Abstract
Granular Anaerobic Membrane Bioreactor (G-AnMBR) is a promising bioprocess for energy-positive domestic wastewater treatment through the combination of anaerobic digestion, granular biomass and ultrafiltration membrane separation. If properly exploited, the synergy between these three technologies would allow the implementation of treatment systems that are both efficient, energy self-sufficient, low-cost, low-maintenance and low-resource and with a potential for wastewater reuse. Nevertheless, the applicability of G-AnMBR to domestic wastewater treatment is hampered by the difficulty of achieving a positive energy balance (i.e. lower biological activity at ambient temperature, loss of dissolved methane within the effluent, low organic loading rate) and sustaining high filtration fluxes due to membrane fouling. Thus, the overall objective of this thesis is to study a new G-AnMBR configuration in order to (i) maximize the conversion of organic matter into methane and (ii) minimize energy consumption.This G-AnMBR integrates a granular anaerobic digester (UASB type), an ultrafiltration membrane immersed directly in the granular bed and does not have a gas sparging for fouling mitigation. The aim was to demonstrate the interest of this innovative process, compared to a conventional anaerobic reactor (UASB), in terms of effluent quality and energy production. The study revealed the ability of the G-AnMBR to meet better treatment performance and to increase the organic to methane conversion rate, resulting in a positive net energy balance of 0.58 kWh produced per m3 of treated water.Fouling mechanisms were studied by size fractionation of the mixed liquor associated with filtration tests. The supernatant fraction (dp < 0.125 mm) was identified as key driver in G-AnMBR fouling, in contrast to the granules (dp ≥ 0.125 mm) whose effect was negligible. Synergetic interactions between granules and compounds of supernatant lead to a decrease in both fouling intensity and fouling rate, demonstrating the benefit of using granular biomass.The impact of process intensification and changes in operating conditions (i.e. filtration flux, organic loading rate and hydraulic retention time) on treatment efficiencies, biogas production and capital and operating expenditures of the G-AnMBR was evaluated. The intensification of the process is technically feasible as high COD removal efficiencies were achieved (~90%) and 70-77% of the COD was converted to methane. The most economically attractive permeate flux was shown to be an intermediate flux (4.1 LMH). Membranes should not be underutilized to avoid overinvestment, but the permeate flux cannot be too elevated, otherwise membrane fouling leads to excessive additional costs (chemical washing, energy costs, maintenance costs, membrane change).Thus, this research has highlighted the high potential of G-AnMBR with submerged membrane and without biogas sparging for the treatment of domestic wastewater at ambient temperature, and suggests an implementation in decentralized treatment.