Gas Detection in Wastewater Treatment Plant

Overhead view of an outdoor wastewater treatment equipmentWastewater Treatment Plants (WWTP) are designed to prevent pollution and disease by treating wastewater before being released to the environment. Water used to flush and transport liquid waste of residences, businesses, and industries is defined as wastewater. The sewage system also transports the run-off surface water and stormwater. Any wastewater that is potentially dangerous to humans or the environment because of its toxicity, flammability, corrosiveness, chemical reactivity, etc.requires treatment before it is released back to the water system.

Municipal and Industrial Wastewater Treatment Plants

Typical gases to be monitored at a wastewater plant are combustible gases (primarily Methane, CH4), Hydrogen sulfide (H2S), Chlorine (Cl2), and Oxygen (O2). In addition, some plants also require Sulfur Dioxide (SO2), Ammonia (NH3), Ozone (O3), and Chlorine Dioxide (ClO2) gas detection.

RC Systems’ Recommendation:

In-plant pump stations are facilities that consist of pumps and service equipment designed to pump flows from lower to higher elevations to allow continuous and cost-effective treatment through unit processes within the plant. If those pumping stations, lift stations, dry wells, or wet wells are in an enclosed building or structure, additional Methane (CH4) monitoring, Hydrogen Sulfide (H2S) and Oxygen (O2) detectors also will be needed.

Wastewater Treatment Process

In Primary Treatment, where 40 - 50% of the solids are removed, sanitary (or separate) sewers carry wastewater from homes and businesses to the treatment plant. Bar screens let water pass through but filter out the trash. A grit chamber slows down the flow of water, allowing sand, grit, and other heavy solids to settle out. A primary sedimentation tank collects the smaller particles. Lighter-than-water liquids, primarily oil, float on top. These “floaters” and “sinkers” are removed. Gas detection for combustible gases and vapors may be required where surface and stormwater might carry fuel oil, gasoline, or flammable solvent spills.
Diagram of wastewater treatment process
In Secondary Treatment, which removes 85 - 90% of the pollutants, an aeration tank supplies large amounts of air to a mixture of wastewater, bacteria, and microorganisms. Oxygen (aerobic process) speeds the growth of these helpful organisms which consume harmful organic matter. A secondary sedimentation tank (anaerobic breakdown) allows the microorganisms and solid waste to settle out. When aerobic breakdown processes occur in enclosed buildings or structures,gas detection may be required for Carbon Dioxide (CO2) and Oxygen (O2; depletion). In the proess or anaerobic breakdown, gas detection for Methane (CH4) and Hydrogen Sulfide (H2S) may be required.
The sludge collected at the bottom of the clarifier is then recycled to the aeration tank to consume more organic material. The term "activated sludge" is used, because by the time the sludge is returned to the aeration tank, the microorganisms have been in an environment depleted of "food" for some time, and are in a "hungry", or activated condition, eager to get busy biodegrading some more waste. Since the amount of microorganisms, or biomass, increases as a result of this process, some must be removed regularly for further treatment and disposal, adding to the solids produced in primary treatment.

Digestion in Sludge Treatment

One commonly used method of sludge treatment is digestion. Since the material is loaded with bacteria and organic matter; why not let the bacteria eat the biodegradable material? Digestion can be either aerobic or anaerobic. Aerobic digestion requires supplying oxygen to the sludge; it is similar to the activated sludge process, except no external "food" is provided. In anaerobic digestion, the sludgers feed into an air-free vessel and digestion produces a mixture of methane and carbon dioxide. The gas has a fuel value and can be burned to provide heat to the digester tank and even to run electric generators. Digestion can reduce the amount of organic matter by about 30 to 70 percent, greatly decrease the number of pathogens, and produce a liquid with an inoffensive, "earthy" odor. This makes the sludge safer to dispose of on land. In the process of aerobic breakdown, when happening in enclosed buildings or structures, gas detection may be required for Carbon Dioxide (CO2) and Oxygen (O2; depletion). In the process of anaerobic breakdown gas detection for Methane (CH4) and Hydrogen Sulfide (H2S) may be required.

Commonly Used Gases in Wastewater Treatment Plants

Chlorine is the most widely used disinfectant for municipal wastewater because it destroys target organisms by oxidizing cellular material. After disinfection, the water’s Chlorine residual must be reduced to nontoxic levels, and Sulfur Dioxide (SO2) is often used in this de-chlorination process. Alternative disinfectants include ozonation (O3), Chlorine Dioxide (ClO2) and ultraviolet (UV) disinfection. The disinfection process makes the wastewater a safe effluent to flow into rivers, lakes, or seas. These various disinfection processes often require Gas Detectors to monitor for Chlorine (Cl2), Sulfur Dioxide (SO2) or other disinfectant gases which may be dangerous to life and property.
Another WWTP often requiring gas detection is Ammonia (NH3) stripping. This is a simple desorption process used to lower the Ammonia content of a wastewater stream. In Ammonia stripping, lime or caustic is added to the wastewater which converts Ammonium Hydroxide ions to Ammonia gas; often requiring the need for Ammonia gas detectors.

Gas Detection for Wastewater Treatment Plants (PDF version)


Wired Gas Detection System

Gases Detectors Controller
CH4 SenSmart 5300 IR ViewSmart 400 (Up to 4 points)
Cl2, O2, SO2, NH3, O3, ClO2 SenSmart 5100 EC ViewSmart 1600 (Up to 16 Points)
CO2 SenSmart 5400 IR ViewSmart 6400(Up to 64 Points)


Wireless Gas Detection System

Gases Detectors Controller
CH4 SenSmart 7300 IR WNR WaveNet Receiver
Cl2, O2, SO2, NH3, O3, ClO2 SenSmart 7100 EC  (Up to 32 Points)
CO2 SenSmart 7400 IR