Mixing system for anaerobic digesters uses biogas inside the tank to help improve mixing efficiency and increase gas production.

Effective mixing is important to optimizing biogas production in anaerobic digesters. There are various approaches to mixing systems, including mechanical stirring with paddles, pumping jets of material, and pumping biogas through the slurry.

Landia, based in Denmark with U.S. operations in North Carolina, offers a mixing technology to the North American market that combines two of those basic methods — jets of material, and aspirating and injecting biogas. The GasMix technology places all rotating components outside the digester tank, helping to reduce wear and tear and simplifying maintenance.

The technology is suitable for mixing solids from wastewater treatment as well as food processing byproducts, animal manures, and essentially any waste or sludge that contains decomposable organic matter. Soren Rasmussen, director with Landia, talked about the technology in an interview with Treatment Plant Operator.

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TPO: What was the company’s objective in developing and offering this technology?

Rasmussen: The basic intent was to mix the contents of anaerobic digesters, but to do so without having any rotating equipment installed inside the tank. When you have propellers or other devices rotating inside the tank, there is a risk of stringy material catching onto those devices. Having all the equipment outside the tank provides easy access for maintenance and frees operators from entering the tank, which would present some health and safety concerns.

TPO: What components are inside the tank to enable mixing?

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Rasmussen: The main things that are physically inside the tank are nozzles. They are installed through the tank wall and extend only a couple of feet into the tank. They’re solid stainless steel, so they need no maintenance.

TPO: Please describe in basic terms how this system performs efficient mixing.

Rasmussen: The driving mechanism is a Landia chopper pump. A suction pipe pulls sludge from the bottom of the digester and runs it through the chopper pump. The resulting material is then recirculated to the tank through a nozzle at high velocity. There is also a Venturi nozzle in the system. When we run the sludge through that nozzle, it creates high pressure on the front side and a low-pressure area, or vacuum, on the back side. That Venturi chamber is connected to a pipe that extends into the digester headspace. The vacuum pulls biogas down and the nozzle injects it into the tank. The gas then travels up and mixes the sludge on a vertical plane. That makes for energy-efficient and effective mixing of the digester.

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TPO: How many of these nozzle combinations would be required for a typical digester?

Rasmussen: There could be one for a very small tank, or six to eight for larger tanks. We have some tanks with nine to 12. It really depends on the size of the tank and what is inside the tank — municipal sludge, thick manure or some industrial feedstock. The larger the tank and the thicker the feedstock, the more mixing energy is required.

TPO: How would you characterize the energy efficiency of this process?

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Rasmussen: It is among the best in the industry. Several customers have reported that the increase in biogas they get from this system more than offsets the parasitic load that a mixing system imposes. So in effect, it’s an energy-producing system. Some customers have put this system on in addition to an existing mixing system, just for the increase in biogas and therefore the increase in energy production.

TPO: How does mixing a digester in this way increase gas generation?

Rasmussen: Two things positively affect methane production beyond simply mixing the tank completely. First, the chopping of the solids makes the material easier to digest. Second, the high pressure in front of the Venturi nozzle and the rapid decrease in pressure after it actually ruptures the biomass cell walls releases more carbon for methane production. The amount of the increase depends on the nature of the feedstock. At the end of the day, any increase in biogas will add to the revenue stream, provided there is a beneficial use for the gas.

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TPO: Are there any other advantages in terms of energy efficiency?

Rasmussen: An interesting thing that happens when the sludge is subjected to high pressure and rapid depressurization is that the viscosity of the sludge is reduced. Pumping a lower-viscosity sludge naturally requires less energy. So, for example, users can save energy when pumping the sludge through heat exchangers or pumping material to a neighboring plant. Some customers tell us they are saving more energy on sludge pumping than the amount of energy they put into the mixing system.

TPO: How does the capital cost of this system compare with other mixing technologies?

Rasmussen: It is competitive with other systems in the industry. In fact, because it doesn’t put any load on the tank cover, walls or floor, there is no need to invest in extra infrastructure. For a top-mounted mixing system, there is a need to size the cover for that load and any torque or other forces the mixing will impose. That reinforcement can cost more than the mixing system itself. Tank wall strength would also have to be considered.

TPO: Can this system be retrofitted to existing anaerobic digesters?

Rasmussen: Yes, very easily. The vast majority of systems we sell are for retrofit applications. New digesters are being built all the time, but there are many more existing digesters that don’t have adequate mixing. We have developed substantial experience in retrofitting. We treat every digester as unique and review the details of every project to make sure the mixing system is optimized.

TPO: What kind of in-the-field experience stands behind this technology?

Rasmussen: The vast majority of our installations are overseas. We have installed the GasMix system on five continents in hundreds of locations, but only a few in the United States so far. There is a lot of interest in the system, and we are working on a number of projects in the municipal sector.

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