Comparison Performances of Membrane Bioreactor and Conventional Activated Sludge Processes on Sludge Reduction Induced by Oligochaete

Abstract

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.

Sludge reduction potential of the activated sludge process by integrating an oligochaete reactor

“…The average free-swimming worm densities in both the IODVC and the worm reactor were 72 ± 76 and 82 ± 99 no./mgVSS, respectively, and their maximal worm densities occurred on day 58 reaching 211 and 309 no./mgVSS, respectively. The size of A. hemprichi was among 0.5-2 mm, the same as results of literatures [12,15,19]. Compared with previous researches in membrane bioreactor and conventional activated sludge process [12,15,19], the amounts of free-swimming worms in this study as shown in Table 2 were lower than their results, but the free- swimming worm densities, including average and maximal values, were higher than their results.…”

“…During energy transfer from bacteria to higher trophic level organisms, energy is lost due to inefficient biomass conversion and thus the predator may make a large contribution to biomass reduction [2,3]. Recently, many studies have focused on sludge reduction induced by protozoa and metazoa grazing on bacteria [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. It is well known that the presence of protozoa and metazoa in aerobic wastewater treatment processes plays an important role in keeping effluent clear by consuming dispersed bacteria and settled solids.…”

“…Worms are the largest organisms observed during the microscopic investigation of activated sludge [22]. The perfor-mance of oligochaetes for sludge reduction in biological wastewater treatment has been paid more attention than that of protozoa for sludge reduction [2,3,[8][9][10]12,13,[15][16][17][18][19][20][21]. Different worms were found in these observations such as Nais elinguis, Pristina sp.…”

“…Different worms were found in these observations such as Nais elinguis, Pristina sp. and Aeolosoma hemprichi, but either Nais or Aeolosoma was predominant [3,12,13,15,19]. Although the presence of worms in the aerobic wastewater treatment may lead to substantial sludge reduction, the practical application of worms for sludge reduction is still uncontrollable in biological wastewater treatment processes because of unstable worm growth, e.g.…”

“…alternative bloom and absence of worms, and high variation of worm density. In these mentioned researches [2,3,[8][9][10]12,13,15,19], worms grew together with bacteria in the same bioreactor. But bioreactors such as aeration tanks in activated sludge process were designed for bacterial growth rather than for worm growth.…”