Page 52 - March 2020 - WT Site

1.Activated Sludge b) These systems are effective to attain purification in influents
that vary little in terms of composition or supply
Activated sludge is a very common type of suspended growth
aerobic method. In this method the sewage is treated using c) A buffer tank is needed to control the high variance where
aeration and aerobic microbes composed of protozoa and other the wastewater characteristics vary in terms of contamination
aerobic bacteria. This method is based on biological oxidations and volume
with active sludge aiming to remove the organic matter from d) The process can be optimized according to the wastewater,
wastewater. In addition to organic matter removal in this method also needs a continuous supervision, but have very limited
also aids the removal of nitrogen and phosphorus to some extent. maintenance
The active sludge consists of aerobic active micro-organisms
with sufficient oxygen supplied so that these micro-organisms e) For the wastewater to be effectively undergoing biological
sufficiently oxidize the organic matter present in the wastewater degradation, it can be tested in lab for small samples
to CO2 and water. Biological floc is an ecosystem of living biota f) This system requires a relatively large system due to long
subsisting on nutrients from the inflowing primary clarifier retention times in the tank, the relatively low sludge content
effluent. These mostly carbonaceous dissolved solids undergo and the large sedimentation surface
aeration to be broken down and either biologically oxidized
to carbon dioxide or converted to additional biological floc of g) The temperature of wastewater is normally between 15°C
reproducing micro-organisms. Overflow from the activated and 35°C
sludge mixing chamber is sent to a secondary clarifier where the h) A wide range of COD values can be treated
suspended biological floc settles out while the treated water moves
into tertiary treatment or disinfection. Settled floc is returned to i) Highly acidic or alkaline waters must be corrected so that
the mixing basin to continue growing in primary effluent. a pH between 6.5 and 8.5 can be implemented within the
system.
There are two types of implementation of Active sludge System j) Active sludge systems are relatively insensitive, but can
one is conventional type of sludge system and other is Sequencing be inhibited by high concentrations of salts and specific
Batch Reactor. Conventional type consists of aeration tank and a chemicals.
sedimentation tank. Aeration tank used for biological oxidation
and sedimentation tank is where the sludge is separated from 2. Membrane Bioreactors
treated water and moved to tertiary treatment. The aeration
basins are sometimes preceded by a mixing tank (selector), where In membrane bioreactors technology is advanced biological
the influent is intensively mixed with sludge. The aim of this wastewater treatment technology that combines the conventional
is to prevent the growth of thread-forming bacteria whereas, suspended growth aerobic method to membrane filtration rather
in the Sequencing Batch Reactor the purification processes of than sedimentation. Membrane bioreactor can produce high
aeration, sedimentation and discharge are performed sequentially quality effluent enough to discharge to costal area, sea, brackish
in same basin. Thus, Sequencing Batch Reactor is best suitable channels or to be reclaimed and reuse for irrigation processes.
for the plants where waste is received in batches. An SBR can Membrane bioreactors are an activated sludge system that uses a
be used to consecutively perform various biological processes, membrane for liquid-solid phase separation process.
like nitrification and denitrification. An SBR system is better at
preventing thread-forming bacteria because the system acts as a The membrane component uses low pressure microfiltration
selector during the supply phase. membranes and eliminates the need for a secondary clarifier or
filtration. The membranes thus, replace the sedimentation basin
Review: in common biological purification and help to separate the sludge
from the effluent. This helps to ensure that all floating matter is
a) Active sludge systems are flexible, robust and cost-effective. retained, whereby sedimentation is no longer a restrictive factor
A wide range of influent concentrations can be treated for sludge concentration.

54 Water Today - The Magazine March 2020