DISSOLVED OXYGEN

For over 30 years Partech has been a manufacturer and supplier of Dissolved Oxygen measurement systems. During this time we have gained experience of a wide range of measurement techniques and have amassed a huge knowledge of the trials and tribulations of DO measurement in Wastewater treatment.
Since 1997, Partech have been the UK distributor for Oxyguard International of Denmark. During this time we have generated a large installed base of Oxyguard sensors in the UK. We are know proud to continue this association with the integration of the famed Oxyguard probe into our product range.
The combined skills of Partech and Oxyguard with our reputations for simple, common sense solutions to measurement challenges means we can overcome these problems. Use of the Pioneer mounting system helps alleviate the problems of fouling, and ensures that the sensor is located correctly for representative measurement. The unique combination of electrolyte, anode and cathode material, and physical probe design gives real world life of in excess of five years with many lasting much longer. The membrane for the cell is only replaced if it is damaged, there is no internal drift, and the internal sensor design means that the electrolyte and anode are not consumed.

The Oxyguard probe benefits from being left alone, the first line of the maintenance section of the manual is 'Leave it alone'.

The use of sensors to measure Dissolved Oxygen was started over 50 years ago by Dr Clark initially in the medical industry and then in water analysis. The Clark cell and it's close cousin the Makereth cell are electrochemical (galvanic)cells where an electrical circuit consisting of an anode and cathode with an electrolyte sits behind a gas permeable membrane. As the oxygen diffuses through the membrane a chemical reaction takes place that generates an electronic signal that is proportional to the amount of Oxygen present in the sample. Oxyguard have developed their probe so that this electrochemical reaction does not consume the electrolyte or anode, thus providing a measuring cell that has an operational life of 10 years or more. Recently a technology from the 1970's has been revived with a great fanfare, optical DO measurement has been touted as the panacea for all issues relating to monitoring in wastewater environments.
Basically, the meter determines oxygen by measuring light. High-energy blue light is directed onto the sensor surface, which is coated with a luminescent material. Electrons of the luminescent material are excited to a higher energy level before then falling back to their basic energy level, emitting red light as they do so. This light is detected by a photo diode.
Despite the hype surrounding the optical measurement technique it suffers from the same issues that effect traditional electrochemical cells. The electrolyte in an galvanic cell is consumed, so is the luminescent material in an optical cell, indeed the cell need to be protected from sunlight during calibration to prevent early failure. Aging of the optical circuit will necessitate frequent calibration, the frequency will vary but will be several times per year.

Suspended Solids measurement is primarily a sewage and wastewater treatment parameter, where it is an important indicator of the solids loading and the efficiency of the treatment process. In addition discharge consents set by environmental protection agencies are normally set in Suspended Solids units.

MLSS

Measurement can be taken at a number of key locations through the treatment cycle, each has its own purpose and individual requirements, all need reliable and repeatable measurements. The sensors offered by Partech have been designed to cope with the problems of fouling that are inevitably associated with wastewater treatment. The Turbi-Tech 2000 sensor in particular uses an integral cleaning mechanism that ensures accurate measurement without routine operator involvement.

Activated Sludge (Mixed Liquor Suspended Solids)
Suspended Solids in the activated sludge process are commonly referred to as Mixed Liquor Suspended Solids (MLSS). The measurement of MLSS is vital in the control of the activated sludge plant, ensuring that the treatment process is effectively removing pollutants and operating efficiently in terms of aerator usage and chemical dosing. Understanding the trend of the MLSS value enables site operators to change Return Activated Sludge (RAS) and Waste or Surplus Activated Sludge (WAS/SAS) rates in response to changing process conditions.

Typical MLSS values are 2,500 to 5,000 mg/l although some processes run at levels up 20,000 mg/l.

SLUDGE BLANKET MONITORING

Continuous monitoring of the sludge/water interface or sludge blanket is central to the efficient operation of clarifiers, thickeners and settlement tanks. Reliable measurement enables a control system to maintain the optimum depth of sludge in the tank. A sludge interface that is too high can result in pollution incidents caused by carry-over from a final settlement tank into the final effluent channel. The sludge can also become too thick to extract easily or turn septic. If there is insufficient sludge in the tank the settlement process cannot operate correctly and the solids will tend to remain in suspension.
Historically, settlement tanks have been monitored during routine site visits using the operators judgment or a portable blanket method such as the 715 Portable Sludge Blanket Detector.

This one-off method of control leaves the site exposed to changes in the process that occur when the site is unattended or while the operator is busy on other work. This means that if change happens to the site loading, the settlement tank can quickly become overloaded with sludge, or be drained of sludge entirely.
The use of Partech’s Sludge Blanket monitors will enable the site operators to set maximum and minimum interface levels. The control system can then combine this with other control parameter to improve the quality of both the final effluent and the sludge that is produced.
Partech offer two alternative methods of sensing the location of the sludge/water interface in settlement tanks. It is necessary to decide on the most appropriate measurement principle for each application.
The original method offered by Partech is the use of an infrared gap sensor that is mounted on a drum that rotates as the sensor tracks the interface. This method is highly reliable and copes very well when the interface is diffused or air bubbles are present. There is however a requirement to regularly clean the sensor, typically once per fortnight on a final settlement tank.

Partech offer the alternative of look- down ultrasonics or sonar. Here the sensor is located at the surface of the tank and emits an ultrasonic pulse into the tank, by timing the echo from the interface the location can be determined. The ultrasonic principle is currently the method favoured by most users and by using one of our cleaning mechanisms the user need not perform any routine maintenance.
To ensure reliable operation of a Sludge Blanket Monitor, care must be taken to ensure that the correct sensing technique and that the correct sensor operating range is selected. Individual product data sheets and your Partech Sales Engineer will help with this selection.
The principle areas to consider are; type of solids or process, thickness of the sludge, clarity of the supernatant and the definition of the interface.
The size and shape of the particles in the blanket will affect the measurement being made. For infrared sensors, a thick blanket will need a narrow sensor gap while a thin blanket will need a wider gap. With Sonar Transducers dense sludge will need low frequencies while thin sludge needs higher frequencies.
Another process parameter to consider is the settling characteristics of the sludge If the interface that is produced is very well defined with thick sludge and clear supernatant the measurement is relatively straight forward, however when the interface becomes diffused then much greater care is required with the selection of the measuring device.

Also, the size and shape of the tank, the presence of rotating bridges, scum boards and scrapers, will all affect the product selection. In general on a circular tank the Sludge Blanket Monitor should be located half to two-thirds of the way from the centre to the outside of the tank. On half bridge scrapers where the bridge and scraper rotate together round the tank the sensor should be on the leading edge, ahead of disturbance caused by the scraper.
Once the instrument has been installed, the signal must be transmitted to the control system. A rotating bridge can cause problems if there are insufficient slip rings to allow hard wiring. If this is the case, then Partech can offer low power radio telemetry options.

Once the system is up and running it is important that the sensor is kept clean. The infrared sensors will need routine cleaning, typically every fortnight. This is a very quick and simple process and will easily be accommodated in any good housekeeping procedure.

The Sonar Transducers will normally be supplied with a cleaning system that prevents any build up of fats, grease or bubbles on the sonar transducer's face.

ORGANIC POLLUTION

PHOSPHATE

SLUDGE DENSITY

TURBIDITY