Gas Measurement in Wastewater Treatment Plants

The primary applications for flow meters in the wastewater environment are measuring blower air to each pool in the aeration basin and measuring digester gas flow. The aeration basin is a series of treatment pools containing aerobic bacteria that feed and breakdown the sewage moving through the pools. A blower adds the required dissolved oxygen (DO) to maintain the aerobic bacteria in the aeration basin.

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Wastewater Treatment Plant Gas Metering Applications

Flare Gas
Excess biogas is flared, but not before filtering and capturing as much as possible. The trend in flaring is to minimize burning and recover as much methane as possible. This is because methane is valuable fuel and because it a restricted substance to release. Therefore flow to the flare is extremely low flow.

Gas Characteristics:
Low Flow, Low Pressure, Condensing Moisture
Boiler Air
Oxygen is pumped to the boiler and mixed with fuel in carefully controlled ratios. A properly balanced air/fuel ratio is vital for a clean, efficient burn.

Gas Characteristics:
High Volume
Fuel Gas
Bio fuel is pumped to the boiler and mixed with air in carefully controlled ratios. A properly balanced air/fuel ratio is vital for a clean, efficient burn.

Gas Characteristics:
Low Flow, Low Pressure, Condensing Moisture
Digester Gas
Bio-gas is collected from digestion tanks and sent to cogeneration stations for power production or stored for later use. The gas seeps out at extremely low flow rates and contains a high degree of moisture.

Gas Characteristics:
Low Flow, Low Pressure, Condensing Moisture
Aeration Air
Wastewater treatment plants run air through tanks of water containing waste. This aeration air is bubbled up through the water to seperate the solids. Careful control and fast response to the aeration air flow lowers wear on the blowers and permits higher efficiency.

Gas Characteristics:
Low Flow, Low Pressure, Condensing Moisture
Aeration Air
Wastewater treatment plants run air through tanks of water containing waste. This aeration air is bubbled up through the water to seperate the solids. Careful control and fast response to the aeration air flow lowers wear on the blowers and permits higher efficiency.

Gas Characteristics:
Low Flow, Low Pressure, Condensing Moisture
Aeration Air
Wastewater treatment plants run air through tanks of water containing waste. This aeration air is bubbled up through the water to seperate the solids. Careful control and fast response to the aeration air flow lowers wear on the blowers and permits higher efficiency.

Gas Characteristics:
Low Flow, Low Pressure, Condensing Moisture
Aeration Air
Wastewater treatment plants run air through tanks of water containing waste. This aeration air is bubbled up through the water to seperate the solids. Careful control and fast response to the aeration air flow lowers wear on the blowers and permits higher efficiency.

Gas Characteristics:
Low Flow, Low Pressure, Condensing Moisture
Aeration Air
Wastewater treatment plants run air through tanks of water containing waste. This aeration air is bubbled up through the water to seperate the solids. Careful control and fast response to the aeration air flow lowers wear on the blowers and permits higher efficiency.

Gas Characteristics:
Low Flow, Low Pressure, Condensing Moisture

Wastewater Treatment Plant Gas Metering Instruments

Aeration Air

Wastewater treatment plants use aeration basins to seperate and sanitize the waste materials. Forced draft air is bubbled up through tanks containing waste material. The aeration air bubbles provide dissolved oxygen to facilitate the microbial activity. Careful control of the dissolved oxygen is neccessary for a balanced process. Fast response to changes in the air flow lowers wear on the valves and provides higher efficiency.

Flare Gas

Flare gas is excess waste gas with hydrocarbons remaining. It is sent to the flare tip to be burned. Wastewater plants try to limit flaring gas because fuel is valuable and atmospheric gas emissions are restricted. It is common for excess fuel to be stored or used in cogeneration, however sometimes the volume of gas exceeds the capacity to store. Flare gas varies widely in composition and flow rate. Flare gas measuring instruments must be able to handle extreme variance in flow rate, widely differing gas compositions and moisture content.

Digester Gas

As bio material decomposes it emits many organic compounds. Biogas seeps from the wastewater digester at a varying rate, generally very low. The rate of flow is dependent on the rate of decomposition which varies with the type of material being broken down, the amount of material, the temperature, season, and time of day. Not only is the bio gas flow rate affected by these variables, the moisture content is as well. Wastewater gas contains an oscillating amount of moisture. As the temperature decreases more water condenses out of the gas, and vice versa. As the temperature increases more water vapor seeps from the decaying matter. These factors contribute to an erratic, widely varied flow condition. Digester gas is varying in composition, flow rate and moisture content.

Condensing Gas

Biogas from wastewater treatment plants contains moisture which may condense in the process line. Water vapor will condense out of a gas for multiple reasons: as temperature drops the gas’ ability to hold water is diminished and water droplets form. A reduction in pressure will cause condensation to form in the stream. The temperature differential between hot gas and a cold pipe will cause moisture to condense out. As water condenses in a pipe it collects on the walls, eventually running along the bottom.The unpredictable moisture levels caused by outside rain, temperature, and humidity can introduce water vapor through outside air intake. Wet gas contains condensing moisture. It can vary widely in flow rate and composition.