Optimizing hydraulic retention time of high-rate activated sludge designed for potential integration with partial nitritation/anammox in municipal wastewater treatment

The integration of high-rate activated sludge (HRAS), an effective carbon redirection technology, with partial nitritation/anammox (PN/A) is a novel AB treatment process for municipal wastewater. In this study, an airlift HRAS reactor was operated in the continuous inflow mode for 200 d at a wastewater treatment plant. The balance between potential PN/A system stability and peak HRAS performance under decreasing hydraulic retention time (HRT) was optimized. Energy consumption and recovery and CO2 emissions were calculated. The results showed that the optimal HRT suitable with the PN/A process was 3 h, achieving 2-3 g/L mixed liquor volatile suspended solid, 67.8 % chemical oxygen demand (COD) recovery, 81 % total COD removal efficiency, 2.27 ± 1.03 g COD/L/d organic loading rate, 62 % aeration reduction, and 0.24 kWh/m3 power recovery potential. Such findings hold practical value and contribute to the development of the optimal AB process capable of achieving energy autonomy and carbon neutrality.

A method for determining the optimal number and location of biomass energy facilities

The site selection of biomass energy facilities has always been a key part of energy spatial planning. The site suitability evaluation criteria of the existing studies are not comprehensive. On the other hand, most of the existing studies are to determine the only site, while less research on the multiple-facility planning. The aim of this paper is to identify the most effective number and location for biomass energy facilities. To achieve this objective, the Geographic Information System (GIS) is utilized to perform the following tasks: Generate a site suitability map for potential biomass energy facilities and identify suitable site candidates. The standardization of site suitability evaluation indicators is based on fuzzy logic, and indicator weights are determined based on the Analytic Hierarchy Process (AHP) evaluation of experts' opinions. 2. Develop planning schemes for biomass energy facilities for various number of proposed facilities, and subsequently determine the optimal scheme using multi-objective fuzzy comprehensive evaluation. The weight of each indicator is again determined using the AHP method. Following the analysis, it was found that in the case study of Fuxin City, the plans of 1 and 40 biomass energy facilities can achieve the lowest cost and the highest energy self-sufficiency level. However, both options have potential drawbacks that must be considered. The plan of 30 energy facilities has the highest comprehensive benefits, corresponding to the 30,919.75 yuan of transport cost (3748 yuan lower than the average transport cost) and 75.49% of energy self-sufficiency (67.21% of the average value). This work maximizes the comprehensive positive impacts in economic, environmental and social aspects.

Mechanistic assessment reveals the significance of HRT and MLSS concentration in balancing carbon diversion and removal in the A-stage process

Nowadays, the shift toward energy and resource-efficient wastewater treatment plants (WWTPs) has become a necessity rather than a choice. For this purpose, there has been a restored interest in replacing the typical energy and resource-extensive activated sludge process with the two-stage Adsorption/bio-oxidation (A/B) configuration. In the A/B configuration, the role of the A-stage process is to maximize organics diversion to the solids stream and control the following B-stage's influent to allow for the attainment of tangible energy savings. Operating at very short retention times and high loading rates, the influence of the operational conditions on the A-stage process become more tangible than typical activated sludge. Nonetheless, there is very limited understanding of the influence of operational parameters on the A-stage process. Moreover, no studies in the literature have explored the influence of any operational/design parameters on the Alternating Activated Adsorption (AAA) technology which is a novel A-stage variant. Hence, this article mechanistically investigates the independent effect of different operational parameters on the AAA technology. It was inferred that solids retention time (SRT) shall remain below 1 day to allow for energy savings up to 45% and redirecting up to 46% of the influent's COD to the recovery streams. In the meantime, the hydraulic retention time (HRT) can be increased up to 4 h to remove up to 75% of the influent's COD with only 19% decline of the system's COD redirection ability. Moreover, it was observed that the high biomass concentration (above 3000 mg/L) amplified the effect of the sludge poor settleability either due to pin floc settling or high SVI₃₀ which resulted in COD removal below 60%. Meanwhile, the concentration of the extracellular polymeric substances (EPS) was not found to be influenced or to influence process performance. The findings of this study can be employed to formulate an integrative operational approach in which different operational parameters are incorporated to better control the A-stage process and achieve complex objectives.

The effect of biomass raw material collection distance on energy surplus factor

The collection radius of biomass raw materials is an important factor affecting the volume of raw materials for energy utilization. At present, it is usually studied based on a single biomass combined heat and power (CHP) plant. However, as the heat transfer threshold of biomass CHP plant is limited, it is necessary to consider the optimal collection radius and biomass raw material allocation under the distribution mode of multiple power plants to improve the overall utilization rate of raw materials. Biomass raw material collection distance threshold (BCDT) refers to the maximum road length between the resource point (that allows the transportation of raw materials to the biomass CHP plant) and the biomass CHP plant. Under the mode of multi-power plant planning, the greater the BCDT is, the more destinations there will be for raw materials to be transported to from the same resource point, and the more flexible the transportation plans and allocation of transportation volumes will be. This also means more raw materials can be ultimately used for energy utilization, which leads to higher transportation cost. Therefore, determining the appropriate BCDT plays a key role in the unified planning of biomass raw materials. Based on the limitation of heat transfer threshold, this paper carries out multi-power plant planning with Fuxin City as the research object. Based on such planning, ArcGIS is used to generate biomass raw material planning schemes with different BCDTs. Then the transportation cost and energy surplus factor (ratio of renewable resource potential to energy demand) of each scheme are calculated and compared. The results show that there is a positive correlation between BCDT and the energy surplus factor. With the increase of BCDT, the growth rate of the energy surplus factor gradually becomes slower. The study also allows to set the utilization threshold of biomass energy utilization capacity and obtain the corresponding BCDT. In order to achieve a higher energy surplus factor, it is recommended that 40 km be used as the BCDT when carrying out uniform planning for biomass raw materials. At this time, the utilization of biomass energy utilization capacity is 75%, which can achieve a high degree of energy self-sufficiency and ensure its economic competitiveness.

Enhancement of sewage sludge thickening and energy self-sufficiency with advanced process control tools in a full-scale wastewater treatment plant

On their path to becoming sustainable facilities, it is required that wastewater treatment plants reduce their energy demand, sludge production, and chemical consumption, as well as increase on-site power generation. This study describes the results obtained from upgrading the sludge line of a full-scale wastewater treatment plant over 6 years (2015-2021) using three advanced process control strategies. The advanced process control tools were designed with the aim of (i) enhancing primary and secondary sludge thickening, (ii) improving anaerobic digestion performance, and (iii) reducing chemical consumption in the sludge line. The results obtained show that the use of advanced process control tools allows for optimising sludge thickening (increasing solids content by 9.5%) and anaerobic digestion (increasing both the removal of volatile solids and specific methane yield by 10%, respectively), while reducing iron chloride and antifoam consumption (by 75% and 53%, respectively). With the strategies implemented, the plant increased its potential energy self-sufficiency from 43% to 51% and reduced de-watered sludge production by 11%. Furthermore, the upgrade required a low investment, with a return of capital expense (CAPEX) in 1.98 years, which presents a promising and affordable alternative for upgrading existing wastewater treatment plants.

Pushing microbial desalination cells towards field application: Prevailing challenges, potential mitigation strategies, and future prospects

Microbial desalination cells (MDCs) have been experimentally proven as a versatile bioelectrochemical system (BES). They have the potential to alleviate environmental pollution, reduce water scarcity and save energy and operational costs. However, MDCs alone are inadequate to realise a complete wastewater and desalination treatment at a high-efficiency performance. The assembly of identical MDC units that hydraulically and electrically connected can improve the performance better than standalone MDCs. In the same manner, the coupling of MDCs with other BES or conventional water reclamation technology has also exhibits a promising performance. However, the scaling-up effort has been slowly progressing, leading to a lack of knowledge for guiding MDC technology into practicality. Many challenges remain unsolved and should be mitigated before MDCs can be fully implemented in real applications. Here, we aim to provide a comprehensive chronological-based review that covers technological limitations and mitigation strategies, which have been developed for standalone MDCs. We extend our discussion on how assembled, coupled and scaled-up MDCs have improved in comparison with standalone and lab-scale MDC systems. This review also outlines the prevailing challenges and potential mitigation strategies for scaling-up based on large-scale specifications and evaluates the prospects of selected MDC systems to be integrated with conventional anaerobic digestion (AD) and reverse osmosis (RO). This review offers several recommendations to promote up-scaling studies guided by the pilot scale BES and existing water reclamation technologies.

The correlations among wastewater internal energy, energy consumption and energy recovery/production potentials in wastewater treatment plant: An assessment of the energy balance

Given the economic and environmental importance of energy use in wastewater treatment plants (WWTPs), the need to assess the energy balance of WWTPs has become a growing concern. Previous studies have suggested that energy balance or even net energy production may be achieved in WWTPs under specific conditions. However, information regarding the energy consumption and the energy recovery/production potential in WWTPs as a function of the influent characteristics is still very limited. In this paper, by exploring the correlations among wastewater internal energy, energy consumption and energy recovery in WWTPs, a novel net energy consumption (NEC) model was developed for predicting the energy self-sufficiency level of WWTPs. From our results, exponential regression showed a high accuracy in predicting the annual energy consumption, the annual excess sludge production and the bioreactor footprints in WWTPs. Wastewater with more internal energy which is determined by influent chemical oxygen demand (COD) concentration and flow rate, not only leads to higher energy consumption in WWTPs, but also results in an increase in the excess sludge production, bioreactor footprints and wastewater volume. This means that the WWTPs could achieve energy saving or even net energy production by incorporating sludge incineration, photovoltaic (PV) generation and thermal energy recovery. By combing regression analysis with theoretical formula, the annual net energy demand of WWTPs reached -0.187-0.466 kWh·m^-3 in the range of wastewater condition studied (the influent COD concentration range of 60-800 mg·L^-1 and the flow rate range of 1296-100,000 m³·d^-1). The NEC model reveals that the net zero energy consumption may be achieved by integrating the better understandings of wastewater internal energy, energy conversion methods and environmental media energy, which is of value to policy makers for the planning of new WWTPs and provides theoretical support for the selection of available energy recovery methods.

Integrated upflow anaerobic fixed-bed and single-stage step-feed process for mainstream deammonification: A step further towards sustainable municipal wastewater reclamation

The high energy consumption and excessive waste activated sludge (WAS) production have become the major concerns on the municipal wastewater treatment with conventional biological processes. To tackle these emerging issues, this study demonstrated the feasibility of a novel process integrating an upflow anaerobic fixed-bed reactor (UAFBR) followed by a continuous step-feed reactor for mainstream deammonification towards improved energy efficiency, minimized sludge production and cost-effective ammonium removal. The results showed that 48.8% of the influent chemical oxygen demand (COD) was directly converted to methane gas in UAFBR with minimized sludge production, while 80% of total nitrogen (TN) was removed in the step-feed reactor. Mass balance on the step-feed reactor revealed that the oxic chambers contributed 51.6% of the removed ammonium oxidation to mainly nitrite, while the produced nitrite was immediately removed via anammox with the ammonium supplied by the step-feed in the following anoxic chambers where about 87.1% TN removal occurred. Moreover, it was found that sustainable repression of nitrite oxidizing bacteria (NOB) was achieved without compromising the activity of ammonia oxidizing bacteria (AOB). The anammox bacteria were effectively retained in the anoxic chambers and showed a high specific anammox activity of 0.42 g N/(g VSS·day). These suggest that the step-feed configuration can offer a feasible engineering option towards single-stage mainstream deammonification. It appears that the integrated process developed in this study sheds light on the possible way towards sustainable, energy self-sufficient municipal wastewater reclamation.

Mass transfer enhancement and improved nitrification in MABR through specific membrane configuration

One of the main energy consumptions in wastewater treatment plants (WWTPs) is due to the oxygenation of aerobic biological processes. In order to approach to an energy self-sufficient scenario in WWTPs, Membrane Aerated Biofilm Reactors (MABRs) provide a good opportunity to reduce the impact of aeration on the global energy balance. However, mass transfer limitations derived from poor flow distribution must be tackled to take advantage of this technology. In this work, in order to improve mass transfer between biofilm and bulk water, a specific configuration was developed and studied at laboratory scale, aimed at compactness, energy efficiency and high nitrification rates. Nitrification rates were higher in the innovative configuration than in the conventional one, achieving a Volumetric Nitrification Rate (VNR) as high as 575.84 g NH₄-N m^-3 d^-1, which is comparable with confirmed technologies. Regarding energy consumption due to aeration, a reduction of 83.7% was reached in comparison with aeration through diffusers with the same Oxygen Transfer Efficiency (OTE). These results highlight the importance of hydrodynamic conditions and the membranes configuration on treatment performance.

New concepts on carbon redirection in wastewater treatment plants: A review

Wastewater treatment plants (WWTPs) are no longer considered pollution removal systems but rather resources (nutrients and energy) recovery plants. Legislation imposing more stringent effluent requirements and the need energy self-sufficient or even energy-positive plants are the main drivers for the research and development of new WWTP configurations. While a lot of effort has been focused on developing new processes for nutrient recovery, limited efforts have been allocated to maximizing energy recovery from the organic load. Within this context, high-rate activated sludge (HRAS) is the most promising alternative technology to redirect carbon (organic compounds) towards energy as biogas. This is a critical review of the last decade's development of new alternatives for carbon redirection to improve the energy balance of WWTPs on both the laboratory and the industrial scale.

Energy-positive sewage sludge pre-treatment with a novel ultrasonic flatbed reactor at low energy input

The performance of a novel ultrasonic flatbed reactor for sewage sludge pre-treatment was assessed for three different waste activated sludges. The study systematically investigated the impact of specific energy input (200 - 3,000 kJ/kg_TS) on the degree of disintegration (DD_COD, i.e. ratio between ultrasonically and maximum chemically solubilized COD) and methane production enhancement. Relationship between DD_COD and energy input was linear, for all sludges tested. Methane yields were significantly increased for both low (200 kJ/kg_TS) and high (2,000 - 3,000 kJ/kg_TS) energy inputs, while intermediate inputs (400 - 1,000 kJ/kg_TS) showed no significant improvement. High inputs additionally accelerated reaction kinetics, but were limited to similar gains as low inputs (max. 12%), despite the considerably higher DD_COD values. Energy balance was only positive for 200 kJ/kg_TS-treatments, with a maximum energy recovery of 122%. Results suggest that floc deagglomeration rather than cell lysis (DD_COD=1% - 5% at 200 kJ/kg_TS) is the key principle of energy-positive sludge sonication.