Microbial Composition and Structure of Aerobic Granular Sewage Biofilms

Abstract

Aerobic activated sludge granules are dense, spherical biofilms which can strongly improve purification efficiency and sludge settling in wastewater treatment processes. In this study, the structure and development of different granule types were analyzed. Biofilm samples originated from lab-scale sequencing batch reactors which were operated with malthouse, brewery, and artificial wastewater. Scanning electron microscopy, light microscopy, and confocal laser scanning microscopy together with fluorescence in situ hybridization (FISH) allowed insights into the structure of these biofilms. Microscopic observation revealed that granules consist of bacteria, extracellular polymeric substances (EPS), protozoa and, in some cases, fungi. The biofilm development, starting from an activated sludge floc up to a mature granule, follows three phases. During phase 1, stalked ciliated protozoa of the subclass Peritrichia, e.g., Epistylis spp., settle on activated sludge flocs and build tree-like colonies. The stalks are subsequently colonized by bacteria. During phase 2, the ciliates become completely overgrown by bacteria and die. Thereby, the cellular remnants of ciliates act like a backbone for granule formation. During phase 3, smooth, compact granules are formed which serve as a new substratum for unstalked ciliate swarmers settling on granule surfaces. These mature granules comprise a dense core zone containing bacterial cells and EPS and a loosely structured fringe zone consisting of either ciliates and bacteria or fungi and bacteria. Since granules can grow to a size of up to several millimeters in diameter, we developed and applied a modified FISH protocol for the study of cryosectioned biofilms. This protocol allows the simultaneous detection of bacteria, ciliates, and fungi in and on granules.During the last 20 years, intensive research in the field of biological wastewater treatment and other applications has demonstrated that biofilms are often more efficient for water purification than suspended activated sludge. Today, the application of anaerobic and aerobic granular sludge in wastewater treatment is regarded as one of the most useful and promising biotechnologies. Granular sludge was described first for strictly anaerobic systems (26). In the late 1990s, the formation and application of aerobic granules was reported (30). Such granules are spherical compact aggregates of microorganisms, mainly bacteria, and extracellular polymeric substances (EPS). Granules can be described as "biofilm in suspension" and are considered to be a special case of biofilm formation, composed of self-immobilized cells (9,16,20). The application of granules for wastewater treatment shows many advantages. An outstanding feature is the excellent settleability (high settling velocity), which is a prerequisite to handle high liquid flows. Moreover, granular sludge provides high and stable rates of metabolism, resilience to shocks and toxins due to protection by a matrix of EPS (40), long biomass residence times, biomass immobilization i...

Biomass granulation in an aerobic:anaerobic-enhanced biological phosphorus removal process in a sequencing batch reactor with varying pH

“…For example, the observations of Weber et al [46], suggesting that protozoa and fungi were crucial initially in providing the matrix for bacterial growth and EPS production in their granules, are quite diVerent from the situation with those examined here. Consequently, it may not be possible to sustain a uniWed mechanistic theory for granulation, although its common feature is that it seems to occur mainly in SBR systems [34].…”

Filamentous bacteria existence in aerobic granular reactors

“…These bacteria are capable to store sulphur granules and PHA; and have rapid nutrients uptake rates under nutrient deficiency. The outgrowth of these bacteria in aerobic granules cultivated with brewery wastewater has been previously reported [38] and they were thought to be responsible of the bulking and settling problems, probably caused by the high applied OLR (3.6 kg COD s /(m 3 d)), the dissolved oxygen deficiency or the high temperatures of operation reported in that research. In the present research the presence of Thiothrix was observed in the reactor treating fish canning effluent with large salts content.…”

“…Recent studies indicate that fungal hyphae also act like skeletal elements in the structural formation process [29,38]. Weber et al [38] observed that filamentous fungal pellets dominated the sludge population in the first 10 to 15 days and they served to initiate the granular biofilm structure as an immobilizing matrix for bacteria. These authors checked that these fungi were still present in mature granules, where they supported the development and maintenance of the granular structure.…”

Sequencing Batch Reactor and Bacterial Community in Aerobic Granular Sludge for Wastewater Treatment of Noodle-Manufacturing Sector

“…Granules creating aerobic granular activated sludge are considered as aggregates of microorganisms, mainly bacteria, and extracellular polymeric substances [4]. Aerobic granular sludge also functions as a biofilm in suspension, which is composed of combining cells [5]. The application of granules for wastewater treatment has many advantages.…”

“…They settle significantly faster than activated sludge flocs (high settling velocity). In addition, granular sludge provides high and stable rates of metabolism [5].…”