Homogenization in Wastewater Treatment Plants
Krishna Kumar
Deputy Manager of Technical Support,
Grundfos India.

Water being the highly abused natural resource in Indian context, it's time to look for an alternate water resource. Waste-water has come out as a potential substitute, which can be reused in industrial and agricultural purpose after apt and thorough treatment. Here the author has bade for a succinct account of the homogenization procedure

Context: In view of India's deteriorating water crisis, there is an urgent need to explore alternative means of water management. Wastewater treatment has emerged as a leading solution to ease water stress in the country. Wastewater treatment plants can treat the contaminated water and retrieve the treated water in a usable format. This water has high potential in agriculture and industrial applications.

Felonies: Variations of flow in wastewater often presents a problematic situation for wastewater treatment plants. Sludge from wastewater must be kept in suspension because if it settles, the operation of the equipment will be hindered. Even pumps will not be able to operate properly as this semi-liquid could create blockages.

Way-out: Therefore, various stages of wastewater treatment by using submersible mixers is a preferred option to retain the solids in suspension. This this was possible with the help of thrust exerted by the submersible mixers into the liquid medium.

The wastewater generated from our daily activities must be freed from oil, color, and other chemicals present in it before it is let out to the environment. Once the initial screening and processing is completed, the liquid is homogenized to ensure that existing pollutants are evenly distributed. This will simplify the next step of neutralization as the contaminants will then be eliminated by injecting an acid or air flow into the mixture. Sometimes the treatment process also envisages anoxic & anaerobic process in the system, where air cannot be used for homogenizing the liquid & keep the microorganisms in suspension. This would be made possible with the help of submersible mixers.

Scope of homogenization: This process of homogenization is particularly useful in all treatment plants that undergo major variations in maximum and minimum flow rates. Homogenization of liquids in large volumes is usually a voluminous task and requires large amounts of energy consumption to get the final liquid. In simple language, wherever we have solids / sludge, we require an equipment for homogenization.

Methods and procedures: There are three methods of homogenizationmechanical, pneumatic, and hydraulic. The first method carries out this process using a mechanical stirrer. The second method uses bubbling and airlifts. Both these methods are known to have their drawbacks. While both mechanical and pneumatic homogenization consume high levels of energy, pneumatic homogenization also has an expensive cost label attached to it along with reduced efficiency and inability to remove residues. It also makes the tanks more turbulent. Hydraulic homogenization is a standard practice in wastewater plants where a submersible mixer is fitted to the tank to homogenize the liquids. It is more cost effective in comparison to its counterparts and can also be used in homogenization processes without the requirement of air. The efficiency of hydraulic homogenization is decided by a term called 'Thrust to Power Ratio(T/P)', based on which the submersible mixers can be designed.

Mixers and flowmakers are used to homogenize sludge in wastewater treatment plants, sludge treatment systems, sewage pumping stations, centralized and decentralized wastewater treatment plants, industrial processes, and biogas. The equipment can be either direct driven or gear driven. The gear driven mixers would provide more thrust compared to direct driven mixers. This would be a primary reason of using gear driven mixers in huge / lengthy tanks. The mixing velocity in the tank would be governed by influence zones / velocity profiles provided by the manufacturers.

Probable deterrents: Viscosity of the liquid can create a major hindrance in the mixing process, as liquids with different viscosity will require varying levels of power and propeller strength to get the homogenized product. Specific gravity of the individual liquids also challenges the process of homogenization. The specific gravity is the ratio of the density of a substance and a reference substance.

The specific gravity value of a substance will decide whether it will float or sink when placed in the reference liquid.

Critical elements: Submersible mixers and flowmakers are two critical components of the homogenization process. Submersible mixers are mechanical devices designed to mix or homogenize the liquids in the tank in an effective manner. These mixers are equipped with a powerful and energy efficient motor, coupled with a propeller which creates flow in the tank. While most pumps are used to displace water, these mixers use power to create thrust that impacts the sludge in suspension.

Conclusion: There are different types of mixers that can be used for this mixing, depending on the viscosity of the liquids and its applications. Latest mixers in the market are technologically superior to its conventional forefathers with multistage sealing system to prevent any sludge from entering and strong propellers that can dispose sludge that attaches itself to the mixer. These mixers are also equipped with thermal sensors, moisture system with an alarm and a leakage senor that monitors any potential leakages.