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Online Sensors for Water & Waste Water Treatments

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The main objective of this report the study about the sensor. Which used to measure the parameters in water and waste water treatments. In the following to know about that sensors and its working methods. This report will help the users to know about online sensors which is used for treatment of water & waste water. Its main aim is to gain the knowledge and information about the online sensors which used in the water and waste water treatment.


Everyone needs water. Then the used water known as water which is treated in the waste water plant. And after that treatment we use that water in many ways. We know the water is first treat in plant and then come to our home, Industries, Public places & Public sectors (Like Hospitals, Schools, Railway stations, Colleges, Municipal sector, etc.). Where the many instruments is used. Some instruments are worked with computer functions.

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Instrumentation engineering is the combination of mechanical engineering, electronic engineering, control engineering, system design engineering and computer engineering to achieve measurements and control.[1] The application for this instruments Health, Agriculture, Safety, and Processing Sectors.[1]Monitoring and control of wastewater treatment plants rely on four building blocks: (1) insight into the process as summarized in a proper process model; (2) sensors that provide on-line data; (3) adequate monitoring and control strategies and (4) actuators that implement the controller output.[2]

Sensors are classified into, on the one hand, reliable & simple and day-to-day use in monitoring and controlling sensors; on other hand the advanced and maintenance-intensives sensors that are typically found in auditing and model calibration activities.[2]Advanced instrumentation for multiple sensor network s, parameters can be measured throughout an entire process such as wastewater treatment plant with one system.[3] on-line sensors give the on the way result of the process.

That sensors depended upon that instant result. The sensor is an input-output model, where outputs are variables for the supervised person. For the controlled and maintain the sensor properly work in the plant so knowledge about the sensor and their working method is very important. The sensors maintain the process in the operation. It also maintain the flow of water. The sensor sensed data and collected data send to the operator in variable form on the display. The sensor are use in variety of applications. In waste water and water treatment the variety in application of sensor is measurement of Ph, Conductivity, Chlorine level, Redox level, Dissolved oxygen, Ozone etc.[4]

On-Line Sensor

On-Line sensors are directly connected with the water measurements recourse. Which direct contact with water. Now the many type of sensor is available in the market. That sensor is very light in weight but very difficult to use that. The sensor working automatically and immediately display the reading of the parameter on the screen as a variable form. Now, we are discuss about the different types of sensor.



pH and ORP probes both used for measuring the acidic intensity of liquid solutions. A pH probe measures acidity on a scale from 0 to 14, with 0 being the most acidic and 14 being most basic.[5]Similarly, an Oxidation-Reduction Potential (ORP) probe return a voltage proportional; to the tendency of the solution to gain or lose electrons from other substance.

How they Work

Both pH and ORP probes working as small voltaic cell means a battery. Which is between the fluid and the solution they are immersed in the probes. The measured voltage is then converted into pH or oxidation/reduction potential.


Modern pH and ORP probes have the following components:

  1. The body of the electrode, typically made from non-conductive glass or impact resistant plastics.
  2. The reference electrode, usually comprised from the same metals as the main electrode.
  3. The junction zone between the studied solution and the reference solution, usually made from ceramics or a capillary with asbestos or quartz fiber.
  4. The reference solution, usually 0.1 mol/L KCL.
  5. The internal electrode, usually made of silver cloride or calomel (Mercury(I) chloride).
  6. The internal reference solution, neutral buffered solution of 0.1mol/L KCL.
  7. The sensing part of electrode, a bulb made from a special type of glass.
  8. When using a Siver chloride electrode, a small amount of AgCL can precipitate inside the glass bulb.

Electrode care:

  1. Electrode calibration: [5]Since glass pH electrodes measure H+ concentration relative to their reference half-cells, they must be calibrated periodically to ensure accurate, repeatable measurements. Although calibration against one pH reference buffer (one-point calibration) typically ensures accurate pH measurement, frequent two-point or even three-point calibrations ensure the most reliable results. Make sure your pH system includes calibration buffers for a range of pH values.
  2. Conditioning: Prior to using your electrode for the first time, follow these three steps to condition your electrode:1) Remove the protective cap or rubber boot from the bottom of the sensor and rinse the electrode with distilled or deionised water.2) Place the electrode in a beaker containing one of the liquids listed below. Soak 20 minute. 3.8 M or 4.0 M KCL, 4.0 pH buffer & 7.0 pH buffer3) After conditioning the sensor for 20 minutes, rinse the electrode with distilled or deionized water. The electrode is now ready for calibration and to measure pH.
  3. Handling: [5]Electrodes should be rinsed between samples with distilled or deionized water. Never wipe an electrode—wiping can cause erroneous readings due to static charges. Blot the end of the electrode with lint-free paper to remove excess water. Never directly touch the glass membrane of the electrode with your fingers as oils from your hands will damage the electrode.
  4. Storage: Always keep your pH electrode moist. We recommend that you store your electrode in a storage solution. If storage solution is not available, use a pH 4 or 7 buffer solution. DO NOT store electrode in distilled or deionized water—this will cause ions to leach out of the glass bulb and render your electrode useless.
  5. Cleaning: [5]Use the guide below to choose the appropriate cleaning solution options:
  • For general cleaning: Soak the pH electrode in 0.1 M HCl or 0.1 M HNO3 for 20 minutes. Rinse well in tap water before use.
  • For removing stubborn deposits and bacteria: Soak the pH electrode in a 1:10 dilution of household laundry bleach for 10 minutes. Rinse thoroughly before use.
  • For removal of oil and grease: Rinse the pH electrode in mild detergent or methyl alcohol. Wash in water before use.
  • For removal of protein deposits: Soak the pH electrode in 1% pepsin in 0.1 m HCl for 5 minutes. Rinse well before use.
  • For sulfide deposits: Use a solution of 8% thiocarbamide in 1 mol/L HCl. After any of the cleaning procedures, it is good practice to thoroughly rinse the pH electrode with de ionized water.

Spectral Sensors


The chemical-free spectral measurement allows a precise determination of the COD / TOC / DOC / BOD / SAC / UTV and nitrate level directly in process.[3] It also providing the high measurement reliability, low operating costs, simple handling, maximum durability etc.

The measurement makes use of the specific absorbance caused by substances:

  • Xeon flash-lamp light source
  • 256-pixel photodiode array detector measures absorption between 200-750 mm
  • Split light beam provides internal references
  • Adjustable measurement path lengths for different waste streams
  • Full spectrum analysis allows for complete compensation of cross-sensitivities and for compound differentiation
  • No consumables and sample preparation required
  • Allow the direct deployment into waste stream.

IQ Sensors


The SAC sensor measure spectral absorption coefficient (SAC) and UV transmission (UVT) at 254nm for determination of organic loads in water.[3] With the sensor UV 700 IQ Sac this parameter can be detected directly without any chemicals.[3] With the VisoTurbo700IQ and Visolid700IQ sensors, WTW presents a family of optical sensors for turbidity and suspended solids measurements.[3] These sensors incorporate a ultra sound cleaning system.

WTW UV 700 IQ SAC Sensor

The SAC reference measurement is performed for compensation of turbidity influences. Turbidity measurements in aqueous media carried out nephelometrically in compliance with EN ISO 7027. Solid matter measurement is performed according to the principal of scattered light measurement.

WTW UV 700 IQ SAC Sensor

To testing the organic load in the water and waste water.

And also the measurement of the turbid particles and the total solid particles in the water and wastewater in the industry and wastewater treatment plant.

DO Sensor


Dissolved oxygen is the amount of the oxygen dissolved in a unit of water. Oxygen gets into water by: diffusing within the surrounding air, aeration (turbulent movement), and as a waste product from plants through photosynthesis.[10] [image: ]Wastewater treatment has been continuously designing, manufacturing and satisfying the demand for reliable dissolved oxygen measurements with the most advanced online systems available anywhere.[3] here is two type of sensor is available in the market. One is “Galvanic dissolved oxygen sensors” and second is “Optical dissolved oxygen sensors”.

How it work

Galvanic DO sensor is consists of anode and cathode electrodes. Both electrodes immerged in electrolytes inside sensor body. An oxygen permeable membrane separates the anode and cathode from measured water.The permeable membrane allows oxygen from the sample water to diffuse into the sensor, where it is reduced at the cathode.[10] This chemical reaction produces an electrical signal, which travels from the cathode to the anode and then into the dissolved oxygen measuring instrument. Consumption of oxygen at the cathode creates a pressure difference across the membrane that varies based on the partial pressure of oxygen in the sample.

Optical dissolved oxygen sensors feature replaceable caps containing the sensing element: a special dye that will luminesce (glow red) when exposed to light of a particular wavelength. The dye is covered by an oxygen permeable paint layer, which allows oxygen molecules to interact with the dye, while protecting it from other sample constituents. Oxygen interferes with the dye’s luminescence (intensity and lifetime). The sensor emits light and measures the resultant luminescence with a photodiode. This reading is compared to a reference reading using light of a different wavelength. The measurement and reference values are compared to calculate dissolved oxygen in the sample.

A DO level in water improves the taste of drinking water.

Monitoring dissolved oxygen content is essential for ensuring process efficiency, because boiler scale build up inhibits heat transfer.

In a sewage treatment plant, bacteria will decompose the solids. If the DO level is too low, then the bacteria will die and the decomposition will cease; if the DO level is too high, then the energy will be wasted through the aeration of the water.


  1. 2010-2012; CSIR Postgraduate Research Program in Engineering, AEI
  2. 2003; Vol 47 No 2pp 1-34; P.A. Vanrolleghem and D.S. Lee; Water science and technology; IWA publishing.
  8. Gaery Girolimon, P.E. Ted D. Miller Associates, inc., online wastewater process monitoring, Presentation


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