DO Measurement in Waste Water Treatment

Introduction

Controlling the world’s water resources is arguably one of the most critical issues. Water demand from industry and domestic users is set to rise throughout the industrialized world, while water supplies are finite. Legislation constantly demands improvements in potable water quality and reduces the contaminants that may be discharged. Wastewater treatment is one way to get water back. Wastewater is subjected to various treatments in wastewater treatment plants, each intended to remove contaminants and other hazardous substances from the water. The objective is to create water from sewage that is safe to discharge into waterways and won’t endanger the environment or human health. Over the last few years, Yokogawa has been applying minimized maintenance measurement systems to an industry that, more than ever, is concerned with condition-based maintenance and the integrity of the measured variable.

Process Information

Sewage that flows into a treatment plant is sent to a primary sedimentation tank to remove the garbage and sludge. Afterward, it is routed to an aeration tank where activated sludge (sludge containing bacteria, protozoa, and other microorganisms) is added. The air is blown into the mixture to agitate it. The microorganisms in the activated sludge thrive on oxygen, which encourages the growth of the sludge’s organic compounds and the precipitation of produced solids.
A shortage of dissolved oxygen (DO) will kill the microorganisms, so the aeration tank’s DO content needs to be carefully managed. The DO concentration is regularly measured using a DO analyzer.

Product Recommendation

The DO71 and DO72 optical dissolved oxygen sensors use the optical measurement method to measure oxygen dissolved in water continuously.
The Optical Sensors have a fast response time & good long-term stability. These sensors are prone to less maintenance, and the overall cost of ownership is lower.
Sensors are available for different oxygen ranges, wide and ultra-trace range oxygen measurements, ideally suited for oxygen monitoring in industrial applications or for W&WW where they must stand harsh measurement conditions.
The optical sensor is integrated into an exchangeable stainless-steel cap that is screwed to the probe housing.
As a digital interface, they use RS485 and can be connected directly to the FLXA402 4-Wire converter or any other host like a PLC, DCS, or data acquisition unit.

Key Features;
• Integrated with a compact 12 mm stainless steel tube
• Standard PG13.5 fitting for simple integration
• Maximum measurement range of 0…22.5 mg/L O2
• A detection limit of 20 ppb
• Trace oxygen sensor 0…2 mg/L O2 with 1 ppb limit of detection
• DO71 Waterproof design
• Simple integration, No polarization time
• Integrated temperature sensor for temperature compensation
• Bi-directional digital communication via RS485 Modbus RTU
• Low power consumption
• Five years shelf life of sensor end cap

DO Measurement System

Transmitter
FLXA402 4-wire Analyzer or A Modbus host with the use of SENCOMct tool

Sensor
DO71 or DO72-W

Configurations