Anaerobic Digester for Treatment of Organic Waste

Home , Anaerobic digestion, Anaerobic filter, Hybrid reactor, Sewage treatment

ENTE PER LE NUOVE TECNOLOGIE, L'ENERGIA E L'AMBIENTE

Dipartimento Ambiente

ANAEROBIC DIGESTER FOR TREATMENT OF ORGANIC WASTE

V.K. SHARMA Visiting Scientist from Indian Institute of Technology, Delhi (India) Centro Ricerche Trisaia, Matera

F. FORTUNA, M. CANDITELLI, G. CORNACCHIA ENEA - Dipartimento Ambiente Centro Ricerche Trisaia, Matera

R. FARINA ENEA - Dipartimento Ambiente Centro Ricerche “Ezio Clementel”, Bologna

mmmmow of this document is unlimited

RT/AMB/97/17 Testo pervenuto nel luglio 1997

I contenuti tecnico-scientifici dei rapporti tecnici dell'ENEA rispecchiano I'opinione degli autori e non necessariamente quella dell'Ente. DISCLAIMER

Portions of this document may be illegible electronic image products. Images are produced from the best available original document. CONTENTS Page

1.0 INTRODUCTION 9

2.0 PROCESS DESIGN 9

3.0 ANAEROBIC TREATMENT PROCESSES 10

3.1 Mobilised Suspended Cell Systems 11 3.2 Mobilised Cell System 11

3.2.1 Fixed medium system 12 3.2.2 Completely mixed moving medium system 12 3.2.3 Fluidized medium system 12

4.0 START-UP OF ANAEROBIC TREATMENT PROCESS 12

5.0 OPERATION OF ANAEROBIC TREATMENT PROCESS 13

5.1 Single phase operation 13 5.2 Parallel operation 14 5.3 Two-phase operation 15 5.4 Multi-phase digesters 15

6.0 DIFFERENT ANAEROBIC REACTORS 17

6.1 Anaerobic lagoon 17 6.2 Conventional anaerobic digesters 18 6.3 anaerobic contact process 23 6.4 Upflow anaerobic filter 24 6.5 Upflow anaerobic sludge bed reactor 26 6.6 Anaerobic fluidized and attached-film expanded-bed reactor 27 6.7 Anaerobic rotating biological contacter 28 6.8 Anaerobic baffled reactor 28 6.9 Hybrid bioreactors 29

7.0 SOME WORKING DEMONSTRATION PLANT 30

7.1 Sequential batch anaerobic composting 30 7.2 KOMPOGAS - A new system for the anaerobic treatment of source separated waste 31 7.3 Semi-dry anaerobic digestion process of organic solid waste 32 7.4 Dry anaerobic conversion of MSW using DRANCO process 33 7.5 Innovative two-stage anaerobic digestion and aerobic composting process 34

7.6 An industrial plant for MSW treatment 35 7.7 Innovative plug-flow type reactor to treat semi-solid orthofruit waste 35

8.0 CHOICE OF ANAEROBIC TREATMENT PROCESS 37

9.0 BIOGAS AND ITS PRINCIPLE USES 39

9.1 Characteristics of digester gas 39 9.2 Purification of biogas by scrubbing unwanted gases 41 9.3 Principle uses 41

9.3.1 Gas as a cooking fuel 41 9.3.2 Gas as an engine fuel 41 9.3.3 Digester effluent as a fertilizer 42

CONCLUSION 43

ACKNOWLEDGEMENT 43

REFERENCES 44

ABSTRACT

The essential features of both new and more efficient reactor systems and their appropriate applications for various organic waste management situations, description of several working plants, experiences had in the past, etc. are discussed in the present communication. It is hoped that significant development reported here would be useful in opening a new vista to the application of anaerobic biotechnology for the waste treatment of both low/high organic strength and specialised treatment for toxic substances, using appropriate anaerobic methods.

Key words : Anaerobic biotechnology, waste characteristics, process design, anaerobic treatment process, biogas properties and its principle uses.

SOMMARIO

In questa relazione sono riportate le principal! caratteristiche dei reattori pin recent! ancorche piu efficienti, le application! piu appropriate per il trattamento di van zifiuti organic!, nonche le esperienze pregresse.

E’ auspicabile che le specifiche tecniche qui riportate possano essere di valido supporto nelle apphcazioni di biotecnologie anaerobiche per il trattamento di rifiuti a basso e/o ad alto contettuto organico nonche per quello di sostanze tossiche.

1.0 INTRODUCTION

Anaerobic methanogenic digestion, an effective method for the treatment of many organic wastes (industrial, agricultural and municipal wastes) with recovery of methane gas, is a topic of increasing interest throughout the world. Nutrient-deficient residues and wastes, which often occur in ago-industrial processing, can usually be treated anaerobically, without any addition of nutrients. During the last two decades considerable progress has been made in the understanding of the anaerobic process which leads to the development of many new configurations in reactor design.

A number of designs and their performance have already been described by several searchers thus providing insight into the design, performance and operation of various useful digesters which leads to the development of many new configurations in reactor design.

The anaerobic digestion is now a matured technology and a practical tool for practitioners. The significant development has opened a new vista to the application of anaerobic biotechnology for the waste treatment. For example, it is now possible to treat wastewater of very low organic strength with COD 200 mg/1, by anaerobic methods. Its application for concentrated waste with high percentage of suspended matter is well organised. The anaerobic biotechnology also shows promise for specialised treatment and can be accomplished for toxic substances.

Given this context, the essential features of both new and more efficient reactor systems and their appropriate applications for various organic waste management situations, description of several working plants, experiences had in the past, etc. are discussed in the present communication. It is hoped that significant development reported here would be useful in opening a new vista to the application of anaerobic biotechnology for the waste treatment of both low/high organic strength and specialised treatment for toxic substances, using appropriate anaerobic methods.

2.0 PROCESS DESIGN

During early 70 ’s, Lawrence and McCarty [1] selected SRP (Qs) technique as a design parameter for biological waste treatment process. SRP can be measured easily and all process variable could be related to it. The efficiency of organic removal at various SRP levels can be determined for a particular waste from laboratory and pilot-plant studies.

The relationship of effluent quality to SRP is shown in Fig. 1. With the decrease in SRP, the percentage micro-organisms wasted from the reactor is measured as reflected from the poor quality of the affluent. At some minimum SRP (SRP - min), the wastage of micro-organisms is higher then their reproduced which results in process failure. It is posable to operate the process at SRP min, but, with lower efficiency and poor stability. In practice, therefore, the designed SRP is kept 2 to 3 times higher than the SRPmm. At very high SRP a little increase in process efficiency is obtained. However, process reliability is increased in high degree.

9 Henze and Hairemues [2] have proposed design SRP for anaerobic treatments process at 35 °C. The loading rates are based on 90 - 95% removal of organic which in turn is based on effluents dissolved COD and influent total COD.

INFLUENT COD 1000 mg/1 ASSUMPTIONS: INFLUENT COD CONCENTRATION = 1000

u 400

_ 100 TREATMENT EFFICIENCY £ 200 80 H

60 taj

krj

T 100 h 40 O EFFLUENT COD CONCENTRATION m

20 i2i

SLUDGE RETENTION PERIOD, DAYS

Fig.l. Relationship of SRP to effluent quality and treatment efficiency.

Hydraulic Retention Period (HRP) required to achieve desired efficiency for particular waste can be determined from laboratory and pilot plant studies. The ratio of SRP/HRP is an important factor which directly reflects the efficiency and economy of the process. In an anaerobic treatment, because of the slow growth rates of methanogens, a very high SRP is beneficial in providing efficient and reliable treatment. A very high SRP provides caution against any adverse conditions that exists on account of shock loading temperature variations, pH changes and influence of toxic substances.

3.0 ANAEROBIC TREATMENT PROCESSES

The stabilisation of the organic matter of the wastes by anaerobic digestion may be carried out in several different types of treatment systems. Depending on the nature of bacterial growth within the reactor, the different systems are grouped into following two categories[3].

10 3.1 Mobilised Suspended Cell Systems

The mobilised cell systems are those where bacterial cells remains in suspension and in completely mixed stage. The mixing is provided by artificial means. The SRP/HRP ratio of 1:10 can be achieved. As shown in Fig. 2, Anaerobic Lagoon, Conventional Digesters and Anaerobic Contact Process, are the different systems that can be considered under this category.

Influent

(A) C onventional Digester (B) High-Rate Digester

Effluent

D igester Settling Tank

Fig 2. Different anaerobic mobilised (completely mixed) suspended growth system.

3.2 Mobilised Cell Systems

The immobilised cell systems, also termed as fixed film system, are developed in recent years. In these systems, bacterial mass is concentrated within the reactor by attaching themselves to an inert supporting media or by way of attaching themselves together to form large conglomerates which settle rapidly. Three basic processes are characterised by immobilised cell systems. 11 3.2.1 Fixed Medium System: The micro-organisms are attached to the inert stationary media forming bio-films. The systems are generally operated without recycle resulting plug- flow regime of the liquid within the reactor. The reactor may be operated in Upfiow and Downflow feed mode. Anaerobic filter reactor have also been referred to as fixed - bed reactors or packed - bed reactors. Anaerobic Upfiow Filter (UAF), Anaerobic Downflow Filter (DAF) and Downflow Stationary Fixed Film Reactor (DSFF), belongs to this category.

3.2.2 Completely-Mixed Moving Medium System: The micro-organisms are attached to inert media which are in the moving state within the reactor. The inert media and the liquid flow regime are uniform throughout the system Anaerobic Rotating Biological Contactor (ARBC) and Anaerobic Attached Film Expanded Bed (AAFEB), falls under this group.

3.2.3 Fluidized Medium System: The inert media with micro-organisms forming floes are fluidized by high upfiow liquid velocities, generally produced by a combination of the influent and re-calculation flow - rates. Fluidization offers mixin g of the bacterial mass uniformly throughout the reactor. When little or no recycle is required for fluidization, the liquid follows a plug-flow matter while biomass approaches completely mixed state. The rate of liquid flow and the resultant degree of expansion of the bed determines whether the reactor is termed as a fluidized - bed or an expanded - bed reactor. Anaerobic Fluidized Bed (AFB), Upfiow Anaerobic Sludge Blanket (UASB), Hybrid Reactor (combination of the basic types) and Anaerobic Baffled Reactor (ABR), falls under this category.

4.0 START-UP OF ANAEROBIC TREATMENT PROCESS

The major problem during the start up of an anaerobic treatment process is that considerably long period is required for establishing a balanced microbial culture for the waste under treatment. Since the acid-forming micro-organisms are fast growing as compared to methanogens, there is always a large risk of unbalance during start-up, because of the build-up of volatile acids which are not consumed by the methanogens at the same rate of their formation. It is, therefore, always necessary to keep the initial organic loading low, say 1-2 kg COD/m^-d, for a process having sludge concentration of 10-20 kg VSS/nA When the steady state conditions are established at low loading as shown by increased gas production, the loading can be increased by step-wise to reach an optimum loading.

When the acclimatised seed is already present for the waste under treatment, the start-up time will be shortened considerably. Such an seed can be kept for a very long time even for a year. In the absence of acclimatised seed, the reactor can be initially inoculated with municipal digester sludge, cow dung sludge, soil, etc.; along with waste. It takes about 1-2 months to start-up the process and about 4-8 months to reach steady state conditions. The very long time is required due to the slow-growth rates of methanogens. It is , therefore, very effective to provide heavy inoculum of methanogens during start-up. The effectiveness of such conditions in anaerobic filter has already been demonstrated experimentally.

It is very beneficial to minimise the biomass washout in the effluent during the start-up of the anaerobic treatment. This can be achieved by providing temporary solid liquid separation

12 during start-up like ultra-filtration or by high recycle. In starting-up full-scale sewage sludge digester, which are commonly with a volume of several thousands cubic meters, provision of sufficient amount of seed is not feasible. For practical reasons, the volume of seed is only few percent of the total reactor volume and therefore the concentration of methanogens in the seed should be as large as possible. The seed inoculum of 30 - 50% reactor volume is beneficial in speedily start-up of the process.

The temperature is also one of the major consideration in start-up of anaerobic treatment process. The optimum temperature of anaerobic micro-organisms is 35° C. It is, therefore, desirable to keep the temperature during start-up at 35° C irrespective of the temperature, aimed at steady - state operation.

In immobilised cell reactors the attachment of biomass to an inert support is the principle criterion of start-up and operation. The inert carrier with porous surface is found to be good surface for biomass attachment in comparison to smooth surfaces. In general, during start-up period, anaerobic process should be operated as constant as possible and fluctuating regarding temperature, pH, recycle ratio, HRP, etc., should be avoided. The gas production and pH of the effluent should be monitored regularly, under any imbalance situation organic load may be stopped till recovery of the process is gained.

5.0 OPERATION OF ANAEROBIC TREATMENT PROCESS

The anaerobic treatment of wastes can be carried out in different reactor systems, in single phase, in two phase or multiphase configuration, as discussed below.

5.1 Single Phase Operation

In single phase operation, different groups of micro-organisms are developed in the same environment to the extent that they are proportional to the flux of their respective substrates available within the system The conventional single stage anaerobic digestion unit (Fig. 3) is not heated or stirred. Charged material undergoes decomposition in a quiescent state, and the highly liquid substrate tends to stratify into a stabilised solid layer, an actively digesting layer, a supernatant layer and a scum layer.

In a well - balanced digestion, each group of bacteria will establish its own particular population size which in turn depends on the feed material, operating conditions (pH, temperature, retention period) and on the stoichiometry of the reaction involved.

The problems such like low loading rate, long retention time, poor efficiency and not optimal biological action , etc. are to be solved to create a successful single-stage digestion process. The most important modification involves increased contact between the micro-organisms and substrate through mixing. Other problems requiring the much needed attention include (1) elimination of the scum layer, (11) maintenance of uniform temperatures throughout the digestion tank, (111) inhibition of large particle settling, (IV) improved process control and (V) dispersion of potential metabolic inhibitors, such as volatile acids.

13 Gas Withdrawal

Active Layer

grit, etc

Solids Removal

Fig. 3. Schematic diagram of conventional single-stage digester.

Because of the long retention time required and its poor efficiency, this type of operation is rarely used any more. However, the single phase operation can further be carried out either in parallel reactors or in reactors in series (multistage).

5.2 Parallel Operation

In parallel operation the suspended solids of the waste are separated from the bulk liquid and both the liquid and solid wastes are anaerobically digested separately in two reactors in parallel The operation of septic tank is based on this principle. The solubility index of the wastewater is one of the main considerations in selecting single reactor or parallel reactor. In general, wastewater with high solubility index (0.8 - 1.0) and low solubility index (0.0 - 0.2) are treated in a single reactor, while the wastes with medium solubility index (0.2 - 0.8) can advantageously be treated in two reactors in parallel operation [2],

14 The multistage operation means the single phase anaerobic treatment carried out in two or more reactors in series. The multistage operation is advantageous because an ideal plug-flow pattern of the wastes is established which provides staging to the treatment progression. In each stage optimum environmental conditions and micro-biological flora exists to take care of the receiving inflow. The multistage operation may be beneficial for the treatment of complex nature waste.

5.3 Two - Phase Operation

The two - phase operation (Fig. 4), is based on the fact that anaerobic decomposition involves two distinct phases, acid formation and methane formation, which may be carried out in two separate reactors in series.

The solid organic matter is broken down during the first phase of the two-phase digestion by acid forming anaerobic bacteria, whereas in the second phase, a methanizer helps to convert the volatile fatty acids produced during the first phase into biogas. The associated methanogenic bacteria convert acetic acid and hydrogen to methane and carbon dioxide. The separation is possible because kinetics of the two phases are different.

It has been claimed by several researchers that two-phase digestion has a number of advantages over single-stage digestion, and that phase separation also leads to production of biogas with a higher methane content [4-6]. It has also been observed experimentally that phase separation is most significant in those cases in which acid formation is so rapid that methnaogenesis is suppressed in the acidification reactor by low pH values, high VFA concentrations and short retention times.

To conclude it can be stated that the two-phase operation may be applicable for all kinds of wastes with improved treatment efficiency at reduced overall reactor size. The sulphate bearing wastes can well be handled by two-phase operation to overcome the toxicity due to hydrogen sulphide. The influence of the toxic materials present in the waste can be minimised by two-phase operation such as the treatment of pulp and paper industry wastes.

5.4 Multi-Stage Digesters

The unique concept behind multistage digesters is based on the fact that anaerobic digestion is not merely a two-phase process, but rather multi-stage one involving many bacteria, having specific function in the step-by-step degradation of long chain polymers. Since optimal environmental conditions for micro-organisms vary from one group to another and from one species to another, it is essential that various biological reactions occurs separately under different environmental conditions. In other words, they must be isolated from each other. In order to achieve this goal, the concept of multi-stage digestion was suggested by many researchers.

15 Gas Witdrawal Gas Witdrawal

Sludge Mixed Supernatant Liquor Removal

Supernatant Mixing

Active Active Layer

Zone

Drain Solids Removal / Drain

Fig. 4. Schematic diagram of two-stage anaerobic digester.

Multi-stage unit basically consists of a series of reactors combining any number of digesters. The most commonly used multi-stage digestion system involves a series of continuously stirred tank reactors, controlled to provide variation in temperature and pH. A more sophisticated multi-stage digester (Fig. 5) has been designed, developed and experimented by J. Restrepo, in Italy [7],

Stage 1 Stage 2 Stage3 Stage 4

Fig. 5. General layout of multiple stage digester : 1 -grating and grease trap; 2-multichamber digesting septic tank; 3-anaerobic filter; 4- phytopedological filter

16 Based on the R&D work done so far on multi-stage digesters, it may be stated that no doubt the system increases gas yield, but the economic problems associated with the system are yet to be solved.

6.0 DIFFERENT ANAEROBIC REACTORS

Available results indicate that anaerobic digestion has a promising future. This section presents the description and essential features of most appropriate designs used for organic waste treatment. Emphasise would be given to both traditional and more efficient and environmentally attractive methods.

6.1 Anaerobic Lagoon

The anaerobic lagoon is the simplest form of anaerobic treatment device which is based on natural ecosystem. It is an open basin with a depth of 2.5 - 3.0 m and made up of earthen works only. The liquid through the depth is maintained anaerobically by controlling the BOD load. Low strength wastewater with BOD concentrations above 500 mg/L can successfully be treated by anaerobic lagoon. A high concentration of suspended solids in the waste is not preferred in this system.

The anaerobic lagoon provides 70-80 % reduction in BOD at the detention period of about 30 -35 days. The simplified anaerobic lagoon developed by Balsari et. aL [8] is shown in Fig. 6.

Studies on biogas production using anaerobic lagoons have also been conducted by Safely and Westman [9], The system designed and developed by these researchers is illustrated in Fig. 7.

GAS HOLDER GAS COLLECTION FLOATING COVER PIPE

Fig. 6. Schematic view of the covered lagoon biogas system.

17 0.97 cm ID Tygon tube

5.0 cm ID sch 40 PVC

Gas wet test meter Located on bank

fig. 7. Lagoon biogas collection and measurement apparatus developed by Safely and Westerman.

A very simple and low-cost design for biogas recovery using anaerobic lagoons has been developed and patented by ENEA together with AGRISILOS [10] S.n c. (Cremone, Italy), as shown in Fig. 8. This version is designed for big lagoons when higher gas pressure is required (max. 800 - 1000 mm H20). The only disadvantage is that production is related to ambient temperature which makes gas recovery from lagoons more suitable for mild conditions. Requirement of large land area, odour nuisance and chances of ground - water pollution, etc. are other main drawbacks of the system.

6.2 Conventional Anaerobic Digester

The conventional digester (Figs. 9 - 11), is the simplest man made ecosystem for anaerobic digestion. Prior to 1950, it was widely used and accepted as a reliable process for the stabilisation of primary and secondary sewage sludge. It is a closed system with the provision of gas collection. In conventional digesters, HRP is equal to SRP (SRP/HRP = 1). The process is more suitable for concentrated wastes, especially with relatively high levels of suspended solids. The retention periods of conventional digesters are generally kept longer in the order of 20 to 60 days, depending on the temperature at low loading conditions. A - impermeable sheet B - separating membrane C - biogas chamber D- air chamber G- separating membrane connecting point with impermeable sheet H - separating membrane ballast I - air pump L - gas outlet M- air tube P - gas collecting chamber Q - floating sheet holder R- one-way valve S- gas tube T - one-way valve U-gas holder V - impermeable sheet.

Kg. 8. Schematic of gas collection systemfloating on anaerobic lagoons. Medium pressure (800/1000 mm H20) model

During fifty’s "High-rate digesters" were developed to lower the retention period with higher loading conditions. The maintenance of digester at high-rate basis is obtained by agitation of the digester contents so that uniform mixture of bacteria and wastewater is achieved.

The mixing can be provided either by gas re-circulation or mechanical stirring. External re circulation of the digester contents was also employed but it was observed that internal re circulation is more efficient and practical. The detention period in high-rate digesters can be reduced to as low as 3-10 days.

19 GAS HOLDER

GENERATOR

WATER TRAP

DIGESTER

Fig. 9. Schematic view of batch designed by Balsari et al. hi India, the first digester for stabilisation of waste was reported to be operating at Lapefs Colony, Matunga, Bombay in 1897. The digester tank was also provided with gas collection facility and the gas generated has been utilised to derive the gas engine.

REMOVABLE MANHOLE COVER SEALED WITH CLAY INLET GAS OUTLET LOOSE COVER

GAS 1000 mm max

DISPLACEMENT TANK SLURRY

OUTLET PIPE

Fig. 10. Fixed dome digester (Chinese type).

20 gasoutlet

tiling pit gas older

.central guide outlet

•ound level

SLURRY SLURRY

outlet pipe

partition

Fig. 11. Floatinggas holder digester (Indian type).

It is true that much of the early work on digestion was done in India and China but in the successive years, as a result of significant R&D efforts throughout the World, considerable variation in terms of the type of the digester employed in various countries, has been obtained.

Designs such as bag and plug-flow types (Figs. 12-14) having significant potential to produce biogas with lower capital investments and higher efficiency levels, have been found both in developed and under - developed countries. But further development and demonstration of these systems is still needed to meet the reliability and economic conditions set by the World market.

21 5 0 mm GAS PIPE

LEVELLED

SURFACE SLURRY INLET OUTLET LAYER OF ABOUT 70 mm OF COMPACTED BACKFILL

Fig. 12. Bag-type digester.

GAS OUTLET FLEXIBLE MEMBRANE

INLET PIT OUTLET PIT

4\ \

COMPACTED BAKFTLL INSULATI SECTION

Fig. 13. Plug-flow digester

22 manure transport tunnel greenhouse

insulation robber Oner biogas to supplementary I storage

miring and pumping

heating pipes baffles waste

supplementary biogas storage

condensation trap hydraulic venting pipe security *- valve

Fig. 14. Schematic of plug-flow plant: longitudinal section of the reactor developed by ENEA, Italy.

6.3 Anaerobic Contact Process

The process is similar to aerobic activated sludge process widely used for the treatment of domestic sewage. The anaerobic contact process comprises of a completely - mixed digester tank unit followed by settling tank unit (Clarifier). The separated biomass from the settling unit is recycled to the digester unit to maintain solid retention time exceeding hydraulic retention time.

A major problem of the contact process is the poor settle ability of the biomass due to gas bubbles adhering to the sludge floes. The gas adhering to the floes can best be removed by de gasification prior to sedimentation. The best example of the application of contact process is the operation of a full-scale plant for the treatment of packing house waste which uses vacuum de-gasification to enhance the settling characteristics of sludge floes.

The "Anamet Process", based on the anaerobic contact process has been successfully used for combined anaerobic-aerobic treatment of wastewater from the food and fermentation industry [11]. The sludge from both anaerobic and aerobic stages are recycled to anaerobic stage. In another modification of anaerobic contact process, "Bioenegy Process", thermal shock treatment is applied to stop gas production temporarily just prior to entry of mixed liquor into settling unit [12]. The anaerobic contact process is suitable for the treatment of wastewater with intermediate strength (2000 - 10000 mg/L COD). The most promising advantages of the process are relatively low investment costs and capability of handling relatively high concentrations of suspended solids. The soluble wastes can also be treated successfully by this method. The only major drawback of the contact process is its limited loading capacity.

23 6.4 Upflow Anaerobic Filter

Coukier and co-workers in their exploratory work on domestic sewage treatment observed that an upflow rock filled column improves the effluent quality of an upflow contact digester. Later, Young and McCarty, extensively studied the rock-filled column for anaerobic treatment. It was the first reactor developed in the series of immobilised cell processes.

The anaerobic filter, is essentially a tall reactor (H/D = 8 - 10) provided with fixed - media matrix over which retention of anaerobic biological sludge is achieved to maintain any longer SRP. The treatment occurs when the wastewater flow is kept in an upward direction through the media matrix and comes in contact with the active micro-organisms present in the sludge. The media used can be of any material which provides a high surface area per unit volume, such as rock (void volume = 0.5) to plastic media (void volume = 0.9). Since than, anaerobic filters have received tremendous enthusiasm all over the World and many researchers as Arora and Routh [13], Raman [14], Kaul [15], Tilche [16-17], etc. have been engaged in laboratory experiments with this system

Laboratory scale research was carried out on this type of reactor (Fig. 15), fed with swine slurry by Bonfonti [18]. It gives excellent results with soluble wastewater even of low strength by combining a high loading capacity. The external solids separation and re-circulation is not necessary and treatment is possible at reduced temperatures due to very long SRP. The process is highly stable and needs minimum control. It can accept variable and severe shock loads and can be restarted within a short period even after prolonged period of shut-down. The energy requirement of the system is low.

G holder;

Fig. 15. Schematic of anaerobic filter.

24 The potential problem of upflow anaerobic filter is the clogging of the voids due to sludge build-up which causes short-circuiting of the waste flow. The major portion of the sludge produced in the process remains loose in the interspace of media matrix rather getting them attached to the media. After some period of operation, anaerobic filter will eventually get clogged. The problem of clogging due to sludge build-up in the interspace can be overcome either by periodic washing of the filter-bed or by draining of the sludge entrapped in the interspace.

On the other hand, the problem could also be solved by operating the anaerobic filter in a down-flow mode. The system, as shown in Fig. 16, is thus called as Down-flow Stationary Fixed Filter (DSFF). Upflow Anaerobic Filter actually performs as combination of Fixed-film and Upflow Sludge Blanket Process where the biological activities is mainly associated with the unattached sludge present in the voids [19]. The DSFF actually represent anaerobic filter as a true biofilm reactor. The efficiency of DSFF is somewhat less as compared to the Upflow Anaerobic Filter. This is mainly due to loss of suspended sloughing into the effluent.

GAS

SUPPORT MATERIAL FOR FILM DEVELOPMENT

Fig. 16. Sketch of downflow stationary fixed film reactor.

The Down-flow Stationary Fixed-Film is capable of handling a wide variety of wastes because of its design and the fact that waste is added at the top of the reactor. The suspended growth developed in the reactor is removed with the effluent by operating the reactor in the down mode. The most important aspect of this reactor is the formation and stability of an active biomass film on the surfaces provided. This reactor is not suitable for the treatment of highly diluted wastewater.

25 Despite the feet that a considerable amount of research has been done on anaerobic filter, relatively few commercial systems appears to have been installed. Most research has been conducted in the industrialised countries, and there is no full scale anaerobic filter in operation in the developing countries.

6.5 Upflow Anaerobic Sludge Bed Reactor

This type of reactor (Fig. 17), was developed to avoid the problem of accumulation in the solids in the anaerobic filter packing. The packing was reduced to a simple gas collection device that encouraged the settling of the suspended solids. The process designed by Lettinga et. el, is based on the principle of natural flocculation and agglomeration of the micro organisms, so as to form discrete microscopic granules of biomass[20-23]. The reactor consists of a circular tank (HZD = 2), in the lower portion of which biomass granules are concentrated and form sludge- bed which provides active zone of microbial metabolism.

GAS

SEDIMENTATION AREA

COLLECTOR

BLANKET

Fig. 17 Sketch of upflow anaerobic sludge bed.

26 The sludge-bed also acts as a filtration media for up going waste and is capable of withstanding high mixing process. Results of investigation carried out by Lettinga and co-workers have made it possible for Upflow Anaerobic Sludge-Bed Reactor (Fig. 17), to work with high concentration of biomass, and this has produced very high loading rates.

The main problem with this reactor is the development of granular sludge, in the same way that a few wastes such as sugar processing wastes and wastes consisting mainly of volatile adds, often results in the formation of this sludge. It is therefore necessary to have gas/fiquid/solid separation device either internally or externally to separate the sludge floes from the effluent.

6.6 Anaerobic Fluidized and Attached-Film Expanded-Bed Reactor

The concept of film formation by micro-organisms is further extended in the development of Anaerobic Attached-Film Expanded-Bed Reactor (AAFEB). The reactor is a closed tall column partly filled with inert support media such as sand, gravel, anthradte or plastic which are expanded by the upflow velocity of the waste assisted by recycle [24].

The reactor (Fig. 18), is very similar to suspended growth reactors in which active biomass becomes an aggregate bed that settles easily. A rapid, even flow of liquid is used to keep the particles in suspension. The reactor is called a fluidized or expanded bed reactor depending on the rate of liquid flow and the degree of expansion of the bed. It has been reported that the performance of this type of reactor depends upon the uniformity of the liquid flow. Thus, a high and very uniform upflow of liquid has to be maintained in the reactor. No fluidized or expanded bed reactor has been put into operation on a large scale, however. GAS

DIGESTED LIQUID

SAND TRAP

FLUIDIZED

EXPANDED BED

RECYCLE PUMP

FEED

Fig. 18 Sketch of anaerobic fluidized or expanded bed.

27 6.7 Anaerobic Rotating Biological Contacter (ARBC)

The Anaerobic Rotating Biological Contacter [25], is essentially the same as that used for aerobic system except that is completely closed. The circular discs made of plastic mesh, PVC, polystyrene foams or just plain asbestos, are enclosed in a closed tank filled with wastewater. The discs are rotated at a speed of about 3-6 rpm The micro-organisms are attached to the discs surfaces thereby providing larger surface area that are exposed for contact with the waste. The discs are spaced at least 2 - 3 cm apart and they are installed in number of compartments to facilitate plug-flow pattern to the liquid within the reactor.

6.8 Anaerobic Baffled Reactor

Anaerobic Baffled Reactor (ABR) is a modification of Upper-flow Anaerobic Sludge - Bed Reactor. This reactor shown in Fig. 19, consists of a simple rectangular tank divided into five or six compartments of equal volumes by means of walls running from the root to the bottom of the tank.

The liquid flows upwards and downwards between the walls. On its upward passage, the waste flows through an anaerobic sludge blanket, hence, the waste is in intimate contact with the active biomass, but because of the baffled nature of the design, most of the biomass is retained within the reactor, even with large hydraulic shocks.

Experimental work on the design of this reactor was carried out in China. This type of reactor appears to be able to treat wastes with high solid content, and hence, it may be a viable alternative in certain situations observed in developing countries. A 10 m^ unit has been created by Stuckey to treat distillery wastes [26-27].

Fig. 19 Anaerobic baffled reactor.

28 6.9 Hybrid Bioreactors

The Hybrid Bioreactor [28-29], represents the recent generation of reactor with potential to incorporate the advantages of both biofihn and suspended solid reactors The reactors, in question, offer the advantages of the UASB concept associated to the ones of the Anaerobic Filters, and nowadays can be considered more suitable for the treatment of a series of soluble or partially soluble wastewater than can many other reactor systems.

Several different designs of hybrid reactors have been proposed up to now. The majority of the laboratory and full scale examples of hybrid reactors have been realised following a simpler design, where the filter is located in the upper part of the reactor without any gas/solid/liquid separation device.

Recent studies have been carried out on anaerobic digestion of the liquid fraction of beef cattle waste using this type of reactor. The reactor had a suspended growth zone at the bottom and an upflow filter at the top. The plant used was a 4 m high and 0.63 m wide (diameter) vertical reactor. The working volume was 1.12 m^, and specific filling surface was 100 m^/nP (Fig. 20).

A - suspended growth zone; B - upflow filter; C - unloading pipe; D - biogas exit pipe.

Influent

Fig. 20. Sketch of the hybrid bioreactor.

29 Experiments have also been carried out on a Hybridised Anaerobic Baffled Reactor operating on molasses stillage evaporator condensates (F. De Poll et al, 1988). The plant consisted of three chambers and a final setter, as shown in Fig. 21. The working volume of the three chambers was 150 fitters. The settler volume was not considered as an active volume, because any sludge was always recycled back to the first chamber. From an efficiency point of view, this type of reactor resulted in the destruction of up to 98% of COD.

Gas Meter

Effluent Flow Meter

Nutrients NaOH

Sampling Port

Recycle

Peristaltic Pump

Fig. 21. Schematic of hybridised anaerobic baffle reactor

7.0 SOME WORKING DEMONSTRATION PLANT

7.1 Sequential Batch Anaerobic Composting

The concept of sequential batch anaerobic composting (SEBAC), developed at Agricultural Engineering Department, University of Florida (USA), has been used to overcome the limitations of most designs for anaerobic digestion, such as, requirement for heavy inoculation, mixing, possibility of instability, etc. [30].

30 A pilot plant, shown, in Fig. 22, with three batch reactors (2.4 m high by 0.6 m ID) and all ancillary equipment was constructed and operated at the University of Florida. The plant was used to treat two fractions of municipal solid waste (MSW), the organic fraction of the processed MSW and yard waste. The sequential batch anaerobic composting of the two primary organic fractions has been reported to be stable, reliable and effective. However, thorough understanding of the physical, chemical, and biological dynamics of high solids, batch systems, etc. has been recommended.

Hydrolysis Products and Volatile Adds From Stage 1 ------—

Stage 3 Stage 1 Stage! New Mature Old

Fig. 22 Schematic of the sequential hatch anaerobic composting(SEBAQ process.

7.2 Kompogas-A New System for the Anaerobic Treatment of Source Separated Waste

KOMPOGAS - a new digester system, designed specifically to treat fruit, yard and vegetable wastes with 15 - 40 % total solids, has been reported by INFOSOLAR (Switzerland). As shown in Fig. 23, the system is a CSTR-type horizontally positioned Cylindrical digester of volume 15 m3 equipped with a hydraulically driven stirrer which takes up strong shear forces [31]-

31 Based upon the preliminary experimental results, it has been concluded by the authors that the digester performed best at TS-concentrations of around 27%. From the test run it was observed that the system under investigation is best suited to treat substrate with low dry matter content.

Encouraged by the satisfactory experimental results, a first full - scale installation with a digester volume of 200 m3, was installed in January 1992. The system capable to treat 3000 tons of fruit, vegetable and yard wastes per year, is in operation. A combination of anaerobic, aerobic, mechanical and chemical treatments for the reduction of CBS, is currently under investigation. The results obtained from the types and order of the treatments currently under experimentation will be used to minimise the energy requirement and the cost.

Heat m Ex.

Digester Shredder Hi *>>1 rj==ji „ ' Piston , Piston Cogenerator / h Compost

Fig. 23 Flow sheet of the KOMPOGAS process.

7.3 Semi-Dry Anaerobic Digestion Process of Organic Solid Waste

Based upon the concept of semi-dry (20% of dry matter) anaerobic process two full-scale industrial plants have been installed in Italy [32-33]. The first plant managed by the General Municipal Services Agency of Verona (AGSM), an anaerobic thermophilic semidry digestion plant, is able to handle nearly 500 tons of MSW each day (designed capacity). When operating at a daily capacity of 200 tons/day, the plant is able to produce 26.000 Nm3 /day of biogas and 42.2 tons/day of dewatered & dried sludge for 7 days/week.

The scheme for the mechanical dewatering of the digester effluent is shown in Fig. 24. The co generation section provides 2 Mwe of energy at the terminal clamp, about 1.4 Gcal/h like hot water.

The second plant installed at Avezzano (Central Italy), is used for anaerobic thermophilic senri- dry co-digestion of sludges and organic fraction with total capacity of nearly 40 tons/day[33]. The organic fraction fed to the anaerobic co-digestion is 15 t/d to which are added 22t/d of sludges from sewage treatment plant.

32 Digester Gas holder Digester

////-/ C h Digested Dewatering i matter system m n e

Heat y Heat Boiler Exchanger Exchanger

Fig. 24. The scheme for the mechanical dewatering of the digester effluent

The digester has been planned assuming that the concentration of dry matter could vary between 12% and 20%. These concentrations are guaranteed by recycling of water from dewatering of digested sludges. The digestion has been optimised using mammoth pumps and introducing the fresh sludge in three different points at the base of the reactor.

From the experiments conducted, it has been observed that the plant is able to produce nearly 2.500 Nm3 /day of biogas and 39 t/d of digested sludges (10% dry matter) dewatered by press filter until the concentration of 30% of dry matter (out flow 14 t/d). The dewatered digested sludges are then co-composted with part of the fresh organic fraction (38 t/d), producing about 41 t/d of rich composts (55% of dry matter).

7.4 Dry Anaerobic Conversion of MSW using Dranco Process

The Dranco process was developed for the conversion of solid organic wastes, specifically the organic fraction of municipal solid waste (MSW), to energy and a humus-like final product, called Humotex. A demonstration plant of 56 m3 is in operation since several years in Gent, Belgium using mixed garbage as feedstock between 1984 and 1991, and source separated waste since July 1991 [34].

33 Another Dranco installation for treating 10.500 tons of source separated waste per year has also been constructed at Brecht, Belgium. The digester have a volume of 808 m3 with a diameter of 7 m and a height of 21 m The plant is already in operation since springtime of 1992. The biogas produced from the plant is stored in a has bag and partially (about 80%) used to produce steam needed for process heating. The rest of the biogas is mainly transformed into electricity by means of a 280 kW gas engine. The electricity is partially used for the operation of the installation and partially sold to the grid.

7.5 Innovative Two-Stage Anaerobic Digestion and Aerobic Composting Process

A relatively recent advance in the field of anaerobic digestion is the development of the high- solids anaerobic digestion process, sometimes called “dry fermentation”. In this process, anaerobic digestion takes place at total solids concentration greater than 20%. It is however to be noted that since the introduction of the high - solids anaerobic digestion process for waste stabilisation by Jewell in 1979 [35], only limited research activity has taken place. A recent study of the high-solids anaerobic digestion process has been conducted by Chynoweth et aL, in 1990 [36].

Pilot investigations of an innovative two-stage bioconversion process have recently been reported by M. Kayhanian et aL, in USA [6], The high - solids anaerobic digestion/aerobic composting process, shown in Fig. 25, is a two-stage process.

Heat Biogas Heat Air

Soil amendment Organic fraction of ------► MSW High - solids anaerobic Aerobic ------► digester Composter

------► Complete - mix Complete-mix Fuel for reactor reactor power plant

Fig. 25 Basic flow diagram for the UC Davis complete-mix high-solids anaerobic digestion/aerobic composting process.

The first stage involves the high solids anaerobic digestion of the organic fraction of MSW to produce biogas whereas the second stage involves the aerobic composting of the anaerobically digested solids to increase the solid content from 25 to 65 percent or more depending on the final use of the compost.

34 The pilot-scale experimental plant has been demonstrated for various mass retention time over a period of one year. From the experiments conducted, it has been observed that the process was stable and relatively easy to operate.

7.6 An Industrial Plant for MSW Treatment

The first world industrial plant installed at Amiens in France, is now operating for more than eight (8) years[37]. The Valorga process is a technique for processing biodegradable organic wastes by anaerobic digestion. The plant is particularly suited for treating the fermentable fraction of Municipal Solid Wastes (MSW) coming from source separation, but it permits also to treat bulk MSW because of its optional sorting line. The process runs at industrial scale for biological aspects.

The plant is treating the total production of household wastes of the city (55,000 t/y). But the maximum treatment capacity reaches 65,000 to 72,000 t/y according to the kind of collection. Performances control and process monitoring have shown good average results in stabilised conditions. The average production of biogas is 146 Nm3 per ton of sorted MSW introduced into the digester or 210 to 240 m3 methane per ton of volatile solid introduced into the digester.

7.7 Innovative Plug-Flow Reactor to Treat Semi-Solid Orthofruit Waste

Designs such as bag and plug-flow types having significant potential to produce biogas with lower capital investments and higher efficiency levels, have been found both in developed and under - developed countries. The principle objective of innovative plug flow reactor developed at ENEA research centre, Trisaia (southern Italy), was to provide low initial investment, high efficiency and relatively simple operational and maintenance operations [38].

As shown in Fig. 26, Cylindrical shaped anaerobic reactor, is 350 cm long, 70 cm in diameter and has an internal volume of nearly 1.35 m3. Contrary to the conventional practices, the reactor has been installed at an inclination of 20° with respect to the ground level. The body of the reactor is heated using auto-controlled electrical resistance fitted along the length of the reactor.

To measure the quantity of biogas produced, two gas meters (commercial (ELKRO mod. BKP) and delicate (ELSTER)), have been fitted on the pipe line meant for the exist of the biogas produced. On the other hand, to quantify both the percentage of CO2 and CH4 produced and controlling O2 pollution, if any, a gas analyser (ADC mod. LFG-20) has been provided. The experimental data has been recorded regularly using a Data logger, mod. 605 fromDATATAKER.

A set of experimentation have been conduced. The semi-solid wastes available from wholesale fruit and vegetable markets mixed together with sewage sludge were used as input feedstock material. The data obtained from the experimental observations has been analysed and the results obtained have already been submitted for publication.

35 Data acquisition system Gas flame

jp Gas analyser

Gas meter Grinder Hydraulic check Hydraulic check

Hopper

Plug-flow type reactor Weighing machine

Fig. 26 Pilot-scale experimental plug-flow type reactor installed at ENEA research centre Trisa ia, in Italy.

36 8.0 CHOICE OF ANAEROBIC TREATMENT PROCESS

With the availability of different anaerobic treatment systems, question arises as to which treatment system should be used. The answer to the question is not simple because anaerobic treatment is applicable to the different wastes with broad range of characteristics. The selection of the system will therefore be dependent on individual consideration and the solution will depend specifically on the characteristics of the waste to be treated.

In order to express the advantages and disadvantages of different systems, an attempt has been made to compare the different anaerobic treatment systems as shown in Table 1[39],

Table 1. Comparison of different anaerobic treatment processes.

AL CD CP AF AAFEB UASB ABR An Fluidized RBC bed High organic loading P F F G G G G G capacity High hydraulic loading P P F G F F G G capacity Tolerance for suspended F G F P P P F F solids Tolerance for shock loads F P F G G G G G Little need for control G P F F F F F F Short start - up time P P P P P P P P Tolerance for toxic shock P P F G G G G G loads Low investment costs G F F P P P P P

AL: Anaerobic Lagoon; CD: Conventional Digester; CP: Contact Process; AF: Anaerobic Filter; AAFEB: Anaerobic Attached Film Expanded Bed; UASB: Upflow Anaerobic Sludge Blanket; ABR: Anaerobic Baffled Reactor; An RBC: Anaerobic Rotating Biological Contacter. G: Good, F: Fair and P: Poor

It is now very well recognised that efficient anaerobic treatment of waste is strongly dependent on attainment of longer sludge retention. Because of the slow growth rate of methanogens, a sudden loss of sludge cannot be tolerated and therefore immobilisation of biomass in one or another form is advantageous to the anaerobic treatment.

The anaerobic lagoon is the simplest treatment device with low investment cost and minimum maintenance requirement. It may be considered only where sufficient land area is available, hi man-made reactors system, number of better alternatives are now available to conventional

37 digesters. The low loading capacity and the problem of stability makes the conventional digesters unattractive where there is possibility of sludge retention. It may be considered only for waste materials with very high degree of suspended solid where sludge retention is practically impossible.

The anaerobic contact process is a straight forward process and can be a system of choice for many instances. It is particularly suitable for wastes containing moderate concentration of suspended solids. Due to its low investment cost, longer treatment time can be allowed. The longer treatment is advantages for waste which contains significant amount of complex organic matters. The later are hydrolysed to a large extent and results in higher overall efficiency when compared with more highly loaded treatment systems.

The anaerobic filter has very good stability against any adverse conditions/situations and requires minimum control. The process is especially suitable for dissolved wastes only. The drawback of this process is its very high investment cost due to costly packing material. The upflow sludge blanket, expanded and fluidized bed and baffled reactors are working on principle of fluidization and are capable of undertaking very high organic loading as compared to other reactor systems. The drawback of the process is the danger of sludge loss in the effluent during very high hydraulic loading. The processes are suitable for treatment of relatively soluble wastes.

Anaerobic rotating biological contactor is an attractive alternative treatment system as it combines the phenomenon of biofilm attachment and mixing of the reactor contents. However, the complex construction of the process together with high capital investment restricts its proper utilisation.

Anaerobic Baffled Reactor (ABR) is a modification of Upper-flow Anaerobic Sludge - Bed Reactor. This type of reactor appears to be able to treat wastes with high solid content, and hence, it may be a viable alternative in certain situations observed in developing countries. The Hybrid Bioreactor represents the recent generation of reactor with potential to incorporate the advantages of both biofilm and suspended solid reactors. Recent studies have been carried out on anaerobic digestion of the liquid fraction of beef cattle waste using this type of reactor. From an efficiency point of view, this type of reactor resulted in the destruction of up to 98% of COD.

38 9.0 BIOGAS AND ITS PRINCIPLE USES

Biogas can be used in similar ways to natural gas or other gaseous fuels to produce heat or to run an internal combustion engine to produce mechanical or electric energy. Before discussing the uses of the gas as an energy source, it is important to discuss the characteristics of biogas and methods for its purification and storage.

9.1 Characteristics of Digester Gas

Digester gas mainly consists of methane (60-70 %) and carbon dioxide (30-40 %) with very small amounts of other gases like hydrogen sulphide, hydrogen, carbon monoxide, nitrogen, oxygen, ethane and other low molecular weight hydrocarbons. The gas is combustible with a calorific value of about 26 MJ/m-.

The physical properties of the principal gases, ie. methane and carbon dioxide are given in Tables 2-3 [40-42]. The amount of hydrogen sulphide present in the digester gas shows a wide variation. Normally, the expected value for a digester working well on sewage sludge is between 100 and 3000 mg B^S/m-*, as shown by Edward [43]. It reacts readily with metals or their oxides. It is very toxic and even in low concentration, can be hazardous. The physiological effects of H2 S gas are given in Table 4 [44].

Physical constant Amount

Molecular weight 16.04

Boiling point at 1 atm - 164.0 °C Freezing point at 1 atm - 182.5 °C Density at 0°C, 1 atm 0.7 kg/m 3 Critical temperature - 82.5 °C Critical pressure 4.63 MN/m3 Heat capacity Cp at 15 °C, latm 2.22 kJ/kg °C Ratio Cp/Cv 1.31 Flammable limits in air 5.3-14% by volume Calorific value at 15 °C and 101.325 kPa 37.7 MJ/M3

Table 2. Some physical constants of methane

39 Physical constant Amount

Molecular weight 44.01

Triple point at 5.11 atm - 56.60 °C Sublimation point at 1 atm - 78.50 °C Density at 0°C, 1 atm 1.97 kg/m 3 Critical temperature 31. 0°C Critical pressure 72.8 atm Heat capacity Cp at 0° C, latm 0.822 kJ/kg °C Ratio Cp/Cv 1.30

Table 3. Some physical constants of carbon dioxide

Concentration of H2S (%) Physiological effects

0.0002 Just detectable by smell 0.002 Maximum allowable concentration for prolonged exposure; at this concentration, causes eye irritation. 0.007 - 0.015 Slight symptoms 0.017 - 0.03 Can be inhaled for 1 h without serious lasting effect 0.04 - 0.06 Dangerous if inhaled for 1/2 - 1 h 0.06 Fatal after 1/2 h 0.1 Paralyses sense of smell and causes instant unconsciousness

Table 4. Concentration of H%S in air and its physiological effects.

40 9.2 Purification of Biogas by Scrubbing Unwanted Gases

In large scale applications of biogas, the excess gas has to be either stored or transported efficiently. Another way to utilise the gas is to inject it into the natural gas pipeline system which requires purification of the gas to meet pipeline specifications. There is a need to remove impurities, such as hydrogen sulphide, carbon dioxide and moisture. Before the purified gas is piped into the transmission lines, its compression is also necessary. There are various methods available for commercial application for scrubbing unwanted gases from biogas.

(A) Method for CO2 Removal: (I) Absorption into liquid, (II) adsorption on a solid, (HI) cryogenic separation, (IV) chemical conversion to another compound and (V) membrane separation.

(B) Methods for H2 S Removal: (I) Dry oxidation process and (II) liquid phase oxidation process.

(C) Dehydration methods: These process can selectively remove either CO2 or H2 S alone or both the impurities simultaneously, but these differ in their effectiveness for removing impurities. There is a need to select an appropriate process for a particular application which involves consideration of the gas composition and amount of gas to be treated, impurity control and the degree of purity required. Most commercially available process are proprietary in nature.

A preliminary survey of various gas purification technologies indicates that viable methods for the treatment of digester gas include physical absorption, chemical absorption, adsorption, and membrane separation process.

From the results of an economic and technical analysis of these systems, water scrubbing and membrane separation processes are the most economical for the treatment of the digester gas to meet pipeline specification.

9.3 Principle uses

9.3.1 Gas as a Cooking Fuel: Stoves capable of burning biogas efficiently are designed differently, as it has different properties compared to other commonly used gases, and it is available only at quite low pressures.

9.3.2 Gas as an Engine Fuel: Besides operating heating equipment with biogas, the second most important way of using it may be for fuelling engines to generate electricity or mechanical energy. However, so far, only in a few cases, the biogas engines have been actually used for water lifting, pumping, electricity generation or running a car.

As the gas output from a digester may vary, there is a need to have a stand-by system to provide the farm or works with power when demand exceeds gas supply. Such dual flue type engines are good for static operation, since flue can be stored in quantity close by.

41 Motive engines requires transportable flue, which often rules out biogas since it is difficult and expensive to compress it to an acceptable form. However, small farm tractors have been run for limited periods using gas stored in 1-2 nP capacity biogas mounted on the roof of the tractor. It is to be noted that tests have also been performed with a generator set composed of a 6.5 kW three-phase alternator and a commercially available biogas fuelled single cylinder auto engine of480 cm3 displacement and 11 kW rated power [45].

The engine's carburettor cylinder head, valves and control bearings have also been modified. Engine endurance tests and laboratory tests on exchangers to develop simple, reliable equipment without expensive control system have also been carried out. por exhaust gas, a cross - flow exchanger was used, which could easily yield 80-85 °C water. The cooling air exchanger, instead operated at 40-45 °C, and its output was limited to 3 kW heat, overall efficiency attained was 40-60 %, depending on the fuel used.

Biogas can be used for lighting purposes, especially as a stand-by facility. It can also be used indirectly to power a generator to provide electricity for fighting and other electric appliances. A gas pressure of 40 cm water (5.91 kPa) is important to maintain a gas lamp, and this is only possible with the fixed dome type digesters.

The benefit of gas driven engine is that, besides generating electric or mechanical power, the total efficiency can be increased by recovering heat from the engine cooling systems and also from exhaust gases. This can be used to heat the digester or for nay other heating requirement at the form.

9.3.3 Digester Effluent as a Fertiliser and Soil Conditioner: There are various methods of utilising the residues produced in the anaerobic digestion systems. The most widely used disposal system is to use it as a fertiliser. It is not easy to give precise data reporting the fertiliser value of slurry as it depends upon the total solid content of the fresh slurry. It is possible to estimate the fertiliser value of the slurry, knowing its total solid contents, ie.

N(g/t) = 0.53 (%TS)- 3.16

P2 05 (g/1) = 0.7 (% TS) - 1.25

K20 (g/1) = 0.13 (% TS) -1.7

Losses of nitrogen during storage of slurry after digestion is related to the type of treatment given to it. Losses of P2 0$ and K20 are generally due to imperfect sealing of the storage pit. Direct application of the various wastes on land as a fertiliser, results in a few problems like smell due to the presence and putrefaction of some of the organic materials and the survival of pathogenic organisms and weed seeds which may be transmitted through spreading on the land.

Anaerobic digestion appears to be the most promising method of utilising waste, reducing the solid organic content by about 50% and resulting in a stable, non-odorous digester effluent. A few studies have demonstrated that digester effluent improves the physical and chemical properties of the soil and stimulate biological activities.

42 There are some drawbacks, too, while using the digester effluent as fertiliser. During the digestion process, The pH increases, and ammonia nitrogen NH4+ breaks down, giving more free ammonia (NH3) . Another problem observed is the changes in the fertiliser value of the digester effluent. La monitoring a digester, ADAS [46], found decreases of 11.9 and 18 % of nitrogen, phosphorusand potassium, respectively.

In fact, more research is to be undertaken before any firm conclusion could be drawn, regarding the nutrient contents of the digester effluent. Last but not the least, it could be stated that besides the usefulness of the products of anaerobic fermentation systems, the system itself serves many useful purposes such like impact on public health and sanitation and ecological uses.

CONCLUSION

During the last two decades considerable progress has been made in the understanding of the anaerobic process which leads to the development of many new configurations in reactor’s design. Needless to say that significant advantages of these developments has opened a new vista to the application of anaerobic biotechnology for the treatment of both solid and liquid wastes. It is hoped that the knowledge gained in respect of the reactor’s designs, different applications and other important aspects, discussed in the present communication will provide a sound base to the actual application of the technology, in question, in the field.

ACKNOWLEDGEMENT

One of the authors (VKS) would like to express his sincere thanks to Prof. G. Furlan from International Centre for Theoretical Physics, Trieste (Italy), for constant encouragement and useful discussion while preparing this documents. The text used in the preparation of this review report is gratefully acknowledged.

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