Excess sludge treatment and disposal currently represents a rising challenge for wastewater treatment plants (WWTPs) due to economic, environmental and regulation factors. There is therefore considerable impetus to explore and develop strategies and technologies for reducing excess sludge production in biological wastewater treatment processes. This paper reviews current strategies for reducing sludge production based on these mechanisms: lysis-cryptic growth, uncoupling metabolism, maintenance metabolism, and predation on bacteria. The strategies for sludge reduction should be evaluated and chosen for practical application using costs analysis and assessment of environmental impact. High costs still limit technologies of sludge ozonation-cryptic growth and membrane bioreactor from spreading application in full-scale WWTPs. Bioacclimation and harmful to environment are major bottlenecks for chemical uncoupler in practical application. Sludge reduction induced by oligochaetes may present a cost-effective way for WWTPs if unstable worm growth is solved. Employing any strategy for reducing sludge production may have an impact on microbial community in biological wastewater treatment processes. This impact may influence the sludge characteristics and the quality of effluent. r

Filamentous Chloroflexi species are often present in activated sludge wastewater treatment plants in relatively low numbers, although bulking incidences caused by Chloroflexi filaments have been observed. A new species-specific gene probe for FISH was designed and using phylum-, subdivision-, morphotype 1851- and species-specific gene probes, the abundance of Chloroflexi filaments were monitored in samples from 126 industrial wastewater treatment plants from five European countries. Chloroflexi filaments were present in 50% of the samples, although in low quantities. In most treatment plants the filaments could only be identified with phylum or subdivision probes, indicating the presence of great undescribed biodiversity. The ecophysiology of various Chloroflexi filaments was investigated by a suite of in situ methods. The experiments revealed that Chloroflexi constituted a specialized group of filamentous bacteria only active under aerobic conditions consuming primarily carbohydrates. Many exo-enzymes were excreted, e.g. chitinase, glucuronidase and galactosidase, suggesting growth on complex polysaccharides. The surface of Chloroflexi filaments appeared to be hydrophilic compared to other filaments present. These results are generally supported by physiological studies of two new isolates. Based on the results obtained in this study, the potential role of filamentous Chloroflexi species in activated sludge is discussed.

Pilot-scale experiments were carried out to compare sludge reduction induced by Oligochaete in a submerged membrane bioreactor (MBR) and a conventional activated sludge (CAS) reactor for 345 d. Worm growth in the CAS reactor was much better than in the MBR. The average worm density of the aeration tank in the CAS reactor was 71 total worms/mg of volatile suspended solids (VSS), much higher than that in the MBR (10 total worms/mg of VSS). Worms did not naturally produce in the MBR, and the dominant worm type in the MBR depended on sludge inoculation from the CAS reactor. Only two types of worms were found in the MBR, Aeolosoma hemprichicii and Nais elinguis. Worm presence and disappearance in the MBR alternated. Worms in the CAS reactor occurred nearly throughout the operating period and were continuously maintained at over 30 total worms/mg of VSS in the aeration tank for 172 d. Three types of worm were found in the CAS reactor, A. hemprichicii, Pristina aequiseta, and N. elinguis, but P. aequiseta was present only occasionally. The alternating dominance of worm types in both reactors changed between Aeolosoma and Nais, and the time of Aeolosoma dominance was longer than that of Nais dominance. Worm growth in the MBR contributed to neither sludge reduction nor improvement of sludge settling characteristics because of low density. But worm presence and bloom in the CAS reactor greatly decreased sludge yield and improved sludge settling characteristics at high density. Both the average sludge yield (0.17 kg of suspended solids (SS)/kg of chemical oxygen demand removed (CODremoved)) and sludge volume index (60 mL/g) in the CAS reactor were much lower than those in the MBR (0.40 kg of SS/kg of CODremoved and 133 mL/g). Nais had more potential for sludge reduction than Aeolosoma. Worm growth had little impact on effluent quality in the MBR but affected effluent quality very much in the CAS reactor.

Complete biodegradation of azo dyes requires an anaerobic and aerobic step, in the anaerobic step sulfonated azo dyes (SADs) are reduced, yielding (sulfonated) aromatic amines ((S)AAs) which can be degraded aerobically. The complete biodegradation of the SAD Mordant Yellow 10 (MY10) was studied in a sequential anaerobic and aerobic bioreactor. Anaerobically, MY10 was reductively cleaved and the resulting aromatic amines, 5-aminosalicylic acid (5-ASA) and sulfanilic acid (SA), were both recovered in high stoichiometric yields. One of the AAs, 5-ASA, was readily degraded under aerobic conditions. However, SA was not degraded aerobically in the continuous experiment because no SA-degrading bacterial activity was present in the system. Therefore, a SA-degrading enrichment culture derived from Rhine sediment was used as an inoculum source. This enrichment culture was bioaugmented into the aerobic reactor by increasing the hydraulic retention time (HRT), thus enabling SA-degrading activity to develop and maintain in the aerobic reactor. After decreasing the HRT, the SA-degrading activity remained in the bioreactor and the stoichiometric recovery of sulfate (a SA biodegradation product) indicated the mineralization of SA after bioaugmentation. Batch experiments with aerobic reactor sludge confirmed the biodegradation of SA and 5-ASA. The sequential anaerobic and aerobic bioreactor was able to completely remove the sulfonated azo dye MY10 at a maximum loading rate of 210 mg MY10 (lreactor d)-1 after the appropriate microorganisms for aerobic degradation of SA were bioaugmented into the aerobic bioreactor.

Filamentous members of the Bacteroidetes are commonly observed in activated sludge samples originating from both municipal and industrial wastewater treatment plants (WWTP), where they occasionally can cause bulking. Several oligonucleotide 16S rRNA-targeted probes were designed to target filaments with a needle-like appearance similar to Haliscomenobacter hydrossis. The design of these probes was based on an isolate and a sequence obtained from a micromanipulated filament. The abundance of filamentous Bacteroidetes was determined in 126 industrial samples applying already published and the newly developed probes. Small populations were found in 62 % of the WWTP investigated. However, only relatively few WWTP (13 %) contained large populations of filamentous Bacteroidetes potentially responsible for bulking incidences. The identity of the most abundant filamentous Bacteroidetes with H. hydrossis morphology could be detected by probes CFB719, SAP-309 and the newly designed probe HHY-654. A comprehensive study on the ecophysiology of probe-defined Bacteroidetes populations was conducted on Danish and Czech samples. The studies revealed that they were specialized bacteria involved in degradation of sugars, e.g. glucose and N-acetylglucosamine, and may participate in the conversion of lipopolysaccharides and peptidoglycan liberated by decaying cells. Many surface-associated exo-enzymes were excreted, e.g. chitinase, glucuronidase, esterase and phosphatase, supporting conversion of polysaccharides and possibly other released cell components. The role of filamentous bacteria with a H. hydrossis-like morphology in the activated sludge ecosystem is discussed.