Real-time monitoring of volatile fatty acids using a novel VFAs analyser and automated high solids separation system

Volatile fatty acids (VFAs) are key intermediates in anaerobic digestion (AD), serving as essential indicators for process control and biogas yield optimization. This study presents and evaluates an innovative, fully automated system for real-time VFAs monitoring, which combines a VFAs analyser with an automated solids separation unit (SSU). The VFAs analyser is based on capillary electrophoresis and is capable of quantifying acetate, propionate, and butyrate over a wide concentration range (1–50 mg/L, extendable to 1–5000 mg/L through the integrated auto-dilution feature), with high linearity (r 2 > 0.999) and precision (RSD < 3 %). A built-in auto-dilution algorithm dynamically adjusts the sample dilution factor, enabling continuous, unattended operation. To ensure accurate analysis of complex AD samples, the SSU automatically prepares solids-free filtrates using disposable syringe filters and includes automated filter replacement. The SSU processed 2106 digestate samples with high reliability and minimal manual intervention. Furthermore, the impact of common anions and matrix components on analytical performance was investigated to assess robustness. The fully integrated system enables reliable, high-frequency VFA measurements directly from digestate, providing a valuable tool for enhancing AD process monitoring, stability, and control. • VFAs can be quantified from 1 to 50 mg/L, extendable to 5000 mg/L with high linearity. • High accuracy with RSD < 3 % and low LODs for acetate, propionate, butyrate • Common anions do not interfere; accuracy remains across diverse matrices. • SSU's robust performance enables accurate, consistent VFA monitoring. • Continuous 12-min sampling with up to 4.2 % solids for up to 3 days unattended