For tanks at wastewater treatment plants that contain solids slurries, either for anaerobic digestion or sludge storage, mixing is a key factor in the proper design, operation and maintenance of the system.

Correct mixing is one of the most important control parameters for the reactors used in every biological treatment step in a plant. It is also critical to maintaining uniform solids concentration in the digester, ensuring that concentrations in any sample taken, at any time, will not vary by more than 10 percent from the average of all samples taken, except for deposits on the tank bottom and floating scum[1].

Anaerobic digesters routinely operate with solids concentration from 3 to 5 percent. Sludge storage tanks can have solids concentrations exceeding 5 percent dry solids and, in some extreme cases, settled solids at the bottom can be as high as 10 to 12 percent dry solids.

 

Preventing settling

Municipal wastewater consists of both biodegradable and non-biodegradable solids, including inert material like grit and silt. These dense materials tend to settle in areas where there is less turbulence. Where the solids concentrations are elevated and grit and inert solids are present, special considerations must be given to a mixing system[2].

When mixing is not carefully considered, inefficient mixing can take place, hindering performance of biological reactors and leading to operational problems.

A well-designed mixing system must be used in anaerobic sludge digesters to ensure proper biological reactions for optimum digester performance, as measured by volatile solids reduction and gas production, and to reduce maintenance made necessary by solids deposition in the digester tanks.

Some commonly used mixing technologies include mechanical, gas (biogas injection alone or with recirculation sludge), draft tube and hydraulic. All can help eliminate settling while keeping digester contents homogeneous.

 

Selecting the right system

Each mixing system is used for certain applications according to digester shape. The accompanying table[3] outlines the pros and cons of each mixing technology that should be considered, along with energy consumption rates, in system selection.

 

Gas injection

Gas injection systems used in cylindrical tanks can be classified as unconfined or confined. In unconfined applications, biogas is collected at the top of the digester and cleaned with a coarse gravel filter and a fine cartridge filter. It is then compressed and sent back to the digester via a diffuser system at the bottom of the reactor or through lances at the vertical top perimeter of the digester. Injected biogas rises to the top of the digester and creates bottom-to-top circulation that mixes and homogenizes the contents.

In confined gas injection applications, biogas is collected at the top of the digester, then treated and compressed in the same way as in the unconfined method. The major difference is that in confined applications, the biogas is released or discharged through a pipe or tube at the center of the digester. The compressed gas is then released from the pipe and gas bubbles rise to the surface, creating an airlift effect.

 

Mechanical mixing

In mechanical mixing systems, vertical-shaft, low-speed turbines or vortex fans are used. Mechanical systems can be unconfined (mixer directly positioned in the center of the digester) or confined (mixer in a pipe or tube positioned vertically at the center of the reactor). Since the motor and gearbox of the mixer are located at the top of the digester, mechanical mixers are mostly used for fixed-cover or floating-cover digesters.

The shaft in a mechanical mixer consists of two or more turbines — as opposed to vortex-type mixers that have only one turbine — to ensure proper mixing at various depths. Provided that the mixing pattern is similar to that of a gas injection system, the rotating turbine or vortex fan generates downward or upward movement on the bulk of the material. The pipe or tube used for the confined version is similar to a case on a pump, while the mixer acts as a propeller, causing movement in only one direction.

 

Hydraulic mixing

A variety of hydraulic mixing systems are on the market that use a pump to recirculate flow from the tank through nozzles strategically located inside the digester. Properly designed systems provide effective mixing and help to improve anaerobic digester efficiency and the re-suspension of solids in applications such as sludge storage tanks that are allowed to sit idle for months at a time.

When this technology is properly designed, it addresses common issues that can arise in mixing digesters and storage tanks. Tanks containing solids slurries are notorious for having fibrous material re-weave that creates ragging issues in pumps and on impeller blades. Hydraulic mixing helps to overcome this, as well as dead spots in the tank where solids tend to accumulate, and accumulation of scum and foam. All these conditions can contribute to operational issues, requiring labor to maintain the mixing system and clean out the tank.

 

Case in point

In 2006, the Wheaton (Ill.) Sanitary District installed a hydraulic mixing system at its wastewater treatment plant after its former gas mixing system was destroyed when the tank cover was taken out of service. The 1,350 gpm hydraulic mixing system used a 25 hp chopper pump to agitate sludge within the anaerobic digestion process, optimizing digestion and methane production. The system provided efficient mixing that kept the tank floor clean and broke up the scum layer. The solids concentration was 4 to 5 percent.

The district chose a hydraulic mixing system because it was sized efficiently, with only two nozzles and a 25 hp motor. The circular digestion tank re-suspended settled solids when the system was turned on. Plant operators were able to schedule mixing times, reducing power usage by about 90 percent without decreasing methane gas production or negatively affecting volatile solids reduction.

Hydraulic mixing uses moving parts that are outside the system and readily accessible. After installing the system, the Wheaton facility experienced minimum scum blanket buildup, improved volatile solids destruction, and better overall digestion.

The plant continued to optimize by consistently reducing the mixing period until it could maintain anaerobic digestion performance by operating one four-hour period, once a week. This allowed the plant to maintain high gas production with an average of only 0.6 hp per day. The district uses the methane produced for heating digesters in the support buildings.

 

About the author

Argun Olcayto Erdogan, Ph.D., is director of anaerobic digestion product management at Siemens Industry in Waukesha, Wis. He can be reached at 262/521-8472 or argun.erdogan@siemens.com.

References

[1] Schlicht, Arthur C. Digester Mixing Systems, Can You Properly Mix with too Little Power. April 1999.

[2] Roehl, Marc. Water & Wastes Digest, Getting the Right Mix. August 2011.

[3] Tchobanoglous, G., Burton, Franklin L., Stensel, H. Davis. Wastewater Engineering Treatment and Reuse. 2004 Fourth Edition, McGraw-Hill.