construction provisions

All the various systems adopted (once the above limitations have been allowed for) are feasible from the viewpoint of foundations and civil engineering works however, their feasibility will vary according to the site, i.e.:

• sub-soil geology;
• presence of an aquifer;
• position in relation to a risk of flooding;
• conditions applicable to effluent discharge into the natural environment;

Construction costs will vary widely and, very often, there will only be one optimum solution.

Note: civil engineering costs often account for more than 60% of total project costs).

The most stringent limitation is often imposed by the need for a structure’s hydraulic stability to be compatible with essential scheduled maintenance that call for the structure to be drained.

In fact, excluding cases where intermediate lifting can be used to re-instate "auto stability" in these structures for a reasonable energy outlay, compact processes (lamellar settling, membrane bio-reactors) with structures requiring little or no ballast, once again have a clear advantage over those that have a large water surface area (aeration tanks and clarifiers). The latter demand special geotechnical precautions such as lowering the aquifer, isolating the structure with sheet piles and pumping, or even reinforcing tank floors and ballasting.