Protection of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 204

Industrial pollution is generally treated on-site where it is produced, but as
some installations are very large, internal pipelines are a source of leakage
and the underlying aquifers receive sometimes-enormous infl uxes (for
example tens of Kg·day -1 of polycyclic aromatic hydrocarbons towards an
alluvial aquifer being exploited for drinking water). The majority of sewer
networks are combined, meaning that they collect sanitary sewage and
stormwater runoff (from roads and car parks). If the quantity of wastewater
depends only on consuption, the quantity of runoff depends on rainfall.
High precipitation can therefore cause high water levels in sewers, thereby
“cleaning” them.
7 WASTEWATER TREATMENT
Wastewater treatment systems are diverse and adapted to the nature
of the pollution to be treated: urban sewage, having a fairly standard
composition, is subjected to treatment processes based on a small number
of basic principles. Industrial wastewater, however, has a highly varied
composition, and treatment is accomplished with procedures specifi c to
the nature of the pollutants to be treated (organic material, hydrocarbons,
solvents, toxic substances, heavy metals).
The treatment is also adapted to the number of inhabitant-equivalents
to be treated, and therefore to the size of the community being served, as
well as to its seasonal variability (tourist destinations).
7.1 Independent wastewater treatment
Independent treatment systems are the solution chosen for industry, small
communities (hamlets far from the nearest agglomeration), or a single
residence far from the collection network. The type of system depends on
the soil permeability: lagoons require impermeable, easy-to-excavate soils
(clay, marl), whereas such an environment would not be conducive to
mechanical fi ltration through the soil, which requires a certain minimum
soil permeability (0.3 mm.mn -1 ).
Lagoons and all other “surface water” treatment systems must let their
treated product out into the hydrographic network. Mechanical fi ltration
systems use either the soil in place (if the permeability is appropriate), or a
substitution soil in order to treat effl uent from a septic tank or a decanter-
digestor. The role of the soil porosity is to absorb the hourly variations in
hydraulic charge coming from the residence, and to allow a long enough
residence time for the effl uent, with a large contact surface area between
the effl uent, the solid particles, and the soil atmosphere. Mechanical