Upgrading Denitrification by Optimal Adsorption of SCFAs from Sludge Alkaline Fermentation Liquid by Acid-Modified Sepiolite

Sludge alkaline fermentation liquid (AFL) is a potential carbon source for biological denitrification. However, its effectiveness is limited due to the presence of nutrients and heavy metals. In this study, acid-modified sepiolite (MSEP) was used to extract short-chain fatty acids (SCFAs) from AFL under optimized conditions and then with the prepared MSEP-AFL as a carbon source for denitrification. The optimal condition with an MSEP dosage of 1.96 g/L and pH 7.93 at 30 °C was obtained based on single-factor experiments and response surface methodology (RSM). Carbon balance revealed that 96.2% of the SCFAs, including 43.7% acetate and 23.5% propionic acid, was retained in the MSEP, demonstrating its high selectivity. The adsorption process followed the pseudo-second-order kinetic and Langmuir isothermal model, indicating dominant physical adsorption on the surface or in the fiber pores. This was further supported by the changes in the morphological features and surface properties of the MSEP. In the batch nitrate utilization experiments, the prepared MSEP-AFL was proven to be efficient as a carbon source, with a nitrate removal efficiency of 88.7% and a specific denitrification rate of 8.2 mg NOx-N/g VSS·h, which was 22% higher than that of the AFL. This was due to the establishment of a delicate “release–utilization” balance. These findings contribute to our understanding of the use of AFL for denitrification.