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VALUE CHAIN AND TECHNOLOGIES
Wastewater treatment involves the biological processing of the combination of
the influent and activated sludge
Wastewater
REMOVAL OF
LARGE VISIBLE
OBJECTS
PROCESS DESCRIPTION
SEDIMENTATION
CLARIFICATION
BIOLOGICAL :
In the activated sludge 1 process, micro-organisms degrade
organic waste under aerobic conditions. The micro-organisms grow,
clump together and eventually settle at the bottom of the tank
(clarifier). Activated sludge is used to clear organic and suspended
solids from the influent 1 and reduce Biochemical Oxygen Demand 2 .
Some of the sludge is reused (RAS in illustration), while the rest is
disposed of (WAS) and can be digested to produce biogas. The
treatment can be performed by different technologies, including:
• Conventional activated sludge process (in the illustration).
• Sequencing batch reactor with a one-tank settling system only.
• Membrane biological reactor, in which a membrane replaces the
settlement process.
ACTIVATED
SLUDGE
FILTRATION
TECHNOLOGY PROS CONS
Activated sludge process
Sequencing batch reactor
Membrane biological
reactor
• Very efficient process
• Low Capex and Opex
• Reliable remote operations and maintenance
• One tank: small space requirement
• Does not need tertiary treatment
• Higher flow rate than settling method
• Small space requirement
DISINFECTION
CHEMICAL
BALANCE
• Difficulty to adjust to variations in waste composition
• Large space requirement (secondary clarifier or tank)
• High energy consumption
• Sludge needs to be disposed of frequently
• Higher capital and operational cost for same throughput
than the other two
• High energy consumption
Note: Picture credits: ewisa (online); 1 “Activated sludge refers to a mass of micro-organisms cultivated in the treatment process to break down organic
matter into carbon dioxide, water, and other inorganic compounds” World Bank (online); 2 See slide 40 and 49.
Source: GE power and water (online), membrane bioreactor; EPA (2007), “Wastewater management fact sheet, membrane bioreactors”; Pipeline (2003),
“Explaining the activated sludge process”; The World bank Sanitation Hygiene and Wastewater (SHW) resource guide (online)
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