Cover Story
TREATING INDUSTRIAL WASTEWATER: Anaerobic DigeSlio Aerobic Anaerobic ---' Effluent, Effluent, C02, ) -"\.4% 10% Comes of Age ~48% / , ~ . ~ i 4JI'// ~ Anaer bic tr atment systems offer import nl ~ / : . . advant ges over conventionally applied .:' . -1.1 m.llkg-COD +4.3 M.llkg-COD aerobic processes for removing org nic +31 g NH4-N/kg-COD +3.2 g NH4-N/kg-COD pollutant from aler-based stre ms FIGURE 1. This comparison shows the respective fate of organic materials that are biodegraded under aerobic versus anaerobic conditions. Aerobic treat Robbert Kleerebezem , Technical University of Delft ment requires energy input for aeration, whereas a and Herve Macarie, Institut de Recherche pour le Developpement net energy surplus is generated during anaerobic treatment, in the form of methane-bearing biogas that can be used to power utility boilers onsite n the absence of molecular oxygen, wastewater constituents on anaerobic the production of byproduct methane natural environments depend on microbes. As a result, it is the opinion rich biogas. The use of the methane for the activity of anaerobic microor of these authors that such systems are energy generation elsewhere at the ganisms for the biological degrada not currently being applied as widely plant allows for conservation of more tion of organic substrates. In as they could be within the global than 90% of the caloric value of the or Ianaerobic environments where, other chemical process industries (CPl). ganic substrates being treated. In addi than carbon dioxide, no inorganic elec This article aims to address some of tion, modern anaerobic bioreactors do tron acceptors are present, the final the lingering misconceptions associ not require large energy input for me degradation of organic compounds is ated with anaerobic wastewater treat chanical mixing (which is required to achieved by their conversion to ment. It provides a description of the maintain adequate aeration during aer gaseous methane and carbon dioxide. microbiological basis of anaerobic obic treatment). Ai; discussed here, wastewater-treat biodegradation, and defines the By comparison, during aerobic ment systems that rely on the anaero boundary conditions required to en treatment, most of the caloric value of bic digestion process for the removal able anaerobic treatment. the organic substrates is dissipated as of organic pollutants in industrial A brief discussion of the pros and non-recoverable heat. And, aerobic wastewater streams offer important cons of today's leading anaerobic bioreactors require significant advantages over conventionally ap bioreactor designs is also provided, amounts of energy for aeration. plied aerobic processes. along with some simple calculations 2. Typically, anaerobic digestion Discussed in detail below, and sum and basic guidelines for selecting the produces only one-fifth to one-tenth as marized in Figure 1, are several of the best bioreactor design for the waste much biomass per unit of organic sub most compelling operational advan stream at hand. Finally, the article strate converted as comparable aero tages and sources of cost savings asso provides a list of organic compounds bic processes. Since the disposal or ciated with using anaerobic biological that have proven to be degradable treatment of waste sludge may ac treatment systems. using anaerobic digestion, and in count for 50% or more of the total Despite these demonstrable perfor cludes several tables that summarize waste-treatment costs, the ability to mance and cost-saving advantages the current status of commercial reduce sludge production is a major and the fact that nearly 1,600 com scale, anaerobic-treatment systems advantage of anaerobic treatment. mercial installations are currently in throughout the global CPI. 3. All microorganisms require nutri operation around the world - anaero ents, such as nitrogen, phosphorus and bic wastewater treatment still re Primary advantages sulfur, for growth. However, many mains something of a mystery to Listed here are some of the leading op chemical and petrochemical waste many chemical process operators. erational advantages and sources ofcost water streams contain relatively low Many operators still harbor miscon savings associated with using anaerobic levels of these nutrients. ceptions and prejudices about the poor biological-treatment systems: If insufficient nutrient concentra biodegradability and presumed toxic 1. Anaerobic treatment is in principle tions are present, additional nutrients ity of chemical and petrochemical an energy-generating process through must be dosed into the wastewater to
56 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2003 TABLE 1. ANAEROBIC SCORECARD: NU.MBE,R Of SYSTEMS, BY REACTOR TYPE Number of commertrcil scale reaclors buill in Ihe world by January 2003 for Ireal- ing diflerenl kind of Indusiriâî waslewalers and sewage: Type of wastewater Type of reactor see Figure 4 tor reactor types) Low- lAC Fixed- Moving- UASB EGSB Total sustain the process, Since the amount rate bed bed number of nutrients to be dosed is directly re- 1 Food & related Industries lated to the amount ofbiomass formed, Brewery & mail 2 6 4 185 88 285 the nutrient loads required for anaero· - Dislillery & elhanol 25 31 40 - 76 9 181 bic treatment systems are signifi- ûlher beverage - 3 11 2 88 15 119 cantly lower than for aerobic systems. Suger produclion - 49 7 1 32 3 92 4. Despite the relatively low bio- Potalo processing 14 4 2 46 10 76 mass production that typifies most - Dairy, ice-cream & cheese 12 10 10 2 27 6 67 anaerobic digesters, higher biomass Slarch production 2 9 10 2 34 7 64 concentrations can be achieved III Veasl production 7 8 6 25 8 54 today's anaerobic bioreactors. In gen- - Candy & confectionery 4 - 3 15 2 24 eral, higher biomass concentrations - Citric acid production 2 3 1 1 3 5 15 enable higher volumetric treatment Coflee processing - - 7 4 1 12 capacities, thereby minimizing the re- - Wine processing 6 -- 1 3 1 1 11 actor volume and footprint required. Fish & seafood processing 1 4 - - 2 1 8 Miscellaneous 10 22, 40 5 112 25 214 Heading for a breakdown Non·food industries
Thanks ta the voracious appetites of 1 Pulp-&-paper 1 16 5 3 75 37 137 sa many types of aerobic microorgan- (Pelro)chemical 3 4 43 1 20 20 91 isms, and the robustness of the tradi- Leacholes - 6 18 - 24 tional aerobic activated-sludge - - Pharmaceulical 2 16 4 1 6 1 3 process, aerobic wastewater treat- - Pig, cow manure & poullry 5 3 6 1 - 15 ment has long reigned as a leading - Texlile - - 1 4 2 7 method for managing organics-laden - Nalural rubber - 3 3 6 industrial wastewater streams. The - - - Siudge & sludge Iiquor 1- 2 1 1 5 activated-sludge process takes place - Tobacco manufacture 4 4 in open basins, equipped with surface - -- - - Tannery - 3 - 3 or submerged aeration facili ties for - -- Fluegas desulfurization - 1 - 1 1 transferring molecular oxygen from - -- Eleclronic components -- 1 1 - 1 air ta the liquid phase, - - Sewage - - 2 1 64 - 67 During respiration, aerobic microor- Number of reactors ganisms usemolecular oxygen to con- per type 93 167 219 24 852 244 1,599 vert complex organic compounds into 'NOTES: Low-rate reaetoN include CSTR, lagoons and BVF reactors from the firm ADI. 80% of the carbon dioxide, water, and large quan- plants reported correspond to BVF, sinee ADI offers one ofthe few low-rate systems commercialized as a turnkey package, and for which statistics exist. AC = Anaerobic Contact; UASB = Upflow tities of residual biomass, Whereas Anaerobic Siudge Berl; EGSB = Expanded GranulaI' Siudge Bed. The fixed-bed systems reported roughly 50% of the organic substrate is correspond for 44% to upflow anaerobic filters (UAF); 260/< to downflow filters (DAF); and 30% to hy- brid reactors. UASB reactors include reverse DOIT-Oliver clarigesters. Both upflow fluidized-bed re- respired, the remaining 50% is con- actors and the AnAerobies l\Iobilized Film Technology are classified as moving·bed reactors. They verted to biomass. The low levels of reprcsent, respectively, 79% and 21% ofthis group ofreactors. The Biothane Biobed EGSB (43% of the units) and Paques IC (57% of the units) systems conform to the expanded sludge bed class residual organic compounds that are ŒGSB} of reactors. The data presented here have been compiled l'rom the January 2003 reference list of ADI (Canada), Biotec (Colombia), Biotecs Œrazil), Biothane mSAINetheriands), Entec (Aus- typical in aerobically treated effluent t.ria), Global Water Engineering (ex-Enviroasia, Germany), Paques (Netherlands), Proserpol often allow for direct discharge of the (France), Purac (Sweden), Sinko Pantec (Japan) and oIder reference lists of AnAerobies (2001, USA), Applied Technologies (1999, USA), Badger (1994, USA), Biotim (2000, Belgium), Grontmij treated stream ta surface waters. (1998, The Netherlands) and Ondeo-Degrémont (1999, France), as weil as data from the literature By comparison, anaerobic microor- and the authors' files. Care has been taken not to double count units reported in different reference list.s due ta commercial agreement between companies (Shinko Pantec with Badger and Grontmij; ganisms convert organic compounds Biotecs with Biotim and Global Water Engineering). The data reported here cOITespond to treat- into biogas consisting of methane and ment plants. This means that various units on one site have been considered as one sole l'eactor, un- Icss the technology lIsect changed at the occasion of a treatment expansion. As for the low-rate re- carbon dioxide. Only 5-10% of the or- adors, the number of plants treating sewage is probably mllch mOl'e important than the given one. Actually, most of these plants are desigoed by a large variety of 10caI engineering companies diffi- ganic compounds are converted ta bio- cult to identif'y, which is particuJarly true for Latin America. mass. Despite the lower volume of bio- mass produced per unit of substrate chemical oxygen demand (COD) per also has sorne limitations. Forinstance, converted, higher biomass concentra- cubic-meter reactor per day; see the the lower biomass production per unit tians (20-35 kg ofdried solids per cubic box on p. 59 for more on COD]. of substrate removed is an advantage meter) can be achieved III so-called Higher treatment capacities also when considering the amount of sur- "high-rate" anaerobic bioreactors. allow for the design of compact reac- plus sludge to be disposed, but becomes Since the volumetric treatment ca- tors, with small footprints. a disadvantage when considering the pacity is directly related to the bio- time needed for startup ofan anaerobic mass concentration achieved, such Anaerobie limitations treatment system. If no specifically high-rate bioreactors result in While the cast and performance advan- suitable biomass is available in suffi- higher volumetric treatment capaci- tages of anaerobic digestion, as noted cient quantities, startup of the system ties [on the order of 5 ta 40 kg of above, can be cornpelling,thisapproach mayre quire up ta several months. This
CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2003 57 TABLE 2. A SAMPLING Of ANAEROBie SYSTEMS BROUGHT ONLINE 1989-EARLY 2003 Commercial-scale anaerobic wastewater treatment plants built in the chemical & petrochemical industries (for a detailed list k>f plants built from 1981 to 1999, see Ref. (10»: Reactor Year Company Industrial produc- Type Organic COD Constructor, number of con- and tion generating of Load removal references struction location the wastewater reactor Ton COD/d "10 1 1989 K. Chemical, Dyestuff raw material UAF 4.8 80 Shinko Hyogo, Japan (BOD~) Pantec 2 1992 UndiscJosed, PET UASB 6 -- Global Water Taiwan Engineering 3 -- Changzho Worldbest PET Hybrid 1.7 -- Biotim Radici, China 4 -- BP Amoco, Cooper PTA DAF -- 80-85 BPAmoco River, SC, USA (TOC) 5 1994 Baek Hwa Co. Alcohols EGSB 2.5 75" Biothane Korea 6 1996 APR, 1pH Iiquor rayon pulp UASB 40 65" Paques Kamalapuram, India :185 (BOD~) 7 1996 Copenor Formaldehyde, formic acid, !EGSB 2.2 60" 1Biothane Brazil sodium formate, polyols, HexamethyJeneamine 8 1997 Eastman Chem. Co. Oxo chemicals EGSB 12.5 92 Biothane Singapore 9 1997 Hyosung Industries Plastics EGSB 0.9 75" Biothane Korea 1 10 1997 Procter & Gambie Soaps, detergents Hybrid 3 70" ADI Indonesia'" 11 1999 M. Gas Chemical PTA UASB 6 75 Shinko Okayama, Japan (BOO!'» (BOO!'» Pantec 12 2000 BP-Amoco Chem., PTA IC -- -- Paques Kuantan, Malaysia - 13 2000 SKW Trotsberg Metallurgical EGSB 6.7 80'· Biothane Germany 1 14 2000 Borsudchem Co. PU, PVC EGSB 10.7 80'· Biothane Hungary 15 2000 Reliance Industries, PlA EGSB 25.2 70'· Biothane Maharastra, India 16 2000 Sam Nam Petro- PTA Hybrid 6 75" ADI chemicals, Korea 1 17 2000 PT Bakrie Kasei Corp., PTA Hybrid 18.8 75'· ADJ Indonesia 1 18 2001 Getec, Sorbitol, manitol, food IC 7.5 80 Poques sao Gonçalo, Brazil taste additives 19 2001 Yizheng Chem. Fiber PlA UAF 81.6 80'· Global Wafer China Engineering 20 2001 Interquisa, Canada PTA EGSB 26.3 75" Biothane 21 2002 Toray Industries PTA IC 16.8 75 Paques Tokai, Japan 22 2002 SUTIWWT Various (13 chem. plants) EGSB 20.8 60" Biothane Singapore 23 2002 Procter & Gambie Soaps, detergents Hybrid 2.7 75" ADI Thailand 24 2003 Celanese, Cangrejera Alcohols, derivatives Hybrid 9 6 > 90" Celanese 1 . Mexico of acetic acid, amines, ketones and acrylates 1 * NOTES: The abbreviations are the same as in Table 1; PET = polyethylene terephthalate; PTA = purified terephthalic acid; PU = polyurethane;PVC = polyvinyl chloride; TOC = total organic carbon. Plant #4 must be operated similarly to the other DAF designed by BP Amoco at three of its PTA production facilities. The TOC removal is estimated from the one previously given for these units [lOI. This plant is planned to be replaced by a UASB system. Plants number 16 & 17 correspond to expansions of the treatment capacity of already existing anaerobic plants. The information reported in this table cornes from the same sourcc as in Table l, and from Yan et al. [13]. ,',. Design values. **', Factory closed down. effect may become even more pro generally longer for anaerobic systems ofvery dilute wastestreams (those with nounced because most specific anaero than for aerobic ones. less than 0.5 kg ofCOD pel' cubic meter bic microorganisms are capable of de Another point to be considered is that of wastewater). grading a Iimited range of substrates. the treated effluent from an anaerobic Higher concentrations of organic This means that seeding an anaerobic bioreactor typically has (slightly) higher compounds in anaerobically treated bioreactor for the treatmentofa specific effluent concentrations oforganic mate effluent also implies that sorne form of type of wastewater with biomass ob rials, compared to that treated aerobi post-treatment - such as aerobic bio tained from a reactor treating another cally (0.1 to 0.5, versus 0.05 to 0.2, kg of logical treatment, or various types of type ofwastewater may still take up to COD pel' cubic meter of treated waste physical-chemical treatment - will be one month. Analogously, the time re water). Ingeneral, this makes anaerobic required before discharge to surface quired to recover from a process upsetis digestion less suitable for the treatment waters. In general, anaerobic diges-
58 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2003 CHEMICAL OXYGEN DEMAND (COD)
he COD concept is 0 lumped concept thot is used to represent the concentration and Caver Story the oxidation state of organic materials that are present in a water-based stream T[1]. The COD concentration corresponds to the concentration of oxygen that is re quired for the full oxidation of ail organic carbon into carbon dioxide and water. The units that are typically used to describe the COD concentration of a wastewater are tion is better-suited for removing the kg of COD per cubic meter of wastewater, or kg of 02 per cubic meter of wastewater. bulk of organic materials from rela For solutions of a known composition, the COD-equivalent concentration can be caleu tively concentrated wastestreams lated from the stoichiometry of the combustion reaction of the organic compounds. Com (those containing more than 4 kg of bustion of ethanol for example can be described as: COD pel" cubic meter ofwastewater). ~ C2HjJH + 302 2C02 + 3Hp (1) Still, the evident advantages out weigh these potential shortcomings of Consequently, to obtain COD-equivalent concentrations the conversion foctors of 3 mol anaerobic wastewater treatment, as 02 per mol ethanol, or 2.09 9 02 per 9 ethanol, have to be applied. In general terms, the combustion reaction of an organic compound containing the atoms C, H, 0, and N reflected by the nearly 1,600 ful!-scale (with the oxidation state of ammonia being -III) can be wriHen as: reactors that are now in commercial 4x + - 2z - 31' - U - 3v - u () service worldwide (Table 1). CxHpzN~+- 4 ·Or-l x·C02+ 2Hp+vNH~+u-v·H" (2) To date, anaerobic treatment has evolved to become the dominant treat and the corresponding conversion factors can be caleulated. ment method for brewery, distillery, and numerous food-processing waste It should be noted that lag periods Toxicity waters, and has demonstrated great prior to the degradation of sorne an Numerous organic and inorganic potential for treating more-complex thropogenic organic by anaerobic inoc chemicals may have either a re waste streams. Examples include ula from natural environments can be versible or irreversible toxic effect on sewage, and wastewater produced by very long. The natural inocula contain microorganisms. Irreversible toxic the chemical, petrochemical and pulp a very limited number of microorgan properties are generally related to the and-paper industries. isms that can degrade these sub presence of highly reactive functional Still, since less than lS% ofal! anaer strates, and they often need several groups, such as aldehyde, or nitro obic wastewater-treatment systems months of growth before measurable groups. Temporary exposure to ele are currently in use by such CPI facili conversion rates are achieved. vated concentrations of sorne cations, ties, a large potential expansion seems For example, for the anthropogenic organic acids or unfavorable pH val possible in the future. A summary of phthalic acid isomers, it has been ues generally has a reversible toxic ef anaerobic bioreactors constructed be shown that the lag periods prior to fect on microorganisms. tween 1989 and early 2003 in the CPI is quantifiable degradation were in the Anaerobie processes have a notorious shown in Table 2. This table is an up range of 20 to 100 days, depending on reputation for being very susceptible to date of a longer compilation [la]. the origin of the inoculum and the ori process upsets as a result ofinhibition by The implementation of anaerobic entation of the carboxylic groups on toxicants. However - contrary to a still wastewater treatment in chemical the benzene ring [7]. common belief- anaerobic microorgan and petrochemical applications has, Meanwhile, compounds that were isms, particularly methanogenic Ar for decades, been hampered by the found to be persistent in anaerobic chaea, are not more susceptible to toxic presumed limited biodegradability of environments include several mole compounds than aerobic bacteria, ex numerous substrates, and the pre cules containing tertiary-substituted cept in the case ofchlorinated aliphatic sumed susceptibility of primarily carbon or ether bonds [i.e., methyl hydrocarbons [2]. Still, wastewaters methanogenic Archaea toward poten tert-butyl ether (MTBE), tert-amyl containing elevated concentrations of tial toxicants in petrochemical waste methyl ether (TAME), ethyl tert-butyl potential toxicants will require specifie waters. For both limitations, commer ether (ETBE)]. It should be noted, pretreatments before either anaerobic cial experience has demonstrated that however, that several of these com or aerobic biological treatment. In sorne this is true only to a very limited ex pounds are similarly hard to degrade cases, it was found to be possible to tent; this is discussed further below. in aerobic environments. treat toxic wastewater by acclimation of It should also be noted that some the biomass or by application of specifie Target compounds compounds that are persistent in aero reactor designs. The range ofcompounds that have been bic environments are susceptible to found to be degradable in methanogenic partial conversion or even complete Environmenta! factors environments has increased enor degradation in methanogenic environ Petrochemical and chemical waste mously during the past three decades ments. Examples include polyols (such waters typically have a temperature (Table 3). In general, most aliphatic and as pentaerythritol, trimethylol of 30 to 70°C. Anaerobie digestion has homocyclic aromatic compounds can be propane, azo-dyes, nitroaromatics and successfully been applied within two degraded in methanogenic environ polychlorinated aromatic and aliphatic temperature ranges: 30-40°C and ments, as long as they have at least one compounds, including trio, tetra-, pen SO-6SoC. In both zones, specialized oxygen-containing functional group. tachlorophenol and tetrachloroethyl microorganisms develop, enabling the Even sorne aromatic hydrocarbons, ene). In sorne cases, a sequential treat application of the process in the full such as toluene and o-xylene, are com ment scheme, which involves the use of range of wastewater temperatures pletely fermented to methane and car both anaerobicand aerobic digestion, is that are typical of CPI operations. bon dioxide in an anaerobic biodegra required to achieve full degradation of By comparison, at elevated temper dation system [3]. these substrates. atures, aerobic treatment systems can
CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2003 59 TABLE 3. ORGANIC COMPOUNDSTHATARE CANDIDATES FOR ANAEROBIC DIGESTION Partial Iist of organic compounds known to be biodegradable under Cover Story methanogenic conditions and susceptible to be present in the effluents of chemi cal and petrochemicalindustries.· Aromatic compounds homocyclic and heterocyclic
Benzene 1 Formylbenzene (benzaldehyde) (cont.) suffer, due to decreased oxygen solu methylbenzene (toluene) 4-hydroxy-3-methoxy- bility (partly compensated by higher l ,2-dimethylbenzene (o-xylene) benzaldehyde oxygen mass-transfer rates), higher Carboxybenzene (benzoate) Hydroxybenzene (phenol) heat production, and the large amount 0, m, p-amino, chloro, iodo, methyl o-aminophenol ofwater that will be evaporated. and methoxybenzoate 0, m, p-chloro, hydroxy, methoxy o-nitrobenzoate and nitrophenol Optimal conversion of organic sub mono, di, tri-hydroxybenzoate 2,4 -dichloro, 3,4-dichloro strates in anaerobic environments oc 3,5-dichlorobenzoate and 3,5-dichlorophenol curs at around neutral pH values. 3-chloro,4-hydroxybenzoate pentachlorophenol Since acidic carbon dioxide is one of the 2-acetylbenzoate (acetylsalicylate) trihydroxybenzene end products ofthe anaerobic digestion 3,4,5-trimethoxybenzoate m, p-methylphenol (m, p-cresol) 4-hydroxy-3- methoxybenzoate 2,6-dimethoxyphenol process, a basic compound needs either 4-hydroxy-3,5-dimethoxybenzoate Nitrobenzene to be present in the wastewater or it 0, m, p-dicarboxybenzene 3-nitrobenzene sulfonate needs ta be dosed in, typically in the (phthalates) phenylacetate form ofsodium hydroxide or carbonate. dimethyl o-phthalate and p-phthalate phenylpropenoate (cinnamate) In presence of sufficient neutralizing diethyl o-phthalate phenylpropionate (hydrocinnamate) capacity (alkalinity), both gaseous car di-n-butyl o-phthalate 3-methoxy-4-hydroxy cinnamate Butylbenzyl o-phthalate 4-hydroxyphenylalanine (tyrosine) bon dioxide and bicarbonate will be Formylbenzene (benzaldehyde) IBenzyl alcohol formed, resulting in a strongly buffered 4-hydroxy-3,5-dimethoxy- 4-hydroxy benzyl alcohol system with a pH around 7. Depending benzaldehyde 2-furaldehyde (furfural) on the oxidation state ofthe organic pol lutants, and the strength of the waste Aliphatic compounds water, the biogas produced will typi Hydro en cyanide Alcohols Cl to C8 n-hydroxyalkanes cally contain about 30% carbon dioxide. Acids Cl to Cl8 n-carboxyalkanes ;-butanol In order ta achieve a pH around 7 at a 4-aminobutyric l-amino-2-propanol carbon dioxide concentration in the bio glyoxalic 3-methylbutanol gas of30%, rougWy 40 equivalents ofbi 2-hydroxypropanoic (Iactic) butylene, ethylene and propylene glycol (diols) carbonate per cubic meter of waste 3-hydroxybutanoic and propanoic ;-butyric and ;-valeric di, tri and polyethylene glycol water must be present in the reactar. acrylic, B-methylacrylic (crotonic) (up to MW' 20,000) It should be noted that the actual pH and 2-propenylacrylic (sorbic) Iycerol (triol) inside the anaerobic reactor can be sig C2 to C6 n-dicarboxyalkanes Esters nificantly different trom the influent 4-aminoadipic ethyl, methyl and vinyl acetate c;s and trans-l,2-ethylenedicarboxyli butyl, ethyl and methyl acrylate pH. Wastewaters containing high con (maleic and fumaric) methyl n-, ;-butyrate and propionate centrations of biodegradable organic 2-hydroxy-l ,2,3-propanetricarboxylic Ethers acids (>100 eq. per cubic meter of (citric) ethylene glycol monomethyl ether wastewater) and a low pH (around 4), Aldehydes (2-methoxyethanol) for example, can be treated in a well Cl to C4 n-formylalkanes ethylene glycol monoethyl ether mixed reactor. Treatment is possible as 2-butenal (crotonaldehyde) (ethoxyethanol) 2-propenal (acrolein) methyl butyl ether long as at least about 40 eq. ofthe acids Amines Ketones per cubic meter ofwastewater are neu sec-butylamine acetone tralized. Assuming that ail organic methyl, dimethyl, ethyldimethyl methyl èthyl ketone and trimethylamine acids are degraded in the reactor, this " MW: molecular weight. will result in the formation of 40 eq. bi triethanolamine carbonate per cubic meter of waste water and a pH around 7. nicipal solid waste. tention time (SRT) from the HRT can The hydraulic retention time (HRT) increase the volumetric treatment ca Engineeringaspects in CSTR-type reactor is determined by pacity of an anaerobic bioreactar treat Figure 4 provides a comparison of six the specific growth rate of the slowest ing dissolved substrates. Initially, bio widely used bioreactor designs, and growing microorganism in the system. mass separation from the provides optimal values for design This generally means that very high biomass-wastewater mixture in the boundary conditions that need to be HRT values are required to achieve an CSTR-type reactor was achieved by observed during design. Briefdescrip acceptable level of degradation gravitational sedimentation in an ex tions are provided here. (roughly 25 days). The high HRT val ternal settler. During operation, set Continuously stirred tank reac ues make the CSTR concept less feasi tled biomass is fed back ta the CSTR tors (CSTR) were the first anaerobic ble for treatment of the wastestreams type reactor, increasing the volumetric bioreactors to be built, and are still considered here, containing primarily degradation rates in the system. This widely used for anaerobic digestion of dissolved organic compounds at mod approach is called the Contact wastes with a high concentration of erate concentrations (4-30 kg of COD process, and is fully comparable to the particulate matter, such as sewage per cubic meter of wastewater). aerobic activated-sludge process. sludge and the organic fraction of mu- Uncoupling the biomass or solid re- However, the maximum biomass
60 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2003 MICROBIAL ASPECTS Of ANAEROBIC DEGRADATION n environments where inorganic electron acceptors are absent, microorganisms de (Particulate) polymers FIGURE 2 (Ieft). Shown here is the pend on fermentation reactions to biode stepwise degradation of organic l + Hydrolysis materials in an anaerobic or grade organic substrates. In these methanogenic treatment environment methanogenic environments, the final prod Monomers ucts of the fermentation reactions are Acidogenesis ~ )-.Â' w - " •. - ~\ 1 ~ methane, carbon dioxide and ammonia. . ~ .• ~.J' Volatile fatty+ acids, 1 . /.- 1 . ./ ...---' '"• 1 • • From an energy perspective, the fermenta alcohols , -'.. tion of organic substrates to methane and car ~'I/:-:-: ~Acetogenesis .,;. "..-- 1 -' ;/1 ') ban dioxide yields much less energy for the • :>z.r... ..•. microorganisms thon the oxidation with inor -- . FIGURE 3. This microbial culture - a ganic e'l'ectron acceptors (Table 4). This is the Acetate Hydrogen 1 main reason why methanogenic degradation mixed, anaerobic culture enriched on an ortho-phthalate - consists of at least of organic substrates only occurs in the ab ~ethanOgeneSiS three different types of microorganisms: sence of externai electron acceptors. anaerobic, phthalate-oxidizing bacteria; Furthermore, the free energy yields are di Methane/carbon dioxide and acetate-fermenting and CO2-reduc reetly coupled to the biomass yields of the mi ing methanogenic Archaea croorganisms. Consequently, the amount of biomass formed per unit of organic substrate TABLE 4. ENVIRONMENTALLY IMPORTANT ELECTRON ACCEPTORS converted is much higher in aerobic or deni AND THEIR STANDARD REDOX POTENTIALS (EOI) AND STANDARD FREE trifying environments compared to ENERGY CHANGES ""-GO1 FOR REDUCTION * methanogenic environments. Since the rate of Electron couple Reaction EOl electron transfer per unit of biomass is basi V colly independent of the electron acceptor [6], man anese reduction the different energy yields form the basis of aerobic respiration 0.82 1 the main differences between biological iron reduction 0.77 wastewater-treatment concepts for the re denitrification 0.75 moval of organic compounds. proton reduction -0.41 Degradation of complex substrates in methanogenic environments involves a complex sulfate reduction -0.26 network of different types of microorganisms. methano enesis -0.24 Full conversion to methane and carbon dioxide * Corrected for a biologically favorable pH of7.0. results from a biological chain reaetion, where one type of microorganism generates the substrate consumed by the ited number of alcohols and volatile fatty acids. In the subsequent step subsequent organism in the chain. A schematic overview of the indi of the chain readion, acetogenic bacteria anaerobically oxidize 01 vidual steps in the degradation of polymerie substrates in cchols and fatty acids to the two main precursors of methane pro methanogenic environments is shown in Figure 2. duction; namely, acetate and molecular hydrogen. Methanogenesis Hydrolysis of polymeric substrates (such as carbohydrates, proteins through acetate fermentation, and carban dioxide reduction with and fat) to the corresponding monomers is generally assumed to be molecular hydrogen, are bath catalyzed by Archaea, which repre catalyzed by extracellular enzymes that are excreted by acidogenic sent a distinct group of microorganisms. An example of a mixed bacteria. These acidogenic baderia ferment the monomers to a lim- methanogenic culture degrading a phthalate is shown in Figure 3. :J concentration that can be achieved in water mixture ta avoid preferential tributed ta the spontaneous formation these systems is usually limited ta 4--B channeling in the sludge bed, and ex oflarge, dense biomass conglomerates kg per cubic meter ofreactar. This rela cludes the need ofmechanical mixing. in the reactor compartment. This sa tively low value for the biomass concen Biogas is collected in the three called methanogenic granular sludge tration is the result of the poor settling phase separator that is operated at a (shawn in Figure 5) plays a key role in characteristics ofmethanogenicsludge, Iow overpressure ta increase the gas the high SRT values that can be partly due ta biogas formation in the ta-liquid-exchange surface area. The achieved in this type of reactors. settler. Consequently, this technology sludge-water mixture flows ta the set Methanogenic granular sludge has a has only been applied on a very limited tler section in the top of the reactor, diameter of 0.5 ta 3.0 mm and a bio scale for treatment ofindustrial waste where biomass is allowed ta settle and mass concentration of approximately water containing dissolved organic return ta the reaction compartment. 100 kg dry matter per cubic meter. Due compounds. Nevertheless, the contact It was found in the 1970s that in ta the absence ofinertcarrier material, process remains a good option for this type of reactor, very high biomass the density of such granules is slightly wastewaters containing high concen concentrations (on the arder of 20-30 higher than water (about 1.05 kg/m3), tratians ofsuspended solids and/or fats. kg of biomass per cubic meter of reac enabling bath good mixing in the reac Upflow anaerobic sludge bed tor) could be achieved with moderate tor compartment and goad settling in (UASB) reactors [9J combine a reac strength wastewaters (about 5 kg of the internaI settler. These characteris tian compartment with an internaI set COD per cubic meter of wastewater) tics make granularsludge a perfectbio Uer and a biogas separator. Waste at Iow HRT-values (around 10 h), but mass carrier. The combinatian ofa sim water is evenly distributed in the very high SRT-values (around 50 d). ple construction and a high volumetric bottom (reaction) section of the reactor The effective uncoupling of the SRT treatment capacity has made the and flows through the sludge bed. In the and HRT in such systems resulted in VASB reactor concept the dominant sludge bed, the organic pollutants are higher treatment capacities. anaerobic bioreactor type, with more converted into biogas. The biogas pro The high biomass concentrations in than 800 currently in use worldwide. vides adequate mixing of the sludge- VASB-type reactors can largely be at- Expanded granular sludge bed
CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2003 61 r Biogas r Biogas ruluent Elfluent Effluent ;:~-://~/0~ //~'" ~$/~';~ Coyer Story /// / A""" // // //,/ ~~ .;// / // (EG8B) reaetor. A novel variation on t t t the VASB reactor concept is the Influent Influent Influent EGSB reactor. This is comparable to Reaetor type ; Upflow anaerobie Hybrid Anaerobie filter VASB reactors, except that much 1 sludge bed (UASB) (AF) higher liquid upflow velocities are ap Max. biomass 25 25 15 3 plied: typically between 5 and 20 rnIh, eone., kg/m compared to less than 1 rnIh in VASB Max upflow 1-2 reactors. To achieve this, EGSB reac veloeity, m/h tors are constructed as tall reactors Reaetor Ileight, m 6 6 6 with a limited diameter and relatively Biogas small footprint. ÂB~ The high liquid upflow velocities c c can avoid preferential channeling in Elfluent -{]'''-~ ---+-~ the sludge bed, making wastewater ~ ffi distribution in the bottom of the reac tor less critical. Consequently, treat Influent ment capacities and biomass concen Influent trations are general!y higher than Reaetor type Expanded granular Contact proeess Continuously those associated with VASB reactors. sludge bed (EGSB)I stirred tank Other bioreactor types that have fluidized-bed (FB) reaetor (CSTR) been investigated intensively use bio Max. biomass 35/20 5 3-12% of mass immobilized on either a stag eone., kg/m3 influent COD nant support material (i.e., fixed,. Max. upflow 15/30 film reactors) or a mobile support veloeity, m/h material (i.e., (luidized-bed reae Reaetor height, m 15 tors). Both of these reactor types do not have the hydraulic limitation en FIGURE 4. Shown here is a comparison of the reactors most often used for commercial-scale anaerobic treatment of industrial wastewater, and the optimal countered in VASB-reactors. How boundary conditions for each ever, the desired biomass quantity and quality control is hard to achieve substrates (8) in the wastewater is conserved and utilized for biomass using these reactor types. Further • The maximum specific growth rate production. An example calculation more, the volume occupied by the car (pmax) of the slowest-growing organ for prediction of the biomass yield is rier supports inside the reactor re ism in the system, and shown in the box titled SampIe Prob duces the effective volume that is • The maximum active biomass con lem, on p. 63. available for biomass formation; centration that can be achieved in a The second step in the procedure is hence, volumetric treatment capaci biomass-retention reactor systems the definition of the slowest-growing ties are generally lower. (al! reactor types shown in Figure 4, organism in the system. For anaerobic Hybrid reactor. A final variant except the CSTR-type reactor). bioreactors treating readily degradable comprises a combination of the VASB The biomass yield
62 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2003 SAMPLE PROBLEM: ESTIMATION OF THE BIOMASS YIELD FOR ANAEROBie BIOMASS DEGRADING ETHANOL or estimation of the biomass yield, the stoichiometry of the energy lationship belween these Iwo variables and 6.GAN was established: Fgenerating (catabolic) reaction equation needs to be derived: C2H'pH ---) 1.5CH 4 + O.5C02 (3) 'G~, 200.18 (6-CI" "'~[( 3.8- C~ '1']''i36'0'ClI151 6.GCAT = -91.6kJ'reacfion (4) For ethanol, a 6. GAN value of 706 kJ per mole of biomass formed From the stoichiometry of the chemical reaction, the standard con be calculated. free energy change under standard conditions con be calculated Now, the following equation for the biomass yield con be de from the free energy of formation values of the reactants that con rived as a function of 6. GAN, 6.G~T and the COD-equivalence be found in review papers and textbooks [5, 12]. Standard free en values for the substrate (eOO )and biomass (COOxl ergy change values (6.CO) need to be corrected for the biologically s favorable pH value of 7 (6.C0 1). Y - -6.GCAT (61 Free energy generated in the catabolic reaction is invested in the XjS- COO-- conversion of a carbon source to biomass (anabolic reaction). From 6.G AN - COO" . 6.GCAT s an extended literalure review, Heijnen [6) propased that the amount of free energy required for biomass formation (6.GAN) de Using a general molar biomass composition of CH1.800.5NO.2, pends only on the carbon chain length (e) and the COD-equiva a COOx value of 1.05 mole O 2 per mole of biomass con be cal lence of the carbon source (COD, mol O 2 per mole of carbon culated. Using this equation, the estimated biomass yield for source) for growth. The 6.GAN value was found to be independent anaerobic growth of biomass on ethanol of 0.12 C-mol/mol is of the electron donor-acceptor couple. The following empirical re- 0.032 9 biomass per 9 ethanol-COD. 0
25 FIGURE 6. This comparison shows the hydraulic retention time required as a 20 function of the COD concentration to be removed for different types of high-rate s: 15 anaerobic bioreactors. Lower Iimits of the t-=' a: / curves are set by the hydraulic operating :I: 10 Iimits. Assumptions used are Il =0.04ld, /' and Yx/s =0.06 kg/kg COD 5 /. are capable ofhandling high hydraulic o loads (EGSB, fluidized-bed designs), FIGURE 5. UASB-type anaerobic o 5 10 15 20 by recirculation of the reactor efflu 3 reactors often produce granular biomass, COD removal, kg/m ent. For an example, see Ref. [15]. such as that shown here UASB = Upflow anaerobie sludge bed In other specific cases, a plug-flow - AF = Anaerobie filler anaerobic bioreactor. A mass balance - EGSB = Expanded granular sludge bed regime is preferred over a completely for substrate over the reactor can be - FB = Fluidized bed mixed regime. A treatment system written as: that enables a plug-flow regime can ~(Sinf limits of the curves are the result of readily be achieved either by placing dS = -S)__LX R (7) dt V Yx/s the hydraulic operational limit. The two or more high-rate anaerobic biore where: figure shows that the UASB and hy actors in series, or by compartmental Q =Influent liquid flowrate, m3jd brid reactors concepts are not very at izing the anaerobic bioreactor. V = Reactor volume, m3 tractive for the treatment of strongly In general, a plug-flow regime is pre In steady state, this equation can be dilute wastewaters, due to their lim ferred in case of: (a) a wastewater that solved to obtain the hydraulic reten ited hydraulic capacity. contains a mixture of slowly and tion time (HRT, in days) required to rapidly degradable substrates, or (b) a achieve an effluent concentration S: Flowregime wastewater that contains a substrate The mixing regime in the reaction that gives rise to inhibition of the HRT =Y- =(Sinf - S). yX'-'; compartments of UASB, EGSB, hy degradation of other substrate(s). One Q tJ. X (8) R blid and fluidized-bed reactor configu particular type of wastewater that ex A boundary condition that needs to rations approaches completely mixed hibits both of these characteristics is be checked for the different types of at full scale. Consequently, spatial the wastewater generated during puri biofilm reactors is the maximum value variations in both the substrate con fied terephthalic acid (PTA) production. for the hydraulic load. Based on the centration and the biomass composi PTA wastewater consists of an acetate, typical reactor height (h) of the reac tion in the reactor compartments of benzoate, terephthalate and para-tolu tor types shown in Figure 4, the liquid these competing designs are smal!. ate mixture. The first two are readily upflow velocity (vu, mIh) can be cal cu Still, elevated concentrations of or degradable in methanogenic environ lated using the folfowing equation: ganic substrates may locally occur in ments, whereas the latter two are only reactors that are operated at low hy degraded after long lag periods and at a Vup =hjHRT (9) draulic loading rates, such as UASB considerably lower rates. It has been If the calculated liquid upflow ex and hybrid reactors. In case of toxic demonstrated that pr~-removal of ac ceeds the value shown in Figure 4, bio but biodegradable compounds (such etate and benzoate enables higher rates mass washout may impede sufficient as formaldehyde) in the wastewater, of degradation of terephthalate in the solids retention in the reactor. smail spatial-concentration variations second stage [8]. Two specific character Figure 6 presents the HRT values may give rise to progressive toxifica istics of methanogenic terephthalate required as a function of the COD con tion of the sludge bed [4]. This limita explain this observation: centration to be removed. The lower tion can be overcome in reactors that • Terephthalate degradation is
CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2003 63 EXAMPLE: REACTDR DESIGN TABLES. s an example, the bioreactor volume required to treat a EXAMPLE WASTEWATER COMPOSITION AND FLOW wastewater with the composition and How shown in Table 5 is Name Abbr. Unit Value calculated. It is assumed that a UASB-type reaelor is de A Waste~ater strength Sint kg-COD/m3 5 signed, and that a COD-removal efficiency of 95% bls = 0.95) Influent flowrate Q 3 must be achieved. The organic materiai in the influent is assumed m /d 2,000 to be fully biodegradable in methanogenic environments. The re Biomass yield yx/s 9/g-COD 0.04 quired reaelor volume can then be calculated according to: Biomass growth rate ft d-' 0.04 V = Q. T/s .Sinf' YXIJ.. = 2000· 0.95 5· 0.04 (10) steady-state biomass concentration of 25 kg/m3, as proposed for f.1XR 0.04·25 UASB-type reactors. The SRT can be calculated from the actual giving a required reaelor volume of 380 m3. biomass growth rate, according to: Assuming a typical reactor height for UASB-type reactors of 6 m, = (11) the corresponding liquid upflow velocity amounts to 1.3 m/h. SRT 1/f.1 Thus, the maximum upflow velocity exceeds the proposed maxi giving an SRT value of 25 days. Given that the hydraulic retention mum value of 1 m/h shown in Figure 4. Relatively high upflow ve time (HRT) amounts to only 4.5 hours, this means that the biomass locities may lead to limitations in biomass retention in UASB-type needs to remain in the reaelor roughly 130 times longer than the reaelors; consequently, an EGSB, f1uidized-bed or fixed-film reac liquid. Provided that the biomass is present as methanogenic tor would be preferred for this example. granuler sludge (such as that shown in Figure 5, this can readily Another point that is useful to check is the solid-retention time be achieved in UASB-type reaelors, demonstrating the potential (SRT) that needs to be achieved in the reaelor, in order to enable a of the system. 0 strongly inhibited by benzoate and ac ious chemical and petrochemical cal ofmany CPI wastewater streams etate, which both are intermediates of wastewaters. Compared to aerobic The full potential of anaerobic treat terephthalate degradation treatment, anaerobic treatment offers ment has only in the past two decades • As described above, the microorgan the following advantages: begun to be recognized. This has re ism with the lowest specifie growth • Lower biomass production per unit sulted in the current construction rate rate determines the actual SRT that of organic substrate removed of roughly four new full-scale anaero needs to be achieved in the bioreac • Lower nutrient requirements, which bic treatment plants per year. Ongo tor. Since terephthalate-degrading are specifically important for chemi ing research and engineering efforts biomass grows even more slowly cal and petrochemical wastewaters, will surely expand the reach of anaer than methanogens, they determine which tend to be nutrient-deficient obic biological treatment in the years the actual SRT required • Production of a potential energy to come. • source (methane-bearing biogas), Edited by Suzanne Shelley Insummary which can often be used onsite, as Authors Anaerobie biological wastewater opposed to energy consumption for Robbert Kleerebezem is a re searcher at the TechnicaJ UlÙver• treatment is, in many cases, a highly aeration in aerobic systems sity Delft, K1uyver Laboratory of attractive option for treatment of var- •A temperature optimum that is typi- 1 Biotechnology, Faeulty of Applied Sciences, Julianalaan 67, 2628 BC Delft, Netherlands; Phone; References 9. Lettinga, G., vanVelsen, A.F.M., Hobma. +31-15-2782425; Fax: +31-15 S.W., de Zeeuw, W., and K1apwijk, A., Use of 2782355; E-mail: r.kleerebezem 1. AFRA, "Standard Methods for the Examination the upflow sludge bJanket (USB) reactor con ®tnw.tudelft.IÙ). In his current of Water and Wastewater," 16th 00. American cept for biological wastewater treatment, es position, K1eerebezem is work Public Health Assn., Washington D.C., 1985. peciaUy for anaerobic treatment, Biolechnol. ing on fluegas desulfurization 2. Blum, D.J.W., and Speece, RE., A database of Bioeng., 22: 699-734, 1980. using inorganic biotechno!ogy. chemical toxicity to environmental bacteria 10. Macarie, H., Overview of the application of Before that, he conducted post-doctoral research and its use in interspecies comparisons and anaerobic treatment to chemical and petro at the University of Santiago de Compostella in correlations, Res. J. WPCF, 63:198-207, 1991. chemical wastewaters, Waler Sei. Tech., 42 Spain and at the Moscow State University. He (5-6): 201-213, 2000. holds an undergraduate chemical engineering 3. Edwards, E.A., and Grbié-Galié, D., Anaero from Engineering College Enschede (Enschede, bic degradation of to!uene and o-xylene by a 11. Pavlostathis, S.G., and Giraldo-Gomez, E., Ki Netherlands); and a master's degree and a doctor methanogenic consortium, Appl. Environ. netics of anaerobic treatment: A critical review, ate in environmental sciences from Wageningen Microbiol., 60:313-322, 1994. CrilicalRev. Environ. Control,21:411490,1991. University (Wageningen, Netherlands). He has 4. Gonzalez Gil, G., K1eerebezem, R., and Let· 12. Thauer, RK, Jungermann, K., and Decker, K., severa! professional publications to his name. tinga, G., Formaldehyde toxicity in anaero Energy conservation in chemotrophic anaero Hervé Macarie is a research bic systems, Waler Sci. Tech., 42 (5-6): 223 bic baeteria, Bacterio!. Rev., 41:100-179,1977. officer at the Institut de 229,2000. 13. Yan Y.G., Wong, P.C.Y., Tan, C.G., and Recherche pour le Développe 5. Hanselmann, KW, Microbial energetics ap Tang, K.F., lntegrated centralized utility ment (Laboratoire de Micro plied to waste repositories, Experienlia, services to a chemical complex on Juroog Is biologie IRD, Universités de 47:645-687,1991. land, Singapore, Waler Sci. Tech., 47 (1),15 Provence et de la Méditer 6. Heijnen, J.J. Bioenergetics of microbial 20,2002. ranée, ESIL case 925, 163 Av growth, in "Encyclopedia of Bioprocess Tech enue de Luminy, 13288 Mar 14. Zehnder, J.B., Huser, B.A., Brock, T.D., and seille cedex 09 France; Phone: nology," Flickinger, M.C. and Drew, S.W. Wuhrmann, K., Characterization of an ac (Eds.), John Wiley, New York, 1999. +33 (0)491828581; Fax: +33 etate·decarboxylating, non-hydrogen-oxidiz (0)4 91 82 85 70; E-mail; 7. K1eerebezem, R,HulshoffPol, L.W., and Let ing methane bacterium, Arch. Microbiol., [email protected]). tinga, G., Anaerobic biodegradability of ph 124;1-11, 1980. ln this position since 1994, Macarie is responsible thalic acid isomers and related compounds. 15. Zoutberg, G.R, and Been, P.d., The Biobed for !RD's llJlaerobic-treatment program. From 1995 Biodegradalion, 10;63-73, 1999. EGSB (Expanded Granular Sludge Bed) sys to 2000, he was assigned as visiting professor at the 8. Kieerebezem, R, Ivalo, M., Hulshoff Pol, tem covers shortcomings of the upf10w UlÙversidad Aut6noma Metropolitana-Iztapalapa L.W_, and Lettinga, G., High-rate treatment anaerobic sludge blanket reactor in the in Mexico City in the frame ofa Mexican-French Cf} of terephthalate in anaerobic hybrid reac chemical industry, Waler Sei. Tech., 35 operation program. He holds a biochemical engi tors, Biolechnol. Prog., 15: 347-357,1999. (10):183-188, 1997. neering degree and a Ph.D. in microbiology from the Université de Provence (Marseilles, France), and was a post-doctorate fellow at the Biotechnol· Acknowledgements Marc de Pijper (Biothane), Marc Eeckhaut (En ogy Research Institute, Canadian National Re The authors thank Connie Smith and Graham viroasia), Haruki Ikemoto (Shinko Pantee), Bo search Council (NRCC; Montreal) From June 1992 Brown (ADI), Leo Habets (Paques), Yann Hallin (Purac) and Alfredo Luna Leôn through April 1994. Macarie is a member of the In Mercier (proserpol), Philippe Conil and Gustavo (Celanese) for sending information on the anaer ternational Water Assn. (IWA), and participated in Arroyave (Biotec), Richard Moosbrugger (Entec), obic reactors designed by their companies and the managing committee ofIWA's Anaerobic Diges Gilberto Salerno (Biotecs), Dennis Korthout and for kindly answering Our questions. tion working group from 1997-2001.
64 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2003