Measurement of Biomass in Small-Scale Microalgal and Microalgal–Bacterial Systems for Wastewater Treatment: Mini Review and Experimental Evaluation

Accurate biomass quantification is important for evaluating growth kinetics and performance of microalgal and microalgal–bacterial wastewater treatment systems. However, small-scale studies frequently encounter methodological limitations due to low biomass concentrations, limited sampling volumes, and/or interference from non-biotic solids in complex wastewaters. This work adopts a two-fold approach: (i) a concise review of current biomass quantification methods for bench-scale systems, and (ii) an experimental evaluation of a gravimetric protocol for complex wastewaters. The review discusses commonly applied techniques, highlights their strengths and weaknesses, and identifies research gaps in data comparability and reproducibility. The laboratory investigations evaluated the effects of key factors, namely culture volume (250 mL to 1 L), test aliquots (2.5 mL to 10 mL), and the absolute weight of total suspended solids (3.43 g to 14.5 g) on total suspended solids measurements. Aliquots containing <5 mg total suspended solids produced statistically significant variability, whereas reliable and reproducible results were obtained when >8–10 mg absolute total suspended solids per aliquot was present. In complex wastewater matrices, approximately 18% of total suspended solids consisted of non-volatile solids, demonstrating that the method can systematically over-estimate true dry cell weight in microalgal–bacterial systems. The findings emphasized the need for procedural standardization. Finally, a practical gravimetric protocol is proposed for both axenic and consortium-based small-scale studies dealing with complex wastewater, providing an evidence-based roadmap for obtaining more reliable biomass estimations.