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Technical Resource Files
Improving Aerobic Digestion by PreThickening, Stages Operation, and Aerobic-Anoxic Operation
Four full scale aerobic digesters were studied to determine the effects of prethickening, staged operation, and aerobic-anoxic operation on digester performance.
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Membrane Thickeners in Controlled Aerobic Digestio
This paper describes the PAD®-K process. Enviroquip has incorporated our digestion
expertise with membranes to develop a controlled aerobic digestion process using Kubota
membranes for thickening the sludge. The sludge flows in a loop between a digester, a
membrane thickener, and an anoxic tank before being discharged from a second digester.
By combining the thickener and an anoxic tank with the aerobic digestion process several
key concerns are resolved.
Thicker sludges reduce the tank volume required to meet time and temperature criteria
and they also produce more heat with less water to warm thus increasing the operating
temperature. Smaller tanks also reduce the energy costs for aerating the digesters. A
built-in unaerated zone provides automatic anoxic conditions stimulating denitrification
and pH adjustment.
The membrane thickener operates unattended twenty-four hours per day to thicken the
solids without polymers. Regular cleaning is automated with the exception of a chemical
cleaning approximately once every six months for two hours.
The PAD®-K process can be used for the WAS from an MBR or conventional treatment
process but is especially applicable to plants with low nutrient discharge limits. The
integral nitrification/denitrification cycle reduces total nitrogen in the recycle back to the
liquid stream. Additionally, because the membranes provide near perfect capture
efficiency no phosphorus is returned in solid form.
Several PAD®-K processes have been approved by local regulatory agencies and are
currently under construction. Membrane thickenershave an extensive history in Europe
and Japan in both liquid and sludge applications.
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The Metamorphosis of Aerobic Digestion
DESIGN GUIDELINES FOR CONTROLLED
AEROBIC DIGESTION OF THICKENED
SLUDGES
Aerobic Digesters must be considered as a biological
process consisting of a number of different steps and the
process design must provide a suitable environment to
facilitate each step of the process.
The most critical step is to provide sufficient oxygen to
allow nitrification to occur. With thickened sludges the
means used to bring the oxygen in contact with the sludge
is critical. Research has clearly shown that it is essential to
combine vigorous mixing and shearing of the sludge while
air is being introduced. This allows the oxygen to come in
contact with the smallest particle sizes possible, rather than
agglomerating and passing out of the basin in large bubbles.
This mixing and shearing can best be achieved in deep tank
digesters utilizing shear tubes and draft tubes in conjunction
with Enviroquip’s non clog aeration system.
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