Digestion.  Several EPA-approved stabilization technologies are available for anaerobic and aerobic digestion, including: 

• Heaters, heat exchangers, digester covers, gas and hydraulic mixing systems, all important components in conventional anaerobic digestion systems.
• Temperature-phased Anaerobic Digestion (TPAD) systems optimize anaerobic digestion through a heat recovery system that pre-heats raw material and simultaneously cools the digested biosolids.
• Membrane gas storage systems include an expandable membrane cover that provides variable digester gas storage, optimizes digester gas utilization for heating and electrical generation, and increases storage capacity. 
• Hydraulic mixers use a multi-port discharge valve to greatly improve biosolids mixing in the digestion process.
• Air diffusers and aerators can be incorporated in any aerobic digester configuration.

Lime Stabilization.  Adding lime can stabilize biosolids by raising the pH and temperature.  While adding sufficient amounts of lime to wastewater solids produces Class B biosolids, adding higher amounts will yield Class A biosolids.  Combining low amounts of lime with anoxic storage can also yield Class A biosolids.

Understanding 40 CFR Part 503
Class A & B Technologies from Siemens Water Technologies
Class A & B Biosolids
Class A Technologies

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