Anaerobic Process Intensification With Vacuum and Combined Pretreatment Technologies

Abstract

Anaerobic digestion and fermentation are essential processes for transforming wastewater treatment plants into robust energy, nutrient, and water recovery facilities. However, these processes' slow microbial growth rate and low yields limit their application to larger facilities. To improve their efficiency, intensifying anaerobic digestion and fermentation (to a lesser extent) has been the subject of numerous research studies. This dissertation explored the principle of vacuum evaporation for combining sludge thickening, decoupling of hydraulic and retention times, and nutrient recovery into a single treatment unit. Gaps, including the efficiency of vacuum sludge thickening at different solids content, its impact on anaerobic bacteria and the overall physio and biochemical processes, were investigated. In batch fermentation, establishing 400 mbar of absolute pressure led to 49% solubilization of primary sludge in 72 hours compared to the conventional fermentation in which 11% solubilization was achieved. During fermentation of TWAS at 400 mbar, deterioration of solubilization and VFA yield was observed. The vacuum technology was used to decouple the hydraulic and solids retention time (HRT/SRT) in semi-continuous fermentation of mixed sludge. At SRT of 3 days and HRT of 1.5 days, the vacuum-assisted fermenter yielded 660 mg sCOD/g VSS compared to the conventional fermenter which achieved 513 mg sCOD/g VSS with equal HRT and SRT of 3 days. Additionally, 50% of the ammonia produced in the fermenter was extracted in the condensate with no chemical alterations. The results led to the development of IntensiCarb, a patent-pending technology that combines sludge thickening, digestion, and dewatering into a single unit. This dissertation also investigated the use of combined chemical-mechanical and chemical-chemical pretreatment technologies with free nitrous acid (FNA) to enhance the anaerobic digestion of TWAS. FNA is an environment-friendly chemical that can be produced in wastewater treatment facilities. In this study, FNA concentrations from 0.7 to 2.8 mg HNO2-N/L were combined with ultrasonication energy input from 600 to 3,000 kj/kg TS to explore the feasibility of lower energy input for effective sludge disintegration and enhanced anaerobic digestion. The energy input for ultrasonication could be halved from 3,000 kj/kg TS to 1,500 kj/kg TS by adding 2.8 mg HNO2-N /L while improving solubilization by 43% and methane yield by 18%. FNA was also combined with hydrogen peroxide, an oxidizing chemical that has shown effectiveness in improving biodegradability at low pH. The results showed that the pretreatment combination with the lowest chemical concentrations of 0.7 mg HNO2-N /L and 25 mg H2O2/g TS was the most effective for maximizing methane yield, increasing it by 50% compared to the untreated TWAS.