10.1007_s10098-001-0123-x

7/16/2019 10.1007_s10098-001-0123-x

http://slidepdf.com/reader/full/101007s10098-001-0123-x 1/17

Engineering concerns and new developments in anaerobicwaste-water treatment

M. Gavrilescu

Abstract A non-comprehensive review of several devel-opments in the field of anaerobic biological waste-watertreatment engineering is carried out, considering theactive role engineers have to play in this field. The analysisis done as a result of the fact that the general performanceof anaerobic systems and the diversity of wastes that thesecan treat are increasing as a result of new reactor design,operating conditions, and use of specialized microorgan-isms. This paper illustrates a few examples of conventionaland new applications of anaerobic treatment and several

advantages, such as no energy-intensive oxygen transfer,less excess sludge, production of combustible biogas, andspace saving. Such an overview of biological waste-watertreatment also precedes comments on some importantaspects on process microbiology as well as considerationsof the application of fundamentals and kinetics to theanalysis of the biological processes used most commonly for anaerobic biological waste-water treatment.

Models developed for this reaction type are discussed,considering four overall steps. Also, models for a contin-uous-stirred tank bioreactor, a fluidized bed reactor, and apacked bed reactor as basic configurations are described.

Major anaerobic biological treatment processes used

for waste-water treatment, both suspended-growth pro-cesses and reactors (anaerobic stirred tank, anaerobiccontact process, upflow anaerobic sludge-blanket process)and attached-growth processes and reactors (anaerobicfilter process, expanded-bed process), are considered. Itwas also shown that the breakthroughs dealing withreactor design and operating conditions offer practicalsolutions to many of the drawbacks that were initially thought to limit the purpose of anaerobic processes.

Finally, the paper includes some aspects regarding thecontrol of anaerobic biological systems.

List of symbols

BC bicarbonate concentration [M L–3

]

[CO2]d concentration of dissolved CO2 [M L–3]CO2½ Ã

d equilibrium value of dissolved CO2 concentra-tion [M L–3]

CTR CO2 mass transfer rate [M L–3 T–1]C TOX toxin concentration [M L–3]c cation concentration [M L–3]D dilution rate [T–1]F influent flow rate [L3 T–1]F R circulation flow rate [L3 T–1]

G biogass flow rate [L

3

T

–1

][H +] protons concentration [M L–3]H S non-ionized substrate [M L–3][H 2] dissolved hydrogen concentration [M L–3]K 1 bicarbonate dissociation constant [–]K a acid dissociation constant [M L–3]K I substrate inhibition constant [M L–3]K H Henry constant for CO2 [M2 L–4 T–2]K H 2 K S value for H2 [M L–3]K NO2

K S value for nitrite [M L–3]K NO3

K S value for nitrate [M L–3]K La mass transfer coefficient [T–1]K S Monod constant [M L–3]

K t toxic death constant [T–1][NH 4

+] ammonium ion concentration [M L–3][NO3

–] nitrate concentration [M L–3][NO2

–] nitrite concentration [M L–3]P CO2

carbon dioxide production ratefrom biomass [M L–3 T–1]

P CO2ð ÞBcarbon dioxide production rate from bicarbonate[M L–3 T–1]

P CH 4 methane production rate [M L–3 T–1] pCO2

CO2 partial pressure [M L–1 T–2]Q volumetric flow rate [L3 T–1]r(S) specific substrate consumption rate [M L–3 T–1]

r max maximum specific substrate consumption rate[M L–3 T–1]

S substrate concentration [M L–3]Sin input substrate concentration [M L–3]S1, S2 stoichiometric parameters [M M–1]T time [T]V , V R reactor volume [L3]V A volume of absorption tank [L3]V FB volume of fluidized bed reactor [L3]V m mole volume of ideal gases [L3 M–1]vmax maximum reaction rate [M L–3 T–1]

X biomass concentration [M L–3]

Received: 1 February 2001 / Accepted: 13 September 2001Published online: 24 January 2002Ó Springer-Verlag 2002

M. GavrilescuTechnical University ‘‘Gh. Asachi’’ Iasi,Faculty of Industrial Chemistry,Department of Environmental Engineering,Mangeron Street 71, 6600 – Iasi, RomaniaE-mail: [email protected]: 4032271311

Original paper Clean Techn Environ Policy 3 (2002) 346–362

DOI 10.1007/s10098-001-0123-x

6

7/16/2019 10.1007_s10098-001-0123-x


z net cation concentration [M L–3]Y yield factor [M M–1]Y C/X yield factor, CO2/biomass [M M–1]Y m/X yield factor, methane/biomass [M M–1]Y X/S yield of biomass based on substrate consump-

tion [M M–1]

Greek symbolsl specific growth rate [T

–1

]lmax maximum specific growth rate [T–1]l*max true specific growth rate [T–1]b specific maintenance metabolism rate [–]

Subscripts0 inlet concentration

f feedi componentmax maximum

IntroductionWaste-water engineering, as a part of environmentalengineering uses the basic principles of science and tech-nology together with engineering principles to carry outthe ultimate goal – waste-water management, which in factensures the protection of the environment in a mannercommensurate with public health, economic, social, andpolitical concerns (Tchobanoglous and Burton 1991;Sincero and Sincero 1996).

Waste-water is a combination of the liquid- or water-carried wastes removed from residences, institutions,commercial, and industrial places, together with suchground-water, surface-water, and storm-water.

Methods for waste-water treatment were first developedin response to the concern for public health and theadverse conditions caused by the discharge of waste-waterto the environment. If untreated waste-water is allowed toaccumulate, the decomposition of the organic materials itcontains can lead to the production of large quantities of malodorous gases. In addition, the pathogenic or disease-causing microorganisms living in waste-water lead tohuman diseases and infections. Also the nutrients presentin industrial waste-water can stimulate the growth of aquatic plants, that is eutrophication.

Almost all waste-waters can be treated biologically, if proper analysis and environmental control are carried out.There is a growing interest to depollute high-strengthindustrial waste-waters, which more often does not pro-ceed optimally because the composition of these ef fluentsis typically time-variable and nutritionally imbalanced. Inthese circumstances it is essential that the environmentalengineer understand the characteristics of each biologicalprocess to ensure that the proper environment is producedand controlled effectively (Dunn et al. 1992; Snape et al.1995; Armenante 1998; Wiesmann and Libra 1999). Thetreatment of waste-water can be performed so that changesare brought about by means of biochemical reactionsproduced as a result of the presence of microorganisms,under aerobic or anaerobic conditions. Those processesare known as biological unit processes and they aredesigned to coagulate and remove the non-settable

colloidal solids and to transform the hazardous organicmatter as well as to stabilize it (Snape et al. 1995; Ma-coveanu et al. 1997; Gavrilescu and Macoveanu 1999,2000). Anaerobic decomposition of organic matter occursin the absence of oxygen. An anaerobic process is lookedupon as a viable alternative to the aerobic system fortreatment of medium strength wastes. The anaerobicprocess can be considered as one of the oldest technologies

for stabilizing waste-waters. It has been applied since theend of the 19th century, mainly for the treatment of household waste(water)s in septic tanks, treatment of slurries in digesters and for the treatment of sewage sludgein municipal treatment plants. The request for more cost-effective treatment systems for the growing food industry,combined with the occurrence of an international oilcrisis, was the driving force that stimulated the mostimportant research achievements of the seventies in thefield of anaerobic treatment. Particularly, the introductionof the modern ‘‘high-rate’’ reactors, in which hydraulicretention times (HRTs) are uncoupled from the solidsretention time (SRT), led to a world-wide acceptance of theanaerobic technology as a cost-effective alternative forconventional waste-water treatment systems.

A comparison of anaerobic and aerobic treatmentprocesses shows that the anaerobic process offers someadvantages (Table 1).

The purpose of this paper is to analyze several con-ventional and advanced problems involved in the principaltypes of anaerobic biological treatment processes that havebeen developed for the treatment of waste-water, regardedas biological unit processes, combining the authors’ ex-perience in the field of process engineering with that of specialists in the world. It is not our aim to give a com-prehensive review of these topics, but to give a feel for thetypes of processes involved in biological anaerobic waste-water treatment together with modelling and simulationapproaches.

Anaerobic microbiologyAnaerobic treatment converts the organic pollutants(COD, BOD) in the waste-water into a small amount of sludge and a large amount of biogas (methane, carbondioxide), while leaving some pollution unremoved(Tchobanoglous and Burton 1991; Kupferle et al. 1995;Quasim 1999). Aerobic processes, in contrast, produce alot of excess sludge, and no biogas, while also leaving somepollution, though less than after anaerobic treatment(Tchobanoglous and Burton 1991; Snape et al. 1995;

Quasim 1999).In the absence of oxygen, many different groups of

anaerobic bacteria ‘‘work’’ together to degrade complexorganic pollutants to methane and carbon dioxide. Themicrobiology is more complex and delicate than in thecase of aerobic processes, where most bacteria ‘‘work’’individually. As a result, anaerobic systems require morecontrol and monitoring systems to operate successfully.

The biological degradation of complex organic com-pounds takes place in several consecutive biochemicalsteps (chain reaction), each performed by different groupsof specialized bacteria, which have been studied in detail(Speece 1983; Tchobanoglous and Burton 1991; Dunn et al.

M. Gavrilescu: Engineering concerns and new developments in anaerobic waste-water treatment

7/16/2019 10.1007_s10098-001-0123-x


1992; Snape et al. 1995; Sincero and Sincero 1996; Quasim1999). These steps can take place simultaneously in onebioreactor (‘‘one phase systems’’), or partially separated intwo consecutive tanks (‘‘two phase systems’’). Severalintermediate products are continuously generated andimmediately processed further. Simplifying the degrada-

tion process, four major steps can be distinguished, asshown hereby (Fig. 1). In practice it is important to realizethat all steps have to occur at matching rates, in order toavoid a build-up of intermediate products. Without good‘‘teamwork’’ of all the microbial communities involved, nocomplete degradation is possible.

The consortium of anaerobic organisms that worktogether to bring about the conversion of organic sludgesand wastes (Holland et al. 1987; Wiesmann 1988;Schurbuscher and Wandrey 1991) can be grouped asfollows:

• Organisms responsible for hydrolyzing organic poly-mers and lipids to basic structural building blocks, such

as monosaccharides, amino-acids, and related com-pounds. This step is carried out by extracellular enzymesof facultative or obligate anaerobic bacteria, e.g. Clos-tridium (degrading compounds which contain celluloseand starch) and Bacillus (degrading proteins and fats).

• Anaerobic bacteria, which ferment the breakdownproducts to simple organic acids, the most common of which in an anaerobic digester is acetic acid (acidogensor acid formers). These bacteria are described as non-methanogenic and can be Clostridium spp., Peptoccocusanaerobus, Bifidobacterium spp., Desulphovibrio spp.,Corynebacterium spp., Lactobacillus, Actinomyces,Staphylococcus, and Escherichia coli.

• Organisms which converts the hydrogen and acetic acidformed by the acid formers to methane gas and carbondioxide, designated as methanogens or methane form-ers. The most important microorganisms that have beenidentified include the rods ( Methanobacterium, Methanobacillus) and spheres ( Methanococcus,

Methanosarcina).Engineered anaerobic consortia are needed to expand

the catabolic diversity of sludge and shorten the period of adaptation to recalcitrant and toxic substrates. The spe-cialized microbial consortia can be ‘‘biochemically rero-uted’’ by the induction of a desirable biochemical pathway and the repression of an undesirable pathway.

A narrow thermodynamic window must therefore bemaintained with respect to the hydrogen concentration inorder to permit methane formation but not inhibitpropionic acid degradation. This bottleneck can best beprevented by use of a microorganism population partic-ularly active with respect to hydrogen utilization. Mea-

surements of the hydrogen concentration can provideimportant information on the interaction of the varioustypes of microorganisms and may be used for processcontrol (Archer et al. 1986; Schurbuscher and Wandrey 1991).

A number of contributions report studies of the start-up and transition behaviour of anaerobic reactors with theaid of mathematical models for the essential degradationsteps of acid hydrolysis and methane formation(Kleinstreuer and Poweigha 1982; Bryers 1985; Denac et al.1988; Ozturk et al. 1989; Ryhiner et al 1993).

Most models developed for this reaction type consid-ered the four overall steps of the process (Yang and Okos

Table 1. Comparison of aerobic and anaerobic systems (Enviroasia 2001)

Aerobic treatment Anaerobic treatment

ConditionsPreferably after preclarification Also without presettlingBest for waste-waters with lower concentration Only for medium and high concentration concentrated waste-watersAlso for rather cold waste-water Only for warm waste-water (>20 °C)Toxic components often acceptable Few toxic compounds allowedNeutralization for alkaline waste-waters required Treatment of alkaline waste-waters without neutralization

ProcessOnly continuous, no long shut-downs allowed Seasonal operation possibleLow ef fluent values can be attained through

multi-stage or cascade designLow ef fluent values only with additional aerobic post-treatment additional

aerobic post-treatment (polishing)Simultaneous N and P removal possible No significant N or P removalHigh excess sludge production Very small excess sludge productionClogging danger, when using carrier material No clogging danger from sludge growthLow volumetric loading rates High volumetric loading rates possibleHigh maintenance (equipment) Low maintenance costsPossible odour problems, or high volumes

of waste air to be treatedNo odour problems and waste air in case of systems using closed tanks

ByproductsA lot of excess sludge Valuable biogasCostsRelatively low investment costs Often higher investment costs

High operational costs for: Low operational costs for: Aeration (power) Power consumptionNutrients (N, P) No or little nutrientsSludge disposal No or little excess sludgeSmall plants also feasible Small plants less economical

Clean Techn Environ Policy 3 (2002)

8

7/16/2019 10.1007_s10098-001-0123-x


1987; Denac et al. 1988; Dunn et al. 1992; Snape et al. 1995)(Fig. 1):

1. Enzyme-mediated hydrolysis of carbohydrates, pro-teins, and/or lipids, resulting in monomer compoundssuch as amino acids, sugars, and fatty acids

2. Fermentation of organic acids (bacterially assisted),through which organic acids (acetic, propionic, andbutyric) can be obtained (acidogenic step)

3. Conversion (bacterially assisted) of organic acids con-taining more than three carbon atoms to acetic acid andhydrogen (acetogenic step)

4. Conversion of acetic acid to methane and reduction of carbon dioxide with hydrogen to methane (methano-genic step)

The necessity of an interlinkage of sequential degra-dation steps in the anaerobic process by various micro-organisms means that the various steps must proceed atthe same speed in order to avoid disturbances. This ex-plains the higher sensitivity of the anaerobic processes todisturbances compared to the aerobic processes.

Anaerobic reactor technology

GeneralIn the past decades, the number of different anaerobicprocesses and the range of waste types that can or arebeing treated via anaerobic processes have been expanded,mainly due to the improvement of the classical reactorsand new reactor designs. Also, aerobic treatment, such asactivated sludge or trickling filters showed high costs(U.S.$50 per inhabitant equivalent per year), while treat-ment costs are halved when anaerobic treatment is used(Verstraete and Wandevivere 1999). The major anaerobicprocesses used for waste-water treatment are identified inTable 2. It is evident that anaerobic treatment processes

can occur in suspended and attached-growth systems, inanaerobic filters and expanded-bed bioreactors, appliedfor the treatment of carbonaceous organic wastes, as wellas for denitrification. Some performance data of thesesystems are presented in Table 3.

The more common processes and reactors now in usecan be grouped as follows (Fig. 2):

• Completely mixed anaerobic digestion• Anaerobic contact processes• Upflow packed bed• Downflow packed bed• Fluidized bed• Expanded bed• Upflow anaerobic sludge blanket (UASB)• UASB incorporating separate settler• Baf fled reactor• Two-stage leaching-bed leachate filter• Membrane solids separation• Anaerobic contact coupled with aerobic polishing

The advantages and disadvantages of the anaerobictreatment of an organic waste, as compared to aerobictreatment (Table 1), stem directly from the slow growthrate of the methanogenic bacteria. Slow growth ratesrequire a relatively long detention time in the system to

perform an adequate waste stabilization. An extensivestudy on anaerobic reactions can lead to these disadvan-tages being overcome by advanced reactor design andcontrol of feed rate (Harada et al. 1994; Grant-Allen andLiu 1998).

Because of the low rates of growth and low specific ratesof anaerobic organisms, it is important for the design of ef ficient treatment processes to achieve high biomassconcentrations and to retain the active biomass within thereactor (Tchobanoglous and Burton 1991; Snape et al.1995). Also, the range of waste types that can or is beingtreated via anaerobic treatment has been expanding due tonew reactor designs (Driesen et al. 1996; Habets et al. 1997;

Fig. 1. Schematic diagram of the patternsof the carbon cycle in anaerobic processes


7/16/2019 10.1007_s10098-001-0123-x


Weiland 1997; Grant-Allen and Liu 1998; Boulenger et al.1999; Wiesman and Libra 1999).

Operational modes available for industriesIndustries are facing tougher ef fluent limits each year andoften find it hard to know the best treatment method tomeet their needs. Anaerobic treatment can be applied to avariety of industrial waste streams. Generally, industries

that discharge streams with high organic concentrationsare prime candidates for anaerobic treatment (Weiland1997). Other characteristics of the waste stream (e.g. fat,oil, grease – FOG – and total suspended solids – TSS –content, soluble versus insoluble organic fraction) willaffect the selection of the most appropriate anaerobictechnology (Chian and De Walle 1977; Esemag 2001).Sometimes, space limitations can also dictate systemselection.

Not all industries have the same waste-water character-istics, and there is no ‘‘one technology fits all’’ available onthe market. There is, however, a range of anaerobic treat-ment technologies that will effectively meet the needs of

industries having small flows and limited space. There arealso anaerobic technologies for those industries with largerflows but still constrained by space, as well as those withlarge flow and large space to install on-site pretreatment.

Low-rate anaerobic processes

Anaerobic lagoonThe anaerobic lagoon is the oldest low rate anaerobictreatment process. Due to their limitations, anaerobiclagoons are limited to the digestion of municipal sludge(Tchobanoglous and Burton 1991; Grant-Allen and Liu1998).

Some technologies are especially applied on particularwaste streams. An example is the low-rate anaerobic pre-treatment. This technology is very successful at treatingdairy and dairy-type waste-waters that contain highquantities of FOG.

ADI bulk volume fermenter (BVF) reactorThis patented low-rate anaerobic system combines fea-tures of the upflow anaerobic sludge blanket and anaerobiccontact processes. It seems to be the leading low-rateanaerobic technology in the world (Brinkman and Schultz1994; Paques ADI 2001). This system is well suited to treatdif ficult industrial waste-waters including pharmaceutical,brewery, distillery, dairy, potato, and other food process-ing wastes. It can also be designed to handle streams thatare very high in TSS and FOG, such as DAF floats andsludges (Paques ADI 2001).

One industry that has benefited from the patented low-rate anaerobic ADI-BVF technology offered by Paques ADIis a candy bar manufacturing facility in Canada (PaquesADI 2001). This plant has a design average flow of 1135 m3/day, average BOD of 1 600 mg/l, average TSS of 410 mg/l, and FOG of 200 mg/l. The on-site waste-watertreatment plant now allows the manufacturing facility tomeet ef fluent limits of 300 mg/l BOD and 350 mg/l TSS. Inaddition to the low-rate anaerobic part, the treatmentpackage comprises an insulated equalization tank ahead of the anaerobic reactor, feed pumps and in-line grinder, amagnesium hydroxide feed system for pH-alkalinity adjustment, and a control building. The ef fluent dischargesinto a street sewer and flows for a short distance to thepublicly owned treatment works (POTW). The entireinstallation is controlled through a PLC system, which canbe accessed to change operating variables via theoperator’s PC workstation in the control building.

Table 2. Major anaerobic biological treatment processes and reactors used for waste-water treatment

Type Use

Suspended growthAnaerobic digestion– Standard rate, single stage Stabilization, carbonaceous BOD removal– High-rate, single stage Stabilization, carbonaceous BOD removal– Two stage Stabilization, carbonaceous BOD removalAnaerobic contact process Carbonaceous BOD removal

Upflow anaerobic sludge-blanket (UASB) Carbonaceous BOD removal Attached growth

Anaerobic filter process Carbonaceous BOD removal, waste stabilization (denitrification)Expanded bed process Carbonaceous BOD removal, waste stabilization (denitrification)Anaerobic rotating biological contactor (RBC) Carbonaceous BOD removal

Table 3. Typical process and performances for anaerobic processes used for waste-water treatment

Process Influent COD (mg/l) Hydraulic detentiontime (h)

Organic loading(kg COD/m3 day)

COD removal (%)

Anaerobic contact process 1,500–5,000 2–10 0.480–2.403 75–90Upflow anaerobic sludge

blanket (UASB)5,000–15,000 4–12 4.005–12.014 75–85

Fixed bed 10,000–20,000 24–28 0.961–4.806 75–85Expanded bed 5,000–10,000 5–10 4.806–9.611 80–85


0

7/16/2019 10.1007_s10098-001-0123-x


Another successful installation is in Virginia, U.S.A.(Paques ADI 2001). The treatment facility comprises ascreen, a raw waste pumping station, equalization tank,calamity tank, an ADI-BVF reactor, and a proprietary ADI-SBR (sequencing batch reactor). This raw waste-water was

a challenge as the incoming pH ranged from 2 to 12,average TSS was 1000 ppm, and peak day FOG concen-trations reached 1200 ppm. Nevertheless, a high-quality ef fluent is being produced with a BOD of 30 mg/l, SS of 30 mg/l, and FOG of 20 mg/l. The ef fluent discharges into

Fig. 2. Typical processes inuse for anaerobic wastetreatment


7/16/2019 10.1007_s10098-001-0123-x


a sewer and flows by gravity 3 miles (4.8 km) to thePOTW. These low-rate systems can be designed asin-ground basin reactors or as above-ground tanks,depending on site restrictions.

High-rate anaerobicThe sensitivity and the low growth rates of the anaerobicbacteria were seen as an important risk factor of anaerobicsystems. Important factors for high rate anaerobic pro-cesses are solids retention, sludge activity, temperature,and reactor design.

Anaerobic contact processOne of the simplest ways to retain biomass is to settle thereactor ef fluent and recycle the sludge, as in the aerobicactivated sludge process (Gavrilescu and Macoveanu1999). The equivalent anaerobic system is referred as theanaerobic contact process, developed in the 1950s, and wasthe first high rate anaerobic treatment system. This systemis suitable for treating ef fluents containing a high con-centration of suspended solids (Grant-Allen and Liu 1998;Habets 1999).

In the anaerobic contact process (Fig. 2), untreatedwastes are mixed with recycled sludge solids and thenprocessed in a reactor sealed off from the entry of air(Speece 1983; Tchobanoglous and Burton 1991; Snape etal. 1995). After digestion, the mixture is separated in aclarifier and the supernatant is discharged as ef fluent andsent for further treatment. The settled anaerobic sludge isrecycled to seed the incoming waste-water. Because of thelow synthesis rate of anaerobic microorganisms, the excesssludge that must be disposed of is minimal. Typical pro-

cess loading and performance data are reported in Table 3.Its ef ficiency is limited by the dif ficulty in achieving sludgeconcentrations in the sedimentation tank, owing to thenature of the anaerobic sludge.

Biobulk is a conventional anaerobic contact process(Biothane Corporation 2001) that is applicable for waste-streams containing high strength COD/BOD concentra-tions and FOG concentrations greater than 150 mg/l(Fig. 3). Biobulk is a ‘‘medium loaded’’ system withvolumetric loadings of 2–5 kg/COD m3 day.

The Biobulk Continuously Stirred Tank Reactor (CTSR)has a specially designed mixing capability to ensure thatthe waste-water is in constant contact with the biomass

(Biothane Corporation 2001). CTSRs are cost effectivewhen flows are low, i.e. 75 m3/day or less. With these low flows, longer hydraulic retention times are possible al-lowing for degradation of the solids. The Biobulk processis an excellent treatment choice for ice cream plants andother food processing facilities which discharge ef fluentshigh in biodegradable fats and oils. Removal ef ficiencieswith this technology have been found to consistently av-erage above 90% with respect to COD, FOG, and BOD andclose to 75% with respect to the organic fraction of TSS(Biothane Corporation 2001).

Upflow anaerobic sludge-blanket processThe treatment of waste-water with a high degree of oper-ational reliability is now in demand all over the world. Atthe same time, preconditions are becoming more impor-tant. These apply to the limitation of sludge yield, energy consumption and space need. Through fundamental andapplied research, the following hypothesis was confirmed:as a result of carefully controlled processing conditions,anaerobic bacteria can effectively remove organic com-pounds from industrial waste-water. In addition, thephenomenon of ‘‘granule formation’’, which has sincebecome an essential part of the process, was also formu-lated. This granule formation, the result of the aggregationof anaerobic bacteria, makes possible a high degree of sludge retention, particularly useful in the treatment of industrial waste-water.

In these circumstances, the UASB reactor was inventedin the mid-1970s at the University of Wageningen(Lettinga et al. 1979) and was applied at full scale in theDutch sugar industry. Over several years, it was continu-

ally improved in order to treat high amounts of waste-water flow and load with important daily fluctuations.Some operating data and recommendations resulted fromthe experience, which enables the reactor to handle thesevery different situations.

The UASB reactor is a high-rate system that operatesentirely as a suspended growth system. The high biomassconcentration renders UASB more tolerable of toxicants. Itis essentially an open column through which liquid wasteis passed with a very low upflow velocity. The sludge-blanket is composed of biological formed granules orparticles and the waste-water, introduced in the bottom of the reactor, flows upwards through them (Fig. 4)

Fig. 3. Schematic diagram of the Biobulkprocess


2

7/16/2019 10.1007_s10098-001-0123-x


(Biothane Corporation 2001). The granular sludge innoc-ula necessary for the startup of a UASB are already com-mercially available throughout the world. Treatmentoccurs at the contact with the granules. The gases pro-duced under anaerobic conditions cause internal circula-tion, which helps in the formation and maintenance of thebiological particles, on which some gas adheres. The freegas and the gas released from the granules are retained inthe gas collection zone at the reactor top. Liquid con-taining some residual solids and biological granules passesinto the settling device where the residual solids are sep-arated from the liquid. The solids fall back through thebaf fle system to the top of the sludge blanket. Properoperation requires the formation of a flocculating biomass,and startup may require inoculation of large amounts of sludge from another operating system. Some of the UASBprocess loading and performances are reported in Table 3.The UASB concept can be applied to a wide variety of industrial waste-waters. For example, the UASB Biothaneprocess (Biothane Corporation 2001) is normally designedto operate at high COD loadings (10–15 kg COD per cubicmeter of reactor volume per day), and it is very spaceef ficient. The high loading translates into a remarkably short hydraulic retention time (less than 48 h for mostapplications). As with other biological waste-water treat-ment technologies, certain pretreatment steps (i.e., flow equalization, TSS, and oil and grease removal, pH cor-rection, etc.) are required prior to the waste-water enteringthe UASB reactor. With such pretreatment in place, theUASB can effectively convert 70–95% of the biodegradableorganic matter in a given waste stream to recoverablebiogas. Hence, higher ef ficiency is made possible by adequate pre- or post-treatments and by various types of additives or co-substrates that improve sludge retention,composition, and resistance toward toxicants (Archer et al.1986; Driessen et al. 1996; Habets et al. 1997).

The high-rate UASB technology, which relies on thegrowth of granular sludge and a three phase separator(biogas–liquids–solids), has been a commercial successwith over 300 installations from various suppliers

operating throughout the world (Zoutberg and Been 1997).The food industry all over the world has become an activeuser of this anaerobic treatment technology. In the foot-steps of the sugar processing industry, breweries, andpotato processing plants have set the trend. The paper andcellulose industry also has been profiting for years fromthe advantages of the IHI-UASB systems (GEC 2001). Forthe past few years, IHI-UASB installations have also beenused in the textile, chemical, and pharmaceutical indus-tries.

However, UASB reactors have frequently been replacedby taller and more slender column reactors to providebetter circumstances for mixing throughout the reactorvolume and saving in space. Although the trend to usehigh column anaerobic reactors was initially startedmainly for cost reasons, these reactors are now more andmore considered technological improvements.

Anaerobic filterThe anaerobic filter, also known as fixed bed or fixed film,is suitable for the treatment of only ef fluent containing low concentrations of suspended solids (Lettinga et al.1979;Speece 1983; Kindzierski et al. 1995; Jordening andBuchholz 1999). It is similar to the trickle filter in that amicrobial film grows of an inert solid support. Thiscontactor is a column filled with various solid media(Fig. 2). The waste flows upwards through the column,contacting the media on which anaerobic bacteria grow.The mean cell-residence times are of 100 days. In thisbioreactor, a large value of the detention time can beachieved with short hydraulic retention times. In theseconditions the anaerobic filter can be used for the treat-ment of low-strength wastes at ambient temperature. Itsutilization is summarized in Table 3. The advantage of thissystem is that it produces high concentrations of activebiomass without the use of a settler (Lim and Lee 1991).The disadvantage is that plugging may develop with highload conditions or if suspended solids are present. Modi-fications of this reactor have given rise to many otherreactor configurations (Grant-Allen and Liu 1998).

Fig. 4. UASB system


7/16/2019 10.1007_s10098-001-0123-x


ADI-Hybrid reactorA technology proven to be particularly suitable for treatingchemical and chemical-type waste-waters is the high-rateanaerobic ADI-Hybrid. This technology combines featuresof the upflow anaerobic sludge bed and upflow anaerobicfilter processes and has been successfully employed atseveral complex chemical plants around the world. Onesuccessful installation is at a purified terephthalic acid

(PTA) plant in Taiwan (Paques ADI 2001). In this case, thepretreatment system consists of two ADI-Hybrid reactors,one being 5,000 m3 in volume and the other, 4,000 m3.They are designed to operate in series and give excellentperformance in treating this complex waste-water. Pre-liminary treatment includes screening, equalization, andpH adjustment for PTA solids dissolution prior to treat-ment in the anaerobic reactors. The pretreatment facility isdesigned to handle 7,800 m3/day of this dif ficult-to-treatwaste-water at a design COD concentration of 6,500 mg/l(50,500 kg COD per day), including 8,400 kg/day of te-rephthalic acid solids. The hybrid reactors remove themajority of the biodegradable organics in the waste-water

and convert them into biogas. The biogas is captured andutilized in process boilers at the site (Paques ADI 2001).The pretreated waste-water discharges to an existing

deep-shaft activated sludge treatment facility for final pol-ishing. Hybrid reactors are suitable where land is limited.

ADI-AMR modular reactorThe proprietary ADI-AMR is a packaged anaerobic systemfor high strength waste-water. It combines both suspendedand fixed-film processes in an upflow hybrid reactor. It isideally suited for flows of warm, high strength/solublewaste streams under 750 m3/day.

Fluidized-bed systemThe reactor is a column with fluidized suspended particleswhich serve as a support for biomass immobilization(Fig. 2). The influent flows upward through a medium,usually sand, at a velocity suf ficient to keep the medium insuspension. The process usually requires recycling of theef fluent from the reactor to keep the medium fluidized(Snape et al. 1995; Jordening and Buchholz 1999).

Some of the fluidized bed process loading and perfor-mances are reported in Table 3. Operating costs areelevated since recycling of ef fluent inside reactorconsumes a large amount of power.

Ultra-high-rate anaerobic

Internal circulation reactor (IC)Specially the UASB process has proven to be the system of choice among numerous systems available. To overcomethe limitations of this process, an ultra-high-rate type of anaerobic reactor, called the internal circulation (IC)reactor, was developed. The IC process is based on theUASB concept of biomass granulation and use of threephase separators. Its special features is the separation of produced biogas in two stages within a reactor with a largeheight/diameter ratio. The gas collected in the first stage isused to generate a gas lift and an internal liquid circula-tion, which gives the reactor its name. The two-stage

design of the IC reactor allows up to three times highervolumetric loading rates and less than half the hydraulicretention times when compared to conventional UASBreactors (GEC 2001). The accompanying figure (Fig. 5)shows a schematic diagram of the IC reactor.

Using the IC reactor, the anaerobic treatment of low concentration waste-water at low temperature is alsoeffective. The influent is pumped into the bottom of the

reactor using a distribution system and mixes with gran-ular sludge. The first, high loaded reactor compartmentcontains an expanded granular bed. Here, most of theCOD is converted into biogas, which is then collected by the lower level phase separator. The collected biogasgenerates a gas lift, which forces the water up, through theriser, into the gas/liquid separator in top of the reactor.Through this separator the biogas leaves the reactor, whilethe water/sludge mixture returns through the downcomerback to the bottom of the system. The ef fluent from thefirst, lower reactor compartment is post-treated in thesecond, upper compartment. The biogas formed here iscollected by the upper level three phase separator and thefinal ef fluent leaves the reactor at the top.

Four basic process units that make up the specialcharacteristics of the IC reactor include the mixing section,the expanded bed section, the polishing section, and the

Fig. 5. IC reactor


4

7/16/2019 10.1007_s10098-001-0123-x


recirculation system. In the bottom of the reactor, in-coming waste-water is effectively mixed with the sludgeand ef fluent from the internal circulation stream. Thisresults in a direct dilution and conditioning of the influent.The distribution system ensures an even distribution intothe expanded bed section by use of special hoods. Theexpanded bed section contains a highly concentrated, ex-panded granular sludge bed. Bed expansion/fluidization is

created by the elevated upward flow of influent, recircu-lation, and waste-water, and biomass results in a highsludge activity, making high organic loading rates andconversion rates possible. Comparison tests showed thatsludge granules from IC reactors have about two timeshigher methanogenic activities than UASB granules. Thehighly mixed and active biomass in this section makes theIC reactor suitable for treatment of moderate and high-strength waste-waters. In the polishing section, an effectivepost-treatment and biomass retention takes place due to alow sludge loading rate, a relatively long hydraulic reten-tion time and a plug flow like regime. As a result of thealmost complete removal of biodegradable COD in thebottom expanded bed section and the gas collection by thefirst separator, the turbulence produced by biogas in thepolishing section is low. Also, the superficial liquid ve-locity in the polishing section is relatively low, since theinternal circulation flow does not pass through this reactorsection. Both factors provide optimal biomass solids re-tention comparable to UASB reactor conditions despite ahigh overall reactor loading rate. Because sludge concen-tration in this second reactor compartment is low, space isavailable for extra expansion of the expanded sludge bed.This avoids sludge loss during high peak loads. The in-ternal circulation is based on the gas lift principle, createdby the difference in gas hold-up between the ‘‘riser’’ andthe ‘‘downcomer’’ (no pump is needed). Driven by thereactor gas flow, the recirculation flow rate depends on theinfluent COD and is therefore self-regulating: higher in-fluent CODÞhigher gas flow Þmore circulationÞmoreinfluent dilution. The recirculation flow under these cir-cumstances is approximately 2.5 times the gas flow. Thereactor features a two-stage separation/collection of biogaswithin a tall cylindrical vessel and uses the gas lift prin-ciple to induce internal circulation of treated ef fluent.

The unique system of the self-regulating internal cir-culation offers considerable advantages in the operation of the system. For example, no extra pumps are required forrecirculation; the influent COD automatically determinesthe amount of circulation liquid.

In production processes in which a chance of suddentoxicity occurs, an IC reactor offers the advantage of directautomatic dilution. The same is true for strong fluctua-tions in pH: the IC reactor adds the alkaline anaerobicef fluent to the influent, which decreases the customary neutralization costs.

Other advantages of the UASB system are its very small footprint, making it especially suitable for a factory located in an urban area having no space for expansion,low chemical needs, stable operation, suitability forwaste-water which contains a COD of more than1000 mg/l. Countermeasures are necessary for odoremission.

More than 70 IC plants are being operated successfully all over the world. Brewery and food processing industriesare the major users (Driessen et al. 1999; GEC 2001).

BIOPAQ internal circulation (IC) reactorA very popular technology is the ultra-high-rate anaerobicBIOPAQ-IC (internal circulation) reactor (Habets 1999;Paques ADI 2001). This system has become, for some, the

preferred technology over the standard UASB.This technology is particularly appealing to pulp and

paper industries and breweries (Grant-Allen and Liu 1998;Gorur et al. 1999; Habets 1999; Paques ADI 2001). Onesuch reactor is successfully installed at a kraft mill inAlabama where it is treating foul condensates, originatefrom the pulp digesters and evaporators and containmainly methanol, reduced sulfur compounds, and someterpenes. In this case, the kraft mill was faced with havingto remove methanol from the condensates to comply withthe MACT I portion of the Cluster Rules. This 6.5-mdiameter reactor (water depth of 20 m) typically removesmore than 98% of the methanol at a hydraulic retention

time of 6–7 h. The granular biomass inside the IC reactorconverts methanol into methane and carbon dioxide; thisbiogas is separated inside the reactor and recovered. Thebiogas contains approximately 85% methane and is usedin the mill’s combo boiler. The COD, which is typically between 4,500 and 5,000 mg/l, is reduced by 75–80%. Theanaerobically treated condensates are discharged to themill’s aerated lagoons. The tall cylindrical design of thisreactor makes it very suitable for applications where landis at a premium.

As stated, one technology does not fit all waste-watertreatment needs, and industries should investigate all theiroptions before committing to a major capital investment.

Every situation is unique, and a pilot study should beconsidered whenever there is a question as to the appli-cability of a technology or the treatability of the waste.On-site pretreatment with an anaerobic system that canproduce biogas for reuse in the plant as a means of offsetting energy requirements should be given seriousconsideration.

Compact BIOPAQ IC reactorThis is a small version of the BIOPAQ IC reactor. Itsmodular design allows it to be completely shop fabricatedand shipped in two pieces (Paques ADI 2001).

Expanded-bed processThe waste-water to be treated in expanded-bed process(Fig. 2) is pumped upward through a bed of an appro-priate solid support made of sand, coal, or expandedaggregates, on which the biological growth occurs. Thisexpansion has the advantage that channeling is reduced,liquid mixing is improved, and the blockage by the solidsis eliminated. Ef fluent is recycled to dilute the incomingwaste and to provide an adequate flow to maintain thebed in an expanded condition. As the quantity of sludgeproduced in the expanded-bed process is considerably less than that produced in aerobic systems, it is in agreater use for the treatment of municipal waste-water.


7/16/2019 10.1007_s10098-001-0123-x


The usable gases recovered from this process, such asmethane, are another important advantage of this an-aerobic process.

As an out growth of successful operating experiencewith UASB technology, an Expanded Granular Sludge Bed(EGSB) process that incorporates the best features of twotechnologies: growth of granular sludge and three-phasefluidized bed have been combined resulting in expanded

granular sludge bed (EGSB) (Versprille et al. 1994; Zout-berg and Been 1997) (Fig. 6). The ultrahigh loading ratesof the fluidized bed process are achieved with granularbiomass, while no carrier material is required for start upor operation. This process can therefore be perceived ei-ther as an ultrahigh rate UASB or a modified conventionalfluidized bed. The process is especially suitable to treatwaste-water that contains compounds that are toxic inhigh concentration and that can only be degraded in low concentrations (such as those in chemical industry). It isalso possible to operate the reactor as an ultrahigh loadedreactor (to 30 kg COD/m3 day) for applications in othersectors of industry.

The Biobed EGSB process offers several inherent ad-vantages:

• Simplicity – the Biobed EGSB process (althoughextremely effective), consists of only two major com-ponents within the reactor: those being the settlers inthe top of the tank, and the feed distribution in thebottom of the tank.

• Flexibility – the influent feed distribution system in thebottom of the reactor has several feed inlet points toensure the maximum waste-water to sludge contact: thisensures that channeling or ‘‘rat holing’’ of the sludge willnot occur.

• The settler in the top of the reactor has been designed toaccept high liquid and gas velocities, without the needfor external clarification.

• Volumetric loading of 15–30 kg COD/m3 reactor volumeis routinely achieved with industrial waste-water. Evenhigher loading rates are possible depending on thecharacteristics of the waste-water. The high volumetricloadings lead to a small footprint reactor, with a slender

and vertical construction.• High recirculation ratios create inherent hydraulicbalancing capacity, and allow for treatment of waste-waters containing inhibitory, but biodegradablesubstances.

More than 20 full-scale Biobed EGSB reactors are inoperation world-wide. These reactors can be exploited atextremely high conversion rates.

Modelling for anaerobic process design and control

Survey on anaerobic treatment modelsDuring years, different authors have presented models foranaerobic treatment.

Ryhiner et al. (1993) presented an interesting model forthe batch anaerobic degradation of whey during threesteps. The stoichiometry of this process is represented inTable 4.

Molasses consists of a variety of sugars, mainly sucrose,which can be converted anaerobically to methane by asequence of four reaction steps comprising hydrolysis,acidogenesis, acetogenesis, and methanogenesis. Followingthe hydrolysis of complex sugars to simple sugars, thefurther degradation is known to proceed via rapidly growing and pH insensitive bacteria to form organic acids(butyric, propionic, and acetic acids) carbon dioxide, andhydrogen. In the next step, bacteria further oxidize thehigher acids to acetic acid, carbon dioxide, and hydrogen.Methanogenesis involves the reduction of carbon dioxidewith hydrogen to methane by autotrophic bacteria. Themodel development starts by considering glucose to be thestarting reactant (Denac et al. 1988).

Anaerobic process models can also be developed fordenitrification processes. Kurt et al. (1987) modelled thedenitrification of drinking water using an autotrophicorganism ( Micrococcus denitri ficants). The model allowsthe possibility of monitoring the dissolved hydrogenconcentration, which is directly related to the reactionrate.

Bhadra et al. (1984) proposed various models andcompared their predictions for acetate degradation withthe measured values (acetic acid, dissolved carbon dioxide,bicarbonate, and carbonate ions).

Droste and Kennedy (1988) applied a model for acidand methane production to a solid film reactor.

Simeonov (1999) considered some problems of themathematical modelling and computer simulation of theanaerobic (methane) fermentation of animal waste instirred tank bioreactors. The models developed allow theestimation of dynamic model parameters and can be usedfor process kinetic investigations, process optimization,and optimal control.Fig. 6. Schematic diagram of the EGSB system


6

7/16/2019 10.1007_s10098-001-0123-x


Converti and coworkers (1994, 1999) have modelled theanaerobic digestion of the various components of thevegetable fraction of municipal refuses (lignin, cellulose,and hemicellulose) as well as of starch, based on batchexperimental tests carried out under both mesophilic andthermophilic conditions and in an immobilized-cell reac-tor, respectively.

Benitez et al. (1997) investigated the purification of

olive mill waste-waters by a single anaerobic digestion in abatch reactor containing immobilized microorganisms, by combining an ozonation pretreatment followed by theanaerobic digestion. A kinetic model is developed by usingthe Monod model combined with the Levenspiel model,which allows the determination of the kinetic parametersof this anaerobic treatment.

Various models have also been developed for describ-ing anaerobic toxicant degradation (Andrews 1968;Edwards 1970; Lewandowski 1985; Gaudy et al. 1988;Brown et al. 1990). Sathish et al. (1993) developed aprotocol for measuring the kinetic parameters needed tomodel the rate of biodegradation of toxic organicchemicals in anaerobic waste-water treatment processes.

Owing to the low growth rates and low specific rates of anaerobic organisms, it is important for the design of anef ficient treatment process to achieve high biomass con-centrations and to retain the active biomass within thereactor.

Key factors for a successful design of an anaerobicreactor are: high sludge activity, effective sludge reten-tion, and right material quality (Driessen et al. 1996).Sludge activity is affected by the contact of the anaerobicsludge with the waste-water. The reactor capacity isdetermined by the amount of active biomass inside thereactor. Effective sludge retention ensures high

concentrations of active biomass resulting in a high re-actor capacity.

In the following sections they were presented severalanaerobic process models.

Modelling the standard continuous-stirred tank bioreactorIn this system (Fig. 3), a mixed population of bacteriastabilizes suspended organic matter in the absence of ox-

ygen.The process is slower than an activated sludge processand therefore requires larger reactor volumes. The mostcommon reactor configuration is the continuous tank re-actor, sometimes with recycling or two stages (Negulescu1985; Ognean and Vaicum 1987; Tchobanoglous andBurton 1991; Snape et al. 1995). The reactor model derivesfrom Andrew ’s model (Fig. 7).

Biological kineticsThe biomass increases due to growth, and is flushed outduring continuous operation. This process can be mathe-matically described as:

dX dt

¼ l Sð Þ À D½ X ð1Þ

The substrate is fed into the reactor convectively. Theresidual substrate flows out of the reactor with the sameflow rate. Part of the substrate is converted by the mi-croorganisms as follows:

dS

dt ¼ Sin À Sð ÞD À r Sð Þ X ð2Þ

Gases are produced during degradation of the substrate:

G ¼ 2V mV Rr Sð Þ X ð3Þ

Table 4. Stoichiometry of the anaerobic degradation of whey (Ryhner et al. 1993)

Step 1: Hydrolysis and acidification of whey:C6H11.1O5.12 (whey)+0.57H2Ofi0.75CH3(CH2)2COOH+0.24CH3(CH2)2COOH+0.5CH3COOH+1.38CO2+1.44H2

Step 2:Organic acid degradation to produce acetic acid:CH3(CH2)2COO–+2H2O,2CH3COO–+2H2+H+

DG’0=71.7 kJCH3CH2COO–+2H2O,CH3COO–+3H2+CO2 DG’0=48.3 kJ

Step 3:Methane production from acetic acid, hydrogen, and carbon dioxide:CH3COOHfiCH4+CO2

Fig. 7. Major processes in Andrews’ model of anaer-obic digestion


7/16/2019 10.1007_s10098-001-0123-x


The following parameters can be calculated:

l ¼ D ð4Þ

r Sð Þ ¼Sin À Sð Þ

X D ð5Þ

Y X =S ¼ X Sin À S ð6Þ

Measurable biomass growth and the yield of biomassbased on substrate consumption can accordingly representthe specific substrate consumption rate (Eq. 7):

r Sð Þ ¼l Sð Þ

Y X =S

ð7Þ

Coupling the substrate consumption rate to the substrateconcentration S by means of the Monod kinetics (Eq. 8):

r Sð Þ ¼ r max

S

K S þ S ð8Þ

the classical form of evaluating such kinetics is repre-sented by methods that convert the non-linear kineticsinto a graphically easily interpretable straight line by means of linear transformations. The best-knownapproach is the double reciprocal plot (Eq. 9):

1

r Sð Þ¼

K Sr max

1

Sþ

1

r maxð9Þ

If the maintenance metabolism is taken into consider-ation, the substrate consumption rate results from thegrowth of biomass and its maintenance metabolism(Eq. 10):

r Sð Þ ¼l Sð Þ

Y X =S¼

l Sð Þ

Y X =Smax

þ m ð10Þ

with m as the maintenance metabolism coef ficient. Non-linear optimization of Eq. (10) results in the parametersY X =Smax

and m. The growth rate can be calculated accordingto:

l ¼ r maxY X =Smax

S

S þ K SÀ mY X =Smax

ð11Þ

or more briefly according to:

l Sð Þ ¼ lÃmax

S

S þ K SÀ b ð12Þ

The measurable growth represents the growth convertedinto biomass; the concept of true growth is a fictive growthincluding the maintenance metabolism (Schurbuscher andWandrey 1991).

In the Andrews model (Fig. 7) the growth of biomass isalso assumed to follow Monod kinetics with inhibitioncaused by high substrate concentration and by the pres-ence of toxin (Andrews 1989). The rate limiting substanceis assumed to be non-ionized substrate:

l ¼lmax

1 þ K SH S

þ H SK i

ð13Þ

A mass balance on the biomass in the reactor gives thefollowing equation:

dX

dt ¼ D X 0 À X ð Þ þ l X À K t C TOX ð14Þ

where D is the dilution rate:

D ¼F

V ð15Þ

A mass balance for substrate gives the following relation-ship:

dS

dt ¼ D S0 À Sð Þ À

l X

Y X =S

ð16Þ

Gas production ratesThe rates for carbon dioxide and methane are related to

the biomass growth rate by constant yield factor:

P CO2¼ Y C = X X l ð17Þ

P CH 4 ¼ Y m= X X l ð18Þ

Liquid phase balanceThe liquid phase can contain the substrate in both anionized (S) and non-ionized (HS) form. The concentrationof non-ionized substrate results from the hydrogen ionconcentration and the dissociation constant, K a:

H S ¼S H þ½

K að19Þ

with:

pH ¼ À log H þ½ ð20Þ

and:

H þ½ ¼K 1 CO2½ d

BC ð21Þ

The carbon dioxide resulting from a growth process can

exist as a dissolved gas (bicarbonate ions), as well as in thegaseous form. The rate of CO2 transfer between the gas-eous and the dissolved forms can be modelled by using themass transfer coef ficient, K La:

CTR ¼ K La CO2½ Ãd À CO2½

À Áð22Þ

where:

CO2½ Ãd ¼ K H pCO2

ð23Þ

The bicarbonate concentration is needed to calculatethe hydrogen ion concentration, which can be found by


8

7/16/2019 10.1007_s10098-001-0123-x


comparing the positive and negative ions in solution andbalancing for electroneutrality (Snape et al. 1995).

A mass balance for the ammonium ions, assumingnegligible microbial utilization is in the following form:

d NH þ4½

dt ¼ D NH þ4

Â Ã0À NH þ4

Â Ã ð24Þ

If the simplified ion balance is written as:

z ¼ S þ BC ð25Þ

with the difference between the cation and anion con-centrations z expressed as:

z ¼ c À a ð26Þ

then differentiating the simplified ion balance gives:

dBC

dt ¼

d NH þ4½

dt À

dS

dt ð27Þ

The mass balance on the bicarbonate ions in the reactorresults in the following expression:

P CO2ð ÞB¼ D BC 0 À BC ð Þ þ

dS

dt À

d NH þ4½

dt ð28Þ

The overall mass balance for CO2 dissolved in the liquidphase becomes:

d CO2½ d

dt ¼ D CO2½ d 0À CO2½ d

À Áþ P CO2

þ P CO2ð ÞBþ CTR

ð29Þ

Also, the mass balance for the non-consumed toxicsubstance in the reactor gives:

dC TOX

dt ¼ D C TOX 0 À C TOX ð Þ ð30Þ

Gas phase balanceUnder the hypothesis that all the methane produced by reaction enters the gas phase, the volumetric gas flow rateswill be:

QCH 4 ¼VP CH 4

qð31Þ

QCO2¼ À

V CTR

qð32Þ

In Eq. (32) the negative sign designates a reversal in thetransfer direction, CO2 flowing from the liquid into the gas

phase.The percentage of carbon dioxide in the exit gas is

frequently used as a parameter for judging the state of theanaerobic digestion and can be calculated from the CO2

and total gas flow rates:

CO2 %ð Þ ¼QCO2

Qtot 100 ð33Þ

where:

Qtot ¼ QCH 4 þ QCO2ð34Þ

All these equations form the mathematical model that

represents the basis for design and simulation purposes.

Fluidized bed reactorThis system (Fig. 8) can contain a conical tapered fluidizedbed reactor containing sand particles. For example,anaerobic denitrification is one of the processes than canbe carried out in this system according to the followingreactions:

2NOÀ3 þ 2H 2 ! 2NOÀ

2 þ 2H 2O2NOÀ

2 þ 3H 2 ! N 2 þ 2H 2Oð35Þ

Hydrogen absorption occurs in a separate tank, and thefluidized bed reactor and the absorption tank are linked by a pumped recycle loop, which supplies the hydrogen to thebed (Fig. 8). This allows the monitoring of the differencesin dissolved hydrogen concentration across the bed, whichis directly related to the reaction rate. In addition therecycle loop make it easy to adjust the pH and temperatureof the liquid recycle and hence the reactor.

The kinetics for each reaction step accounts for adouble Michaelis–Menten relationship with hydrogen andnitrite or nitrate as limiting substrate respectively:

Fig. 8. Schematic diagram for the anaerobic fluidizedbed reactor


7/16/2019 10.1007_s10098-001-0123-x


r 1 ¼ vmax 1

NOÀ3

Â ÃNOÀ

3½ þ K NO3

H 2½

H 2½ þ K H 2

ð36Þ

r 2 ¼ vmax 2

NOÀ2

Â ÃNOÀ

2½ þ K NO2

H 2½

H 2½ þ K H 2

ð37Þ

The system can be assumed to act as well-mixed stages.

Mass balance for nitrate and nitrite for a combined systemcan be written as:

V A þ V FBð Þd NOÀ

3

Â Ãdt

¼ F NOÀ3

Â Ã0À NOÀ

3

Â Ã À r 1V FB

ð38Þ

V A þ V FBð Þd NOÀ

2

Â Ãdt

¼ F NOÀ2

Â Ã0À NOÀ

2

Â Ã

þ r 1V FB À r 2V FB ð39Þ

A hydrogen balance around the fluidized bed can alsobe applied as follows:

V FBd H 2½

dt ¼ F R H 2½ 0À H 2½

À ÁÀ r 1S1 þ r 2S2ð Þ ð40Þ

Packed bed reactorThe process of anaerobic degradation of molasses canoccur in a packed bed reactor with clay spherical packingand operated with liquid recycle. The fraction of glucoseconverted to the indicated acids must be known. The ratesof biomass growth and substrate utilization are assumed tofollow Monod kinetics with constant yield factors anddiffusional effects assumed to be negligible:

r xi ¼lmaxSi X iSi þ K S

À K d ;i X i ð41Þ

r si ¼lmaxSi X i

Y ðSi þ K SÞð42Þ

The total biomass is given by:

X ¼X

X i ð43Þ

The following liquid phase balance can be written foreach component:

V RdSi

dt ¼ F S fi À Si

À ÁÀ r siV R ð44Þ

Allowing for the spherical nature of clay packing, abalance for the forming biomass film gives:

n R p þ R f

À Á3ÀR3

p

4pq

3¼ XV R ð45Þ

Control of anaerobic reactorsThe primary objectives of control systems are to providequality assurance and economic incentives. Biologicalprocesses exhibit some very specific behaviours and thecontrol needed is not the same as it would be usually understood by a control engineer. In anaerobic treatments,slow dynamics associated with biological reactors canresult in unforeseen disturbances in a bioreactor, such as a

failure (washout), requiring new start-up. As a directconsequence, the synthesis of control systems for opti-mizing the functioning of these processes has beenextensively studied in the last decade. Extensive surveyshave been published and several international conferenceshave been held on this problem (Andrews 1989; Shimizu1993; Andrews 1994; Bastin and Van Impe 1995).

The purpose of the control is to manipulate the controlvariables to achieve the following results:

• Maintain the desired outputs at a constant desired value• Stabilize unstable or potentially unstable processes• Optimize the performances as defined by measures such

as yield, productivity, or profit

These objectives are to be achieved under variousconstraints such as safety, environmental regulations,limited resources, and operational constraints.

The control of anaerobic reactors has been attemptedon the laboratory and pilot scale, as well as on simulatedsystems, but little experience has been obtained on full-scale experiments (Ryhiner et al. 1993; Snape et al. 1995).

Although adjusting parameters requires a certainamount of trial and error, a simulating model is invaluablefor determining an approximate controller setting. Someexperience was gathered on the feedback control on thefollowing variables (Snape et al. 1995):

• pH• Propionic and acetic acid• Product gas flow rate• Methane and carbon dioxide content in biogas• Dissolved hydrogen concentration

Models for the methane stage of waste-water purifica-tion in the stirred tank have been developed (Schurbu-scher and Wandrey 1991). Models including masstransport permit, within limits, the calculation of the pHvalue.

Empirical correlations capable of predicting effects of small changes in control variables on the response of microbial systems are very useful, and they can be effec-tively utilized in adaptive control.

The last decades saw significant cost reductions andimprovements in the speed and reliability of computerhardware and increased availability of software. Thesefactors have resulted in an increase in the use of com-puters for control and optimization of processes. However,the extent of computer application depends on the devel-opment of better on-line instrumentation that can monitorcellular physiology, as well as on reliable mathematicalmodels that can describe various cellular dynamics.


0

7/16/2019 10.1007_s10098-001-0123-x


Concluding remarksThis work is devoted mainly to the process engineeringinvolved in the development of the processes included inbiological anaerobic waste-water treatment. Using thisprocess presents a number of advantages, in particular, theorganic matter is degraded into a gas mixture of carbondioxide and methane and thus produces valuable energy.In addition, this process is able to operate under severe

conditions: high strength ef fluents and short hydraulicretention time.Taking into account the fact that the engineering con-

cepts are applied in the conception, planning, evaluation,design, construction, operation, and maintenance of thesystems that are needed to meet waste-water managementobjectives, it was stressed that the main role of the envi-ronmental engineer is – among others – to understand thecharacteristics of each biological process involved in bio-logical treatment of waste-waters. In these circumstances,progress is needed and is being made in the followingareas: finding mathematical simplifications of the analysesin order to facilitate the use of the developed models and

theories in the field of the biological unit processes forbioreactor design; producing more complex models toprovide insight into important processes; development of procedures for model verification and parameter mea-surement for a particular waste-water.

Engineering aspects and modelling concepts encoun-tered in anaerobic waste-water treatment are linked by theadvantages occurring by application of these processes: alow sludge production, that is one-tenth comparative toaerobic treatment; no aeration energy requirement; closedsystems, which leads to odour reduction; associatedmethane energy generation.

The models proposed for anaerobic waste-water treat-

ment processes should overcome the dif ficulties in thesynthesis of the large volume of material as well as prob-lems and should be able to predict how even the simplestprocess or bioreactor will behave under different condi-tions.

Special attention must be paid to the physical, processrelated, and geometric factors affecting bioreactor opera-tion conditions and the ef ficiency of treatment plants on atechnical scale. The purpose of the control is to manipulatethe control variables to achieve the following results:maintain the desired outputs at a constant desired value;stabilize unstable or potentially unstable processes; opti-mize the performances as defined by measures such as

yield, productivity, or profit. These objectives are to beachieved under various constraints such as safety, envi-ronmental regulations, limited resources, and operationalconstraints.

The combination of anaerobic treatment reactors withan aerobic post-treatment or a waste stabilization pond asa hybrid system is developed to warm the climate. Suchanaerobic/aerobic treatment of waste-water is beingdeveloped with a variety of configurations, being moresustainable than the conventional aerobic systems becauseof low excess sludge production, low energy input, andcomplete biodegradation of certain xenobiotics that arenot degraded in a single stage (anaerobic or aerobic).

ReferencesAndrews JF (1968) Modelling of biological treatment process. Bio-

technol Bioeng 10:707–723Andrews JF (1989) Dynamic, stability and control of the anaerobic

digestion process. In: Patry GG, Chapman D (eds) Dynamicmodelling and expert systems in wastewater engineering. Lewis,New York

Andrews JF (1994) Dynamic control of wastewater treatment plants.Environ Sci Technol 28:434A–439A

Archer DB, Hilton MG, Adams P, Wiecko H (1986) Hydrogen as a

process control index in a pilot plant anaerobic digester. Bio-technol Lett 8:197–202Armenante PM (1998) Suspended-biomass and fixed-film reactors.

In: Lewandowski GA, Defillipi LJ, (eds) Biological treatment of hazardous wastes. Wiley, New York, pp 1–33

Bastin G, Van Impe JF (1995) Nonlinear and adaptive control inbiotechnology: a tutorial. Eur J Control 1:1–37

Benitez FJ, Beltran-Heredia J, Torregrosa J, Acero JL (1997)Improvement of the anaerobic biodegradation of olive millwastewaters by prior ozonation pretreatment. Acta Biotechnol17:169–175

Bhadra A, Mukhopadhyay SN, Ghose TK (1984) A kinetic model formethanogenesis of acetic acid in a multireactor system. BiotechnolBioeng 26:257–264

Biothane Corporation (2001) Anaerobic. http://www.biothane.comBoulenger P, Driessen W, Werfhorst E, Tielbald M (1999) Anaerobic

ef fluent treatment by a pilot and full-scale plant at a chemicalindustrial complex. In: Proceedings of the IAWQ Conferenceon Waste Minimization End of Pipe Treatment in Chemical andPetrochemical Industries, Merida, Mexico, 14–18 November,pp 467–473

Brinkman I, Schultz CE (1994) The Paques anaerobic digestionprocess: a feasible and flexible treatment for solid organic waste.In: Paques Solid Waste Systems BV: Communications with dif-ferent waste treatment organizations and government institutionsin The Netherlands, Germany and Switzerland

Brown SC, Grady CPL, Tabak HH (1990) Biodegradation and kineticsof substituted phenols. Wat Res 24:853–861

Bryers J-D (1985) Structural modelling of the anaerobic digestionof biomass particulate. Biotechnol Bioeng 27:638–649

Chian ESK, De Walle FB (1977) Treatment of high strength acidicwastewater with a completely mixed anaerobic filter. Wat Res

11:295–304Converti A (1994) Modelling of continuous corn starch hydrolisatefermentations in immobilized-cell reactors. Starch 46:260–265

Converti A, Del Borghi A, Zilli M, Arni S, Del Borghi M (1999)Anaerobic digestion of the vegetable fraction of municipal refuses:mesophilic versus thermophilic conditions. Bioprocess Eng21:371–376

Denac M, Miguel A, Dunn IJ (1988) Modeling dynamic experimentson the anaerobic degradation of molasses wastewater. BiotechnolBioeng 31:1–10

Driessen WJBM, Habets LHA, Groeneveld N (1996) New develop-ment in the design of upflow anaerobic sludge bed reactors. In:2nd Specialized IAWQ Conference on Pretreatment of IndustrialWastewaters, Athens, Greece, 16–18 October, pp 1–9

Driessen WJBM, Habets LHA, Zumbragel M, Wasenius CO (1999)Anaerobic treatment of recycled paper mill ef fluent with internalcirculation reactor. In: 6th IAWQ Symposium on Forest Industry Wastewaters, Tampere, Finland, 6–10 June, paper 181

Droste RL, Kennedy KJ (1988) Dynamic anaerobic fixed-film reactormodel. J Environ Eng 114:606–620

Dunn IJ, Heinzle E, Ingham J, Prenosil JE (1992) Biological reactionengineering. VCH, Weinheim

Edwards VH (1970) The influence of high substrate concentrations onmicrobial kinetics. Biotechnol Bioeng 12:679–712

Enviroasia Ltd (2001) Worldwide experience. Anaerobic waste-watertreatment. http://www.enviroasia.org

Esemag (2001) Anaerobic wastewater treatment reviewed.http://www.esemag.com

Gaudy AF, Ekambaram A, Rozich AF (1988) Practical methodology for predicting critical operation range of biological systemstreating inhibitory substances. J Water Pollut Control Fed60:77–85


7/16/2019 10.1007_s10098-001-0123-x


Gavrilescu M, Macoveanu M (1999) Process engineering in biologicalaerobic waste-water treatment. Acta Biotechnol 19:111–145

Gavrilescu M, Macoveanu M (2000) Attached-growth process engi-neering in waste-water treatment. Bioprocess Eng 23:95–106

GEC (2001) IC reactor for producing energy from high and low strength wastewater. http://nett21.unep.or.jp

Gorur S, Vereijken T, Tielbaard M (1999) In: 6th IAWQ Symposiumon Forest Industry Wastewaters, Tampere, Finland, 6–10 June,paper 153

Grant-Allen D, Liu HW (1998) Pulp mill ef fluent remediation. In:

Meyers RA (ed) Encyclopedia of environmental analysis andremediation. Wiley, New YorkHabets L (1999) The application of anaerobic waste-water and process

water treatment in the paper industry. In: 6th IAWQ Symposiumon Forest Industry Wastewaters, Tampere, Finland, 6–10 June,paper 180

Habets LHA, Engelaar AJHH, Groeneveld N (1997) Anaerobic treat-ment of inuline ef fluent in an internal circulation reactor. Wat SciTechnol 35:189–197

Harada H, Momomoi K, Yamazaki S, Takizawa S (1994) Applicationof anaerobic-UF membrane reactor for treatment of a wastewatercontaining high-strength particulate organics. Wat Sci Technol30:307–319

Holland KT, Knapp JS, Shoesmith JG (1987) Anaerobic bacteria.Chapman and Hall, New York

Jordening H-J, Buchholz K Fixed film stationary bed and fluidizedbed reactors. In: Rehm H-J, Reed G (eds) Biotechnology, vol 11a.Wiley-VCH, Weinheim, pp 373–415

Kindzierski WB, Fedorak PM, Gray MR, Hrudey SE (1995) Activatedcarbon and synthetic resins as support materials for methano-genic phenol-degrading consortia – comparison of phenol-de-grading activities. Wat Environ Res 67:108–117

Kleinstreuer C, Poweigha T (1982) Dynamic simulator for anaerobicdigestion processes. Biotechnol Bioeng 24:1941–1951

Kupferle MJ, Chen TC, Gallardo VJ, Lindberg DE, Bishop PL,Safferman SI, Bishop DF (1995) Anaerobic pretreatment of hazardous waste leachates in publicly owned treatment works.Wat Environ Res 67:910–920

Kurt M, Dunn IJ, Bourne JR (1987) Biological denitrification of drinking water using autotrophic organisms with H2 in a fluidizedbed biofilm reactor. Biotechnol Bioeng 29:493–501

Lettinga G, van Velsen AFM, Hobma SW, de Zeeuw W, Kapwijk A(1979) The use of the upflow sludge blanket (UASB) reactor forbiological wastewater treatment, especially for anaerobic treat-ment. Biotechnol Bioeng 22:699–734

Lewandowski Z (1985) Denitrification in packed bed reactors in thepresence of chromium(VI). Wat Res. 21:147–153

Lim HC, Lee K-S (1991) Control of bioreactor systems. In: Schugerl K(ed) Biotechnology, vol. 4. Wiley-VCH, Weinheim, pp. 509–560

Macoveanu M, Teodosiu C, Duca GH (1997) Epurarea avansata aapelor uzate continand compusi organici nebiodegradabili. Gh.Asachi Press, Iasi, Romania

Negulescu M (1985) Municipal waste treatment, Elsevier, BucharestOgnean T, Vaicum L-M (1987) Modelarea proceselor de epurare

biologica. Romanian Academy Press, Bucharest

Ozturk SS, Palsson BO, Thiele JH (1989) Control of interspecieselectron transfer flow during anaerobic digestion: dynamic dif-fusion reaction models for hydrogen gas transfer in microbialflocs. Biotechnol.Bioeng. 33:745–757

Paques-ADI (2001) Anaerobic and aerobic wastewater treatmenttechnologies. http://www.paquesadi.com

Quasim SR (1999) Wastewater treatment plants, Technomic,Lancaster

Ryhiner GB, Heinzle E, Dunn IJ (1993) Modelling and simulationof anaerobic wastewater treatment and its application to control

design: case whey. Biotechnol Prog 9:332–343Sathish N, Young JC, Tabak HH (1993) Protocol for determining therate of biodegradation of toxic organic chemicals in anaerobicprocesses. In: Tedder DW, Pohland FG (eds) Emerging technol-ogies in hazardous waste management III. ACS Symp Ser 518.American Chemical Society, Washington, D.C., pp.203–218

Schurbuscher D, Wandrey C (1991) Anaerobic waste water processmodel. In: Schugerl K (ed) Biotechnology, vol 4. VCH, Weinheim,pp.441–484

Shimizu K (1993) An overview on the control system design of bioreactors. Adv Biochem Eng Biotechnol 50:65–84

Simeonov IS (1999) Mathematical modelling and parameters esti-mation of anaerobic fermentation processes. Bioprocess Eng21:377–381

Sincero AP, Sincero GA (1996) Environmental engineering. A designapproach. Prentice Hall, Englewood Cliffs, N.J.

Snape JB, Dunn IJ, Ingham J, Prenosil JE (1995) Dynamics of envi-ronmental bioprocesses. VCH, Weinheim

Speece RE (1983) Anaerobic biotechnology for industrial wastewatertreatment. Environ Sci Technol 17:9–15

Tchobanoglous G, Burton FL (1991) Waste-water engineering: treat-ment, disposal and reuse (3rd edn). McGraw-Hill, New York

Versprille AI, Frankin RJ, Zoutberg GR (1994) Biobed, a successfulcross-breed between UASB and fluidized bed. In: Seventh Inter-national Symposium on Anaerobic Digestion. RSA, Goodwood,pp. 587–590

Verstraete W, Vandevivere P (1999) New and broader applications of anaerobic digestion. Crit Rev Environ Sci Technol 28:151–173

Wiesmann U (1988) Kinetik und Reaktionstechnik der anaerobenAbwasserreinigung. Chem Ing Technol 60:464–474

Wiesmann U, Libra J (1999) Special aerobic waste water and sludgetreatment processes. In: Rehm H-J, Reed G (eds) Biotechnology,vol 11a. Wiley-VCH, Weinheim, pp.373–415

Weiland P (1997) Codigestion of agricultural, industrial and munic-ipal wastes. In: Anaerobic conversions for environmental protec-tion, sanitation and re-use of residues. REUR Technol Ser 51.FAO, Rome, pp 42–52

Zoutberg GR, Been P (1997) The Biobed EGSB (expanded granularsludge bed) system covers shortcomings of the upflow anaerobicsludge blanket reactor in the chemical industry. Wat Sci Technol35:183–188

Yang ST, Okos MR (1987) Kinetic study and mathematical modelingof methanogenesis of acetate using pure culture of methanogens.Biotechnol Bioeng 30:661–667


2

10.1007_s10098-001-0123-x

Documents

Transcript of 10.1007_s10098-001-0123-x