So Now You Need to Cool the Sludge

Thermal hydrolysis process (THP) has gained more and more attention recently in the wastewater industry. The THP process conditions sludge under high temperature and high pressure environment which solubilizes the extracellular polymeric substances and rupture the cell walls. The conditioned sludge, referred to as thermally hydrolyzed sludge (THS), is more easily digestible in the downstream anaerobic digestion process. Applying THP as a digestion pretreatment process can result in increased digester loading, increased biogas production, increased cake dewaterability, and Class A biosolids.The THS cooling and digester feeding system is critical to optimize digester performance, reduce system down time, and maximize the benefits of THP process. Design of the THS cooling and digester feeding system must take into consideration of the unique characteristics of the THS and site specific situations. The key design considerations are summarized as follows:The THS transfer and digester feeding system must provide continuous and even THS feed to downstream digesters. Sufficient digested sludge must be recirculated to mix with THS prior to the heat exchangers (HEXs) to increase cooling efficiency and reduce fouling of the HEXs. Adequate access for flushing, draining, and cleaning of the THS transfer and digester feeding system must be provided for regular maintenance and emergency situations.Concentric tube HEXs are by far the most popular HEXs used for THS cooling due to their minimal risk of clogging and relatively easy of maintenance. Stainless steel HEXs have been traditionally used for THS cooling. However, carbon steel HEXs have been evaluated in recent THP installations.Cooling technologies work on different principles to provide cooling water. Therefore, selection of cooling technology must be determined based on site specific conditions.