Membrane bioreactor operation at short solids retention times: performance and biomass characteristics

• Published in: Water research (Oxford) Vol. 39; no. 6; pp. 981 - 992
• Main Authors: Ng, How Y., Hermanowicz, Slawomir W.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.03.2005; Elsevier Science
• Subjects: activated sludge; Applied sciences; Biological and medical sciences; Biological treatment of waters; Biomass; Bioreactors; Biotechnology; Carbohydrates - analysis; Completely mixed activated sludge; Environment and pollution; equipment design; Exact sciences and technology; Exocellular polymeric substances; Flocculation; Fundamental and applied biological sciences. Psychology; General purification processes; Industrial applications and implications. Economical aspects; Membrane bioreactor; Membranes, Artificial; microbial biomass; microorganisms; Nitrification; Nitrites - chemistry; Nitrites - metabolism; Non-flocculating microorganisms; Pollution; Proteins - analysis; Sewage - chemistry; Sewage - microbiology; Short solids retention times; Time Factors; Waste Disposal, Fluid - methods; wastewater treatment; Wastewaters; Water Purification - methods; Water treatment and pollution; Membrane bioreactor; Short solids retention times; Completely mixed activated sludge; Nitrification; Non-flocculating microorganisms; Exocellular polymeric substances; Activated sludge; Mixed microorganism culture; Retention time; Microbial biomass; Extracellular; Mass transfer; Biological purification; Membrane reactor; Bioreactor; Biopolymer; Waste water purification; Performance
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2004.12.014

This study investigated the performance and biomass characteristics of a membrane bioreactor (MBR) and a completely mixed activated sludge (CMAS) system operated at short solids retention times (SRT) ranging from 0.25 to 5 d and hydraulic retention times of 3 and 6 h. The lab-scale reactors were fed with synthetic wastewater to ensure consistency in feed composition. Results show the MBR was capable of achieving excellent quality effluent regardless of the extremely short SRT. The MBR removal efficiencies ranged from approximately 97.3–98.4% (TCOD) in the MBR, compared to 77.5–93.8% (TCOD) and 94.1–97.0% (SCOD) in the CMAS. Nitrification completely ceased when SRT was<2.5 d. The MBR biomass was composed of small, weak and uniform-sized flocs with large mass of short filamentous organisms and mainly dispersed microorganisms at SRT of 5 and 0.25 d, respectively. In contrast, the CMAS sludge was composed of large flocs with filamentous organisms as a backbone at SRT >2.5 d. The CMAS flocs were smaller and weaker at shorter SRT. The MBR sludge contained a much higher fraction of non-flocculating microorganisms. This fraction increased significantly with decreasing SRT. It was found that the concentrations of protein and carbohydrates in the exocellular polymeric substances for both the MBR and the CMAS decreased with increasing F/M ratio or decreasing SRT. The combination of increasing amounts of non-flocculating microorganisms and a reduction of EPS at shorter SRT in both reactors contributed to deteriorating sludge settling properties. A significant presence of dispersed biomass and small flocs in MBR contributed to better reactor performance probably due to less mass transfer resistance.