BIOLOGICAL TREATMENT (AEROBIC TREATMENT)
The biological technology allows transforming the organic and inorganic contamination, contained
in waste water, into carbon dioxide and water. This is achieved by injecting oxygen into the water,
thereby making p
ossible the survival of the bacteria necessary to complete this process.
AIM OF THE TREATMENT
The biological treatment has for objective the removal of the pollutant substances in the
The biological technology presents the following advantage
Minimum plant operational costs
Working stability of the plant and its capacity to adapt to the variations of quality and quantities
Management easiness and minimum manpower requested.
Minimisation of waste sludge at the outlet of the pro
cess which anyway is NO TOXIC sludge
and can be used as fertilizer in agriculture
Waste water Lifting
Storage and homogenizing
Nutrient salt dosing
ion with lamella clarifier
circulation and excess sludge extraction
Excess sludge extraction
Thickening of excess sludge
Description of each step
Waste water lifting
The row liquors coming from the factory are collec
ted, through a collector, in a tank of initial lifting.
It is located at a lower level than the liquors contained in the storage and homogenisation tank, for
this reason it is necessary to lift and transfer liquors by means of pumps.
The switch on and the
switch off of pumps are driven by limit
switches which are positioned in a
specific way in order to maintain the liquor level under the height of drainage points of the factory.
Inside of the tank is installed a switch with a high alarm level.
s are dimensioned in order to have the capacity to transfer peak of flows which are
even twice of the nominal flow per hour of the plant, and the lifting tank guarantees a such storage
in order to assure a lasting working of pumps.
has the purpose of separating coarse and medium
fine solids at the inlet, avoiding thus their
sedimentation in the next stages and clogging of the equipment (pumps, mixers etc.)
There are several models of screens, depending on the type of plant: rotary dr
um, arch with
brushes, arch with comb, static, cascade, disk, step, coclea etc.
In all cases the screen is made of stainless steel and however in material, suited to be constantly
in contact with water without suffering corrosion.
The solids are separated
from the water and forwarded to special containers, which must be
The cleaning of the screening surface is automatic and continuous and it is done by brushes,
combs or counter current pressured water jets. Pre
set time switches or lev
el sensors control the
In the case of static screens, cleaning occurs manually.
Storage and homogenizing
The quality and quantity of water discharged during the day is not constant. The flow rate and the
quality depend on the type of
work and rate of work happening throughout the day.
To feed a biological plant it is necessary to homogenize the water. The water has to result
constant from the point of view of quantity and quality. This is in order to grant the bacteria a
ng (necessary for their survival) with consequently high performances in the
abatement of polluted substances.
Furthermore it will be necessary to first collect the water and then dose it into the biological plant
in a constant way in order to avoid hydrau
lic problems in different parts of the plant that might
compromise its proper functioning.
Inside of the homogenizing tank it will also be important to oxygenate lightly the effluent just to
transform the conditions from anaerobic to aerobic, so to create
the ideal habitat for bacteria.
The oxygenation of effluents is also important because it prevents smells which are usual in
effluents in anaerobic condition.
A neutralization step takes place because of the wide
pH range in the wastewater.
The biological oxidation needs a neutral or slightly alkaline pH value. Neutralization is carried out
automatically by adding soda or acid, depending on the characteristics of the wastewater, by
dosing pumps and a pH meter equipped with alarm signal con
The pumps take the reagents from storage tanks of a suitable capacity.
Their start is automatic and set to provide for a pH value between 7 and 8.
The pH measurement is carried out by a glass made probe, equipped with potentiometer, easy to
n case of failure or wear.
Nutrient salts feeding
In some cases wastewater discharged from industries doesn’t have the right composition of
nutrients which are required for the survival of bacteria
For this reason it is necessary to install a plant
for dosing those nutrients which if lacking would
cause many problems to the survival of bacteria. This plant is the same used during the start
period and during holiday (periods in which the industry is not working).
It is the “
“ of the plant.
The Oxidation Tank is where all transforming phenomena of pollution contained in the waste water
takes place, thanks to bacteria and micro
C, H, N, O, P, S,
Organic acids etc.
Bacteria in the course o
f their life they grow and multiply themselves. This is possible through two
processes: the energetic and the synthesis one.
In the wastewater treatment plants both are used. The oxidation of a part of the organic substance
furnishes energy, while the rema
ining part is used as feeding substance.
In the synthesis process the polluted materials removed constitute new organic matter of bacteria,
while with the energetic process are obtained stabilized final materials such as: H
E CAN SIMPLY
SAY THAT BACTERIA FE
EDS ON POLLUTED SUBS
TANCES CONTAINED IN
DISCHARGED FROM THE
AND TRANSFORM THESE
IN SUBSTANCES WHICH
ARE NOT DANGEROUS
FOR ENVIRONMENT SUCH
So the oxygen supply has a big importance for the su
rvival of Bacteria and for the transforming
This is also the phase where the running costs of the whole plant are
The system permits high performances of the oxygen transfer in
particular if supported by mixers that impro
ve the contact time
between micro air
The best way to supply oxygen inside the tank, is by installing on the
bottom, diffusers that produce micro air
bubbles furnished through
The quantity of air supplied, is autom
atically controlled by a dissolved
meter which is positioned inside of the oxidation tank and
drives the blowers permitting to furnish the exact quantity of oxygen
required by increasing or reducing the rated power of blowers.
The aerated mixture in the oxidation tank is forwarded, in consequence of gravity, to the clarifier.
The clarifier is a tank that, in particular conditions of quietness, allows the settlement of the sludge
flocks with a consequently good separation be
tween clean water and sludge (bacteria).
The sludge flocks settled on the floor are collected again in the oxidation tank.
Sedimentation with lamella clarifier
Instead of the traditional static circular or longitudinal shaped decanters, sometimes a more
compact and efficient decanter such as a lamella clarifier elements can be a better choice for the
sedimentation of solids in wastewater.
The flexible elements are a sequence of plane or corrugated overlapping plates, which run parallel
to each other insid
e a frame and are arranged in metallic or concrete tanks.
In order to provide for a good sedimentation of solids, the plate interceptor elements are inclined,
so that the plates form a 60° angle with the horizontal plane.
When the wastewater flows throu
gh the plate lamellar pack, all the suspended particles of the
water are immediately intercepted and settled on the surface of the flexible elements, avoiding in
this way to be swept away by the main stream of water, which flows through the flexible elemen
from the bottom to the top.
Due to the gravity effect, all the particles settled on the surface of the plates reach the lower part
of the sink where the plate interceptor elements are installed. Here, they arrange themselves in
strata and grow thick. Af
terwards, they will be extracted by means of pipes and valves.
The advantage of plate interceptor elements sedimentation is that they are remarkable space
Compared to the traditional decanters where the sedimentation surface is as large as the de
surface, the working sedimentation surface is given, in our project, by the projection of the
horizontal plane of the plates forming the plate interceptor elements, which means a space
reduction of about 90%.
The back pumping of the aerated liquor
avoids the slipping away into the sedimentation tank of a
part of the nitrates. The back pumping of the sludge avoids the sludge to become anoxic inside
the clarifier, with as a consequence flotation and its outlet from the very same clarifier.
Description of each step
circulation sludge pumping station
The sludge re
circulation is a very important plant stage.
It works 24 hours a day and it is needed to extract sludge settled on the decanter floor to put it in
again in the oxidation
Usually this operation is made by means of submersible pumps at low speed (to avoid breaking
the sludge flock) with a specific design of impeller that avoids the obstruction.
The extraction of excess sludge from the plant derive
s from the necessity to maintain a constant
concentration of bacteria, once the right working conditions are reached.
The quantity of excess sludge to be removed, is equal to the mass of suspended solids and is
produced in consequence of the growth of ba
cteria due to the flocculation phenomena.
Thickening of excess sludge
To reduce the quantity of water contained in sludge in order to permit the settlement, a static tank
with vertical flow called
is used, it permits to increase the dry substanc
e from 1% till
To improve the thickening of sludge 0,4 ppm of cationic polyelectrolyte is usually dosed.
The water so decanted returns inside the biological plant.
The thickened sludge extracted from the floor is sent directly at dismalting or alt
further thickened by means of a filter
press in order to improve the dry content till 25% and make it