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Metropolitan St. Louis Sewer District | Pre-Design Report

BLACK & VEATCH |Process Evaluation 2 November 2012 85

49.1D.O. PROBE AND SUSPENDED SOLIDS PROBE TECHNOLOGIES49.1.1 D.O. Probe TechnologyThere are two types of Dissolved Oxygen (DO) probe technologies, membrane sensor and

luminescent sensors. Membrane sensors use an electrode system where the anode and cathode are

immersed in an electrolyte and oxygen diffuses into the solution. The sensor converts oxygen into a

voltage that is proportional to the amount of DO. The electrodes are made of lead. The membrane

sensors have a response time of 5 minutes and can read low levels of DO. The electrolyte must be

changed every few months and the calibration is time consuming. There are membrane sensors

that have end caps with electrolyte in them that can be changed quickly. This technology was not

chosen due to the slow response time, calibration difficulties and the amount of maintenance.

Luminescent DO probes are the most popular technology in the industry. Luminescent sensors use

a blue LED that excites luminescent material in the probe. The luminescent material then emits red

light. A photo detector measures this emitted light, which is proportional to the amount of DO in the

water. MSD has these sensors installed in six of the eight tanks in the aeration system. The sensors

are not affected by the flow velocity, pressure changes or electrical interference. They do not

require calibration and have a response time of 90 seconds. The end cap on these sensors may

need replacement every 1-3 years, but most sensor caps exceed that time frame. MSD has been

pleased with the performance of the current sensors. District Operations staff clean the sensors

daily to keep the readings accurate.

49.1.2 Suspended Solids Probe TechnologySuspended Solids (SS) Probes are proposed for installation in the last pass of each aeration basin

for measuring the amount of solids in the mixed liquor channels to assist in balancing solids

concentrations and achieving an equal food to mass (F/M) ratio in all aeration tanks. The SS probes

use infrared LEDs emitting light that is scattered by suspended solids. Photo detectors receive the

scattered light which has a proportional response to the amount of suspended solids in a liquid.

This technology can provide accurate results and the sensors are factory calibrated for

replacement. The SS probe connects to a transmitter, which takes the signal from the probe and

converts it to a 4-20mA signal. Most manufacturers have transmitters which can accept two(2)

probes. The SS probes can be installed near the DO probe in the last pass and can be connected to

the same transmitter

The challenge with SS probes is keeping the optical sensors clean. MSD has tested SS probes with

automated air cleaning systems with poor results. The other options for cleaning the SS sensors are

ultrasonic and mechanical wiper systems. The options for sensor cleaning systems will be

investigated during detailed design to determine the cleaning system that best fits the maintenance

needs of MSD. The Secondary Improvements project will include installation of the SS probe and

cleaning system that best meets the Districts operation and maintenance criteria.