Biogas Production Performance Of Anaerobic Co-Digestion Of Kitchen Waste And Cattle Manure And The Optimization Study On Operating Parameters

Anaerobic digestion (AD) is a complicated process using anaerobic microorganism to convert organic matter into methane and carbon dioxide under anaerobic condition. As compared to aerobic treatment, AD has apparent advantages such as conversion of organic pollution, biogas recovery in a renewable form. Kitchen waste (KW) and cattle manure (CM) are two main organic solid wastes (OSW) with huge amount of production in China. If not treated timely, they will cause serious problem for the health of human being and living environment. Anaerobic digestion of KW or CM as single-substrate always resulted in over-acidification, low pH and low biogas production rate, etc. In this paper, anaerobic co-digestion was conducted to treat two organic solid wastes simultaneously and obtain higher biogas production. The main results are as follows.1. Firstly, the characteristics of three kinds of KW from breakfast (KW-B), lunch(KW-L), and dinner (KW-D) were determined, respectively, and then anaerobic batch tests were carried out with these three KWs to evaluate their anaerobic digestibility. Results showed that there were large differences in characteristic between the KW-B and KW-L/KW-D. In the KW-B, the VS/TS ratio and lipids contents were far lower than those of KW-L and KW-D, while the inorganic salts of Na+、Cl-were higher than other two KWs. The methane yield of KW-B was only212.2ml/g VS, which is36～41%lower than KW-L and KW-D. KW-L and KW-D could be mixed and treated together as their similar characteristics; however, KW-B should not be mixed with KW-L and KW-D due to its low biodegradability.2. Secondly, the characteristics of CM from China and USA were compared. The digestion performance of completely stirred tank reactor (CSTR) and plug flow reactor (PFR) with the digestion of CM were comparatively studied. It was found that the VS removal efficiencies in CSTR with TS concentrations of40,80, and128g/L were increased17.4～21.5%,3.9～21.5%, and0.5～5.3%as compared with PFR, respectively.3. Thirdly, through anaerobic batch fermentation, the digestibility of KW, CM and the mixtures of KW and CM were investigated, respectively. The results indicated that co-digestion could significantly enhance the methane yield, and there was synergistic effect existing in the co-digestion of KW and CM. In order to explore the synergistic effect in the co-digestion, the effects of alkalinity and ammonia nitrogen presented in CM and the KW acidification on the methane yield were further studied. The results showed that the alkalinity in CM is crucial for the increase of biogas in the co-digestion. In the acidogenic phase, the high alkalinity could prevent methanogens from the violation of low pH, which is favorable to the microbial growth. The addition of KW could increase the hydrolysis rate of CM by two times, which provided more substrates for methanogensis and enhanced the methane production. Therefore, in the co-digestion of KW and CM, two substrates have synergistic effect and this enhanced the methane yield.4. In two-phase anaerobic digestion system with co-digestion of KW and CM, operating parameters such as mixing ratio, organic loading, and hydrolytic retention time (HRT) were investigated respectively.(1) In the study of mixing ratio on methane yield, there mixing ratio of1:1,2:1, and3:1were tested comparatively. Among three mixtures, the mixture of KW and CM at3:1obtained the highest biogas production and its methane yield was3.8and1.6times of that of single-substrate digestion of CM and KW, respectively.(2) In the study of organic loading rate on methane yield, the loadings of1.2～6.1g VS/L d were tested in the co-digestion of KW and CM. The results showed that the biogas production rate increased with the increasing of KW proportion but the biogas yield decreased from0.7L/g VS to0.58L/g VS. Considering all of indicators including biogas production rate, biogas yield, effluent VFA, and the ratio of VFA/TA, the optimal loading rate was believed to be2.2g VS/Ld with the mixing ratio of1:1. When the KW proportion was so high as at the ratios of3:1and6:1, the propionate concentration would be high in the acidogenic and methanogensis phase, and it could be the potential inhibitor for the digestion process.(3) Through two-phase system for the co-digestion of KW and CM, the effects of the same overall HRT of13d but the different HRT in acidification and methanogensis (1d+12d,2d+11d, and3d+10d) on the biogas production rate were investigated. The results showed that under the same overall HRT the required acidification HRT was short for the readily biodegradable organic matter, while longer for the slowly biodegradable organic matter. There was close relationship between the TS and VS removal and biogas production rate.(4) Through two-phase system for the co-digestion of KW and CM, the effects of the same acidfication HRT but different HRT in methanogensis on the biogas production rate were investigated. Under the same acidification HRT, the longer the methanogenisis HRT was, the higher biogas production rate obtained.5. Through the simplified model with two-phase digestion, the kinetics in the co-digestion of KW and CM were investigated, and kinetic parameters for4kinds of mixtrures were obtained also. It was found that the hydrolysis rate of k1are within0.43～0.83/d, which was increasing with the proportion of KW in the mixtures. Besides, the specific growth rates and saturation concentrations were0.8～5.0/d and0.11-0.19g C/L, respectively.