anaerobic treatment of semi-solid organic waste

Anaerobic treatment of semi-solid organic waste

V.K. Sharma 1, *, C. Testa, G. Castelluccio

ENEA Research Centre, Trisaia, AMB-TEIN-RIF, 75025 Policoro (MT), Italy

Received 21 November 1997

Abstract

To illustrate the potential of anaerobic digestion for di�erent organic wastes treatment, the presentpaper focuses on the essential features of both new and more e�cient reactor systems and theirappropriate applications for various organic waste management situations. # 1998 Published byElsevier Science Ltd. All rights reserved.

Keywords: Anaerobic digestion; Anaerobic treatment process; Plug¯ow reactor; Semi-solid organic waste

1. Introduction

Anaerobic methanogenic digestion, an e�ective method for the treatment of many organicwastes, is a topic of increasing interest throughout the world. Nutrient-de®cient residues andwastes, which often occur in agro-industrial processing, can usually be treated anaerobicallywithout any addition of nutrients. Anaerobic digestion is now a matured technology and apractical tool for practitioners. During the last two decades, considerable progress has beenmade in understanding the anaerobic process. A number of designs and their performance havealready been described by several researchers [1±5]. However, the fact remains that anaerobictreatment processes have not been utilised as widely as aerobic processes, both at pilot and fullscale. It may be due to the fact that anaerobic digesters are not yet readily available on themarket, and those models that are available do not always function properly. In addition,standardisation of procedures for waste characterisation, expansion of anaerobic digestiontechnology to a wider range of waste available, improvement in reactor design, understandingof a consensus on the utility of monitoring data in delineating operational control etc. are theprinciple problems yet to be solved.

Energy Conversion & Management 40 (1999) 369±384

0196-8904/99/$ - see front matter # 1998 Published by Elsevier Science Ltd. All rights reserved.PII: S0196-8904(98)00128-9

PERGAMON

1 Visiting Scientist, ENEA Research Centre, Trisaia, 75025 Policoro (MT), Italy* Corresponding author.

With this in mind, the main objective of the present communication is to discuss theexpansion of anaerobic digestion technology to a wider range of organic waste available anddescribe several working plants and possible improvements in existing reactor design. It ishoped that the signi®cant development of anaerobic biotechnology reported in the presentcommunication will be useful for the waste treatment of both low/high organic strength andspecialised treatment for toxic substances.

2. Operation of anaerobic treatment processes

The anaerobic treatment of wastes can be performed in di�erent reactor systems, in singlephase, two phase or multiphase con®guration. In single phase operation, di�erent groups ofmicro-organisms are developed in the same environment to the extent that they areproportional to the ¯ux of their respective substrates available within the system. In a wellbalanced digestion, each group of bacteria will establish its own particular population sizewhich, in turn, depends on the feed material, operating conditions (pH, temperature, retentionperiod) and on the stoichiometry of the reaction involved.The problems, such as low loading rate, long retention time, poor e�ciency and non-optimal

biological action etc., must be solved to create a successful single stage digestion process. Themost important modi®cation involves increased contact between the micro-organisms andsubstrate through mixing. Other problems requiring much needed attention include (1)elimination of the scum layer, (2) maintenance of uniform temperatures throughout thedigestion tank, (3) inhibition of large particle settling, (4) improved process control and (5)dispersion of potential metabolic inhibitors, such as volatile acids. Because of the longretention time required and its poor e�ciency, this type of operation is rarely used any more.However, the single phase operation can further be performed either in parallel reactors or inreactors in series (multi-stage).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 inparallel. The solubility index of the wastewater is one of the main considerations in selecting asingle reactor or a parallel reactor. In general, wastewaters 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 mediumsolubility index (0.2±0.8) can advantageously be treated in two reactors in parallel operation [2].The two phase operation, involving two distinct phases (acid formation and methane

formation), may be performed in two separate reactors in series. It has been claimed by severalresearchers that two phase digestion, having a number of advantages over a single stagedigestion, leads to production of biogas with a higher methane content [3±5]. The operationmay be applicable for all kinds of wastes with improved treatment e�ciency at reduced overallreactor size. Sulphate bearing wastes can well be handled by two phase operation to overcomethe toxicity due to the hydrogen sulphide. The in¯uence of the toxic materials present in thewaste can be minimised by two phase operation, such as the treatment of pulp and paperindustry wastes.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 occur

V. K. Sharma et al. / Energy Conversion & Management 40 (1999) 369±384370

separately under di�erent environmental conditions. In order to achieve this goal, the conceptof multi-stage digestion was suggested by many researchers. A multi-stage unit basicallyconsists of a series of reactors combining any number of digesters [6]. The most commonlyused multi-stage digestion system involves a series of continuously stirred tank reactors,controlled to provide variation in temperature and pH. A more sophisticated multi-stagedigester has been designed, developed and experimented in Italy by Restrepo [7].Based on the R&D work done so far on multi-stage digesters, it may be stated that while the

system certainly increases gas yield, the economic problems associated with it are yet to be solved.

3. Anaerobic reactors for waste treatment

Available results indicate that anaerobic digestion has a promising future. This sectionpresents the description and essential features of the most appropriate models used for organicsolid waste treatment. Emphasis will be given to new, more e�cient and environmentallyattractive methods.

3.1. Anaerobic lagoon

The anaerobic lagoon is the simplest form of anaerobic treatment device, which is based ona natural ecosystem. Low strength wastewater with BOD concentrations above 500 mg/L cansuccessfully be treated by an anaerobic lagoon. Studies on biogas production using anaerobiclagoons, shown in Fig. 1(a), have been conducted by Sa¯ey and Westman [8].As shown in Fig. 1(b), a very simple and low cost design for biogas recovery using anaerobic

lagoons has been developed and patented by ENEA, together with AGRISILOS [9] S.n.c.(Cremone, Italy). This version is designed for big lagoons when higher gas pressure is required(max. 800±1000 mm H2O). Here, the production is related to ambient temperature whichmakes gas recovery from lagoons more suitable for mild conditions. The requirement of largeland area, odour nuisance, changes of ground water pollution etc. are other main drawbacks ofthe system.

3.2. Anaerobic contact process

The anaerobic contact process comprises a completely mixed digester tank unit followed bya settling tank unit (clari®er). The process is suitable for the treatment of wastes withintermediate strength (2000±10000 mg/L COD). Comparatively low investment costs and thecapability of handling relatively high concentrations of suspended solids are the principleadvantages, whereas the limited loading capacity and the poor settleability of the biomass arethe major drawbacks of the contact process.

3.3. Up¯ow anaerobic ®lter

Anaerobic ®lters have been used for almost twenty years to treat a variety of industrialwastes, yet until recently, little was known about the factors a�ecting their design and

V. K. Sharma et al. / Energy Conversion & Management 40 (1999) 369±384 371

performance. The anaerobic ®lter is essentially a tall reactor (H/D=8±10) provided with a®xed media matrix over which retention of anaerobic biological sludge is achieved to maintaina longer SRP. The process is highly stable and needs minimum control. It can accept variableand severe shock loads and can be restarted within a short period, even after a prolongedperiod of shut down. The energy requirement of the system is low.The up¯ow anaerobic ®lter basically is a contact process in which wastes pass over or

through a mass of biological solids contained within the reactor by ®xed media. The up¯owanaerobic ®lter actually performs as a combination of the ®xed-®lm and up¯ow sludge blanket

Fig. 1. (a) Lagoon biogas collection and measurement apparatus developed by Safley and Westerman.


processes, where the biological activities are mainly associated with the unattached sludgepresent in the voids. A number of full scale up¯ow anaerobic ®lter systems have beenconstructed in di�erent countries.The potential problem of an up¯ow anaerobic ®lter is clogging of the voids due to sludge

build-up, which causes short-circuiting of the waste ¯ow. The problem of clogging can,however, be overcome, either by periodic washing of the ®lter bed or by draining of the sludgeentrapped in the interspace. The problem could also be solved by operating the anaerobic ®lterin a down-¯ow mode. The system is, thus, called down-¯ow stationary ®xed ®lter (DSFF).The DSFF actually represents an anaerobic ®lter as a true bio®lm reactor and is capable of

handling a wide variety of wastes. The most important aspect of this reactor is the formationand stability of an active biomass ®lm on the surfaces provided. The e�ciency of the DSFF issomewhat less than that of the up¯ow anaerobic ®lter. This is mainly due to the loss ofsuspended matter sloughing into the e�uent. In addition, the reactor is not suitable fortreatment of highly diluted wastewater.

Fig. 1. (b) Schematic of gas collection system ¯oating on anaerobic lagoons. Medium pressure (800/1000 mm H2O)model.


Despite the fact that a number of full scale up¯ow anaerobic ®lter systems have beenconstructed, especially in the industrialised countries [10±13], the fact remains that there is nofull scale anaerobic ®lter in operation in developing countries. It is, therefore, necessary thatpilot plants be designed to provide data suitable for design use.

3.4. Up¯ow anaerobic sludge bed reactor

The design of UASB-reactors was discussed for di�erent types of wastewater. Anaerobicwastewater treatment using the UASB-reactor concept can be considered as a maturetechnology. The system has found successful application for a wide variety of industrialwastewaters with proven technology for the treatment of domestic wastewater at temperaturesexceeding nearly 228C. The process designed by Lettinga et al. [14] is based on the principle ofnatural ¯occulation and agglomeration of the micro-organisms. The results of investigationsconducted by Lettinga and co-workers have made it possible for the up¯ow anaerobic sludge-bed reactor to work with a high concentration of biomass, and this has produced very highloading rates.However, as with conventional systems, undoubtedly, speci®c improvements in the design,

the process layout and the operation remain possible. The main problem with this reactor isthe development of granular sludge, in the same way that a few wastes, such as sugarprocessing wastes and wastes consisting mainly of volatile acids, often result in the formationof this sludge. It is, therefore, necessary to have gas/liquid/solid separation devices eitherinternally or externally to separate the sludge ¯ocs from the e�uent.

3.5. Anaerobic ¯uidized and attached-®lm expanded-bed reactor

The concept of ®lm formation by micro-organisms is further extended in the development ofthe anaerobic attached-®lm expanded-bed reactor (AAFEB). The reactor is a closed tallcolumn partly ®lled with inert support media, such as sand, gravel, anthracite or plastic, whichare expanded by the up¯ow velocity of the waste, assisted by recycle.The reactor is very similar to suspended growth reactors in which the active biomass

becomes an aggregate bed that settles easily. The reactor is called a ¯uidized or expanded bedreactor, depending on the rate of liquid ¯ow and the degree of expansion of the bed. It hasbeen reported that the performance of this type of reactor depends upon the uniformity of theliquid ¯ow. Thus, a high and very uniform up¯ow of liquid has to be maintained in thereactor. No ¯uidized or expanded bed reactor has been put into operation on a large scale,however.

3.6. Anaerobic rotating biological contacter (ARBC)

The anaerobic rotating biological contacter is essentially the same as that used for an aerobicsystem, except that it is completely closed. The circular discs, made of plastic mesh, PVC,polystyrene foams or just plain asbestos, are placed in a closed tank ®lled with wastewater. Thediscs are rotated at a speed of about 3±6 rpm. The micro-organisms are attached to the discsurfaces, thereby providing larger surface areas that are exposed for contact with the waste.


The discs are spaced at least 2±3 cm apart, and they are installed in a number of compartmentsto facilitate a plug ¯ow pattern to the liquid within the reactor.

3.7. Anaerobic ba�ed reactor (ABR)

The anaerobic ba�ed reactor (ABR) is a modi®cation of the upper-¯ow anaerobic sludge-bed reactor. This reactor consists of a simple rectangular tank divided into ®ve or sixcompartments of equal volumes by means of walls running from the roof to the bottom of thetank [15]. The liquid ¯ows upwards and downwards between the walls. On its upward passage,the waste ¯ows through an anaerobic sludge blanket. Hence, the waste is in intimate contactwith the active biomass, but because of the ba�ed nature of the design, most of the biomass isretained within the reactor, even with large hydraulic shocks.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 designo�ers the advantages of reactors in series, i.e. high e�ciency, low bypass, resistance to shockloading together with high biomass retention capacity and high speci®c activity of themethanogenic acetoclastic biomass. However, the necessity to build shallower reactors,relatively higher building complexity and di�culties to distribute the in¯uent evenly are themain drawbacks of the design.

3.8. Hybrid bioreactors

The hybrid bioreactor [16, 17] represents the recent generation of reactor. The reactors inquestion o�er the advantages of the UASB concept associated with the ones of the AnaerobicFilters and, nowadays, can be considered more suitable for the treatment of a series of solubleor partially soluble wastewaters than can many other reactor systems.Several di�erent 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 simplerdesign, where the ®lter is located in the upper part of the reactor without any gas/solid/liquidseparation device. Recently, studies have been performed on anaerobic digestion of the liquidfraction of beef cattle waste using this type of reactor. The reactor had a suspended growthzone at the bottom and an up¯ow ®lter at the top. The plant used was a 4 m high and 0.63 mwide (diameter) vertical reactor. The working volume was 1.12 m3, and the speci®c ®llingsurface was 100 m2/m3.Experiments have also been performed on a hybridised anaerobic ba�ed reactor (Fig. 2)

operating on molasses stillage evaporator condensates [17]. The plant consisted of threechambers and a ®nal settler. The working volume of the three chambers was 150 litres. Thesettler volume was not considered as an active volume, because any sludge was always recycledback to the ®rst chamber. From an e�ciency point of view, this type of reactor resulted in thedestruction of up to 98% of COD.

3.9. Sequential batch anaerobic composting

The concept of sequential batch anaerobic composting (SEBAC), developed at theAgricultural Engineering Department, University of Florida, USA, has been used to overcome


the limitations of most designs for anaerobic digestion, such as the requirement for heavyinoculation, mixing, possibility of instability etc. [5]. A pilot plant, shown in Fig. 3, with threebatch reactors (2.4 m high by 0.6 m ID) and all ancillary equipment was constructed andoperated at the University of Florida. The plant was used to treat two fractions of municipalsolid 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 e�ective. However, thorough understanding of the physical,chemical and biological dynamics of high solids, batch systems etc. has been recommended.

3.10. KOMPOGASÐa new system for anaerobic treatment of source separated waste

KOMPOGAS, a new digester system, designed speci®cally to treat fruit, yard and vegetablewastes with 15±40% total solids, has been reported by INFOSOLAR (Switzerland). As shownin Fig. 4, the system is a CSTR-type horizontally positioned cylindrical digester of volume

Fig. 2. Schematic of hybridized anaerobic ba�ed reactor.


15 m3 equipped with a hydraulically driven stirrer which takes up strong shear forces [18].Based upon the preliminary experimental results, it has been concluded by the authors that thedigester performed best at TS-concentrations of around 27%. From the test run, it wasobserved that the system under investigation is best suited to treat substrates with low drymatter content.Encouraged by the satisfactory experimental results, a ®rst full scale installation with a

digester volume of 200 m3, was installed in January 1992. The system, capable to treat 3000tons 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 underinvestigation. The results obtained from the types and order of the treatments currently underexperimentation will be used to minimise the energy requirement and the cost.

Fig. 3. Schematic of the sequential batch anaerobic composting (SEBAC) process.


3.11. Semi-dry anaerobic digestion process of organic solid waste

Based upon the concept of a semi-dry (20% dry matter) anaerobic process, two full-scaleindustrial plants have been installed in Italy [19, 20]. The ®rst plant, managed by the GeneralMunicipal Services Agency of Verona (AGSM), an anaerobic thermophilic semi-dry digestionplant, is able to handle nearly 500 tons of MSW each day (design capacity). When operating ata daily capacity of 200 tons/day, the plant is able to produce 26.000 Nm3 day of biogas and42.2 tons/day of dewatered and dried sludge for 7 days/week. The scheme for the mechanicaldewatering of the digester e�uent is shown in Fig. 5. The cogeneration section provides 2MWe of energy and the terminal clamp, about 1.4 Gcal/h like hot water.The second plant, installed at Avezzano (Central Italy), is used for anaerobic thermophilic

semi-dry co-digestion of sludges and organic fractions with total capacity of nearly 40 tons/day [20]. The organic fraction fed to the anaerobic co-digestion is 15 t/d to which are added 22t/d of sludge from the sewage treatment plant. The digester has been planned assuming thatthe concentration of dry matter could vary between 12% and 20%. These concentrations areguaranteed by recycling of the water from dewatering of the digested sludges. The digestionhas been optimised using mammoth pumps and introducing the fresh sludge at three di�erentpoints in 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 biogass and 39 t/d of digested sludges (10% dry matter), dewateredby press ®lter to the concentration of 30% of dry matter (out¯ow 14 t/d). The dewatereddigested sludges are then co-composted with parts of the fresh organic fraction (38 t/d),producing about 41 t/d of rich composts (55% dry matter).

3.12. Dry anaerobic conversion of MSW using Dranco process

The Dranco process was developed for the conversion of solid organic wastes, speci®callythe organic fraction of municipal solid waste (MSW), to energy and a humus-like ®nalproduct, called Humotex. A demonstration plant of 56 m3 has been in operation for several

Fig. 4. Flow sheet of the KOMPOGAS process.


years in Gent, Belgium, using mixed garbage as feedstock between 1984 and 1991 and usingsource separated waste since July 1991 [21].Another Dranco installation for treating 10.500 tons of source separated waste per year has

also been constructed at Brecht, Belgium. The digester has a volume of 808 m3 with a diameterof 7 m and a height of 21 m. The plant has been in operation since the spring of 1992. Thebiogas produced from the plant is stored in a gas bag and partially (about 80%) used toproduce steam needed for process heating. The rest of the biogas is mainly transformed intoelectricity by means of a 280 kW gas engine. The electricity is partially used for the operationof the installation and partially sold to the grid.

3.13. Innovative two-stage anaerobic digestion and aerobic composting process

A relatively recent advance in the ®eld 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 concentrations greater than 20%. It is, however,to be noted that, since the introduction of the high solids anaerobic digestion process for wastestabilisation by Jewell in 1979 [22], only limited research activity has taken place. A recentstudy of the high solids anaerobic digestion process has been conducted by Chynoweth et al. in1990 [23].Pilot investigations of an innovative two stage bioconversion process have recently been

reported by M. Kayhanian et al., in the USA [5]. The high solids anaerobic digestion/aerobiccomposting process is a two-stage process. The ®rst stage involves the high solids anaerobic

Fig. 5. Semi-dry anaerobic digestion process of organic solid waste.


digestion of the organic fraction of MSW to produce biogas, whereas the second stage involvesthe aerobic composting of the anaerobically digested solids to increase the solid content from25 to 65 percent or more depending on the ®nal use of the compost. The pilot scaleexperimental plant has been demonstrated for various mass retention times over a period ofone year. From the experiments conducted, it has been observed that the process was stableand relatively easy to operate.

3.14. An industrial plant for MSW treatment

The ®rst world industrial plant, installed at Amiens in France, has now been operating formore than eight years [24]. The Valorga process is a technique for processing biodegradableorganic wastes by anaerobic digestion. The plant is particularly suited for treating thefermentable fraction of municipal solid wastes (MSW) coming from source separation, but itpermits also treating bulk MSW because of its optional sorting line. The process runs atindustrial scale for biological aspects.The plant is treating the total production of household wastes of the city (55,000 t/y).

However, the maximum treatment capacity reaches 65,000 to 72,000 t/y according to the kindof collection. The performance control and process monitoring have shown good averageresults in stabilized conditions. The average production of biogas is 146 Nm3 per ton of sortedMSW introduced into the digester or 210 to 240 m3 methane per ton of volatile solidsintroduced into the digester.

3.15. Innovative plug ¯ow reactor to treat semi-solid orthofruit waste

It is true that semi-solid orthofruit waste, characterised by both high water content (>80%)and a high C/N ratio, after dewatering or addition of municipal sewage sludge/zootechnic,could be composted aerobically, but anaerobic digestion, without any speci®c pre-treatment ofresidues and with rather likely energy recovery, seems to be the most attractive method fortreatment of the above mentioned semi-solid waste. However, the fact is that very littleattention has been paid to the concept under investigation.Designs, such as a bag and horizontal plug ¯ow types having signi®cant potential to produce

biogas with lower capital investments and higher e�ciency levels, have been found both indeveloped and under developed countries. Contrary to conventional practices, Fig. 6(a), thepresent plug ¯ow reactor has not been installed horizontally but in an inclined position to theground level. The principle objective of the innovative inclined plug ¯ow reactor developed atthe ENEA Research Centre, Trisaia (southern Italy), was to provide low initial investment,high e�ciency and relatively simple operational and maintenance operations [25].As shown in Fig. 6(b), the cylindrical shaped anaerobic reactor is 350 cm long, 70 cm in

diameter and has an internal volume of nearly 1.35 m3. The reactor has been installed at aninclination of 208 with respect to the ground level. The body of the reactor is heated, usingauto-controlled electrical resistance ®tted 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 ®tted on the pipe line meant for the exit of thebiogas produced. On the other hand, to quantify both the percentage of CO2 and CH4


Fig.6.(a)Schem

aticofplug-flow

plant:longitudinalsectionofthereactordeveloped

byENEA.


Fig.6.(b)Pilot-scale

experim

entalplug-¯ow

anaerobic

reactor.


produced and controlling O2 pollution, if any, a gas analyser (ADC mod. LFG-20) has beenprovided. The experimental data has been recorded regularly using a Data logger, mod. 605from DATATAKER.A set of experimentations has been conducted. The semi-solid wastes available from

wholesale fruit and vegetable markets mixed together with sewage sludge were used as inputfeedstock material. The data obtained from the experimental observations has been analysed,and the results obtained are presented elsewhere [25].

4. Conclusion

Needless to say, the signi®cant advantages of the recent developments have opened a newvista to the application of anaerobic biotechnology for the treatment of both wastewater andsolid organic wastes. It is hoped that the innovative plug ¯ow reactor, designed by the ENEAResearch Centre, Trisaia, in Italy, will provide a sound base for the treatment of semi-solidorganic wastes available from orthofruit markets.

Acknowledgements

One of the authors (VKS) would like to express his sincere thanks to Prof. G. Furlan fromthe International Centre for Theoretical Physics, Trieste, Italy, for useful discussion whilepreparing this document. The text used in the preparation of this paper is gratefullyacknowledged.

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anaerobic treatment of semi-solid organic waste

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