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A net-zero energy model for sustainable wastewater treatment Peng Yan, Rongcong Qin, Jinsong Guo, Qiang Yu, Zhe Li, You-Peng Chen, Yu Shen, and Fang Fang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04735 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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Environmental Science & Technology

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A net-zero energy model for sustainable wastewater treatment

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Peng Yan1* Rong-cong Qin1*, Jin-song Guo1**, Qiang Yu1, Zhe Li1, You-peng Chen1, Yu Shen1, Fang

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Fang2

1

5

1

Key Laboratory of Reservoir Aquatic Environment of CAS, Chongqing Institute of Green and

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Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China

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2

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400045, China

College of Urban Construction and Environmental Engineering, Chongqing University, Chongqing

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* The two authors contributed equally to this work

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**Corresponding Author

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Prof. Jin-song Guo

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Fax: +86-23-65935901, Tel: +86-23-65935901

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Email: [email protected]

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1

ACS Paragon Plus Environment


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Abstract: The large external energy input prevents the wastewater treatment from being

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environmentally sustainable. A net-zero energy (NZE) wastewater treatment concept based on

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biomass energy recycling was proposed to avoid wasting resources and promote energy recycling in

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wastewater treatment plants (WWTPs). Simultaneously, a theoretical model and boundary condition

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based on energy balance were established to evaluate the feasibility of achieving NZE in WWTPs;

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the model and condition were employed to analyze data from 20 conventional WWTPs in China.

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Six WWTPs can currently export excess energy, eight WWTPs can achieve 100% energy

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self-sufficiency by adjusting the metabolic material allocation, and six municipal WWTPs cannot

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achieve net-zero energy consumption based on the evaluation of the theoretical model. The NZE

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model offset 79.5% of the electricity and sludge disposal cost compared with conventional

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wastewater treatment. The NZE model provides a theoretical basis for optimization of material

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regulation for effective utilization of organic energy from wastewater, and promotes engineering

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applications of the NZE concept in WWTPs.

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Keywords: net-zero energy; wastewater treatment; biomass energy recycling; theoretical model; anaerobic digestion

1. INTRODUCTION

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Wastewater purification consumes a large amount of electricity.1 Treatment of wastewater

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currently consumes approximately 4% of all electrical power produced in the United States,

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whereas the electricity consumption of wastewater treatment plants (WWTPs) in China is 1 × 1011

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kWh.2,3 The electricity required for wastewater treatment will increase by 20% in the next 15 years 2


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in developed countries, leading to significantly increased CO2 emissions and resource consumption,

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because WWTPs have a large carbon footprint.4,5,6 The consequences of the massive energy

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consumption of WWTPs include damage to the environment, depletion of natural resources, and

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significant economic burden.7,8,9 Therefore, sustainable wastewater treatment processes must be

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developed to decrease electricity consumption and the greenhouse gas (GHG) footprint of

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WWTPs.4,10,11

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Wastewater contains a significant amount of potential energy.12,13 The organic energy in

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wastewater is approximately 9–10 times greater than that used to treat it.14 Wastewater treatment is

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a complex microbial metabolic process, in which microbes utilize pollutants to achieve sustained

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growth and propagation.15 Some organic energy is converted into biomass energy, while remaining

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organic energy dissipates as heat.16 In conventional WWTPs, biomass energy is wasted with sludge

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discard.5 Because the substantial energy in organic matter currently lost in conventional treatment

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processes,1 intensive attention has been focused on recovering energy from wastewater to power

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wastewater treatment and evaluating the energy efficiency of WWTP. Several energy recovery

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methods developed with the aim of achieving energy self-sufficiency in WWTPs have been

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discussed in previous reports.17 However, the most feasible approach to recovering internal energy

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in existing treatment plants is using CH4 biogas produced during sludge anaerobic digestion as

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biofuel to generate power and heat by cogeneration.12,18 If more organic energy is captured from

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wastewater than is used for wastewater treatment, external energy input is not required; thus, an

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independent and self-sufficient energy recycling system, with the characteristic of “net-zero energy”

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(NZE), may be established. NZE has been partially or fully achieved in some WWTPs.19,20

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Although these practical cases of energy self-sufficiency in WWTPs have been discussed in depth 3



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in previous studies, there have been no reports including statistical analysis of such cases and

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mathematical derivation of energy utilization patterns; additionally, a theoretical model is urgently

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needed to be developed to evaluate the feasibility of achieving NZE in WWTPs.

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As earlier studies,21-22 the attention has been devoted to direct in-plant energy consumption and

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generation in this paper (indirect energy use, energetic value of N and P as fertilizers, chemicals

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input, etc. are not considered), and it seems to be more pertinent with respect of the direct energy

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exploitation in WWTPs and the practical interest of stakeholders. The objective of the present study

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is to establish a NZE theoretical model based on substance transformation and energy utilization to

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systematically evaluate the feasibility and level of energy self-sufficiency of WWTPs, as well as to

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obtain optimal organic matter allocation between catabolism and anabolism. The theoretical model

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was employed to analyze data from 20 WWTPs in China to evaluate energy self-sufficiency based

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on anaerobic digestion of excess sludge. Finally, a statistical analysis of NZE cases was conducted.

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The results of this study provide a foundation for engineering applications of the NZE concept.

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2. METHODS AND MATERIALS

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2.1. Material-energy cycle of conventional and NZE WWTPs

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Metabolism is typically divided into two categories: catabolism, the decomposition of organic

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matter by cellular respiration with energy release; and anabolism, the synthesis of components of

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microbial cells with energy utilization. Microorganisms decompose organic matter to sustain life

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and reproduction via material-energy transformation in municipal WWTPs. Chemical energy from

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organic matter is converted into biomass energy during biochemical treatment. 23

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Organic matter, the main exploitable energy source in wastewater [represented as chemical 4


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oxygen demand (COD)], is completely biodegraded into simple molecules, such as CO2 and H2O,

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during wastewater treatment. Two pathways transform COD into CO2 and H2O: (1) a conventional

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pathway, in which COD is oxidized directly into CO2 by aerobic processes that require significant

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input of external electrical energy; and (2) a sustainable pathway, in which COD is maximally

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transformed into biomass (sludge) by bio-synthesis processes, after which the biomass is

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biodegraded into CH4. CH4 is ultimately utilized to produce electricity, heat, and simple molecules

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(CO2 and H2O). The conventional pathway is widely applied in existing WWTPs, in which most of

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chemical energy from organic matter is expended by consuming electric energy in the pathway,

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with CO2 escaping and energy depletion. The chemical energy in sludge is squandered with sludge

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waste in conventional pathway (Figure. 1a); In contrast, in the sustainable pathway, most of the

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chemical energy from organic matter is effectively utilized by sludge anaerobic digestion; therefore,

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external energy consumption is offset by the electricity and heat generated by cogeneration,

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reducing the cost and energy consumption associated with sludge treatment and disposal. Biomass

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recycling and energy-saving are achieved by the sustainable pathway; thus, the NZE mode of

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WWTPs is postulated based on the matter-energy cycle (Figure. 1b). A part of organic pollutants as

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substrate were completely oxidized (O2) into simple, stable molecules (CO2 and H2O) by

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microorganisms via catabolism with energy release. Remaining organic matter was directly used to

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synthesize structural molecules and reproduce cells via anabolism; the energy required for

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anabolism was provided by the energy released by catabolism. In order to ensure smooth

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metabolism of microorganism, the organic matter allocated to anabolism cannot exceed 2/3 of the

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total organic matter in bio-treatment unit.15 In the NZE WWTP model, an independent

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energy-recycling system (no need for external energy input) for wastewater treatment was 5



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developed based on the biomass energy utilization. In addition, a mathematical formulation of the

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energy supply-demand balance of wastewater treatment in the NZE wastewater treatment system

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was established.

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2.2 Establishment of the NZE WWTP theoretical model

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2.2.1 Specific energy consumption and COD flow of anabolism

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COD is more directly related to energy and carbon substrate is usually used in energy balance

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calculation, therefore specific energy consumption µ was defined as the electricity consumption per

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unit of COD removed (kWh/kgCOD), and it can be calculated with Eq. [1]:

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µ =

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where Ew represents the electricity consumption of wastewater treatment [kWh]. Mo (ton) represents

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the total mass of organic matter removed as measured by COD (Equation [3]).

Ew 1000M o

(1)

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In the NZE mode, sludge handling is the process of energy production based on self-energy

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balance, and the Ew is only contributed by the energy for aeration (Eaera); therefore, Eaera can be

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obtained by the function of the removed COD Mo (ton) with the specific energy consumption as

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follows:

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Eaera =1000µ M o

(2)

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As presented in section 2.1, there are two material flows of COD, catabolism and anabolism,

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which are represented by CODcata and CODana, respectively. Thus, the total mass of organic matter

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was divided into two parts, which were consumed by catabolism Mcata and anabolism Mana, and

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their quantitative relation was derived as Equation [3]. In addition, the total mass of organic matter

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Mo was related to influent CODinf, effluent CODeff, and wastewater flow Qw (m3/d), which was 6


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expressed as Equation [4]:

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M o = M cata + M ana

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Mo =

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where the units of COD and M were mg/L and ton/d, respectively.

(3)

(

Qw CODinf − CODeff 10

)

(4)

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In this paper, the MLVSS/MLSS ratio of sludge in practical municipal WWTPs is considered

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to be 0.75.24 The relationship between sewage sludge amount Mts (ton/d), organic matter Mvs (ton/d),

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and inorganic matter Mis (ton/d) was derived as Equation [5] and Equation [6]:

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M ts = M is + M vs

(5)

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M vs = 0.75M ts

(6)

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where Mvss is the sludge reproduced by anabolism of microorganisms, and it is assumed that

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CODana contributes completely to MLVSS.

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Microbial cells in sludge are typically expressed as C5H7O2N;16 the amount of oxygen required

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to oxidize one microbial cell to carbon dioxide, water, and ammonia is given by:

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C5 H 7 O2 N + 5O2 → 5CO2 + 2 H 2O + NH 3

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(7)

According to Formula [7], one gram of C5H7O2N is equivalent to 1.42 grams of COD; thus,

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Mana in sewage sludge was expressed as Equation [8]:

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M ana = 1.42 M vs

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2.2.2 Electricity and heat requirements of the anaerobic digestion system

(8)

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Thermophilic digestion is more beneficial for energy utilization than mesophilic digestion. 25,26

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Therefore, thermophilic digestion was adopted to generate energy, and the digestion temperature

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was assumed to be 50 °C. The energy requirement of digestion stirring is related to the excess

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sludge volume Qs (m3/d), which is a function of Mts (ton/d) (Equation [9]). In the calculation, the

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moisture content of the sludge was assumed to be 96% (a common value), whereas the density of

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the digested sludge ρs was assumed to be 1000 kg/m3 (approximately equal to that of water).

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Equations [5] and [8] were substituted into Equation [9] to simplify the Equation. A digestion time

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of 25 days and typical power per unit volume of stirring of 0.008 kW/m3 were chosen;27 after which

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the energy for stirring Ediges (kWh) per day was obtained by Equation [10]:

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Qs =

1000(kg/t) ⋅ M ts (t ) ρ s (kg / m3 ) ⋅ (1 − 96%)

(9)

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= 25(0.7 M ana + M is ) (m / d ) 158

Ediges = 25( d ) ⋅ Qs ( m 3 / d ) ⋅ 0.008( kW / m 3 ) ⋅ 24( h / d ) = 4.8Qs ( kWh / d )

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(10) The heat requirement includes heat loss of digesters and the heat necessary for raising the

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incoming sludge temperature. To obtain the total heat demand of sludge digestion, an inlet ambient

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sewage sludge temperature of 15 °C was assumed. Before feeding, the sludge was pre-heated to

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30 °C through heat convection with high-temperature sludge discharged from digestion tanks.28 The

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heat requirement Hs (MJ) for raising the sludge temperature to 50 °C was calculated using Equation

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[11]. The heat loss of digesters is usually 2–8% of the heat requirement of the sludge.29 Thus, the

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total heat requirement Ht (MJ) of complete digestion of 1 m3 of sludge was obtained by Equation

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[12] (with 8% heat loss assumed):

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H s = ρ s Qs cs ⋅ ( t s − to )

(11)

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H t =( 1 + 8%) H s = 90.4Qs ( MJ )

(12)

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where cs is the specific heat capacity of sludge (96% moisture), Qs is the volume of sludge, and ts

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and to represent the final and initial temperature, respectively, of the input sludge.

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2.2.3 Energy recovery of CHP system 8


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Energy production by anaerobic processes can be expressed in a simplified model as a function

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of the mass of biodegradable organic compounds (Mana). Generally, the biogas yield of sludge and

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its calorific value are assumed to be 0.35 m3/kg COD and 11 kWh/m3, respectively; thus, total

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energy recovery Pt (kWh) is calculated as Equation [13].18,29-31 Fifty percent heat efficiency ηh and

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40% electricity efficiency ηe of CHP are assumed, achieving an overall efficiency of 90%.32 All

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design parameters of the NZE WWTP model are provided in Table S1.

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Electricity recovery Pe (kWh) and heat recovery Ph (MJ) are calculated as Equation [14] and Equation [15], respectively:

Pt = 1000(kg / t )M ana × 0.35(m3 / kg ) ×11(kWh / m3 ) = 3850M ana (kWh)

Pe = ηe Pt

(13)

(14)

= 1540M ana (kWh) Ph = η h Pt × 3.6 ( MJ / kWh)

(15)

= 6930 M ana ( MJ )

2.2.4 NZE WWTP theoretical model

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In the NZE WWTP model, energy self-sufficiency of 100% is assumed; thus, there exists an

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energy balance between electricity production Pe and electricity consumption Eaera and Ediges

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(Equation [16]). At the same time, the heat requirement should be provided by heat recovery, as

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expressed by Inequality [17]. However, only some WWTPs can achieve 100% energy

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self-sufficiency in reality. Anaerobic digestion was applied to recover energy based on the premise

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that electricity production Pe and heat recovery Ph should meet the digestion energy consumption

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(Ediges and Ht), presented as Inequality [17] and Inequality [18]. Inequality [18] always holds based

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on Equations [16] (Eaera ≥0), therefore, Inequality [18] is not considered in this model. Equation [19]

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and [20] can be obtained by substituting Equations [2], [9], [10], [12], [14], and [15] into Equations 9



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[16] and [17].

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Pe = Eaera + Ediges

(16)

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Ph ≥ Ht

(17)

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Pe ≥ Ediges

(18)

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1456M ana = 120 M is + 1000 µ M o

(19)

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2.37 M ana ≥ M is

(20)

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Inequality [20] is an energy boundary condition in the NZE WWTP model. The minimum of

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Mvs/Mts (organic matter content of sludge) was obtained by solving Inequality [20] with Equation [5]

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and Equation [8], yielding minimum of 0.23 which was selected as a comprehensive energy

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boundary condition of energy self-sufficiency in the NZE WWTP model. The Mvs/Mts is generally

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less than 0.9;33 therefore, an inequality regarding Mvs/Mts can be obtained (Inequality [21]).

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Ultimately, Equation [19] and Inequality [21] constituted the NZE theoretical model.

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0.23 ≤ M vs / M ts ≤ 0.9

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Furthermore, the observed yield of excess sewage sludge was defined as follows:

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Yobs =

(21)

M vs CODt

(22)

Where CODt is the total of removed COD in the wastewater treatment, and a large value of

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Yobs indicates that material and energy flow are directed towards anabolism, yielding sludge.

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3. RESULTS AND DISCUSSION

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3.1. Case studies

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Twenty municipal WWTPs in China (Table S2) were investigated to validate the theoretical

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model. Characteristic parameters of the WWTPs are shown in Table S3. In the matter-energy cycle, 10


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metabolic energy allocation corresponds to COD flow (material flow), and the amount of removed

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COD is a relatively constant value during wastewater treatment. It is clear that increasing the

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amount of COD allocated to anabolism benefits energy recovery; however, the maximum benefit is

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achieved when anabolism of COD utilizes 2/3 of the total COD in the bio-treatment process.15

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Figure 2 shows the COD flow of anabolism and catabolism in twenty WWTPs. The CODana/CODt

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ratios of municipal WWTPs were in the range of 0.34–0.66 (average = 0.53). Only 6 WWTPs had

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CODana/CODt ratios greater than 0.6 (close to the extremum). These results indicate that there is

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significant potential to reduce energy dissipation by improving the CODana/CODt ratios in the

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WWTPs. To avoid energy waste, WWTPs need optimized strategies to regulate the COD flow

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between catabolism and anabolism; the NZE model provided a theoretical basis to optimize such

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allocation.

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According to the NZE theoretical model, when given a specific value of Mvs/Mts, the

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relationship between specific electricity consumption µ and the material allocation ratio Mana/Mo

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(represented as the CODana/CODt ratio) may be obtained by substituting Eqs. [5] and [8] into Eq.

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[19]. Consequently, in the range of 0.23 to 0.9, each specific Mvs/Mts corresponds to an NZE

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equation for µ and the CODana/CODt ratio, which can be shown as a line in a graph. As the Mvs/Mts

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ratio changed, a series of lines was drawn to show the relationship between µ and CODana/CODt

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(Figure 3A). The frontiers of these curves are the lines obtained when Mvs/Mts was its most extreme

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values of 0.23 and 0.9. Changing the value of CODana/CODt can alter the amount of recovered

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energy so that it meets the energy needs of water treatment.

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The Mvs/Mts ratio (organic matter content of the sludge) in the wastewater treatment process is

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usually determined as 0.75, and the corresponding zero energy curve was drawn to analyze the 11



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sample points (Figure. 3B). Because the maximum of Mana/Mo is 2/3 (0.67), the maximum specific

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energy consumption was calculated to be 0.95. The sample points were categorized as type A, type

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B, or type C according to the probability of realizing NZE status (Table 1).

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Type A cases were those in which specific energy consumption was greater than 0.95

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irrespective of CODana regulation [region (a) in Figure 3B]; 100% energy self-sufficiency cannot be

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achieved in these WWTPs, which require external energy. The low influent CODinf and high Ew led

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to the phenomenon in these WWTPs (Table S3). However, biomass energy can be recovered by

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anaerobic digestion with cogeneration to compensate for some external power consumption by

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these WWPTs. The energy self-sufficiency rates of the municipal WWTPs of Degan, Xiyong,

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Jijiang, Changshou, Jiuquhe, and Jiangdong are 94.6%, 79.4%, 73.9%, 93.8%, 87.4%, and 88.0%,

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respectively.

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Type B cases can currently achieve NZE status and export excess energy [region (b) in Figure

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3B]. The energy self-sufficiency rates of the municipal WWTPs in Zhouping, Xiaojiahe, Wushan,

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Chengebei, Dajiu, and Chayuan are 158.2%, 127.9%, 103.4%, 122.4%, 101.7%, and 112.9%,

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respectively.

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Type C cases are those in which municipal WWTPs can achieve 100% energy self-sufficiency

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by adjusting CODana; these WWTPs must increase the value of CODana to increase sludge yield.

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Ultimately, the energy produced by increasing sludge yield will offset the energy requirement of

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wastewater treatment.

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There are eight municipal WWTPs can achieve 100% energy self-sufficiency and six

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municipal WWTPs can export excess energy in the investigated twenty municipal WWTPs in this

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study. The theoretical model of NZE wastewater treatment was successfully employed to evaluate 12


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the feasibility and level of energy self-sufficiency in practical wastewater treatment cases, and the

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mode can be utilized to conserve resources by biomass energy recycling during wastewater

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treatment, allowing WWTPs to achieve environmental sustainability.

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3.2. Statistical analysis of specific energy consumption and CODana/CODt in the NZE model

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In order to assess the predicted values of µ and CODana/CODt calculated using the NZE

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WWTP model using real-life examples, the µ (kWh/kgCOD) values of 53 municipal WWTPs

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(Table S4) were subjected to normality tests and statistical description. The Shapiro-Wilk test

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yielded the following results: w = 0.97118, p-value = 0.2268 (>α, 0.05). Therefore, the null

266

hypothesis can be accepted, indicating that specific energy consumption is distributed normally

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(Figure 4a; mean = 0.69, standard deviation = 0.28).

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According to the µ distribution, the sample size was enlarged to 1000 by generating random

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numbers, which were substituted into the NZE WWTP model to obtain corresponding

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CODana/CODt ratios. The obtained CODana/CODt ratios were filtered under the condition of 0

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