Flux and performance improvement in a submerged anaerobic membrane bioreactor (SAMBR) using powdered activated carbon (PAC)

Abstract

A three litre submerged anaerobic membrane bioreactor (SAMBR) was used to examine the effect of powdered activated carbon (PAC) on COD removal and flux. 98% COD removal efficiency was achieved at a high loading rate (16 gCOD l −1 d −1) and a low hydraulic retention time (HRT) of 6 h in the presence of 3.4 g l −1 of PAC. PAC addition improved start-up and performance during a hydraulic shock by buffering the volatile fatty acids (VFAs), however, a large amount of biomass was important in accommodating the higher loading rate in the SAMBR. Batch assays showed that PAC addition improved methane potential both in terms of amount and inoculation time. COD retention inside the SAMBR for three experiments was proportional to the soluble microbial products (SMPs) contribution to COD, as (VFAs) passed through the membrane at high sparging rates (5 litres per minute—LPM). Size exclusion chromatography showed that the membrane acted as an ultra-filter (<30 kDa) as the cake or gel layer significantly reduced the amount of organics passing through the membrane. PAC adsorbed slowly biodegradable low and high MW residual COD from the bulk liquid, and hence improved COD removal and flux. Particle size distributions demonstrated that PAC also adsorbs fine colloids, and flux data showed that only a thin biofilm was formed due to the high backtransport velocity of PAC. The combined effects of adsorption of fine colloids and dissolved organics, and the formation of a thin cake layer resulted in significant flux improvement from 2 to 9 litres per square metre per hour (LMH) in the presence of 1.67 g l −1 PAC. However, addition of 3.4 g l −1 PAC reduced the flux to 5 LMH by the combined effect of viscosity and its inability to completely adsorb the dissolved organics and fine colloids that resulted in high internal fouling. Under the given operating conditions, addition of 1.67 g l −1 PAC appears to be the optimum quantity in terms of cost, flux and soluble COD removal.