Anaerobically digested sludge (ADS) can be difficult to separate because fine particles and hydrated extracellular polymeric substances (EPS) contribute to sludge dispersion and water retention. Although particulate conditioners and bio-based flocculants have each been investigated, their combined effects on EPS distribution and sludge conditioning mechanisms are still poorly understood. This study evaluated algal biochar as an auxiliary conditioner in combination with grafted cationic starch (GCS) or etherified cationic starch (ECS), with particular attention to EPS fractions. The combined treatments achieved supernatant turbidity removals of 77.1% to 84.1%, exceeding that achieved with the corresponding starch-based flocculants alone. Although the two conditioning systems showed similar turbidity removal, they followed different zeta-potential and EPS responses. The algal biochar and ECS system shifted to less negative values and reduced soluble microbial products (SMP) and loosely bound EPS (LB-EPS). In contrast, the GCS combinations remained negatively charged and increased tightly bound EPS (TB-EPS) by 52% to 54% compared with GCS alone. These findings show that comparable supernatant clarification can be accompanied by contrasting electrokinetic and operationally extracted EPS profiles. The results highlight the value of examining EPS fractions alongside turbidity when comparing combined conditioning formulations. The implications of these profiles for sludge dewaterability and filterability remain to be established through direct performance measurements.
- Open Access
- Article
Combined Algal Biochar–Starch Bioflocculant Conditioning Reveals Distinct Responses of Extracellular Polymeric Substances in Anaerobically Digested Sludge
Author Information
Received: 20 Aug 2026 | Revised: 15 Sep 2026 | Accepted: 08 Oct 2026 | Published: 19 Oct 2026
Abstract
Graphical Abstract
Keywords
algal biochar | anaerobically digested sludge | cationic starch | extracellular polymeric substances | combined conditioning
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