The document discusses online monitoring of wastewater parameters like COD and TSS in a Common Effluent Treatment Plant using UV/VIS spectroscopy. It describes how a local calibration procedure using samples from the specific plant can provide more accurate readings compared to a global calibration. Maintaining proper cleaning frequencies for the monitoring equipment is also important for avoiding measurement drift. The results showed that UV/VIS analyzers can reliably and accurately measure COD and TSS levels with a local calibration approach.

1. Online Monitoring of Wastewater in Common Effluent Treatment Plant
1. Introduction
Monitoring of wastewater parameters in a Common Effluent
Treatment Plant is a challenging task due to intake of wastes, having
diverse properties & composition, from various industries. Hence it is
difficult to set boundary conditions on the measurement. UV/VIS
spectroscopy is an efficient method for continuous monitoring. This
method is based on Beer-Lambert’s absorption law;
A= Ɛbc
Where, ‘A’ is absorbance (no units, since A = log10 P0 / P)
‘Ɛ’ is the molar absorbtivity with units of L mol-1 cm-1
‘b’ is the path length of the sample - that is, the path length of the
cuvette in which the sample is contained.
‘c’ is the concentration of the compound in solution, expressed in mol
L-1.
Correlations between absorption and equivalent concentrations are
calculated using Multivariate Linear Regression Analysis. These
correlations if calculated for limited effluent matrices, environmental
conditions & industrial operations will be insignificant to establish
correct correlation as it does not allow understanding & evaluating
complete dynamics & variability of different parameter
concentrations. Hence, it is important to collect most of the
wastewater information in the training set for the analyzer. Accuracy
of the measurement showcases the efficacy & reliability of Real
Spectrum Analyzer for application in Common Effluent Treatment
Plants
2. Materials & Methods
Apparatus
An extraction based UV-VIS Spectrometer has been used for
measurement of COD, BOD & TSS in PRIA Common Effluent
Treatment Plant, Patalganga, Maharashtra. The spectrometer
analyzes the sample over complete UV-VIS spectrum (200 – 750
nm), wavelengths being configurable on field, with sample cell path
length as 2mm. Deuterium & Tungsten light sources are used for
generation of ultraviolet & visible light, respectively. Deuterium light
source is quite resistant to stray light for interfering with the
measurements. The sensor uses just one photodiode for
measurement regardless of how many wavelengths are used. For
multiple & full spectrum scanning, a diffraction grating is rotated
through the desired measurement wavelengths one at a time. The
use of single sensor design allows the realization of several optical
measurement improvements & achieves a wavelength resolution
down to 1nm.
The Real Spectrum Analyzer works on innovative Split-Sense Pro
technology. It is designed to take measurement readings through
water sample & reference readings through air. By continuously
moving the sample cell from the test position between the lamp &
sensor and outwards to allow for a reference reading, lamp readings
are inherently compensated for by the measurement process itself,
therefore improving accuracy.
An automatic pump/clean system is used for sample extraction &
analyzer cleansing process; it is advantageous over other pumping
systems as the effluent matrix remains unaffected to a large extend
& it can be used to cleanse the sample lines in the reverse direction
also for any blockages. However, peristaltic pumps are limited to a
maximum vertical lift of about 10-12 ft.
The system utilizes automatic chemical cleaning technique to
cleanse the sample cell & sample tubing, therefore enables choice of
various cleaning fluids suitable for different industrial applications
based on their wastewater composition. The cleaning mechanism is
initiated on user-configured frequency besides a manual trigger.
Software Platform
envirologicIQ is a cloud based remote monitoring software platform onto
which data from field devices is collected. envirologicIQ enables real-time
monitoring & reporting of the data. Smart GPRS/LAN based Cloud
connector is used to capture data from field devices.
Intelligent alarm rules can be set for the field devices based on parameter
readings enabling the user to take preventive measures to ensure proper
observance of regulations.
User management is well defined in the software so that authority is
properly distributed among various stakeholders, besides proper
maintenance of audit logs.
Device management & diagnostics are built in the software for calibration,
diagnosis & authentication of devices. Automatic alerts are generated in
case devices are not working properly.
Calibration
Two calibration methods exist for online analyzers, viz., Global Calibration
& Local Calibration. Global calibration is the default calibration that is
provided by the manufacturer from a similar plant type to get some
readings in the beginning just after the installation. Global Calibrations do
not have relevant weightage in measurement accuracy due to different
composition of wastewaters. Even similar types of industry/plant may have
significant difference in their effluents due various factors like weather
conditions, industrial processes, etc. Therefore, Global calibrations only
give a rough correlation between measured absorption and parameters of
interest.
The local calibration is able to account for specific wastewater compositions
and possible matrix effects. Provisioning to acquire sample data points over
vast operational & environmental conditions, local calibration enhances
measurement quality & accuracy.
Real Spectrum Analyzer is calibrated using Local Calibration procedure.
For this purpose wastewater samples are physically collected from the
plant & different parameters are determined by laboratory analysis using
standard methods & procedures. For the case-study under discussion,
eight samples were collected over a period of 5 days, (Table1) to get a
training set for establishing best correlations for equivalent parameter
measurements. On completion of laboratory analysis of samples, UVA data
from the analyzer is downloaded onto an external storage device.

2. Online Monitoring of Wastewater in Common Effluent Treatment Plant
Spectra of measured absorbencies is correlated to laboratory results
to achieve a correlation matrices for different parameters (COD,
TSS, etc.) using statistical regression models expressed as;
𝐶𝑒𝑞 = ∑ (𝜆𝑛
𝑖=1 𝑖. 𝐴𝑖) + K
with ‘n’ the number of used wavelength ‘λi’ depending on the
measuring system (for the case under study, n=390). The weighing
factors ‘Ai’ are determined after fitting to laboratory analysis. The
constant offset K can be used for the specific wastewater matrix
adjustment over all single wavelengths. Every calibration model
results from a compromise between generality and robustness.
Usually, particularities in calibration data can be better reproduced by
increasing the value of ‘n’. Therefore, minor deviations between
calculated equivalence values and laboratory values can be
determined.
3. Operational Experiences
After carrying out the local calibration, correlations were fed into the
system. The spectrometer worked absolutely fine for first two days of
operation. The results of spectrometer & laboratory analysis are
compared as in Table 2.
A tremendous overshoot in readings of different parameters
happened on 17th
July. A detailed site and instrument study was
carried out to find the reason for this abrupt change. Primary reason
was found to be the heavy rainfall that happened. Since the effluent
is collected in an open tank before pumping out, rains increased the
component of suspended solids. Second reason was the fouling of
flow cell that occurred due to unsuitable cleaning frequency set for
automatic cleaning. Frequency was set to 12 hour; however the
recommended cleaning frequency for a common effluent treatment
plant is 3 - 4 hours due to high concentration of fouling agents. Third
reason was found to be the improper working of oven in the laboratory that
consequently added to error in readings.
With another unit installed in a CETP at Ahmadabad, Gujarat we faced a
challenge for measuring TSS with visible spectrum method as the effluent
mainly consisted of wastes from pharmaceutical, textile and dye industries.
Therefore, sensor based on IR 90º Scattering principle was used to
measure the TSS accurately. Also, training set for the instrument needs to
cover vast operational changes for better accuracy of COD & BOD
measurements.
4. Results & Discussion
It is of high importance to investigate whole concentration range to get
reliable absorbance-concentration correlation for the system to produce
accurate results. Plot 1 shows laboratory concentration of Chemical
Oxygen Demand & its equivalent online reading.
After local calibration and adjustment work based on taken reference
laboratory samples, the deviations produced by Global calibration could
be reduced. Investigating whole concentration range to get reliable
absorbance-concentration correlation for the system is therefore important
if high accuracy of the spectrometer equivalent values is needed.
However, the dry wastewater matrix adjusted local calibrations does not
inevitably deliver reliable data during wet weather conditions due to the
different composition of wastewater.
For long-term effective operation, cleaning systems are necessary for the
spectrometer itself to prevent measurement drift due to accumulation of
waste material on the measurement optics and for the channel & tubing to
prevent sedimentation & blockages.
Plot 2. depicts the online & laboratory readings for Total Suspended
Solids.
The cited results demonstrate the practicability of UV/VIS analyzers for
online effluent monitoring as the equivalent concentrations for the
parameter COD and TSS measured by the UV/VIS spectrometer are
reliable & accurate.
COD TSS
1 23/06/15 12:05 PM 208 1000
2 24/06/15 11:05 AM 171 340
3 24/06/15 4:00 PM 191 100
4 25/06/15 12:20 PM 235 140
5 25/06/15 4:20 PM 205 960
6 26/06/15 4:15 PM 233 660
7 27/06/15 11:46 AM 541 960
8 27/06/15 3:50 PM 463 1080
Training Set
Table 1. Laboratory Reports for COD & TSS
(PRIA CETP, Patalganga,Maharashtra)
Sample Date
Parameters
Time