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Conventional wastewater treatment processes are energy-intensive and focus on removal of organics and nutrients rather than recovery. In response to these challenges, a great deal of research has been conducted on alternative wastewater treatment technologies that recover or produce energy during wastewater treatment. As researchers attempt to balance the potential gains in net energy production with performance and economic trade-offs of each technology, the range of configurations for anaerobic and phototrophic systems continues to grow.
Suspended growth processes are characterized by complete-mix conditions to prevent biomass from settling and to facilitate contact between the microorganisms and the wastewater. Attached growth anaerobic technologies rely on packing material in the reactor to provide surfaces for biofilm formation. AFB reactors are operated at high upflow velocities in order to suspend particulate media such as granular activated carbon (GAC) in the reactor,20 with wastewater treatment achieved by biofilms attached to the media.
METs (also referred to as bioelectrochemical systems, BES) leverage microorganisms capable of extracellular electron transfer29,59 to produce electrical energy from wastewater. There is extensive literature on phototrophic growth in stirred tank reactors (open, completely mixed reactors lit from overhead). The cost of biomass harvesting (including flocculation, centrifugation, and sedimentation82) remains a key barrier to the broad implementation of suspended growth algal systems.88 Although sedimentation is often the most inexpensive approach, it achieves low (50–90% (ref. A comprehensive literature review was conducted on the technologies listed above, with a focus on studies demonstrating treatment of municipal-strength wastewaters (COD <500 g COD per m3). 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 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. An estimation of energy consumption for each technology was included in order to evaluate the feasibility of net energy positive wastewater treatment. 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. In the review of the peer-reviewed literature a total of 225 anaerobic and 86 phototrophic papers were screened and assessed according to the inclusion criteria (Fig. Table 1 Average and range of percent COD or nutrient (N or P) removal for each technology used in Fig. Advertising Press on Your Side Creating a BIG Promotion Tutorial Unique Ideas Promotion Articles Online Tools This is our article database. 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 m?3 of wastewater treated), energy consumption, and treatment efficacy. Re-envisioning 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.
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 fixation during growth and carbon storage.
As the phototrophs (including algae and cyanobacteria) take up inorganic carbon and grow, they also take up N and P from the wastewater and achieve nutrient recovery via assimilation. To better understand the status and relative potential of each configuration, 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 management. The primary characteristics that differentiate reactors within this category are the packing material type and degree of bed expansion.32 For example, the packed bed and fluidized bed configurations are operated at increasing upflow velocities, with fluidized bed being the highest. While AFBs are particularly effective for low strength wastewaters, the main shortcoming is minimal solids capture.32 AFBs are therefore more appropriate for wastewater streams with primarily soluble COD. Higher SRTs correlate to greater volatile fatty acid (VFA) and soluble COD removals.32 Additionally, AnMBRs allow for a much smaller footprint by enabling higher solids concentrations in the reactor.
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.
There are relatively few studies that examine PBRs in conjunction with wastewater treatment, largely because of high costs compared to other treatment technologies94 and because axenic cultures are generally not targeted for municipal wastewater treatment. Although published studies using stirred tank reactors cover a range of operational conditions (including various lighting schemes, batch vs. For phototrophic systems, comparison studies have found that synthetic wastewater, though displaying comparable nutrient removal rates, generates more biomass than wastewater-based studies85 and was thus excluded from this review.

Biomass was either reported as total, maximum, or average VSS (g m?3), as productivity (g per m3 per day), or as aerial productivity (g per m2 per day). 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. 2 (a) Energy content [kJ fuel per g COD removed] for each paper studying anaerobic technologies with an influent COD below 500 g m?3 (for synthetic wastewater) or using actual domestic wastewater.
26), the average energetic content of cultivated phototrophic biomass across all technologies (kJ per capita per day) was 130–510% of the energy saved from offsetting fertilizer production (assuming 100% nutrient recovery and 69 kJ per g N (ref. Rich portfolio, good prices, hosting, important refferences, order on-line!, advertising, internet, outdoor, media, publishing, TV, commercials, promotion, romania, romanian, Bucharest, poster, promovare, reclama, campanii media, campanie, companie, company, network, franchising, communication, banner, baner, banere, piata, market research, marketing, professional, romanian advertising company, studii de fezabilitate, promovare, investitie, investment, marketing campaigns, marketing campaign, international advertising, eastern europe advertising company, indoor, trade publications, advertising rates, ce ofera gama completa a serviciilor profesionale de reclama si marketing pentru companiile romanesti si straine ce doresc sa atinga pozitia de leader pe piata romaneasca.
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). To this end, anaerobic technologies can recover the chemical energy of organic carbon in wastewater as methane, hydrogen, and electricity. In addition to the production of bioenergy products such as methane, hydrogen, or electricity,28–31 anaerobic processes are expected to be less energy intensive than aerobic processes due to a lack of aeration and a reduction in sludge wastage.32 Although published studies have analyzed the performance of one or a small number of anaerobic system designs,28,33–37 an in-depth comparison of technologies focusing on their potential in domestic wastewater management is still needed. Based on available data, we quantified 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. Methane (CH4) and hydrogen gas (H2) are possible bioenergy products from anaerobic systems.
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.
For methane producing systems, studies using synthetic wastewater with relevant COD concentrations were included because differences in performance (between synthetic and real wastewater) were not readily observed. Results for each technology were compared on the basis of per capita and per m3 of wastewater treated using the conversions discussed in Section 4.3.
The SRT, experiment length, and reactor volume were leveraged to convert all numbers to an average daily productivity (g per m3 per day). 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 5.3). Phototrophic technologies included were the high rate algal pond (HRAP), photobioreactor (PBR), stirred tank reactor, waste stabilization pond (WSP), and algal turf scrubber (ATS). 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.
42 and 43) and have even examined the potential for energy positive wastewater treatment,38 but such studies have been limited to single cultivation technologies.
Seeking a deeper understanding of the potential energy balance of each technology, we also quantified usable energy yield (based on downstream conversion of bioenergy feedstocks) and anticipated energy consumption (based on experimental conditions in published studies).
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.
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. Lastly, we examined the dichotomy between emerging (energy production) and traditional (effluent quality) objectives for treatment technologies (Section 6).
When including typical percent COD removals for each technology, this range would equate to roughly 40–1200 kJ per capita per day (Fig. All energy products (methane, hydrogen, electricity) are reported as kJ using theoretical unit conversions (see ESI†). In this review we evaluate the strengths and weaknesses of these technologies and project a path toward energy positive wastewater treatment.
To our knowledge there has not been a comprehensive comparative assessment of existing and emerging phototrophic technologies as tools to enable energy positive domestic wastewater management.
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.
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.
Once relevant studies were identified, many were excluded from further analysis due to insufficient data that prevented the calculation of energy production normalized to contaminant removal.

To convert biomass productivity to energetic potential, reported VSS were converted to units of COD (see Table S2, ESI†). 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). Individual points represent distinct experimental data sets, with error bars extending to ±standard deviation (if reported). When including typical percent chemical oxygen demand (COD) removals by each technology, this range would equate to roughly 40–1200 kJ per capita per day or 110–3300 kJ m?3 of treated wastewater. 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 production24–26 that can have a net benefit for the environment27 – to gain traction at a broad scale. Given these constraints, we identify technologies with the greatest potential to enable energy positive carbon, nitrogen, and phosphorus management and present operational and performance targets for anaerobic and phototrophic treatment technologies to improve their net energy balance. If a required value was not explicitly stated but prerequisite values were given, the unknown values were calculated (see Fig. 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.
After conversion of gases to usable electricity in a fuel cell (42.3% efficient), these values represent recoveries of roughly 2–20%. The average bioenergy feedstock production by phototrophic technologies ranged from 1200–4700 kJ per capita per day or 3400–13000 kJ m?3 (exceeding anaerobic technologies and, at times, the energetic content of the influent organic carbon), with usable energy production dependent upon downstream conversion to fuels. Any insights gained may be applied to the design of larger-scale batch or sequencing batch reactors for both wastewater treatment and algal biomass production. 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 noted (fuels are converted to electricity; biomass is converted to heat, methane, biodiesel, and biocrude oil).
Finally, WSPs were excluded from the energetic analysis due to a lack of biomass productivity data. 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 energy-producing technologies are identified, and the role of integrated anaerobic and phototrophic bioprocesses in energy positive wastewater management is discussed.B. This range equates to 3400–13000 kJ m?3 of treated wastewater, or 280–400% of the potential recovery from methane-producing anaerobic technologies. Although MFC power production from wastewater was limited by substrate conductivity and strength, power densities from single chamber MFCs fed optimized synthetic solutions (?1.4 kJ per g COD)67 would have still been only 19–55% of the average reported energy recovery rates for methane-producing technologies.
As expected, the ratio of energy yield to P uptake was 2–14? higher as compared to N uptake (except for PBR data). Brian's current research focuses on the advancement of anaerobic membrane bioreactor technology via quantitative sustainable design. More broadly, his interests are in process modeling and improving energy recovery from methanogenic wastewater treatment processes.I. In the case of PBR experiments, data analysis was limited to two wastewater-relevant studies with adequate data.
Ian's current research focuses on algal growth for biofuel feedstock production in conjunction with wastewater treatment.
He is interested in biological systems and process design for water and sanitation systems in the developed and developing world.R. His research focuses the development of electrochemical technologies for nutrient and energy recovery from wastewater as well as natural and engineered salinity gradients.J. GuestDr Guest is currently an Assistant Professor in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign. Dr Guest's interests center on biotechnology development for resource recovery from sanitation media, with particular emphasis on microalgae and methane-generating 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.

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