Biogas production by anaerobic digestion of fruit and vegetable waste. A preliminary study

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<ul><li><p>J . Sci. Fd Agric. 1978, 29, 822-830 </p><p>Biogas Production by Anaerobic Digestion of Fruit and Vegetable Waste. A Preliminary Study </p><p>Wieger Knol, Michael M. van der Most and Jacobus de Waart </p><p>Central Institute Jbr Nutriiion and Food Reseurch TNO, Zeisr, The Ne~heria~ds </p><p>(Manuscript received 23 January 1978) </p><p>Waste of apples, asparagus, carrots, green peas, French beans, spinach and straw- berries from a canning factory have been screened on mesophilic anaerobic digestion in 90-day experiments at loading rates varying between 0.80 and 1.60 kg volatile solids (VS) m-3 day-l at a retention time of 32 days. Average biogas yields varied from 0.30 to 0.58 m3 kg-l VS day-1. High percentages of reduction in VS, carbohydrate and crude fibre were obtained in most experiments. Some waste materials showed un- balanced digestion, as might be expected from carbohydrate-rich substrates. In those cases alkali addition, feed interruption and mixing with a nitrogen-rich substrate were used to overcome unbalanced digestion. Residual solids in the digested sludges were removed by flocculation with a polyelectrolyte and centrifugation; liquids with lower CODs remained after flocculation. </p><p>1. Introduction </p><p>Biogas production by anaerobic digestion of waste materials has been in use for more than a century. In some Asian countries biogas obtained from animal and domestic waste provides the fuel needed by small farmers.l In developed countries interest in anaerobic digestion revived when circumstances made energy supply uncertain; for example in Germany shortly after World War 11.2 Although biogas production is an important aspect of anaerobic digestion, the process has other advantages in waste treatment, such as reduction in dry matter, smell and pathogenic organisms and the better dewatering properties of the digested sludge. Therefore, it has been used to stabilise sewage sludge in domestic sewage plants for at least 60 years. </p><p>It is not surprising that in the last decennium anaerobic digestion has come into focus again. Rising fuel prices have led to serious consideration of the profit to be expected from practical appli- cation of the process. On the other hand, adequate treatment of growing quantities of agricultural and domestic waste materials is becoming more and more urgent in present day society. In this connection, much attention has been given in recently published papers to the disposal of the enormous quantities of animal waste from intensive pig </p><p>In the fruit and vegetable canning industry, about 10% of the raw material is, generally speaking, wasted in the course of production, the actual amount depending on the product involved. Part of the relevant waste can be used as feed. The rest is transported to a municipal dump site. In view of the increasing charges for this kind of disposal, production of biogas by anaerobic digestion would be an interesting alternative. The biogas produced might cover an important part of a factory's energy demand. While in domestic sewage sludge digestion approximately 50 % of the biogas yield is needed to heat the digester contents in order to obtain optimal digestion, in the fruit and vegetable industry cooling water might serve this purpose so that the total gas production could be utilised, e.g. for the generation of steam to be applied directly in the factory. However, detailed data on the anaerobic digestion of fruit and vegetable waste are not available. The aim of the present study was to get preliminary information on this matter. </p><p>Many papers have been published on the microbiology and biochemistry of anaerobic digestion </p><p>0022-5142/78/0900-0822 $02.00 0 1978 Society of Chemical Industry 822 </p></li><li><p>Biogas from fruit and vegetable waste 823 </p><p>Waxes Plastics Hydrocarbon 011s </p><p>Nan-protein N Corbohydrates T SO4 NO3 </p><p>Long chain sugars SH NH3 </p><p>V o l a t i l e f a t t y </p><p>7 </p><p>Bacteria, sa l ts </p><p>residues </p><p>Figure 1. Reactions involved in anaerobic digestion according to Hobson et aL6 </p><p>in digesters as well as in the rumen of herbivores. In the survey by Hobson et a1.6 these topics are discussed in detail. Briefly, anaerobic digestion can be defined as a two-stage process, carried out by a mixed bacterial population in a continuous culture, the optimal temperature for mesophilic digestion being approximately 32C. The first stage, defined as liquefaction, comprises the hydrolysis of high molecular weight compounds into metabolites, from which volatile acids, hydrogen and carbon dioxide are formed. In the second stage, defined as methanogenesis, the intermediates are further transformed to methane and residual carbon dioxide (Figure 1). The two stages are balanced so that little of the intermediate acids accumulate. </p><p>Addition of a new substrate to a stable digestion could for a time result in unbalanced growth until a new bacterial population is established. Therefore, new substrates may need a certain adaptation period. Recovery of a balanced flora may be prevented when rapid acid formation lowers the pH from its optimum of about 7.2, caused by much easily digestible carbohydrate in the substrate, as in fruit and vegetable waste. Methane fermentation is then disturbed and even complete digestion failure may occur. Digestion of nitrogenous compounds leads to ammonia which is partly converted to bacterial protein. Accumulation of ammonia, due to much nitrogen in the substrate, will have a neutralising effect upon excess acids. Inhibition by accumulated ammonia is not likely to O C C U ~ . ~ A better balanced digestion may be obtained by mixing wastes high in carbohydrate with wastes high in nitrogen. A temporary increase in acidity may be counteracted by cessation of loading or addition of alkali.638 These measures were applied in this study. </p><p>In addition, this investigation comprises preliminary experiments concerning removal of residual solids from the digested wastes and the pollutional value of the remaining sludges. </p><p>2. Experimental 2.1. Materials Samples of fruit and vegetable waste were collected at Veluco Conservenfabrieken BV, Gelder- malsen, The Netherlands. From each sample an aliquot was analysed for ash, crude protein, total carbohydrates and crude fibre (Table 1). Directly after arrival the samples were homogenised, frozen and stored at - 20C. The samples were thawed and diluted to feed the digesters at a loading rate of approximately 1 kg volatile solids per m3 digester volume per day and to maintain a retention time of 32 days. </p><p>Digested sludge from the anaerobic digesters of the municipal sewage works at Zeist, The Nether- lands, was used to start the experiments. </p><p>2.2. Anaerobic digestion The experiments were carried out in 1 litre digesters. Stirring speed was set at 200 rev min-l. Each digester was heated by a 33C water bath. Biogas production was measured with Mariotte bottles </p></li><li><p>824 </p><p>Table 1. Origin and chemical composition of the fruit and vegetable wastes </p><p>W. Knol et al. </p><p>Waste samplea Origin, definition </p><p>Total carbohydrates </p><p>( %) Crude protein Crude fibre </p><p>( %) ( %) </p><p>Group I Spinach-waste Raw and blanched spinach, </p><p>Asparagus-peels Peels removed by hand Group 2 French bean-waste Mixture of samples collected </p><p>from waste containers along production line </p><p>Strawberry-slurry Strawberry waste with small amounts of asparagus and peas </p><p>Apple-pulp Remains after blanching and pressing in apple-sauce production </p><p>Apple-slurry Mainly apple particles with small amounts of asparagus </p><p>Carrot-waste Mixture of carrot-skin-sludge and rejected carrots </p><p>Group 3 Green peas-slurry Mainly whole and damaged </p><p>peas </p><p>dropped along production line 12.0 1.74 </p><p>8.9 0.72 </p><p>13.4 1.49 </p><p>11.5 1.75 </p><p>23.2 0.46 </p><p>6.7 0.29 </p><p>5.3 0.48 </p><p>12.8 0.45 </p><p>0.66 (0.05)b </p><p>0.50 (0.06) </p><p>4.01 (0.29) </p><p>2.42 (0.21) </p><p>5.57 (0.24) </p><p>1.41 (0.21) </p><p>1.58 (0.29) </p><p>5.70 (0.44) </p><p>4.22 (0.35)b 2.11 </p><p>1.98 (0.22) 2.14 </p><p>2.37 (0.18) 2.10 </p><p>2.25 (0.19) 2.38 </p><p>1.28 (0.05) 3.74 </p><p>0.60 (0.08) 1.29 </p><p>0.38 (0.07) 0.67 </p><p>3.49 (0.27) 1.68 </p><p>fC Waste materials are classed under groups 1, 2 or 3 on account of their carbohydrate content in the dry weight. 1 Figures in brackets are the percentages on dry weight basis. </p><p>connected with the gas outlet of the digester and filled with acidified water to prevent solubilisation of carbon dioxide (Figure 2). The digesters were filled initially with 1 litre digested sewage sludge. Each working day, except Friday, the digesters were fed 40 ml influent sludge, i.e. 160 ml diluted waste over 4 days. Then half the amount for 3 days, i.e. 60 ml was added. In this way, an average liquid retention time of 32 days was obtained. Simultaneously with loading the same volume of digested waste was discharged without allowing air to enter the digester. Biogas production was recorded every day. Examinations of the effluents were carried out periodically and comprised determination of pH, total solids, ash, volatile fatty acids; the amount of CH4 in the biogas was also estimated. The duration of the experiments was about 90 days, i.e. three times the retention time. The loading rate of each experiment varied as is indicated in Table 2. At the end of the experi- ments the digested waste was dried for analysis. </p><p>In addition, preliminary experiments were carried out with regard to the removal of residual solids and the pollutional value of the digested waste. Residual solids were removed by (a) floccula- tion of 100 ml final sludge with 15 ml of a 0.1 % solution of a polyelectrolyte (Preastol 444K) and (b) centrifugation of 100 ml sludge at 1500 xg. Chemical oxygen demand of the filtrate and super- natant was determined. </p><p>2.3. Analysis Definitions of the terms to be used are given below to make comparison with other investigations possible. Total solids content (TS) is defined as the percent dry weight of residue obtained after drying the sludges for 16 h at 105C. Ash is the weight of material remaining after ashing of dry matter and heating of the residue for 3 h at 550C. VolatiZe solids (VS), i.e. organic matter, is calculated as total solids minus ash. Total nitrogen was estimated according to the Kjeldahl method. Crude protein was calculated by multiplying total nitrogen content by 6.25. </p></li><li><p>Biogas from fruit and vegetable waste 825 </p><p>Total carbohydrate estimation was carried out according to Van de K a ~ n e r . ~ Tn this procedure the material is boiled with water and subsequently exposed to pancreatin amylase. After acid hydrolysis the reducing sugars obtained are determined. Total carbohydrate is expressed as glucose. Crude fibre analyses were performed according to the methods of the Rijkslandbouwproefstation, Maastricht, The Netherlands,lo a modification of the Weender method based on successive boiling in dilute sulphuric acid and dilute sodium hydroxide, washing and drying. Loss of weight after incineration of the dry material at 550C is reported as crude fibre. Volatilefatty acids (VFA) were estimated according to a method developed at the institute (Wijsman, J. A., personal communication), and carried out as follows. After removing the solids by centrifu- gation, alkali is added to the supernatant. The salts of the acids are dried and suspended in organic solvents. By addition of concentrated phosphoric acid the fatty acids are reformed and the different acids (CZ-CS) are quantitatively and qualitatively estimated by gas chromatographic analysis. </p><p>7 </p><p>Figure 2. One-litre digester. (1) Funnel and tube-clamp for charge; (2) tube and clamp for discharge; (3) gas outlet; (4) Mariotte bottle; (5 ) heat exchanger connected to water bath. The digester was immersed in a water bath at 32C. </p><p>Extent of decomposition. The percentage decomposition of volatile solids, total carbohydrates and crude fibre is derived from the analysis of the waste materials before and after digestion. The pH was measured with an Electrofact pH meter. Methane. The methane content of the produced biogas was determined on a Carlo Erba gas chroma- tograph, type Fraktovap M, with a 80-100 mesh silica gel column and a hot wire detector. </p><p>Chemical oxygen demand (COD) was determined by the bichromate sulphuric acid procedure, according to the methods of the Dutch Normalisation 1nstitute.ll </p><p>3. Results and discussion 3.1. General In Table 1 the chemical compositions of the different fruit and vegetable waste samples are given. The characteristics of the various experiments are listed in Table 2. </p></li><li><p>Tab</p><p>le 2</p><p>. C</p><p>hara</p><p>cter</p><p>istic</p><p>s of</p><p> the </p><p>expe</p><p>rim</p><p>ents</p><p> and</p><p> ext</p><p>ent </p><p>of d</p><p>econ</p><p>lpos</p><p>ition</p><p> by </p><p>anae</p><p>robi</p><p>c di</p><p>gest</p><p>ion </p><p>Ext</p><p>ent </p><p>of d</p><p>ecom</p><p>posi</p><p>tion </p><p>(%) </p><p>Load</p><p>ingb</p><p> B</p><p>ioga</p><p>s V</p><p>FA</p><p> (mg </p><p>litre</p><p>-')" </p><p>kg V</p><p>S m</p><p>-3 </p><p>m3 k</p><p>g-lV</p><p>S bi</p><p>ogas</p><p> P</p><p>H </p><p>- </p><p>Aft</p><p>er </p><p>Tot</p><p>al </p><p>rate</p><p> pr</p><p>oduc</p><p>tion </p><p>CH</p><p>I in</p><p> TS (</p><p>%) </p><p>Exp</p><p>erim</p><p>enta</p><p> da</p><p>y-1 </p><p>day-</p><p>' (%</p><p>) In</p><p>fl. </p><p>Effl.</p><p> In</p><p>A. </p><p>Effl.</p><p> T</p><p>otal</p><p> 50 d</p><p>ays </p><p>VS </p><p>carb</p><p>ohyd</p><p>rate</p><p>s C</p><p>rude</p><p> fibr</p><p>e </p><p>Gro</p><p>up I</p><p> Sp</p><p>inac</p><p>h-w</p><p>aste</p><p> A</p><p>spar</p><p>agus</p><p> pee</p><p>ls </p><p>Gro</p><p>up 2</p><p> Fr</p><p>ench</p><p> bea</p><p>n-w</p><p>aste</p><p> St</p><p>raw</p><p>berr</p><p>y-sl</p><p>urry</p><p> A</p><p>pple</p><p>-pul</p><p>p A</p><p>pple</p><p>-slu</p><p>rry </p><p>Car</p><p>rot-</p><p>was</p><p>te </p><p>Gro</p><p>up 3</p><p> G</p><p>reen</p><p> pea</p><p>-slu</p><p>rry </p><p>Mixe</p><p>d su</p><p>bstr</p><p>ates</p><p> Pi</p><p>g m</p><p>anur</p><p>e + a</p><p>pple</p><p> pul</p><p>p </p><p>0.83-1.18 </p><p>0.74-1.06 </p><p>0.40 </p><p>0.30 </p><p>77-80 </p><p>64-82 </p><p>3.1-4.5 </p><p>2.5-3.6 1.3-2.0 </p><p>I .7-2.4 </p><p>4.9 </p><p>4.3 </p><p>7.2-7.5 </p><p>7.0-7.2 </p><p>15-1580 </p><p>430-3740 </p><p>15-220 </p><p>430-2670 </p><p>70 </p><p>40 </p><p>80 </p><p>90 </p><p>70 </p><p>20 </p><p>0.47 </p><p>0.34 </p><p>0.49 </p><p>0.38 </p><p>0.58 </p><p>95 </p><p>95 </p><p>95 </p><p>95 </p><p>100 </p><p>60 </p><p>55 </p><p>20 </p><p>55 </p><p>70 </p><p>0.96-1.15 </p><p>1.02-1.15 </p><p>1.02-1.60 </p><p>0.80-0.90 </p><p>0.83-1.15 </p><p>71-75 </p><p>72-82 </p><p>5&amp;75 </p><p>72-75 </p><p>71-73 </p><p>3.444.0 </p><p>3.9-4.3 </p><p>3.5-5.1 </p><p>2.7-3.8 </p><p>2.8-3.2 </p><p>1 .5</p><p>-1. </p><p>9 2.0-2.7 </p><p>2.6-2.8 </p><p>1.5-2.2 </p><p>1.0-1.5 </p><p>4.5 </p><p>4.7 </p><p>4.1-7.0 </p><p>3.8-7.0 </p><p>4.2 </p><p>7 .O-7.2 </p><p>6.4-7.0 </p><p>5.3-7.1 </p><p>6.5-7.1 </p><p>6.6-7.0 </p><p>7-165 </p><p>92-4090 </p><p>1560400 </p><p>0-16</p><p>10 </p><p>43-3300 </p><p>7-150 </p><p>92-2370 </p><p>2570-3350 </p><p>36-490 </p><p>0-340 </p><p>70 </p><p>50 </p><p>40 </p><p>60 </p><p>75 </p><p>0.87-1.25 </p><p>0.42 </p><p>66-8 1 </p><p>2.8-4.0 </p><p>1 .2-1.6 </p><p>4.5 </p><p>5.6-6.9 1750-5460 </p><p>3930-5350 </p><p>75 </p><p>95 </p><p>80 </p><p>0.90-1.12 </p><p>0.45 </p><p>72-75 </p><p>3.2-4.3 </p><p>2.1-</p><p>2.9 </p><p>6.9 </p><p>7.1-7.3 </p><p>22-1430 </p><p>22-1 55</p><p> 50</p><p> 95 </p><p>60 </p><p>a W</p><p>aste</p><p> mat</p><p>eria</p><p>ls a</p><p>re c</p><p>lass</p><p>ed u</p><p>nder</p><p> gro</p><p>ups 1,</p><p> 2 o</p><p>r 3 </p><p>on a</p><p>ccou</p><p>nt o</p><p>f th</p><p>eir </p><p>carb</p><p>ohyd</p><p>rate</p><p> cont</p><p>ent. </p><p>See </p><p>text</p><p>. * M</p><p>axim</p><p>um a</p><p>nd m</p><p>inim</p><p>um v</p><p>alue</p><p>s ar</p><p>e gi</p><p>ven,</p><p> exc</p><p>ept f</p><p>or th</p><p>ose </p><p>of b</p><p>ioga</p><p>s pr</p><p>oduc</p><p>tion,</p><p> bei</p><p>ng a</p><p>vera</p><p>ge v</p><p>alue</p><p>s. </p><p>c V</p><p>FA a</p><p>re g</p><p>iven</p><p> in </p><p>two </p><p>colu</p><p>mns</p><p>; in </p><p>the </p><p>first</p><p> col</p><p>umn </p><p>VF</p><p>A d</p><p>urin</p><p>g th</p><p>e w</p><p>hole</p><p> exp</p><p>erim</p><p>ent, </p><p>in th</p><p>e se</p><p>cond</p><p> vol</p><p>ume </p><p>VFA</p><p> aft</p><p>er a</p><p>n as</p><p>sum</p><p>ed a</p><p>dapt</p><p>atio</p><p>n pe</p><p>riod</p><p> of </p><p>50 d</p><p>ays.</p></li><li><p>Biogas from fruit and vegetable waste a21 </p><p>To facilitate the discussion of the results the waste samples have been divided into three groups. The division is based on the carbohydrate content in the dry weight of the materials, because the carbohydrate content may affect digester functioning, as stated in the introduction. </p><p>Spinach-waste and asparagus-peels can be classed under group 1, the carbohydrate content in the dry weight being 0.05 and 0.06 %. The larger group 2 comprises apple-pulp, apple-slurry, carrot- waste, French bean-waste and strawberry-slurry with a carbohydrate content varying between 0.21 and 0.29%. Group 3 consists of green pea slurry only, having a carbohydrate content in the dry weight of 0.44 %. </p><p>3.2. Waste sample...</p></li></ul>

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