Biological treatment of leachate in a submerged anaerobic membrane bioreactor

Biological treatment of leachate in a submerged anaerobic membrane bioreactor

In this chapter, 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 microfiltration module. 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 Chemical Oxygen Demand (COD) removal was greater than 90% at a Hydraulic Residence Time (HRT) of 1.6-2.3 days at a maximum Organic Loading Rate (OLR) of 20 g COD/L.day. Even though the influent COD of the leachate was constantly changing giving rise to a transitory Food to Microorganisms ratio over time, the permeate COD from the SAMBR was typically between 300 and 500 mg/L, which can therefore be defined as a stabilised leachate. Because of the fluctuating properties of the leachate produced in the HR, the process was deemed more representative because the SAMBR treated a leachate with varying organic strength which is what can happen on a full scale.
The COD removal in SAMBR2 was 94.5% on average, and only 1.6% in the subsequent aerobic polishing bioreactor (AMBR-Aerobic Membrane Bioreactor), so that a total COD removal of 96.1% was achieved at 0.4 day HRT. On average, 26% of the recalcitrants from SAMBR2 could be degraded aerobically in the AMBR. However, as in SAMBR1, at HRTs lower than 2 days, particulate solids in the leachate built up at the bottom of the SAMBR, eventually leading to the diffuser blocking. At MLTSS beyond 20 g/L, the transmembrane pressure (TMP) culminated at 850 mbar and the flux dropped to 0.5 L.m-2.hr-1 LMH until the end of the experiment. The permeate of the SAMBR was low in COD and relatively constant which promoted the growth of autotrophic bacteria in the subsequent AMBR, so that 97.7% of the NH4+-N was removed at a maximum nitrogen loading rate of 0.18 kg NH4+-N/m3.day.
Calcium in the leachate was found to precipitate in the AMBR because of the higher pH. A sample taken from the membrane consisted most likely of pure hydroxylapatite Ca5(PO4)3(OH) which had a needle shape, whereas the background of the precipitate consisted more of nodules of calcium carbonate with traces of manganese, iron, magnesium, aluminium, sulphur and sodium.
The very high molecular weight (MW) aromatic organics in the leachate fed to the SAMBRs were almost fully degraded in the bulk of the SAMBRs. Moreover, their permeate was absolutely free of them which indicates a full rejection of these compounds by the membrane. The medium MW compounds in the range of 395 - 646 kDa were more likely to be rejected by the membrane while the MW ≤ 395 kDa were observed in the permeate. Regarding the evolution of the medium and low MW compounds over the 200 days, it can be stated that overall there was no build-up in the absorbance in any effluent.