Tratamento de Esgoto Doméstico Energeticamente Positivo: Tecnologias Anaeróbicas e Fototróficas - Revisão e Análise

PERGUNTAS PARA SEREM RESPONDIDAS 1 Quais foram qual foi os resultados mais significativos 2 O que se discutiu sobre os resultados Energy positive domestic wastewater treatment the roles of anaerobic and phototrophic technologies B D Shoener I M Bradley R D Cusick and J S Guest The negative energy balance of wastewater treatment could be reversed if anaerobic technologies were implemented for organic carbon oxidation and phototrophic technologies were utilized for nutrient recovery To characterize the potential for energy positive wastewater treatment by anaerobic and phototrophic biotechnologies we performed a comprehensive literature review and analysis focusing on energy production as kJ per capita per day and as kJ m3 of wastewater treated energy consumption and treatment efficacy Anaerobic technologies included in this review were the anaerobic baffled reactor ABR anaerobic membrane bioreactor AnMBR anaerobic fluidized bed reactor AFB upflow anaerobic sludge blanket UASB anaerobic sequencing batch reactor ASBR microbial electrolysis cell MEC and microbial fuel cell MFC Phototrophic technologies included were the high rate algal pond HRAP photobioreactor PBR stirred tank reactor waste stabilization pond WSP and algal turf scrubber ATS Average energy recovery efficiencies for anaerobic technologies ranged from 16 MFC to 475 ABR When including typical percent chemical oxygen demand COD removals by each technology this range would equate to roughly 401200 kJ per capita per day or 1103300 kJ m3 of treated wastewater The average bioenergy feedstock production by phototrophic technologies ranged from 12004700 kJ per capita per day or 340013 000 kJ m3 exceeding anaerobic technologies and at times the energetic content of the influent organic carbon with usable energy production dependent upon downstream conversion to fuels Energy consumption analysis showed that energy positive anaerobic wastewater treatment by emerging technologies would require significant reductions of parasitic losses from mechanical mixing and gas sparging Technology targets and critical barriers for energyproducing technologies are identified and the role of integrated anaerobic and phototrophic bioprocesses in energy positive wastewater management is discussed Environmental impact Conventional wastewater treatment processes are energyintensive and focus on removal of organics and nutrients rather than recovery Reenvisioning wastewater as a renewable resource may enable energy positive treatment creating economic incentives for increased access to sustainable sanitation in both developed and developing communities To this end anaerobic technologies can recover the chemical energy of organic carbon in wastewater as methane hydrogen and electricity In a complementary fashion phototrophic technologies could address a major limitation of anaerobic treatment nutrient removal while increasing the energetic potential of wastewater resources by leveraging nutrients for biomass growth and organic carbon storage In this review we evaluate the strengths and weaknesses of these technologies and project a path toward energy positive wastewater treatment 1 Introduction The sanitation industry is facing a confluence of events that are straining utility budgets and reducing their ability to provide reliable protection of public health and the aquatic environment Critical challenges include rapid and localized population growth and decay aging infrastructure deterioration of surface waters resulting from excess nutrient N and P loading and a reliance on expensive energyintensive treatment processes These pressures are exacerbated by decreased resilience of ecosystems and increased variability in renewable freshwater resources resulting from climate change with current energyintensive approaches to wastewater treatment consuming roughly 0306 kW h m3 of wastewater treated or 3 of US electricity demand further contributing to climate change via greenhouse gas emissions from electricity production Ultimately these stressors have intensied the need to address the waterenergy nexus in wastewater management Given that upgrades to US infra structure are expected to cost roughly 300 billion over the next 20 years6 the industry has an unprecedented opportunity to re envision wastewater streams as resourcerich sanitation media In particular treatment strategies enabling nutrient recovery as well as energy recovery and generation should be advanced enabling resource positive sanitation the management of wastewater as a renewable resource for nutrient recovery and net energy production2426 that can have a net benet for the environment27 to gain traction at a broad scale In response to these challenges a great deal of research has been conducted on alternative wastewater treatment technolo gies that recover or produce energy during wastewater treat ment Most notably anaerobic technologies can recover usable energy from organic carbon typically measured as chemical oxygen demand COD and phototrophic technologies can increase the chemical energy of a wastewater through CO2 xation during growth and carbon storage In addition to the production of bioenergy products such as methane hydrogen or electricity2831 anaerobic processes are expected to be less energy intensive than aerobic processes due to a lack of aeration and a reduction in sludge wastage32 Although published studies have analyzed the performance of one or a small number of anaerobic system designs283337 an indepth comparison of technologies focusing on their potential in domestic wastewater management is still needed The limited literature on anaerobic treatment of domestic wastewater is partially due to lower methane production lower COD removal and higher methane solubility all of which stem from the fact that domestic wastewater is a relatively dilute resource stream28 Ultimately it is unclear whether the conversion of organic carbon to usable energy will be adequate for energy positive treatment using existing and emerging technologies2038 Beyond COD nutrient nitrogen N phosphorus P limits are becoming increasingly common across the US3940 In Brian Shoener is a Jack Kent Cooke Foundation Graduate Scholar and MS student in Environmental Engineering at the University of Illinois at UrbanaChampaign He holds dual BS degrees in Civil Engi neering and Management for Engineers from Bucknell University Brians current research focuses on the advancement of anaerobic membrane bioreactor tech nology via quantitative sustainable design More broadly his interests are in process modeling and improving energy recovery from methanogenic wastewater treatment processes Dr Cusick earned his BS in Environmental Engineering from the University of Cal ifornia Riverside holds an MS and PhD in Environmental Engineering from the Pennsyl vania State University and is now an Assistant Professor of Environmental Engineering at the University of Illinois at UrbanaChampaign His research focuses the develop ment of electrochemical tech nologies for nutrient and energy recovery from wastewater as well as natural and engineered salinity gradients Ian Bradley is a Deuchler Fellow and PhD student in Environ mental Engineering at the University of Illinois at Urbana Champaign He earned a BS degree in Industrial and Enter prise Systems Engineering and a MS degree in Civil and Envi ronmental Engineering both from the University of Illinois Ians current research focuses on algal growth for biofuel feed stock production in conjunction with wastewater treatment He is interested in biological systems and process design for water and sanitation systems in the devel oped and developing world Dr Guest is currently an Assis tant Professor in the Department of Civil and Environmental Engineering at the University of Illinois at UrbanaChampaign He holds BS and MS degrees in Civil Engineering from Buck nell University and Virginia Polytechnic Institute and State University respectively and a PhD in Environmental Engi neering from the University of Michigan Dr Guests interests center on biotechnology development for resource recovery from sanitation media with particular emphasis on microalgae and methanegenerating processes His group works in developed and developing communities and he currently serves as Thrust Leader for Sanitation and Resource Recovery for the Safe Global Water Institute This journal is The Royal Society of Chemistry 2014 Environ Sci Processes Impacts 2014 16 12041222 1205 Critical Review Environmental Science Processes Impacts Open Access Article Published on 18 March 2014 Downloaded on 10182022 71141 PM This article is licensed under a Creative Commons AttributionNonCommercial 30 Unported Licence View Article Online addition to high capital costs of wastewater treatment plant WWTP upgrades eg 336396 billion for the for plants discharging to the Chesapeake Bay watershed11 the addition of biological nutrient removal BNR incurs higher operational costs that create or exacerbate nancial challenges for utili ties41 As an alternative to conventional BNR processes that leverage chemotrophic bacteria phototrophic processes rely on light typically sunlight to promote growth and nutrient assimilation As the phototrophs including algae and cyano bacteria take up inorganic carbon and grow they also take up N and P from the wastewater and achieve nutrient recovery via assimilation Studies have examined the potential for energy production using algae eg ref 42 and 43 and have even examined the potential for energy positive wastewater treat ment38 but such studies have been limited to single cultivation technologies To our knowledge there has not been a compre hensive comparative assessment of existing and emerging phototrophic technologies as tools to enable energy positive domestic wastewater management In fact studies focusing on bioenergy feedstock production with phototrophs have been largely disjointed from the wastewater literature oen using high strength synthetic media for cultivation eg ref 44 and 45 The prospect of using phototrophs for nutrient recovery andor bioenergy feedstock production holds signicant promise however and warrants further discussion As researchers attempt to balance the potential gains in net energy production with performance and economic tradeoffs of each technology the range of congurations for anaerobic and phototrophic systems continues to grow To better understand the status and relative potential of each conguration we undertook a critical review of the literature to characterize the demonstrated energy production by and critical barriers to a range of anaerobic and phototrophic technologies that have the potential to contribute to energy positive wastewater manage ment Based on available data we quantied the typical performance of technologies in terms of treatment efficacy and bioenergy or bioenergy feedstock production including the production of methane hydrogen gas electricity biocrude oil biodiesel and heat Seeking a deeper understanding of the potential energy balance of each technology we also quantied usable energy yield based on downstream conversion of bio energy feedstocks and anticipated energy consumption based on experimental conditions in published studies To be considered energy positive a wastewater treatment scheme was required to produce energy in excess of the energy required to operate the process while also discharging water that meets regulatory standards Given these constraints we identify technologies with the greatest potential to enable energy posi tive carbon nitrogen and phosphorus management and present operational and performance targets for anaerobic and phototrophic treatment technologies to improve their net energy balance 2 Anaerobic systems Anaerobic processes for bioenergy production are most commonly leveraged for industrial wastewater treatment or for solids management at domestic wastewater treatment facili ties32 Limited application of these systems stems from a perceived need for high organic loading rates and mesophilic temperatures31 However anaerobic technologies have been demonstrated at psychrophilic temperatures314647 and have the potential to be applied more broadly for lowstrength waste water treatment25 Anaerobic wastewater treatment processes can generally be categorized as suspended growth sludge blanket attached growth membranebased or microbial elec trochemical systems3248 The rst step of COD degradation in anaerobic treatment systems is the fermentation of complex organic matter into long chain volatile fatty acids carbon dioxide and hydrogen by acidogenic microorganisms Long chain fatty acids are then further fragmented into acetic acid and hydrogen Methane CH4 and hydrogen gas H2 are possible bioenergy products from anaerobic systems In methaneproducing reactors acetoclastic methanogens ferment acetic acid to methane and carbon dioxide and hydrogenotrophic methanogens convert hydrogen and carbon dioxide to methane49 In microbial electrochemical technolo gies exoelectrogenic bacteria oxidize acetate to carbon dioxide and produce electrical current transferring electrons to a conductive surface50 The anaerobic systems considered in this review are described below 21 Suspended growth processes Suspended growth processes are characterized by completemix conditions to prevent biomass from settling and to facilitate contact between the microorganisms and the wastewater The most common processes include anaerobic sequencing batch reactors ASBRs completelymixed anaerobic digesters and the anaerobic contact process32 Of these only ASBR had adequate peerreviewed data ie 5 papers on the treatment of domestic wastewater 211 Anaerobic sequencing batch reactor ASBR The ASBR progresses through four stages similar to the aerobic sequencing batch reactor settle decant feed and react51 ASBRs oen have higher solids residence times SRTs compared to continuous ow processes and enable more precise operational parameter eg hydraulic retention time HRT control52 However their suitability for the treatment of lowstrength wastewaters has been questioned due to low gas production on dilute streams although intermittent mixing has been suggested to improve gasliquid separation and to enhance sludge settling53 22 Sludge blanket processes Successful operation of anaerobic sludge blanket processes relies on the aggregation of organisms into diversely populated granules capable of settling54 The granules form naturally from reactor operation and consist of a mixed population of bacteria and archaea that are able to carry out the overall fermentation and gas production from organic carbon substrates55 The gas bubbles produced from methanogenesis help to uidize the granules enhancing mass transfer without mixing55 Technol ogies with adequate 5 peerreviewed studies included the 1206 Environ Sci Processes Impacts 2014 16 12041222 This journal is The Royal Society of Chemistry 2014 Environmental Science Processes Impacts Critical Review Open Access Article Published on 18 March 2014 Downloaded on 10182022 71141 PM This article is licensed under a Creative Commons AttributionNonCommercial 30 Unported Licence View Article Online upflow anaerobic sludge blanket UASB and the anaerobic baffled reactor ABR 221 Upflow anaerobic sludge blanket UASB In a UASB reactor wastewater enters the reactor and is distributed across the bottom traveling upward through the sludge blanket Granular sludge in the reactor allows for high volumetric COD loadings as compared to other wastewater treatment technologies To enable better solids capture and to prevent loss of granules modifications to the basic design have added packing material or a settling tank UASBs are advantageous because of their simple construction scalability and small footprint though downstream processing is usually necessary to reduce effluent particulate organics 222 Anaerobic baffled reactor ABR The ABR utilizes a sequence of baffles to impede the flow of wastewater as it passes through the reactor Flow patterns and gas production force sludge in the reactor to rise and settle slowly Since its inception the ABR has undergone several modifications in an effort to improve performance such as changes to baffle design including a settler in the system or achieving solids capture using packing material Advantages of this process include simplicity of construction and operation prolonged retention of influent solids staged operation and insensitivity to shock loads Disadvantages include having to construct shallow reactors to accommodate gas and liquid upflow velocities as well as difficulty with distributing the influent flow evenly 23 Attached growth processes Attached growth anaerobic technologies rely on packing material in the reactor to provide surfaces for biofilm formation The primary characteristics that differentiate reactors within this category are the packing material type and degree of bed expansion For example the packed bed and fluidized bed configurations are operated at increasing upflow velocities with fluidized bed being the highest Because of the similarity of packed beds to the UASB and the availability of data for the fluidized system only the anaerobic fluidized bed AFB was included 231 Anaerobic fluidized bed AFB AFB reactors are operated at high upflow velocities in order to suspend particulate media such as granular activated carbon GAC in the reactor with wastewater treatment achieved by biofilms attached to the media While AFBs are particularly effective for low strength wastewaters the main shortcoming is minimal solids capture AFBs are therefore more appropriate for wastewater streams with primarily soluble COD 24 Membranebased processes Membranes have been used in water treatment for over half a century and are becoming increasingly common in applications ranging from wastewater treatment to desalination Microfiltration and ultrafiltration membranes are primarily used for particulate removal and can be arranged as flat sheets or hollow fibers One of the main benefits of using membranes in biological treatment processes is the completely independent control of SRT and HRT SRT values have been reported as high as 300 days where biomass was only removed from the system during sampling 241 Anaerobic membrane bioreactor AnMBR An AnMBR is an anaerobic reactor coupled with membrane filtration The membrane can be configured as external crossflow internal submerged or external submerged Inclusion of a membrane allows for robust solids capture while also improving effluent quality over other mainstream anaerobic processes This increase in quality comes about because of the decoupling of SRT from HRT Higher SRTs correlate to greater volatile fatty acid VFA and soluble COD removals Additionally AnMBRs allow for a much smaller footprint by enabling higher solids concentrations in the reactor 25 Microbial electrochemical technologies METs METs also referred to as bioelectrochemical systems BES leverage microorganisms capable of extracellular electron transfer to produce electrical energy from wastewater Like all electrochemical technologies such as fuel cells and batteries METs are composed of an anode where electrons for current are generated and a cathode where electrons are consumed In METs anaerobic bacteria naturally present in most wastewaters oxidize biodegradable organic matter and continuously transfer electrons to the anode Electrons flow from the anode through an external circuit to a cathode where electrical current is consumed in a reduction reaction Current production in METs is dependent on the redox potential difference between organic matter oxidation at the anode Eo 032 V and current consumption at the cathode If the anode is more negative than the cathode as in the case with oxygen reduction Eo 082 V in a microbial fuel cell MFC electrical current production is spontaneous If the cathode reaction occurs at a redox potential that is more negative than the anode such as hydrogen production Eo 0414 V in a microbial electrolysis cell MEC then additional cell voltage must be applied to drive current production in the cell Although a multitude of cathodic reactions eg caustic production and hydrocarbon electrosynthesis have been paired with anodic oxidation of organic matter the review will only focus on electricity and hydrogen production in METs 251 Microbial fuel cell MFC The most commonly investigated MFC architecture is a single chamber reactor in which both the bioanode and oxygen reduction cathode operate in the same solution The cathode electrode which acts as the barrier between reactor solution and air is coated with hydrophobic diffusion layers to allow oxygen transport but prevent water loss Although a variety of wastewaters have been evaluated for electricity generation power production has been significantly lower 05 W m2 cathode area than reactors fed synthetic and well buffered solutions 1043 W m2 due to low solution conductivity as well the dilute concentrations and complex nature of organic substrates in domestic wastewater Additionally cathodic materials are often expensive due to the need of precious metals eg platinum 252 Microbial electrolysis cell MEC MECs produce hydrogen from substrate by coupling a hydrogen evolution electrode to the bioanode Hydrogen is a promising fuel for meeting future energy demand because it only produces water when combusted or oxidized in a fuel cell and has a high energy yield 14235 kJ g1 Since MEC current production is not spontaneous voltage must be applied to produce hydrogen 0612 V in practice Due to cathode catalyst electrolyte and substrate deficiencies energy consumed by applying voltage can exceed the energy recovered as hydrogen gas Also to prevent hydrogen losses due to hydrogenotrophic methanogenesis that occur in single chamber architecture a membrane or gas diffusion electrode is required to separate anode and cathode 3 Phototrophic systems Simple passive phototrophic processes cultivating algae and or phototrophic bacteria such as open ponds are commonly used to treat municipal and agricultural wastewaters To date the objective for these technologies tends to be nutrient and often COD removal from wastewater rather than nutrient recovery or bioenergy feedstock production Alternatively more capitalintensive systems such as photobioreactors have been studied for phototroph cultivation but this work has most often focused on bioenergy feedstock cultivation rather than wastewater treatment eg ref 78 and 79 Both types of systems predominantly operate with suspended cultures in open eg ponds or closed systems eg photobioreactors that allow for sunlight penetration and nutrient assimilation to promote growth and carbon storage before biomass is harvested Alternative systems consist of attached or immobilized phototrophs for easier harvesting Ultimately the energetic benefit of phototrophic systems stems from the fact that they can increase the energetic content of wastewater through the conversion of light energy to chemical energy as organic carbon In order to evaluate the relative potential of phototrophic technologies in achieving energy positive municipal wastewater treatment only published studies using actual wastewater as the growth medium have been included in the analysis 31 Suspended systems Conventional phototrophic systems consist of suspended cultures that are operated in either continuous batch or semibatch mode The most common largescale phototroph cultivation systems are waste stabilization ponds WSPs high rate algal ponds HRAPs stirred tank reactors and tubular photobioreactors PBRs At laboratoryscale a wider variety of reactor configurations have been evaluated including flat panel aka flat plate and annular PBRs as well as more basic wellmixed systems that are simply lit from overhead these studies were classified as Stirred Tank Reactors for this review 311 High rate algal pond HRAP While open raceway ponds are used commercially for the production of algal biofuels and health products a subset of published studies use HRAPs for wastewater treatment eg ref 80 and 90 HRAPs are open raceway ponds first proposed in the 1950s with the goals of providing improved wastewater treatment over traditional WSPs and algal biomass for potential biofuel applications Although they have the potential to be a more cost effective solution than PBRs for wastewater treatment HRAPs have relatively low biomass productivity and thus require larger land areas as compared to reactorbased technologies 312 Photobioreactor PBR Another widely used technology for cultivating algal biomass is the PBR These closed array systems allow for high biomass productivity as well as axenic growth conditions for monoculture maintenance Although a range of configurations have been evaluated at the labscale larger systems tend to be tubular PBRs due to economies of scale There are relatively few studies that examine PBRs in conjunction with wastewater treatment largely because of high costs compared to other treatment technologies and because axenic cultures are generally not targeted for municipal wastewater treatment Most PBR studies focus on pure species with high lipid productivities and consequently higher energy potential and revenue generation 313 Stirred tank reactor There is extensive literature on phototrophic growth in stirred tank reactors open completely mixed reactors lit from overhead Although published studies using stirred tank reactors cover a range of operational conditions including various lighting schemes batch vs continuous vs semicontinuous operation etc and a subset have been performed at the pilotscale the majority of these studies have been at the laboratoryscale eg ref 98100 In order to look for general trends in performance of stirred tank reactors data from these studies have been aggregated to identify performance trends and enable comparisons to largerscale more broadly applied technologies eg HRAPs Any insights gained may be applied to the design of largerscale batch or sequencing batch reactors for both wastewater treatment and algal biomass production 314 Waste stabilization pond WSP WSPs are the most widely used phototrophic treatment technology In the US alone there are 7000 WSPs in use which accounts for over onethird of all centralized treatment systems During the day phototrophs in these systems produce dissolved oxygen which facilitates COD degradation by aerobic heterotrophs and promotes photooxidative damage for pathogen removal Although WSPs are often a cost effective solution for wastewater management utilities they are used almost exclusively in rural areas due to large land requirements With the exception of early visionary proposals linking wastewater to bioenergy with algae WSP literature has focused almost exclusively on wastewater treatment removal of COD N P heavy metals with little discussion of biomass production or potential biofuel applications Despite limited literature linking WSPs to bioenergy feedstock cultivation this technology represents one of the easiest opportunities to transition from an existing energy neutralconsuming technology to an energy producing process given that algal biomass is already generated 32 Attached growth systems The cost of biomass harvesting including flocculation centrifugation and sedimentation82 remains a key barrier to the broad implementation of suspended growth algal systems88 Although sedimentation is often the most inexpensive approach it achieves low 5090 ref 80 and 113 biomass recoveries and is typically used when low value biomass is being removed from the system114 Technologies that seek to achieve high percentages 95 of suspended biomass recovery for use as biofuel feedstock would add significantly to the cost of operation80115116 As an alternative to suspended growth attached growth systems restrict algal growth to physical structures resulting in aggregated biomass that either sloughs off the structures or can be removed through cleaning117 While there are a number of different attached growth systems available eg Algaewheel and other industrial solutions118 as well as various immobilized gel matrices119 the data necessary to perform the energetic analysis for most attached growth technologies was lacking One exception was the algal turf scrubber ATS which has been the focus of a number of studies and which reported adequate data for its inclusion in this study120121 321 Algal turf scrubber ATS ATSs consist of long inclined beds typically constructed of landfill liner that support mixed community biofilms that include cyanobacteria filamentous periphyton and epiphytic diatoms117122123 As water flows down the beds into a concrete sump nutrients are taken up by the biofilm supporting microbial activity and reducing the concentrations of nutrients in the effluent124 When biomass accumulates harvesting is often performed by machinery such as a loader driven across the bed117 Although it is not a common process there are several private companies operating ATSs on a large scale notably Aquafiber Technologies 75 million gallons per day MGD and HydroMentia capacity 30 MGD118 required value was not explicitly stated but prerequisite values were given the unknown values were calculated see Fig S1 and S2 for inclusionexclusion decisionmaking ESI Ultimately these data were used to report the effluent COD and energy as kilojoules kJ recovered by anaerobic treatment as well as effluent NP and energy produced by phototrophic technologies Fig 1 42 Energetic analysis 421 Anaerobic technologies Anaerobic technologies recovered energy either in the form of methane mol CH4 per g COD hydrogen gas mol H2 per g COD or electricity kJ per g COD In order to compare the data objectively each was normalized to kJ recovered per g COD removed by converting each energy source to kJ using standard conversion factors based on energetic content 803 kJ per mol CH432328 6 kJ per mol H2125 and by converting electricity reported in kW h to kJ by multiplying by 3600 s h1 eqn S1S3 Results for each technology were compared on the basis of per capita and per m3 of wastewater treated using the conversions discussed in Section 43 422 Phototrophic technologies Phototrophic technology data were compiled from articles that reported both biomass generated and nutrient N andor P removal Biomass was either reported as total maximum or average VSS g m3 as productivity g per m3 per day or as aerial productivity g per m2 per day The SRT experiment length and reactor volume were leveraged to convert all numbers to an average daily productivity g per m3 per day Biomass productivities were then normalized by the average nutrient removal per day g per m3 per day to achieve g biomass produced per g nutrient removed from the treatment system To convert biomass productivity to energetic potential reported VSS were converted to units of COD see Table S2 ESI Two scenarios were considered using macromolecule content lipidscarbohydratesproteins LCP within typical ranges from the literature126128 a low CODVSS ratio of 147 g COD per g VSS assuming 104050 lipidscarbohydratesproteins LCP8698126128 and a high CODVSS ratio of 184 g COD per g VSS assuming a ratio of 302050 LCP86126128 COD calculations were performed assuming lipids could be represented as stearic acid C18H3602 carbohydrates as glucose C6H12O6 and proteins as C16H2405N4129 Although higher CODVSS ratios would be possible if higher lipid content were achieved eg 70 lipids81 the ratios used here represent a reasonable range of expected compositions8698 to avoid overly optimistic ratios that would artificially increase calculated energy yield Although it has been reported that some species can obtain greater than 80 lipids by dry biomass weight130131 mixed algal wastewater cultures routinely see far less lipid accumulation with an average around 108698 Once biomass productivities were converted to COD the energetic potential of the biomass was then calculated using a theoretical value of 139 kJ per g COD132 Results for each technology were compared on a per capita basis as described in Section 43 423 Conversion to usable energy Although the energetic content of treatment system products may provide insight into 5 Results and discussion In the review of the peerreviewed literature a total of 225 anaerobic and 86 phototrophic papers were screened and assessed according to the inclusion criteria Fig S1 and S2 Of the papers reporting on anaerobic technologies only 32 met the necessary criteria for energetic and treatment analysis with a total of 122 experimental data sets ie if the study reported multiple experimental conditions or replicates all that met the inclusion criteria were included in this review Published data on phototrophic technologies were less complete with only 23 papers meeting the necessary criteria for treatment analysis with a total of 33 and 58 data sets for N and P removal respectively Of these papers 13 had the necessary biomass productivity for energy analysis resulting in 21 and 25 experimental data sets for energy production per g nutrient N or P removed across 37 independent data sets Furthermore 9 of these 37 datasets were excluded because they reported greater than 50 g or 225 g of algal biomass grown per g N or g P removed respectively which was deemed to be outside the likely range of feasible biochemical compositions Finally WSPs were excluded from the energetic analysis due to a lack of biomass productivity data 51 Energetic analysis The energetic analysis began by determining fuel anaerobic or bioenergy feedstock phototrophic production from each study and the associated caloric content Section 511 Energy consumption Section 512 of each technology was then estimated based on experimental conditions in published studies and on additional assumptions detailed in Section S4 of the ESI An energy balance between consumption and production was then detailed for anaerobic systems to estimate net energy given typical experimental conditions in order to identify key barriers to energy positive treatment Section 52 An energy balance was excluded for phototrophic technologies because of the uncertainty associated with downstream conversion to usable fuels but available data were leveraged to set targets for cultivation and downstream fuel conversion processes discussed in Section 53 Lastly we examined the dichotomy between emerging energy production and traditional effluent quality objectives for treatment technologies Section 6 511 Energy yield The average energy recovery by anaerobic systems ranged from 048 kJ per g COD MFC to 73 kJ per g COD ABR and was highest for gas producing technologies Fig 2a The average percent energy recovery as methane hydrogen or electricity from degraded COD by each technology was as follows from greatest to least average standard deviation ABR 475 45 AnMBR 354 268 AFB 338 129 UASB 240 114 ASBR 177 101 MEC 143 144 and MFC 16 14 When including typical percent COD removals for each technology this range would equate to roughly 401200 kJ per capita per day Fig 2b or 1103300 kJ m3 of treated wastewater After conversion of gases to usable electricity in a fuel cell 423 efficient these values represent recoveries of roughly 220 UASBs and AnMBRs had the two highest reported energy recovery data sets 122 and 97 kJ per g COD degraded respectively but AnMBRs also had the greatest variability standard deviation of 43 kJ per g COD The energy recovery by MECs was statistically different from most of the methaneproducing technologies pvalues 0024 α 005 twotailed unpaired ttest except ASBRs pvalue 0077 which could not be shown to be statistically different MFCs did however exhibit significantly lower energy production pvalue 0048 with average per capita energy recovery 515 fold lower than gas producing technologies or 2365 fold lower after gas conversion to electricity Although MFC power production from wastewater was limited by substrate conductivity and strength power densities from single chamber MFCs fed optimized synthetic solutions 14 kJ per g COD67 would have still been only 1955 of the average reported energy recovery rates for methaneproducing technologies Fig 2 a Energy content kJ fuel per g COD removed for each paper studying anaerobic technologies with an influent COD below 500 g m3 for synthetic wastewater or using actual domestic wastewater b Energy content kJ fuel per capita per day determined by multiplying values from a by 180 g COD per capita per day and by the respective average percent COD removals for each technology Table 1 All energy products methane hydrogen electricity are reported as kJ using theoretical unit conversions see ESI Individual points represent distinct experimental data sets with error bars extending to standard deviation if reported Fig 1 Flow chart showing the process of data acquisition and analysis used in the manuscript along with conversion factors and their location in the ESI the fundamental limitations of a given technology the question regarding the feasibility of energy positive treatment can only be answered by determining the usable energy eg electricity heat liquid fuel provided by each treatment system For anaerobic systems the outputs include methane hydrogen and electricity Given that the predominant form of energy consumed by treatment plants is electricity methane and hydrogen were converted to electricity in a fuel cell at a 423 conversion efficiency133 In order to predict the production of usable energy from phototrophic biomass the energy yield from four different conversion processes hydrothermal liquefaction HTL transesterification anaerobic digestion and combustion were also calculated Although anaerobic digestion has a long history in the conversion of algal biomass to methane108109134 direct combustion of algal biomass has been proposed as more energetically favorable than converting biomass to any biofuel38135 For the conversion of phototrophic biomass into liquid fuels both transesterification and HTL were considered with HTL representing an emerging process of interest to the algaetobiodiesel community136137 HTL has been applied to wastewatergrown biomass eg ref 138 and 139 although energy balances have identified biomass harvesting and dewatering as key barriers to achieving energy positive systems38 The list of assumptions and values used for these calculations can be found in Table S2 ESI 424 Energy consumption An estimation of energy consumption for each technology was included in order to evaluate the feasibility of net energy positive wastewater treatment However the published studies analyzed did not include energy consumption data with the exception of Sturm and Lamer 201138 In order to quantify energy consumption of each process the energy demand from various activities eg pumping mechanical mixing gas sparging etc32140 was estimated using standard design equations and the published range of design and operational parameters see ESI Section S4 for a detailed explanation 43 Unit conversions and efficiency calculations Data were normalized and reported in one of four ways as energy per gram of pollutant removed energy per capita energy per cubic meter of wastewater treated and as a percent of energetic potential recovered Energetic data normalized to pollutant removal kJ per gCOD g N or g P was calculated directly from the published data sets included in the review These data in units of kJ per g pollutant removed were then normalized to per capita values by multiplying i by the average percent removal of that pollutant by a given technology and ii by the average daily per capita production of that pollutant 180 g COD 13 g N 21 g P141 Next energy productions were also reported per cubic meter of wastewater treated by assuming a wastewater production rate of 036 m3 per person per day resulting in a wastewater composition of 500 g COD per m3 36 g N per m3 58 g P per m3 For efficiency calculations eg percent of chemical energy recovered COD was assumed to contain roughly 139 kJ per g COD132 resulting in an influent energetic content of 7000 kJ m3 This conversion factor is lower than more recent values reported in the literature 177287 kJ per g COD142 but was used throughout the manuscript to provide a consistent framework for energy conversions All energy values in units of kJ represent the energetic content of produced fuel methane hydrogen or electricity for anaerobic systems or produced biomass for phototrophic systems unless otherwise Table 1 Average and range of percent COD or nutrient N or P removal for each technology used in Fig 2 and 3 Technology Average percent removal min max COD ASBR 581 33 91 UASB 676 54 85 ABR 903 887 925 AFB 82 72 897 AnMBR 867 82 90 MEC 78 337 967 MFC 455 19 83 N P HRAP 671 36 872 521 32 729 PBR 785 68 897 932 85 99 Stirred tank 623 782 7 100 ATS 705 181 907 786 583 957 Fig 3 a Energy potential of phototrophic technologies kJ algal biomass per g nutrient removed showing relative bioenergy feedstock production based on nutrient removed N or P b Energy content kJ algal biomass per capita per day determined by multiplying values from a by 13 g N per capita per day or 21 g P per capita per day and by the respective average percent N and P removals for each technology Table 1 Energy products are reported as kJ using theoretical unit conversion see ESI Individual points represent distinct experimental data sets with error bars extending from high to low CODVSS assumptions discussed in Section 422 Table 2 Ranges of energy consumption for anaerobic technologies based on experimental conditions in examined literature kJ m3 wastewater treated Technology Mixing Pumping Heating Applied voltage ASBR 48009400a 2831b 4200f UASB ABR AnMBR 42 00058 000c 36120d AFB 55130e MEC 28007900 MFC a Mechanical mixing Section S41 and Table S3 b Effluent pumping Section S45 and Table S3 c Biogas sparging Section S42 and Table S3 d Permeate pumping Section S45 and Table S3 e Recirculation pumping Section S45 and Table S3 f Energy required for each increase in C not included in final energy balance Section S46 and Table S3 Table 3 Ranges of energy consumption for phototrophic technologies based on experimental conditions in examined literature kJ m3 wastewater treated Technology Mixing Pumping Harvestinga HRAP 3296b 34170 PBR 630013 000c 5558d Stirred tank 7703100e 2831f WSP ATS a Low value is coagulationflocculation with belt press filter for dewatering high value is gravity settling with centrifugation Section S44 and Table S3 b Paddlewheel mixing Section S43 and Table S3 c Aeration Section S42 and Table S3 d Influent lift pump Section S45 and Table S3 e Mechanical mixing Section S41 and Table S3 f Effluent pumping Section S45 and Table S3 g Although minimal energy would be required for the physical harvesting of algae from ATS it was not estimated due to lack of available data Fig 4 Energy recovery consumption and theoretical maximum energy yield for each technology Blue circles represent energy production per gram of COD removed in experimental data sets from the literature Red boxes indicating the range of energy consumed that needs to be overcome for energy positive treatment excluding heating requirements of the wastewater were calculated based on volumetric energy requirements Table 2 coupled with typical COD removal of each technology Table 1 and an assumed influent of 500 g COD per m3 Blue horizontal lines show the maximum energy that can be generated for methane solid hydrogen dotted and electricity dashed based on thermodynamics calculations shown in the Section S3 of ESI Fig 5 Influent vs effluent COD g m3 from anaerobic treatment technologies treating real and synthetic wastewaters with influent COD concentrations 500 g m3 Points and error bars represent averages standard deviations from experimental data sets Plots a e are separated by technology type suspended growth sludge blanket etc The solid line is no COD removal ie 0 removal the dotted line is 80 removal and the dashed line is 90 removal effluent P concentrations below 03 g P per m3 but these studies had inuent P concentrations below 1 g P per m3 When compared to energy consumption data it can be seen that the technologies that require more energy PBRs stirred tank reactors tend to perform better in meeting traditional treatment objectives such as N and P removal from wastewater They also generate more biomass and more energy per gram nutrient removed Fig 3 with which to offset this energy consumption Balancing increased nutrient removal and biomass yields and thus energy production with higher energy demands will be a key challenge in the design and development of energy positive phototrophic systems 6 Navigating a path to energy positive wastewater management A striking conclusion of this review was that phototrophic processes have the potential to produce 280400 of the amount of energy as anaerobic processes on a per m3 basis given existing pollutant removal efficiencies and downstream conversion technologies The energy recovery by anaerobic technologies reported in this manuscript 247 assumes an energetic content for COD of 139 kJ per g COD which has recently been found to be a low estimation142 A higher ener getic content would further reduce anaerobic energy recovery efficiency whereas cultivating algae on nutrients and con verting to fuels could exceed the original energetic content of the inuent wastewater Additionally the use of nutrients for phototrophic cultivation may result in 130510 of the energy production as would be offset by the use of nutrients for fertilization An unfortunate nding of this review was the lack of adequate data to enable a coordinated analysis of both energy production and wastewater treatment Of the 311 papers screened in the initial literature search 82 could not be included because they did not measure or report adequate data From the available data it is clear that the potential exists for energy positive wastewater treatment and that both anaerobic and phototrophic may play a role However there are several critical barriers that must be overcome Anaerobic processes must balance reduced energy consumption with increased treatment efficacy and fuel recovery and we must develop a deeper understanding of phototrophic bioprocesses to enable process optimization To this end we examine the implications of this work and propose areas for future research Fig 6 Influent vs effluent a total N concentrations g N per m3 and b total P concentrations g P per m3 for suspended and attached growth systems on a loglog scale The solid line identifies no nutrient removal the dotted line 80 removal and the dashed line 90 removal Table 4 Energy yield kJ fuel per g nutrient removed for phototrophic cultivation technologies and select conversion processesab Technology Nutrient HTL Anaerobic digestion Transesterication Combustion HRAP N 75160 32160 34100 90130 P 7301600 3201500 330980 8801300 PBR N 270590 120580 120370 330500 P 230500 100490 100310 280420 Stirred tankc P 9001900 4001900 4001200 11001600 ATS N 110240 47230 49150 130200 P 5801300 2501200 260790 7001100 a Calculations and assumptions can be found in Table S4 b WSP could not be included due to lack of available biomass productivity data c Data was not available to estimate kJ fuel per g N removed 1216 Environ Sci Processes Impacts 2014 16 12041222 This journal is The Royal Society of Chemistry 2014 Environmental Science Processes Impacts Critical Review Open Access Article Published on 18 March 2014 Downloaded on 10182022 71141 PM This article is licensed under a Creative Commons AttributionNonCommercial 30 Unported Licence View Article Online 61 Implications of this work This review examines the potential of various biotechnologies to directly treat domestic wastewater with a positive operating energy balance For anaerobic technologies influent COD is an important determinant of fuel production higher COD concentrations lead to more energy recovery and less energy consumption per gram of COD degraded Since freshwater serves as a carrier for human waste in developed countries domestic wastewater is often dilute limiting the amount of energy that can be recovered during secondary treatment For phototrophic technologies a similar relationship exists between influent N and P concentrations and biomass yields Despite limited energy recovery and production values replacing energy intensive COD and nutrient removal processes could enable treatment plants that have already established solids digestion and onsite electricity generation to achieve energy positive operation At the forefront of energyconscious wastewater treatment with aerobic COD removal and BNR an activated sludge WWTP in Strass Austria has achieved energy selfsufficiency by implementing a high rate aerobic process anammox treatment of nutrient rich side streams and onsite electricity generation from biogas generated by solids digestion A published COD mass balance and energy analysis of the plant indicated that 75 of the COD entering the plant is fed to a digester 61 primary and high rate solids and 14 waste solids from biological nutrient removal and 36 is converted to biogas166 The aerobic BNR process in which 31 of the influent COD and 80 of the N is removed accounted for 45 of energy consumption at the plant The Strass WWTP COD mass balance was used to simulate the energetic potential of replacing the existing aerobic processes with anaerobic and phototrophic wastewater treatment If the BNR process was replaced with an ABR to remove COD and a HRAP to remove nitrogen total plant biogas production could potentially increase by 39 and energy recovery from COD could reach 41 Section S5 of ESI The energetic content of biomass produced in the HRAP during N removal 2200 kJ per capita per day was estimated to be more than twice as much as recovered biogas 1020 kJ per capita per day If PBRs were employed rather than HRAPs the estimated biomass energy content alone 7800 kJ per capita per day assuming Nlimited growth could be more than three times the caloric energy content of wastewater entering the plant 2500 kJ per capita per day More broadly combined anaerobic and phototrophic processes could reduce energy demand and achieve energy recovery and production on the order of 5092 kW h m3 using higher values for UASBs and PBRs well above the wholeplant energy demand of conventional WWTPs 0306 ref 20 and 21 Though achieving energy and resource positive treatment in developed countries is an important goal for future treatment far more urgent is the need to deploy sanitation infrastructure in developing and underdeveloped communities where an estimated 25 billion people lack access to improved sanitation167 Even in cases where individuals have access to bathroom facilities and collection systems it is estimated that globally 15 billion people connected to sewerage infrastructure have their wastewater discharged without treatment168 In developing communities in tropical regions mainstream anaerobic treatment of domestic wastewater has been shown to be a viable means of achieving treatment goals while simultaneously producing biogas169 This biogas if utilized properly could be an invaluable resource providing a consistent supply of electricity In developing countries effluent from anaerobic treatment processes eg UASB can be fed to WSP for further treatment170171 The data analysis presented in this review indicates that converting WSPs to HRAPs is a path toward more meaningful energy production from wastewater management Ultimately one of the greatest opportunities to advance wastewater treatment in developing communities is to recover resources that make wastewater management energy positive and financially viable 62 Future research needs anaerobic technologies The experimental results complied in this review clearly show that energy recovery in the form of methane gas is significantly higher than energy recovery by MECs and MFCs While methaneproducing technologies do not require electrodes or applied voltage to generate fuel converting biogas to electricity requires expensive auxiliary equipment ie gas conditioning storage prime movers or fuel cells and is currently only feasible at high flow wastewater treatment facilities 30 MGD133 Of the more the 1300 treatment plants that employ anaerobic digestion for solids management in the US only 364 are sites generate enough biogas to make combined heat and power CHP financially viable of which 104 currently generate electricity from biogas133 Primary anaerobic treatment would make CHP accessible to smaller WWTPs but it remains to be seen at what scale economic feasibility could be reached Though methane is relatively insoluble in water Henrys constant 776 bar L mol1 loss of dissolved methane in the wastewater effluent continues to be a critical challenge for anaerobic processes172173 This loss of fuel removes much of the potential for anaerobic processes to be energy positive especially since energy savings from psychrophilic operation are in tension with increased energy losses due to higher methane solubility at lower temperatures47 Finding alternative methods to recover dissolved methane without excessive energy input ie using an amount of energy less than the amount recovered will be pivotal to achieve energy positive treatment with AnMBR In terms of energy recovery MFC bioelectricity was significantly lower than gaseous products However when fuel conversion to electricity was considered the discrepancy between MFCs and gasproducing technologies was less substantial indicating that MFCs may be a favorable option for distributed electricity production from wastewater To capitalize on this potential research efforts should focus on anode and passiveair cathode fabrication without the use of expensive materials as well as evaluation of power production from source separated waste streams METs can also be designed to operate in concert with methaneproducing processes to enhance treatment efficiency and recover nutrients Allocating a portion of soluble organic energy to produce electrical current with MET electrodes could be leveraged toward electrolytic pH adjustment to volatilize and concentrate ammonia174176 or recover N and P as struvite177178 Ionic current produced by MET could also be used to polarize capacitor electrodes and remove charge mole cules such as nutrients and minerals from wastewater179 63 Future research needs phototrophic technologies Although the predominant focus of nutrient research in the wastewater eld has been on improving the efficiency of BNR by chemotrophic bacteria the energetic potential of phototrophic processes warrants further development of these processes for energy positive nutrient management In particular more highly engineered systems that minimize footprint like PBRs and stirred tank reactors may have potential in advancing nutrient removal initiatives while also increasing the energy independence of treatment facilities A critical challenge in achieving reliable and resilient phototrophic treatment systems however is a lack of understanding of how process design and operational decisions inuence effluent quality biomass productivity and biochemical composition144 Devel oping a deeper understanding of mixed community photo trophic biotechnology in the context of wastewater treatment will require longterm experimentation with real wastewaters under natural light or simulated natural light conditions with diurnal cycles Targeted experimentation and modeling may enable process optimization but a priority should be to determine how complex models will need to be to enable reliable predictions of performance across climates and wastewaters180181 Harvesting and downstream processing to usable fuels are also opportunities for technology advancement including research furthering the development of processing technologies that do not require complete drying of biomass prior to pro cessing anaerobic digestion and HTL hold particularly high potential in this regard In addition to fundamental advance ments to HTL and the management of waste products182 a critical challenge is to link process design decisions with downstream processing to usable energy Without a mecha nistic understanding of the links among cultivation decisions biochemical composition harvesting and processing to fuel any attempts at process optimization are likely to result in trade offs that may be obscured by energetic impacts of design and operational modications 7 Conclusion The pursuit of energy positive domestic wastewater treatment is a necessity due to both the nancial costs and the broader environmental impacts incurred by energy consumption Beyond economic and environmental drawbacks energy intensive treatment processes may also be infeasible for devel oping communities that may even lack the energy infrastructure to reliably treat wastewater aerobically Based on the results of this review it is clear that WWTPs can be net energy producers especially if phototrophic technologies are leveraged to increase the energetic potential of wastewater through inorganic carbon xation In the search for energetically favorable technologies however there is a critical point to be made we should not compromise traditional sanitary engineering objectives for wastewater treatment systems ie effluent quality to achieve energy positive performance but rather seek to develop tech nologies that achieve equivalent or superior effluent quality by leveraging biological chemical and physical processes whose treatment efficacy is not in direct tension with their energy balance Therefore we should seek to advance technologies that have synergies between effluent quality and energy production such as anaerobic and phototrophic technologies where every gram of pollutant removed increases the potential energy yield from the system Acknowledgements This work was partially funded by the King Abdullah University of Science and Technology KAUST Academic Partnership Program UIeRA 201206291 and by the Center of Advanced Materials for the Purication of Water with Systems Water CAMPWS under NSF Agreement Number CTS0120978 The authors would like to acknowledge the Jack Kent Cooke Foun dation for partial funding for BD Shoener and the Safe Global Water Institute SGWI at the University of Illinois at Urbana Champaign UIUC for partial funding for IM Bradley We would also like to thank Cheng Zhong UIUC and Anthony Greiner Hazen and Sawyer for their assistance in energy consumption estimation Shijie Moses Leow UIUC for discussions on hydrothermal liquefaction and the anonymous reviewers for their 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a Creative Commons AttributionNonCommercial 30 Unported Licence View Article Online PERGUNTAS PARA SEREM RESPONDIDAS 1 Quais foram qual foi os resultados mais significativos Dentre os resultados mais significativos das pesquisas destacase a possibilidade de utilização dos WWTPs como produtores líquidos de energia sobretudo quando as tecnologias fototróficas são promovidas para aumentar o potencial energético das águas residuais através da fixação de carbono inorgânico Para tal foi realizada inicialmente uma análise energética determinando a produção de combustível anaeróbio ou matériaprima de bioenergia fototrófica de cada estudo e o conteúdo calórico associado Após isso foi possível determinar o rendimento energético e o consumo energético das tecnologias anaeróbica e fototróficas Em seguida determinouse o balanço energético e eficácia do tratamento das tecnologias anaeróbias e das tecnologias fototróficas Com isso podese observar que as tecnologias que exigem mais energia tendem a ter melhor desempenho no atendimento aos objetivos tradicionais de tratamento e também são capazes de gerar mais biomassa e mais energia por grama de nutriente removido para compensar esse consumo de energia 2 O que se discutiu sobre os resultados Os autores apresentam uma crítica em relação à procura por tecnologias energeticamente favoráveis Nesse sentido eles defenderam que não se deve comprometer os objetivos tradicionais de engenharia sanitária para sistemas de tratamento de águas residuais no intuito de atingir desempenho energético positivo Em contrapartida eles defendem que devese procurar desenvolver tecnologias capazes de atingir qualidade de efluentes equivalente ou superior impulsionando processos biológicos químicos e físicos cuja eficácia do tratamento não esteja em tensão direta com seu balanço energético Dessa forma a partir dos resultados os autores sustentam que devese visar o avanço de tecnologias que tenham sinergias entre a qualidade do efluente e a produção de energia como as tecnologias anaeróbica e fototrófica uma vez que cada grama de poluente removido seria capaz de aumentar o potencial rendimento energético do sistema