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and oxygen. The final products of the process are treated water and excess
sludge. Treated water is usually discharged into water bodies such as lakes and
rivers, while excess sludge ends up mostly as a fertilizer in agriculture or it is
disposed of on land. Some countries like Germany and Switzerland forbade
the use of secondary sludge in agriculture and excess sludge is incinerated
together with hazardous wastes. In any case, the processing of this sludge,
which includes operations like thickening, anaerobic stabilization, chemical
conditioning, dewatering and thermal reduction [260], represents a cost and
a problem that has to be dealt with. Land application of sewage sludge in agri-
culture is very restricted owing to the presence of potentially toxic substances,
i.e., heavy metals, pathogens, persistent organic pollutants, etc. Critical short-
age of available land coupled with new, more-stringent regulations for design
and operation of landfills have caused prices of their sighting, building, and
operating to rise sharply. Incineration is usually the final option for sewage
sludge treatment due to an abundant ash generation, which has a high content
of heavy metals and is generally toxic.
Therefore, high sludge production is one of the main drawbacks of CAS.
Currently, reduction of sludge wasting is a major challenge of biological
wastewater treatment. Excess sludge processing and disposal could account
for about 50–60% of the total cost of wastewater treatment [261, 262]. The
ideal way to solve the problem of sludge post-treatment and disposal is to
reduce its production. To reduce the production of biomass, the wastewater
process must be engineered in such a way that substrate utilization is diverted
from assimilation of carbon for biosynthesis to non-growth activities of a mi-
crobial community. In activated sludge plants, the sludge-yield coefficient (Y)
is typically 0.5. [263]. According to Urbain et al. [264], the yield coefficient
for an aerobic membrane separation process treating municipal wastewa-
ter (488 ± 143 mg COD/L) was 0.23 kgSS kgCOD
–1
removed
. Pollice et al. [99]
reported a production of sludge in an MBR of 0.12 gVSS gCOD
–1
removed
,
which was in accordance with previously reported yields for MBRs [34, 42,
65, 263]. This advantage of MBR, together with the abandonment of energy-
demanding sludge recirculation loop in CAS, contribute to better competi-
tiveness of membrane technology compared to the conventional one.
The limiting step in the conventional treatment is the separation of sludge
from the treated water. Without a good sedimentation in secondary settler,
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