Study On Citrus Pulp Fermentation In Anaerobic Digestion Process At Ambient Temperature

Chongqing locates in citrus advantage industry park along Yangtze River strip, which possesses the ability to process orange juice 250 thousands tons, firstly of Asia, and also 250 thousands tons citrus pulp may be produced every year. Plenty of citrus pulp were still treated as the mode of landfill up to the present, this mode occupied mass soil resources, and threatened the entironment of Three Gorges Region. How to deal with so many citrus pulp comes being an urgency problem to our country, explore new technology to dispose all these residues become necessary. Anaerobic digestion is an ideal technique to deal with organic wastes. It was reported that anaerobic digestion technique had applied in treatments of citrus pulp abroad, but only a few research had developed, analogous work still hasn't developed in China. This research explore to seek for the feasibility of citrus pulp anaerobic digestion, select main technics parameters, and provide foundation of building high effective anaerobic digestion technics citrus wastes process. The main results list as follows:1. Water content, volatility solid content, C/N compare and trace element were analyzed, contrast to materiel characteristics of swine feces, it was determined to apply swine feces to promote anaerobic digestion of citrus pulp, and adjust fluid C/N compare to 25: 1. Acidification experiment of citrus pulp was developed in 40 d, pH value hadn't been modulated during the period, VFA content were examined every 3 days. The results showed that citrus pulp was acidified with high speed, short cycle, high acid production and much low pH value. So, proposals were put forward that applied lye to modulate pH value and reduced fluid concentration to resolve the problems mentioned above;2. Sludge, collected from biogas digester that fermented with swine feces and producing gas stably, inoculated citrus pulp, domesticated with lixivium of citrus pulp every 10 days, the cycle was 40 days. Results indicated that after domestication, sludge configuration and predominant microbes were changed, revealed domestication gave positive effects, made sludge adapt to materiel characteristics;3. Four treatments of citrus pulp: swine feces (8:2, 7:3, 6:4, 5:5) were set to investigate effects of different proportion of swine feces on fermentation process. The results indicated that when fluid concentration was 6%, pH value was about 7.0, and inoculum size was 20% of dry material weight, the biogas production and quality in every treatment except pulp: swine feces 8:2 showed no significance. The treatments of pulp: swine feces 8:2, 7:3 utilized organic matter best. Totally, the treatment of pulp: swine feces 7:3 ensured production and quality of biogas, and owned high proportion of citrus pulp, which gained total gas production, daily average gas production, TS gas generation, VS gas generation and tank volume gas generation were 11918.67 ml,397.29 ml,49.60 ml·g^-1,58.56 ml·g^-1,0.10 ml·ml·d^-1 separately, methane content was 53.23%, (in the normal range of methane content 50-70%), removal of COD was 40.93%, (44788.80 mg·L^-1, the beginning concentration, the same below). So the treatment of pulp: swine feces 7:3 was chosen for later experiments;4. A fermentation experiment under room temperature was carried out for about 30d, which set the fluid concentration 2%, 3%, 4%, 6%, 8%, pulp: swine feces 7:3, and other condition same the study before. Results showed that if fluid concentration was 8%, the fermentation process. blocked because of high organic loading, while too low concentration exhibited negative effects on gas production, gas quality and utilization of microbes. Treatment of fluid concentration 6% was best, its total gas production, daily average gas production, TS gas generation, VS gas generation and tank volume gas generation were 10481.44 ml,349.38 ml,49.96 ml·g^-1,49.34 ml·g^-1, 0.09 ml-ml^-1d^-1 separately, methane content was 51.94%, removal of COD was 44.12%(45145.58 mg·L^-1). So 6% of fluid concentration was chosen for later experiments;5. In pre-process of pile fermentation, when pile fermented 5 d, the total gas production, daily average gas production, TS gas generation, VS gas generation and tank volume gas generation were promoted significantly, which reached 12058.21 ml, 401.94 ml, 50.16 ml·g^-1, 57.61 ml·g^-1; 0.10 ml·ml^-1d^-1 differently, and methane content reached 54.91%, removal of COD was 47.79%(44419.33 mg·L^-1). Once the pile fermentation time was beyond 10d, gas production declined, and methane content and COD removal rate didn't increased. It can be concluded that a timing pile fermentation can boost anaerobic digestion of citrus pulp;6. Organic loading of citrus pulp was examined, and the adaptive one was about 1.0 g VS·L^-1·d^-1 , the total gas production, daily average gas production, TS gas generation, VS gas generation and tank volume gas generation were reached 13773.32 ml, 459.11 ml, 69.07 ml·g^-1, 79.68 ml·g^-1, 0.11 ml·ml^-1·d^-1 differently, methane content was 54.91%, much higher than in treatment of 0.5 g VS·L^-1·d^-1 If feed-adding quantity reached, appeared organic loading too high, gas production and quality decreased on the contrary;7. Effects of different pH modulator on gas production and quality were examined. The results revealed under different organic loading, high gas production and low methane content were gained when using NaHCO₃, impacts of NaOH was similar to NaHCO₃, but the methane content was much higher than the latter. So, NaOH was chosen as the best modulator;8. Orthogonal test was used to check the effects of pile fermentation time, feed-adding time and feed adding quantity on anaerobic digestion on fresh citrus pulp. The results showed that at the process of citrus pulp anaerobic digested continuously in single-phase system at ambient temperature, feed-adding quantity was the key factor to influence the ratio of gas production, at the range of 2.5-7.5% feed-adding quantity, the daily average gas production promoted as feed-adding quantity increased. In addition, pile fermentation was another important factor affecting gas production, the daily average gas production was enhanced significantly (P<0.05) when pile fermented 5 d. Feed-adding impacted least, which had no significant effects on daily average gas production when feed-adding 8-12 d, CH₄ content in every treatments exhibited no significance. The best resultant lead to highest daily average gas production was pile fermentation 5 d, feed-adding 10 d, and feed-adding quantity 7.5% of total efficiency capacity,which possessed the total gas production, daily average gas production, TS gas generation, VS gas generation and tank volume gas generation were 18140.18 ml,604.67 ml,71.79 ml·g^-1: 78.95 ml·g^-1,0.15 ml·ml·(-1)d^-1, CH₄ content 57.22%, removal of COD was 46.77%(46010.00 mg·L^-1);9. On the foundation of experiments former, effect of temperature on gas production was studied. Results revealed that temperature change affected gas production significantly, they had fine linear relation between each other, indicated stable and high temperature benefit fo'r fermentation process. Considering the cost, it was suggested that control temperature to improve fermentation efficiency;10. Above all, the best technics can be advanced: adding swine feces 30% in fresh materiel, pile fermentation 5 d, with fluid concentration 6%, inoculating sludge domesticated by citrus pulp lixivium 20% (the percentage of inoculant in dry fermenting material) , feed-adding every 10d, and the quantity is 7.5% of total efficiency capacity.