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

Abstract

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.

Popular Abstract

How to recover energy from wastewater treatment? Or how to turn wastewater into gold? Well you have to know that every year, a lot of industries (like slaughterhouse or dairies and beverage industries producers) release a lot of wastewaters into the environment without any valuable reuse. It’s a big waste, because wastewater can be valorise. One good way to do it is to produce biogas from it, thanks to the anaerobic digestion (fermentation in a closed environment, without any oxygen). That process allows to recover energy (with the biogas released) from the wastewater treatment, which can also decrease the energy cost!
However anaerobic digesters can’t handle large volume of feeding due to the washing out of the micro-organisms producing methane into the outlet. One solution is to use an anaerobic membrane bioreactor, which is a reactor coupled with a membrane that maintain particles and micro-organisms into the reactor, allowing to treat larger volumes of wastewater. Moreover, the use of an anaerobic membrane bioreactor working at ambient temperature (25°C) instead of higher temperatures (37°C) is interesting as it would decrease the energy consumption of the reactor.
Nevertheless, how to start that kind of reactors while keeping the process stable is still unknown. In one hand, a too quick start-up would lead to the failure of the reactor, as the anaerobic digestion would be saturated .In the other hand, a too slow start-up wouldn’t be cost-effective. Then a suitable speed of start-up for the wastewater feeding has to be determined.
The aim of the work was to evaluate the stability of the reactor during two different start-ups, using stability indicators found in the scientific literature. Two anaerobic membrane bioreactors were used (see the photo), fed with synthetic wastewater (prepared with milk powder, in order to mimic a real wastewater). One reactor was first used to perform a slow start-up (with several step-increases of the feeding) after a long adaption time for the micro-organisms. The second reactor was used to perform a quick start-up from a short adaption time, in order to figure out if both long adaptation time and slow start-up are really needed in order to avoid the failure of the reactor.
The first reactor fulfilled all the stability indicators during its operation, which means that the reactor has handled the start-up without any failure in the anaerobic process. However, the second reactor didn’t meet any of the biological benchmarks; it failed. Finally, for both reactors, fouling of the membrane was found to be limiting in order to continue the start-up. The membrane fouling rate was too high to properly operate the reactors. Reasons for the membrane fouling were investigated (accumulation of fat in the reactor, thickness of the sludge, use of fat-free milk powder), without any relevant results… Investigation must continue! In conclusion, a step-increase start-up of the feeding was successfully carried out while monitored by stability indicators, which found out to be powerful monitoring tools to control the anaerobic digestion process.

• author: Monhonval, Jean
• supervisor: Hamse Kjerstadius
• organization: Chemical Engineering (M.Sc.Eng.)
• course: VVA820 20151
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: environmental engineering, anaerobic membrane bioreactor, stability indicators, ambient methanization, stable increase organic loading rate, vattenförsörjningsteknik, avloppsteknik, water engineering
• language: English
• additional info: Contact : [e-mail]
• id: 7521385
• date added to LUP: 2015-08-19 15:47:50
• date last changed: 2015-08-19 15:47:50