A plant-wide energy model for wastewater treatment plants: application to anaerobic membrane bioreactor technology

Interface behavior and removal mechanisms of human pathogenic viruses in anaerobic membrane bioreactor (AnMBR)

Effective removal of human pathogenic viruses is an indispensable yet rarely studied aspect for sustainable treatment of domestic wastewater by anaerobic membrane bioreactor (AnMBR). In this study, the interface behaviors and removal mechanisms of norovirus genogroup I (GI), genogroup II (GII), and rotavirus A from domestic wastewater was systematically investigated in a one-stage AnMBR. On average, norovirus GI, GII and rotavirus were reduced by 4.64, 5.00, and 2.31 logs, respectively. Viruses tended to be transferred to larger-sized suspended solids from sewage influent to the mixed liquor, and the weight-specific concentration of the virus in >100 μm particles of the mixed liquor was significantly higher than that of sewage, indicating a particle scale-dependent affinity with the virus. In-series membrane filtration test showed the main contribution of the membrane retention, which was dominated by the bio-cake layer and the pristine membrane, while the membrane and associated pore foulants can retain viruses in a filtration resistance-efficient way. An unsteady-state mass balance model revealed that free viruses in the bulk liquid of AnMBR were minimally attached to the cake layer but mainly retained by the membrane and pore foulants (>99%). In addition, despite the small virus decay rates in the mixed liquor, the associated contribution increased with run time due to the prolonged sludge retention time. These insights into virus behaviors and removal mechanisms may provide novel regulation strategies for enhanced virus removal by AnMBR.

Mass and heat balances for biological nitrogen removal in an activated sludge process: to couple or not to couple?

Models adapt constantly, usually increasing the degree of detail describing physical phenomena. In water resource recovery facilities, models based on mass and/or heat balances have been used to describe and improve operation. While both mass and heat balances have proven their worth individually, the question arises to which extent their coupling, which entails increased model complexity, warrants the supposedly more precise simulation results. In order to answer this question, the need for and effects of coupling mass and heat balances in modelling studies were evaluated in this work for a biological nitrogen removal process treating highly concentrated wastewater. This evaluation consisted on assessing the effect of the coupling of mass and heat balances on the prediction of: (1) nitrogen removal efficiency; (2) temperature; (3) heat recovery. In general, mass balances are sufficient for evaluating nitrogen removal efficiency and effluent nitrogen concentrations. If one desires to evaluate the effect of temperature changes (e.g. daily, weekly, seasonally) on nitrogen removal efficiency, the use of temperature profiles as an input variable to a mass balance-based model is recommended over the coupling of mass and heat balances. In terms of temperature prediction, considering a constant biological heat generation term in the heat balance model provides sufficient information - i.e. without the coupling of mass and heat balances. Also, for evaluating the heat recovery potential of the system, constant biological heat generation values provide valuable information, at least under normal operating conditions, i.e. when the solids retention time is large enough to maintain nitrification.

Widening the applicability of AnMBR for urban wastewater treatment through PDMS membranes for dissolved methane capture: Effect of temperature and hydrodynamics

AnMBR technology is a promising alternative to achieve future energy-efficiency and environmental-friendly urban wastewater (UWW) treatment. However, the large amount of dissolved methane lost in the effluent represents a potential high environmental impact that hinder the feasibility of this technology for full-scale applications. The use of degassing membranes (DM) to capture the dissolved methane from AnMBR effluents can be considered as an interesting alternative to solve this problem although further research is required to assess the suitability of this emerging technology. The aim of this study was to assess the effect of operating temperature and hydrodynamics on the capture of dissolved methane from AnMBR effluents by DMs. To this aim, a commercial polydimethylsiloxane (PDMS) DM was coupled to an industrial prototype AnMBR (demonstration scale) treating UWW at ambient temperature. Different operating temperatures have been evaluated: 11, 18, 24 and 30 °C. Moreover, the DM was operated at different ratios of liquid flow rate to membrane area (Q_L:A) ranging from 22 to 190 Lh^-1m^-2 in order to study the resistance of the system to methane permeation. Methane recovery was maximized when temperature raised and Q_L:A was reduced, giving methane recovery efficiencies (MRE) of about 85% at a temperature of 30 °C and a Q_L:A of 25 Lh^-1m^-2. The study showed that high Q_L:A ratios hinder methane recovery by the perturbation of the DM fibers, being this effect intensified at lower temperatures probably due the higher liquid viscosities. Also, the performed fouling evaluation showed that not significant membrane fouling may be expected in the DM unit at the short-term when treating AnMBR effluents. A resistance-in-series model was proposed to predict the overall mass transfer of the system according to operating temperature and Q_L:A, showing that methane capture was controlled by the liquid phase, which represented up to 80-90% of total mass transfer resistance. The energy and environmental evaluation performed in this study revealed that PDMS DMs would enhance energy recovery and environmental feasibility of AnMBR technology for UWW treatment, especially when operating at low temperatures. When MRE was maximized, the combination of AnMBR with DM achieved net energy productions and net greenhouse gas reductions of up to 0.87 kWh and 0.216 kg CO₂-eq per m³ of treated water.

Sludge yield and degradation of suspended solids by a large pilot-scale anaerobic membrane bioreactor for the treatment of real municipal wastewater at 25 °C

Sludge yield and suspended solid are important factors concerned in the anaerobic treatment of municipal wastewater. In this study, a large pilot-scale anaerobic membrane bioreactor (AnMBR) was constructed for effectively treating real municipal wastewater at an ambient temperature of 25 °C. The sludge yield and the degradation of influent suspended solids were evaluated during the long-term operation of the AnMBR. This reactor with 5.0 m³ effective volume is the largest one-stage submerged AnMBR that has ever been used to treat municipal wastewater. During the long-term operation of 217 days, this AnMBR obtained excellent COD and BOD₅ removal efficiency over 90%. Stable biogas production was also successfully obtained from treating municipal wastewater. The sludge yield of the AnMBR was approximately 0.19-0.26 g MLSS g^-1 COD removed for the treatment of real municipal wastewater. The shortest SRT of the AnMBR was calculated as 29 days for an HRT of 6 h at an empirical MLSS of 10 g L^-1. While the influent suspended solid (SS) contained in the municipal wastewater was completely removed by the AnMBR, only 57%-66% of the influent SS was degraded. The rest of influent SS was directly converted to MLSS instead of being degraded. The AnMBR maintained a stable membrane filtration using a hollow-fiber membrane with a total area of 72 m², realizing a flux of 2.75-17.83 LMH, and the mean transmembrane pressure (TMP) was 0.9-23.5 kPa. An online chemical backwash cleaning system helped to lower the TMP timely using sodium hypochlorite and citric acid when the TMP increased rapidly and reached the rated limit of membrane. This is the first report on demonstrating the successful operation and detailed performance of a large pilot-scale AnMBR applied to the treatment of real municipal wastewater.

Plant-wide modelling in wastewater treatment: showcasing experiences using the Biological Nutrient Removal Model

Plant-wide modelling can be considered an appropriate approach to represent the current complexity in water resource recovery facilities, reproducing all known phenomena in the different process units. Nonetheless, novel processes and new treatment schemes are still being developed and need to be fully incorporated in these models. This work presents a short chronological overview of some of the most relevant plant-wide models for wastewater treatment, as well as the authors' experience in plant-wide modelling using the general model BNRM (Biological Nutrient Removal Model), illustrating the key role of general models (also known as supermodels) in the field of wastewater treatment, both for engineering and research.

Performance of a membrane-coupled high-rate algal pond for urban wastewater treatment at demonstration scale

The objective of this study was to evaluate the performance of an outdoor membrane-coupled high-rate algal pond equipped with industrial-scale membranes for treating urban wastewater. Decoupling biomass retention time (BRT) and hydraulic retention time (HRT) by membrane filtration resulted in improved process efficiencies, with higher biomass productivities and nutrient removal rates when operating at low HRTs. At 6 days of BRT, biomass productivity increased from 30 to 66 and to 95 g·m^-3·d^-1 when operating at HRTs of 6, 4 and 2.5 days, respectively. The corresponding nitrogen removal rates were 4, 8 and 11 g N·m^-3·d^-1 and the phosphorous removal rates were 0.5, 1.3 and 1.6 g P·m^-3·d^-1. The system was operated keeping moderate specific air demands (0.25 m³·m^-2·h^-1), resulting in reasonable operating and maintenance costs (€0.04 per m³) and energy requirements (0.29 kWh per m³). The produced water was free of pathogens and could be directly used for reusing purposes.

Process schemes for future energy-positive water resource recovery facilities

There are numerous successful studies on optimizing the performance of conventional activated sludge (CAS)-based wastewater treatment plants. However, recent studies have shown that a more significant improvement of the plant performance is achievable through integration of established technologies in novel process schemes. High-rate activated sludge system, chemically enhanced primary treatment, partial nitritation-anammox, partial nitrification-denitrification over nitrite and anaerobic digestion are integrated in two process schemes to determine to which extent energy savings and energy production can be achieved with these new process layouts compared to a CAS-based process scheme. The results presented in this paper show that there is potential for achieving future energy-positive water resource recovery facilities through novel integration of mature technologies for municipal wastewater treatment.

A review on anaerobic membrane bioreactors (AnMBRs) focused on modelling and control aspects

The use of anaerobic membrane bioreactor technology (AnMBR) is rapidly expanding. However, depending on the application, AnMBR design and operation is not fully mature, and needs further research to optimize process efficiency and enhance applicability. This paper reviews state-of-the-art of AnMBR focusing on modelling and control aspects. Quantitative environmental and economic evaluation has demonstrated substantial advantages in application of AnMBR to domestic wastewater treatment, but detailed modelling is less mature. While anaerobic process modelling is generally mature, more work is needed on integrated models which include coupling between membrane performance (including fouling) and the biological process. This should include microbial factors, which are important to generation of specific foulants such as soluble and particulate inert organics. Mature and well-established control tools, including better feedback control strategies are also required for both the process, and for fouling control.

Current status of the pilot-scale anaerobic membrane bioreactor treatments of domestic wastewaters: A critical review

This review presented the performances of the pilot-scale anaerobic membrane bioreactors (AnMBRs) treating domestic wastewater. High COD removal efficiencies and low biosolids productions were achieved at HRTs comparable to conventional aerobic processes under ambient temperatures. The energy demands for fouling control in the pilot-scale AnMBRs ranged from 0.04 to 1.35kWh/m³, which is lower than those of lab-scale AnMBRs and aerobic MBRs. The energy demands for fouling control were in the order of gas sparging>particle sparging>rotating membrane AnMBR. Two major factors affecting the energy demand in gas sparging AnMBRs were specific gas demands (SGD_m) and operating flux. The energy potentials in wastewater were significantly affected by the influent sulfate concentrations. Energy balances indicated that five out of nine pilot-scale AnMBRs was energy positive. However, further improvements of the AnMBRs are required to implement the energy positive wastewater treatment process.

Economic and environmental sustainability of an AnMBR treating urban wastewater and organic fraction of municipal solid waste

The objective of this study was to evaluate the economic and environmental sustainability of a submerged anaerobic membrane bioreactor (AnMBR) treating urban wastewater (UWW) and organic fraction of municipal solid waste (OFMSW) at ambient temperature in mild/hot climates. To this aim, power requirements, energy recovery from methane (biogas methane and methane dissolved in the effluent), consumption of reagents for membrane cleaning, and sludge handling (polyelectrolyte and energy consumption) and disposal (farmland, landfilling and incineration) were evaluated within different operating scenarios. Results showed that, for the operating conditions considered in this study, AnMBR technology is likely to be a net energy producer, resulting in considerable cost savings (up to €0.023 per m(3) of treated water) when treating low-sulphate influent. Life cycle analysis (LCA) results revealed that operating at high sludge retention times (70 days) and treating UWW jointly with OFMSW enhances the overall environmental performance of AnMBR technology.