Potential operating savings through controlled wastewater and sludge treatment
By Franz Winter, Endress+Hauser
In recent years, wastewater treatment
plant operators’ efforts have
increasingly been aimed at devising
economically efficient processes
in wastewater and sludge treatment
besides actually eliminating pollutants.
Target figures for eliminating carbon
and reducing nutrients such as phosphorus
and nitrogen play an important
role in this since they are a major factor
in the eutrophication of our waters.
There is also a financial aspect linked
with sewage treatment, sludge
biosolids treatment and disposal, to be
taken into consideration. Use of modern
instrumentation and control systems
in various stages of the wastewater
treatment plant processes supports,
or rather permits in many cases,
controlled wastewater and sludge treatment.
Inlet Load Measurements
Measurements in the wastewater
treatment inlet make it possible to
determine the load carried and to track
the load development, i.e. the capacity.
The organic load is of special interest.
Recognising shock loading in good
time allows continuous measurement
of the TOC content. The “Sum
Absorption Coefficient” (SAC) is
gaining increasing acceptance in this
respect since it is easy to use.
Nowadays it is possible to measure the
SAC using sensor technology.
Automated Sludge Discharge
Volumes of sludge in the mechanical
treatment stage are subject to wide
fluctuations. This is attributable to
waste-water composition, weather
conditions, seasons and the daily
rhythm of dischargers in the catchment
area. Normally, primary sludge drawoff,
according to a given time pattern,
will not be capable of handling varying
fluctuations in solid matter volumes.
This is the first point where it is
possible to prevent the undesirable
entrainment of water into the downstream
sludge treatment process.
Sludge which is too thin causes
increased costs in heating digestion
tanks, in the sludge dewatering
process, and reduces the hydraulic
capacity of the sludge treatment.
Primary sludge discharge.
The start of sludge discharge, i.e.
opening the sludge draw-off gate
valves, can take place as before within
a time pattern based on a daily load
curve or using sludge level measuring
equipment which works on ultrasonic
principles. The fact that there are now
four-channel systems for measuring
sludge levels is proving to be a benefit
in respect of capital investment
requirements.
The use of continuously operating
instruments determining suspended
solids content is aimed instead at ending
sludge draw-off in good time.
Suspended solid sensors are
installed directly in the sludge discharge
pipe in order to reduce maintenance
effort, increase operational safety
and obtain representative measurements.
In a typical cycle the solids
content increases to approximately 5%
dry mass (DM) after the discharge gate
is opened, then declines to approximately
2% DM over a period of about
five minutes, before sudden water
inrush occurs. The limit value for suspended
solid content is set according
to this characteristic curve to halt
sludge draw-off at about 1 - 2% DM.
Suspending the sensors directly in
the sludge hopper has not proved successful.
The risk of sludge depositing
on the hopper walls is too great. The
sensor would then signal a high sludge
level although the water would have
already been drawn off due to tunnelling.
Biological Nitrogen Elimination by Denitrification/Nitrification
In recent years it has become
impossible to imagine not using online
measuring technology for optimum
operation and for transparency. This is
particularly relevant since approximately
65% of the entire energy
requirement for a sewage treatment
plant is used during the biological
stage of the process. Approximately
80% of this is used in denitrification.
Various methods are used, the most
frequent being upstream denitrification.
Therefore the water flows first
into a biomass culture in anoxic conditions.
The nitrate content at the outlet
of this denitrification tank is measured
by immersion sensors using the UV
absorption principle. This nitrate content
measurement also directly controls
the quantity of recirculated
sludge, i.e. the nitrate return from the
downstream nitrification tank. If there
is a low level of nitrate, the quantity of
recirculated sludge is increased, or
vice versa. For example, if the nitrate
content of 2 mg/l in the denitrification
discharge is exceeded, the denitrification
stage is overloaded. In this case,
the recirculated sludge is either
reduced or carbon dosing may be activated
in order to “feed” the nitrate
organisms, thereby increasing its performance/
activity. If, instead, the
nitrate content falls below 0.5 mg/l,
this indicates that the denitrification
stage still has decomposition capacity
and the recirculated sludge can be
increased accordingly.
The microbiological decomposition
of organically linked nitrogen to nitrate
occurs during the aerobic stage - nitrification.
The microorganisms involved
in the nitrification stage require dissolved
oxygen to metabolise; this is
introduced by a blower. Overriding
control according to ammonium content
at the exit from the nitrification
stage can lead to significant energy
savings. Furthermore, the efficiency of
the denitrification stage is increased in
that, under controlled conditions, less
free oxygen is introduced into the tank.
In this cascade control of the ammonium
and oxygen concentration, the
ammonium analyser controls the oxygen
flow within the 0.5 mg/l to 2.0
mg/l range.
The oxygen control operating at
this level limits the air supply intensity,
since oxygenation becomes uneconomical
at values over 2 mg/l. If the
ammonium is not completely oxidised
at this value, there is an overload or
wastewater pollution load.
Suspended solids measurement in
the activated sludge basin serves to
ensure the biomass necessary for
decomposition of the organic matter.
The quantity of return activated sludge
from the secondary clarification stage
is set by the return sludge ratio.
A ratio of 1:1 is normal (incoming
wastewater:return sludge). The return
activated sludge ratio is controlled by
the solids measurement in the activated
sludge basin.
As a result of the larger basin volumes,
the reaction time is correspondingly
high, so that step controls have
proved correct.
Automated Waste Activated Sludge Discharge
There are two possible measuring
parameters for the sedimentation
process stage in the secondary clarifier:
measurement of the sludge profile,
and solids content measurement in the
return activated sludge pipe.
a) Sludge level concentration measurement
(profile):
Waste activated sludge draw-off is
controlled by continuous measurement
of the sludge level using a
minimum/maximum contact for the
sludge level.
The operator can obtain a wealth of
additional information by installing an
optical measuring system where a suspended
solid sensor tracks the separation
zone. This determines the sedimentation
behaviour by traversing a
probe up and down the depth of the
basin and by assigning the suspended
solid content measured by the sensor
to the sensor position. The resulting
depth/concentration profile provides
information on sedimentation behaviour,
sludge quality and, in the broadest
sense of the term, on the sludge
age.
The deposition of extremely fine
suspended matter, caused by anaerobic
processes, and undesired denitrification,
can also be detected in good time.
Control of circulated sludge.
Control of required dissolved oxygen.
b) In-line suspended solid measurement:
Just as in the preliminary settled
sludge draw-off process, the suspended
solids of the return activated sludge
can be used as a control parameter. The
photometric sensor is normally
installed using a ball valve assembly
which allows the operator to fit and
remove the sensor for maintenance
purposes without interrupting the
process. The signal from the continuous
suspended solid measuring system
is evaluated against a minimum limit
in order to stop discharge of waste activated
sludge in good time.
Optimized Sludge Dewatering
The digested sludge still contains
approximately 95% water. In order to
burn the sludge, or to enable it to be
stored at a disposal site, additional
water must be extracted from it in
thickeners, belt filter presses, screen
belt presses and centrifuges. Solids
content, turbidity, pH, flow and hydrostatic
pressure, as well as differential
speed when decanters are used, are the
control parameters required. When the
suspended solid sensor is installed it is
especially important to ensure that
measurements are taken before the
polymers are added.
Flocculant dosing causes
a direct change in the
optical characteristics of
the sludge, which are
also dependent on the
reaction time. An excessive
change in the optical
quality of the sludge
will lead to measuring
errors.
Control of return/waste activity sludge.
Flocculant dosing
according to the incoming
dry mass weight,
determined from the
product of flow and
solids content, is referred
to as “Feed forward”
control. Feedback,
i.e. regulation, is possible by
using turbidity sensors in the centrifugate/
filtrate. The foam and air bubbles
must be extracted from the centrifugate
in a de-aeration vessel in order to
obtain representative measured values.
Flocculants (polymers) are used to
destabilise the charge equilibrium in
the sludge/water mixture and thereby
to promote the coagulation of
microflocs. The reaction of polymers
with the sludge to be dewatered
depends on several factors, e.g. pH
value, temperature, particle size, and
the water retention capability of the
sludge. Polymers change the charge
balance in the suspension and thus permit
the suspended solids to coagulate.
Measuring the charge on the surface of
the suspended solid is an obvious way
of determining effectiveness. Streaming
current monitors are used to measure
the charge. The control target (set
point) is located close to the isoelectrical
point.
The principle of the economic optimum
applies here. In other words,
overdosing of polymers must be avoided
on the one hand, since the motto
“the more the better” does not apply
here; it is rather the case that too much
flocculant makes the dewatering result
worse. On the other hand, it is appropriate
to consider whether a centrifugate
lightly loaded with solids should
not be accepted if this means that polymer
consumption can be significantly
reduced and thereby great cost savings
can be achieved over the year.
Summary
If one starts with the guiding principle
of avoiding causes instead of remedying
them, it is impossible to do
without online measuring systems as
described above.
The continuous measurement of
level, pressure, flow, oxygen, pH,
solids content, sludge profile, sludge
level, SAC, ammonium, nitrate and
phosphate parameters has become
standard equipment in a sewage treatment
plant.
They are a suitable means not only
for finding out more about the constitution
of the sludge and wastewater but
also about the process stages and, thus,
to reduce operating costs through regulation
and control, and, at the same
time, to increase operational reliability.
For more information contact Jean
Rivers, Endress+Hauser Canada. Email:
jean.rivers@ca.endress.com.
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