Recovery of Freshwater from Wastewater: Upgrading Process

Critical Review pubs.acs.org/est

Recovery of Freshwater from Wastewater: Upgrading Process Configurations To Maximize Energy Recovery and Minimize Residuals Yaniv D. Scherson*,†,‡ and Craig S. Criddle†,‡ †

Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, United States Woods Institute for the Environment, NSF Engineering Research Center ReNUWIt, Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, United States

‡

S Supporting Information *

ABSTRACT: Analysis of conventional and novel wastewater treatment configurations reveals large differences in energy consumed or produced and solids generated per cubic meter of domestic wastewater treated. Complete aerobic BOD removal consumes 0.45 kWh and produces 153 g of solids, whereas complete anaerobic treatment produces 0.25 kWh and 80 g of solids. Emerging technologies, that include short-circuit nitrogen removal (SHARON, CANON with Anammox, CANDO) and mainstream anaerobic digestion, can potentially remove both BOD and nitrogen with an energy surplus of 0.17 kWh and production of 95 g of solids. Heat from biogas combustion can completely dry the solids, and these solids can be converted to syngas without imported energy. Syngas combustion can produce ∼0.1 kWh with an inorganic residue of just 10 g. If salt is removed, freshwater can be recovered with net production of electrical energy from methane (0.03−0.13 kWh) and syngas (∼0.1 kWh) and an inorganic residue of ∼0.1-0.3 kg as brine. Current seawater desalination requires 3−4 kWh (thermodynamic limit of 1 kWh) and results in an inorganic residue of ∼35 kg as brine. required for removal of salt is 0.5 kWh per m3 (including pretreatment),11 with a thermodynamic limit of just 0.02 kWh per m3 (80% recovery).12 Energy is still needed for delivery of treated water to users, but transport distances are much shorter. At present, the transport and treatment of domestic wastewater imposes a significant energy demand, consuming ∼3%13 of the U.S. electrical energy supply. Values for other countries range from 3 to 5%.14 This demand can be viewed as the legacy of aerobic treatment processes deployed in the 20th century. An example is activated sludge, the most commonly used biotechnology in the world. Conventional activated sludge treatment requires 0.40−0.65 kWh per m3 of which about half the energy is required for O2 delivery.9,15−23 For municipalities, treatment of wastewater is the largest energy consumer, comprising 30−60% of municipal demand.18 Thermodynamic calculations indicate that this demand is unnecessary: the chemical energy in biodegradable COD (bCOD), inert COD (iCOD), and reduced nitrogen (NH4+, organic N) in municipal wastewater (medium strength) is 1.96 kWh per m3 (Table 1),

1. INTRODUCTION The sun distills approximately 107000 km3 per year of freshwater.1 This enormous volume would seem more than sufficient to meet projected human demand (domestic, industrial, and agricultural) of 6900 km3 per year in 2030.2 But precipitation is hugely uneven,3,4 and rapidly growing urban centers are often located in arid regions5,6 where droughts of increasing severity are predicted.7,8 The conventional solution to water scarcity is transport of large volumes of freshwater from water-rich to water-poor regions, but the energy needed to move water across watersheds is significant. The California State Water Project uses 2.0−2.6 kWh per m3 to transport 12 billion m3 per year of freshwater from the waterrich North to the arid South.9 An alternative to the transport of “solar-treated” water over long distances is the treatment and use of local supplies, such as seawater (3−5% dissolved solids), brackish water (0.05−3% dissolved solids), and wastewater (0.3−0.9% dissolved solids). The energy required to desalinate seawater is similar to the energy needed to transport freshwater over long distances: state-of-the-art desalination of seawater by reverse osmosis requires 3−4 kWh per m3, and the thermodynamic limit is 1 kWh per m3.10 The energy needed to recover freshwater from seawater is due to its high ionic strength. For domestic wastewater effluent, the salinity is much lower, and the energy © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8420

April 7, 2014 June 18, 2014 June 25, 2014 June 25, 2014 dx.doi.org/10.1021/es501701s | Environ. Sci. Technol. 2014, 48, 8420−8432

Environmental Science & Technology

Critical Review

then add “upgrades” for progressively higher levels of water quality and/or improvements in energy recovery. We assume that energy recovery is achieved by methane fermentation followed by combustion with combined heat and power. Where thermodynamically feasible, we assume biosolids gasification for additional energy recovery and to minimize volume for disposal. Assumptions are summarized in Supporting Information (SI) Table S4. For configurations 1 and 2, the required level of treatment is removal of bCOD. Configuration 1 does not include energy recovery. Configuration 2 adds energy recovery as an “upgrade”: particulate bCOD from enhanced primary and conventional secondary treatment is digested anaerobically. We assume that 50% of the influent bCOD (a theoretical maximum) is diverted to anaerobic digestion, and that the remainder is removed aerobically.24 Configuration 3 assumes removal of bCOD and ammonium oxidation. Ammonium is converted to nitrate, and nitrate is discharged in the effluent. Energy recovery is achieved by anaerobic digestion of primary and secondary biosolids. Configuration 4 upgrades configuration 3, adding an effluent standard for removal of total nitrogen. This is achieved using conventional anoxic-aerobic treatment, with anaerobic digestion of primary and secondary biosolids for energy recovery. Nitrogen-rich side-streams from digestion are returned to the headworks. Configurations 5−7 are possible upgrades of Configuration 4 where the nitrogen-rich side-streams are treated. Nitrogen removal from the side-stream decreases nitrogen load on the mainstream. Aeration energy requirements decrease, and less bCOD is required for denitrification. This allows more efficient capture of organic matter in primary treatment and increased biogas production. The three side-stream treatment processes evaluated are Single Reactor System for High Activity Ammonium Removal Over Nitrite (SHARON), Completely Autotrophic Nitrogen Removal Over Nitrite (CANON), and Coupled Aerobic-anoxic Nitrous Decomposition Operation (CANDO). Configurations 8−11 assume anaerobic removal of bCOD in the mainstream and anaerobic digestion of biosolids, with recovery of energy as methane. Nitrogen is removed from the mainstream by conventional nitrification-denitrification (configuration 8), by CANON (configuration 9), by CANDO (configuration 10), or it is left in the effluent as fertilizer (configuration 11). 2.2. Consumption and Production of Energy. For each of the 11 configurations in Table 2, we use mass and energy balances for the physical-chemical and biological unit operations to estimate net energy consumed or produced (kWh) and biosolids and TS produced (g) by treatment of one cubic meter of wastewater. We assume treatment of medium strength domestic wastewater, pretreated to remove inorganic suspended solids, with an influent bCOD of 320 g per m3 (175 g of soluble bCOD per m3 plus 145 g of particulate bCOD per m3) and iCOD of 110 g per m3 (60 g of soluble iCOD per m3 plus 50 g of particulate iCOD per m3), a Total Kjeldahl Nitrogen of 40 g-N per m3, and total dissolved inorganic solids of 300 mg per L.24 For both primary treatment and secondary solids separation, we assume ideal separation of suspended organic matter from dissolved organic matter. Primary bisolids are assumed to be 90% digestible, and secondary biosolids 50% digestible.31,32 We also assume that nitrogen removal during primary treatment is linearly proportional to bCOD removal: 10% N-removal

about four times the energy required for treatment using conventional technology.24,25 Table 1. Maximum Theoretical Energy Potential in Medium Strength U.S. Wastewater24 constituent total COD bCOD dissolved suspended iCOD dissolved suspended nitrogen free (NH3−N) organic (N) total energy

concentration (g per m3)

maximum potential energy (kWh per m3)

430 320 175 145 110 60 50 40 25 15

1.66a,b 1.24 0.56 0.67 0.42 0.23 0.19 0.3c 0.19 0.11 1.96

a

Based on theoretical 3.86 kWh per kg COD (complete COD oxidation).24 bMeasured reported value for total COD is 1.75 kWh per m3.25 cBased on the higher heating value for ammonia combustion with oxygen to N2.

Decision-making on wastewater treatment plant upgrades and configurations is complex and requires consideration of many factors in addition to energy, including the scale of treatment (centralized, satellite, hybrid), economic considerations, footprint, life cycle impacts, and other factors important to local communities.26 In this study, however, we limit our analysis to the impacts of centralized treatment train configurations on two key performance metrics per m3 of treated wastewater: (1) energy consumed or produced and (2) biosolids and total solids (TS) produced. We use these key metrics to quantitatively compare existing configurations to new and possible future configurations. For energy recovery, we evaluate the effects of enhanced primary treatment, a shift to complete anaerobic removal of soluble bCOD, impacts of heat recovery, and biosolids gasification. For nitrogen removal, we compare conventional nitrification-denitrification to new technologies that have been demonstrated at full-scale27,28 and to a promising technology under evaluation at pilotscale.29,30 Finally, to estimate energy requirements for desalination in the case of water reuse, we add an incremental energy demand for reverse osmosis removal of salt from wastewater after removal of organic matter and nitrogen. We compare this value to the value required for desalination of seawater.

2. TREATMENT OF ONE CUBIC METER OF WASTEWATER 2.1. Overview. Our aim is to evaluate wastewater treatment configurations that produce freshwater of differing quality from domestic wastewater. We limit our analysis to medium strength domestic wastewater and to the 11 configurations listed in Table 2. For each configuration, we compute two key performance metrics per unit volume of wastewater treated: (1) energy consumed or produced and (2) mass of biosolids and TS generated. We also consider three levels of treatment: (1) removal of biodegradable organics; (2) removal of biodegradable organics and nitrogen; and (3) removal of biodegradable organics, nitrogen, and salt. We begin with a configuration that simulates conventional secondary treatment 8421

dx.doi.org/10.1021/es501701s | Environ. Sci. Technol. 2014, 48, 8420−8432

8422

mainstream

11

10

9

8

7

6

5

bCOD and nitrogen bCOD and nitrogen bCOD and nitrogen bCOD

anaerobic digestion

anaerobic digestion

anaerobic digestion

anaerobic digestion

enhanced primary anaerobic digestion



conventional primary anaerobic digestion

conventional primary aerobic digestion enhanced primary anaerobic digestion enhanced primary anaerobic digestion

unit operations for particulate bCOD

anaerobic

anaerobic

anaerobic

anaerobic

conventional anoxic-aerobic




conventional aerobic conventional aerobic conventional aerobic

unit operations for soluble bCOD

none

mainstream CANDO

mainstream CANON

mainstream nitrification and denitrification

side-stream CANDO

side-stream CANON

side-stream SHARON

mainstream nitrification and denitrification

mainstream nitrification

none

none

unit operations for total N

−0.10

−0.25

42

−0.11

−0.11

47

−0.11

−0.07

42

−0.11

−0.11

49

−0.10

−0.17

40

−0.09

7

23

21

37

37

−0.09

−0.11

72

biosolids producedd g VSS per m3

N/A

net electrical energy produced (−) from syngasc kWh per m3

0.10

0.08

0.06

0.11

0.12

0.27

0.09

0.45

net electrical energy consumed (+) or produced (−) kWh per m3

b

80

97

95

113

120

125

120

127

117

153 (not digested) 114

digested solids for disposale g TS per m3

9

10

10

12

14

14

14

14

13

13

N/A

digested and thermally treated solids for disposalf g TS per m3

Energy consumed or produced and solids produced are summarized for each configuration. Additional energy recovered from solids thermal treatment shown. bNet electrical energy required for treatment of 1m3 of wastewater. cAdded electrical energy recovered from solids thermal treatment with gasification (values from Table 3). d“Biosolids Produced” assumes 1.42 g COD per g VSS (see SI Section S.5). eTo estimate “Digested Solids for Disposal”, the “Biosolids Produced” are added to (1) Inorganic suspended solids associated with the biosolids produced; (2) Influent inert volatile suspended solids and the associated inorganic fraction; and (3) Salt residue from dissolved inorganic solids that results from water evaporation during the solids drying. VSS are given in the table above. Inorganic suspended solids are assumed to be 10% of the biosolids produced and are calculated by the product of VSS and the ratio 10:90. The mass of influent inert volatile suspended solids is equal to equal to 64 g per m3 and is calculated by assuming they account for 40% of the influent VSS (160 g per m3); the inorganic mass associated with influent inert volatile suspended solids is 7 g per m3 and is calculated by assuming it accounts for 10% of the mass.24,33 The mass of dissolved inorganic solids is calculated by the product of the volume of water evaporated and the concentration (300 mg per L).24 f Assumes “heat neutral” thermal treatment (see Section 2.4) for configurations 2−11. The mass of digested and thermally treated solids for disposal is equal to the product of TS and the mass fraction of inorganic solids.

a

bCOD and NH4+ bCOD and nitrogen

3

bCOD and nitrogen bCOD and nitrogen bCOD and nitrogen

bCOD

2

side-steam

bCOD

1

config

conventional

4

effluent criteria

Table 2. Eleven Configurations for Removal of bCOD and Nitrogen (Without Salt Removal)a

Environmental Science & Technology Critical Review



Critical Review

correlates with 25% bCOD removal, and 20% N-removal correlates with 50% bCOD removal (adapted from Siegrist et al.).33 For secondary treatment and anaerobic digestion, we perform an oxygen equivalents balance on the electron donor, terminal electron acceptor, and biomass (assumed empirical formula of C 5 H 7 O 2 N) to estimate biomass production, oxygen use, nitrate or nitrite production and consumption, and methane production (SI Table S1). We assume complete hydrolysis of particulate bCOD, complete ammonification of organic nitrogen, conventional nitrification for ammonium removal (configuration 3), and conventional nitrification-denitrification for removal of total nitrogen (configurations 4 and 8). For configurations 5−7 and 9−10, we assume unconventional nitrogen removal in which ammonium is partially oxidized to nitrite. Nitrite is then reduced to N2 with bCOD as electron donor (SHARON), to N2 with ammonium as electron donor (CANON), or to N2O with subsequent use of N2O as a co-oxidant with oxygen of methane (CANDO). For removal of bCOD, we assume oxidation of 8 g COD releases one mole of electrons. We further assume that a fraction of electrons is used for cell synthesis ( fs) and the remaining fraction (fe) is transferred to the terminal electron acceptor for energy. The resulting electron balance fs + fe = 1 is a balance on oxygen equivalents.34 The overall stoichiometry R is given by combining the half reaction for oxidation of the donor (Rd), with the weighted half reactions for synthesis (Rs), and energy (Ra): R = R d + fs ·R s + fe ·R a

net energy demand = energy demand for aeration − energy recovered from biogas combustion + energy of baseline plant operation

To estimate electrical energy demand for aeration, we multiply fe by input bCOD and assume that 1 kWhr of electrical energy is used to deliver 1 kg of oxygen.24 To estimate electrical energy recovery from methane, we multiply fe by input bCOD, then by the following three conversion factors: 0.25 g methane per g COD, the heat of combustion of methane (0.0155 kWh per g when O2 is the oxidant or 0.0212 kWh per g when N2O is oxidant), and an electrical energy recovery efficiency of 40%. For all configurations, we assume that the baseline energy required for plant operation (pumping, screening, settling, disinfection) is 0.23 kWh per m3.17 Configurations 2−11 recover energy by converting oxygen demand in the wastewater (bCOD, NOD) into biosolids, N2O (in the case of CANDO), and methane. Calculations used to estimate energy recovered from biogas combustion are summarized in Section 2.4. For configurations 3−10, we calculate the theoretical minimum energy required for wastewater effluent desalination using SI Equations S1−S4. This value is used to project a plausible future energy requirement for desalination, assuming that the future requirement is three times the thermodynamic minimum. For desalination calculations, we assume influent total dissolved solids (TDS) of 600 ppm and effluent TDS of 10 ppm. We calculate a greenhouse gas (GHG) penalty for each configuration equal to the greenhouse gas emissions associated with imported electrical energy and N2O emissions (expressed as CO2 equivalents) minus the electrical energy recovered from combustion of biogas and syngas (also expressed as CO2 equivalents). We assume that all GHG emissions are associated with electricity consumed and N2O released. CO2 emissions associated with aeration and baseline plant operation are calculated using the U.S. EPA emission factor of 0.706 kg-CO2 per kWh.35 We assume that CO2 emissions from the oxidation of bCOD do not contribute to GHG emissions because this organic matter is of recent origin and can be regenerated over short time scales via photosynthesis.36 We further assume that N2O release to the atmosphere contributes 310 kg CO2 per kg N2O.37 Finally, we assume that nitritation releases 1.7%38 of the influent nitrogen as N2O, and that conventional nitrification/ denitrification releases 0.1%39 of the influent nitrogen as N2O. 2.3. Production of Biosolids and Total Solids. For the purposes of this analysis, we define “biosolids” as the biologically generated volatile suspended solids. A portion of the biosolids is biodegradable; the remainder is inert. Biosolids removed by primary treatment are combined with biosolids generated by secondary treatment. To estimate the biosolids generated by secondary treatment and by anaerobic digestion, we multiply the appropriate value of fs by the influent bCOD (g per m3) then divide by 1.42 g COD per g volatile suspended solids.34 We estimate the digested volatile suspended solids as the inert influent volatile suspended solids plus inert volatile suspended solids produced biologically. Digested solids for disposal is the sum of digested volatile suspended solids and inorganic solids, assuming a ratio of inorganic solids (ash) to

(1)

The synthesis fraction (fs) is a dimensionless measure of yield. It is a function of the maximum fraction for synthesis fs°, defined as the oxygen equivalents assimilated in biomass divided by the bCOD or NOD consumed, specific decay rate b (unit of day−1), and solids residence time (SRT, units of days),34 where ⎛ 1 + 0.2·b·SRT ⎞ ⎟ fs = fs o ⎜ ⎝ 1 + b·SRT ⎠

(3)

(2)

We focus this study on dispersed growth reactors where SRT is the mass of solids in the bioreactor divided by the solids removal rate. For attached growth reactors SRT is the reciprocal of detachment rate.34 SI Table S2 summarizes the half reactions used for all analyses. SI Table S3 summarizes the stoichiometric parameters, assumed solids residence time, and decay rate for each unit operation. Effluent bCOD and nitrogen from the primary clarifier is the influent to the first unit of a secondary treatment system. This system consists of aerobic or anoxic/aerobic processes in series and is assumed to provide complete removal of bCOD and complete removal of nitrogen (ammonium or total nitrogen) from the aqueous phase. The fraction for energy (fe = 1 − fs) is used to estimate aeration energy requirements and nitrogen conversion stoichiometry. The net energy demand (kWh) per m3 of treated wastewater is calculated for each configuration as 8423



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volatile suspended solids of 10:90.34 The mass of digested and thermally treated solids for disposal (i.e., ash) is computed as inorganic suspended solids from the digester plus the inorganic residue resulting from evaporation of water containing dissolved solids. 2.4. Recovery of Energy from CH4, N2O, and Biosolids. Figure 1 illustrates the methodology used to maximize energy recovery with combustion of methane and thermal treatment of residual solids. We assume combustion of CH4 with either O2 or N2O as oxidants, where 40% of the combustion enthalpy is recovered as electricity and 60% as heat.40 A fraction of the recovered heat is used to warm water for anaerobic digestion, and the remainder to partially or completely dry solids. To compute heating requirements for anaerobic digestion, we calculate water volume assuming 4% influent solids by mass and warming from 15 to 35 °C. We assume mechanical dewatering to 20% dry solids cake followed by drying with the heat generated by methane combustion.

3. WASTEWATER TREATMENT PLANT UPGRADES The effects of upgrades on energy consumed or produced and solids produced per cubic meter of wastewater treated are summarized in Table 2 and SI Figures S2−S12. The choice of configurations has major impacts upon energy recovery and residual solids production (Table 2). For most configurations, energy is invested and significant levels of biosolids are produced. For configurations that convert soluble organic matter into methane, however, net energy recovery is achieved, and biosolids production is greatly reduced. In the sections that follow, we highlight 7 of the 11 configurations (configurations 1, 2, 4, 5, 9, 10, 11) in Table 2, considered in order of efficiency of energy recovery. We begin with the least energy-efficient configuration with bCOD removal under fully aerobic conditions (configuration 1). Nitrogen is not removed, and no measures are taken for energy recovery. We then carry out a series of “upgrades”. We first add anaerobic digestion (configuration 2). This is followed by a nitrogen removal upgrade in which conventional nitrification− denitrification is used to remove nitrogen (configuration 4). Less efficient primary settling is needed in order to provide sufficient bCOD for denitrification, so less bCOD remains for energy production. In the next upgrade (configuration 5), we decrease nitrogen loading by removing nitrogen in the sidestream with a short-circuit nitrogen removal process. This enables more efficient primary treatment, and increases energy production. To estimate the upper bounds on energy production for potential future configurations, we conclude with mainstream anaerobic treatment coupled to mainstream nitrogen removal, where nitrogen is removed with an unconventional process (configurations 9−10), or it remains in the effluent for use as fertilizer (configuration 11). For each configuration, we use flow diagrams to illustrate the fate of oxygen equivalents. Arrows indicate the conversion of oxygen equivalents inputs (bCOD, NOD, O2) and outputs (CH4, H2O, N2, N2O, biosolids) where line width is proportional to flux. Methane combustion in combined heat and power recovers electrical energy and heat. The recovered electricity offsets electrical energy demand for aeration and baseline plant operation. The recovered heat is used to warm water for anaerobic digestion, and the remaining heat is used to dry residual biosolids for syngas production, enabling “heatneutral” thermal treatment. Details of the bCOD and nitrogen flux for each unit operation of a treatment configuration are provided in SI Section S.4. 3.1. Conventional Removal of bCOD. Figure 2a illustrates aerobic removal of bCOD with no provisions for nitrogen removal or energy recovery. Aerobic oxidation consumes oxygen and produces large quantities of biosolids. Influent bCOD reducing equivalents are ultimately transferred to water and biomass. Nitrogen is only removed by incorporation into biomass. This configuration is the least energy efficient and produces the most biosolids. Configuration 1 can be upgraded by diverting bCOD from the mainstream to anaerobic digestion (Figure 2b). In this configuration, soluble bCOD is removed by aerobic treatment, where a fraction is oxidized, and the balance assimilated. The oxidized fraction depends upon the SRT (eq 2): at long SRT, more is oxidized and less assimilated; at short SRT, more is assimilated and less oxidized. Operation at short SRT followed by anaerobic digestion can recover approximately one-fourth of the energy originally present in the soluble and colloidal

Figure 1. Methodology for optimization of energy recovery. Electrical energy and heat is recovered by production of biogas methane followed by its oxidation with combined heat and power. A 40:60 split is assumed between electricity and heat production. Some heat is used to warm water for anaerobic digestion, and the remainder for drying of solids. Residual solids are mechanically dewatered to 20% dry solids. Dried solids enter the reformer where a fraction is combusted to provide heat for gasification, and the remainder is converted to syngas.

Heat for drying is used to increase temperature of the cake from 15 to 100 °C and to evaporate water to the extent possible. After drying, solids enter a reformer for syngas production. Combustion of a fraction of the volatile solids generates sufficient heat for conversion of the remaining fraction to syngas. This energy balance can be considered “heatneutral” because no imported energy is required. When solids are dried to a greater extent, less energy is needed for heating and more can be converted to syngas. Syngas production is calculated as a function of dry solids content with the thermochemical equilibrium solver Cantera41 implemented in Matlab. We assume residual solids have an empirical formula C5H7O2N, with a lower heating value of 12.7 MJ per kg (3.5 kWh per kg) for dry ash-free solids (representative of stabilized solids) (Phyllis2, Database for Biomass and Waste), and reformer temperature of 850 °C. Details are provided in SI Sections S.2 and S.3. 8424



Critical Review

Figure 2. Flow of oxygen equivalents for (a) aerobic removal of soluble and particulate bCOD with no anaerobic digestion as depicted in SI Figure S2; (b) removal of suspended solids with a separation device (clarifier or microscreen) followed by aerobic removal of soluble bCOD and anaerobic digestion of suspended solids as depicted in SI Figure S3; (c) removal of soluble bCOD and nitrogen by conventional nitrification-denitrification and anaerobic digestion of suspended solids; (d) removal of soluble bCOD and mainstream nitrogen by conventional nitrification−denitrification, removal of side-stream nitrogen by the SHARON short-circuit process, and anaerobic digestion of suspended solids as depicted in SI Figures S6−S8.

bCOD. For this configuration, the energy produced from digestion is 0.62 kWh per m3, a value that compares favorably with a measured value of 0.67 kWh per m3 for a full-scale treatment plant with this configuration.43 One strategy to enhance energy recovery is to maximize assimilation by operation at short SRT followed by cell separation, thickening, and anaerobic digestion. A challenge is that cells can be difficult to lyse, limiting the fraction that can be converted to methane to ∼50%.31 Strategies that lyse cells can increase this fraction and decrease biosolids produced.44−46 These strategies require an energy investment, but reportedly enable net energy surplus in some applications, where the reported ratio of net energy gain to electricity consumed is 2.5.46 3.2. Upgrades for Nitrogen Removal. Conventional nitrogen removal (configuration 4) requires addition of an electron donor to reduce nitrate to N2. This requirement can be met by operating a primary clarifier inefficiently, releasing sufficient bCOD to the secondary treatment system for denitrification.33,47,48 Figure 2c illustrates this scenario: primary treatment settles only 33% of the influent bCOD, and the remaining 67% is used for mainstream denitrification. Nitrate replaces oxygen as the oxidant for bCOD removal, decreasing aeration energy requirements, but complete nitrification increases oxygen demand. In fact, the oxygen demand for nitrification is comparable to that required for aerobic oxidation (Figure 2a). While configuration 3 removes bCOD and nitrogen, energy recovery is limited by the bCOD required for denitrification. If this bCOD were not used for

denitrification, it could be used to produce methane and thereby increased energy recovery. The effluent side-stream of the anaerobic digester is rich in ammonium. It is typically returned to the headworks where it increases the mainstream nitrogen load by 15−30%. When trucked wastes are also digested, side-stream nitrogen can constitute 50% or more of the mainstream load.24,49 Nitrogen removal from side-streams is attractive because flow rates are small, and the nitrogen concentrated. The bCOD required for nitrogen removal from side-streams can be decreased using “short-circuit” processes that prevent formation of nitrate. Nitrite, rather than nitrate, is reduced to N2. Because denitrification of nitrite requires less bCOD, more bCOD can be routed to methane. Use of short-circuit processes to treat side-streams decreases nitrogen and bCOD loadings on the mainstream, decreases the energy required for O2 delivery, and increases bCOD available for methane production. Several short-circuit processes are used for side-stream treatment. Two that have been vetted at full-scale are the SHARON process27,50,51 and the CANON process with Anammox.28,52−56 In the SHARON process, bCOD is added to drive nitrite reduction to N2. In the CANON process, roughly half the ammonium is oxidized to nitrite; the balance is oxidized to N2 and used to reduce nitrite to N2. A third short-circuit technology, not yet scaled-up, is CANDO.29,30 Ammonium is partially oxidized to nitrite, and the nitrite is partially reduced to N2O with bCOD as the source of reducing equivalents. N2O is used as a methane co-oxidant, and N2O is converted to N2 during combustion. CANDO decreases demand for bCOD because denitrification is 8425



Critical Review

Figure 3. Flow of oxygen equivalents assuming: (a) bCOD removal by anaerobic treatment/anaerobic digestion followed by mainstream nitrogen removal by CANON, as depicted in SI Figure S10; (b) bCOD removal by anaerobic treatment/anaerobic digestion followed by mainstream nitrogen removal by CANDO, as depicted in SI Figure S11; and (c) removal of all bCOD by anaerobic digestion. Nitrogen in the effluent is used as a fertilizer. (See SI Figure S12).

in processes that better separate SRT from HRT, with accumulation of active biomass. The anaerobic membrane bioreactor (AMBR) completely separates SRT and HRT enabling high COD removal at short HRT.60 The major technical challenge is membrane fouling: a condition that requires energy intensive gas scouring.61 In fact, the energy required (0.25−1.0 kWh per m3) can exceed the energy required for aerobic MBRs (0.3−0.6 kWh per m3) and even exceeds the electrical energy required for activated sludge treatment (0.2−0.4 kWh per m3).60 Recently, a new system: the staged anaerobic fluidized membrane bioreactor (SAF-MBR) has achieved high bCOD at pilot-scale removal treating domestic wastewater at short HRT, with minimal membrane fouling over extended periods of operation, even at cool temperatures.62−65 Sufficient methane was generated to offset energy demand. Some methane remained dissolved in the effluent. Gas stripping could remove this methane with less than 0.05 kWh/m3. Such a system could achieve the soluble organic removal envisioned for configurations 8−11. Microbial fuel cells (MFC)66,67 are an alternative for methane fermentation for electrical energy recovery. In these reactors, microorganisms oxidize organic material at an anode and transfer electrons through an external circuit to an electron acceptor (typically O2) at a cathode. At present, however, energy capture efficiencies for MFC treatment of municipal wastewater are low (∼4%).68 This is largely due to voltage losses that occur when O2 is reduced at the cathode and columbic losses that occur when O2 mixes with bCOD from the anode compartment, enabling microbial growth without power production.66,69 A new technology that circumvents these issues is the microbial battery, a device that eliminates oxygen at

incomplete, and energy is recovered by coupling the oxidation of methane to the reduction of N2O to N2, an energy-yielding step. As shown in Figure 2d, sides-stream treatment removes a fraction of influent nitrogen by reduction of nitrite; the remaining influent nitrogen is removed by reduction of nitrate in the mainstream. Primary settling diverts 50−55% of the bCOD to anaerobic digestion (depending on side-stream process used), while satisfying bCOD demand for mainstream denitrification. Such configurations increase energy recovery, reduce aeration demand, remove bCOD and nitrogen, but still do not achieve net energy positive treatment. 3.3. Upgrades for Nitrogen Removal and Maximum Energy Recovery. Figure 3a and b show potential future configurations that enable energy recovery from both soluble and particulate bCOD while also enabling nitrogen removal from the mainstream. These configurations shift the energy balance from net energy consumption to net energy production, with significantly fewer residual biosolids. A variety of processes, established and emerging, can recover energy from soluble bCOD. These processes generally rely upon the production of methane by concentrated methanogenic biomass attached to a surface as biofilms or held within the bioreactor by a membrane. Most notable is the upflow anaerobic sludge blanket (UASB) reactor where dense biomass is fluidized to achieve high rate conversion of mainstream wastewater organics to methane.57,58 For this process, however, washout of microorganisms and insufficient bCOD removal is observed, particularly at the lower temperatures typical of temperate climates, and intense energy usage is required for downstream polishing.59 More efficient bCOD removal occurs 8426



Critical Review

removal of 0.6% TDS with 80% recovery (see SI Section S.6). We assume that the practical lower limit is 0.04 kWh per m3 (three times the theoretical minimum). These calculations indicate that complete anaerobic treatment and mainstream nitrogen removal with CANON or CANDO can potentially achieve energy positive treatment with removal of organics, nitrogen, and salt. 3.6. Biosolids Management. In a typical operation, the biosolids removed during primary treatment (particulate bCOD) and secondary settling (biomass) are mixed, thickened, and subjected to methane fermentation in an anaerobic digester. Anaerobic digestion stabilizes biosolids and decreases volume for disposal, but transport and disposal or reuse of the remaining inert and inorganic solids is still required. Minimizing the volume of such materials decreases energy requirements for transport. Stabilized biosolids are typically dewatered mechanically from roughly 4% to 15%−20% dry solids, then transported to a landfill or burned in an incinerator. Disposal in landfills is becoming less attractive because of reduced capacity, increasing hauling costs, and increasingly stringent regulations on the types of materials that can be landfilled.84 Incineration requires a large energy investment and releases air pollutants.85−87 SI Figure S13 summarizes three management options for biosolids: disposal, reuse, and thermal treatment. Disposal refers to options that do not involve recovery of a resource of value, that is, transport to a landfill or incineration. Reuse refers to use of biosolids as a soil amendment.88 In the U.S., biosolids treated to class A standards can be used as a soil amendment− enabling slow release of nutrients, improving soil water retention, and decreasing demand for both phosphorus and nitrogen. The nitrogen offsets demand for ammonia produced by the energy-intensive Haber-Bosch process.89 In many cases, however, use of biosolids is limited by processing time for composting, a large process footprint, low resale value, and the presence of contaminants, such as helminth eggs, metals, pharmaceuticals, steroids, hormones, pesticides, PCB, PBDE, and others.84,90−95 Thermal treatment reduces solids volume by up to 90%, kills pathogens, destroys unwanted organics, and enables recovery of fuel or heat via combustion,86 pyrolysis,96 gasification,97 or supercritical water oxidation (SCWO).9885 These processes generate combinations of heat, syngas (gas), bio-oil (liquid), or char (solid) as useful products. SI Figure S14 shows the relative distribution of thermal products in terms of enthalpy for combustion, pyrolysis, and gasification. Combustion and SCWO are used to produce heat; pyrolysis (with flash pyrolysis) produces bio-oil; and gasification produces syngas.99−101 Bio-oil and syngas are considered high value products because the energy is stored and transportable. Combustion refers to the oxidation of volatile solids with O2 to CO2 and H2O. Combustion releases heat, but if the solids are wet, the energy invested for drying often exceeds the energy recovered as heat.102 Removing water from residual solids before processing is therefore critical. SCWO is similar to combustion, but oxidation of the solids occurs in solution at temperatures and pressures above the critical point of water (374 °C, 22 MPa).103 An advantage of SCWO is that drying of the solids is not necessary. A disadvantage is that a significant amount of energy is still required for heating and pressurization. SCWO has been demonstrated at pilot- and full-scale and can be energy positive.104,105 Pyrolysis and gasification can both produce a fuel and a phosphorus-rich fertilizer from the residual ash. During

the cathode, enabling efficiencies of energy recovery comparable to biogas combustion with combined heat and power.70 Another promising system couples an MFC with an air cathode to an anaerobic fluidized bed membrane bioreactor, achieving 92.5% COD removal from municipal wastewater and a net energy surplus.71 These innovations have yet to be tested at pilot-scale. To project the upper performance limits of future treatment systems capable of removing soluble bCOD, particulate bCOD, and nitrogen, we envision anaerobic secondary treatment of the mainstream, anaerobic digestion of solids, and mainstream nitrogen removal by processes currently limited to side-stream treatment. Figure 3a illustrates energy recovery from all influent bCOD and removal of all nitrogen with CANON. This configuration enables the greatest energy surplus and produces the least residual biosolids. Aeration demand is significantly reduced because only half of the influent ammonium is oxidized to nitrite. Anammox bacteria anaerobically oxidize the remaining ammonium, consuming nitrite and producing N2. They also produce a small amount of nitrate requiring some bCOD for its reduction. Concerns with this process are the slow growth rates of the Anammox bacteria (10−12 day doubling time), process stability, robustness, and the effects of inhibitors.56,72−78 While these concerns have slowed adoption,76,78 Anammox-based side-stream nitrogen removal is now implemented at full-scale in over a dozen installations,79 mostly in Europe, and efforts are underway to extend application to the mainstream.80−83 A second promising technology for nitrogen removal from the mainstream is CANDO (Figure 3b). Compared to CANON, CANDO generates a smaller energy surplus because more oxygen is required for partial-nitrification and more bCOD is consumed for partial-denitrification. On the other hand, CANDO relies upon faster growing heterotrophic organisms, potentially increasing robustness, decreasing SRT, and enabling more rapid start-up and recovery from disturbances. Pilot-scale tests of CANDO are underway. 3.4. Upgrade for Maximum Energy Recovery and Nitrogen Reuse. As shown in Figure 3c (configuration 11), energy recovery is maximized when nitrogen in the effluent is used as fertilizer.64 All bCOD is routed to production of methane, and energy for aeration is avoided entirely. The energy recovered as methane (1.2 kWh per m3) compares favorably with an independently estimated value of 1 kWh per m3.64 This configuration yields the greatest energy surplus (0.25 kWh per m3 net energy produced) and the fewest solids for disposal (9 g TS per m3). 3.5. Upgrades for Salt Removal. Desalination of treated wastewater can provide a stable supply of water similar in quality to desalinated seawater (i.e., without bCOD, nitrogen, or appreciable salt), but with a much lower energy investment. Figure 4 compares the energy requirement for desalination of seawater to the net energy required or produced when treated wastewater (configurations 3−10) is desalinated. Also shown are the greenhouse gas “penalties”. Not shown are the solid residues as brine for wastewater (∼0.1−0.3 kg) and seawater (35 kg). For treatment of seawater by reverse osmosis, the electrical energy currently required is 3−4 kWh per m 3 . The thermodynamic limit is 1 kWh per m3. Removal of bCOD, nitrogen, and salt from wastewater can be achieved with electrical energy inputs well below 1 kWh per m3. The theoretical thermodynamic limit is 0.013 kWh per m3 for 8427



Critical Review

Figure 4. Net energy consumed or produced for removal of bCOD, nitrogen, and salt from one m3 of domestic wastewater compared to desalination of seawater (blue). Also shown are the associated greenhouse gas emissions expressed as CO2 equivalents (red). Labels refer to the 11 configurations in Table 2. The greenhouse gas penalty for each configuration is calculated as the greenhouse gas emissions associated with imported electrical energy and N2O emissions (expressed in terms of CO2 equivalents) minus the electrical energy recovered from combustion of biogas and syngas (also expressed in terms of CO2 equivalents). See section 2.2 for details.

Figure 5. (a) Summary of energy invested as electricity for baseline plant operation and aeration, and energy recovered as electricity and heat from combustion of methane with O2, and N2O for CANDO. Heat is used for anaerobic digester heating, with remaining heat used for biosolids drying. (b) Dry solids content (%) of residual solids after evaporation of water from recovered heat with methane combustion. Assumes initial 20% DS from centrifuge or belt press of anaerobic digester effluent before evaporative drying.

treatment. Primary and secondary biosolids are higher in energy content than stabilized solids, as expected. Figure 5a summarizes energy invested as electricity and energy recovered as electricity and heat from methane combustion. Heat is used to warm water in the anaerobic digester and to partially, or completely, dry residual solids by evaporation (red bar). Figure 5b illustrates the solids content after drying where the resulting %DS (y-axis) is plotted against the heat available for drying per unit mass of solids (x-axis). Table 3 summarizes the key parameters needed to determine syngas recovery when the operation is heat-neutral (See Section 2.4). Recovered syngas is reported as the stored energy ultimately derived from the organic matter preset in one cubic

pyrolysis, solids are heated in the absence of oxygen yielding bio-oil, char, and syngas.106 Bio-oil production is maximized at 500−550 °C through short residence time “flash pyrolysis”.107−109 During gasification, solids are heated with limited oxygen to produce syngas and residual char. This process is generally performed at 850 °C with some oxygen and low water content.97,110,111 When oxygen is used, a fraction of the volatile solids combust to offset the heat required for gasification, but the resulting syngas has a lower heating value. The higher heating value (HHV) is the net enthalpy change between combustion products (CO2, N2, condensed water) and reactants (organic matter, O2) at standard conditions per unit mass of reactant. SI Table S5 summarizes reported HHV and ash fractions for solids at different process stages in wastewater 8428



Critical Review

Table 3. Conversion of Solids to Syngas with Heat Neutral Thermal Treatment configuration

kWh per g-DSa

%DSb

syngas (kWh per kg-DS)c

volatile solids produced (g-DS per m3)d

total energy recovered from syngas (kWh per m3)e

electrical energy produced from syngas (kWh per m3)f

2 4 5 9 10

1.5 1.6 1.8 6.6 5.7

39 40 46 100 100

2.4 2.4 2.6 3.3 3.3

97 108 101 83 84

0.23 0.26 0.26 0.27 0.28

0.09 0.10 0.11 0.11 0.11

a

Ratio of the available heat for drying to the digested solids produced. bThe dry solids content of the solids after drying. cThe energy content of the syngas produced per kg of dry solids supplied to reformer. Syngas energy is derived from SI Figure S1 for heat neutral thermal treatment. See SI for calculation details. dThe volatile solids for thermal treatment are calculated as the total solids (TS) minus inorganic suspended solids. (See Table 2 for details). Volatile solids can undergo combustion or conversion to syngas. eRecovered syngas is calculated as the product of the energy content of produced syngas (kWh per g-DS) and the volatile solids produced (g-DS per m3). fThe assumed conversion of syngas to electricity is 40%.

and the syngas can produce ∼0.1 kWh/m3 with only ash for disposal (∼10g/m3). We note that our analysis is limited to energy recovery from the volatile organics and nutrients in wastewater. Greater energy benefits could accrue from integrated waste management where other waste streams (i.e., green waste, plastics, industrial waste) are converted to biogas and syngas.

meter of wastewater (kWh per m3 of wastewater treated). Details of this calculation are given in SI Section S.3. Configuration 1 consumes the greatest electrical energy and does not recover energy because biosolids are not digested. Configurations 2, 4, and 5 consume less energy, but do not recover enough electrical energy for net energy production, and the large quantities of solids diverted to anaerobic digestion limit the heat available for drying. The heat from methane combustion is only sufficient to partially dry residual solids. Mainstream anaerobic treatment (configurations 9 and 10) generates the most energy, the smallest mass of residual solids, and the most heat for dryingenough to completely dry the residual solids. Syngas production for configurations 9 and 10 is comparable to the other configurations with roughly 30% fewer solids feedstock. Production of large amounts of methane and low solids means that solids can be completely dried. When solids are completely dried, fewer need to be combusted and more can be converted in syngas. For all configurations, the energy potentially recoverable as syngas (Table 3) is appreciable. The energy from syngas is roughly two-thirds of the energy recovered from methane for configurations 2−7 and one-third of the energy recovered from methane for configurations 8−11 (Figure 5). Increased energy production is largely due to syngas derived from inert organics. Syngas production strongly depends upon the ratio of volatile solids to ash. We assume that influent inorganic suspended solids (i.e., grit) are removed by preliminary treatment through grit removal or screening. If these solids are not removed, production of syngas is not feasible for configurations 2−7. This is because the additional solids are associated with water, and dried to a lesser extent with the heat recovered from methane combustion. Solids with high moisture and ash content do not release enough heat for thermal treatment.

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ASSOCIATED CONTENT

* Supporting Information S

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Corresponding Author: Yaniv D. Scherson, Department of Civil and Environmental Engineering, Stanford University, Email: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Support was provided by the U.S. NSF Engineering Research Center (ReNUWIt) (EEC-1028968), the Woods Institute for the Environment at Stanford, a gift from Veolia, and a grant from the U.S. NSF Partnership for Innovation Research Translational Technology Program (IIP-1312359). We thank Professor Perry McCarty and Dr. Eli A. Goldstein for their support.

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