Technical and economic evaluation of anaerobic membrane bioreactors for municipal wastewater treatment

Abstract

Anaerobic membrane bioreactor (AnMBR), which is a combination of membrane separation and anaerobic digestion, is an emerging biotechnology for municipal sewage treatment. The application of AnMBRs in the mainline of wastewater treatment plants (WWTPs) can provide several advantages compared with conventional activated sludge processes, such as no aeration requirements, biogas production and reduction in the sludge management costs. However, despite these advantages, mainstream AnMBR application still presents challenges, whose resolution requires considering both technical and economic aspects.

The goal of this thesis is to evaluate the technical and economic implications of implementing AnMBRs for municipal sewage treatment. Specifically, the thesis covered the techno-economic implications of two main topics: (i) forward osmosis (FO) pre- concentration before AnMBR, and (ii) plant-wide impact of AnMBR implementation in a WWTP.

In the first part of this thesis, the techno-economic effects of combining FO and AnMBR technologies have been evaluated. First, a lab-scale mesophilic AnMBR operated at pre- concentration factors of 1, 2, 5 and 10 achieved chemical oxygen demand (COD) removal efficiencies above 90% for all the conditions. The differences between the soluble COD concentration of the permeate and digester suggested that membrane biofilm contributed to COD removal efficiency. Second, the techno-economic analysis of combining FO, reverse osmosis (RO) and AnMBR was conducted. The results showed that the wastewater treatment cost of the FO-RO+AnMBR system ranged between 0.80 and 1.40 € per m3 of wastewater treated. A sensitivity analysis illustrated that FO fluxes above 10

L m-2 h-1 (LMH) would improve the economic competitiveness of the FO-RO+AnMBR system. Finally, the impact of the draw solute and FO membrane material on the economic balance of this system was evaluated. The membrane material had a high impact on the economic balance since thin film composite (TFC) membranes substantially reduced the net cost when compared with cellulose triacetate (CTA) membranes. Conversely, the draw solute featured a moderate impact on the net cost. CH3COONa and CaCl2 were the most economically favourable draw solutes for CTA

membrane, whereas MgCl2 was the most economically favourable draw solute for TFC

membrane.

In the second part of this thesis, the plant-wide impact of implementing AnMBRs in WWTPs has been evaluated. First, the effect of specific gas demand (SGD) and flux on membrane performance and process economics of granular AnMBRs was analysed. SGD and membrane flux impacted membrane fouling, but they did not impact organic matter rejection. The economic evaluation of granular AnMBRs showed that the most competitive strategy for fouling control relied on operating the membrane at normalised fluxes and SGDs of 7.8 LMH and 0.5 m3 m-2 h-1, respectively. Second, the economic feasibility of implementing mainstream AnMBR in a WWTP was evaluated for five different WWTP layouts. The results showed that the net treatment cost ranged between 0.33 and 0.43 € m-3 (100-1200 mg COD L-1) for WWTP layouts combining AnMBR, degassing membrane, primary settler and anaerobic digester. However, when partial nitritation-anammox and chemical phosphorus precipitation were included for nutrients removal, the net treatment cost increased from 0.33-0.43 to 0.51-0.56 € m-3. Finally, the techno-economic implications of co-digesting food waste with sewage sludge in the sidestream anaerobic digester of an AnMBR-WWTP were analysed. Co-digestion reduced the net cost of the sludge line when the nutrients backload was treated in the mainstream. However, when the nutrients backload was treated in the sidestream with partial nitritation-anammox and struvite crystallisation, the electricity revenue did not offset the additional costs of these two processes. The results also indicated that biosolids disposal cost represented the highest cost contributor in the sludge line of an AnMBR- WWTP.

El biorreactor anaeróbico de membranas (AnMBR, por sus siglas en inglés) es una tecnología emergente para el tratamiento de aguas residuales municipales. El AnMBR no requiere de aeración, produce biogás y reduce la producción de fangos en comparación con procesos convencionales de lodos activos. Sin embargo, la aplicación del AnMBR en estaciones depuradoras de aguas residuales (EDAR) es muy limitado, ya que la tecnología debe superar barreras técnicas y económicas antes de una implementación generalizada. El objetivo de esta tesis es evaluar las implicaciones técnicas y económicas de implementar el AnMBR para el tratamiento de aguas residuales municipales. En concreto, la tesis aborda dos temáticas relacionadas con el AnMBR: (i) preconcentración del agua residual municipal mediante osmosis directa (FO, por sus siglas en inglés), e (ii) impacto global de implementar un AnMBR en una EDAR.

En la primera parte de esta tesis se ha evaluado la combinación de las tecnologías de FO y AnMBR. El sistema experimental AnMBR alcanzó eliminaciones de demanda química de oxígeno por encima del 90% para el tratamiento de aguas residuales preconcentradas. Un análisis tecno-económico sobre la combinación de la FO, osmosis inversa y AnMBR mostró que el coste de tratamiento osciló entre 0,80 y 1,40 € por m3 de agua residual tratada. Los resultados también mostraron que el material de la membrana de FO tuvo un gran impacto sobre el balance económico del sistema.

En la segunda parte de esta tesis se ha evaluado el impacto de implementar un AnMBR en una EDAR. La demanda específica de gas y el flujo de permeado tuvieron un impacto directo sobre el ensuciamiento de la membrana en un sistema AnMBR con fango granular. El análisis tecno-económico de la implementación de un AnMBR en una EDAR mostró que el coste de tratamiento se situaba entre 0,33 y 0,43 € m-3 para configuraciones que combinaron AnMBR, membrana de desgasificación, sedimentador primario y digestor anaeróbico. Finalmente, se determinó que la codigestión de fangos y residuos alimenticios tiene el potencial de reducir el coste de la línea de fangos de una EDAR que utiliza un AnMBR para tratar las aguas residuales.