Simultaneous in-situ sludge reduction and nutrient removal in an A(2)MO-M system: Performances, mechanisms, and modeling with an extended ASM2d model.

Among the existing in-situ sludge reduction processes, the oxic-settling-anaerobic (OSA) process is of particular interest because it has shown significant sludge reduction with several advantages. However, an ideal process for practical application must simultaneously incorporate effluent quality with sludge reduction. In this study, an improved OSA system, the stage-aerated anaerobic, anoxic, micro-aerobic, and oxic system combining a micro-aerobic starvation tank (abbreviated as A(2)MO-M system) was developed. Compared with OSA3# (hydraulic retention time (HRT) of 12 h), the A(2)MO-M2# system with optimized HRT of 9 h yielded almost 16.3% less sludge. The average total nitrogen (87.3%) and total phosphorus (91.9%) removal efficiencies in A(2)MO-M2# were 20.6 and 42.2% higher than those in OSA3#. Investigation of the mechanisms of sludge reduction revealed that, except for the main factors of energy uncoupling metabolism (16.7%) and sludge decay (21.2%), enrichment of slow-growing bacteria and lysis-cryptic growth metabolism analyzed by high-throughput 454 pyrosequencing were shown to contribute to sludge reduction in the A(2)MO-M system. On the basis of effluent organic matters (EfOM) measurements, soluble microbial products (SMP) were the major components in EfOM; and different reduction-oxidation (redox) potentials controlled in the OSA and A(2)MO-M systems led to different SMP formation mechanisms. To explore the mechanism and kinetics of SMP formation under different redox potentials, three new components (SUAP, SBAP, and XEPS) were integrated in an extended ASM2d model. Experimental and modeling results revealed that biomass-associated products (BAP) supported a substantial population of SMP that were quite sensitive to different redox potentials. The extended ASM2d model further illustrated that more BAP produced in the alternating anaerobic and aerobic conditions in the OSA system adversely affected its effluent quality.