Journal of Scientific & Industrial Research

Vol. 6 1 . March 2002. pp I 84-207

Waste Management and Production of Future Fuels Neena Raizada, V Sonakya, Vanita Anand and V C Kalia*

Centre for Biochemical Technology (CS IR), Delhi Uni versity Campus, Mall Road. Delhi II 00 07. India

The ever-increasing demand for energy. the dimini shing energy source and the problem of environmental pollution have raised public awareness to the need for a non-polluting renewable energy source. Biowastes, a potential renewa l energy source of different ori gi ns arc associated with a negati ve value due to disposal and pollution cost. Biological wastes of different origins (agricultural. industri al. municipal and domestic) undergo slow and uncontrolled degradation , wh ich leads to environmental pollution and their disposal is a bi g problem due to high transportati on costs and scarcity of dump ing sites. Anaerobic degradation of these wastes to useful products like energy rich fuel gases can stabi li ze them and also serve as renewable energy source. Mi crobial production of methane from different biological wastes has been studi ed on a wide range of wastes . Thus. wastes utili zati on. rather than its treatmen t emphas izes upon shifting the process from reducing the potenti al for pollution to synthesis of useful products. like gases and chemicals. Biomass amenabi lity to conversion depends largely on the characteristi c of the biomass (substrate) and the process req uirements for conservation technology under considerat ion. Among the various by-products. whi ch can be obtained by the fermentation of waste biomass. hydroger. has gained importance. It is regarded as the strongest contender as the clean fuel of the future. Microbial production of hydrogen has been demonstrated but the yields are quite low. large scale and continuous production is still in the incipient stage. In nature, wherever organic materi al is degraded microbially, under anaerobic conditi on and in the absence of sulpha! and nitrate. meth ane is produced. When organic matter decompose wi thout oxygen, the anaerobic bacteria produce methane and carbon dioxide. Anaerobic diges tion provides appropriate solutions to problem associated wi th waste disposal and also generates biofuels as by-products.

Waste generation is a necessary evil of modern daily life as almost all human activiti es result in the some quantity of was te generation (Figure 1 & 2) 1

•

Consequent ly. its disposal and management has assumed alarming magnitude because waste generat ion rate more than the resources and abi I iti es to dispose them off. The available disposal fac ilities are very scarce. The indi scriminate use of natural resources has onl y added to the worsening scenario. Rap id industri ali zati on and urbanization is largely responsibl e for rapid dep leti on of fossi l fuels . Hi gh rate of rossi l fuel consumption and emiss ion of obnox ious gases therefrom has pushed the whole humanit y into a polluted environment and also crea ted the energy cri sis. Hence, attempts are being made to manage the waste in an organi sed manner. reduce was te generati on, conserve energy and look for altern ati ve sources of energy.

Was tes can be grouped based on the source of ori gin : (i) Domestic , (ii) Muni cipa l, (iii ) Agricultural and (iv) Industri al. The wastes encountered in garbage include: paper. textiles, glass. wood, metal, rubber, leather. plastics, animal carcass, abattoir wastes. rl oor sweepings, etc. The quantum of wastes

*Aut hor for correspondence.

varies with region and cultural background of an area or country. Di fferent agencies arrive at different figures for waste generation and according to one such estimates, the producti on of waste in Indi a2 is given in Table 1. Agriculture and municipal sectors were major contributors to the waste produced. As the waste product ion was increas ing at the rate of 1.33 per cent, more expenditure on its di sposal and management could be anticipated. In fact the industrial sectors have not been able to handle and treat their wastes or control emi ss ion of gases into the atmosphere. Various laws have, however, been enacted in air and water pollut ion control and haza rdous waste management. Nati onal Standards and upper limits for di scharge of pollutants in the environment have a lso been drawn up . The environmental impact analysis has a lso been made mandatory for large and medium scale industries. However, the industria l firms are apparently not in a position to take up effec tive steps to fa ll in line with the statutory requirements primaril y due to lack of awareness of pollu tion problems and the ir effects, awareness of and access t0 cos t-effective technologies and services and finan ce at affordable cos t. Biologica l methods are certainly the best for treating bi ological wastes in particular.

RAIZADA et. a/.: WASTE MANAGEMENT & PRODUCTION OF FUTURE FUELS 185

SUN .c------ C0

2 LN ATMOSPHERE ... -----,

Harvcsling ~ -------'-------+ ~

Conversion + Harvesting Conversion

+ I Consumption ~

------------.~

Figure !-Schematic representation of synfu le production from Biomass and waste 1

•

Resource - .. Activity ---+ Waste

t Reclamation t Pollut1on 'I Fig. 2- Schematic representat ion from waste to resourc1

In general the waste disposal methods include: (i) transportation of waste to low lying areas/landfill s3

, (ii ) burning of waste on site or in the incinerators, (iii) compost ing, (iv) briquetting, (v) recycling of waste matter, and (vi) microbi al treatment, aerobic and anaerobi c digestion, etc . Each of these methods has its own advantages, and can be employed to certain types of wastes. Hence, for compl ete disposal of wastes , it is important to take an integrated approach.

Landfills

Collection of wastes an d their transportati on to low lyi ng areas on the outskirts of inhab ited areas and loca lities is cos tl y, req uiring around Rs 300-500 crore an nuall y. The ava ilability of landfill s is also decreas ing. The problem is further compounded by rapid urbanization and population explos ion . Increased awareness among people about the ill effects of landfill s has become a cause of concern among waste managers as unorgani zed landfill s cause envi ronmental pollution owmg to slow and uncontrol led fe rmentat ion generatin g gases I ike carbon dioxide4

, methane and hydrogen sulphide. Unass isted natural degradation occu rs very slowl y in land fi ll s and it goes on for several years5 To control fermentati on for effective degradation of landfill

Table !-Estimated quantity of waste generation in India-

Waste

Municipal solid waste

Municipal liquid waste ( 121 class I & II cit ies)

Di still ary (243 nos.)

Sugar cane press mud

Food & Furit Processing waste

Willow dust

Dairy industry waste

Paper & pulp industry waste (300 mills)

Tannery (2000 nos.)

Quantity

27.4 mt /y

12 145 mlld

8057 kL /d

9 mt /y

4.5 mt /y

30, 000 t/y

50-60 mL /d

1600 m3 waste /d

52500 m3 wastewater /d

wastes, many factors like temperature, compos ition , moi sture content, pH, alkalin ity, etc are taken in to consideration6

. In spite of all these effort s, one can recover only I 0 to 15 per cent of the ava il abl e energy content. Efforts are being made to understand the metabolism and engineers are also tryi ng to look in to the physical parameters .

Emission of green house gases, like carbon di oxide and methane from landfill s are maj or causes of global warming7

. Although methane constitu tes onl y 0.4 per cent of all greenhouse pollutants, it contributes 20 per cent of the total green house warming of the earth's atmosphere with annual rate of increase of the order of I per cent (Figure 3f These contribute over 40 per cent of the total meth ane emissions and over 60 per cent of the anthropogenic methane emi ssions (Figure 4)8

.

In fac t, each tonne of refuse results in the generation of 2 m3 biogas which may contain 55 per cent methane and 45 per cent carbon dioxide. Carbon dioxide concentration is currentl y ri sing at the rate of 1.8 ppmv (0.5 per cent) per year clue to foss il fue l burning, land use changes and biomass burning. Atmospheric carbon diox ide takes 50-200 y to adju st to changes in sources or sinks, consequentl y, an immediate reduction 111 globa l anthropogenic emissions by 60-80 per cent would be necessary. Methane concentrat ion in atmosphere is increasin g at about 0.0 15 ppmv (0.96 per cent)/y. The global warming is known to influence all li ving beings 2

.

In the production of methane and carbon dioxide from landfill , moi sture in the waste, prec ipitation and

186 J SCI IND RES VOL 61 MARCH 2002

water produced chemically during the waste decomposition and infiltration of rain are the sources of moisture in a conventionallandfill 7

. Water addition accelerates waste stabili zati on processes, which stimul ates methane production and produce leachatcs5

. The leachates pose environmental problem when discharged into environment. Moi sture limitation , poorly shredded waste, high bulk density and lack of necessary inocula contribute to slow waste degradation. Discharge of untreated effluents fro m industries further lead to ground water poll uti on. The efflu ents are loaded with health hazard heavy metals like lead, arseni c and cadmium and studies have revea led presence of these elements in large concentrat ions in fruits and vegetables grown by irrigation with such contam inated waters7

. Heavy metals are also known to cause kidney troubles, heart problems, ski n diseases, mental tension, etc. It is thu s imperati ve to treat wastewater before di scharging and/or reuse fo r different purposes. Landfills need to be properly const ructed where leachate can be collected and treated to be safe and the ground water is not polluted . Gases produced during fermentation in a landfill can be coll ected by lay ing a network of perforated pipes6 and used as energy source.

Burning of Wastes

Burning of wastes onsite is a conventional method of dealing with wastes. According to various surveys, 4000 to 5000 t/d wastes are generated in metropolitan cities, and 60-70 per cent are di sposed off in landfill s. The wastes have up to 65 per cent combustibl e materi als generating around 2700 BTU/kg waste ( I BTU= 2.9x 10-4 kWh)7

. Only dry wastes consist ing of old clothes. used papers, wood, etc can be burnt releasing I xI 07 BTU/t waste which is eq uivalent to 350 L oil or 360 kg coal. Burning waste in absence of oxygen at high temperatures results in breakdown of compl ex organic matter into obnox ious gases wh ich spread over large di stances9

.

Carbon monox ide reduces oxygen carrying capacity of lun gs. Ammonia generated during burning of dry leaves resu lts in allergic disorders. Oxides of nitrogen affect lungs and nervous sys tem. In additi on, sulphur dioxide causes as thma, allergy and other respiratory disorders. Incineration does resu lt in reducing the waste vo lume by 90 per cent and reduction in weight by 75 per cent without any reducti on in the toxicity of such was tes . Rather, the tox ic dry ash particl es accumul ate in polluti on control devices and act as nucleus for the depos ition of dioxins and furans.

Emission of hot HCI from the ex haust of the incinerator corrodes the system. The soot and obnoxious gases emitting from the incinerator also carry mercury, sulphur dioxide, nitrogen oxide, carbon monoxide, etc. Dioxins 10

' 11

, furans , cadmium, lead and mercury adversely affect endocrine system, thyroid in particular, hatching of eggs in birds, fishes and mammals. The sexual behavior is also badly altered. In addition , the incinerators are extremely expensive to in stall and run . The ou tput in terms of energy recovery is only 43 per cent the operating cost. It does not create a large employment for the huge capital in vestment. These are some of the primary reasons that no incinerator has been insta lled in any developed country during the las t 5 y. Most of the proposals to put up incinerators for disposal of wastes have been shelved in the ea rl y stages.

CFCs

Figure 3- Percentage contribution of d1fi'crent Green hou se gasses for Global Wanninl.

Other Fosil

Entric Fermentation

Figure 4 - Different sources of methane emissionH

RAIZADA el. a/.: WASTE MANAGEMENT & PRODUCTION OF FUTURE FUELS 187

Composting

Composting is the hi ghest form of recycling. An organic, di scarded materi al is converted (recycled) for reuse in a manner that only can benefit mankind (Figure 5) 1. The major problem is a lack of appropriate technologies fo r separating com postab le waste from other materi als, but mainl y because landfill s are availabl e. Compostin g is inexpensive, rapidl y implemented and a publicly acceptable treatment process. Sewage sludge compostin g has been fairl y successful. However, maj or prob lems st ill occur at some faci lities, such as odors, poor product quality, long processing times and excessive moisture and material s handling problems. Compost ing is a complex biologica l process 12

. It depends upon a host of phys ical, bi olog ical and chemical factors. Among the physical factors: mi xing, heat evolution, temperature, heat now and control' temperature dynamics, water, available energy density, etc. are some of the important ones. Chemical factors whi ch affect the processing are: interstiti al oxygen concentrat ion, pH, ammonia, etc. Biologically, bacteria , fungi and actinomycetes fun ct ion at different stagesu of the process and at different temperature ranges, 35 to 65 oc. Bacteria and fungi are able to operate favorab ly in a narrow C: rat io range of 20: 1 and I 50: I, respecti ve ly. The process is carried in trapezoid al heaps of garbage, which may be 3 m wide and I m in height. The length of the windrows can be extended even up to 50 m. A mature compost is dark brown or bl ack, granul ar, spongy or fibrous in feel and smell mould or rather like good earth . Marketab le compost is free from contaminants such as glass, plastic. meta ls and odor. It is of uniform and constant size. Additives like phosphates can be used to enhance the va lue of the fina l product. Compost can be used as :

(i) soil conditioner and fe rtili zer,

(ii ) to res tore and maintain so il humus and producti vity.

(iii ) landscaping. turf prod uction and stab ili za ti on of slopes,

(iv) mu shroom culture,

(v) as media fo r green house prod ucti on of seedli ngs. rlowers and vege tables.

(v i) bi olog ical agents of pest con trol. pa rti cu lar ly fungi and bacteria antagon1 st1c to plan t pathogens may be grown and marketed in a compost medium.

(vii ) as biofilters to trap and biotransform foul odors from an imal houses, abattoirs and industry into harmless products.

Briquetting

Agro-industri al wastes, though rich in organic matter, constitute a tremendous disposal probl em. The problem is compou nded by the irregular size, low density, large volumes, difficulties in collect ion, handling and transportation of the materi al. Dry waste such as wood , straw, etc. can be either burnt to give heat or subjected to thermo-chemical processe; to produce a variety of energy rich prod ucts , li ke gas, char, oil , methanol , ammonia, coal. etc.

Briquetting converts hi ghl y voluminous and troublesome waste material s into clean, non-pollut ing fue l. Being compact, tran sportati on and storage cost~ are reduced. It is environmentall y superior technol ogy for handling different ligno-cellulos ic materials 14

.

Feedstocks consist of al! the forest and agricultural wastes like pine needles 1

), wild bushes 16, grasses, ri ce husk, saw du st. tea was tes. etc. The quality and quantity of fuel produced (Table 2) 17 depends upon the type of raw materi als used and conditi ons prevailing during process ing. The process consists of heatin g feedstocks under controll ed conditi ons. without any contact with air. During carboni zation. char, gases and liquid products are also obtained which are burnt during continued pyrolysis to provide heat for carbon izat ion , so that ad ditional fuel is not required. The char so obtained cooled and mixed with inert materi als. like clay, is useful for briquetting and also gives good combust ion character istics. Briquettes, on drying are hard enough to withstand impact durin g handling and storage. One sure way to avoid smoke with its attendant pollution is to convert woody biomass into pe llets that eliminates all the undes irable component s. The briquettes can be used in gas ifier production units, industri al boil ers and furnaces. cerami c units, brick or I i me ki Ins, bakeries. potteries , etc. These are eq uall y effecti ve in edibl e oil extracti on units, paper mill s, etc. Domesti ca ll y. it can be used for cook in g in restaurants, hotels, hospitals. bakeries and kitchens. So. it wi ll not onl y release pressure on non-renewabl e sources but also control the firewocd demand ami consumpti on considerablv and simult:meously help protect the environment. -

Microbial Treatment of Wastes

Bio-deteriorati on or bio-d isintegrat ion 1x·1'' is effective in recyc lin g and stab ili;.ati on of waste

188 J SCIIND RES VOL 61 MARCH 2002

Waste

Biodisintc<>ration / ~ Biodetcrioration "'7 Biosynthesis ~

Resource Products

Fig. 5- M icrobial recycling of wastes 1•

Table 2-Calori fie value of briquettes of different biowastes 14"

17

Calorific value (klcal/kg DM) Biowaste

3300- 3700

3900-4200

4300 - 5000

DM= Dry matter

Rice husk, sarkanda grass, sugarcane trash . sugarcane waste.

Bamboo dust , bark (wood), coffee husk, jute sti ck, maize sta lk, wheat straw.

Bagasse. coconut coir. cotton stalk, groundnut shell , musta rd stalk, pine needles. saw dust

Table 3- Compari son of aerobic/anoxic and anaerobic '0 sys tem·

System

Feature Aerobic/ anoxic Anaerobic

Ef"Ouent quality Excellent Moderate to poor

sludge production High Low

lutrient requirements High Low

Energy requirement High Low

Temperature sensiti vity Low Hi gh

Methane production 0 Yes

lutrient removal Possible Negli gib le

through anaerobic digestion which is a better approach for treating wastes than the aerob ic variant or composting (Tab le 3, Figure 6)20

. Importance of anaerobic digestion is given in Table 42 1

. Unlike uncontrolled and slow fermentation of wastes in a landfi11 5

, bi odegradation in a factory system results in energy generat ion and production of nutrient rich biomanure.

Anaerobic Digestion

Anaerobic digestion stabilizes organic matter, present in the biological wastes by anaerobic bacteria . It involves breakdown of almost all kinds of biomass

(waste) into soluble compounds whi ch is carried out by the consorted action of a wide range of microorganisms in the absence of oxygen or other strong oxidizing chemicals. Methane and carbon dioxide are the principal prod cts and minor quantities of nitrogen, hydrogen , ammonia and hydrogen sulphide are also generated. Thus, it is a complex sequence of biological reactions where product by one group of organisms serves as substrate for the next group or organisms and the methanogens are the terminal organisms in the microbial food chain22

. The success of a decomposition process depends on the interaction among of metabolicall y different bacteria. Effective conversion of organic matter to biogas through fermentation is a result of combined and coordinated metabolic activity of anaerobic bacterial populations. The generally accepted relationships and pathways, active in anaerobic digestion , are shown in Figure 7 and further detailed below:

( I) Hydrolysis- In the first st p. the complex insoluble macromolecules of a substrate are hydrolyzed into simpler and more so lubl e intermediates by putrefying bacteri a. The process depends on extracellul ar enzymes. e g, cellulase, amylase, protease and I ipase that degrade biopolymers (carbohydrates, protein and lipids) into smaller units , sugars, amino acids. fatty acids and glycerol, respectivel / 3 It is normal ly a rate limiting step for the anaerobic treatment of insoluble wastes such as biomass2~ · 25 .

(2) Acidogensis- In the second step the fermentation/acidification of soluble substrates into more oxidized intermedi ates occur by hydrogen producing acidogens and primarily the volatile fatty acids (VFAs) are produced. These bacteria added or inoculated in the feed utili ze oxygen present in water, to produce acetic acid. They also reduce low molecular weight compounds into alcohol, propionate, butyrate, hydrogen and carbon dioxide. Some of these reactions are endergonic under standard conditions, hence the metabolism of acidogenic bacter ia demands a low partial pressure of hyd rogen. Thus it is apparent that metabolic interaction, in the form of inter-species hyd rogen transfer reactions, IS operative m anaerobic environment which maintains a low partial pressure of hydrogen of I o·6M. As a co ~equence, the anaerobic degradation of substrate tends to shift to the formation of acetate by passing the formation of

RAIZA DA et. a/.: WASTE MANAGEMENT & PRODUCTION OF FUT URE FUELS 189

10

100 1000 10000

ANAEROBIC DIGESTION

100000 1000000

BIODEGRADABLE COD Cone. mg I L

Figure 6 - Operating ranges for aerobic/anox ic and anaerobic suspended growth bi ochemical opcrration20

hi gher ac ids. The step however, is the fastest in the anaerobic convers ion of complex materi al in the liquid phase di gesti on26

. Acidogens are fast growing bacteri a, with minimum doubling time around 40

27 mll1

(3) Acetogenesis-The third maj or reaction in the anaerobic di ges ti on process is acetogenes is. About 76 per cent of organic matter are degraded via reduced organi c in termedi ates, whic h can be oxidi zed further into methanogenic substrates by homoacetogenic and hydrogenogenic bac te ri a. Therefore . acetogenes is is a key process in the minerali za ti on of organic matter in methanogenic environment. The acetogens are s low growing with doubling time about 14 d and depends on the low parti a l pressure of hydrogen to make the conversion yie ld e nerg/ . Thus. the reacti on occurs in syntrophic assoc iati on, whe re severa l anaerobic mi croorgani sms can share the energy ava i I able in the bi o-convers ion of a molecule to methane and carbon di oxide and thu s can ac hi eve intermedi ate reacti ons whi ch are endenw ni c under standard conditi ons (T able 5)24

.

(4) Methanogenesis- It is a major step in anaerobic digesti on, where methanogens cataboli ze acetate and one carbon compound to methane . Methanogens are unique among prokaryotes, and are c lass ifi ed as me mbers of archae bacte ri a, a group of phytogeneti call y different mi croorgani sms. These obli gate anaerobes are very sensiti ve to inhibiti ons. All methanogens can utili ze hydrogen and carbon dioxide as growth substrates and some of the m can also catabo li ze acetate and one carbon compounds to methane . Other unique prope rti es of methanogens inc lude, a diffe rent autotrophic growth in which carbon di oxide fixation does not in volve ca lvin cycle and the presence of co-enzyme M, unu sua l e lectron carri er like co-enzyme F420

29 and fac tor F420 and pseudomuri n in ce ll walls .

About 72 pe r cent of meth ane from organic matter are produced vi a acetoclasti c methanogens though the bacteri a grow very s lowly with doubli ng time of 2-6 d, generally making it the rate limiti ng step. Other pathway is of hydrogenotrophic where hydrogen from fermentati on and acetogenic reac tion

190 J SCIIND RES VOL 6 1 MARCH 2002

Figure 7- Pathway of Anaerobic Biodegradation.

IS converted to methane by hydrogen consuming methanogens. And only 28 per cent methane is produced via hydrogen pathway. This syntrophic coup ling of many acetogenic/acidogen ic reactions is critica l as 80 per cent of methane production occurs via this pathwai0

. The doubling time for the hydrogenotrophic methanogens is 6 h. Due to the reason. hydrogen IS rarely detectable In final product- biogas -although a large amount of hydrogen is produced in this process. Moreover, the pathway is more energy yielding and plays a major role in regulation of anaerobic digestion process. These methanogens have a high affinity for hydrogen, as they are the only anaerobic organisms capable of utili zing electron in the form of hydrogen. Thus to maintain a low partial pressure of hydrogen makes thermodynamically unfavorable reactions to occur or in other words the hydrogen is of special concern as a key metabolite in anaerob ic process and its potenti al for i"cedback contro l of acetogenic reacti on.

Table 4- Advantages of anaerobic digest ion process

Advantages of anaerobic diges ti on process

High efficiency Good COD removal efficiency can be achieved in system even at high loading rates and low temperature.

Economical

Flexibi lity

Low space req uirement

Low energy consumption

Low sludge production

Low nutri ent

Production of Biofertilizer

Pathogen removal

Post treatment

Aerat ion requirement

The construction and operation of these reactors is relatively simple and inexpensive.

It can be app li ed at any place and at any scale.

When hi gh loadi ng rates are accommodated. the area needed for the reactor is small.

Indeed the consuming energy for heating influent and plant operati on . useful energy is prod uced in the form of biogas .

The sludge production is 3-20 times lower as compared to aerobic methods, due to slow growth rate of anaerobic bacteria.

Macronutrient and micronutrient can be fulfilled by waste biomass itself.

The effl uen t produced is odorless and rich inN. P, K and biofertilizer other metal contents which makes it a good fertilizer.

It can remove plant. animal and human fecal pathogens.

Anaerobic digestion can destroy the greater part of the COD allowing the efflu ent to be post treated aerobically. post treatment can be combi ned with the methods of recovering useful products like NH, and S.

0 2 requirement is eliminated . as the process is anaerobic.

Anaerobic Digestion Processes (Figure 8)31:

I Up flow Anaerobic Sludge Blanket ( UASB)

The technology comb ines the well-mixed attributes of the contact sys tem with an internal gas separation and clarificat ion mechanism . The UASB is an anaerobi c reactor fed from the bottom. It can be divided into four compartments-bottom to the top­the sludge bed, the fluidi zed zone, the gas- liquid separator and the settling compartmen t. In the sludge bed, the organic compounds present in wastewater are hydrol ysed and acidified by hyd rol yt ic and ac idogenic bacteria. These further converted into hydrogen, methane an d carbon dioxide' 2

. The reactor contains no mechan ical components, but does have a top works baffle arrangement which ac ts to separate the gas, liquid and solid phases.

RAIZADA et. a!.: WASTE MANAGEMENT & PRODUCTIO OF FUTURE FUELS 191

GAS

Jt .------------' ~UENT Gf

.---------___JI ~VENT

INFLU~'

PLUG FLOW REA CTOR HIGH RATE DIGESTOR

INFLUENT t ANAEROBIC FILTER

GAS

ANAEROBIC LAGOON

i SLUDGE REC!RCU TION I .. ..._ ___ ___,~ 1---.___---1 INFLUENT

CONTACT UEACTOR FLUDISED BED REACTOR

Figure 8 -Types of anaerobic processes·"

2 Completely Mixed Stirred Tank (CMRT)

A CMRT (a lso ca lled CSRT) is s imil ar to the conventiona l acti vated sludge aerobi c process, wide ly used in the treatment of municipal wastewater. It became indu stry standard in I 970s and represents an effl uent to im prove des ign of an anaerobic lagoons. The reactor conta in s a mi xer to maintain good contact between biomass and the organic materi a l to be d igested, and a post-c larificati on step with biomass return to in sure steady quantity of mi xed liquor suspended so li ds within the reactor33

.

3 Anaerobic Filter

The anaerobic fi lter was commerc iali zed in late I 970s and , as the name implies , re li es on a media substrate to retain bi omass within the reactor vessel33

.

Different types of substrate materi a ls have been uti li zed for the purpose, and different flu sh out

methods have been developed to reduce the possibility of plugging.

4 Upflow Fluidized Bed

These processes reduce loading rates and reactor size s ignifi cantl y. Earli e r systems attached bi omass to heavier parti cul ate matte r (e g, sand) so the bacteria would not be swept out of a reacto r by strong hydrauli c fl ow-through rates2 1

. A late r gene rati on of "ultra high rate" reac tor e liminates the need for carri er materi a l, and still maintain a fluidi zed or expended bed to fac ilitate bi omass contact33

.

5 Dry Continuous Digestion

The process involves a continuous ly fed diges tion vessel with 20-40 pe r cent di gestate dry matter. Both completely mixed and plug flow sys tem are ava ilabl e2 1

. Plug fl ow systems re ly on ex tern al recycle of a proportion of the outgoing di gestate to

19'2 J SC II ND RES VOL 6 1 MARCH 2002

Table .'i - Acetogeni c and methanogen ic reacuons: free ~ energies (I'>G) 2~ ~

Aceto~en i c and

methanogcnic reactions

Propionate:

CI I,C H2COO II + 211 20

Hutyrate: CH1Cll 2

CII ,COO II + 211 20

Ethanol: CH,Cll 20 11 +

11 ,0

Lactate:

Cll 1Cl-IOII COO II + 2H20

Acetate: C II.,COO l-1 +

11 ,0

Carbon dioxide and

hydrogen: C02 + 4 11 2

L'>G". kJ

Acetate: CH,COO H + 76 .1

c o , + 311 ,

Acetate: 2C l-1 1COO H 4R. I

+ 211 ,

Acetate: CH ,COOI-I + 9.6

211 ,

Acetate: C l-l ,COO H + -4.2

co,+ 211 ,

Methane: CH~ + C0 2 - 104.6

Methan e: CH.1 + 3H20 - 135.6

inoculate the incomin g raw feed materi als30 In both the cases, the requirement for onl y minimal water add iti ons makes the overa ll heat ba lance very favorable for operat ion at the thermoph ilic di ges tion temperature (50-55 °C).

6 Drr Batch Digestion

It is closest to the accelerated landfill concept. Whil e the containment vessel is being loaded with raw MSW, it is inoculated with digestate from another reactor. It is then sealed and left to digest naturall y, and during the closure period leachate from the base of the vesse l is re-circulated to maintai n a uniform moi sture and redistribute soluble substrates (VFA) and methane bacteri a throughout the mass of MSW within the vessel. When the digestion is complete, the vessel is reopened , uploaded and refill ed with fres h charge of raw MSW feedstock. Its main advantages are simplicity of the containment vessel and the need for only minimal feed preparation and mechanical handling33

.

7 Leach-Bed Processes

It is genera ll y similar to dry batch digestion , except that the leachate from the base of diges ti on vessel is exchanged between establi shed and new batch to facilitate start-up, inoculation and removal of volat ile ac ids in the reactor2t. After a while, when methanogenesis is establi shed in the solid waste, the industrial digester is uncoupled and reconnected to fresh solid waste in a second vessel. It has also been described as "sequential batch anaerobic composting" (SEBAC)13

.

8 Wet Continuous Digestion

MSW feedstock involves slurrying with a large portion of water to provide a dilute ( I 0 per cent dry solids) feedstock that can be fed to a conventional complete ly mixed digester similar to those commonly used for sewage sludge or fa rm slu rries. Effective remova l of glass and stones is req uir d in the feed preparat ion stage to prevent their rapid accumulation in the bottom of the main digesti on tank. When used for MSW digestion alone. filter pres ing of the wet digesti on to recover liquor to recyc le for feed preparation is required to avoid generating and excess ive vo lu me of diluted di gesta~e for disposal, altern ati ve ly. thi s concept lends itself towards cod igestion of MSW with more dil ute feedstocks such as sewage sludges or animals manures·1·

1. The

concept of codigestion is especia ll y well establi shed in Denmark where there is a maj or program of cooperat ive biogas plants for animal manures.

9 Mu ltistage Wet Digestion

lt includes · a range of proprietary multi stage wet digestion process where the MSW is slu rri ed with water or recyc led and fermentative bacteri a to release VFA which are then converted to bi ogas in a speciali st high rate indust ri al anaerobic di gester. usuall y an anaerob ic filter or an UASB reactor33

.

Improved Anaerobic Sludge Bed (ASB) Reactor System

Expanded Granular Sludge Bed (EGSB) Reactor

The EGSB reactor was the fi rst modifi ed form of a USAB reactor. The concept was introduced by De Man et a. to increase hydraulic mixing intensity, which is low in a UASB reactor. A EGSB reactor is characteri zed by an expansion of the granul ar sludge bed. This expansion is due to the applicati on of a high upflow velocity (5- I 0 m.h-t) which is 5- l OX that of a UASB reactor34

.

UASB-Septic Tank

The ori ginality of the UASB-septic tank compared to the conventi onal UASB reactor is that the former includes accumulation and stabili zati on of the sludge and operates in uptlow mode. In practice the UASB-septic tank operates as a continuous system with respect to the liquid and as well as a batch system with respect to the sol ids. The concept for the treatment of was tewater was first in vestigated by Bogte et al. , in the Netherlands35

.

RAIZADA et. nl.: WASTE MANAGEMENT & PRODUCTION OF FUTURE FUELS 193

Hydrolysis Upflmr S/11dge Bed (HUSB) Reactor

The HUSB reactor concept for raw domestic wastewater treatment was proposed in 1985 and tested first on a pilot scale at the Gao Be idian (China) wastewater treatment plant. The system achieved a very sat isfactory suspended solids removal of 75-84 per cen t at a short HRT of 2.5-5 h and ambient temperature of 18-31 °C. However, the total COD removal in the system was only 40-48 per cent and the soluble COD remova l was very low36

.

TH •o-stage Reactor Concept

There is a confusion between the expressions­two-phase, and two-stage or two-step reactor-while the former implies a process configuration employing separate reactor, for acidification and methanogenesis , connected in series. The latter is so designed that acidification in the first reactor is incomplete (Wang 1994). The main advantage of the two-phase system appears to be the improvements in process control and low accumulation of biomass in the methanogenic phase37

Anaerobic Hybrid (A H) Reactor

The AH reactor is a combination of a UASB reactor or an EGSB reactor and an anaerobic fi lter (AF) in one reactor. The reactor bottom is a s ludge bed and top is a filter on which biomass can be attached. Although eight hybrid reactor plants have been built in Mexico since 1997, the data about the system are still scarce. Recently, some workers studied an A H reactor for the treatment for wastewater under low temperature condition ( 13 °C) usin g small sludge granules instead of large sludge granu les. The main advantage of AH system seem to be re lated to the prevention of sludge floatation and the substantial removal of suspended solid s38

Some of the merits and demerit s of these reactors39 are li sted in Table 6.

Volatile Fatty Acids (VFAs) From Municipal Solid Waste (MSW)

The complex organic matter present in wastes are degraded by a microbial chain to VFAs as a first stage product and then the VFAs are used to produce biogas in the second stage of anaerobic digestion. Furthermore, the sol ids residue after fermentation can be used for the production of a soil averment. For the

treatment of organic fraction of Municipal Solid Waste (OFMSW) the following possible integrated plants could be used (Figure 9)27 and various transformation process are used to manage solid wastes (Table 7)40

:

A. The OFMSW could be used as the substrate for anaerobic digestion. Biogas would be obtained and yields would depend on the characteristics of the refuse. The outlet sludge cou ld be composted at a later stage for its final stabilization.

B. The ·oFMSW cou ld be the substrate for the first stage (ac idogenic step) of the two-phase anaerobic digestion process. The effl uents from the first reaction wou ld be tran sported to a second stage where the biogas would be obtained. The out let sludge wou ld be finally treated by composting process. This approach could be of interest under thermophi I ic conditions, when the C : N ratio is low.

C. Another possible strategy could be to obtain valuable products, such as VFA, by anaerobic digestion. Thus the OFMSW could be pretreated in a fermenting acidogenic step for obtaining maximum levels of VFA. The outlet sludge of the acidogenic process leads to screw press and then VFA would be extracted from the liquid phase and converted to products , such as methyl- or ethyl-esters, for commercial purposes. The screwed sludge from the acidogenic treatment cou ld go directly to a composting step or it would even be poss ible to recover part of the energy, digesting the degradable organic fractions still remaining in the residue. Studies on the bi ogas production potential of the out let s ludge of the ac id ogenic reactor would be necessary to assess feasibility of the approach (Figure 7)27

.

Ghosh et att 1 demon strated bi-phasic process consisting of so lid state, acidogenic fermentation of the organic reaction of MSW followed by biomethanation of acidic hydrolysates in a separate methane fermenter. Solid state fermentation of MSW with effluent recirculation resulted in rapid hydrolysis, acidification and denitrification , with aacumu lation of soluble COD and VFA to inhibitory levels. The bi-phasic process effected carbohydrates, lipids and protein conversion efficiencies of 90, 49 and 37 per cent, respectivel/2

. Two-phase anaerobic digestion produced a higher methane digester gas than obtained by single-stage41

.

194 J SC II ND RES VOL 6 1 MARCH 2002

Table 6- Anaerobic treatment process comparison - organic stab ilization·'

Process

Anaerob ic digestion (AD)

Low- rate anaerobic processes

Anaerobic contact (AC)

Up fl ow anaerobic sludge blanket (UASB)

Benefit s

Suitab le for a wide range of wastewater

Effi ciently handl es high suspended soli ds wastewater Easy to mix , there by creating uniform reaction environment Large bioreactor volume to dilute inhi bitors Performance not dependent on sludge settleability Capable of accepting waste aerobi c biomass Simple and rel ati vely economical construction

Suit able for a wide range of wastewaters Efficientl y hand les high suspended solids wastewaters

Large bioreactor volume to dilute inhibitors Good performance possible Performance not dependent on sludge settleabi lity Capable of accepting waste aerobic biomass Suitable for Concentrated wastewaters

Easy to mix. there by creating uniform react ion envi ronment Relati vely high effluent quality achievable achievable Reduced bioreactor volume compared to AD Significant process control capabi lity available Hi gh bi omass concentration and long SRTs achievable

Small bioreactor volume due to high volumetric organic loading load ing rates High-quality effl uent achievable Mechanicall y simple compact system, relatively small land area well mi xed conditions produced in bioreactor

Anaerobic fil ter (A F) High biomass concentrat ion and long SRTs achievable Small bi oreactor volume due to high volumetric organic loading rates Hi gh-qu ality efflu ent achi evable Mechanically simple Compact system. rel ati vely small land area Performance not dependent on development of dense, settable solids Well mi xed conditions produced in bioreactor

Hybrid UASB/AF Hi gh biomass concentration and long SRTs achievabl e Small bioreactor volume due to high volumetric organi c loading rates High-qu ality efflu ent achievable Mechanically simple Compact system, relatively small land area Performance not dependent on sludge settleability land area well mixed conditions produced in bioreactor

Drawbacks

Large biorcactor volumes required

Effluent quality can be poor if nondegradable organic matter is present or ir a large concentrati on of anaerobic organisms is generated Process s~abilit y and performance poor at short SRTs Requires separate mechani ca l mixi ng

Relati vely large bioreactor volumes required

Large land area required Poorl y controlled conditions with1n biorcactor reduce eiTicicncy Limited process cont rol capabilit)

Biomass Settleabi lity critical to success ful performance Most suitable for waste with low to moderate levels of suspended solids System is relati vely complex mechanically

Shorter bioreactor HRTs mean less eq uali za ti on and dilution of inhibition Performance dependent on development of dense. settable sol ids Much lower process loading reqUi red if was tewater contains suspended solids Special bioreactor configurati on required which is based on ex perience Little process control possible Shorter bioreactor HRTs mean less eq uali zation and dilution of inhibition Suspended solid accumul ation may negative impact performance

ot suitable for high suspended solids wastewater

Little process control possible High cost for media and su pport Shorter bioreactor HRTs mean le s eq uali zation and dilution of inhibitors

Lower process loadings required if wastewater contains suspended solids Little process control possible

Little process control possible Shorter bioreactor HRTs mean less equali zation and dilution of inhibitors

Contd

RA IZADA et. a/.: WASTE MA AGEMENT & PRODUCTIO N OF FUTU RE FUELS 195

Table 6- Anaerobic treatment process compari son-organic stabi li zat ion39- Contd

Process

Downllow stati onary fixed film (DSFF)

Benefits

Red uced medi a cost

High biomass concentration and long SRTs achi evab le

Drawbacks

Biodegradable suspended solids not generall y degraded Hi gh cost for med ia and support

Small bioreactor volumes due to hi gh volumetric organi c loading rates

Organic removal rate genera ll y lower th an other hi gh­rate processes Little process control possib le

Fluidized Hi gh-quality efnu ent achi evable

Shorter bioreactor HRTs mean less eq uali zati on and dilution of inhibition Lengthy stary-up period required

bed/ex paneled bed (FB/EB)

Mechanicall y simple compact system. re lati vely small land area

High power req uirements for bed fluidization and expansion

Performance not dependent on sludge settleabilit y land area well mixed conditi ons produced in bioreactor

Not suit able for high suspended solids wastewater Mechanicall y more compl ex th an other hi gh-rare processes

High bi omass concentration and long SRTs achievable

Increased process con tro l req uired Cost of carrier media is high

Small bioreactor volumes due to high volumetri c organi c loading rates

Shorter bioreactor HRTs mean less equalization and dilution of inhibition

Excell ent mass transfer characteri stics Hi gh-qu ality efnu ent achievable, often better then other high-rate processes Most compact of all hi gh-rate processes; requires small est land area Performance not dependent on development of sellable sol ids Very well mi xed conditions generall y produced in boreactor Increased process control capability relative to other hi h-rate rocesses

Bio-ethanol From Waste

Transformation of cellulosic wastes , especially those resulting from paper mills-as cellulosic suspension through hydrolysis and fer mentation to useful products (sugar, ethanol) represents the most profitable way for thei r recover/3

. Although development of an economical process for bi o-energy (ethano l) production through enzymatic hydrol ys is of lignocelluloses is hindered because of the high cost of cellul ase production, low cellulase activity per unit volume and low concentration of sugar syrup obtained by the hydro lys is of ce llulose44

. A few processes for ethanol production have been reported and a ll of them suffer with these bottlenecks. A criti ca l analysi s of literature on enzymatic hydrolysis revealed that hi gh cellul ase activity per unit volume of ferme ntati on broth is the most important factor in obtai ning 20-30 per cent sugar from the hydrolysis of cellul ose for a process for ethanol production from such a materia l45

.

Many other wastes of organic nature (like vegetable , fruit, agricu lture wastes) are digested anaerobicall y to stabilize them. The organic ac id s

produced during acetogenesis stage in the process of anaerobic digestion works as a substrate for methanogens for conversion into methane and carbon dioxide (Table 8).

Government of India is funding waste treatment projects through var ious agencies like Mini stry of Non-conventional Energy Sources (MNES) and Indian Renewable Energy Development Agency Limited (IRADA) (Table 9)53

. Some of the waste treatment plants generating energy are li sted in Table 10.

Problems Leading to Instability of Anaerobic Digesters

Most common disturbances causing imbalance in anaerobic digestion process are organic load ing rates, the presence of inorganic or organic tox ins or other disturbances in process conditi ons such as temperature, pH and substrate composition57

.

However, combination of hydraulic and organic overloading (high influent concentration) of the reactor are the major causes of process failure, which are indicated by:

196 J SC II ND RES VOL 61 MARCH 2002

OFMSW

/~ composting ~ anaerobic ___. bioga~

digestion

(a)

OFMSW ----il•-""'cidogenic phase

~ ~ composting +--- methanogenic ______.. biogas phase

(b)

OFMSW _____..acidogenic ______.. phase

~ outlet sludge

I ~

volati.le fatty acids

composting ~ anaerobic -----..biogas digestion

(c)

Figure 9- Possible strmegies for the treatment of the Organic Fraction of Municipal Solid Waste (OFMSW)27

Mixed liquor volatile suspended olid (MLVSS) wash out.

2 Increase in total VFA , and VFA/alkalinity ratio of the system58

.

3 Reduction in the system COD removal efficiency.

4 A drop in the pH of the system . 5 A reduction in methane production, and 6 An in crease in effluent total VFA~ry .

Organic Loading

When the digesters are overloaded by an increase in influent concentration equivalent to 15.5 kg COD/m3/d, the concentration of hydrogen and short chain fatty ac id increases with the concomitant decline in gas production and COD removal efficienc/0

· 6 1 result ing in the inhibition of

stimulated methanogenes is. It may be due to a shift in metabolic pathway to a less favorable one, causing imbalance for VFA producers (Acidogens and acetogens) and consumers (methanogens and sulphate reducing bacteria) inside a reactor62

. Under these conditions, intermediates such as VFAs and alcohol accumulates leading to the process failure.

Table 7-Transformation processes used for the management of solid wastes

Transformation Process

Physical Component separation

Volume red uct ion

Size reduction

Chemica l Combusti on Pyrol ys is

Gasificat ion

BIOLOG ICAL Aerobic composti ng Anaerob ic digestion

Transformation means and methods

Manual and/or mechanical

Application of energy in the form of a force or pressure Application of energy in the form of shredding. grindi ng, mining

Thermal oxidation Destructive distillation

Starved air combustion

Aerobic biological convers ion Anaerobic biological

Transformation or principal conversion product(s )

Individual components found in commingled municipal waste The original waste reduced in volume

The ori gi nal waste component altered i form and reduced in size

C02• S02 and other oxidation products, ash A gas stream contai nin g a variety of gases. tar. oi l and a char A low-BTU gas. a char containing carbon and the inert originally in the fuel and pyrolytic oi l

Compost CH4, C02, trace gases. digested humus or sludge

Table 8-Potent ial of different wastes to generate biogas and methane

Waste category Type of waste Biogas, Llkg TS Methane, per cent Reference

Vegetables Asparagus, peas. cabbage. carrots, cassava peel , 300-450 50-70 46.47,48 cau lifl ower etc

Fruit waste Apple pomace, Mango peel, Apricot. Banana 400-550 50-60 47.48.49 stem & leaves. Orange etc

Agricul tural Corn cob, Cotton stalks. Rice straw. Wheat 300-500 50-70 50.51 waste straw. Sugar cane etc. Other wastes Lantana, Hydri ll a etc. 250-500 6-80 52

TS =Total so li ds

RAIZADA el. a/.: WASTE MANAGEMENT & PRODUCTION OF FUTURE FUELS 197

The degradation rate may decrease at low substrate concentration due to mass transport limitation and degradation of acetate is also in hi bited by low pH (due to acid accumulation) and moreover dissociated acid cannot penetrate the ce ll ular membrane6

-' . Thus anaerobic digestion process at short retention time is restricted by:

I The re lative slow rate of conversion of organic matter by anaerobic bacteria, so that the degree of waste conversion to methane Is reduced significantl y as its retention time is decreased below 10 d.

2 The growth rate of methanogens is relatively slow. The maximal rate of slowest growing bacteria is about 0.08 to 0.15 per day. Hence requires a minimal retention time of7-12 d and if waste retention time is reduced below 5 days it will lead to washou t of bacteria and process fai lure6

-'.

3 There is also a possibili ty of process fa ilure at short retention time through the accumulati on of gaseous of hyd rogen or/and VFA which inhibits further waste degradat ion either directly or through reducing the digester pH.

Pre-t reat111 ent of Agro- industrial and Municipal So lid Waste

Substrate Composition- Other major phys iochemical features limiting anaerobic digestion IS polymeric nature of waste biomass. Cellulose,

Table 9 - Methane recovery projects sanctioned by IR EDA: Industry wise di slribuiaiion53

S. No.

I

2

3

4

5

6

Industry No. of projects

Distillery 54

Yeast (baking) food processing

Pulp and paper

Pharmaceut ica l

Miscell aneous (Biogas boiler, mn f. loan for equipment)

Power generati on

Total

I

2

3

62

hemicellulose and amylopectin can be directl y fermented by anaerobic bac teria, whereas li gnin is tota ll y recalcitrant;. Lignin is a hi ghl y irregu lar polymer, with no precise chemical structure and conseq uently not read il y depolymerized compared to cellu lose and hemice ll ulose66

. The important fact is that lign in physically surrounds and not chemically (covalentl y) bound to ce ll ulose in biomass67

. Thus ligni n li mits the activity of microbial cellu lases and hem icell ulases by stearic hindrance. Moreover as it does not conta in any readi ly hydrolyzable inter­monomer bonds. it requires extremely ox idative conditions for biologica l depolymerization. Anaerobic microorganisms apparently have not evolved effective extracellul ar enzymes needed for depolymerizing lignin. Thus biomass rich in lignocell ulosic materia l must be pretreated to remove li gnin pri or to fe rmentat ion.

Several chemica l and physica l pretreatment meth ods to depolymerize li gnin without removing it from li gnocell ulosic structures have been employed. Most common are chemical methods I ike acid and alka li treatment and phys ical treatment meth ods like exposure to hi gh temperature and high pressure68

· 69

.

Other methods include pretreatment with sul phu r dioxide gas, ozone, pure ammoni a and H20 2Mn 2

+

solution, hydrogen flu oride vapours70, steam

exposure, hydrolys is of was te at higher temperatu re with aerati on, and reduction of particle size71

· 72

.

All the methods empl oyed so fa r tried to increase di gesti on of lignin ri ch li gnocellulos ic materi al. Dilute ac id treatment does not remove li gnin from the substrate but modi fies the lignin carbohydrate linkages. Therefore, li gnin still remains a barrier for enzymatic attack, thereby affecting the overall fermentation process. On the other hand, treatment with a higher acid concentration (e g, 70 per cent H2S04) leads to signi ficant loss of polysaccharides due to the secondary reacti ons. These reactions in turn could lead to the accumulati on of sugar degradati on products (furfurals) and release of some toxi c substrates which may act as inhibitors in microbial fermentation, leading to low enzyme

Table I 0- Energy generation capaci ti es of di ffereni anaerobic treatment plants

S.No.

2 3 4

Waste

Distill ary efnu en t Black liquor Bagasse Paddy waste

Quant ity of waste genetated

12000 m3/d

450 m3/d 520000 tly 30000 tly

Energy production Governing body Reference

IMW KM sugar mill s Ltd. 54

6050 KWh Paper and pul p industry 55 88 10 KW Sugar pl ant at Aj nala (Panjab) 56

4920 Sugar pl an t at Aj nala (Panjab) 56

198 J SCIIND RES VOL 61 MARCH 2002

production n Other problems include, scum format ion. cloggi ng of digester, high bacteri al sensitivity to environ mental conditi ons (mainl y pH, temperature and toxic compounds), long start-up time req uired which is clue to slow growth rate of methanogens and higher sensiti vityn.

Sulphate Reducing Bacteria (SRB)-It plays an im portant role in methanogenes is as in marine or brack ish waters where sulphate is abu ndant and cellulose is converted to carbon dioxide and hydrogen sul phide through SRB . These bacteria have a parallel metabo li sm to the methanogens and are able to utili ze hyd rogen and sulphate to produce hyd rogen sulphide. In sewage treatment facil ities and in freshwater logs where sulphate levels are low, the SRB enter into a symbiotic rel ati onship with methanogens wherein the SRB produce hydrogen from organi c ac ids and alcohol. And the methanogens in turn convert the hydrogen to methane and carbon dioxide75

. The process can be described in terms of the followin g equat ion 76 (Eq I ). The activity of SRB depends primarily on the availability of inorganic su lphate which is utili zed by these microorgani sm as the final electron acceptor in the respiratory chain . It follows that SRB can be used both for the reduction of sulphate and the removal of organic substance77

(Eq 2).

Equation 1

8H+ + 8e- + SO/· -----+IJJio

Equation 2

s2- + 4H o 2

Since key intermedi ates like hydrogen and acetate are e·-donors fo r methanogenes is and also for sulphate reducti on. The free energy change for acetate (-59.9 kJ by SRB and -31.0 kJ per reaction by MPB ) and hydrogen uti I izati on ( - 151 .9 kJ by SRB and -1 35.6 kJ per reacti on by MPB) is such that theoretically SRB should outcompete MPB. In practi ce, the free energy chan ges are dependent upon the activities of reactants and products. MPB can outcompete SRB if e·-donors (acetate and hydrogen): S04 ratio becomes high or when a build up of su lphide occurs. Sulphate-reducers have the abi li ty to outcompete methanogens in both natural habitats and

in defined anaerobic consorti a where su lphate is not limiting. Greater affinities fo r hyd rogen or acetate7s. the effect of hydrogen sulphide on methanogens bacteria and energy reasons 79 have been invoked to ex plain the competi tive advantage of sulphidogens over methanogens. Robinson et a/80 proves thi s by certain experiments and by Michaelis-Menten parameters of methanogenic and SRB. Hydrogen K111

es timates for most of the sulphidogens is between 0.7 to 1.9 11M which is lower than that for methanogenic bacteri a which is 2.5 to 13.0 11M.

On the other hand accumul ati on of sulphide, which can be toxic to other mi crobes because hydrogen sulphide is corrosive. Tox ic compound s may interfere with vari ous co-enzymes A & M sulphide linkages caus ing inhibition of these key metabolic protein s. The Sulphate- reducing bacteria are dependent on the reduction of su lphate as a mode of anaerobic energy-yielding metab !ism. Sulphate is therefore required as a nutrient at high levels, since it serves as the terminal electron acceptor. The range of utilizable organic substances is narrow, being confined for most strain s to a few organic acids, notably lactate, malate and pyruvate. The end product of the an aerobic oxidation of organi c substances in all cases is acetic ac id , since these bacter ia do not posses a functional tricarboxylic acid cyc le. Under favorabl e conditions Desulphovibrio can form very large amounts of sulphide during growth the concentration may attain a level of as high as I 0 giL. Unlike nearly all other strictly anaerobic chemoheterotrophs, Desulphovibrio spp contain heme pigments: speciall y, a cytochrome of 'c' type, which participates in the anaerobic electron transport system. The cells also contai n a colored protein , Desulphovibridin , the functi on of which is yet unknown .

Biological Hydrogen Production

Hydrogen is also an important source of energy. which represents a hi ghl y effic ient energy carri er. Production of hydrogen by anaerobic, facultative anaerobes 81

, methylotrophs, and photosynthet ic bacteria is possible. Hydrogen compares favo rab le with other fuels , which on combustion results in water and hence non-pollutin g and environmentall y safe. Hydrogen is an efficient fuel ( 122KJ/g). Hydrogen can be converted to steam, heat, electri city. etc. and therefore it is the most versati le fue l. It has the potenti al to be used as a fue l for automobiles, trucks, buses, aircraft, etc. Man y chemotrophs

RAIZADA el. a/.: WASTE MANAGEMENT & PRODUCTIO OF FUTURE FUELS 199

produce hydrogen from different carbohydrates. Hydrogen production from simple molecules like glucose, xylose, maltose and lactose is observed in Bacillus8 1

, Clostridium spp. and several anaerobes. Anaerobic mi crobes produce hydrogen and may degrade several other organic compounds including fatty acids82

• as well. In addition to simple sugar and carbohydrates . fermentation of raw starch of corn , potato and cassava pee l resulted in hyd rogen generation. Among the various wastes employed for hydrogen generati on. biowastes coul d produce 40L to I OOL of H2/ kg dry matter.

Criteria for Considering the Future Fuel Options Systematically

Trnnsportntion- Fuel shoul d be as li ght as poss ibl e and also take up as li ttle a space as poss ibl e for convenient transportation. Hydrogen being the lightest fuel with the bes t motivity facto r (of unity) is a staple fuel for a space program83 24

. Hydrogen is also a fuel of choice fo r aircrafts as well fo r automobiles84

.

2 Versa tility-Fuel must be converted with ease to other form s of energy at user end. Hydrogen has a maxi mum versati I i ty factor ( <1> V) of unity as compared to other fuels (<j>Vf = 0.4) as they can be converted th rough onl y one process i e, combustion. Hence hydrogen can be used fo r thermal and electric energy generati on, and also used fo r refrigeration and air-condi tioning83

· 84

· 85

.

3 Utilization Efficiency-For utili zation by the users, fuels transferred to vari ous energy fo rms such as mechanical, electri cal, and thermal and hydrogen can be converted to any des ired fo rm of energy more efficientl y than other fuels. Moreover, hydrogen is the most efficient fuel and hence resu lts in conservation of resources in add ition to conserving energl3

· ~4 . Hydrogen can also be used as chemical feed stock in industry as an ingredient in manufacturing chemica ls li ke ammoni a and methanol, foods (Vegetable oil ), and electroni cs24

.

4 Environmental compatibiliry-S ince utili zation of fuels affects the environment it is important th at fu el mu st not have an adverse effect on it. Hyd rogen combusti on when carried out in presence of air will lead to the fo rmati on of H20 and a very less amount of nitrogen ox ides (NOx) . Hence does not lead to the phenomenon of ac id rain or green house effect. Moreover when

hydrogen IS used for refri geration and air­conditioning its leakage will not lead to ozone layer dep letion and global warming8r' .

5 Scife-Thi s aspect of fuel invol ves their toxicity and fire hazards. The fuel combustion product may be tox ic poll utant and toxicity increases as the C/H ratio increases. While hydrogen and tt s ma in combustion product (water or water vapor) are not toxic. Although NOx produced through flame combustion of hydrogen displays toxic effect, but when the amou nt of toxic pollutants produced per uni t of energy consumed is considered, it is clear that hydrogen is the safest fuel fo ll owed by methane s1. 83

. Hydrogen has wide range of ex pl os ive concentration in ai r ( 4-75 per cent), which makes it a potentiall y good all around fuel87

. Moreover due to its hi gh volati I ity. rapid di ssemination in the event of sp ill age makes it safe to use83

· 88

.

6 Economics-The economica l comparison competing fuels and energy systems shoul d be based on the effective cost of the servi ces these fuels provide. The effec tive cost inc ludes the utili zati on efficiency, cost of fuel production, the amount of the physical damage done ro environment and living beings due to harm ful emi ss ions oil spill s and leaks etc. as we ll as expenditure for polluti on abatement.

On comparing all the candidates hyd rogen stands out as best poss ible fuel with unmatched unique properti es (Table I I )87

. Moreover hyd rogen also serves as an important feedstock, and its demand as feedstock has been proposed to increase by a factor of 27 over the next 45 l 8

. Although after energy cri sis of 1970 the hydrogen has become popul ar fuel source and it is considered as fuel for the fu ture85

. But as the prices of coal dropped other alternative energy technologies have no longer been persuaded. In 1990 due to concerns about green house effect, a new crisi s

Tab le 11 - Ph ysical characteristi cs of future fuel (1-12)

Propert y Uni t Va lue for I l l

Specific energy kJ/kg 119 19

Mass densi ty kg!m ' 70.8

Energy densi ty mJ!m' 8480

Volume kJ/m' 0.112

Boil ing point "C -253

200 J SCIIND RES VOL 61 MARCH 2002

regained interest in hydrogen as a fuel. as it is a long­term soluti on to the dep leti on of conventional fuels, as we ll as for globa l environmental prob lems84

•

As hydrogen does not exist naturall y in the earth crust in un combined state, there is a need to produce molecular hyd rogen ei ther by sp lit ting of water or by utili zin g bi omass82

. Various processes involved in biologica l hydrogen producti on are:

Direct Biophotolysis - This photosynthetic hydrogen production process is a two step process , where hydrogenase produces hydrogen and ferridoxin shutt le electrons from the photosynthetic membrane to the hydrogenase IEq. (3)1.

In thi s process, the reductant generated by the photosynthes is is direct ly transferred to hydrogenase via reduced ferridoxin. But a major obstacle for thi s process is that these two steps are incompatibl e i e, in the first step, water is spl itted to prod uce oxygen, whereas in the second step reducing power of electron is passed to protons to produce hydrogen via hydrogenase, which is strongly inhibited by 0 2.

Hence. process showspl a inherent feedback inhibition mechanism89

· 90

· 9 1

.

Heterocysrrons Nitrogen Fixing Cyanobacteria - In thi s process photobiological hydrogen production occurs by algae, that compartmentali zes the two separate reactions and uses carbon dioxide as intermediate to shuttle between two compartments. For e.g,. Anabaena cylindrica, a fi lamentous cyanobacteria compartmentalizes these into

IIJoPhotosystem fenidoxin

Equation 3

vegetat ive ce ll s, which generate oxygen from H20 and fi xing carbon dioxide, and specia li zed nitrogenase contam 111g heterocyst, which evolve hyd rogen when nitrogen reduction is blocked90

·92

fEq. (4)1.

Indirect Biophotolysis Non-heterocystrons Nitrogen Fixing Cynobacteria - This process makes use of non-heterocystron nitrogen fixing cyanobacteria that separates the hydrogen and 0 2

evolution steps temporaril y such as day-night cycle, especiall y through separate bioreactors rather than two ce ll types. Here too, carbon dioxide acts as an intermed iate, which joins two steps th rough carbon dioxide fixation and get released [Eq. (5)1.

The problem with nitrogen fixing bacteria for hyd rogen production is that nitrogenase has a hi gh ATP requirement, whi ch lowers potenti al solar energy conversion efficiencies to unacceptable leve l.

Photofermen tation PhotosyntheTic Bacteria- In this process photosynthetic bacteria in the' presence of li ght converts organ ic substrates, quantitatively into hydrogen and carbon dioxide fEq. (6) 1.

In principle, re latively littl e energy inputs are required and thus only small photo bioreactors shou ld be required. However measured photosynthetic efficiencies have been disappointing due to the high energy demands by the nitrogenase catalyzing hydrogen evolution in these bacteria and the bacteria operates at relatively low light intensiti es hence preventing efficient use of full sunlight intensity.

hydrogenase

C02 i NADPH i r H

20 ___. Photosystem ---l~~ [CH20h--•lll [CH20 2h___. Ferredoxin ----<IIJo• Nitrogenase

Vegetative cells Heterocysts

Equation 4

RA IZADA et. a/.: WASTE MANAGEMENT & PRODUCTION OF FUTURE FUELS 201

0 2 C02 C02

i l l NADPH

Hp ... Photosystem • [CH;..Oh ... [CH202h ... Ferridox.in ... Nitrogenase

151 stage 2nd stage t Photosystem ... ATP

Equation 5

NADP [CH20h • Ferridoxin ... Nitrogenase

I t ATP ...,..,....__ Bacterial ... ATP

Photosystem

Equation 6

Ferridoxin -----~•• Hydrogenase ___ ..,..,. H2

Equation 8

Microbial shift Reaction

Photosynthetic Bacteria - This is an another process. in volving photosynthetic bacteria, which acts as a cata lyst in the dark conversion of carbon monoxide to hydrogen [Eq. (7)1.

CO+HzO

Equation 7

Such a microbial "shi ft reaction " can accomp li sh the conversion at room temperature and in one step , in contrast to chemical cata lyst that requires high temperatures and multipl e stages. But thi s process is limited due to mass transfer limitations. which cou ld be overcome with gas phase in the reactors. Hence useful for small-scale effective classi fication of biomass to produce hydrogen.

Dark Fermentation - Another potentially eco­nomicall y viable approach is dark ferme ntation, which can convert waste biomass at higher yields to

hydrogen93. The model for such a process is

fermentation of waste biomass to produce methane. Hydrogen fermentation can use sim il ar hardware. Hence makes the process promi sing as in addition to waste treatment credits it could cover much of the costs of waste handling and processing. One rea l term option in this regard is to produce a mixture of hydrogen and methane in a two-stage process [Eq. (8)1.

The f irst step would produce hydrogen and organic ac ids, which would be converted to methane in second fermentation stage90

.

Classification of All Hydrogen Producing Organisms

Gray and Gest94, categoried a ll hydrogen

producing microorganisms into four groups:

Category I - Strict anaerobic heterotrophs I ike Clostridia, Micrococci, Methanobacteria etc. are categori zed in this category. These bacteria do not contain a cytochrome sys te m but have hydrogenase

202 J SCIIND RES VOL 61 MARCH 2002

enzyme and electron carri er ferr idoxin protein. The following scheme represents the reacti on of pyruvate degradation in Clostridium pasteurianwn: jEq. (9) ].

The presence of oxygen severe ly inhibits the generation of hydrogen, which indicates the interference or compet ition by oxygen with hydrogen as the termin al electron acceptor.

Category II - Heterotrophi c facu ltative anae­robes l ike E.co li that contain cytochromes and lyse formate to produce hydrogen in these microorgani sms. Formate is ox id ized to carbon dioxide and hydrogen by formate hydrogenase, which is a complex of formate dehyd rogenase and hydrogenase enzyme. The following scheme represents this sys tem: rEq. ( I O) j.

Category Ill - On ly one microorganism is categorized in this category because of its unique characteri st ic. Desulphovibrio desulphuricans is a heterotrophic strict anaerobe with hydrogen a

cytochrome system of low redox potenti al (E0 = -205 mY). Thi s can also use sulphate and hydrogen as terminal electron oxidant. The hyd rogenase enzyme is more like category I type. whereas the ac tual process of pyruvate degradat ion produces formic ac id intermediate like the mechanism f category II. Hence enzymes in D desulphuricans might represent a transition state.

Category IV - A ll photosynthetic micro-organi­sms are pl aced in thi s ca tegory. Photosynthetic bacteri a evo lve hydrogen from ADH and the process depends on l ight. Thi s category includes photosynthet ic su lphur bacteria and the anaerobic algae95

.

Hydrogen Production by Chemotrophs

Chemotrophic hydrogen producing bacteri a are widely distributed in nature. Some of them are symbiotic with human and other animals, while some species are pathogens of human, other animals or

------~~~ CH3CHOX ---1~~ 2Ferridoxin H2

Leo, l CHrCO-Co-enzyme-A

~

Equation 9

Cytochrome ... ~t-----­C552

Equation 10

1

reductase

RA IZADA et. a/.: WASTE MANAGEME T & PRODUCTI ON OF FUT URE FUELS 203

plan ts. Generall y, obligate and fac ultat ive anaerobic bacteria evolve hydrogen as a result of degradati on of organi c substrates resulting in ATP synthes is at subst rate level. The production of H2 hydrogen in some processes all ows microorganism to oxidi ze particu lar substrates more efficientl y, but not all subs trates can be direct ly oxidized by micro­organisms with the producti on of hydrogen.

ln many cases. degradati on steps are in volved leading to the fo rmati on of intermedi ates that are oxi di zed with the evolution of hydrogen. However fac ul tat ive anaerobic bacteri a evolve hydrogen in anaerobi c condition, but when these microorgani sms are ex posed to oxygen, hydrogen production is inhibited, although thi s inhibition may be reversible. Finall y. the accumulati on of hydrogen inhibits its own formation in many mi croorgani sms and in many cases where degradation of organic compounds is obligatory rel ated to hydrogen evolution . High partial pressure of hydrogen inhibits the growth of these mi croorganisms.

The inhibitory effect of hydrogen is usually strong when hydrogen formation is linked to the oxidation of NADH. As the redox potential of NADH/NAD+ (E'o = -320 mY) is more positive than that of H2/H+ (E' 0 = -420 m V) . For this reason, the oxidation of NADH to NAD+ and hydrogen is possible only at a very low partial pressure of hydrogen (about 1.5 X I 0'3 atm). In nature the low partial pressure of hydrogen is maintained as a result of interaction between hydrogen producing and hydrogen consuming microorganisms. This type of interaction between microorganisms is very important in rumen where up to 80 I of hydrogen may be formed in rumen of a cow everyday as a result of the fermentation of cellulose and other plant materials by different microorganisms. But there is no accumulation of molecular hydrogen because the methanogenic bacteria consume it71

•

Table 12 shows the amount of hydrogen produced by several chemotrophic bacteri a from pure simple substrate and was tes . But ca lculati on shows that effi ciency of energy conversion to hydrogen does not exceed 33 per cent of the combustibl e energy in organic substrates. In particul ar hydrogen yields are lower and effi ciency of energy conv:ersion is approximately 20 per cent.

Thus the producti on of hydrogen is limited and is not produced on a large sca le. However. if hydrogen producti on is coupl ed to the prod uction of other important compounds or with the removal of wastes (Table 13), it may be useful.

The producti on of hydrogen by different microorganisms is intimately linked with their energy metabolisms. In aerobic microorgani sms, the rel eased electrons from substrate oxidation are transferred to oxygen as ultimate oxidant whil e in anaerobic organisms, where the supply of energy is limited, electrons released from the anaerobic cataboli sm use many terminal oxidants such as nitrate, sulphate. However, obligate and facultative anaerobes use as the terminal electron acceptor. Thus here hydrogen production is one of the specific mechanism to

Table 12- Amount of hydrogen produced by several anaerobic bacteria from glucose as the substrate

Bacteria H2 yield mole/mole References

Bacillus licheniformis 0.58 96, 97

C beijerincki AM21 B 1.8-2.0 98

Clostridium spp. WO 2 1.9 99

Enterobacter aero genes 100 H039 Enterobacter aero genes 0.8-1.1 101

Pseudomonas fluores cens 0.03 102

Rhodobacter sphaeroides 0. 19 102

Syhechococcus cedrorum 0.01 103

Table 13 -Amount of hydrogen produced by anaerobic bacteria from waste biomass

Bacteri a

Mixed hydrogen culture

Bacillus lichenifo rmis

Rhodobacter sphaeroide

Waste Biomass

Pea shells

Damaged wheat grains Apple pomace, Tamarind effluent

Distillery waste. 5 per cent Distillery waste, 10 per cent

Rhodopseudomonas Dairy and sugarcane waste

#OS : Organic Solids ; *Red. : Reduction ; $Sub. : Substrate

H2 yield

119 L/kg OS# red *

74 L/kg OS red. 67 L/kg OS red

6 LIL Feed

266 mLIL sub$. 996 mLIL sub.

6 mUh/g dry cell

References

82

8 1 104

89

103

204 J SC IIND RES VOL 61 MARCH 2002

dispose of excess electrons through enzyme hydrogenase present 111

. . 90 mtcroorgantsms .

the activity of H2 producing

ow-a-days microbial H2 production by f"ermentative hydrogen producers is being extensive ly inves ti gated. Strict anaerobes had a stoichiometry of I :4 compared to I :2 for facultative anaerobes with glucose as substrate. Considering all these facts about production of hyd rogen by anaerobi c micro­organisms and their optimal stoi chi ometry, the later process is comparative ly simpler than former, hence reduces the cost of hydrogen production and thus makes the process economica ll y feas ible.

In las t 25 years hydrogen energy has moved in all fronts maki ng roads in all areas of energy. And in next 20 years the progress wi II be many fo ld greater and hydrogen energy sys tem will provide the planet earth with the energy sys tem, she deserves, wh ich is hospitable to life, clean and efficient. Hence hydrogen energy system will enhance the quali ty of l ife for the people or the world and help to preserve our biosphere. India has also recogni zed hydrogen as a fuel of great potential and l ikely to be the only source of energy avai lab le to man in years to come.

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