Bioenergy from Wastewater

Bioenergy from wastewater
Name:
Institution:
Date:

BIOENERGY FROM WASTEWATER
Introduction
In order to build a sustainable society, reduction of reliance on fossil fuels is required. Waste disposition in the wrong places causes devastating impacts on cultures as well as ecosystems all over the world. Hence, a new practical approach has been taken to develop or enhance the wastewater treatment. The approach is usable in production of liquid bio fuels and electric power, as well as a revenue stream to offset the improvement of infrastructure (Klass, 1998). This approach is known as Photosynthentacally Oxygenated Waste-to-Energy Recovery-system (POWER).
How This Technology Works
There is a realization that there are particular types of algae that are efficient in removing nitrogen, phosphorous compounds and carbon, from agricultural and municipal wastewater. These algae are cultivated in the outdoors of the sewage water, in large and enclosed containers made of plastic, that is known as ‘closed photo bioreactors,’ which hinders evaporative water loss. Such kinds of photo bioreactors also hold of carbon dioxide, which is also algae’s nutrient. This results to fostering the production of high-density algae (Wall & Harwood & Demain, 2008).
Photo bioreactors also help to contain the potential destructive microbes and maintain wastewater smells. The main contributions of this process in addition to sun light are the algae, the wastewater and the carbon dioxide. The photo bioreactors then produce large quantities of biomass in a very short time, if given suitable strains of algae. The production of these photo bioreactors becomes a broth that is later isolated into biosolids, which move into the components of fuel production. Efficiency at every stage is mandatory in order to come up with a sustainable approach for the system (Angenent & Karim & Al-Dahhan & Wrenn & Domíguez-Espinosa, 2004).
How It Was Applied To Solve a Particular Problem
This technique was applied to solve the problem of treating wastewater without necessarily incurring the high effort of electricity, which is required by regular sewage treatment plants. The technique was as well applied to help the nutrients from the wastewater to apply photosynthesis in producing renewable surplus energy. Moreover, the POWER technique was used in offering maintainable technique for treating wastewater. It was used as a workable method of producing the electric energy and biofuels liquid. The Desert Southwest provided a favorable environment for the application of the technique. This is because of the algae production due to mild temperatures, abundant sunlight as well as wide-open locations for the facilities of alga cultivation. This has resulted to attraction of many states to the area.
What are the strengths and weaknesses of the new technology relative to one other technology that is currently used to address the same problem?
The best thing about this new technology is the fact that it enhances the wastewater treatment process by offering a credible method of the electricity power as well as biofuels liquid. This method is advantageous than the use of traditional wastewater plants where there bacteria is used for this purpose. This is because the use of algal photosynthesis in processing wastewater, yield more biomass compared to sludge output of the previous systems. The POWER technique also converts algal biomass into electric power and liquid fuel. However, the disadvantage is that the enclosed photobioreactors in desert surroundings prevents the intense of the hot days from destroying the algae. The other disadvantage is the higher usage of electricity in regulating the water’s temperature using a cooling system, unlike the method of traditional wastewater plants where electricity is not an issue (Gopalakrishnan & Van & Brown, 2012).
Conclusion
Ultimately, the POWER technique is usable in production of liquid bio fuels and electric power, as well as a revenue stream to offset the improvement of infrastructure. This technique was applied to solve the problem of treating the wastewater without necessarily incurring the high effort of electricity, which is required by regular sewage treatment plants (Khanal & Environmental and Water Resources Institute (U.S.), 2010).
References
Gopalakrishnan, K., Van, L. J., & Brown, R. C. (2012). Sustainable bioenergy and bioproducts:
Value added engineering applications. London: Springer.
Khanal, S. K., & Environmental and Water Resources Institute (U.S.). (2010). Bioenergy and
biofuel from biowastes and biomass. Reston, Va: American Society of Civil Engineers.
Klass, D. L. (1998). Biomass for renewable energy, fuels, and chemicals. San Diego: Academic Press.
Wall, J. D., Harwood, C. S., & Demain, A. L. (2008). Bioenergy. Washington, D.C: ASM Press.
 HYPERLINK "/science/article/pii/S0167779904001933" /science/article/pii/S0167779904001933
Production of bioenergy and biochemicals from industrial and agricultural wastewater

The building of a sustainable society will require reduction of dependency on fossil fuels and lowering of the amount of pollution that is generated. Wastewater treatment is an area in which these two goals can be addressed simultaneously. As a result, there has been a paradigm shift recently, from disposing of waste to using it. There are several biological processing strategies that produce bioenergy or biochemicals while treating industrial and agricultural wastewater, including methanogenic anaerobic digestion, biological hydrogen production, microbial fuel cells and fermentation for production of valuable products. However, there are also scientific and technical barriers to the implementation of these strategies.
The bioprocesses that will be used to treat wastewater in the future will be chosen as they have been in the past: according to technical feasibility, simplicity, economics, societal needs and political priorities. The needs and political priorities of a sustainable society, however, provide pressure that will shift the focus on wastewater treatment from pollution control to resource exploitation. Many bioprocesses can provide bioenergy or valuable chemicals while simultaneously achieving the objective of pollution control. Industrial wastewaters, for example, from food-processing industries and breweries, and agricultural wastewaters from animal confinements are ideal candidates for bioprocessing because they contain high levels of easily degradable organic material, which results in a net positive energy or economic balance, even when heating of the liquid is required. In addition, they have a high water content, which circumvents the necessity to add water. Such wastewaters are potential commodities from which bioenergy and biochemicals may be produced. Recovery of energy and valuable materials might reduce the cost of wastewater treatment, and somewhat reduce our dependence on fossil fuels.
This review describes four different bioprocessing strategies that can be used to treat industrial and agricultural wastewater, with the generation of valuable products. Three of these bioprocessing strategies result in the production of bioenergy (methane, hydrogen, electricity), and the fourth processing strategy produces valuable biochemicals by fermentation. Although phototrophic processes can also be used to produce hydrogen and valuable products, this review focuses exclusively on chemotrophic (i.e. dark) processes; in particular, on recent technological developments, but barriers to implementation and unresolved scientific questions will also be discussed. For each of the bioprocessing strategies, we will consider whether the technology is ready for full-scale implementation, whether the products can be easily separated from the treated wastewater, whether mixed, pure or well-defined co-cultures are preferable, and whether the products have sufficient value to justify the added complexity over conventional wastewater treatment processes.
1. Biological methane production from organic material in industrial and agricultural wastewater
Methanogenic anaerobic digestion of organic material in wastewater ( HYPERLINK "/science/article/pii/S0167779904001933" \l "tbl1" Table 1, reaction 11) has been performed for about a century and is advantageous over aerobic active sludge systems because of its high organic removal rates, low energy-input requirement, energy production (i.e. methane), and low sludge production. The food web of anaerobic digestion is reasonably well understood ( HYPERLINK "/science/article/pii/S0167779904001933" \l "tbx1" HYPER14Box 1HYPER15). An important breakthrough was made ∼30 years ago, with the development of the upflow anaerobic sludge blanket [UASB] reactor HYPER13 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib1" [1], which efficiently retains the complex microbial consortium without the need for immobilization on a carrier material (for example, as a biofilm) by formation of biological granules (i.e. granulation; self-immobilization) with good settling characteristics. The mean cell residence time, that is, the average time a typical microbial cell remains in the reactor, of UASBs is much longer than the hydraulic residence time (the average time the wastewater remains in the reactor), due to this self-immobilization process. Performance depends on the mean cell residence time and reactor volume depends on the hydraulic residence time, therefore, UASBs can efficiently convert wastewater organic compounds into methane in small ‘high-rate’ reactors. Approximately 60% of the thousands of anaerobic full-scale treatment facilities worldwide are now based on the UASB design concept, treating a diverse range of industrial wastewaters  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib2" 2 and  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib3" 3.
Table 1. Reactions for the different (bio) process strategies
Biotic or abiotic processReactionNumber (letter, if shown, is that in HYPERLINK "/science/article/pii/S0167779904001933" \l "fig1" Figure Iin Box 1)Hydrogen fermentation to acetic acidC6H12O6+2H2O ⇆ 4H2+2CH3COOH+2CO2
1 (a)
Hydrogen fermentation to butyric acid
C6H12O6⇆2H2+CH3CH2CH2COOH+2CO2
2 (a)
Fermentation to ethanol
C6H12O6 ⇆ 2CH3CH2OH+2CO2
3 (a)
Propionic acid production with hydrogen
C6H12O6+2H2 ⇆ 2CH3CH2COOH+2H2O
4 (a)
Ethanol production with hydrogen
CH3COOH+H2 ⇆ CH3CH2OH+H2O
5
Syntrophic propionic acid oxidation
CH3CH2COOH+2H2O ⇆ CH3COOH+3H2+CO2
6 (b)
Syntrophic butyric acid oxidation
CH3CH2CH2COOH+2H2O ⇆ 2CH3COOH+2H2
7 (b)
Syntrophic acetic acid oxidation
CH3COOH+2H2O ⇆ 4H2+2CO2
8 (c)
Hydrogenotrophic methanogenesis
4H2+CO2 ⇆ CH4+2H2O
9 (d)
Acetoclastic methanogenesis
CH3COOH ⇆ CH4+CO2
10 (e)
Methane formation from glucose
C6H12O6 ⇆ 3CH4+3CO2
11 (a,b,c,d,e)
Catalytic methane conversion to syngas
CH4+H2O ⇆ 3H2+CO
12
Catalytic gas-shift reaction
CO+H2O ⇆ H2+CO2
13
Hydrogen fuel cell
2H2+O2 ⇆ 2H2O+electricity
14
Methane fuel cell
CH4+2O2 ⇆ CO2+2H2O+electricity
15
MFC
C6H12O6+6O2 ⇆ 6CO2+6H2O+electricity
16
Cellulose bioconversion
[–C6H11O6–]n+aH2O ⇆ bCH3COOH+cCH3CH2OH+dCO2+eH2
17
Polyhydroxyalkanoates formation
aC6H12O6+bO2 ⇆ [–COOH(CH2)3COO–]n+cH2O
18
Dyes formation
[–C6H11O6–]n+aNH3−+bH2O ⇆ cC23H26O5+dC22H27O5 N+eC21H22O5
19
Table options
Box 1
Anaerobic bioconversion of complex organic material to methane requires four major steps and five physiologically distinct groups of microorganisms. Elements of the food web of methanogenic anaerobic digestion are expected to occur also for biological hydrogen production, microbial fuel cells and biochemical production. As shown in  HYPERLINK "/science/article/pii/S0167779904001933" \l "fig2" Figure I, complex organic polymers (e.g. proteins, polysaccharides) are hydrolyzed to monomers by fermentative bacteria (a), which ferment the monomers to a mixture of low-molecular-weight organic acids and alcohols. These fermentation products are further oxidized to acetic acid and hydrogen by obligatory hydrogen-producing acetogenic bacteria (b) through a process called acetogenesis. Acetogenesis also includes acetate production from hydrogen and carbon dioxide by acetogens and homoacetogens (c). Hydrogen-producing acetogenic bacteria (b) grow in syntrophic associations with hydrogenotrophic methanogens (d), which keep the hydrogen partial pressure low enough to allow acetogenesis to become thermodynamically favorable (this process is referred to as interspecies hydrogen transfer)  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib58" [58]. Finally, acetoclastic methanogens (e) convert the acetate to methane and carbon dioxide (methanogenesis). Although ∼70% of methane produced in many natural and engineered systems is due to acetoclastic methanogens, it is increasingly clear that many stressed and thermophilic systems use an alternative pathway: syntrophic oxidation of acetate to carbon dioxide and hydrogen by acetogenic or homoacetogenic bacteria (c) coupled to hydrogen consumption by hydrogenotrophic methanogens  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib8" 8 and  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib59" 59.
 HYPERLINK "/science/article/pii/S0167779904001933" \l "gr2" \o "\"Full-size image (20 K)\" 

Figure I. Intricate food web of methanogenic anaerobic digestion. Several trophic groups of microorganisms work together to convert complex organic material into methane and carbon dioxide
 HYPERLINK "/science/article/pii/S0167779904001933" Figure options
Box 1
Anaerobic bioconversion of complex organic material to methane requires four major steps and five physiologically distinct groups of microorganisms. Elements of the food web of methanogenic anaerobic digestion are expected to occur also for biological hydrogen production, microbial fuel cells and biochemical production. As shown in  HYPERLINK "/science/article/pii/S0167779904001933" \l "fig2" Figure I, complex organic polymers (e.g. proteins, polysaccharides) are hydrolyzed to monomers by fermentative bacteria (a), which ferment the monomers to a mixture of low-molecular-weight organic acids and alcohols. These fermentation products are further oxidized to acetic acid and hydrogen by obligatory hydrogen-producing acetogenic bacteria (b) through a process called acetogenesis. Acetogenesis also includes acetate production from hydrogen and carbon dioxide by acetogens and homoacetogens (c). Hydrogen-producing acetogenic bacteria (b) grow in syntrophic associations with hydrogenotrophic methanogens (d), which keep the hydrogen partial pressure low enough to allow acetogenesis to become thermodynamically favorable (this process is referred to as interspecies hydrogen transfer) [58]. Finally, acetoclastic methanogens (e) convert the acetate to methane and carbon dioxide (methanogenesis). Although ∼70% of methane produced in many natural and engineered systems is due to acetoclastic methanogens, it is increasingly clear that many stressed and thermophilic systems use an alternative pathway: syntrophic oxidation of acetate to carbon dioxide and hydrogen by acetogenic or homoacetogenic bacteria (c) coupled to hydrogen consumption by hydrogenotrophic methanogens  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib8" 8 and  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib59" 59.
 HYPERLINK "/science/article/pii/S0167779904001933" \l "gr2" \o "\"Full-size image (20 K)\" 

Figure I. Intricate food web of methanogenic anaerobic digestion. Several trophic groups of microorganisms work together to convert complex organic material into methane and carbon dioxide
 HYPERLINK "/science/article/pii/S0167779904001933" Figure options
It was originally thought that a continuous upflow-hydraulic pattern was required for granulation, but this phenomenon was recently observed in a continuously-fed horizontal-flow bioreactor that incorporated a migrating blanket within a compartmentalized reactor (a multi-vessel; anaerobic migrating blanket reactor [AMBR])  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib4" [4]. Due to the AMBR's inherent dynamic conditions, the organic removal rates are higher than those in UASB reactors  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib5" 5 and  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib6" 6.
A significant limitation of UASB reactors is the interference of suspended solids in the incoming wastewater with granulation and reactor performance  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib7" [7]. Hence, other high-rate systems, such as the anaerobic sequencing batch reactor (ASBR), were developed to better handle high-suspended solids in wastewater. ASBRs are single-vessel bioreactors that operate in a four-step cycle: (i) wastewater is fed into the reactor with settled biomass; (ii) wastewater and biomass are mixed intermittently; (iii) biomass is settled and; (iv) effluent is withdrawn from the reactor. ASBRs are particularly useful for agricultural waste, and it has recently been scaled up for on-farm treatment of dilute swine waste  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib8" [8].
The methane that is produced by anaerobic digestion has traditionally been used as a fuel source, usually, for on-site heating or electricity production. Recently, methane has also been converted to other useful products, such as methanol for use in production of biodiesel, for example, by production of syngas (a mixture of hydrogen and carbon monoxide;  HYPERLINK "/science/article/pii/S0167779904001933" \l "tbl1" Table 1, reactions 12 and 13) in downstream chemical processes HYPERLINK "/science/article/pii/S0167779904001933" \l "bib9" [9]. Production of syngas requires the removal of impurities, such as hydrogen sulfide, in the digester biogas, which can poison the catalyst. In addition, direct conversion of methane to electricity in solid-oxide fuel cells after a single-step anaerobic digester might soon become feasible ( HYPERLINK "/science/article/pii/S0167779904001933" \l "tbl1" Table 1, reaction 15)  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib10" [10].
2. Biological hydrogen production
Much recent interest has been expressed in the biological production of hydrogen from wastewater by dark fermentation, due to its potential importance in our economy  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib11" 11,  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib12" 12,  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib13" 13 and  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib14" 14. Biological hydrogen production shares many common features with methanogenic anaerobic digestion, especially the relative ease with which the two gaseous products can be separated from the treated wastewater. The mixed communities involved in both bioprocesses share some common elements but with one important difference: successful biological hydrogen production requires inhibition of hydrogen-using microorganisms, such as homoacetogens (see  HYPERLINK "/science/article/pii/S0167779904001933" \l "fig2" Figure I in  HYPERLINK "/science/article/pii/S0167779904001933" \l "tbx1" Box 1, group c) and methanogens (see  HYPERLINK "/science/article/pii/S0167779904001933" \l "fig2" Figure I in  HYPERLINK "/science/article/pii/S0167779904001933" \l "tbx1" Box 1, group d). Inhibition is commonly accomplished by heat treatment of the inoculum to kill all microorganisms except for spore-forming fermenting bacteria (for example, species from the families Clostridiaceae,Streptococcaceae, Sporolactobacillaceae, Lachnospiraceae, and Thermoanaerobacteriacea  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib15" 15,  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib16" 16 and  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib17" 17) (see  HYPERLINK "/science/article/pii/S0167779904001933" \l "fig2" Figure Ia in  HYPERLINK "/science/article/pii/S0167779904001933" \l "tbx1" Box 1). Other methods that have been used include the operation of reactors at high dilution rates  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib18" [18] or low pH  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib19" [19].
Considerable effort has been devoted to optimizing operational environmental conditions to maximize hydrogen production (examples of optimization efforts are given in  HYPERLINK "/science/article/pii/S0167779904001933" \l "tbl2" Table 2). Conceptually, important efforts are those that prevent consumption of hydrogen by, for example, propionic acid-producing bacteria, ethanol-producing bacteria, and homoacetogens ( HYPERLINK "/science/article/pii/S0167779904001933" \l "tbl1" Table 1, reaction 4, reaction 5, and the reverse of reaction 8, respectively); and those that channel more reducing equivalents towards reduction of protons by hydrogenases (see  HYPERLINK "/science/article/pii/S0167779904001933" \l "tbx2" Box 2). Operating bioreactors at low hydrogen partial pressure, perhaps by sparging with nitrogen gas to strip hydrogen from the solution as fast as it is produced  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib19" 19 and  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib20" 20, accomplishes both efforts simultaneously.
Table 2. Maximum hydrogen yields achieved from organic material by a mixed culture performing dark fermentation during optimization efforts
Optimization effortReactor typeSubstrateMax. Hydrogen yield (mol/mol) based on hexoseReferencesInitial pH and acetic/butyric acidBatchSucrose/starch1.8 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib61" [61]Reactor configurationFluidized bed reactorSucrose1.3 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib62" [62]Hydrogen partial pressureCSTRWheat starch1.9 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib20" [20]Inhibition of acetic/butyric acidBatch-fedGlucose2.0 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib63" [63]Reactor operation, temperatureUpflow reactorWastewater2.1 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib13" [13]Immobilized biomassBatchSucrose2.0 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib64" [64]Immobilized biomass, granulesFermentorSucrose2.1 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib16" [16]pHFermentorGlucose2.1 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib65" [65]Hydrogen partial pressure, substrateBatchSucrose, lactate0.5 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib11" [11]Hydraulic retention timeCSTRGlucose, sucrose2.2 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib66" [66]Peptone additionBatch/chemostatCellulose2.0 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib15" [15]Nitrogen sourceBatchGlucose2.4 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib67" [67]pH and substrate levelsBatchSucrose2.5 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib21" [21]Hydrogen partial pressureCSTRGlucose1.4 HYPERLINK "/science/article/pii/S0167779904001933" \l "bib68" [68] HYPERLINK "/science/article/pii/S0167779904001933" Table options
Box 2
Hydrogen production from organic substrates is limited by the thermodynamics of the hydrogenase reaction, which involves the enzyme-catalyzed transfer of electrons from an intracellular electron carrier molecule to protons. Unfortunately, protons are poor electron acceptors (
 mV); so, the electron donor must be a strong reducing agent. Ferredoxin is a low-potential (
 mV, depending on source) iron-sulfur protein that is capable of reducing protons to hydrogen  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib60" [60]. Another important intracellular electron carrier, NADH, has a higher redox potential (
 mV). The ability of reduced ferredoxin and NADH to reduce protons is determined by the redox potential of the net reaction under actual conditions. Assuming the intracellular concentrations of the oxidized and reduced forms of ferredoxin and NADH are about equal, hydrogen production becomes thermodynamically unfavorable at hydrogen partial pressures greater than:


where 
 is the redox potential of the electron donor, F is Faraday's constant, R is the ideal gas constant, and T is the absolute temperature. For ferredoxin, hydrogen production can continue as long as the hydrogen partial pressure is less than ∼0.3 atm (3×104 Pa); for NADH, the partial pressure of hydrogen must be less than ∼6×10−4 atm (60 Pa). Note that these values assume equal concentrations of the oxidized and reduced forms of the electron donors. Higher hydrogen partial pressures can be achieved if the ratio of reduced ferredoxin to oxidized ferredoxin is greater than one. The free energy change of the pyruvate-ferredoxin oxido reductase reaction (ΔGo;′=−2.1 kcal/mole) is sufficient to allow the reaction to proceed with more than a tenfold excess of products over reactants. Hence, the frequent observation of more than 30% hydrogen in reactor headspace is not unexpected.
In most systems for biological production of hydrogen, all of the observed hydrogen can be attributed to electrons derived from a single reaction: oxidative decarboxylation of pyruvate by pyruvate:ferredoxin oxidoreductase ( HYPERLINK "/science/article/pii/S0167779904001933" \l "fig3" Figure II). Hexoses can be metabolized to pyruvate through several pathways, often involving the Embden-Meyerhoff-Parnas (i.e. glycolysis) or the Entner-Doudoroff pathways. Both of these pathways produce two moles of pyruvate and two moles of NADH for every mole of hexose that is transformed  HYPERLINK "/science/article/pii/S0167779904001933" \l "bib60" [60]. Therefore, hexose metabolism by bacteri...