Impact of co-digesting fruit juice processing wastewater with primary sludge on VFA and me- thane production

<title>Abstract</title> <p> Fruit juice processing wastewater (FJPWW) is rich in readily biodegradable COD and suitable for biogas production, but its low buffering capacity and nutrient imbalance may accelerate acidification and destabilize anaerobic digestion. Co-digestion with well-buffered, nutrient-rich substrates can mitigate these limitations and support process stability. This study evaluated whether co-fermenting and co-digesting FJPWW with primary sludge (PS) improves VFA production and methane yield. Batch acidogenic fermentation was conducted for 100 hours in 1 L bottles using FJPWW alone and FJPWW + PS at 97:3 and 90:10 (volume to volume) at a substrate-to-inoculum ratio (S:I) of 2:1 g COD g VS ^− 1 and initial pH 6.0. After settling, supernatants were assessed in BMP assays (OxiTop®) at 35°C for 120 hours, with a S:I of 0.25 gCOD gVS ^− 1 and initial pH adjusted to 7.0. Degree of Acidification (DoA) peaked at 24 hours (33%, 31%, and 15%), alongside maximum VFA yields of 0.89, 0.86, and 0.45 g netTVFA-COD g VS ^− 1 ; final VFA profiles were acetate-dominated (51–57% of total VFAs). In BMP tests, methane yields were similar for FJPWW (504.3 NmL CH4 g VS − 1) and 97:3 (504.5 NmL CH ₄ g VS ^− 1 ), while 90:10 was lower (483.7 NmL CH ₄ g VS ^− 1 ). Overall, PS at the tested ratios did not lead to a significant enhancement in either VFA accumulation or methane yield, however, co-fermentation with primary sludge improved the COD:N ratio of the substrate mixture. Further research is needed to investigate primary sludges from different municipal WWTPs, and evaluate the influence of controlled pH conditions. </p>