Effect Of Organic Loading Rate On Process Performance In Dry Anaerobic Digestion Of Food Waste

Food waste was characterized by high organic loading rate, high moisture and high fat content. As a green treatment, anaerobic digestion stands out because it could not only dispose the food waste effectively, but also transform the bio-energy in organic waste into biogas. Dry anaerobic digestion of food waste(TS=21.73 %~26.33 %) has a few good features like low energy consumption, high utilization rate of the reactor volume and high organic load holding capacity. Currently, the study on the dry anaerobic digestion were very few, and the related engineering projects was seldom heard. The main reason is that the stability of the dry anaerobic digestion is hardly to keep, of which organic loading rate plays an important role. This study will focus on the effect of organic loading rate on process performance in dry anaerobic digestion of food waste, and run the system a long time under different OLR. Finally, obtain the optimum OLR, which make the system keep stability and have high efficiency.This thesis based on the mesophilic(T=37℃) dry(TS=23.2%) anaerobic digestion of food waste. The experiment was divided into 5 stages that the running time were 45 d, 35 d, 25 d, 20 d and 15 d,and the organic loading rate of the 5 stages are 3 kg VS·m-3·d-1、4.1 kg VS·m-3·d-1、5.5 kg VS·m-3·d-1、6.8 kg VS·m-3·d-1、8.5 kg VS·m-3·d-1 respectively. During the experiment, a large number of parameters were be monitored including p H, Volatile Fatty Acid(VFA), alkalinity, ammonia in the lipid phase, methane content, carbon dioxide content and biogas production in the gas phase, TS and VS content in the solid phase, to evaluate the stability of the anaerobic digestion system. At the end of the experiment, build the linear relation based on the methane yield, VS removal efficiency and volumetric biogas productivity under different organic loading rate, and get optimum organic loading rate through solving equations.Phenomena of the study were as follow, 1) the VFA content increased with the increase of the organic loading rate, which changed from 357 mg·L-1 to 2942 mg·L-1 during the former four stages, but when the organic loading rate up to 8.5 kg VS·m-3·d-1, the VFA content exceeded 3000 mg·L-1. 2) Alkalinity in the liquid phase was gradually increased from 2858 mg·L-1 to 15280 mg·L-1 with the increase of organic loading rate. At the beginning of the feed, alkalinity in the liquid phase grew faster, but the growth of the alkalinity was slowing down when it was higher than 10000 mg·L-1. 3) Ammonia concentration in the liquid phase increased from 664 mg·L-1 to 2728 mg·L-1 with the increase of organic loading rate, at the beginning of the anaerobic digestion, ammonia concentration increased rapidly, but it slew down when the organic loading rate was higher than 5.5 kg VS·m-3·d-1. When the organic loading rate was 8.5 kg VS·m-3·d-1, liquid ammonia concentration exceeded 4500 mg·L-1rapidly, and there may appear ammonia inhibition in the anaerobic digestion system.Results of the study were as follow, 1) The p H value is affected by a lot of parameters, it changed slowly and cannot timely indicate the stability of the anaerobic digestion system, the indicating effect of the VFA/TA ratio is superior to the p H value. 2) When the organic loading rate was low, the VFA/TA ratio was very low and less than 0.35, which showed that the stability of the anaerobic digestion was good. When the organic loading rate up to 8.5 kg VS·m-3·d-1, VFA/TA ratio greater than 0.35 and the stability of anaerobic digestion system reduced. 3) During the anaerobic digestion process, the system of the biogas volume rate ranged from 3.1 L·L-1 to 6.6 L·L-1, the methane production rate range was 4.2 m3CH4·kg VS-1 ~ 5.5 m3CH4·kg VS-1. 4) According to the fitting equation of the CH4 yield, VS removal efficiency and volumetric biogas productivity, the optimum organic loading rate of the dry anaerobic digestion of the food waste 7 kg VS·m-3·d-1.