Optimal biogas sparging strategy, and the correlation between sludge and fouling layer properties in a submerged anaerobic membrane bioreactor (SAnMBR)

Biogas sparging is considered to be an effective way to control membrane fouling in an anaerobic membrane bioreactor, but is primarily used with a constant gas flow. This paper examined whether different gassing strategies could lead to better fouling control, and tried to understand the correlation between sludge properties and the fouling layer by disaggregating them into supracolloidal (1-5 µm), colloidal (0.45 µm-1 µm) and solute (<0.45 µm) fractions. This study found that the “critical constant sparging rate” was 6 L per minute (LPM) at a flux of 26 L per square meter per hour (LMH). However, a sparging strategy of 3 LPM for 30 s alternating with 5 LPM for 30 s (Alter 3/5) reduced membrane fouling by 150% compared with a constant 4 LPM strategy. The viscosity, colloid and floc sizes of the suspended sludge increased with decreasing sparging rate, while the concentration of supracolloidal particles, soluble microbial products (SMPs) proteins and carbohydrates, and extracellular polymeric substances (EPS) decreased with decreasing sparging rate. Colloids were the major foulants above 2 LPM, while HPLC-SEC analysis showed that one main SMP fraction was above 1522 kDa, and the other between 72 and 500 kDa. The high MW compounds accumulated on the membrane, while the later permeated through into the effluent. With the Alt 3/5 strategy, SMPs were the primary foulants, and the particle sizes were a lot smaller than in the supernatant. The foulant SMPs not only originated from the reactor supernatant, but were also produced by biomass in the membrane fouling layer. Chemical analyses reflected the effect of different sparging rates on sludge properties, the production of colloids, SMPs and EPS, and the foulant components.