Dear Alexandra,

My idea is to use the cylindrical part of the 3-liter PET bottles, which are roughly the same diameter as 11-cm PVC pipe. They also get slit up one side, such that they COULD open up flat. A hole gets melted with a hot nail in each corner. A 1-cm-diameter rope gets tied spirally around the perforated drain pipe. The PET gets stitched with string around the pipe and rope, such that the gap runs along the bottom. The crushed tops and bottoms of the bottles are placed under this gap, on the bottom of the excavation.

With this, sand and silt cannot filter, with water flow and gravity, into the pipe. They would have to be carried up 5 cm vertically, but not directly, due to the rope, the tops and the bottoms.

This is a bit labor-intensive, but doable, and the worst thing would be to build the wetland and have it plug. Recylers would be glad to provide the bottles at some low cost, and it would not matter if they had previously been crushed.

Good luck with your project. Let us know how things go.

Best wishes,
Chris Canaday

Dear Juan

Thank you for the reference. I managed to find the related description in English and it was quite helpful.

Greetings from Nairobi

Dear Chris

Thank you for your answer. The project is going on well, we are still at the construction phase,trying to finalise the decentralised treatment plants as best as possible. For the filter material in the wetland we have opted for crushed stones (ballast) but only for the locations where round gravel (from rivers) are not available. It would be very interesting to study and compare the efficiency of the different alternative materials you are suggesting. I would be interested to hear about your idea of protecting the drainage pipe with PET plastic. Are you speaking about the plastic pellet or the actual bottle?

Dear Alexandra,

How is your project going?

In the interest of brainstorming, here are some ideas of materials that could replace the gravel in your wetlands:
-- the thick shells from African Oil Palm (Elaeis guineensis),
-- intact, crushed or chopped PET plastic bottles (with their caps removed), since this plastic is heavier than water (Here they do not accept certain colors in normal recycling),
-- the caps from the PET bottles (I guess these might be polypropylene.)
-- other plastic bottles that do not float,
-- unbroken disposable glass beer bottles (Do these get recycled there? Do they even exist? How much are they worth on the market?),
-- building rubble that has been screened to classify the size,
-- broken ceramic tiles from the factory,
-- charcoal or biochar, screened and graded (maybe Sanivation's fecal charcoal briquets that are made in Kenya).

The main reason for using this gravel is to keep sand from filling and plugging the pipes. I have an idea of how to greatly contribute to this protection with the PET plastic of 3-liter bottles. I worked it out for a project out in the jungle, where there is no gravel, but we still have not done the project, for lack of funding. I can explain, if this sounds interesting.

Best wishes,
Chris Canaday

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!

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

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.

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

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.

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

Hi Alexandra.. i wannt to ask you something! How big is your filter bed?!

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