Hydraulic characterization of an activated sludge reactor with recycling system by tracer experiment and analytical models

A new substrate equalization method for optimizing the influent conditions and fluid flow patterns of a multifed upflow anaerobic sludge blanket reactor with mature anammox granules

To improve nitrogen removal efficiency (NRE) and achieve homogenous distribution of anammox sludge and substrate, a new substrate equalization theory and a cumulative overload index was proposed for multifed upflow anaerobic sludge bed (MUASB) reactors with mature anammox granules. The performance and flow patterns of MUASB reactors were investigated under various influent conditions. The results showed that the nitrogen removal performance and stability of MUASB reactors could be optimized by minimizing the cumulative load. The NRE gradually increased from 83.3 ± 2.2 %, 86.8 ± 4.2 % to 89.3 ± 4.1 % and 89.7 ± 1.6 % in feeding flow tests and feeding port tests, respectively. Furthermore, the flow patterns were compared based on residence time distribution and computational fluid dynamics, indicating that a better equilibrium distribution of microorganisms and substrates could be achieved in the MUASB reactors under the lowest cumulative load. Therefore, substrate equalization theory can be used to optimize the nitrogen removal performance of MUASB reactors with low-carbon footprints.

Designing process and operational effect of modified septic tank for the pre-treatment of rural domestic sewage

Structure and performance of a newly designed four-chamber septic tank were investigated in current study. A three-layer orifice plate was fitted in the first compartment in an upward direction, and a baffle was arranged in the second compartment of the septic tank. The filter placed in the third and fourth compartment was used to collect water for field irrigation or connect subsequent devices for further processing. Flow field distribution in the first chamber was numerically simulated by Fluent software, and simulation results were verified by residence time distribution tracer experiment. The improved treatment effects of the modified septic tank with optimal parameters of 20 mm (diameter), 60 mm (gap) and 180 mm (distance) were validated by simulation experiments. High removal rates of chemical and biological oxygen demand reflected the effectiveness of modified septic tanks.

Enhanced estrogen removal in activated sludge processes through the optimization of the hydraulic flow pattern

The removal of β-estradiol (E2) and α-ethinylestradiol (EE2) in biological wastewater treatment plants (WWTP) would need to be improved in order to comply with prospective Environmental Quality Standards (EQS) of 0.4 and 0.035 ng.L^-1 respectively. The effluent concentration of a micropollutant in an activated sludge process is a function of the removal rate, the hydraulic retention time (HRT) and the flow pattern, which is usually overlooked. In order to better understand this aspect, we carried out tracer studies in eight WWTPs in the UK and found that relatively modest changes in aeration tanks would translate into tangible improvements in their flow pattern. We further evaluated the degradation rates for E1 (estrone), E2, E3 (estriol) and EE2 in each WWTP and we estimated that the modification of the flow pattern would be sufficient to place effluent concentrations of E2 (23.2 L∙g_VSS^-1∙d^-1<k_bio<210 L∙g_VSS^-1∙d^-1) far below the prospective EQS, while EE2 (0.3 L∙g_VSS^-1∙d^-1<k_bio<2.9 L∙g_VSS^-1∙d^-1) would have to rely on river dilution for compliance. Regarding E1 and E3, with no prospective legislation, the modifications would place E3 (9.9 L∙g_VSS^-1∙d^-1<k_bio<39.5 L∙g_VSS^-1∙d^-1) effluent concentrations easily below its predicted no-effect concentrations (PNEC = 60 ng.L^-1) while for E1, (2.6 L∙g_VSS^-1∙d^-1<k_bio<19.2 L∙g_VSS^-1∙d^-1) it would very much depend on the degradation rates of the specific WWTP (PNEC = 6 ng.L^-1). Improvement in flow pattern had the additional benefit of improving the effectiveness of other plausible changes in HRT or biological removal rates. Managing the flow pattern of existing WWTPs is a cost-efficient tool for managing the fate of estrogens.

Nitrification-denitrification of municipal wastewater without recirculation, using encapsulated microorganisms

In most municipal wastewater treatment plants, there is need for the removal of nitrogen, which usually takes place using the combined nitrification - denitrification process. Vigorous recirculation between the aeration and the anoxic tanks is enforced, to ensure complete denitrification. The scope of the present work was to investigate the possibility for nitrification-denitrification process in once-through systems (i.e.: without recirculation), without the need for the addition of extra carbon source (i.e. using the BOD in wastewater as carbon source), using encapsulated microorganisms. The primary aim was to increase the concentration of nitrifiers in the aerated reactor with parallel operation at hydraulic retention times (HRTs) below the doubling time of heterotrophic microorganisms, thus ensuring high ammonia oxidation rate and minimal reduction of organic carbon. The preserved organic carbon may be then used as carbon source at the downstream anoxic reactor. Coagulated and clarified wastewater from the effluent of the primary clarifier of a municipal wastewater treatment plant was used as feed to the system. The system comprised of two reactors (with 2 L working volume each) configured in series. The first one (aerated tank) contained encapsulated nitrifiers, while the second one (anoxic tank) contained encapsulated denitrifiers. The system operated at HRTs 8, 4 and 3 h (calculated individually for each reactor). The experiments indicated that at HRT equal to 8 h, almost all N-NH4+ was converted to N-NO3- in the aerated reactor, while the total nitrogen (TN) concentration was below 2 mg L-1 at the exit of the system. At HRT of 4 h a slight decrease in N-NH4+ removal was observed at the exit of the aerated tank (N-NH4+ concentration was measured 3.7 ± 0.1 mg L-1). At HRT equal to 4 h, N-NH4+ concentration did not change significantly during the downstream treatment in the anoxic tank, while N-NO3- concentration at the exit of the system was 1.4 ± 0.1 mg L-1. At the lowest HRT (3 h), N-NH4+ concentration was measured between 10 and 11 mg L-1, both, at the exit of the aeration tank and at the exit of the system; while, N-NO3- was measured 2.6 ± 0.2 mg L-1 at the exit of the system. On the other hand, BOD and TOC removal in the aeration tank decreased with the decrease of the HRT. BOD concentration at the exit of the anoxic tank measured 30.3 ± 2.2 and 19.4 ± 1.7 mg L-1 for HRTs 8 h or 4 h, respectively, while it was measured 51.6 ± 7.6 mg L-1 at HRT 3 h. On the other hand, TOC concentration at the outlet was measured 17.5 ± 1.2 and 13.2 ± 0.6 for HRTs 8 or 4 h, respectively, while it was measured 31.1 ± 5.6 mg L-1 at HRT equal to 3 h. Analysis of variance (ANOVA) showed significant variations of all measured parameters with the applied HRT, apart from N-NO3- concentration at the exit of the aerated tank. The latter was attributed to the complete oxidation oft N-NH4+ in the aerated tank at all HRTs. Based on the efficiency of the system, the volume of the aeration and denitrification tanks of a wastewater treatment plant using encapsulated microorganisms may be designed 16 times smaller, compared to conventional activated sludge plants, while the need for recirculation between the aerated and anoxic tanks may be completely eliminated.

CFD simulation of fluid dynamic and biokinetic processes within activated sludge reactors under intermittent aeration regime

Due to the aeration system, biological reactors are the most energy-consuming facilities of convectional WWTPs. Many biological reactors work under intermittent aeration regime; the optimization of the aeration process (air diffuser layout, air flow rate per diffuser, aeration length …) is necessary to ensure an efficient performance; satisfying the effluent requirements with the minimum energy consumption. This work develops a CFD modelling of an activated sludge reactor (ASR) which works under intermittent aeration regime. The model considers the fluid dynamic and biological processes within the ASR. The biological simulation, which is transient, takes into account the intermittent aeration regime. The CFD modelling is employed for the selection of the aeration system of an ASR. Two different aeration configurations are simulated. The model evaluates the aeration power consumption necessary to satisfy the effluent requirements. An improvement of 2.8% in terms of energy consumption is achieved by modifying the air diffuser layout. An analysis of the influence of the air flow rate per diffuser on the ASR performance is carried out. The results show a reduction of 14.5% in the energy consumption of the aeration system when the air flow rate per diffuser is reduced. The model provides an insight into the aeration inefficiencies produced within ASRs.