Chiaki Niwa

Maximum organic loading rate for the single-stage wet anaerobic digestion of food waste.

Anaerobic digestion of food waste was conducted at high OLR from 3.7 to 12.9 kg-VS m(-3) day(-1) for 225 days. Periods without organic loading were arranged between the each loading period. Stable operation at an OLR of 9.2 kg-VS (15.0 kg-COD) m(-3) day(-1) was achieved with a high VS reduction (91.8%) and high methane yield (455 mL g-VS-1). The cell density increased in the periods without organic loading, and reached to 10.9×10(10) cells mL(-1) on day 187, which was around 15 times higher than that of the seed sludge. There was a significant correlation between OLR and saturated TSS in the sludge (y=17.3e(0.1679×), r(2)=0.996, P<0.05). A theoretical maximum OLR of 10.5 kg-VS (17.0 kg-COD) m(-3) day(-1) was obtained for mesophilic single-stage wet anaerobic digestion that is able to maintain a stable operation with high methane yield and VS reduction.

Effect of temperature on VFA’s and biogas production in anaerobic solubilization of food waste

The effectiveness of methane fermentation treatment used in food waste processing is currently limited by solubilization and acidogenesis. In efforts to improve the treatment process, this study examined the effects of temperature on solubilization and acidogenesis. The solubilization rate of food waste, which was based on suspended solid removal, was 47.5%, 62.2%, 70.0%, 72.7%, 56.1% and 45.9% at 15 degrees C, 25 degrees C, 35 degrees C, 45 degrees C, 55 degrees C and 65 degrees C, respectively. Solubilization rate was accelerated from the middle to late experimental periods under mesophilic (35 degrees C and 45 degrees C) conditions. In contrast, overall solubilization rate was significantly lower under thermophilic (55 degrees C and 65 degrees C) conditions than under mesophilic conditions, although solubilization occurred rapidly in the early experimental period. The production of biogas was high under mesophilic conditions of 35 degrees C and 45 degrees C, at 64.7 and 62.7mL/g-VS, respectively, while it was scarce under thermophilic conditions. Solubilization of food waste was accelerated under both mesophilic and thermophilic conditions; however, solubilization rate was observed to be particularly high under mesophilic conditions, and a shortening of the hydraulic retention time is expected under thermophilic conditions.

The effect of the labile organic fraction in food waste and the substrate/inoculum ratio on anaerobic digestion for a reliable methane yield.

Influence of the labile organic fraction (LOF) on anaerobic digestion of food waste was investigated in different S/I ratio of 0.33, 0.5, 1.0, 2.0 and 4.0g-VSsubstrate/g-VSinoculum. Two types of substrate, standard food waste (Substrate 1) and standard food waste with the supernatant (containing LOF) removed (Substrate 2) were used. Highest methane yield of 435ml-CH4g-VS(-1) in Substrate 1 was observed in the lowest S/I ratio, while the methane yield of the other S/I ratios were 38-73% lower than the highest yield due to acidification. The methane yields in Substrate 2 were relatively stable in all S/I conditions, although the maximum methane yield was low compared with Substrate 1. These results showed that LOF in food waste causes acidification, but also contributes to high methane yields, suggesting that low S/I ratio (<0.33) is required to obtain a reliable methane yield from food waste compared to other organic substrates.

Mesophilic Anaerobic Digestion as a Post-treatment for Biofouling Blue Mussels

In the last 80 years, the blue mussels of the genus Mytilus have been introduced by shipping and anthropogenic activities to the Japanese waters, resulting in biofouling which lead to lower plant efficiency and problems in treatment and disposal. Despite many on-going researches into the development of preventive, removal, and control strategies, treatment and disposal of the removed blue mussels are still ecologically, socially, and economically challenging. In this study, the possibility of using anaerobic digestion as an alternative to direct disposal and incineration of blue mussels was examined, and the effect of salinity (0 g/L, 10 g/L, 20 g/L, 3 5g/L, and control) on solubilization and methanogenesis of these marine biofoulers was evaluated in five batch reactors maintained at mesophilic temperature (37degC) over a period of 100 days. Rapid progression in the hydrolysis and acidogenesis of complex organic matter in mussel tissues, denoted by a sharp decrease in pH and increases in VFAs and SCOD in the first four days, were observed in all blue mussels-containing batch reactors. This suggests that solubilization of blue mussels in anaerobic digestion was not rate limiting. Salinity was also not found to have a significant effect on the degree of solubilization. A high cumulative methane production of above 8 L was obtained from batch reactors of low salinity (0 g/L and 10 g/L), in contrast to that of less than 3 L of methane from those of high salinity (20 g/L and 35 g/L). These results have thus proven that the concept of using anaerobic digestion for the treatment of the removed blue mussels will be feasible, and methanogenesis can be best optimized under salinity conditions below 10 g/L.