Assessing the Usefulness of Clostridia Spores for Evaluating Sewage Sludge Hygienization.

•Clostridia and E. coli were monitored during sewage sludge anaerobic digestion.•Results question the usefulness of clostridia as sewage sludge treatment indicators.•Alkali pre-treatment (35.3gNaOH/kgTS; 24h) increased clostridia levels.•Clostridia also increased after digesting NaOH or ultrasound pre-treated sludge.•E. coli levels did not increase under any of the tested conditions.