Start-up and operation strategies on the liquefied food waste anaerobic digestion and a full-scale case application

A model predictive optimal control system for the practical automatic start-up of anaerobic digesters

The optimal automatic start-up of anaerobic digesters has remained an elusive problem over the years to be solved at the lowest possible costs, including that of process monitoring. In this work, a non-linear model predictive control (NMPC) system was developed, under two proposed configurations, for the optimal start-up of anaerobic digesters treating soluble non-recalcitrant substrates. The minimum set of low cost practical control variables (CVs) selected for process start-up include (i) the effluent quality as acetate COD, (ii) the level of aceticlastic methanogenic biomass in the reactor, and (iii) the methane production rate (only for one of the NMPC configurations). The manipulated variables (MVs) consist of the volumetric inflow rates of the organic substrate, dilution water, and of a possible concentrated alkali addition. To be able to apply the above selected CVs (technically and economically feasible to measure/estimate), a simplified tailored AD model was specifically designed as the prediction model, integral part of the NMPC system. The NMPC system developed was evaluated for a case scenario consisting of the automatic start-up of a high rate AD reactor treating a readily biodegradable carbohydrate based substrate. The AD plant was virtually represented by the complex Anaerobic Digestion Model No. 1. Compared to other manual start-up strategies, the two configurations of the NMPC developed appeared to reach the target methane production rate faster (39 and 18 days for the NMPC versus 70-75 days for the manual strategies) together with an overall superior CV set-point tracking error performance. Interestingly, the two configurations of the NMPC developed appear to propose two very different, almost opposite, start-up feeding strategies to both eventually start-up the reactor successfully with no process destabilizations throughout. A number of practical scenarios were also considered to evaluate the NMPC configurations for robustness and any possible improvements. These tests indicate that the NMPC objective function formulation is a key factor of the success and robustness exhibited during start-up.

Insights into biomethane production and microbial community succession during semi-continuous anaerobic digestion of waste cooking oil under different organic loading rates

High content of lipids in food waste could restrict digestion rate and give rise to the accumulation of long chain fatty acids in anaerobic digester. In the present study, using waste cooking oil skimmed from food waste as the sole carbon source, the effect of organic loading rate (OLR) on the methane production and microbial community dynamics were well investigated. Results showed that stable biomethane production was obtained at an organic loading rate of 0.5-1.5 g VS L-1 days-1. The specific biogas/methane yield values at OLR of 1.0 were 1.44 ± 0.15 and 0.98 ± 0.11 L g VS-1, respectively. The amplicon pyrosequencing revealed the distinct microbial succession in waste cooking oil AD reactors. Acetoclastic methanogens belonging to the genus Methanosaeta were the most dominant archaea, while the genera Syntrophomona, Anaerovibrio and Synergistaceae were the most common bacteria during AD process. Furthermore, redundancy analysis indicated that OLR showed more significant effect on the bacterial communities than that of archaeal communities. Additionally, whether the OLR of lipids increased had slight influence on the acetate fermentation pathway.

A model based on feature objects aided strategy to evaluate the methane generation from food waste by anaerobic digestion

A model based on feature objects (FOs) aided strategy was used to evaluate the methane generation from food waste by anaerobic digestion. The kinetics of feature objects was tested by the modified Gompertz model and the first-order kinetic model, and the first-order kinetic hydrolysis constants were used to estimate the reaction rate of homemade and actual food waste. The results showed that the methane yields of four feature objects were significantly different. The anaerobic digestion of homemade food waste and actual food waste had various methane yields and kinetic constants due to the different contents of FOs in food waste. Combining the kinetic equations with the multiple linear regression equation could well express the methane yield of food waste, as the R² of food waste was more than 0.9. The predictive methane yields of the two actual food waste were 528.22 mL g^-1 TS and 545.29 mL g^-1 TS with the model, while the experimental values were 527.47 mL g^-1 TS and 522.1 mL g^-1 TS, respectively. The relative error between the experimental cumulative methane yields and the predicted cumulative methane yields were both less than 5%.

Enhancing anaerobic digestion performance of crude lipid in food waste by enzymatic pretreatment

Three lipases were applied to hydrolyze the floatable grease (FG) in the food waste for eliminating FG inhibition and enhancing digestion performance in anaerobic process. Lipase-I, Lipase-II, and Lipase-III obtained from different sources were used. Animal fat (AF) and vegetable oil (VO) are major crude lipids in Chinese food waste, therefore, applied as substrates for anaerobic digestion tests. The results showed that Lipase-I and Lipase-II were capable of obviously releasing long chain fatty acid in AF, VO, and FG when hydrolyzed in the conditions of 24h, 1000-1500μL and 40-50°C. Compared to the untreated controls, the biomethane production rate were increased by 80.8-157.7%, 26.9-53.8%, and 37.0-40.7% for AF, VO, and FG, respectively, and the digestion time was shortened by 10-40d. The finding suggests that pretreating lipids with appropriate lipase could be one of effective methods for enhancing anaerobic digestion of food waste rich in crude lipid.

Electricity generation from food wastes and characteristics of organic matters in microbial fuel cell

The microbial fuel cell (MFC) was evaluated as an alternative way to recover electricity from canteen based food waste. Characteristics of the organics in food waste before and after the MFC treatment were analyzed to investigate how the organic matters were biodegraded and transformed during the MFC treatment. A maximum power density of 5.6W/m(3) and an average output voltage of 0.51V were obtained. During the MFC operation, the hydrophilic and acidic fractions were more readily degraded, compared to the neutral fractions. Additionally, aromatic compounds in the hydrophilic fraction were more preferentially removed than non-aromatic compounds. The MFC could easily remove the tryptophan protein-like substances in all fractions and aromatic proteins in hydrophilic and hydrophobic neutral fractions. Additionally, the hydrophobic amide-1 proteins and aliphatic components were readily hydrolyzed and biodegraded in the MFC. These findings may facilitate the pretreatment and posttreatment choices for MFC system fed with food waste.

Evaluating biomethane production from anaerobic mono- and co-digestion of food waste and floatable oil (FO) skimmed from food waste

Batch anaerobic digestion was employed to investigate the performance of the floatable oil (FO) skimmed from food waste (FW) and the effect of different FO concentrations (5, 10, 20, 30, 40 and 50g/L) on biomethane production and system stability. FO and FO+FW were mono-digested and co-digested. The results showed that FO and FO+FW could be well anaerobically converted to biomethane in appropriate loads. For the mono-digestions of FO, the biomethane yield, TS and VS reduction achieved 607.7-846.9mL/g, 69.7-89% and 84.5-92.8%, respectively, when FO concentration was 5-40g/L. But the mono-digestion appeared instability when FO concentration was 50g/L. For the co-digestions of FW+FO, TS and VS reductions reached 70.7-86.1% and 87.5-91.4%, respectively, when FO concentration was 5-30g/L. However, the inhibition occurred when FO concentrations increased to 40-50g/L. The maximal FO loads of 40g/L and 30g/L were hence suggested for efficient mono-digestions and co-digestions of FO and FO+FW.

Effect of lipase addition on hydrolysis and biomethane production of Chinese food waste

The lipase obtained from Aspergillums niger was applied to promote the hydrolysis of food waste for achieving high biomethane production. Two strategies of lipase additions were investigated. One (Group A) was to pre-treat food waste to pre-decompose lipid to fatty acids before anaerobic digestion, and another one (Group B) was to add lipase to anaerobic digester directly to degrade lipid inside digester. The lipase was used at the concentrations of 0.1%, 0.5%, and 1.0% (w/v). The results showed that Group A achieved higher biomethane production, TS and VS reductions than those of Group B. At 0.5% lipase concentration, Group A obtained experimental biomethane yield of 500.1 mL/g VS(added), 4.97-26.50% higher than that of Group B. The maximum Bd of 73.8% was also achieved in Group A. Therefore, lipase pre-treatment strategy is recommended. This might provide one of alternatives for efficient biomethane production from food waste and mitigating environmental impact associated.