Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
4.2 Off-site wastewater treatment systems
Off-site treatment is the treatment of wastewater that has been
conveyed using a sewerage system (Section 2 (3)). Activated sludge treatment is
now considered the conventional means of large-scale off-site treatment of
sewage, and is described first. Trickling filtration is an alternative that was
developed earlier than the activated sludge process, and this is described next.
There have traditionally been other more simple, but as effective methods of
treating sewage. These include the use of ponds or lagoons, land based treatment
(sewage farming), and aquaculture. The first two are described in this section,
while aquaculture is described under wastewater reuse (Section 2 (6)), because
wastewater is generally treated first prior to aquaculture.
Several general principles common to treatment systems will be
discussed first. The main aim of treatment is to reduce biochemical oxygen
demand (BOD) and suspended solids (SS) to acceptable levels. This is achieved by
removing solids and aerating the wastewater to satisfy the oxygen demand of the
wastewater. The different treatment systems achieve the removal of solids and in
providing oxygen in different ways. It should be noted that if the systems are
properly designed, constructed, operated and maintained, they should all achieve
the required standard of treatment. The latter is generally a reduction of BOD
to less than 20 mg/L, and SS to less than 30 mg/L.
Nutrients (nitrogen and phosphorus) may need removal if the
wastewater is discharged to water environments sensitive to enrichment by
nutrients. The North America and Western Europe Regional Overviews contain
details of methods for removing nutrients, because nutrients have been found to
be a problem in many receiving waters. Heavy metals and other pollutants are not
generally a problem unless the sewerage system receives industrial discharges.
In this case treatment of industrial wastes prior to discharge to the sewerage
system is the solution to this problem.
Removal of SS and BOD produces sludge, and the sludge has to be
treated prior to reuse or disposal (Section 2 (5)). Anaerobic treatment has
recently been suggested for wastewater. The main reason for the use of an
anaerobic process is the recovery of energy (in the form of methane) from the
wastewater (see Section 2 (2.3)) for explanation of the anaerobic process). The
upflow anaerobic sludge blanket process is described at the end of this section.