Vand og spildevand

Sewage treatment

The preliminary treatment process removes debris through a pumping action. Pumping stations and lift stations are usually unmanned and designed to handle raw sewage that is fed from underground gravity distribution pipelines. Fixed gas detectors to monitor gases such as O₂, H₂S and CH₄ are needed for these spaces and, depending on the site, other gases may form part of the overall requirement.

Activated sludge is transferred to anaerobic digesters, where it is mixed with a different culture of microorganism, which thrives in an oxygen-deficient environment and further breaks down the sludge into biogas. This process takes 20 to 30 days, and gases are released during this process.

Biogas is high in CH₄ (54%) and CO₂ (40%), but it also has over 3000 ppm of H₂S. High levels of H₂S can cause corrosion and mechanical failure in generators, so biogas must be treated before it can be used as a source of energy.

Two different types of gas sensor are used to monitor the biogas – infrared for methane and electrochemical for hydrogen sulphide. The methane measurement is in the range 0-100% by volume and is continuously monitored, to provide accuracy and stability, vital in areas of high concentrations of methane.