Energy benchmarking in wastewater treatment plants: the importance of site operation and layout

Unlocking energy potential: Decarbonizing water reclamation plants with salinity gradient energy recovery

Climate change, together with the ecological droughts suffered by a large part of the European Union's territory, calls for joint environmental solutions. In this regard, water reclamation is a promising way to alleviate the pressure on existing water resources. However, reuse strategies are penalized by the extra energy consumed in urban wastewater treatment plants (UWWTPs), facilities mainly powered by fossil fuels. The opportunity to integrate renewable sources of energy into the energy-intensive UWWTPs holds great promise towards decarbonization of the sector. In this context, the energy harvested from a Salinity Gradient (SGE) has attracted great interest in the last decade. This work aims at the analysis of opportunity of implementing integrated processes for water reclamation and SGE recovery in the coastal EU UWWTPs. According to the selection criteria, a total of 281 potential sites located across eighteen coastal countries of the EU have been inventoried attending to the current state of the art. The water reclamation potential has been estimated at 3.7 million m3/day. As a consequence, the environmental burdens of the reclamation process could result in the reduction of 1.5·105 t CO2/year. The Mediterranean region, highly affected by hydrological drought, has proved to be a hot spot for water reclamation, with the highest number of plants inventoried in the study and a predicted potential for SGE harvesting of 60 Wh/m3 of reclaimed water. These results highlight a niche of opportunities to encourage water reclamation, avoid water bodies' degradability due to effluent discharge and the further decarbonization of reclamation processes.

Towards low energy-carbon footprint: Current versus potential P recovery paths in domestic wastewater treatment plants

With the unprecedented exhaustion of natural phosphorus (P) resource and the high eutrophication potential of the associated-P discharge, P recovery from the domestic wastewater is a promising way and has been putting on agenda of wastewater industry. To address the concern of P resource recovery in an environmentally sustainable way is indispensable especially in the carbon neutrality-oriented wastewater treatment plants (WWTPs). Therefore, this review aims to offer a critical view and a holistic analysis of different P removal/recovery process in current WWTPs and more P reclaim options with the focus on the energy consumption and greenhouse gas (GHG) emission. Unlike P mostly flowing out in the planned/semi-planned P removal/recovery process in current WWTPs, P could be maximumly sequestered via the A-2B- centered process, direct reuse of P-bearing permeate from anaerobic membrane bioreactor, nano-adsorption combined with anaerobic membrane and electrochemical P recovery process. The A-2B- centered process, in which the anaerobic fixed bed reactor was designated for COD capture for energy efficiency while P was enriched and recovered with further P crystallization treating, exhibited the lowest specific energy consumption and GHG emission on the basis of P mass recovered. P resource management in WWTPs tends to incorporate issues related to environmental protection, energy efficiency, GHG emission and socio-economic benefits. This review offers a holistic view with regard to the paradigm shift from "simple P removal" to "P reuse/recovery" and offers in-depth insights into the possible directions towards the P-recovery in the "water-energy-resource-GHG nexus" plant.

Long-term performances and membrane lifespan of full-scale MBR treating filtrate from sludge ultra-dewatering

An MBR treating filtrate from sludge ultra-dewatering (FSD) was studied to evaluate the real applicability to concentrated effluents. The MBR operation is comparable to conventional wastewater MBRs in terms of F/M and nitrogen to sludge ratios, SRT and MLSS in biological tanks. On the contrary, the volume treated is lower with a comparable pollution load, the effluent being concentrated in nitrogen and carbon. Very high and stable ammonium (97.7 ± 2.4%), total phosphorus (81.8 ± 11.9%), chemical (89.5 ± 2.3%) and biochemical oxygen demands (98.8 ± 0.7%) removals are observed despite a significant modification of the FSD composition. The MBR removal performances are whether comparable or greater than those reported at full-scale for Sharon or Anammox processes. The evolution of membranes properties, in particular, the decrease of permeability induced by the irreversible fouling, leads to a decrease of the daily permeate volume produced and an increase of the chemical cleaning need. The membrane lifespan was determined to be 5.5 years based on both the permeability loss and TSS in permeate. The permeability is directly correlated to the cumulated filtered flux of colloidal matter and a total of 350 kg O₂/m² of the membrane (COD in the supernatant) is needed to reach a permeability below 100 LMH/bar. The specific energy consumption is comparable to other wastewater MBRs in kWh/kgCOD removed but the intensive chemical cleanings need to be 2.5-4.5 times more frequent. Overall, it can be concluded that MBR is adapted to treat FSD efficiently.

The performance evaluation of wastewater service: a protocol based on performance indicators applied to sewer systems and wastewater treatment plants

This research aimed to identify a tool to objectively analyse the performance and the environmental contextualisation of sewer systems (SwSs) and wastewater treatment plants (WWTPs). This procedure performs assessment by calculating performance indices which could be subsequently applied to SwSs and WWTPs with different characteristics. The proposed tool can be applied conveniently over the years by managers of integrated urban water management systems for the analysis of different realities also allowing the evaluation of the effects of upgrades carried out during the management phases. The proposed analysis allows the optimisation of SwSs and can profitably guide the choice and the priority among possible interventions for the sewerage infrastructure and WWTPs providing a verification and evaluation protocol as well as a financial planning tool.

Low-Head Hydropower for Energy Recovery in Wastewater Systems

Hydraulic turbines for energy recovery in wastewater treatment plants, with relatively large discharges values and small head jumps, are usually screw Archimedes or Kaplan types. In the specific case of a small head jump (about 3 m) underlying a rectangular weir in the major Palermo (Italy) water treatment plant, a traditional Kaplan solution is compared with two other new proposals: a Hydrostatic Pressure Machine (HPM) located at the upstream channel and a cross-flow turbine (CFT) located in a specific underground room downstream of the same channel. The fluid mechanical formulations of the flow through these turbines are analyzed and the characteristic parameters are stated. Numerical analysis was carried out for the validation of the HPM design criteria. The efficiency at the design point of the CFT and HPM are estimated using the ANSYS CFX solver for resolution of 3D URANS analysis. The strong and weak points of the three devices are compared. Finally, a viability analysis is developed based on several economic indicators. This innovative study with a theoretical formulation of the most suitable turbomachine characterization, the potential energy estimation based on hydraulic energy recovery in a real case study of a wastewater treatment plant and the comparison of the three different low-head turbines, enhancing the main advantages, is of utmost importance towards the net-zero water sector decarbonization.

Greening Urban Areas with Decentralized Wastewater Treatment and Reuse: A Case Study of Ecoparque in Tijuana, Mexico

In rapidly growing urban areas, such as Tijuana, Mexico, the presence of urban green spaces (UGSs) can help stem soil erosion, improve infiltration, slow runoff, decrease flooding, reduce air pollution, and mitigate climate change. In many water-scarce parts of the world, where centralized wastewater treatment is not accessible or practical, decentralized wastewater treatment systems (DEWATSs) have the potential to supply the water needed for irrigating UGSs. Here, we first review UGS systems supported by DEWATSs and the water quality guidelines and challenges associated with implementing DEWATSs for urban greening in different countries, including Mexico. We also critically examine the linkages between the lack of UGSs in Tijuana, Mexico, extensive soil erosion, and failing sanitation infrastructure that has led to the infamously poor water quality in the Tijuana River. Tijuana’s Ecoparque Wastewater Treatment Facility, a low-energy, aerobic DEWATS, which collects, treats, and discharges residential sewage for localized landscape irrigation, demonstrates how DEWATSs can meet the water demands for urban greening in rapidly urbanizing cities. The aerobic decentralized treatment using a gravity-fed trickling biofilter resulted in a >85% removal of chemical oxygen demand and dissolved organic carbon. Prior to treatment facility upgrades, there was a ~2 log reduction in total coliform and Escherichia coli and a <20% decrease in ammonia from the influent to final effluent. After the addition of a maturation pond in 2020, the effluent met Mexico standards for irrigation reuse, with a ~4 log reduction in fecal coliforms from the influent to final effluent. Case study results demonstrated the potential for decentralized wastewater treatment to meet effluent standards for landscape irrigation, provide water for urban greening, and prevent pollution in the Tijuana River and other urban waterways.

Structure and indicators of electric energy consumption in dairy wastewater treatment plant

The aim of the research was to determine the indicators of electricity consumption in every stage of the dairy sewage treatment process in relation to the sewage flow and the load of removed organics (BOD₅, COD) and nutrients (TN, TP). The research was conducted in a dairy wastewater treatment plant (WWTP) consisting of mechanical treatment, averaging tank, dissolved air flotation (DAF) and biological treatment with sequence batch reactors (SBRs). Energy consumption was measured with the help of transducers. Indicators of unit electricity consumption were determined on the basis of 95 measurement series of energy consumption, sewage flow and removed load. The mean value of total unit energy consumption relating to the flow for the entire WWTP was 2.29 kWh·m^-3, while for biological treatment 1.17 kWh·m^-3 and 0.05 kWh·m^-3 for DAF. The mean values of indicators relating to removed pollutants load for the entire WWTP were: 1.89 kWh·kg_rem BOD₅^-1, 1.30 kWh·kg_rem COD^-1, 48.61 kWh·kg_rem TN^-1 and 160.01 kWh·kg_rem TP^-1. During biological treatment, energy consumption indicators were on average: 1.65 kWh·kg_rem BOD₅^-1 and 1.19 kWh·kg_rem COD^-1, 52.90 kWh·kg_rem TN^-1 and 141.26 kWh·kg_rem TP^-1, while for DAF: 0.12 kWh·kg_rem BOD₅^-1, 0.09 kWh·kg_rem COD ^-1, 3.85 kWh·kg_rem TN^-1 and 16.17 kWh·kg_rem TP^-1. It was found that the biological treatment in SBRs was responsible for 54.1% of the total energy consumption of dairy WWTP. Aerobic sewage sludge treatment accounted for 17.0% of total consumption, mechanical treatment 17.1%, deodorization 2.6%, and other (social, lighting etc.) 6.9%, while DAF only 2.3%. The real-time electricity metering system enabled the optimisation of the electricity consumption in the WWTP, taking into account its consumption in unit processes and the removed pollutants load. The application of this system enabled to make corrections that reduced energy consumption while maintaining the required treatment efficiency.

Optimizing the system-wide layout of sewage treatment plants based on enumeration and the orthogonal test

Optimizing the locations of sewage treatment plants has enormous practical significance. In this study, a large-system mathematical model was developed for optimizing the locations of sewage treatment plants within a system and designing the associated pumping station pipe network. Head loss of pipe segments in the pipe network was the coupling constraint, the economic flow rate of pipe segments was determined by the feasible region constraints of decision variables, and the design variables were the sewage treatment plant locations, the design head of the pumping stations, the pipeline economic life, and the pipe diameter of divided pipe segments. The minimum total annual cost of the sewage treatment plant(s) and the pumping station pipe network was the objective function. A large-system quadratic orthogonal test-based selection method was used with a discrete enumeration comparison and selection method to determine pipeline economic life. A dynamic programming method was used to determine the pipe diameter of the divided pipe segments. By comparing the total annual cost of the sewage treatment plants and the associated pumping station pipe network corresponding to different pipeline economic lifetimes, the optimal solution that generates the minimum total annual cost can be identified. The sewage treatment plant and pumping station pipe network in Taizhou, China, was used as an example to compare and analyze optimization results. The new optimization method would have produced much lower annual cost than that of the existing system. This study provides valuable theoretical references for probing the layout design of urban sewage treatment plants corresponding to different pipeline economic lifetimes.

Analysing water-energy-GHG nexus in a wastewater treatment plant of Mumbai Metropolitan Region, India

Developing nations are advancing towards improving municipal wastewater treatment infrastructure. However, there have been limited efforts in these countries to understand Water-Energy-GHG nexus for sustainable wastewater treatment. This study evaluates operational energy, total embodied energy and corresponding GHG emissions of a widely implemented municipal wastewater treatment technology in Mumbai Metropolitan Region of India. Data for wastewater inflow, total energy consumption and materials used in the unit processes of the treatment plant were collected and analysed. Direct GHG emissions from the plant were estimated using the method proposed by the intergovernmental panel for climate change (IPCC). The energy embodied within the materials used in the construction and operations of the plant was estimated using country-specific information. The results show that the energy intensity for plant operation and chemical oxygen demand (COD) removal was 0.23 ± 0.05 kWh/m³ of wastewater treated and 1.12 ± 0.77 kWh/kg of COD removed, respectively. The embodied energy in chemicals and building materials during the plant's life cycle contributed 19% of the total embodied energy. Wastewater inflow with specific average energy consumption in wet-well pumping and blowers showed a negative and positive correlation, respectively. The total GHG emissions (direct and indirect) from the plant were ~0.22 kgCO₂eq/m³ of wastewater treated. The study presents the energy accounting of a wastewater treatment system and extensive analysis of infrastructural data, which is scarcely accessible and available in developing countries.

Reducing energy costs of the wastewater treatment plant by improved scheduling of the periodic influent load

The wastewater treatment plant (WWTP) is a major actor of the water-energy nexus. This study proposes to partially store in available WWTP tank infrastructure the wastewater received during the day-time and schedule the purification of the stored wastewater at night-time. The intended operational approach aims to shift part of the WWTP electrical energy consumption from day-time into the night-time period when the energy has lower prices, also contributing to the balance of the electrical power generation system. This research presents the case study of a Romanian WWTP with Anaerobic-Anoxic-Oxic (A²O) process configuration. A proposed control strategy was implemented and tested on the dynamic calibrated WWTP model, based on the Activated Sludge Model No. 1 and the secondary settler Takács model. Simulations of the proposed scheduling program for the storing and processing time-periods of the influent wastewater, associated to the designed control strategies, demonstrate the reduction of the operational costs and energy savings, while keeping the effluent quality within the requested regulation limits and improving the plant sustainability. In the most favorable case and considering the overall WWTP performance, the operational costs are reduced by 47% and the effluent quality is improved by 25%. To achieve this performance a part of the influent wastewater is stored from 2 p.m. in the available tanks (day-period) while the beginning of the stored wastewater treatment is scheduled at 12 a.m. (night-period). Air flow rate distribution in the nitrification zone and the two water recirculation flow rates are also found by optimization.

Benchmarking the sustainability of sludge handling systems in small wastewater treatment plants

A benchmarking strategy was developed to assess all aspects of sludge handling in small wastewater treatment plants and tested on a cross-section of Ontario facilities. Using operational data and on-site measurements, sustainability metrics that addressed energy consumption, chemical use, biosolids quality and disposition, and greenhouse gas (GHG) emissions were estimated. Electricity consumption for sludge handling ranged from 0.9 to 3.9 kW-hours per dry kilogram of raw sludge (kWh/dry kg) with thermo-alkali hydrolysis and auto-thermal thermophilic aerobic digestion (ATAD) processes consuming the least and most electricity for stabilization, respectively. Mechanical dewatering processes consumed between 2 and 5% of total sludge handling electricity, however, associated polymer dosages were higher than literature values in some cases. Disposition fuel requirements for plants with dewatering were up to 85% lower than facilities without dewatering. Biosolids contaminant (pathogen/metals) contents were observed to be substantially below Non-Agricultural Source Material (NASM) requirements. The copper content of the hauled biosolids exhibited the highest concentration relative to the NASM limit among all plants studied, ranging from 14 to 37% among facilities practicing land application of biosolids. Four biosolid products met Class A requirements for E. coli content, including one product generated via long-term storage. Carbon emissions ranged from -119 to 299 kg CO₂ equivalents per dry tonne of raw sludge (g CO₂ eq./kg). Six facilities that practiced land application exhibited net-negative GHG emissions; the carbon credits gained from fertilizer production avoidance outweighed emissions associated with sludge processing and transportation. The results provide evidence that this practice is sustainable from a GHG emissions standpoint. The benchmarking approach developed and information gathered is beneficial to plant owners and operators seeking to better understand how their utility is performing relative to peers, identify areas of need and further investigation, and improve the sustainability of their operations.

Mass transfer of nanobubble aeration and its effect on biofilm growth: Microbial activity and structural properties

It is necessary to improve the performance and reduce the aeration cost is of wastewater treatment by aerobic biofilm systems. Nanobubble aeration is supposed to be a promising method to achieve these goals. Compared with coarse bubbles, dissolved oxygen profiling showed that the nanobubbles provided more oxygen to biofilms, offering superior oxygen supply capacity and 1.5 times higher oxygen transfer efficiency. Nanobubble aeration accelerated the growth of the biofilm and achieved better removal efficiencies of chemical oxygen demand and ammonia, with as maximum as six times higher dehydrogenase activity, and more extracellular polymeric substance content than when using the traditional aeration mode. This is attributed to the enhancement of metabolism and the proliferation of microorganisms. Confocal laser-scanning microscopy imaging confirmed that nanobubble aeration affected the components of biofilm by shifting the microbial community and changing its metabolic pathways of biofilms, such as carbohydrate synthesis. Nanobubble aeration resulted in an energy saving of approximately 80%. The assessment of nanobubble aerated biofilm growth suggests that this technique can offer a rapid-initiation, high efficiency, and low-cost strategy for aerobic biofilm systems in wastewater treatment.

Energy intensity of wastewater treatment plants and influencing factors in China

The wastewater-energy nexus is an emerging concern in the wastewater treatment sector. Understanding the energy efficiency of wastewater treatment plants (WWTPs) and the factors that influence it will help to improve planning and managing in order to meet increasing energy conservation demands. In this study, we use a unique big dataset from a pollution source census of all WWTPs in China to establish a quantitative model that relates the energy consumption of WWTPs to major influencing factors. From our results, we found that WWTPs in China are more energy-intensive than their international counterparts. Influencing factors such as treatment scale, technology, treatment degree, load factor, sludge amount, age, topography and wastewater collection area all significantly affect energy efficiency. In terms of energy saving potential, if the influent chemical oxygen demand (COD) concentration is increased to >500 mg/L, the total energy consumption of the wastewater treatment industry can be reduced by at least 20%. Furthermore, potential energy conservation is 5.9% for increasing the load of sewage treatment plants and 3.2% for renovating old WWTPs. We prioritized approaches for WWTP energy conservation and ranked them as follows: 1) establishing rain and sewage diversion facilities and increasing the inlet concentration of pollutants; 2) expanding and improving the sewage treatment pipe network and increasing the utilization rate of WWTPs; and 3) renovating old WWTPs. Our findings provide insights for other countries to improve the wastewater-energy nexus in their wastewater treatment sector.

Proof of concept of wastewater treatment via passive aeration SND using a novel zeolite amended biofilm reactor

The current paper describes a novel passive aeration simultaneous nitrification and denitrification (PASND) zeolite amended biofilm reactor that removes organic carbon and nitrogen from wastewater with low-energy consumption. Next to the ammonium oxidizing bacteria (AOB), this reactor contained naturally enriched glycogen accumulating organisms (GAOs) and zeolite powder to initially adsorb BOD (acetate) and ammonium (NH₄ ⁺-N) from synthetic wastewater under anaerobic conditions. Draining of the treated wastewater exposed the biofilm directly to air enabling low-energy oxygen supply by passive aeration. This allowed the adsorbed ammonium to be oxidized by the AOB and the produced nitrite and nitrate to be reduced simultaneously by the GAOs using the adsorbed BOD (stored as PHAs) as carbon source. Overall, with an operation mode of 1 h anaerobic and 4 h aerobic phase, the nutrient removal efficiency after single treatment was about 94.3% for BOD and 72.2% for nitrogen (NH₄ ⁺-N). As high-energy aeration of the bulk solution for oxygen supply is completely avoided, the energy requirement of the proposed PASND biofilm reactor can be theoretically cut down to more than 50% compared to the traditional activated sludge process.

Benchmarking of energy consumption in municipal wastewater treatment plants - a survey of over 200 plants in Italy

One of the largest surveys in Europe about energy consumption in Italian wastewater treatment plants (WWTPs) is presented, based on 241 WWTPs and a total population equivalent (PE) of more than 9,000,000 PE. The study contributes towards standardised resilient data and benchmarking and to identify potentials for energy savings. In the energy benchmark, three indicators were used: specific energy consumption expressed per population equivalents (kWh PE^-1 year^-1), per cubic meter (kWh/m³), and per unit of chemical oxygen demand (COD) removed (kWh/kgCOD). The indicator kWh/m³, even though widely applied, resulted in a biased benchmark, because highly influenced by stormwater and infiltrations. Plants with combined networks (often used in Europe) showed an apparent better energy performance. Conversely, the indicator kWh PE^-1 year^-1 resulted in a more meaningful definition of a benchmark. High energy efficiency was associated with: (i) large capacity of the plant, (ii) higher COD concentration in wastewater, (iii) separate sewer systems, (iv) capacity utilisation over 80%, and (v) high organic loads, but without overloading. The 25th percentile was proposed as a benchmark for four size classes: 23 kWh PE^-1 y^-1 for large plants > 100,000 PE; 42 kWh PE^-1 y^-1 for capacity 10,000 < PE < 100,000, 48 kWh PE^-1 y^-1 for capacity 2,000 < PE < 10,000 and 76 kWh PE^-1 y^-1 for small plants < 2,000 PE.