In this study, a simulated Organic Fraction of Municipal Solid Waste (OFMSW) was treated in an anaerobic two-stage membrane process. The OFMSW feedstock was fed to a ten litre hydrolytic reactor (HR) where solid and liquid fractions were separated by a coarse mesh, while the leachate was fed to a three litre submerged anaerobic membrane bioreactor (SAMBR) with in-situ membrane cleaning by biogas sparging beneath a flat sheet Kubota membrane. The
aim was to develop and optimize this two-stage process where the use of a membrane in both reactors to uncouple the Solid and Liquid Retention Times (SRT and HRT) would allow us to improve the current performances obtained with single stage designs. The Denaturing Gradient
Gel Electrophoresis (DGGE) technique was used to monitor the microbial population in the reactors and have a better understanding of the archaeal and bacterial distribution in a two-stage process.
It was found that meshes with pore sizes of � 10 microns and � 150 microns were inappropriate to uncouple the SRT and HRT in the HR. In the former case, the mesh became clogged, while in the latter case, the large pore size resulted in high levels of suspended solids in the leachate
that built up in the SAMBR. The most important parameter for Volatile Solids (VS) removal in the HR was the SRT. Maximum VS removals of 70-75% could be achieved when the SRT was equal to or greater than 50-60 days. This was achieved at a HRT of 9-12 days and an Organic Loading
Rate (OLR) of 4-5 g VS.l-1.day-1.Increasing the SRT to beyond 100 days did not significantly increase the VS removal in the HR. However, at an OLR of 10 g VS.l-1.day-1 in the HR the SRT had to be reduced due to a build up of TS in the HR that impeded the stirring. Below 20 days
SRT, the VS removal reduced to between 30 and 40%. With kitchen waste as its main substrate, however, an OLR of 10 g VS.l-1.day-1 was achieved with 81% VS removal at 23 days SRT and 1.8 days HRT.
The SAMBR was found to remain stable at an OLR up to 19.8 g COD.l-1.day-1 at a HRT of 0.4 day and at a SRT greater than 300 days, while the COD removal was 95%. However, the
performance at such low HRTs was not sustainable due to membrane flux limitations when the Mixed Liquor Total Suspended Solids (MLTSS) went beyond 20 g.l-1 due to an increase in viscosity and inorganics concentration. At 35 °C the SAMBR was found to be stable (SCOD removal �greater than or equal to 95%) at SRTs down to 45 days and at a minimum HRT of 3.9 days. The SAMBR could achieve 90% COD removal at 22 °C at an OLR of 13.4 g COD.l-1.day-1 and 1.1 days HRT (SRT = 300 days).
The DGGE technique was used to monitor the archaeal and bacterial diversity and evolution in the HR and SAMBR with varying SRTs, HRTs, OLRs and temperatures in the biofilm and
in suspension. Overall, it was found that stable operation and high COD removal correlated with a high bacterial diversity, while at the same time very few species (2-4) were dominant. Only a few dominant archaeal species were sufficient to keep low VFA concentrations in the
SAMBR at 35 °C, but not at ambient temperatures. It was found that some of the dominant species in the HR were hydrogenotrophic Archaea such as Methanobacterium formicicum and Methanobrevibacter while the other dominant species were from the genus Methanosarcina or Methanosaeta. The presence of hydrogenotrophic species in the HR could be fostered by reinoculating the HR with excess sludge from the SAMBR when the SRT of the SAMBR was greater than 45 days. Among the bacterial species Ruminococcus flavefaciens, Spirochaeta, Sphingobacteriales, Hydrogenophaga, Ralstonia, Prevotella and Smithella were associated with good reactor performances.