Long-term operation of an anaerobic bioreactor effluent gravity driven membrane system

Abstract

Currently, anaerobic treatment technology stands out as a viable solution for wastewater treatment due to its minimal carbon footprint. The key advantage lies in its capacity to directly convert organics into methane, bypassing the energy-intensive oxidation process. Coupled with the cost-effective gravity-driven membrane (GDM) technology, which removes bacteria and extends the solids retention time (SRT) of the system, this approach appears well-suited for regions grappling with limited energy resources. However, the prevailing consensus from most prior studies on GDM systems is that energy demanding backwashing or other cost demanding membrane cleaning procedures are necessary for achieving a long-term stable flux.

This report explores the long-term operation of a gravity-driven anaerobic membrane bioreactor (AnMBR). The primary objective is to investigate whether a stable minimum flux can be attained in the absence of backwashing and other membrane cleaning processes. This investigation involves monitoring the long-term flux of an AnMBR system, incorporating Upflow Sludge Blanket (UASB) and dead-end GDM ultrafiltration (UF) technology with a pore size of 0.04 µm.

The system operated continuously for a total of 308 days, utilizing municipal wastewater sourced from the Grødaland Wastewater Treatment Plant as the feed water. Throughout the project duration, the trans-membrane pressure (TMP) remained at approximately 30 mbar with a few exceptions. The system achieved a minimum flux of 0.4 L/h/m² after four months of operation. Despite occasional accidental drained periods, the flux consistently re-established itself at 0.4 L/h/m² within a few days. It appears that the system could have maintained this flux indefinitely, suggesting that the operation would be limited by the membrane’s lifespan or the gradual filling of the reactor volume as the cake thickness increased.

Upon completion of the project period, a deliberate backwashing of the membrane was carried out, resulting in an augmented flux of 3 L/h/m². This enhanced flux closely mirrors the membranes performance when it was in its pristine state. It is essential to note that this action was performed for observational purposes rather than out of necessity. Following the resumption of system operation, the flux exhibited a gradual decline. Within a span of one week, it dwindled to 0.6 L/h/m², with the expectation of further reduction over the next few months until stabilizing at the initial minimum flux of 0.4 L/h/m².

The membrane effectively captured particulate organics and nutrients, allowing the passage of the majority of dissolved organic, nitrogenous, and phosphorous species through its structure. The findings indicate that the membrane functions as a hygienic barrier, proficiently preventing bacterial pathogens from penetrating. This capability is pivotal in addressing water, sanitation, and hygiene (WASH)-related diseases, where bacteria and parasites pose primary concerns. However, the membrane’s efficacy in capturing viral pathogens remains uncertain.

Currently, anaerobic treatment technology stands out as a viable solution for wastewater treatment due to its minimal carbon footprint. The key advantage lies in its capacity to directly convert organics into methane, bypassing the energy-intensive oxidation process. Coupled with the cost-effective gravity-driven membrane (GDM) technology, which removes bacteria and extends the solids retention time (SRT) of the system, this approach appears well-suited for regions grappling with limited energy resources. However, the prevailing consensus from most prior studies on GDM systems is that energy demanding backwashing or other cost demanding membrane cleaning procedures are necessary for achieving a long-term stable flux.

This report explores the long-term operation of a gravity-driven anaerobic membrane bioreactor (AnMBR). The primary objective is to investigate whether a stable minimum flux can be attained in the absence of backwashing and other membrane cleaning processes. This investigation involves monitoring the long-term flux of an AnMBR system, incorporating Upflow Sludge Blanket (UASB) and dead-end GDM ultrafiltration (UF) technology with a pore size of 0.04 µm.

The system operated continuously for a total of 308 days, utilizing municipal wastewater sourced from the Grødaland Wastewater Treatment Plant as the feed water. Throughout the project duration, the trans-membrane pressure (TMP) remained at approximately 30 mbar with a few exceptions. The system achieved a minimum flux of 0.4 L/h/m² after four months of operation. Despite occasional accidental drained periods, the flux consistently re-established itself at 0.4 L/h/m² within a few days. It appears that the system could have maintained this flux indefinitely, suggesting that the operation would be limited by the membrane’s lifespan or the gradual filling of the reactor volume as the cake thickness increased.

Upon completion of the project period, a deliberate backwashing of the membrane was carried out, resulting in an augmented flux of 3 L/h/m². This enhanced flux closely mirrors the membranes performance when it was in its pristine state. It is essential to note that this action was performed for observational purposes rather than out of necessity. Following the resumption of system operation, the flux exhibited a gradual decline. Within a span of one week, it dwindled to 0.6 L/h/m², with the expectation of further reduction over the next few months until stabilizing at the initial minimum flux of 0.4 L/h/m².

The membrane effectively captured particulate organics and nutrients, allowing the passage of the majority of dissolved organic, nitrogenous, and phosphorous species through its structure. The findings indicate that the membrane functions as a hygienic barrier, proficiently preventing bacterial pathogens from penetrating. This capability is pivotal in addressing water, sanitation, and hygiene (WASH)-related diseases, where bacteria and parasites pose primary concerns. However, the membrane’s efficacy in capturing viral pathogens remains uncertain.