  • JFranciscoDeLeon
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Re: Alternative to gravel stones for filter media of Constructed Wetland


i think with ein PE liner 1 mm tick at least should be ok no matter what type of gravel you choose...

Recently i evaluate, with another colleges, a municipal constructed wetlad in México and all the filter body was fill it with crushed gravel like this picture and the linier is ok and the constructed wetland have filter beds L 40 m x W 20m and the deep is similar as yours.
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  • AquaVerde
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Re: Alternative to gravel stones for filter media of Constructed Wetland

Dear Colleagues,

Nice photo Juan!

Just for the drainage/ground aeration areas, crushed gravel is OK. Crushed stones have sharp edges, therefore protection of HDPE-liner (minimum 2 mm) becomes an important issue too. Please use geotextil (fleece) min. 500 g/m2 better 1.000 g/m2 for liner protection inside and outside. 1 mm is OK just for small domestic CW's. 2 mm is better for larger municipality CW's, 2 mm because it is difficult to handle a heavy weight roll of PE-Liner and danger of liner damage during construction by tools and equipment.

Draining of treated wastewater out CW is just one purpose of drainage pipes and rough gravel. Very important is additional passive aeration by this drainage pipes and rough gravel to BIOFILM (THE "engine" of any CW's or any aerated wwpt) which is connected to the filter-sand/-gravel 0...8 mm above the gravel.

Therefore, please use drainage pipes with minimum 5 mm slots, distances 150 mm and 1/3 staggered or use 8 mm holes, not less (a French standard for all CW's). Do not use standard agricultural drainage pipes with only 1 mm slots! This is often not understand and people wounder why vertical CW's got clogged, even no sludge went to the CW.




Planing suggestions for large scale CW (after pre-treatment) (based on CEMAGREF, France):

- Distribution-Layer
10 cm Gravel
(3 to 10 mm or 2 to 8 mm)

- Filter-Layer (Sand/Gravel mixture)
50 cm mixture of Sand (0.25 mm < d10 < 0.40 mm) and Gravel (2 to 8 mm)
or 0 to 8 mm (screed gravel)
Always (each lorry load) test delivered filter-Sand/gravel by simple infiltration-time test with water before installing in CW pond.

- Transition-Layer: (if you do not use a transition layer, sand particles will "travel" towards drainage layer and will clog over a longer time period (as usual outside warranty time;-)
10 - 20 cm Gravel
(3 to 10 mm or 2 to 8 mm or better 8 to 16 mm)

- Drainage/Aeration-Layer,with drainage/aeration pipes:
20 cm rough Gravel
(20 to 40 mm or 16 to 32 mm)

Filter-Layer Sand/Gravel mixture:
comparing German standard DWA and French standard CEMAGREF, I suggest to go better for French mixture advises, as filter-mixture is more rough/robust, therefore filter-mixture would last longer = "grandchildren fit.." ;-)



Where to find good washed & "round" Sand and Gravel?
In Nairobi or other large cities you have many very tale concrete buildings, to construct tall buildings in a long lasting and save manner it needs well prepared engineering/construction with the right mixture of washed sand and gravel to make good concrete at larger scale concrete plants. Well established local engineering/construction companies can tell you where to find quality gravel plant companies with washed sand and gravel. You will find this gravel plants (not quarries!) on larger rivers and close to former larger river beds.


Good luck
Detlef

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  • alexandra85
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Re: Alternative to gravel stones for filter media of Constructed Wetland

Dear Juan,
Thanks for the confirmation. The crushed gravel will be exactly of this type. This is the PE liner that we found:



Dear Detlef
Thanks for the thourough explanation. About the drainage pipes you are suggesting a hole diameter of 8mm (minimum), what would be the minimum distance (center to center) between each hole? I am under the impression that if we go for 150mm (2 series of holes every at an angle of 30° on each sides of the bottom axle this way:

)

we would compromise the overall strength of the drainage pipe (100mm dia PVC pipe).

Concerning the feeding pipes what holes diameter and distances would you recommend (taking into account that we are using 63mm dia HDPE pipes)?

Regarding the filter media profile you are suggesting I was wondering what is the reason why in the Filter-Layer you have a mixture of sand and gravel? And what is the recommended proportion of both material? We might introduce a transition layer to our filter media profile, but because we are limited by the total depth of the bed we will have to reduce the layer thickness to 5cm (on top of the 15cm drainage layer).

Thanks again for the useful input.

Alexandra Dubois

Technical Advisor
Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH
Nairobi, Kenya
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  • AquaVerde
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Re: Alternative to gravel stones for filter media of Constructed Wetland

Dear Alexandra,

Of course it is much-much more better to have only rounded gravel material for a CW, but in real life, "educated" compromises very often needed...

Transition-Layer:
For a proper Transition-Layer minimum 150 mm, you may compromise just for the time being on the 150 mm Freeboard and later after finishing the implementation, add a little dam (lined too) around to have a Freeboard of 200 mm. I remember on a lesson learned paper from Florian (SKAT, CH) on 2-stage CW's in Macedonia or so, that all layers should have a minimum of 150 mm, as it is not easy to keep on large scale installations like yours a minimum of 100 mm.


Aeration via Drainage Pipes (holes or slots):

For placing the holes you may use this example from France.

If you use holes instead of slots you will for sure not compromise stability of pipe.
Just for reference see attached very robust (very expensive too) drainage pipe product from company REHAU for railway and tunneling construction which is using only slots. You can find this in English language via REHAU-webpage by your self too.

Be careful Alexandra, if yo use 8 to 16 mm gravel around your planed drainage pipes with 15 mm holes, gravel (8 to 14 mm) will "travel" towards your drainage pipes and will clog over a longer time period drainage pipe (as usual outside warranty time;-).

Distribution pipe holes:
I use 6 mm holes, each 300 mm staggered. I am interested in other "educated" advice/experiences too?

Mixture of (rounded) Sand 0 to 2 mm and (rounded) Gravel 2 to 8 mm for "Filter-Layer":
I regret, to give an STANDARD % advice is very difficult, not seeing the granular of the materials involved. I had often do mix it different materials in pre-test by checking the percolation of mix again and again to find out the right mixture. You should do so too, together with your construction engineers & supplier at suppliers plant, so they understand the importance too.
Find attached a simple percolation test from company FANN, Sweden. Unfortunately the translation from Swedish to English language in this paper is a bit misleading.
For sure other suppliers for this kind of simple perculation tests are available. Maybe others know too?

Reason why in the Filter-Layer a mixture of sand and gravel:
Pre-treated sewage should not flow through Filter-Layer (Biofilm!!!)
1. too fast = biofilm would not have time enough to "eat" carbon and nutrients, you would not have effluent results as you was planing for
or
2. too slow = water will stagnate and anaerobic condition (smelly) will start very fast, leading towards --> clogging.

Good Luck.
Detlef

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  • JFranciscoDeLeon
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Re: Alternative to gravel stones for filter media of Constructed Wetland

Dear Alexandra.

The hole diameter, the minimum distance (center to center) between each hole and the angle, are specificified in the standard AASHTO M252/M294.. I have a copy but is in Spanish. I hope the reference will help you!
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