Energy-water Nexus of Wastewater Treatment System - Science Direct

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environment, water is used to generate energy for system input. With the rapid ... 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ... Fig.1 The conceptual model of energy-water nexus in WWTS.
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ScienceDirect Energy Procedia 104 (2016) 141 – 145

CUE2016-Applied Energy Symposium and Forum2016: Low carbon cities & urban energy systems

Energy-water nexus of wastewater treatment system: conceptual model and framework Le Fenga, Bin Chenb* a

Department of Ecology and Urban Environment, Beijing Municipal Research Institute of Environmental Protection, Beijing 100073, P.R. China

b

State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment Beijing Normal University, Beijing100875, P.R. China

Abstract

Energy and water are two interwoven elements of wastewater treatment system (WWTS). Energy is consumed to remove pollutants in wastewater, decrease negative influence on the natural water environment, water is used to generate energy for system input. With the rapid increasing on wastewater quantity, how to improve wastewater treatment efficiency and reduce energy costs has attracted many attentions. However, there is lack of synthesize understanding of the energy-water nexus in WWTS. In this study, a new energy-water nexus conceptual framework is developed, energy used for wastewater extraction, operation of the wastewater treatment process and the waste recycling were explored. Efficiency and redundancy of the WWTS were also examined in structural nexus analysis based on Network Environ Analysis (NEA). The conceptual framework would help to investigate the mechanism and properties of the energy-water nexus for WWTS. © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). responsibility of CUE Selection and/or peer-reviewofunder Peer-review under responsibility the scientific committee of the Applied Energy Symposium and Forum, CUE2016: Low carbon cities and urban energy systems.

Keywords: Water-energy nexus; Structural nexus; Waste water treatment system (WWTS); Network Environ Analysis (NEA)

1. Introduction

* Corresponding author. Tel.:+86 10 58807368; fax: :+86 10 58807368 E-mail address: [email protected].

1876-6102 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the Applied Energy Symposium and Forum, CUE2016: Low carbon cities and urban energy systems. doi:10.1016/j.egypro.2016.12.025

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Le Feng and Bin Chen / Energy Procedia 104 (2016) 141 – 145

Water and energy are closely connected and both are significant for human development [1-2]. Waste water treatment system is central to water-energy interactions as it consumes electrical energy to decrease the pollutants in the wastewater, and water is required to generate electrical energy. Wastewater treatment requires various forms of energy [3-5], while almost every stage in the energy supply chain needs water [6-7]. Abbreviation WWTS Waste water treatment system NEA

Network Environ Analysis

WF

Water footprint

GWF

Grey water footprint

GWFRE Grey water footprint reduction efficiency Symbols

Pi Concentration of pollutants in the wastewater, which including Biochemical Oxygen Demand(BOD), Chemical Oxygen Demand(COD) and Total Nitrogen(TN) in discharge from the WWTS Concentrations of pollutant in the waste water after treatment

Pti V Wastewater volume treated by WWTS Ei Total energy input during the wastewater treatment process (in kWh) Et Energy consumption of pumping waste water(Mtce)

T Conversion coefficient of electricity to Joule(Kj/kW h),valued 3600 J Specific weight of water value 9.8 Kn/m3 H ' Total dynamic head of the WWTS Schnoor pointed out that probably the greatest water story of the 21st century is to treat wastewater through membranes and reverse osmosis for drinking water supplied with significant energy consumption [8]. Moreover, wastewater treatment accounts for about 3% of the electrical energy load in developed countries, and the high energy costs for treatment due to aeration requirement in developed countries cannot be borne by developing countries. Therefore, to balance the trade-off between energy consumption and wastewater cutting loads in the wastewater treatment system is highly needed. As illustrated by Wiedmann and Minx(2008)[9], there are water footprint behind energy footprint input from a life cycle analysis perspective. According to the water footprint theory[10-13], Grey water footprint(GWF) is defined as the volume of freshwater that would be required to dilute the pollutants to meet given natural background concentrations or water quality standards[14]. In this work, GWF is utilized as a wastewater treatment indicator to measure the response relationship between energy consumption and pollutant removing quantity. In this study, water-energy nexus conceptual framework in WWTS is constructed, energy consumption through the life cycle stage of the wastewater treatment is quantified. Grey water footprint, which includes removal quantity of BOD and COD are investigated. Moreover, Network Environ Analysis

Le Feng and Bin Chen / Energy Procedia 104 (2016) 141 – 145

(ENA) is introduced to evaluate the efficiency and redundancy of the WWTS, which would provide reference to the improvement of operation efficiency reduce energy costs for WWTS. 2 Conceptual model 2.1 System boundary The system boundary of WWTS is shown in Fig.1, in which green line represents electrical energy flows, yellow line represents water flow, dotted lines denote the backflow of water fluxes. The lifetime of the WWTS is divided into five different phases, i.e., manufacturing, transportation, construction, operation, and waste treatment. The entire life-cycle electrical energy and water flows of the WWTS are depicted in Fig.1. In the current system, electrical energy use and water consumption not only embraced relative processes during the operation of the WWTS, for example, the direct electrical energy and water consumption for wastewater tube cleaning, but also covered those indirectly linked activities that occur outside, e.g., energy and water embodied in the manufacturing, transportation and construction of WWTS. In the waste treatment process, the treating wastewater and solid waste are reused for water supply and material recycling.

Fig.1 The conceptual model of energy-water nexus in WWTS 2.2 Energy-water nexus Energy input is needed in every stage in the WWTS, such as wastewater collection, physical treatment, chemical treatment, sludge treatment and discharge. In most WWTS, electricity is utilized as the only energy source for pumping and carrying wastewater through tubes, as well as operation of the most treatment equipment. Meanwhile, electricity production needs water withdraws. Generally, grey water footprint of one region can be reduced through WWTS, decreasing the negative impact on the environment. Effluents from the WWTS can be reused for irrigation, landscape and other industrial activities, reducing blue water footprint to realize water footprint compensation. However, there are tradeoffs in water footprint reduction since it may increase energy consumption. 2.2.1 Energy for grey water footprint reduction Grey water footprint is useful metric for quantify the hypothetical dilution volume, according to Hoekstra et al.(2011)[14], wastewater treatment can reduce the GWF down to zero when the

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concentrations of pollutants in the treated effluent are equal to or lower than the local water quality standards. To investigate the role of WWTS in reducing human activities on water resources, grey water footprint reduction efficiency (GWFRE) of WWTS is determined as: PP GWFRE ( i ti ) *V (1) Pi where, Pi represents concentration of pollutants in the wastewater, which including Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and Total Nitrogen (TN) in discharge from the WWTS in this work, Pti represents concentrations of pollutant i after treatment, V represents the wastewater volume treated by the WWTS. The energy used to reduce the GWF in the WWTS is formulated as: eGWFR

'GWF Ei

(

Pi  Pti ) *V Pi Ei

(2)

where, Ei is the total energy input during the wastewater treatment process(in kWh). The higher eGWFR corresponds to increased grey water footprint reduction for per unit energy input into the wastewater treatment system. 2.2.2 Energy for water The energy used for wastewater extraction, utilization and waste water treatment is calculated follow Eqs.(3) and (4). For wastewater extraction, the energy used to pump wastewater from the inlet pipes is considered. Since the extracted water is manually used for construction and equipment cleaning, the energy consumed in the water utilization stage is relatively minor and is ignored in this study. For wastewater treatment process, the coefficient of energy consumption per unit wastewater treatment extracted from Ecoinvent database is adopted (see Eq.(3)). n J Qe H ' (3) Et Ee  Et =T u ¦ i +WW u M i 1 1000Ki More explanations in Eq.(3) can be referred to [15] and [16] . 2.3 Network Environ Analysis To analyze the structure nexus of the WWTS, a holistic approach should be employed to trace the complex energy and wastewater flows in the system. A system-oriented technique known as Network Environ Analysis has received increasing attention due to its obvious strength in examining the structure characteristic and the efficiency and redundancy in the WWTS. A detail introduction of NEA is available in the literatures[17-19]. 3. Conclusion Energy-water nexus couples the energy and water system in WWTS, which earns more investigations to unveil more information of its mechanism and properties. This work depicts the mechanism of energy-water nexus in WWTS by a newly proposed conceptual model. The approach of Network Environ Analysis is also introduced to quantify the nexus’s structure properties. This work may be helpful for wastewater treatment scientist to explore more on the energy-water nexus in municipal wastewater treatment system. 4. Copyright

Le Feng and Bin Chen / Energy Procedia 104 (2016) 141 – 145

Authors keep full copyright over papers published in Energy Procedia Acknowledgements This work was supported by the Fund for Innovative Research Group of the National Natural Science Foundation of China (No. 51421065), Major Research Plan of the National Natural Science Foundation of China (No. 91325302), National Natural Science Foundation of China (Nos. 71573021, 41271543), and Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20130003110027).

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Biography Bin Chen is a professor of energy science at Beijing Normal University. Dr. Chen has published over 200 peer-reviewed papers in prestigious international journals. He is also serving as subject editor of Applied Energy and editorial board member of more than ten journals.

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