ROBERT A. EVANGELISTA

MEMBRANE RECYCLE of ANAEROBIC MICROORGANISMS in the MEMBRANE ANAEROBIC REACTOR SYSTEM (MARS)

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A Membrane Anaerobic Reactor System (MARS) system transformed cheese whey into methane gas for reuse on site as a biofuel at the Kraft cheese manufacturing plant in Melrose, MN. Cheese whey is the green-yellow liquid waste that remains after the curd is separated from whole or skim milk. The curd turns into cheese. The remaining whey contains almost all of the water, salts, and milk sugar or lactose, and about twenty five percent of the protein in the original milk. I provided technical support to the Dorr-Oliver MARS division at Kraft.

 

Anaerobic Metamorphosis of Waste to Biofuel

Anaerobic microorganisms (without oxygen or air) have the ability to digest and convert many types of waste into methane gas, a valuable biofuel that can be used on site to heat or generate electricity, or can be used in modified vehicles. The MARS reactor---where the “bugs” lived and worked---was highly efficient and effective because the free-floating “bugs” were recovered and “returned home.”

 

Ultrafiltration Membrane Recycle of Anaerobic Microorganisms

MARS used a semi-permeable ultrafiltration membrane recycle to recover microorganisms back into the reactor. Semi-permeable membranes, like the lining of your lungs, allow small molecules to pass or permeate the membrane, called the permeate, and retain and concentrate larger molecules or particles behind the membrane, called the retentate or concentrate. At Kraft, the permeate was a whey-free and microbe-free liquid that could be discharged directly into the environment; the retentate or concentrate contained the recycled microorganisms.

 

The MARS group changed to new type of membrane, cellulose acetate, to improve the permeate flow (flux). The flow improved and kept improving, an unusual occurrence since membranes foul and must be periodically cleaned to restore performance. Flow increased again to unseen rates; then suddenly, anaerobic microbes appeared in the permeate. They had digested the cellulose (wood-based) membranes and escaped! Replacement nondigestible polymer membrane promptly restored order.

 

Soon both the permeate flow and the methane generation decreased. The later was a serious problem. I went to the Melrose plant to inspect the operations and maintenance and restored the design permeate flow through a modified operational routine and a different membrane cleaning regiment.

 

Solving the decrease in methane was trickier. Microscopic analysis showed only spherical organisms visible where there should have been the mixed culture of anaerobic microbes needed to digest whey into methane. What did this mean? MARS was selecting a particular microbe based on its shape. A sphere is the three-dimensional geometric shape with the least surface area. The unknown culprit was affecting the membrane surface of the anaerobes and spherical organisms having the least surface area were affected the least. Based on the MARS design, at that time, only the high shear zone within the centrifugal pumps could affect a microbe’s surface area to this degree. So a new pumping system was designed and installed, and a fresh anaerobic culture introduced to restore whey digestion and its subsequent hey generation.

 

This project was for a private industrial client; no report or publication is available.