Protection of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 206

The specifi c drawback of biological systems is their low tolerance of
variations in quantity: they are therefore well-adapted to communities
with little seasonal population variation (rural areas and agglomerations).
The large masses of sludge produced must absolutely not return to the
hydrographic network. This sludge, which has a high agronomic value,
generally has a poor image with farmers who might benefi t from it, and
the surplus must therefore be incinerated.
In combined sewers, the stormwater runoff has a composition that
varies over the course of a period of rainfall: the fi rst water to reach the
ground washes the pavement, picking up suspended material, heavy metals,
and automobile lubricants. After this washing, the stormwater is of better
quality. The inability of treatment plants to handle the peak discharge of
combined sewers during rainstorms was long solved by creating “storm
sanitary sewer bypass”, which bypassed the treatment plant and dumped
the combined sewage and runoff into the environment (river or aquifer).
Today, treatment plants are being equipped with holding basins with
capacities of several thousands of m 3 , capable of holding at least the
beginning of fl ood peaks, in order to treat them later along with normal
urban effl uents, after the peak.