Dear Mary Rose,I agree with ...

Dear Mary Rose,

I agree with Zaid and Anacleto: a constructed wetland is a more reliable solution to treat BOD and SS from grey waters than a sand filter. The latter is more likely to clog, due to microbial biofilm development.

But I would rather advise a vertical flow constructed wetland (VFCW), with one or two stages, depending on the outlet requirements. A VFCW will accept raw water, after coarse screening. Biological clogging is avoided by constructing 2 or 3 beds for each stage. One bed is fed during 3-4 days, and then let for rest during 7 days while feeding the other beds. This way, biofilm remains under control. Of course granulometry of gravel or sand has nevertheless to be carefully selected.

The surface of the beds is planted with macrophytes. Their role is to prevent SS clogging of the surface of the first stage, the layer trapped on the surface being continuously perforated by new sprouts. Species used is adapted to the geographical zone. The organic layer formed on the surface mineralizes with time, and need only be removed after several years.

This process can accept reasonable amounts of grease, i.e. mainly what comes from dishwashing, and most of the products used for the washing, the kitchen, or the body.

Standard outlet quality can be as low as 30 mg/l SS and 25 mg/l BOD. Lower levels can also be achieved with specific design.

With sufficient water head, the process requires no energy.

Feel free to ask me for more information: I would be glad to help you in dimensioning and designing your treatment.

Sincerely yours,

François