Scaled-up experimental biogas production from two agro-food waste mixtures having high inhibitory compound concentrations

The most abundant agro-food wastes in Puglia (Italy) are derived either from olive oil production, e.g., olive pomace (OP) and olive mill wastewaters (OMW), or from diary activity, e.g., milk whey. All of these wastes have an acidic pH (3.5–5.5), high organic matter volatile solids, a (VS) higher than 50 g/L, and chemical substances such as total nitrogen (TN), total ammonia (TAN) and total phosphorous (TP), which are able to alter the properties of the soil and pollute aquifers in scenarios where they were released into the ground without any treatment. Two types of OP exist but have different chemical characteristics: OP from a two phase centrifugation (OPII) and OP from a three phase centrifugation (OPIII). These differ primarily in their water content, which is higher in OPIII, and in their polyphenol and ammonia content. In the present work, two mixtures of wastes from olive oil and dairy production were prepared and initially tested in a 50 L batch digester. Then, in a scaled up 2 m3 anaerobic reactor a test in continuous mode was realized. Two feeds were tested: a first mixture containing OPII and a second mixture with OPIII. The tests were conducted in mesophilic conditions (35 °C) with a total solid (TS) content of approximately 10% w/w in continuous mode. The test including OPIII showed a productivity of 1.23 LCH4/L d against 0.83 LCH4/L d for the test with OPII, as a consequence of the higher organic content and the simultaneous effect of the minor inhibitory compound (ammonia and polyphenols) concentration in OPIII.