1 Integration of Industrial and Agricultural Sectors: Proposed Eco-Park in Burlington, Vermont, USA

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Stage 2: The spent mushroom substrate is placed in earthworm or vermiculture chambers.

The earthworms rapidly convert the materials to enriched compost. The earthworms, a

product of the process, are then blended with aquatic plants, Azolla sp. (water fern) and

Lemna spp. (duckweeds), to produce protein-rich fish feeds.

Stage 3: The mushroom/earthworm-based compost is then utilized in the growing of tropical

plants in pots and the culture of salad greens. No additional fertilization to the compost is

required for the production of greens. After several harvests of salad greens the medium

is then utilized as a soil amendment or as a potting soil.



8.2. Aquaculture

Another key component in the design of integrated food systems for urban settings is

aquaculture. The food team at OAI has designed re-circulating systems based upon four

tank modules for the culture of aquatic animals. To date, OAI has successfully cultured

Oreochromis sp. (tilapia) and Perca flavescens M. (yellow perch) in these systems. The system

is designed to produce feeds for the fish internally, including attached algae turfs and their

associated communities, floating aquatic plants including Lemna and Azolla, zooplankton,

and snails. External feeds to the system include earthworms and commercial feeds. These

ecosystem based fish culture systems have proven to be efficient. The multiplicity of pathways

for nutrients and materials to flow in the production of a diversity of crops is an integral part

of ecological design. If such an approach proves to be economically viable in an urban setting,

the larger issue of food security can be addressed through the application of applied ecological

concepts (16, 40, 41).



NOMENCLATURE

PCB = Polychlorinated biphenyls

PAH = Polycyclic aromatic hydrocarbons

EPA = Environmental protection agency

BOD = Biochemical oxygen demand

COD = Chemical oxygen demand

NAS = National Academy of Sciences

AEES = Advanced ecologically engineered systems

SFS = Surface flow systems

FWS = Free water surface

m = Meter

m3 = Cubic meters

EFB = Ecological fluidized beds

TSS = Total suspended solids

NH4 = Ammonium

NH3 = Ammonia

HFR = Horizontal flow reedbed

VFR = Vertical flow reedbed

PRS = Pond and reedbed system



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CBOD = Carbonaceous biochemical oxygen demand

TKN = Total Kjeldahl nitrogen

NO3 = Nitrate

TN = Total nitrogen

HRT = Hydraulic retention time

VOC = Volatile organic compounds

SBR = Sequencing batch reactor

UV = Ultra violet

ft = Feet

TP = Total phosphorous



REFERENCES

1. Li XZ, Mander U, Ma ZG, Jia Y (2009) Water quality problems and potential for wetlands as

treatment systems in the Yangtze Riverdelta, China. Wetlands 29:1125–1132

2. Mejáre M, Bülow L (2001) Metal-binding proteins and peptides in bioremediation and phytoremediation of heavy metals. Trends Biotechnol 19(2):67–73

3. Abraham W-R, Nogales B, Golyshin P, Pieper HD, Timmis NK (2002) Polychlorinated biphenyldegrading microbial communities in soil and sediments. Curr Opin Microbiol 5:246–253

4. Samanta KS, Singh VO, Jain KR (2002) Trends Biotechnol 20(6):243–248

5. Novotný C, Svobodova K, Erbanova P, Cajthaml T, Kasinath A, Lang E, Šašek V (2004) Ligninolytic fungi in bioremediation: extracellular enzyme production and degradation rate. Soil Biol

Biochem 36:1545–1551

6. U.S. Environmental Protection Agency (EPA) (2004) Report. EPA/600/R-04/075 report, Washington DC

7. Khan GA (2005) Role of soil microbes in the rhizospheres of plants growing on trace metal

contaminated soils in phytoremediation. J Trace Elem Med Biol 18:355–364

8. Ang LE, Zhao H, Obbard PJ (2005) Recent advances in the bioremediation of persistent organic

pollutants via biomolecular engineering. Enzyme Microb Technol 37:487–496

9. Thassitou PK, Arvanitoyannis IS (2001) Bioremediation: a novel approach to food waste management. Trends Food Sci Technol 12:185–196

10. Atlas MR, Bartha R (1993) Interactions among microbial populations. In: Microbial ecology:

fundamentals and applications. The Benjamin/Cummings Publishing Company, Menlo Park, CA,

pp 37–65

11. National Academy of Sciences (NAS) (1985) Oil in the sea: fates and effects. National Academic,

Washington DC

12. Zhu X, Venosa DA, Suidan TM (2004) Literature review on the use of commercial bioremediation

agents for cleanup of oil-contaminated estuarine environments. U.S. Environmental Protection

Agency (EPA) Report, EPA/600/R-04/075, Washington DC, pp 1–5

13. U.S. EPA (1996) A series of fact sheets on nonpoint source (NPS) pollution. EPA841-F-96-004.

Office of Water, U.S. Environmental Protection Agency, Washington DC

14. U.S. EPA (2000) The quality of our nation’s waters: a summary of national water quality inventory; 1998 report to congress. EPA841-S-00-001. Office of Water, U.S. Environmental Protection

Agency, Washington DC



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March Issue

16. Todd J, Brown EJG, Wells E (2003) Ecological design applied. Ecol Eng 20:420–440

17. Mistch JW, Jorgensen ES (2003) Ecological engineering: a field whose tie has come. Ecol Eng

20:363–377

18. Odum HT (1989) Ecological engineering and self-organization. In: Mitsch WJ, Jørgensen SE (eds)

Ecological engineering: an introduction to ecotechnology. Wiley, New York, pp 79–101

19. Todd J (1997) Ecological design, living machines and purification of waters. In: Kibert CJ,

Wilson A (eds) Reshaping the built environments; ecology, ethics and economics. Island Press,

Washington DC, pp 133–149

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109–136

21. Ehrlich KF, Cantin MC, Horsfall FL (1989) Bioaugmentation: biotechnology for improving aquacultural production and environmental protection. In: Murray K (ed) Aquaculture engineering

technologies for the future. Inst. Chem. Eng. UK Symposium Ser. No. 111, pp 329–341

22. Margulis L, Schwartz KV (1988) Five kingdoms; an illustrated guide to the Phyla of Life on Earth.

WH Freeman, New York, NY

23. Odum HT (1971) Environment, power and society. Wiley, New York, NY

24. Gray NJ (1989) Biology of wastewater treatment. Oxford University Press, Oxford, UK

25. Curds CR, Hawkes HA (eds) (1975) Ecological aspects of used water treatment, vol 1. Academic,

London, UK

26. Curds CR, Hawkes HA (eds) (1983) Ecological aspects of used water treatment, vol 2. Academic,

London, UK

27. Barlocher F, Arsuffi TL, Newell SY (1989) Digestive enzymes of the saltmarsh periwinkle Littorina irrorate (Mollusca: Gastropoda). Oecologia 80:39–43

28. Hawkins A, Bayne B (1992) Physiological interrelations and the regulation of production. In:

Gosling E (ed) The mussel Mytilus: ecology, physiology, genetics and culture. developments in

aquaculture and fisheres science, vol 25. Elsevier, Amsterdam

29. Karnaukhov VN (1979) The role of filtrator molluscs rich in carotenoid in the self cleaning of fresh

waters. Symp Biol Hung 19:151–167

30. U.S. EPA (2004) Constructed treatment wetlands: wetland resources; EPA843-F-03-013. Office of

Water, U.S. Environmental Protection Agency, Washington DC

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quality and wildlife habitat; EPA843-B-03-003. Office of Water, U.S. Environmental Protection

Agency, Washington DC

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EPA832-R-93-005. Office of Water, U.S. Environmental Protection Agency, Washington DC

33. Living Technologies Ltd (2010) http://www.ltluk.com/lake_restorers.html

34. Bergen SD, Bolton SM, Fridley JL (2001) Design principles for ecological engineering. Ecol Eng

18:201–210

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permit application. Report to the Town of Harwich, MA, 43 pp

36. K.V. Associates Inc. & IEP Inc (1991) Shallow pond diagnostic and feasibility study. Report to the

Town of Barnstable, MA, 113 pp

37. Todd NJ, Todd J (1994) From eco-cities to living machines: principles of ecological design. North

Atlantic Books, Berkeley, 197 pp



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38. Presidents Council on Sustainable Development (1996) Eco-industrial park workshop proceedings,

17–18 October, Cape Charles, VA, 141 pp

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some case studies from China. In: Etnier C, Guterstam B (eds) Proceedings of the international

conference on the ecological engineering for wastewater treatment, 24–28 March 1991, Stensund

Folk College, Gothenburg, Sweden, pp 80–94

40. Wang LK, Pereira NC, Hung YT, Shammas NK (eds) (2009) Biological treatment processes.

Humana, Totowa, NJ, 818 pp

41. Wang LK, Shammas NK, Hung YT (eds) (2009) Advanced biological treatment processes.

Humana, Totowa, NJ, 738 pp

42. Introduction to greenhouse-wetland treatment systems; http://www.oberlin.edu/faculty (2010)

43. U.S. EPA (2002) Wastewater technology fact sheet: the living machine, EPA 832-F-02-025. Office

of Water, U.S. Environmental Protection Agency, Washington DC



23

Global Perspective of Anaerobic Treatment

of Industrial Wastewater

Kuan Yeow Show, Joo Hwa Tay, and Yung-Tse Hung

CONTENTS

G LOBAL P ERSPECTIVE OF A NAEROBIC T REATMENT

D EVELOPMENT OF THE A NAEROBIC P ROCESSES

A NAEROBIC B IOCHEMISTRY AND M ICROBIOLOGY

C OMPARISON B ETWEEN A EROBIC AND A NAEROBIC P ROCESSES

G LOBAL A PPLICATIONS OF A NAEROBIC T REATMENT

A PPLICATIONS OF A NAEROBIC P ROCESSES FOR I NDUSTRIAL

WASTEWATER

T HE F UTURE OF A NAEROBIC T REATMENT

C ONCLUSION

R EFERENCES

Abstract While anaerobic process had been widely used for stabilizing concentrated solids,

the process long suffered a poor reputation because of lack of understanding regarding its

fundamentals. Nearly a century later, anaerobic treatment is now arguably the most promising

and favorable wastewater treatment system for meeting the desired criteria for future technology in environmentally sustainable development. The development of anaerobic processes,

anaerobic biochemistry and microbiology, global applications, and applications of anaerobic

processes for industrial wastewaters are discussed.



1. GLOBAL PERSPECTIVE OF ANAEROBIC TREATMENT

The anaerobic treatment of wastewaters and sludges has been in practice for more than a

century. While it had been widely used for stabilizing concentrated solids, the process long

suffered a poor reputation because of lack of understanding regarding its microbiology and

biochemical components. The anaerobic treatment was perceived as a sensitive process that

was easily upset and difficult to control. It was also known for producing obnoxious odors,

as well as requiring long initial start-up periods and high temperatures (35◦ C) for effective

From: Handbook of Environmental Engineering, Volume 11: Environmental Bioengineering

Edited by: L. K. Wang et al., DOI: 10.1007/978-1-60327-031-1_23 c Springer Science + Business Media, LLC 2010



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waste stabilization. Another possible reason that the anaerobic process had not found general

acceptance was that the practical feasibility of direct treatment processes had yet to be proven

on specific industrial effluents (1).

Nearly a century later, anaerobic treatment is now arguably the most promising and favorable wastewater treatment system for meeting the desired criteria for future technology in

environmentally sustainable development.

Since 1973, the European Union’s environmental legislation has developed within a framework set by different Environmental Action Programs, which show how the EU proposes to

develop its environmental policy and legislation. The fifth program, titled “Towards Sustainability: A European Community Program of Policy and Action in relation to the Environment

and Sustainable Development,” has already been finished (1993–2000), and a new program,

the sixth (“Environment 2010: Our Future, Our Choice”), has been approved by the European

Commission for the next decade (2001–2010). The general approach and strategy of the fifth

program differs from that of the previous programs. As its title “Towards Sustainability”

implies, the program sets longer term objectives and focuses on a more global approach.

The Sixth Environmental Action Program has the same overall perspective, focusing on areas

where more action is needed.

Similarly, a recent U.S. National Research Council committee (2) stated, “We are convinced

that socially compatible and environmentally sound economic development is possible only

by charting a course that makes full use of environmentally advantageous technologies. By

this, we mean technologies that utilize resources as efficiently as possible and minimize

environmental harm while increasing industrial productivity and improving quality of life.”

Again, the main term of this program is “sustainable development.”

Achieving an integrated prevention and control of pollution requires an integrated control

of emissions to air, water, and land, as well as the efficient use of energy and raw materials.

The anaerobic treatment process would appear to meet these criteria well. The first reason is

the fact that anaerobic treatment is a natural process in which a variety of different species

from two entirely different biological kingdoms, the Bacteria and the Archaea, work together

to convert organic wastes through a variety of intermediates into methane gas, an excellent

source of energy. Methane gas can be used to heat the waste stream to give a higher rate of

stabilization or to supplement in-plant power requirements. Pathogenic microorganisms are

reduced, and objectionable organic matter is eliminated. The net result is the production of

biosolids that are also useful as soil conditioner and are widely used as such. Additionally, with

industrial wastewater treatment, the amount of biosolids produced is far less than with aerobic

treatment, and the biosolids are already stabilized for land application. Nutrient requirements

for anaerobic treatment are smaller in amount than with aerobic treatment.

A unique characteristic of anaerobic treatment by methane fermentation is that no electron

acceptor such as oxygen or nitrate needs to be present or added for the process to work.

Organic matter itself or the carbon dioxide resulting from its destruction serves this need.

As a result, organic loadings to anaerobic reactors can be much higher than to aerobic

reactors because oxygen mass transfer limitations are not involved, and energy requirements

for mixing are greatly reduced. Therefore, reactors can be much smaller. The absence of a need

for an external electron acceptor is also of great advantage when groundwater is contaminated



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with biodegradable organics, since sufficient external electron acceptors are often absent.

Anaerobic processes leading to methane formation make possible the intrinsic bioremediation

of organic groundwater contaminants.

Not only is anaerobic treatment of value for wastewater treatment, but it is also the process

used in sanitary landfills that results in the stabilization of organic wastes, converting them to

methane gas, which is becoming increasingly valued as an energy source. Some opponents

would cap landfills to prevent water from entering and the natural bioconversion to methane

to occur, but this practice needs to be questioned. Methane is a powerful greenhouse gas, but

if captured for use, it acts instead as a good renewable energy source.

As an added advantage, an unexpected scientific finding, over the past several decades, is

that the same anaerobic process is capable of destroying most chlorinated hazardous compounds, including pesticides and chlorinated solvents, and converting polychlorobiphenyls

(PCBs) to less harmful forms. Aerobic processes that are so widely used do not have this

capability.

Of even greater surprise was the finding that some anaerobic organisms obtain energy

for growth from the dehalogenation process (3). Anaerobic processes can also destroy some

inorganic pollutants, such as nitrates and perchlorates.

In summary, anaerobic treatment results in net energy production, produces biosolids that

are good soil conditioners, requires less reactor volume, and destroys troublesome hazardous

chemicals. By itself, the process is capable of meeting the criteria for sustainable development.



2. DEVELOPMENT OF THE ANAEROBIC PROCESSES

2.1. History of Anaerobic Treatment

The first recorded research of anaerobic treatment was accidentally made while evaluating

the fertilizer value of digested and undigested manure. In 1808, Davy collected gas containing

30% methane from the digested manure. At the same time, however, Volta is credited with

having recognized first that anaerobic biological processes result in the conversion of organic

matter to methane (4). In 1776, he showed that “combustible air” was formed from sediments

in lakes, ponds, and streams, and concluded that it was derived from the plant material in the

sediment.

In 1856, Reiset found methane being liberated from decomposing manure piles and proposed that this process be studied to help explain the decomposition of organic material in

general (5).

The first full-scale application of anaerobic treatment was also for domestic wastewater but

in a configuration more like a septic tank. This air-tight chamber was described in the French

journal Cosmos (6) by Mouras, and was called “Mouras’ Automatic Scavenger” in which

suspended organic material was “liquefied.” The article indicated that the invention had been

in use for 20 years, which would place the beginning of its application in the 1860s. Targe

indicated in the article that this was “the most simple, the most beautiful, and perhaps, the

grandest of modern inventions,” and “a complete solution of the problem which for centuries

had been an insolent menace hurled in the face of all humanity.”



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Perhaps, the first hybrid anaerobic system was that described in Metcalf and Eddy’s historical text on American sewerage practice (7). In about 1890 or 1891, Moncrieff constructed a

tank with an empty space below and a bed of stones above. Thus, this was a hybrid of a tank

digester and an anaerobic filter. The wastewater of 10 people entered the tank first and then

passed up through the anaerobic filter. After 7 years of operation, the sludge remaining in the

bottom was removed and readily disposed. Other studies on this system by Houston in 1892

and 1893 confirmed that there was a great reduction in sludge volume to be handled by this

system.

One of the first anaerobic filters was a bed of sand at the Massachusetts experimental station

(8) to which wastewater was applied with a pore space detention time of about 8 days. After

14 years of operation, 89% of the organic impurities applied to it were claimed to have been

removed through biological activity.

Another experiment described was with a filter containing broken stone 0.5–2 in. in

diameter. Domestic wastewater was applied with a surface loading rate of about 2 m/day, and

85% organic removal was indicated. A thin film of bacteria covered the stone, indicating that

the removal was by bacterial action. The Massachusetts State Board of Health also indicated

the advantages of holding wastewater solids for a period of time to achieve hydrolytic or

bacteriolytic action on waste solids, resulting in the conversion of a portion of the organic

matter into inoffensive gases or soluble compounds that pass out with the wastewater (7).

A “septic tank,” which appears to be modeled after the Automatic Scavenger, was constructed in Exeter, England, in 1895 by Cameron to treat about 230 m3 /day of waste water,

for which Cameron was awarded a patent (7). Because of its success, the City of Exeter, in

1897, approved the treatment of the entire city’s wastewater by this means.

A similar system was designed by Talbot for Urbana, Illinois, in 1894, and for Champaign,

Illinois, in 1897. The Talbot design had vertical baffles reaching 0.6–1 m below the surface of

the wastewater in the tank. Thus, a sort of baffled reactor is indicated. Cameron recognized

the value of the methane gas produced in the septic tanks, and at Exeter, the gas was collected

and used for heating and lighting at the disposal works.

In 1897, waste disposal tanks at a leper colony in Matunga, Bombay, were reported to

also have been equipped with gas collectors, and the gas was used to drive gas engines (5).

While septic tanks began to be used widely, the effluents were often black and offensive and

contained indigestible material that clogged contact beds often used for subsequent treatment.

Clark suggested in 1899 that this problem could be reduced if the sludge was fermented by

itself in a separate tank at Lawrence, Massachusetts (9). This is perhaps the first indication of

a move towards separate sludge digestion.

In 1904, Travis put into operation a new two-stage process in which the suspended solids

settled into a separate chamber for digestion (7, 9). Travis believed it was desirable to pass

some wastewater through the “hydrolyzing” chamber, as the sludge digestion chamber was

called, but this created problems with suspended solids and septic conditions in the effluent.

A Travis tank began operation in Emscher, Germany, in 1905, but was modified by Imhoff to

prevent wastewater from flowing through the “hydrolyzing” chamber. The sludge was allowed

to stay in this chamber from a few weeks to several months, after which it was inoffensive and

could be withdrawn and disposed without nuisance.



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The Imhoff tank greatly reduced the cost of sludge disposal and rapidly came into favor.

By the end of 1914, about 75 cities and many institutions in the United States had received

licenses to use the Imhoff tank (7).

The anaerobic process was then beginning to move away from treatment of wastewaters to

the treatment of settled sludge. The Imhoff tank was obviously not the complete solution to

wastewater treatment and had problems that needed addressing. The tanks were tall, and the

digestion chamber had to be connected intimately with the sedimentation tank. Efforts then

began with separate digestion of sludges (10). This was not practically successful until 1927

when the Ruhrverband at Essen-Rellinghausen installed the first sludge-heating apparatus in a

separate digestion tank (11). The efficiency of treatment greatly exceeded that available with

Imhoff tanks, and separate digestion grew rapidly in popularity, particularly in larger cities.

The value of methane gas produced by digestion became more generally recognized. In

addition to its use for heating digesters, it can be used for other purposes such as a medium

for digester liquid mixing through biogas recirculation. In 1923, methane gas was collected

on a large scale by the Emschergenossenschaft and delivered to the municipal gas system at

the Essen-Rellinghausen plant (11).

In 1927, the Ruhrverband utilized the sludge gas in Iserlohn and then in EssenRellinghausen to generate power for a biological treatment plant, and they used the cooling

water from the motors for heating he digestion tanks. Such use of digester gas is now common

practice at wastewater treatment plants throughout the world.

By the 1930s, many German cities added compressing plants to store the gas in steel

cylinders for use as a motor fuel (11). This practice also has been used on and off in modern

times.

Along with these applications, there were many studies during the 1920s and 1930s of the

separate anaerobic sludge treatment process, such that by the end of the 1930s, a sufficient

understanding had developed to allow wide-scale practical applications.

During this period, the use of anaerobic process to treat wastewaters evolved together with

further development to treat sludges. The following section, however, will focus solely on the

history and development of industrial wastewater treatment.



2.2. Industrial Wastewater Treatment

Initial strong interest in applying the anaerobic process for industrial wastewater treatment

can perhaps best be attributed to Buswell. Beginning in the 1920s, he and his colleagues

conducted extensive research on the nature of the process and its potential application for

treatment of industrial wastewaters and agricultural residues (5, 9, 12–16).

These important studies were hampered in application as the single tank anaerobic digester

was generally used, which offered no provision for separating microorganisms from the

wastewater for long residence time in the reactor. Nevertheless, Buswell’s contributions to

anaerobic treatment development are significant.

Recognition of the importance of solids residence time for reducing reactor size and detention time began in the 1950s, drawing from experiences with aerobic treatment in activated

sludge plants and trickling filters.



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One of the leaders of this new movement was Stander (17, 18). By separating the anaerobic

bacteria from the effluent stream and keeping them in the reactor, he demonstrated that the

detention time for efficient treatment of several different wastewaters from the fermentation

industry could be lowered to 2 days, compared with the two weeks or more with conventional

digesters.

Stander (19) later demonstrated the validity of these concepts in full-scale treatment of

winery wastewater in an anaerobic “clarigester,” which employed a settling tank over a tank

reactor. This differed from the Imhoff tank in that wastewater was introduced directly into

the reactor for treatment and then moved upward into the settling tank. Bacteria and other

solids settled in the settling tank and were returned by gravity to the reactor, creating a long

biological solids retention time.

Another approach was to use a reactor followed by a settling tank with organism recycle,

similar to an activated sludge plant (20). Here, dilute packinghouse waste was treated, and

organism recycle allowed reduction of the detention time to less than 1 day.

These applications and those by Stander indicated that efficient treatment of dilute industrial wastewaters was possible by anaerobic processes when solids retention time concepts

were applied. These studies then led to various modifications of anaerobic reactors to achieve

efficient treatment of wastewaters in general.

Since taking advantage of the principles of aerobic-activated sludge treatment (a dispersed

growth reactor) appeared to work well for anaerobic treatment, many researchers in the 1960s

proposed applying a biofilm reactor, which also retained microorganisms, and was also widely

used for aerobic treatment. As noted with early developments, the anaerobic filter for treatment

of dilute municipal wastewaters was one of the first applications of the anaerobic process. It

appeared worthwhile to return to this early concept for modern evaluation.

The first large-scale application of the anaerobic filter was reported in 1972 for the treatment of wheat starch wastewater (21). The process has seen many applications since then.

Another biofilm concept was that of the expanded-bed reactor (22), which McCarty had

earlier applied successfully for denitrification (23). This system is particularly suitable for

very dilute wastewaters because of the large retention of microorganisms, short detention time

potential, and freedom from bioclogging.

The most successful new reactor design in its broad application to a variety of industrial

and municipal wastewaters, however, is the upflow anaerobic sludge blanket reactor (UASB)

process conceived by Lettinga. The developments and applications of anaerobic treatment

stemming from Lettinga’s work have been considerable.



3. ANAEROBIC BIOCHEMISTRY AND MICROBIOLOGY

Several techniques have been developed and adapted to isolate and study anaerobic bacteria

(24). The anaerobic ecosystem is the result of complex interactions among organisms of different species. Generally, there are four major stages in the production of methane and carbon

dioxide from organic matter. The first stage involves hydrolysis of large organic compounds

into smaller sizes. In the second stage, the smaller-sized organic compounds undergo fermentation through extracellular enzymes produced by fermentative bacteria. Acidogenesis occurs



Global Perspective of Anaerobic Treatment of Industrial Wastewater



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with the formation of hydrogen, carbon dioxide, acetate, organic acids, and other organic

intermediates. The third stage involves acetogenesis, in which the organic acids produced in

acidogenesis are converted to acetate and hydrogen. In addition, a proportion of the available

hydrogen and carbon dioxide is converted to acetate by homoacetogenic bacteria. In the final

stage, methanogenic bacteria reduce the carbon dioxide and the decarboxylate acetate to form

methane.

Other organisms may play an important role in the initial fermentative stages. These are

termed “passenger organisms” as they do not become established in the reactor but are

continuously added with the feed. The constant addition of these facultative bacteria does

not significantly change the established hydrolytic anaerobic flora.



3.1. Hydrolysis

Hydrolysis and liquefaction converts complex insoluble organic compounds into smaller,

simpler molecules that may be utilized as an energy source. The biopolymers protein, carbohydrate, and lipid are hydrolyzed to amino acids, simple sugars, and fatty acids, respectively,

by extracellular enzymes.

Starch and cellulose are quantitatively the most important of these polymers. The genera of

bacteria associated with cellulose degradation are Bacteroides, Ruminococcus, Clostridium,

Cellobacterium, and Butyrivibrio. Clostridium, obligate bacteria that are strict anaerobes

sensitive to oxygen, is the major group. It produces spores to survive in aerobic conditions.

Flavobacteriem, Alcaligenes, Achromobacter, and various enteric bacteria are common

facultative microorganisms that have been identified in wastewater treatment systems. Cellulolytic bacteria require ammonia as a nitrogen source, cysteine and sulfides as sources of

sulphur, vitamin B, hemin, menadione, and mineral salts, especially sodium.

The hydrolysis of polysaccharides, such as hemicellulose and pectin, yields hexose and

pentose sugars. Starch is degraded more readily in anaerobic reactors than cellulose. Lipids

are broken down by hydrolysis, 4–5% being incorporated as lipids in the bacteria. The neutral

fats are hydrolyzed to long-chain fatty acids and glycerol. Long chain fatty acids are then

degraded via the betaoxidation cycle.

The extracellular hydrolysis of proteins to polypeptides and amino acids is catalyzed by

proteases. This usually is accompanied by the formation of ammonia, carbon dioxide, and

volatile fatty acids. Deamination is done by fermentative bacteria, Bacteriodes ruminicolai,

peptococcus, and other bacteroides species.



3.2. Acidogenesis

The end products from the first stage are converted into short-chain volatile acids such

as acetic acids, propionic acids, and to a lesser extent, butyric, valeric, and caproic acids

(25). Acetate is considered the most important intermediate formed from the fermentation of

proteins and fats.

Hydrogen and carbon dioxide are formed as well. The final products of the acidogenic

bacterial metabolism depend on initial substrate and environmental conditions, especially

hydrogen partial pressure. Low hydrogen partial pressure favors the formation of acetate,