anaerobic digestion of semi-solid organic waste available from orthofruit market: preliminary...

Anaerobic digestion of semi-solid organic waste availablefrom orthofruit market: preliminary experimental results

V.K. Sharmaa, C. Testa a, G. Cornacchia a, G. Lastella b, R. Farina c

aENEA CR Trisaia, AMB-TEIN-RIF, 75025 Policoro (MT), ItalybUniversitaÁ di Bari, Sede di Taranto, Via Trentino 66, Taranto, Italy

cENEA CR Bologna, AMB-TEIN-DEP, Via Martiri di Montesole, 40129 Bologna, Italy

Received 30 August 1997

Abstract

Design features, both mechanical and operational, as well as preliminary experimental resultsobtained in the context of semi-solid organic waste available from wholesale fruit and vegetablemarkets, mixed together with sewage sludge, are presented here. # 1998 Elsevier Science Ltd. All rightsreserved.

Keywords: Anaerobic digestion; Plug-¯ow reactor; Control parameters; Semi-solid organic waste; Analysis

1. Introduction

During the last two decades, considerable progress has been made in understanding theanaerobic process. A number of designs and their performance have already been described byseveral researchers [1±5]. However, the fact remains that anaerobic treatment processes have

not been utilised as widely as aerobic processes, both at pilot and full scale. It may be due tothe fact that anaerobic digesters are not yet readily available on the market, and those models

that are available do not always function properly. In addition, the standardisation ofprocedures for waste characterisation, expansion of anaerobic digestion technology to a widerrange of waste available, improvement in reactor design, understanding of a consensus on the

utility of monitoring data in delineating operational control etc., are the principal problems yetto be solved.

Having said above, the main objective of the present communication is to focus on both the

concept of the expansion of anaerobic digestion technology to a wider range of organic wasteavailable and improvement in existing reactor design. The treatment of semi-solid wastesgenerated from wholesale fruit and vegetable markets, local orthofruit shops, supermarkets etc.

Energy Conversion & Management 40 (1999) 287±304

0196-8904/99/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved.PII: S0196-8904(98)00106-X

PERGAMON

characterised by both high water content (>80%) and a high C/N ratio, mixed together withsewage sludge, has been attempted.It is true that such residue, after dewatering or addition of municipal sewage sludge, wastes

available from zootechnic or trimming of various plants etc., could be composted aerobically,but anaerobic digestion without any speci®c pre-treatment of residues and with possible energyrecovery seems to be the most attractive method for treatment of the above mentioned semi-solid waste. However, the fact is that very little attention has been paid to the concept underinvestigation. It is in this reference that, in order to augment our limited knowledge on thetopic in question, an attempt has been made to study the treatment of the above mentionedsemi-solid waste experimentally.To achieve the objective, a plug-¯ow type reactor based upon the principle of anaerobic

digestion has been installed at the ENEA Research Centre, Trisaia. A set of experiments hasbeen conducted. 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. The preliminary

experimental results obtained, along with the mechanical and operational aspects of theexperimental installation under investigation, have been reported in the present research paper.

2. Description of anaerobic plug-¯ow type digester under investigation

The principal objective of developing such a design was to provide low initial investment,high e�ciency and relatively simple operational and maintenance operations. As shown inFig. 1 (¯ow diagram), the anaerobic reactor, trituration system, mixer, loading unit, biogasdischarging system, gas analyser, data acquisition, etc. are the main components of the system

Fig. 1. Flow diagram of the anaerobic digester under investigation.

V.K. Sharma et al. / Energy Conversion & Management 40 (1999) 287±304288

under investigation. Brief descriptions of the components mentioned above are given in thefollowing text.Trituration system: The pilot-scale experimental plant comprises two trituration systems, i.e.

(1) screw mincer, and (2) homogenizer grinder. The screw mincer, though capable of treatingfruits with consistent kernel, is very slow. The homogenizer grinder with a capacity of 25 litresis very fast but cannot break the hard fruit kernel. In order to solve the problems mentionedabove, a suitable system was provided to remove the fruit kernel before it reached thehomogenizer grinder on its way to the feeding hopper.Mixing unit: A trapezoidal shaped hopper with a capacity of nearly 500 litres is provided

with a stirrer having blades of approximately 50 cm in length. The stirrer is placed at the top.It is to be noted that, due to the speci®c characteristics of the hopper, vertical strati®cation andlocal mixing of the input waste was observed. To avoid improper mixing, down stream fromthe pump was recycled to the hopper, thus obtaining excellent mixing. The pump is providedwith two valves, one placed between the joint connecting the pump and the reactor and thesecond on the return line to the hopper. To have a sample of the input feedstock, a suitablesocket has been provided on the line.Material input to the reactor: Considering the high pumping power and the loading rate of

nearly 40 kg/day, it was initially thought to feed the reactor continuously by adjusting theopening of the valve appropriately, while operating in a recirculating mode. It is, however, tobe noted that, due to technical problems associated with the pump, feeding the reactorcontinuously was not possible and the reactor was therefore loaded once a day. The loadingsystem consists of a hopper having dimensions 0.8�2� 1 m3. The maximum loading volume isabout 0.5 m3. The average daily loading rate is of the order of 67.5 dm3/day.Anaerobic reactor: The cylindrical shaped anaerobic reactor (Fig. 2) is 350 cm long, 70 cm in

diameter and has an internal volume of nearly 1.35 m3. Contrary to conventional practices, thereactor has been installed at an inclination of 208 with respect to the ground level. To achievehomogenous mixing of the material to be treated, it is furnished with a suitable mixer,comprising a crankshaft provided with blades bolted to the shaft. The iron pipes meant formaterial supply are connected to the reactor at the bottom side, whereas an appropriatearrangement has been made to collect the outgoing material from the reactor (after treatment)at the upper end. In addition, a gas holder (tank) of volume nearly 40 dm3 has been providedat the upper portion of the reactor.To obtain a sample of the digested material inside the digester at di�erent positions along

the length of the reactor, three sampling positions, ®tted with valves, have been provided. Thebody of the reactor is heated using auto-controlled electrical resistance heating, ®tted along thelength of the reactor. To achieve a higher initial temperature, it is obvious that the number ofelectrical resistance heaters would be more at the entrance. To avoid thermal losses from thereactor, it has been well insulated, using wool sandwiched between galvanised steel sheets.System for discharge of biogas: The biogas produced from the reaction taking place in the

reactor is collected at the upper end. It is then conveyed to a pre-set hydraulic check, workingwith a water head of 100±200 cm. The said hydraulic check, in addition to maintaining gaspressure in the gasometer, serves also for its proper clearing before discharging into theatmosphere. Using a ¯oating system, the water head in the hydraulic check is maintained at aconstant level. Another safety hydraulic check with an oil head of nearly 350 mm is provided

V.K. Sharma et al. / Energy Conversion & Management 40 (1999) 287±304 289

with the reactor. The main purpose of this safety hydraulic check is to assist, in case someirregularities arise in the biogas discharge line, with casual plant pressurisation on the biogasdischarge circuit.Quantity and quality of biogas produced: To measure the quantity of biogas produced, two

gas meters (commercial (ELKRO mod. BKP) and delicate (ELSTER)) with ¯ow rates of 40 l/hand 0.2 l/h, respectively, have been ®tted on the pipe line meant for the exit of the biogasproduced. On the other hand, to quantify both the percentage of CO2 and CH4 produced andcontrolling the O2 pollution, if any, a gas analyser (ADC mod. LFG-20) has been provided.Data acquisition system: For procurement of data of the principal process control

parameters, the plant has been equipped with appropriate instruments. To measure thetemperature at di�erent positions, i.e. bottom, middle and top, three sensors have been ®ttedinside the reactor along its length. Another sensor has been provided to measure the ambient

Fig. 2. Pilot-scale experimental plug-¯ow anaerobic reactor.


temperature. The experimental data has been recorded regularly using a Data logger, mod. 605from DATATAKER.

3. Experiment

The plug-¯ow type anaerobic digester was tested in July±November, 1996. A series ofexperiments was conducted using semi-solid organic wastes available from wholesale fruit andvegetable markets. In order to obtain the maximum total solid content possible, the abovementioned semi-solid wastes were mixed with sewage sludge. Below is given the briefprocedural description of the testing of the digester under investigation experimentally. Theperiod of experimentation can be divided into four principal phases, i.e.:

. insemination of digester

. technical problems and their solution

. starting the process using orthofruit wastes

. working observations.

The major problem during the start-up of the anaerobic treatment process is that aconsiderably long period is required for establishing a balanced microbial culture for the wasteunder treatment. To speed up the bio-stabilised reaction, the concept of inoculated micro-organisms in the insemination phase, as suggested by Vallini et al. [6], has been used. Theinsemination phase was started in July, 1996. The reactor was inoculated using pig excrement.The insemination was continued for nearly a month, using just the sewage sludge as thesubstrate. It is very e�ective to provide a heavy inoculum of methanogens during start-up.Subsequently, during the testing phase, a number of technical problems were encountered andthen solved through suitable modi®cations. It was only after such modi®cations that the trueexperimental phase could be started.In the beginning, it is necessary to keep the initial organic loading low. In fact, during the

®rst week of experimentation, approximately 20 kg of input material (90% homogenisedorthofruit waste mixed with 10% sewage sludge) was fed to the reactor. Here, it is to be notedthat it is the initial water content of the waste that represents the principal parameter de®ningthe optimum ratio for semi-solid waste and sludge mixing.From the moment the process is stabilised, i.e. once steady state conditions are established at

low loading as shown by increased gas production, the loading can be increased step-wise toreach an optimum loading. In the present study, the loading was raised to nearly 40 kg/day. Itis hoped that, within a span of time and, of course, after appropriate planning based uponexperience and the problems encountered, the input could be increased to 100 kg/day.So far as the experimental observations are concerned, total solid (ST), total volatile solid

(SV), CODtol, VFA, Alkalinity, pH etc. are the principal parameters determined both for thewaste and sewage sludge. In addition, the parameters mentioned above have also beendetermined for the mixed material before being fed to the digester. The observations wererepeated for both the substrate inside the digester (bottom, middle and top) and the e�uentfrom the reactor. The data on the quantity and quality of biogas produced was also recorded.


4. Methodologies for substrate analysis

The successful operation of anaerobic biotechnology for waste treatment depends upon thecomposition of the substrate. It is the composition of the substrate which dictates the stabilityof the process and the choice for micro-organisms. The important characteristics whichindicate the suitability of waste for anaerobic treatment can be judged from their organicstrength, composition and parameters like pH, VFA, alkalinity etc.There is a large volume of literature available on the subject. It is just to give a very brief

initial input to readers, both new and experienced, that an attempt has been made tosummarise here the most relevant aspects involved in the above mentioned importantparameters.

4.1. Total solids (ST) and volatile solids (VS)

Total solids represent the material residue left in a dish after evaporation of the sample andits subsequent drying in an oven at a temperature of 1058C. Volatile solids, on the other hand,is the weight lost on ignition when the dried residue obtained above is ignited in a furnace at atemperature of 5508C until the weight change is less than 4%. For calculation purposes, theexpressions discussed in Ref. [7] have been used, i.e.

Total solids � �Aÿ B� � 1000�

=�Cÿ B�; �g=Kg�Volatile solids � �AÿD� � 1000

� =�Aÿ B�; �g=Kg�

where A=weight of dried residue+dish, (gm); B=weight of dish, (gm); C=weight of wetsample+dish, (gm); D=weight of residue+dish after ignition, (gm).

4.2. Chemical oxygen demand (COD)

COD is a measure of the oxygen equivalent of the oxygen matter content of a samplesusceptible to oxidation. Open re¯ux and closed re¯ux titrimetric methods discussed in Ref. [7]were used for the determination of COD with values greater and less than 50 mg O2/l.Calculations have been done using the expression,

COD � �Bÿ S� � N � d � fc � 8000�

=Vc

where B=ferrous ammonium sulphate (FAS) titrant used for blank, (ml); S=ferrousammonium sulphate (FAS) titrant used for sample, (ml); N=normality of the ferrousammonium sulphate (FAS) titrant; d=dilution of the sample; fc=correction factor for thetitrant; Vc=diluted sample used, (ml); and 8000=oxygen equivalent weight, (mg).

4.3. Volatile fatty acids (VFA)

The volatile acids can be removed from the aqueous solution by distillation. The distillationmethod discussed in Ref. [7] was used for the determination of volatile fatty acids (VFA). Thecalculations for VFA expressed in mg CH3COOH/l have been done using the expression,


VFA � �Vt � Nt � 60000�=Vc

where Vt=0.1 N sodium hydroxide titrant used for the titration sample, (ml); Vc=Volume ofsample used, (ml); Nt=Normality of titrant; and 60000=Acetic acid equivalent weight, (mg).

4.4. pH value

The parameters, such as pH, alkalinity, volatile acids etc., are closely interrelated inanaerobic treatment. At a given temperature, the intensity of the acidic or basic character of asolution is indicated by pH. It is desirable to maintain the pH of the process between 6.8 and7.2, which is the optimum range for the growth of methanogens. By careful use of a laboratorypH meter with good electrodes, a precision of20.02 pH units and an accuracy of20.05 pHunits can be achieved while determining the pH value.

4.5. Total alkalinity (AT)

Alkalinity, the acid-neutralising capacity, is the sum of all the titratable bases. The majorsource of bicarbonate alkalinity in anaerobic treatment is the formation of ammonia by themicrobial degradation of proteins. Properly operating anaerobic digesters typically havesupernatant alkalinity in the range of 2000±4000 mg CaCO3/l. A titration method has beenused to determine the total alkalinity, expressed in mg CaCO3/l. The expression given belowwas used for the calculation, i.e.,

Total Alkalinity � �Vt � Nt � 50000�=Vc

where Vt=volume of titrant used for titration of sample, (ml); Vc=volume of sample, (ml);Nt=normality of titrant; and 50000=calcium carbonate equivalent weight, (mg).

5. Results and discussion

The main objective of designing the plug-¯ow type reactor installed at an inclination of 208was to improve its performance compared to one installed horizontally. The factors such asgood mixing and decomposition of the organic feedstock (pushed across the length to theopposite extreme) seem to be the possible reasons for the enhanced performance of the reactorunder investigation at the ENEA Research Centre, Trisaia, in Italy.To start, in order to know the fundamental physical as well as chemical characteristics

relevant to their composition, the organic wastes to be treated, i.e. orthofruit waste and sewagesludge, were analysed thoroughly. The results obtained from the above analysis are presentedin Tables 1 and 2. It is clear from the data presented above that the organic wastes availablefrom wholesale fruit and vegetable markets, containing easily biodegradable organic matterand with high dissolved COD values, are of particular interest for their anaerobicdecomposition.


Table 3

Principal parameters obtained from the analysis of input feedstock

Date ST g/kg SV g/kg COD tot. mgO2/l COD diss. mgO2/l

28/10/96 91.5 80.2 119,040 Ð05/11/96 58.5 51.9 92,000 27,000

08/11/96 55.5 48.5 102,080 65,12012/11/96 60.6 53.1 104,000 69,92019/11/96 58.7 51.4 128,000 92,000

22/11/96 69.7 62.7 95,040 50,40025/11/96 Ð Ð 84,000 50,40029/11/96 53.8 Ð 96,000 58,000

Table 1Principal parameters obtained from the analysis of organic waste available from wholesale fruit and vegetable

market


04/11/96 64.8 57.6 108,000 40,320

11/11/96 Ð Ð 102,080 75,68012/11/96 75 67.6 136,160 73,60019/11/96 93.5 85.1 112,000 76,00022/11/96 73.3 66.5 109,120 73,920

26/11/96 Ð Ð 136,000 112,000

Table 4Principal parameters obtained from the analysis of e�uent


28/10/96 45.8 27.1 30,720 Ð04/11/96 67.6 41.2 40,000 5,760

08/11/96 62.8 37.8 52,800 7,04012/11/96 64.2 41.2 47,840 019/11/96 38.5 23.3 48,000 32,00022/11/96 31.5 19.1 21,120 3,360

25/11/96 Ð Ð 20,160 5,04029/11/96 31 Ð 24,000 2,000

Table 2

Principal parameters obtained from the analysis of sewage sludge


31/10/96 14.7 10.41 Ð Ð

04/11/96 23.9 17.05 19,231 Ð11/11/96 Ð Ð 14,080 88019/11/96 10.1 7.1 16,000 1,40822/11/96 25 18 24,640 Ð


Table 6Principal parameters obtained from the analysis of substrate at the middle of digester

Date ST g/kg SV g/kg VFA mgCH3COOH/l AT mgCaCO3/l pH

29/10/96 34.7 20.8 803 8,530 7.904/11/96 36.1 21.2 200 Ð Ð

06/11/96 38.1 22.5 204 9,516 8.111/11/96 35.1 20.3 47 Ð Ð18/11/96 31.5 18.5 481 10,027 7.821/11/96 32 19 142 9,959 7.7

25/11/96 Ð Ð 76 9,609 8.228/11/96 32.3 Ð 74 9,918 8.302/12/96 Ð Ð Ð 9,650 8.3

Table 5Principal parameters obtained from the analysis of substrate at the bottom of digester


29/10/96 43 25.1 1,515 13,500 8.104/11/96 37 21.8 199 Ð Ð

06/11/96 37.2 21.7 173 12,120 8.111/11/96 34.8 20 81 Ð Ð18/11/96 30 17.4 1,200 10,110 7.921/11/96 33.5 20.2 145 9,933 7.7

25/11/96 Ð Ð 73 9,434 8.328/11/96 33.7 104 9,685 8.402/12/96 Ð Ð Ð 9,274 8.3

Table 7Principal parameters obtained from the analysis of substrate at the top of digester


29/10/96 30.6 18.3 2054 11,960 7.904/11/96 36.4 21.1 217 Ð Ð06/11/96 37.9 22.3 182 10,100 8.1

11/11/96 34.5 20 68 Ð Ð18/11/96 32.4 18.8 212 10,333 7.821/11/96 32.4 19.4 263 9,861 7.625/11/96 Ð Ð 61 10,135 8.2

28/11/96 Ð Ð 51 10,280 8.302/12/96 Ð Ð Ð 9,675 8.3


Fig. 3. Trend of pH inside the reactor.

Fig. 4. VFA trend inside the reactor.


Fig. 5. Trend of COD for the input feedstock.

Fig. 6. Trend of COD in the e�uent from the reactor.


Similarly, the samples taken from the substrate (before being fed to the reactor), as well asthe e�uent from the reactor, were also analysed, and the results obtained are presented inTables 3 and 4.The closely interrelated control parameters, such as pH, alkalinity, VFA etc., have been

monitored using samples taken from the digester at di�erent positions, i.e. bottom, middle andtop. The data obtained is presented in Tables 5±7. As shown in the Tables, the parameters canserve as indicators for expected process failure and necessitate steps to overcome such failure.It is to be noted that, while during normal digestion, low molecular weight volatile acids

formed by acidogens are readily utilised by methanogens, the same acids get accumulated inthe system during a process instability re¯ected by a drop in pH (Fig. 3) or an increase in VFA(Fig. 4). In order to solve the problem, it is necessary to provide a high level of alkalinity,providing su�cient bu�er to neutralise a sudden increase in VFA.As is evident from the data relevant to CH4, pH and VFA, recorded within a day or so after

the process instability, thanks to the presence of a large concentration of carbonates and,hence, bu�er capacity, the process is stabilised very soon. To summarise, it can be stated that,for the stable growth of methanogens, it is desirable to maintain the pH of the process between6.8 and 7.2, which appears to be the optimum range in an anaerobic treatment process. Theratio of volatile acids to total alkalinity, a better indicator for the stability of the process,should be less than 0.1.

Fig. 7. Trend of ST inside the reactor.


Fig. 8. Trend of SV in the reactor.

Fig. 9. Trend of total alkalinity inside the reactor.


Observations on the evolution of total solid (ST), total volatile solid (SV), COD, alkalinityetc. of the substrate under digestion recorded at di�erent positions of the reactor, i.e. bottom,middle and top, are presented in Figs. 5±9.A stable treatment process produces gas at a constant rate with a constant methane content.

The deviation from this rate immediately re¯ects the imbalance conditions in the treatment.The relative change in methane production (methane production during unstable conditionscompared with stable conditions) appears to be a reliable control parameter.From Fig. 10, representing the amount of gas produced daily, it has been observed that, on

certain days of experimentation, there was absolutely no gas production from the system.However, on successive days, a net rise in the overall gas production was registered. It is dueto the fact that during the week-end, non-loading of the reactor caused a net signi®cantdecrease in the nutrients used for the production of bacteria inside the reactor and, hence, nogas production. The increase in production of gas on successive days was mainly due to thereloading of substrate in the reactor.Experimental observations regarding the quantity and quality (in terms of its composition)

of biogas produced each day are presented in Table 8. The quantity and quality of biogasbeing produced by microbial activity (Figs. 11 and 12) immediately re¯ect the changes takingplace in the digester. Analysing the gas produced, as shown in Fig. 13, it is clear that,depending on the nature of the input feedstock, methane and carbon dioxide contents of theorder of 25±35% and 65±75%, respectively, were recorded.

Fig. 10. Production of biogas on daily basis.


Table 8Principal parameters obtained from the analysis of the biogas produced

Date Biogas m3/day CO2 % CH4 % CO2 m3/day CH4 m

3/day CH4 Prod. spec. Biogas Prod. spec.

31/10/96 Ð 36.05 63.95 Ð Ð Ð Ð01/11/96 0.77 29.85 70.15 0.23 0.54 0.40 0.5702/11/96 0.81 Ð Ð Ð Ð Ð Ð

03/11/96 0.81 Ð Ð Ð Ð Ð Ð04/11/96 0.81 23.57 76.43 0.19 0.62 0.46 0.6005/11/96 1.40 36.19 63.81 0.51 0.89 0.66 1.03

06/11/96 1.08 38.56 61.44 0.41 0.66 0.49 0.8007/11/96 1.18 40.51 59.49 0.48 0.70 0.52 0.8708/11/96 0.97 41.85 58.15 0.41 0.57 0.42 0.72

09/11/96 1.42 Ð Ð Ð Ð Ð Ð10/11/96 1.42 Ð Ð Ð Ð Ð Ð11/11/96 1.42 42.16 57.85 0.60 0.82 0.61 1.05

12/11/96 1.42 41.36 58.64 0.59 0.83 0.62 1.0513/11/96 1.95 42.54 57.46 0.83 1.12 0.83 1.4514/11/96 2.12 40.68 59.32 0.86 1.26 0.93 1.5715/11/96 1.84 40.77 59.23 0.75 1.09 0.81 1.37

16/11/96 0.72 Ð Ð Ð Ð Ð Ð17/11/96 0.72 Ð Ð Ð Ð Ð Ð18/11/96 0.72 41.12 58.88 0.30 0.43 0.31 0.53

19/11/96 1.82 38.69 61.31 0.70 1.11 0.83 1.3520/11/96 1.76 Ð Ð Ð Ð Ð 1.3121/11/96 1.76 48.65 51.35 0.86 0.91 0.67 1.31

22/11/96 1.05 Ð Ð Ð Ð Ð Ð23/11/96 1.05 Ð Ð Ð Ð Ð Ð24/11/96 1.05 Ð Ð Ð Ð Ð Ð

25/11/96 1.05 33.33 66.67 0.35 0.70 0.52 0.7826/11/96 2.01 41.19 58.81 0.83 1.18 0.88 1.4927/11/96 2.04 36.74 63.26 0.75 1.29 0.96 1.5128/11/96 2.05 29.00 71.00 0.60 1.46 1.08 1.52

29/11/96 2.33 27.83 72.17 0.65 1.68 1.25 1.7330/11/96 1.04 Ð Ð Ð Ð Ð Ð01/12/96 1.04 Ð Ð Ð Ð Ð Ð

02/12/96 1.04 Ð Ð Ð Ð Ð Ð03/12/96 1.04 26.24 73.76 0.27 0.77 0.57 0.7704/12/96 1.33 35.17 64.83 0.47 0.86 0.64 0.99

05/12/96 1.80 28.72 71.28 0.52 1.28 0.95 1.33


Fig. 11. Speci®c production of biogas and methane.


The above studies, conducted at a preliminary level, could not correctly predict thetreatment e�ciency of the process. Detailed and long duration experiments are essential tooptimise the performance of the system discussed above.

6. Summary and conclusions

Designs, such as bag and plug-¯ow types, having signi®cant potential to produce biogas withlower capital investment and reasonably good e�ciency levels, have already been found, bothin developed and under-developed countries. The principal objective of the anaerobic plug-¯owtype reactor reported here was to develop and investigate an experimental unit designedspeci®cally to treat semi-solid wastes available from wholesale fruit and vegetable markets andorthofruit shops, both at higher e�ciency and with fewer operational and maintenanceproblems. Based upon the preliminary experimental results, it has been observed that, duringthe steady state period, while operating at 60 kg/day and 22.5 days HRT, the digester was ableto produce mean daily biogas and methane of 2659 and 1837 litres, respectively. Performancecontrol and process monitoring have also shown good average results in stabilised conditions.Encouraged by satisfactory preliminary experimental results on the treatment of semi-solidwastes, a set of additional experiments using algae (seaweed) has also been conducted, and theresults obtained will soon be submitted for publication.

Fig. 12. Daily production for the di�erent components of biogas produced.


References

[1] Weiland P, Pohland FG, editors. Proceedings of the International Symposium on Anaerobic Digestion of SolidWaste. Venice (Italy), 1992. p. 193.

[2] Cozzolino C, Bassetti A, Rondelli P. Pohland FG, editors. Proceedings of the International Symposium on

Anaerobic Digestion of Solid Waste. Venice (Italy), 1992. p. 551.[3] Premier GC, Dinsdale R, Hawkes DL, Hawkes FR. Paper presented at the 8th International Conference on

Anaerobic Digestion. Sendai (Japan), 1997.

[4] Sharma VK et al. Renewable Energy. An Int. Journal 1997 (in press).[5] Sharma VK et al. Anaerobic digesters for treatment of organic waste. Technical Report ENEA, AMB/TEIN/

RIF, 1997.[6] Vallini GK et al. Pohland FG, editors. Proceedings of the International Symposium on Anaerobic Digestion of

Solid Waste. Venice (Italy), 1992. p. 159.[7] Greenberg AE, Clesceri LS, Eaton AD. Standard Methods for the Examination of Water and Wastewater. 18th

ed. American Public Health Association, Washington, DC 20005, 1992.

Fig. 13. Trend for the composition (%) of the biogas produced.


anaerobic digestion of semi-solid organic waste available from orthofruit market: preliminary...

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