MEMBRANE THICKENERS IN CONTROLLED AEROBIC DIGESTION

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 thickeners have an extensive history in Europe and Japan in both liquid and sludge applications.