The benefits of preheating sludge upstream of thermophilic digesters are well documented. However, doing so can be a costly proposition with conventional methods that rely on bulky and expensive spiral heat exchangers. An alternative is using direct steam injection heaters that handle large volumes of liquid with a negligible pressure drop while maintaining tight temperature control. These heaters are compact, can be retrofitted into existing sludge piping, and can be fabricated with ports to match specific piping. Hard facing is available on the internal surfaces of the housing, and the injector can be surface-hardened to resist erosion when heating sludge containing abrasive grit.