Two-phase anaerobic co-digestion of food waste and sewage sludge

A new approach to enhance the conventional two-phase anaerobic co-digestion of food waste and sewage sludge

BACKGROUND

Two-phase anaerobic co-digestion (TAcoD) is a versatile technology for the simultaneous treatment of organic materials and biogas production. However, the produced digestate and supernatant of the system contain heavy metals and organic substances that need to be treated prior to discharge or land application. Therefore, in this study, an innovative TAcoD for organic fertilizer and high supernatant quality achievement was proposed.

METHODS

In the conventional TAcoD, mixed sewage sludge (SS) and food waste (FW) were first hydrolyzed in the acidogenic reactor, and then the hydrolyzate substrate was subjected to the methanogenic reactor (TAcoD 1). In the modified TAcoD (TAcoD 2), only FW was fed into the acidogenic reactor, and the produced hydrolyzed solid was directly converted to the organic fertilizer, while the supernatant with high soluble chemical demand (SCOD) concentration was further co-digested with SS in the methanogenic reactor.

RESULTS

Although TAcoD 1 produced bio-methane yield and potential energy of 56.18% and 1.6-fold higher than TAcoD 2, the economical valorization of TAcoD 2 was 9-fold of that from TAcoD 1. The supernatant quality of TAcoD 2 was far better than TAcoD 1, since the SCOD, total nitrogen (TN), and total phosphor (TP) removal in TAcoD 2 and TAcoD 1 were 94.3%, 79.4%, 90.7%, and 68.9%, 28%, 46%, respectively. In terms of solid waste management, the modified TAcoD converted FW to organic fertilizer and achieved a solid reduction of 43.62% higher than that of conventional TAcoD.

CONCLUSIONS

This new modification in two-phase anaerobic co-digestion of food waste and sewage sludge provides a potentially feasible practice for simultaneous bio-methane, organic fertilizer, and high supernatant quality achievement.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40201-020-00603-8.

Volatile fatty acids production from saccharification residue from food waste ethanol fermentation: Effect of pH and microbial community

In this study, residue from saccharification and centrifugation of food waste ethanol fermentation was used as substrate to produce volatile fatty acids. The effects of different pH (5.5, 6.5, and uncontrolled) on the VFAs concentration, composition, acidogenic efficiency and microbial community distribution were investigated. The results showed that the highest concentration of VFAs was 267.8 ± 8.9 mg COD/g VS at pH of 6.5, and the highest percentage of butyric acid (79.8%) was followed by propionic acid and acetic acid at the end of the reaction. Microbial analysis showed that the contents of Vagococcus and Actinomyces increased, while the contents of Bacteroides and Fermentimonas decreased during anaerobic fermentation. The comparative high pH induced the accumulation of butyric acid. This study provides a new idea for the step anaerobic fermentation of food waste to produce alcohol and acid simultaneously.

Importance of storage time in mesophilic anaerobic digestion of food waste

Storage was used as a pretreatment to enhance the methanization performance of mesophilic anaerobic digestion of food waste. Food wastes were separately stored for 0, 1, 2, 3, 4, 5, 7, and 12days, and then fed into a methanogenic reactor for a biochemical methane potential (BMP) test lasting up to 60days. Relative to the methane production of food waste stored for 0-1day (285-308mL/g-added volatile solids (VSadded)), that after 2-4days and after 5-12days of storage increased to 418-530 and 618-696mL/g-VSadded, respectively. The efficiency of hydrolysis and acidification of pre-stored food waste in the methanization reactors increased with storage time. The characteristics of stored waste suggest that methane production was not correlated with the total hydrolysis efficiency of organics in pre-stored food waste but was positively correlated with the storage time and acidification level of the waste. From the results, we recommend 5-7days of storage of food waste in anaerobic digestion treatment plants.