A two-stage continuous process was developed for improved silica extraction from rice husk. The two-stage continuous process consists of attrition ball milling and alkaline leaching methods. To find the optimum conditions for the continuous process, the effects of alkaline leaching parameters, such as the alkaline solution type and reaction conditions, on the silica extraction yield were investigated in a batch process. The use of NaOH showed a slightly higher silica yield than KOH. The optimum reaction conditions were found to be 0.2 M, 80 °C, 3 h, and 6% (w/v) for the reaction concentration, temperature, duration time, and solid content, respectively. Attrition ball milling was used to make micron-sized rice husk particles and to improve the fluidity of the rice husk slurry. The two-stage continuous process was performed using optimum conditions as determined based on the results of the batch experiment. The two-stage continuous extraction was stably operated for 80 h with an 89% silica yield. During the operation, the solid content remained consistent at 6% (w/v). The obtained silica was characterized using inductively coupled plasma–optical emission spectrometry (ICP–OES), X-ray diffraction (XRD), and the Brunauer–Emmett–Teller (BET) method.

Antimicrobial air filters are required to protect humans from the risk of secondary bioaerosol pollution as well as airborne particles. Three plant extracts (tea-tree oil, rosemary, and garlic) were selected to replace antimicrobial chemicals in air filters. The antimicrobial activity of plant extracts was investigated using Micrococcus luteus and Escherichia coli. Phytochemicals present in the three plant extracts were identified using a gas chromatograph coupled with a mass spectrometer. The extracts were spray-coated on polyethylene terephthalate filter surfaces using silicate polymeric coating and evaluated via X-ray photoelectron spectroscopy and a scanning electron microscope with energy dispersive spectroscopy. After coating, an increase of 9.1% in the pressure drop was observed. The strain Micrococcus luteus was used to evaluate the antimicrobial activity of the air filter. After bioaerosol exposure, the tea-tree oil-coated filters immediately induced M. luteus cell inactivation (40–55%), whereas the rosemary and garlic coated filters did not. However, 48 h after exposure, a significant M. luteus inactivation of 99.99%, 99.0%, and 99.9% was recorded for concentrations of 2.89, 6.73, and 11.51 mg/cm² for the tea-tree, rosemary, and garlic extracts, respectively. The coated filters exhibited high antimicrobial activity, thereby indicating significant potential for application as self-cleaning air filters.

Rice husks are a feedstock of biogenic silica because of their high silica content. After silica extraction, a solid residue comprising mostly carbohydrates is present. Solid residue valorization is important for closed-loop systems using rice husk and has minimal negative environmental impacts. In this study, we used solid rice husk that was generated by silica extractionto anaerobic digestion for producing biomethane. The rice husk residue was characterized in terms of total solids, volatile solids, pH, composition, and particle size. Changing the characteristics increased biogas production by 2.48-fold compared to that of raw rice husk. The residue produced 166.4 mL-biogas g⁻¹ vs. and 100.4 mL CH 4 g⁻¹ VS, much more than previously reported. Microbial community analysis, which was conducted to investigate the biological reasons for increased biogas and methane, found increased Bacteroidetes levels in the rice husk samples. Among archaeal communities, Bathyarchaeota was more abundant in all rice husk samples than in the inoculum. The rice husk residue contained more operational taxonomic units than other samples. These changes in the microbial community significantly influenced the anaerobic digestion of the rice husk residue and improved methane production. Our findings provide a basis for the cleaner utilization of rice husk residue to produce renewable energy.

The impact of attrition ball-mill pretreatment on food waste particle size, soluble chemical oxygen demand (SCOD), biochemical methane potential, and microbial community during anaerobic digestion was investigated based on milling speed and time. The uniformity of particle size improved with increasing milling speed and time. The SCOD of the pretreated samples increased to 4%, 7%, and 17% at the speeds of 150, 225, and 300 rpm, respectively, compared to the control. Milling time did not significantly change the SCOD. The cumulative methane productions of 430, 440, and 490 mL/g-VS were observed at the speeds of 150, 225, and 300 rpm, respectively, while the untreated sample exhibited the cumulative methane production of 390 mL/g-VS. Extended milling time did not improve methane production much. When the milling times of 10, 20, and 30 min were applied with the milling speed fixed at 300 rpm, the methane productions of 490, 510, and 500 mL/g-VS were observed respectively. Ball-mill pretreatment also increased the total volatile fatty acids. During the anaerobic digestion (AD) of ball-mill treated food waste, acetoclastic methanogens predominated, with a relative abundance of 48–49%. Interestingly, hydrogenotrophic methanogens were 1.6 times higher in the pretreated samples than those in the control. These results showed the potential of attrition ball milling as a food waste pretreatment for improving methane production.