Hi Florian,

reluctance to implement such a system in other places comes more from engineers and authorities (that are used to other systems) than from the public

In my case I´m the engineer .
The problem about the reluctance is severe when you don´t have examples close by. Once you did the first, the next are MUCH easier. In my case the point is to convince the final customer..the one who pays for it... as my work in this in most cases is not donor dependent.
Concerning the slope necessary when you would like to avoid a pump. That is really a condition, you have to have about 1-1,5 m between siphon and bed twice (2 stages), no elegant way to come around that.
If you are working without electricity than you have to go to the plant twice a week, but that is necessary anyhow as you would have a corse screening which has to be cleaned (controled) at least twice a week.
But I agree, a septic tank and a horizontal flow or vertical flow with siphon is less necessity of continuous operation.
Christoph

Hi Christoph,

thanks for the reply and also the hint to the technology review, somehow that has slipped my memory...

For the operation I agree, when you have pumping, it's no big deal to also automate the change of loading to the different filter units. In our projects (eastern europe) I try to avoid the need for pumping wherever possible. This leeds to perhaps another disadvantage of the "French system": more slope required than horizontal beds if you don't want to use pumps.

In your practice, did you have really much problems with the fact that wastewater is loaded on the surface? In France these things are built at a rate of around 100 plants per year, so perhaps reluctance to implement such a system in other places comes more from engineers and authorities (that are used to other systems) than from the public?

Cheers, Florian

Dear Florian,
I do agree with you totally. The French system is a very interesting sytem. We described it in the
Technology review of constructed wetlands
Personally I do see a very, very interesting system and it works extremely well. I´m pretty sure that the first stage of the system in a lot of cases gives sufficient results, at least much better results than a lot of treatments which are poorly operated.
I have been involved in the design, construction and operation of some of these wetlands. They are one of the best options I know as a reliable, excellent wastewater treatment with a very low need for operation.
Concerning your point of the need of visiting the plant on a regular base to switch between the first stages (for those who do not know the system, one bed of three of the first stage receives raw water for some days and then has a resting period to be able to decompose the material). We do solve that with a simple electronic device which changes the alimentation automatically.
The most important point is the careful analysis of the material used in the filter beds.
In regard..why is it no used in larger scale? I see that the raw water aspect on the first stage surface is a concern in many cases. It has been a limiting factor for my intents of selling the system.

Yours
Christoph

Hello,

I uploaded a photo-documentation from a recent trip to France where we visited several constructed wetlands.

The described type of CW seems to be very widespread in France, with most rural wastewater treatment plants currently built being of this type:

2 stages of vertical flow constructed wetlands, often with subsequent inflitration of the effluent. The most interesting feature is that these plants do not have a pretreatment stage such as settling tanks to remove solids, but load the raw wastewater (after coarse screening only) directly onto the surface of first stage vertical filter. There sludge accumulates, dries and is mineralised very similarly to the way sludge is treated in planted drying beds.

I find that option very interesting, as there is no need for desludging of pre-treatment facilities and sludge disposal. In my experience this is very often the most critical point of operation of CWs in developing or transition countries. Desludging is usually a costly action and is very often just not done. Then the pre-treatment fails and solids enter the filters that are not designed to recieve them. And even if desludging is done properly, very often there is no good chain for sludge treatment and disposal in place, sludge ends up being dumped somewhere.

I also see some desadvantages of the described system, e.g. that operators need to be present more frequently on the treatment plant (at least 2-3 times per week) compared to systems with pretreatment...

I'd be interested to hear some opinions on this by people having more experience with constructed wetlands.

It seems that different schools on how to do constructed wetlands have evolved. In Germany and Austria, the standards seems to be pre-treatment in settling tanks and then vertical or horizontal filters, whereas in France the standard ist the above describe set-up without pre-treatment. Why is it that the "French style" is so uncommon elsewhere, dispite the very interesting feature of being a sludgeless system?

Florian