Limitations regarding organic matter
Limitations imposed by environmental agencies
for BOD are usually expressed in terms of effluent discharge standards and minimum removal
efficiencies. These constraints are probably the
cause that has mostly limited the use of anaerobic systems (without post-treatment) for sewage
treatment (see typical values in Table 1).
In view of the limitations imposed by the environmental legislation for the effluent BOD concentration, or also when the receiving body has
limited capacity for assimilating the effluent from
the treatment plant (which is frequently the case),
it is usually necessary to use aerobic treatment to
supplement the anaerobic stage. However, there
are situations in which the combination of different anaerobic processes can meet less restrictive
requirements regarding efficiency and concentration of the final effluent (e.g. 80% and
60 mgBOD/l, respectively). This is the case of systems consisting of a septic tank followed by an
anaerobic filter (usually feasible for small populations, generally fewer than 1000 inhabitants) or of
a UASB reactor followed by an anaerobic filter.
Obviously, the application of these combined
anaerobic systems is conditioned to an appropriate dilution capacity of the receiving body.
In this sense, in situations in which the receiving body presents a good dilution capacity, the
adoption of less restrictive discharge standards
could enable the construction of simpler and
more economical treatment plants in several
small cities by means of a more intensive use of
anaerobic reactors, particularly UASB reactors.
At a later stage, if it becomes necessary to produce a better quality effluent, a complementary
treatment unit can be built after some years. The
high costs of sophisticated treatment systems, designed exclusively to meet BOD discharges standards, make their construction at a single stage
unfeasible for most cities located in developing
countries. On the other hand, the construction in
stages could be decisive and that systems consisting of UASB reactor and a post-treatment unit
become the most feasible ones regarding technical and economical criteria.
2.2. Limitations regarding nitrogen
and phosphorus
The discharge of nutrients into surface water
bodies may cause increased algal biomass as a result of the eutrophication process (abnormal algae growth due to the nutrients discharged). It is
known that 1.0 kg of phosphorus can result in
the reconstruction of 111 kg of biomass, which
corresponds to approximately 138 kg of chemical
oxygen demand in the receiving body (Randall
et al. 1992). Similarly, the discharge of 1.0 kg of
nitrogen can result in the reconstruction of
approximately 20 kg of chemical oxygen demand
under the form of dead algae (Randall et al.
1992). The problem can be even worsened due to
the decreased oxygen levels, by means of th