Fouling and contamination of sensors was studied in waste water applications. Tests were conducted at two waste water treatment plants to analyze fouling on different sensor surfaces and evaluate mitigation techniques. Ultrasound did not effectively reduce fouling and may have worsened it. Certain coatings and ultraviolet light showed promise in mitigating fouling on optical sensors. Further testing of photocatalytic coatings combined with UV light could provide an effective non-fouling solution for optical sensors in waste water applications.

1. FOULING AND CONTAMINATION OF SENSORS
MMEA 2.4.1
Veijo Sutinen (Cemis-Oulu), Markus Riihimäki (UOulu),
Timo J Manninen (Valmet)
1
Faculty of Technology / Environmental and Chemical Engineering research group / Markus Riihimäki 9/17/2015

2. INTRODUCTION
Fouling and contamination of sensors
• Unwanted deposition or contamination from the fluid contact
with sensor element that causes signal noise, drift or delays
leading to error in measurement
• Classification on mechanism: crystallization, solidification,
particle deposition, microbial growth etc. on surface or
contamination of electrolyte in ISE electrodes.
• Classification on type of sensor: optical window, metal
electrode, membrane fouling or contamination in ISE electrodes
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
2

3. MITIGATION OF FOULING
Maintenance and cleaning interval, is this acceptable?
• Discrete or continuous measurement
• Manual or automatic cleaning and calibration
• Sensor installation position
Mitigation and cleaning techniques
• Mechanical: brushers, water or air jets
• Chemical: cleaning solutions
• Physical energy: magnetic, radiation, ultra sound
• Material and coating selection
Compensation of signal drift due to fouling and contamination
• Calibration
• Compensation
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
3

4. TEST CASE IN TASKILA
• Surface material and ultra sound tests at Taskila waste water plant in Oulu
• Reject water from sludge centrifuges
• 10 coatings: 9 for metals, 1 optical for sapphire glass
• Test with and without ultrasound
• Measurement of fouling after a test period c.a. 2,5h: visual, microscopy,
and absorbance measurement for fouling detection
• Chemical analysis from deposited material: SEM-EDS (microanalysis for
elements), IR-DRIFT (chemical identification)
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
4

5. TASKILA SET-UP
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
5
Cell A with
ultra sound
Cell B

6. TASKILA VISUAL OBSERVATIONS
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
6
A)

Ultra sound
B)

Referece
• All material fouled
• More fouling with ultra sound (A)
• YO safir fouled more than ref
• YO metal fouled more than ref
• ATV fouled equally to ref
• AR226 and 229 fouled the least
and cannot make difference
• All CVD coatings fouled markedly
particle or growth crystals observed
• Deposited material consist of
pale transparent deposit containing
dark particles

7. CHEMICAL ANALYSIS OF DEPOSITED MATERIAL
• Particles, X-ray microanalysis  Iron oxides
• Transparent deposit, IR-FTIR  waxes and fatty acids
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
7
Element Weight
%
Atomic%
O K 8.82 71.76
Al K 0.41 2.00
P K 0.34 1.44
S K 0.12 0.48
Ca K 0.27 0.87
Fe K 10.07 23.45
Totals 20.03
Kajaani, Peuraniemi plant
Oulu, Taskila plant

8. TRANSMITTANCE
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
8
• With ultrasound transmittance
dropped more than in ref
• With optical coating
transmittance dropped less
both with ref and ultrasound
9%
15%
22%
27%

9. TASKILA OUTCOME
• Optical windows and metal pieces were tested in reject water of sludge
centrifuges
• Fouling was mainly due to fat/wax and oxidized iron particles depositing on
sample surfaces
• Aim was to make difference between
(1) coatings on fouling susceptibility
(2) effectiveness of ultrasound
• In the tests, all surface fouled in 2,5 h
• Deposited material was found to be mostly fatty acids/wax with iron oxide
particles
• Ultrasound did not mitigate fouling, probably induced fat droplet coalescence
leading to heavier fouling
• Fouling was mitigated by some coating, marked benefit was found with AR
226 and 229 coatings (fluoropolymer coatings)
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
9

10. TEST CASE IN PEURANIEMI
Optical measurements at Peuraniemi waste water plant in Kajaani
• Waste water after biological filtration, treated plant effluent
• 3 optical cells with quartz windows:
(1) air jet (top position),
(2) spectrum measurement as reference (middle),
(3) continuous UV-light (bottom)
• Fouling detection from spectrum through optical cell trough tap
water
• Chemical analysis from deposited material:
SEM-EDS (microanalysis for elements),
IR-DRIFT (chemical identification)
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
10

11. PEURANIEMI TEST SET-UP
9/17/2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
11
Continuous measurement
24/7 (190…1000nm)
UV-light (270nm))
Reference channel
UV quartz glass
Tap water
Sample fluid out
Sample fluid in
Cyclic air jet
2s 3,5h interval

12. TEST 1 PEURANIEMI PLANT IN KAJAANI
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
12
• Top (air-jet) and bottom position (UV light) have small drop in transmittance
• Reference has marked drop especially after 3 weeks

13. TEST 2 PEURANIEMI PLANT IN KAJAANI
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
13
• During test period 2 fouling was weak and difference between UV, air-jet and
reference could not be made
• Test period 2 was during spring having flood waters: high flow and low P

14. PEURANIEMI OUTCOME
• Aim was to find optical window fouling in plant effluent
conditions
• Mitigation of fouling was tested using (1) UV light and (2) air jet
• Test periods (2) were during the spring 2015
• In the first test period UV light and air jet was found to
mitigate fouling
• In the second test fouling was slow and no clear differences
• Further testing of UV light could be beneficial because in
normal operation of waste water plant it was found to mitigate
fouling effectively
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
14

15. CONCLUSIONS
• Ultrasound seem not to be option in fouling mitigation
• Not effective in waste water application
• Expensive structure if integrated to a sensor
• Coatings and especially UV light are promising techniques to
mitigate fouling in optical measurements
• UV led gives low-cost source of radiation to be used in connection
with optical measurements
• Optical coatings are demanding to find, most existing ones are
anti-reflection coatings and are not optimized for non-fouling
• Photo catalytic coatings (non-fouling) for windows are known but
was not unfortunately tested yet in this work
• In further studies, combination of photo catalytic coating and
UV-light would be beneficial to be studied
17.9.2015Faculty of Technology / Environmental and Chemical Engineering group
Markus Riihimäki
15