Protection of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 205

fi ltration allows the physical fi ltration of bacteria through the material,
but not the treatment of nitrogen. There are two variants of this method:
if the soil is impermeable, if the unsaturated zone is nonexistent (almost-
exposed aquifer), or if the soil is hydromorphic, fi ltration occurs over a
slab and the treated water goes out into a ditch or a stream; if the soil has a
permeability between 0.3 and 2.5 mm.mn -1 , the effl uent is fi ltered through
the unsaturated zone.
Independent wastewater treatment systems are an interesting solution
for residents far from collection networks, but as they are the fi nancial
and technical responsibility of the owners, it is diffi cult to ensure that the
installation is done according to adequate standards. The only solution
would be for the community to manage independent systems in the same
manner as shared systems (construction, diagnostic). Small lagoon units,
installed in rural zones, lack qualifi ed personnel, and local communities
have put SATESE ( Technical Assistance Service for Wastewater treatment
Plant Operators) in place, in order to improve the inconvenient situation.
7.2 Urban sewer systems
Town sewer systems use biological or physco-chemical processes depending
on the regularity of the discharge being treated, the price of land, and the
winter temperature.
Physico-chemical systems, based on the principle of adding reactive
substances (ferric chloride, quicklime), can eliminate phosphorus, but not
nitrogen. Adjusting the dosage of the reactants enables the treatment of
variable quantities characteristic of industry and of tourist destinations
(coastlines, ski resorts). The treatment occurs in a reactor; the facility
therefore has a small footprint, which makes it easier to integrate into areas
with high land prices (cities, tourist destinations: ski and coastal resorts).
In addition, the closed facility allows for containment of unpleasant odors.
The inconveniences are the high cost of reactants, the resulting sludge
mass, and the technical complexity of the treatment, which is incompatible
with the lack of qualifi ed operators in small communities. In addition, the
resulting water may require additional treatment downstream in order to
control bacterial contamination.
Originally designed to decant suspended material and to oxidize
organic material, the activated sludge system requires a larger surface of
oxygenation basins. This treatment is now being completed by the tertiary
physico-chemical treatment of phosphorus and nitrogen.
Spatially extensive treatment systems (macrophyte lagoons, infi ltration
beds) require a high surface area per inhabitant, and are therefore used only
in small, rural communities (hamlets, isolated areas).