Influence of hydraulic retention time on fouling in a UASB coupled with an external ultrafiltration membrane treating synthetic municipal wastewater

This paper evaluates the effect of hydraulic retention time (HRT)1 on anaerobic membrane bioreactor (AnMBR)2 performance, biopolymeric substances production and fouling; also, struvite precipitation is analyzed based on solubility product equation. A lab-scale UASB reactor coupled with an ultrafiltration (UF)3 membrane was operated under three different HRTs (4, 8 and 12 h) and fed with synthetic municipal wastewater in two experimental blocks: in the first one, phosphate was supplemented to the influent and in the second one, no phosphate was added. Results show that biopolymeric substances concentration in UASB effluent increased with HRT reduction, probably due to the higher shear forces occurring inside the UASB reactor at lower HRTs, as well as higher upflow velocities and washout conditions. Consequently, chemical oxygen demand (COD)4 removal decreased slightly at HRT of 4 h (81% vs. 89%). Increase in specific fouling cake resistance and decrease in particle size were also a consequence of HRT reduction. These results indicate that AnMBR (UASB-UF) operation at HRT lower than 4 h has an adverse effect on its performance and fouling. Though struvite components were found in UASB effluent, precipitation analysis showed that they were undersaturated, so it is presumed that struvite precipitation did not occur.

Research highlights
► HRT reduction has a detrimental effect on AnBRM performance and fouling.
► HRT reduction caused an increase in EPS, SMP, SMPc, EPSc and COD in UASB effluent.
► HRT reduction promoted the increase in specific fouling cake resistance.
► HRT reduction and pumping encouraged particle size decrease.
► Thus, it is recommended to avoid operating AnMBR at HRTs lower than 4 h.