Overcoming the bottlenecks of anaerobic digestion of olive mill solid waste by two-stage fermentation

Abstract

Two-stage anaerobic digestion (AD) of two-phase olive mill solid waste (OMSW) was applied for reducing the inhibiting factors by optimizing the acidification stage. Single-stage AD and co-fermentation with chicken manure were conducted coinstantaneous for direct comparison. Degradation of the polyphenols up to 61% was observed during the methanogenic stage. Nevertheless the concentration of phenolic substances was still high; the two-stage fermentation remained stable at OLR 1.5 kgVS/m³day. The buffer capacity of the system was twice as high, compared to the one-stage fermentation, without additives. The two-stage AD was a combined process - thermophilic first stage and mesophilic second stage, which pointed out to be the most profitable for AD of OMSW for the reduced hydraulic retention time (HRT) from 230 to 150 days, and three times faster than the single-stage and the co-fermentation start-up of the fermentation. The optimal HRT and incubation temperature for the first stage were determined to four days and 55°C. The performance of the two-stage AD concerning the stability of the process was followed by the co-digestion of OMSW with chicken manure as a nitrogen-rich co-substrate, which makes them viable options for waste disposal with concomitant energy recovery.

Long-Term Evaluation of Mesophilic Semi-Continuous Anaerobic Digestion of Olive Mill Solid Waste Pretreated with Steam-Explosion

“…For example, Serrano et al [13] reported that the methane production in the anaerobic digestion of OMSW subjected to a thermal treatment at 170 • C and 1 h varied from 119 ± 30 mL CH4/(gVS•d) to 172 ± 60 mL CH4/(gVS•d) by reducing the OLR from 2 to 1 gVS/(L•d). Stoyanova et al [12] reported that the biomethanization of two-phase OMSW operated at stable conditions at an OLR of 0.76 gVS/(L•d), but the increase of the OLR up to 1.1 and 1.76 gVS/(L•d) affected the methane production and, finally, caused the total failure of the system. In the same way, Orive et al [22] also reported low methane production rates (77 mL CH 4 /(gVS•d) in the semi-continuous anaerobic digestion of diluted OMSW (COD: 60 g/kg) at high OLRs (around 3 gVS/(L•d).…”

“…Among the processes to stabilize OMSW, anaerobic digestion offers the possibility of obtaining methane, as an energy carrier, and the possibility to recover the remaining digestate as fertilizer [10,11]. According to Stoyanova et al [12], stable single-stage semi-continuous OMSW anaerobic digestion proceeded at an organic loading rate (OLR) of 0.76 kgVS/(m 3 d) (VS, total volatile solids). Recently, Serrano et al [13] determined that the accumulation of VFA was the main reason for destabilization during anaerobic digestion of OMSW subjected to a thermal pre-treatment (1 h at 170 • C) at an OLR of 2 gVS/(m 3 •d).…”

Fitting Biochars and Activated Carbons from Residues of the Olive Oil Industry as Supports of Fe- Catalysts for the Heterogeneous Fenton-Like Treatment of Simulated Olive Mill Wastewater

“…Several treatment processes have been proposed to address this issue, including physicochemical procedures such as coagulation/flocculation [11], adsorption [12] or electrocoagulation [13], aerobic and anaerobic biological processes [14][15][16], as well as a wide range of advanced oxidation processes (AOPs), namely ozonation [17], Fenton-based [18], photocatalytic [19] or electrochemical [20] processes. However, the applicability and efficiency of AOP single-step units is often limited by OMW characteristics, as influent values of chemical and biochemical oxygen demand (COD, BOD 5 ) and total suspended solids (TSS) may be as high as 200, 100, and 35 g/L, respectively [21].…”

Is anaerobic digestion a feasible alternative to the combustion of olive mill solid waste in terms of energy production? A critical review

“…Similarly, some authors have proposed the use of two-stage AD for OMSW, where a first acidification stage is carried out to enhance the hydrolysis of the substrate. 39,40,42 The methane production rate obtained through twostage AD widely varied from 200 to 370 mL CH4/(g VSꞏd) ( Table 1). Therefore, the implementation of an acidification stage is likely not attractive for OMSW as the achieved methane productions were in the same range than for the single-stage AD.…”

“…On these studies, higher OLRs usually implied the inhibition of the process by the accumulation of volatile fatty acids and phenolic compounds. 35,42,44 Other authors reported operations at much higher OLRs than 1 g VS/(L reactor •d) without compromising reactor performance, although in most of the cases the experimentation time was less than 2 hydraulic retention times per condition, [36][37][38]41 which may not be enough for triggering the inhibition of the anaerobic digestion process.…”