Mass transfer in a membrane aerated biofilm.

We present experimental results of mass transfer of a non reactive tracer gas (neon) measured in aerobic heterotrophic biofilm developed from activated sludge. Biofilms are grown in various hydrodynamic conditions and the effective diffusivity is used to quantify the mass transfer through the biofilm. Beyond some cross-flow conditions, the effective diffusivity through the biofilm seems larger than in the bulk. This can be explained by a dispersion generated by convection inside the biofilm, as supported by an analytical flow model and in accordance to the numerical simulation proposed by Aspa et al. (2011).

Virus removal retention challenge tests performed at lab scale and pilot scale during operation of membrane units.

The determination of the virus retention capabilities of UF units during operation is essential for the operators of drinking water treatment facilities in order to guarantee an efficient and stable removal of viruses through time. In previous studies, an effective method (MS2-phage challenge tests) was developed by the Water Research Center of Veolia Environnement for the measurement of the virus retention rates (Log Removal Rate, LRV) of commercially available hollow fiber membranes at lab scale. In the present work, the protocol for monitoring membrane performance was transferred from lab scale to pilot scale. Membrane performances were evaluated during pilot trial and compared to the results obtained at lab scale with fibers taken from the pilot plant modules. PFU culture method was compared to RT-PCR method for the calculation of LRV in both cases. Preliminary tests at lab scale showed that both methods can be used interchangeably. For tests conducted on virgin membrane, a good consistency was observed between lab and pilot scale results with the two analytical methods used. This work intends to show that a reliable determination of the membranes performances based on RT-PCR analytical method can be achieved during the operation of the UF units.

In the search of alternative cleaning solutions for MBR plants.

The objective of the study was to identify alternative cleaning reagents to chlorine for membrane permeability regeneration in MBR applications. Indeed, chlorine is prohibited in some countries because of the formation of by-products such as THM. The study was focused on the comparison of ten cleaning reagents performances and in particular on their ability to remove irreversible fouling. The tests were carried on with the A3 Water Solutions' Maxflow membrane (flat sheet membrane). A specific experimental protocol was defined at lab scale to develop an irreversible fouling by filtering sludge supernatant. The more promising reagents at lab scale were then tested on the A3 membrane continuously immersed in a MBR pilot plant functioning under typical biological conditions (MLSS=11 g/l; SRT=28 days). A full scale test was finally performed with hydrogen peroxide, one of the best reagents. Chlorine was taken as reference for all performed tests. The cleaning performances of the selected reagents were different at the different scales, probably due to the difficulty to obtain an irreversible membrane fouling at larger scales. This testing procedure will be reproduced with other membrane materials to have a better understanding of interactions between irreversible fouling, material nature and chemical reagents.

Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment.

Two parallel membrane bioreactors (2 m3 each) were operated over a period of 2 years. Both pilots were optimised for nitrification, denitrification, and enhanced biological phosphorous elimination, treating identical municipal wastewater under comparable operating conditions. The only constructional difference between the pilots was the position of the denitrification zone (pre-denitrification in pilot 1 and post-denitrification in pilot 2). Despite identical modules and conditions, the two MBRs showed different permeabilities and fouling rates. The differences were not related to the denitrification scheme. In order to find an explanation for the different membrane performances, a one-year investigation was initiated and the membrane performance as well as the operating regime and characteristics of the activated sludge were closely studied. MLSS concentrations, solid retention time, loading rates, and filtration flux were found not to be responsible for the different performance of the submerged modules. These parameters were kept identical in the two pilot plants. Instead, the non-settable fraction of the sludges (soluble and colloidal material, i.e. polysaccharides, proteins and organic colloids) was found to impact fouling and to cause the difference in membrane performance between the two MBR. This fraction was analysed by spectrophotometric and size exclusion chromatography (SEC) methods. In a second step, the origin of these substances was investigated. The results point to microbiologically produced substances such as extracellular polymeric substances (EPS) or soluble microbial products (SMP).