Application Note: Wastewater Sludge Heating

More and more WWTP are considering the use of Anaerobic digestion because it allows for a faster rate of solids destruction, increased solids loading, and a greater through put through the plant. Anaerobic digestion is a biological process that breaks down for organic solids. Heat exchangers can face the following challenges when heating sludge:

• Live steam used to provide heat for heat exchangers creates hot spots where sludge can burn and build-up plugging the heat exchanger tubes. This increases maintenance costs and an increase pressure drop across the heat exchanger.
• Upgrading from mesophillic to thermophilic often require multiple Heat Exchangers in series to achieve the necessary temperature rise.
• Increased pressure drop adds demand to the sludge pumps thus increasing energy usage.
• Temperature control problems can be present as there is a lag time from slurry entering and then being discharged.

To overcome these challenges, Direct Steam Injection (DSI) is a very good approach. There are some correct ways to apply DSI and some approaches that may not produce desirable results. One of the key factors to successful DSI, is to inject steam at sonic velocity to achieve choked flow.