Noise control for Roots blowers in wastewater treatment plants

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Noise control for Roots blowers in wastewater treatment plants – Site overview: The wastewater treatment plant uses the jet aeration activated sludge process, and it is equipped with 9 Roots blowers, each with a capacity of 183 kW. The average noise level is 118.8 DB, exceeding the regional factory noise standards. Now, each Roots blower is equipped with a sound insulation enclosure; the outer wall of this enclosure is made of fiberglass, the inner wall is made of perforated steel plate, and glass wool is filled in between. A silencer and air filter are installed at the inlet of the Roots blower, which is positioned horizontally inside the housing. The air required by the fan is introduced into the sound-insulated enclosure from the outside through a glass fiber reinforced plastic pipe with a diameter of 420 mm; a resistive silencer made of glass fiber reinforced plastic is installed at the outdoor air inlet. See Figure 1: Modification plan: 1. Move the air inlet of the sound insulation enclosure from above to below the enclosure, so that air flow can pass through the motor and the rotor blower casing to cool them (see Figure 1 for details) ； 2. The cross-sectional area of the air inlet increased from 0.126㎡ to 0.384㎡ ； 3. Changing the air intake from outdoor to indoor not only reduces ventilation resistance but also improves the ventilation conditions inside the fan room ； 4. The air inlet is equipped with a folded-plate resistive silencer ； 5. New forced ventilation equipment shall be installed inside the enclosure; an exhaust fan shall be placed above the sound-insulating enclosure, and it operates only when the Roots blower is not in use ； A silencer is installed outside the exhaust fan to reduce the noise emitted from the exhaust outlet ； A simple check valve is installed between the exhaust fan and the silencer to prevent outdoor air from entering through this area while the Roots blower is in operation ； 6. Move the pressure relief port of the Roots blower from the outside to inside the sound insulation enclosure, and equip it with a silencer; its impact on the external environment can be disregarded. After taking the above measures, it is not necessary to open the soundproof enclosure door even during the hottest days of summer, ensuring the noise reduction effect of the enclosure. After the completion of the project, measurements showed that the noise level inside the enclosure was 106.8 DB(A), while the noise level at the air intake outside the enclosure had dropped to 82.5 DB(A) ； In the original design of the Roots blower, although attention was paid to noise control at the inlet of the blower, no consideration was given to noise control at the outlet, which is another major reason for the failure in noise control. Point ① was selected at the location near the exhaust outlet of the rotoary fan inside its sound insulation enclosure, while point ② was selected at the location near the fan’s exhaust pipe outside the sound insulation enclosure. The measurement results are shown in Table 1: As can be seen from the above, reducing the airflow noise at the exhaust of the Roots blower is another important measure for this project. Installing a silencer is an effective way to reduce airflow noise. Based on the noise spectrum of rotary vane blowers, an impedance-combined silencer was designed, as shown in Figure 2. The sound-absorbing material used for this silencer is centrifugal glass wool fabric, with a thickness of 150 mm. At the same time, noise control measures were also taken for the busbar. Tests showed that the noise level at measurement point ② had dropped from 116.2 DB(A) to 87.8 DB(A). After installing an impedance-combined silencer at the inlet of the outdoor main air supply duct, the noise level near the air supply duct in the aeration tank decreased from 116.2 DB(A) to 63.8 DB(A). Measures to improve the sound insulation performance of the fan room’s enclosure involve replacing the ordinary wooden doors with sound-insulating doors, and the windows with sound-insulating windows. Increase ventilation with low-noise exhaust fans to lower indoor temperatures. Fully covered acoustic ceiling. Currently, the average noise level in the fan room has been reduced to 85.5 DB(A), while the noise level at the factory perimeter has dropped to 43.5 DB(A), meeting the **relevant noise standards.