Experimental study on the inhibition of glucose and olive mill wastewater degradation by volatile fatty acids in anaerobic digestion

Olive mill wastewater (OMW) generated during olive oil extraction presents a significant challenge for olive growers due to its pollution potential, high organic and mineral contents, and acidic nature. Anaerobic digestion has emerged as an efficient method to tackle this challenge and valorize this waste, as it offers ecological and energetic benefits by converting pollutants into biogas—chiefly methane, a renewable energy source. This study investigated the inhibition kinetics of two substrates—glucose and milled olive wastewater—by evaluating a group of C1–C4 organic acids in salt form, including formic (C1), acetic (C2), propionic (C3), lactic (C3), pyruvic (C3), and butyric (C4) acids. Batch experimental tests were conducted to assess the impacts of these fatty acids acting as intermediates in anaerobic organic degradation. Each acid was individually used as a substrate in fermentation, with concentrations ranging from 1000 mg chemical oxygen demand (COD)/L to 5000 mg COD/L under mesophilic conditions (37 °C). The kinetic parameters were estimated using the Andrews and Monod equations. The results showed that formate exhibited self-substrate inhibition in anaerobic degradation, with the maximum specific growth rate observed at 3000 mg COD/L for glucose and 2000 mg COD/L for OMW. The inhibition constants were 4900 mg COD/L (glucose) and 2500 mg COD/L (OMW). Similarly, propionate evinced auto-inhibition when glucose was used as a substrate at concentrations above 2000 mg COD/L. However, acetate, lactate, pyruvate, and butyrate did not inhibit methane production, even at the highest concentrations tested (5000 mg COD/L).