Stable increase of organic loading rate in anaerobic membrane bioreactors working at ambient temperature

Sammanfattning: Anaerobic processes are an interesting alternative to treat wastewaters instead of aerobic treatments as it allows recovery of energy with the produced biogas while removing organic matter. In the industry, lots of low-strength industrial wastewaters are available and unused for biogas production due to their low concentration of organic material and high volumes (slaughterhouse wastewaters, dairies and beverages industries, oils and fats producers). However converting low-strength wastewaters into biogas is problematic because the need of expensive thickening and concentration steps for conventional continuously stirred tank reactors. Anaerobic membrane bioreactors (AnMBR’s) are potentially a good method to convert low-concentrated wastewaters into methane, due to their ability to process large amount of influent using a small reactor volume. Moreover using AnMBR’s working at ambient temperature (25°C) instead of higher temperatures are interesting as it would improve the energy balance while processing wastewaters without any expensive heating and concentration steps. Nevertheless, use of industrial AnMBR’s working at ambient temperature is still in its infancy and it is unknown how the process stability is affected by the step-increase in organic loading rate (OLR) during start-up. Suitable OLR increase has then to be defined in order to know this critical operational parameter and perform the start-up of AnMBR as quickly as possible to reduce costs. The aim of this work was to evaluate the stability of the anaerobic digestion process in ambient temperature (25°C) anaerobic membrane bioreactors (AnMBR’s) while increasing the organic loading rates (OLR) during the start-up. Two 180L AnMBR’s fed with synthetic wastewater (milk powder) were used to realize up-scales of the OLR, which were monitored by stability indicators, to determine if the biological process is stable or not according to the indicator’s benchmarks. The stability indicators are constituted by the specific gas production, the pH, the alkalinity ratio, the volatile fatty acid concentrations and the membrane performance. One reactor was first used to perform several step-increases of OLR after a long initial steady state. The other reactor was then used to reach directly the higher OLR reached by the first reactor after a short initial steady state, in order to figure out if a long initial steady state and a step-increase of OLR really were need in order to ensure the stability of the process. The first reactor fulfilled all the stability indicators during its operation. The reactor handled the increase of the OLR without any failure in the anaerobic process. Nevertheless the membrane performance was not met since the membrane was fouled too often. The second reactor did not meet any of the biological benchmarks or the benchmarks for the membrane performance. Fouling of the membrane were investigated in order to enhance membrane performance. Finally, one step-increase of OLR was successfully carried out while monitored by stability indicators, which found out to be powerful monitor tools to control the anaerobic digestion process.