5 wastewater mixing challenges and how to solve them

Mixing is one of the most critical parts of wastewater treatment, but designing and operating a mixing system involves several complex challenges. This article describes some of the most common wastewater mixing challenges, how to solve them, and what you should consider when upgrading an existing mixing system or designing a new one.

Wastewater mixing is one of the most energy-intensive processes at a wastewater treatment plant, and it plays an important role in helping utilities meet regulations regarding nutrient removal. If your system is not optimally designed, however, you could be using too much energy, wasting resources, and spending too much on equipment and repairs.

Before exploring mixing challenges, let’s take a quick look at the purpose of mixing in wastewater treatment plants, and why it is playing an increasingly important role in meeting standards for effluent quality. 

What is wastewater mixing used for?

One of the main purposes of mixing at a treatment plant is to blend wastewater and prevent the sedimentation of solids. As wastewater moves through the treatment plant, mixers ensure that the wastewater has a consistent, uniform composition. The uniformity of the wastewater increases the efficiency of treatment processes, such as biological nutrient removal (BNR) and flocculation.

Treatment plants use mixers to create optimal conditions for nutrient removal, which involves using bacteria to consume nitrogen and phosphorous. It is important to remove these nutrients because, if they are present in wastewater effluent, they can negatively affect water quality and ecosystems. Without mixing, bacteria will settle in the tank, limiting their ability to consume nitrogen and phosphorous. Mixing keeps the bacteria and the nutrients moving to increase their interaction.

Mixing also avoids the problem of “short circuiting” within treatment tanks. Short circuiting refers to those times when new wastewater flows into a tank and then out again without making sufficient contact with the beneficial bacteria.

Common wastewater mixing challenges

Given the financial constraints and increasing regulations that utilities and industries face, the main challenge with wastewater mixing is how to achieve the best results while using the least resources. Here are five common challenges that treatment plants need to address when designing and operating a mixing system.

1. Selecting the right type of mixer

Wastewater treatment plants have several different types of mixers to choose from, such as top-entry mixers and submersible mixers. The type of mixer you select can depend on the treatment stage, the type of fluid to be mixed, and the objectives of the mixing process. When selecting a mixer, it is always important to keep in mind that you want to achieve optimal results with minimal energy usage.

Top-entry mixers are inserted directly into the tank like an upside-down ceiling fan, directing energy toward the bottom of the tank and around its sides. While heavy, slow-running top-entries can achieve this efficiently in tanks of simple shapes ( for example not too flat, wide, or  long), the risk for weak zones with poor mixing is a concern, and often so is the installation cost. 

Submersible mixers, on the other hand, are mounted on the side of the tank at carefully engineered angles. These mixers drive a loop-shaped bulk flow around the tank, , which can be maintained with less energy as the flow reaches full circle. Submersible mixers can deliver optimal results with less energy consumption than a top-entry mixer. When selecting a submersible mixer, it is important to choose a vendor that can help you design your overall mixing solution including the options for mixer positioning.

2. Choosing the optimal mixer size

In the past, many treatment plants selected oversized mixers to ensure optimal mixing regardless of variations in flow. Now that energy consumption and biological nutrient removal are top concerns, treatment plants must be more cautious in selecting mixer sizes. Excessively large mixers can lead to unnecessary energy consumption during periods of low flow, as they are often set to operate at full speed. Oversized mixers can also  cause unwanted surface mixing and have adverse effects on the mechanical stability of installed equipment.

When selecting the size of a mixer, it is important to understand the difference between power and thrust. Power refers to how much energy a mixer uses, which does not necessarily translate into better mixing. A more “powerful” mixer may have a less efficient motor and a less efficient design.

Thrust, on the other hand, refers to the force created by the mixer and imparted to the liquid. Thrust is a much better point of comparison than power when selecting a mixer. The thrust-to-power ratio of a mixer measures how much force is produced by the mixer, divided by the electric input power consumed to create that force. The higher this ratio, the more efficient the mixer.

3. Finding the best mixer position

A mixer’s position and orientation in a wastewater treatment tank are critical to maximizing performance. The objective is to achieve optimal bulk flow, which refers to how fluid is moving in the entire tank volume, with minimal energy consumption. The strength of the bulk flow determines the overall mixing result.

Mixer positioning should be based on the mixing duties, the tank’s layout, the mixer’s thrust, and the fluid’s properties. A submersible mixer, for example, should be positioned so its jet stream smoothly deflects off walls to create a loop-shaped pattern. If the mixer is not correctly positioned, this will create calm zones in the tank were mixing does not occur. 

Given the complexities of mixer positioning, you need a carefully designed solution to achieve the best results. To find the most cost-effective mixing solution, choose a vendor that provides computational fluid dynamics (CFD) modeling. This modeling will help you determine the optimal mixer and mixer position to meet your specific needs.

4. Reducing mixer energy consumption

Optimizing mixing processes to minimize energy consumption, without compromising performance, is a constant challenge. To reduce energy consumption, choose mixers with variable speeds, high-efficiency motors, and hydraulic designs that maximize thrust.

Mixers for wastewater treatment plants are typically selected based on the plant’s peak load. Peak load, however, rarely occurs at most plants, and if you are using a single-speed mixer with an inefficient motor, the mixer will consume far more energy than is required.

An adaptive mixer, in contrast, can operate at variable speeds based on real-time conditions, which reduces energy consumption and operational costs. With an adaptive mixer, a mixer’s thrust can be simply increased or decreased to meet current requirements. To ensure maximum energy savings, choose a variable-speed mixer with a high-efficiency IE3 or IE4 equivalent motor. These mixers come with the bonus of easier mixer standardization between different tanks on a site, or between sites.

The design of the mixer’s propeller also plays an important role in maximizing thrust while minimizing energy consumption. Flygt’s “banana blade” propellers, for example, provide exceptional thrust and high bulk flow to provide high energy efficiency. The propeller’s backswept design is engineered to be self-cleaning and ensure clog-free operation, even in the presence of fibrous materials.

5. Monitoring and controlling mixers

Since the conditions of wastewater in treatment plants are constantly changing, mixers need to be continuously monitored to ensure optimal performance. Operators must monitor parameters such as mixer speed, power consumption, and flow rates, as well as ensure that the mixers are not overheating, leaking or clogging. This can be a time-consuming process, which is why wastewater treatment plants increasingly rely on automated monitoring and control systems.

To minimize manual processes and maximize performance, choose a mixing system that provides real-time data, automatic speed adjustments, and remote monitoring capabilities. This enables operators to easily optimize mixing performance, reduce energy consumption, and quickly troubleshoot any issues.

Flygt Adaptive mixers, for example, can be installed preprogrammed to operate autonomously, at times evenwithout any external control connections. They have programmable ramp rates for soft starting and a wide range of fixed speed settings for reduced power consumption and wear. They also have self-protecting logic that will slow down the mixer, either to dislodge items caught on the propeller or to avoid damage if overheating. The internal controls keep an alarm log for easy trouble shooting.

Get expert help with your mixer solution

Xylem has been developing innovative wastewater mixers for over 60 years, with more than 300,000 Flygt mixers installed worldwide. We have a broad range of mixers, as well as monitoring and control solutions, that ensure that you get the best possible performance for your application. If you would like help designing your mixing solution, including advanced computational fluid dynamics (CFD) modeling, our experts are ready to help.