Improved measurement methods of mixed liquor suspended solids from aerobic granular sludge

Total and volatile mixed liquor suspended solids concentrations (MLSS and MLVSS) are key parameters in activated sludge (AS) process design, monitoring, and modelling. Yet for the emerging AS version of aerobic granular sludge (AGS), representative sampling and pipetting of the granular mixed liquors are challenging, leading to uncertainties when measuring the suspended solids by standard methods. In this study, new MLSS methods based on correct sampling principles were developed and evaluated to determine the suspended solids (SS) of AGS reactors. Method-1 used as a reference consisted of sacrificing the whole mixed liquor (ML) of each reactor as an exhaustive sample that was all ground to determine the total solids (TS) of the slurry; this allowed for accurate estimates of the MLSS and its volatile fractions (ivt_bio), which, in turn, led to the MLVSS. In parallel, Methods-2 and -4 were tested, both based on small homogeneous subsamples of ML, which were ground or drained before measuring the total solids of the slurry or of the drained granule paste. Compared to the reference, these latter methods allowed the MLSS and ivt_bio of AGS with large granules to be determined with much greater accuracy than the current standard Method-3 (direct SS measurement). The study's good sampling practices are readily applicable to laboratory reactors and would henceforth improve solids measurement in AGS research. At large plants, the developed procedures of subsampling, granule grinding or draining, and MLSS calculation formulas would reduce sampling errors and enhance process monitoring.