In wastewater treatment, chemicals are degraded in the aeration tank via the actions of microorganisms in the activated sludge. The slurry which is a mixture of water with sludge and microorganisms then enters the clarifier tank (known in the literature as the secondary clarifier) where the sludge settles and separates from the water. The clean water is then sent to a receiving body while the settled sludge is returned to the aeration tank. This returned sludge is called return activated sludge (RAS).
Some of the settled sludge, known as surplus sludge or waste activated sludge (WAS), is discarded from the wastewater treatment system. WAS is used for the sludge thickening process, the digestion process, or for the dewatering process. In some wastewater treatment plants, the WAS is sent for incineration, although this depends on regulations to maintain environmental and health safety because sludge could contain toxic chemicals.
What is return activated sludge (RAS)?
The general definition of return activated sludge (RAS) is settled activated sludge that is collected from the secondary clarifier and returned to a treatment process that uses aerobic oxidation such as an aeration tank. It is basically sludge that is recycled from the secondary clarifier back to the aeration tank. RAS returns a concentrated population of microorganisms to the aeration tank.
What is the difference between waste activated sludge and return activated sludge?
Waste activated sludge (WAS) is surplus sludge that is removed from the wastewater treatment process, while return activated sludge (RAS) is sludge that is returned from the secondary clarifier to the aeration tank.
RAS, which is returned to the aeration tank, is recycled sludge and because sludge contains microorganisms, RAS recycles microorganisms into the aeration tank to keep the microbial population balanced. WAS, on the other hand, is sludge that is disposed of and thus, WAS removes excess microorganisms to prevent microbial overpopulation in the aeration tank and in the secondary clarifier.
Why return activated sludge?
Activated sludge contains microorganisms that treats chemicals including organic matter in the wastewater. The return of activated sludge from the secondary clarifier to the aeration tank helps to maintain the correct concentration of activated sludge in the aeration tank for the microbial treatment of chemicals.
What equipment is used in return activated sludge?
The equipment used in the return of activated sludge consists of pumps, flow meters, and associated pipes. Overall, the pumps draw sludge from the secondary clarifier and returns it to the aeration basin, and the flow meter measures the rate of return flow. The intricate details depend on the wastewater treatment facilities.
RAS flow rate
RAS flow rate affects the microbial treatment of chemicals in the aeration tanks. High RAS rates can cause a hydraulic overload, which means that the incoming flow rate is more than what the wastewater system can handle. Increasing the RAS flow rate can lead to an increase in the sludge volume index.
How do you measure RAS flow?
RAS contains a high concentration of solids and air bubbles and these characteristics are used in RAS flow meters. The following types of RAS flow meters use sound and magnetic principles:
- Doppler flow meter: The Doppler flow meter transmits high frequency sound that is reflected to the sensor from the air bubbles in RAS to measure the RAS flow rate.
- Ultrasonic flow meter: The ultrasonic flow meter uses sound waves to measure the RAS flow rate. These flow meters are ideal for sludge containing high concentration of solids.
- Vortex flow meter: Vortex flow meters use the principle of vortex shedding. This meter contains a vortex detection sensor.
- Magnetic flow meter: The magnetic flow meter detects electromotive force that is generated from the flow of conductive sludge and converts it into a digital signal that is used to calculate the RAS flow rate.
What is the function of a RAS pump?
RAS pumps move large volumes of sludge with high solid contents from the secondary clarifier to the aeration system. These pumps minimize shear forces to prevent damage to the biological floc in the RAS. The key feature of RAS pumps includes large-passage impellers to allow the passage of the sludge without clogging. The impellers are designed to reduce turbulence and maintain smooth flow of RAS to prevent damage to the flocs.
The different types of RAS pumps include submersible pumps, submersible mixed flow pumps, and submersible propeller pumps for the return of activated sludge from the secondary clarifier to the inlet of the aeration tank. Sludge characteristics in RAS, flow rate and pressure requirements, energy efficiency, and regulatory compliances are some of the issues to consider when selecting a RAS pump.
Challenges and future needs for RAS technologies
There is an increasing prevalence of microplastics, new pharmaceuticals, and endocrine disrupting chemicals in wastewater. This is a concern because microplastics could clog the RAS pumps while new pharmaceuticals and endocrine disrupting chemicals could eliminate the microorganisms that treat chemicals including organic matter in the wastewater. There is a need to develop new RAS pump technologies to prevent clogging due to the microplastics and methods to prevent new pharmaceuticals and endocrine disrupting chemicals from disrupting the microbiological treatment of wastewater.
Furthermore, there is a need for current research to explore ways to improve the RAS for enhancing the efficiency of the microbiological treatment in the aeration tank. This is imperative because the microbiological treatment of wastewater is a core process in wastewater treatment.
