Minimization of excess sludge production for biological wastewater treatment.

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.

Sewage treatment by a low energy membrane bioreactor.

A new membrane bioreactor (MBR) was developed for treatment of municipal wastewater. The MBR was mainly made up of an activated sludge reactor and a transverse flow membrane module, with an innovative configuration being in application between them. As a result, the transverse flow membrane module and low recirculation flow rate created advantages, such as lower energy consumption and more resistance to membrane fouling. The total energy consumption in the whole system was tested as 1.97+/-0.74 kWh/m(3) (permeate) while using periodical backwash with treated water and backflush with mixed liquor daily, being in the same level as a submerged membrane bioreactor, reported to be 2.4 kWh/m(3) (permeate). Energy consumption analysis in the system shows that the membrane module was more energy consuming than the other four parts listed as pump, aeration, pipe system and return sludge velocity lose, which consumed 37.66-52.20% of the total energy. The effluent from this system could be considered as qualified for greywater reuse in China, showing its potential application in the future.

Comparison performances of membrane bioreactor and conventional activated sludge prcesses on sludge reduction induced by Oligochaete.

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.

Determination and discussion hydraulic retention time in membrane bioreactor system.

Based on the microorganism kinetic model, the formula for computing hydraulic retention time in a membrane bioreactor system (MBR) is derived. With considering HRT as an evaluation index a combinational approach was used to discuss factors which have an effect on MBR. As a result, the influencing factors were listed in order from strength to weakness as: maximum specific removal rate K, saturation constant Ks, maintenance coefficient m, maximum specific growth rate mu m and observed yield coefficient Yobs. Moreover, the formula was simplified, whose parameters were experimentally determined in petrochemical wastewater treatment. The simplified formula is theta = 1.1(1/beta-1) (Ks + S)/KX0, for petrochemical wastewater treatment K and Ks equaled 0.185 and 154.2, respectively.