Laboratory scale anaerobic digestion of fruit and vegetable solid waste

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<ul><li><p>Biomass 5 (1984) 245-259 </p><p>Laboratory Scale Anaerobic Digestion of Fruit and Vegetable Solid Waste </p><p>A.G. Lane </p><p>CSIRO Division of Food Research, PO Box 52, North Ryde, New South Wales, 2113, Australia </p><p>(Received 11 October, 1983) </p><p>ABSTRACT </p><p>Anaerobic digestions that were fed waste apple, corn cobs, apple press cake, extracted sugarbeet pulp, pineapple pressings or asparagus waste were stable in trials lasting up to 226 days. Loading rates of 3.5-4.25 kg m-3 day -1 and conversions of 88-96% of the organic solids fed were obtained by ensuring adequate levels of alkalinity, nitrogen and other nutrients during digestion. Gas yields ranged from 0.429 to 0.568 litre (50-60% methaneJ per gram organic solids fed. For reasons not under- stood, gas yields from digestion of apricot waste declined after 63 days from 0.4 77 to 0.13 7 litre g-1 or feedstock. </p><p>Key words: anaerobic digestion, fruit, vegetables, methane yields, conver- sion, supplementation, recycle, alkalinity. </p><p>1. INTRODUCTION </p><p>Disposal of the large quantities of wet, organic refuse generated by fruit and vegetable processing operations creates economic and environ- mental problems to which no fully satisfactory solutions have yet been found. At present, fruit and vegetable wastes are usually disposed of by dumping, spreading on land or by feeding to animals. Earlier reports presented results of laboratory and pilot scale trials carried out to evaluate the anaerobic digestion processes as a means of utilizing these materials, l'z These papers also discussed the potential benefits from commercial application of the process. </p><p>245 Biomass 0144-4565/84/$03.00- Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain </p></li><li><p>246 A.G. Lane </p><p>This report presents results of further laboratory trials carried out to determine the long term stability of digestions fed a range of fruit and vegetable waste solids. Laboratory trials using orange peels as feedstock are the subject of a separate paper. 3 </p><p>2. MATERIALS AND METHODS </p><p>2.1. Digestions </p><p>Digestions were initiated using actively digesting sludge from a municipal sewage digester after screening through wire mesh having 5 X 5 mm openings. Digestions were carried out in 10 litre microbiological fer- menters (L.H. Fermentations, Stoke Poges, Bucks., UK), containing 8 litres of sludge which was stirred for 5 min each half hour at 200 rev min -1 and maintained at 36 -+ IC. The daily charge of feed was delivered to the digester over a 24 h period by means of a motor-driven syringe activated by a timer (Fig. 1). Gas was collected in a football bladder and the volume produced daily was measured by discharge through a gas meter. </p><p>Fig. 1. </p><p>~ ~i~ii~ ~ </p><p>~ ~ ~i~ ~ </p><p>Motor driven syringe (capacity 1 litre) used to deliver fruit and vegetable wastes to anaerobic digester. </p></li><li><p>Anaerobic digestion or fruit and vegetable refuse 247 </p><p>A volume of mixed liquor equal to twice the volume of feed was removed from the digester each day and allowed to settle at IC in a conical 2 litre vessel (apex down). A volume of settled solids equal to the volume of feed was removed daily from the bottom of the vessel and returned to the digester. </p><p>Percentage conversion of feed volatile solids to gas was calculated from the average organic solids contents of feedstock and settled supernatant fluid, over a period of not less than 14 days. </p><p>2.2. Feedstocks </p><p>Waste solids from fruit and vegetable processing were obtained from commercial processing plants. Pineapple pressings were the residue from the pressing of juice from peels and cores, apricot fibre was the residue from ,manufacture of pulp, and sugarbeet pulp was the residue after removal of solubles by countercurrent extraction. 4 Apple cake was the residue from the pressing of fruit for juice extraction, whereas apple waste consisted of peels, cores and rejected fruit. Asparagus waste consisted of the fibrous lower ends trimmed from fresh spears. </p><p>Wastes were hammermilled (12 mm screen) and stored at -20C. Before use, the wastes were diluted with water to 10% total solids then supplemented by addition of (NH4)2 HPO4 (2 g litre -1) and an elements solution (0.2 ml litre -1) containing (g litre -1 in 0.1 M HC1): NaC1, 250: K2SO4, 12.5; MgC12.6H20, 125; CaC12.2H20, 125; FeCI3, 25; ZnO, 1; MnC12.4H20, 0.25; CuSO4, 0.25; (NHa)6MoTO24.4H20, 0.25. All reagents were analytical grade. </p><p>2.3. Alkalinity </p><p>Alkalinity (g litre -1) in digestion supernatant fluids was determined by titration to pH 3.7 using 0.05 M H2SO4, according to the American Public Health Association. s Alkalinity levels in digesting mixtures were adjusted by adding NaHCO3 to maintain pH values above 6.8. </p><p>2.4. Gas analysis </p><p>The carbon dioxide content of digester gases was determined routinely using gas-analysis tubes, type CO2/A, range 5-60% v/v (Drager AG, Lubeck, West Germany). Gas chromatography was used periodically to </p></li><li><p>248 A. G. Lane </p><p>verify the ratio of methane : carbon dioxide. This was done using a Pye Series 104 Chromatograph fitted with a thermal conductivity detector and a glass column (1 m 4 mm) packed with Porapak Q (50-100mesh) and operating at 52C with helium carder gas. </p><p>2.5. Total dry solids (TS), organic solids (OS) and ash </p><p>The TS contents (% w/w) of digesting sludges and feedstocks, other than whole fruit wastes, were determined by drying samples for 24 h at 105 -+ 5C. Whole fruit wastes were dried at 70 -+ 2.5C for 48 h, then for a further 24 h under vacuum at 60 -+ 2.5C. Ash (% w/w of TS) and OS (% w/w) values were determined by weighing before and after the incineration of dried samples at 600 -+ 25C for 2 h. 'Organic solids' (OS) is thus equivalent to the term 'volatile solids' (VS) used by previous authors, but is used in preference as a more accurate descrip- tion of the material lost during incineration. TS, OS and ash contents of supernatant fluids and settled solids were determined either after settling as described under Digestions, or after centrifuging (1150g, 5 min), as specified in the text. </p><p>2.6. Volatile fatty acids (VFA) </p><p>VFA concentrations in centrifuged (1150g, 5 min) digestion super- natant liquors were estimated according to American Public Health Association 5 by absorption onto chromatographic silica gel (May &amp; Baker, Dagenham, UK), elution with acidified butanol-chloroform and titration under nitrogen with 0.02 M NaOH. Acetic acid was used as the standard and results were expressed as g acetic acid litre -1. </p><p>2.7. Chemical oxygen demand (COD) </p><p>COD (g litre -1) of centrifuged (1150 g, 5 min) digestion supernatant fluid was determined according to the Department of the Environment, Great Britain, 6 except that the reaction mixtures in covered vessels were heated in an oil bath at 150C for 2 h and excess dichromate was titrated with ferrous ammonium sulphate (49.01 g litre -1). </p></li><li><p>Anaerobic digestion of fruit and vegetable refuse 249 </p><p>2.8. Total and ammonia nitrogen </p><p>Ammonia nitrogen was determined by titration with H2SO4 (0.05 M) after steam distillation with MgO into boric acid solution (4% w/v). Total nitrogen was determined by distillation after Kjeldahl digestion in which selenium powder was used as catalyst. 7 </p><p>2.9. Phosphorus </p><p>Phosphorus content of fruit and vegetable materials was determined by the colorimetric, phosphomolybdate method of Fiske and Subbarow. 8 </p><p>3. RESULTS </p><p>Results of analyses for total solids, ash, total nitrogen and phosphorus in corn cobs, apple press cake and wastes from processing apricots, apples, pineapples, asparagus and sugarbeet are given in Table 1. </p><p>These waste materials, after hammermilling and supplementation, were fed to digesters for periods of up to 226 days. Data obtained for maximum loading rates, gas yields and percentage conversion of volatile solids to gas for the various materials are given in Table 2. This table </p><p>TABLE 1 Total Solids, Ash, Total Nitrogen and Phosphorus Contents of Fruit and </p><p>Vegetable Wastes </p><p>Waste Total solids Ash Total N P (% w/w) (%)a (%)a (mg%)a </p><p>Apricot fibre 17.7 6.62 1.70 180 Corn cobs 20.9 2.31 0.90 190 Apple cake 16.5 2.82 0.63 110 Apple 12.0 2.08 0.42 72 Asparagus 9-5 1.91 3.51 910 Sugarbeet 7.0 3-36 2.11 130 Pineapple 19.6 3.36 0.72 110 </p><p>a % w/w of total solids. </p></li><li><p>250 A. G. Lane </p><p>TABLE 2 Maximum Loading Rate, Gas Yield and CO2 Content, Average Ammonia Nitrogen Level in Settled Supernatant Fluids and % Conversion of OS During Anaerobic </p><p>Digestion of Fruit and Vegetable Waste Solids </p><p>Waste Duration Maximum Gas yieM C02 NHa-N Conversion of trial load (litre g -a (%) (mg litre -~) (% of OS) (days) (kg TS m -3 TS fed) </p><p>day-l) a </p><p>Apricot b 100 4.0 0.477 38-50 650 96.3 Corn cobs 91 4.0 0.465 36-52 500 95.7 Apple cake 61 4.0 0-454 39-54 510 93-4 Apple 226 3.5 c 0.437 42-56 300 88-1 Asparagus 27 4.25 0.460 48-55 1100 89.7 Sugarbeet 48 4-2 0.445 34-52 400 95-2 Pineapple 83 4.0 0-568 37-50 600 93-2 </p><p>a Maximum loading rates tested. b Results to day 63. e Failed at loading of 4.0 kg m -a day -a. </p><p>also shows ammonia nitrogen levels in settled supernatant fluids during digestion of these materials. </p><p>Table 3 shows the average quantities of sodium bicarbonate added daily to digesters to maintain pH values above 6.8 and ranges of alkalinity values observed during digestion at the maximum loading rates given in Table 1. Table 3 also shows the average VFA levels observed during these digestions. </p><p>The gas yield obtained during digestion of apricot waste was 0.477 litreg -l TS until day 63, when gas production fell sharply to 0.137 litre g-i (Fig. 2). In Table 4, dry matter and ash contents of mixed digesting liquor, settled supernatant fluid and settled solids before (day 23) and after (day 74) this change in gas production are compared with values for fresh digesting sludge from a municipal sewage digester. Average COD values for supernatant fluids from digestions are given in Table 5. </p></li><li><p>Anaerobic digestion or fruit and vegetable refuse 251 </p><p>TABLE 3 Average VFA Concentrations, Average Quantities of Sodium Bicarbonate Added Daily and Alkalinity Ranges During Digestion of Fruit and Vegetable Wastes </p><p>(Loading Rates as in Table 2) </p><p>Waste Average VFA Average NattCOs added Alkalinity (g litre -1) (g litre -1 day -1) (g litre -x) </p><p>Apricot 1.46 0-0 4.4-6-7 Corn cobs 0.99 0-3 5.1-7-8 Apple cake 0.67 0.2 3.1-4.9 Apple 2.12 0.3 4.8-7.6 Asparagus 2.98 0-0 4.6-5.6 Sugarbeet 0-66 0.2 3.5-4.3 Pineapple 1.12 0.0 3.8-4-2 </p><p>600 </p><p>L </p><p>Q . . J 111 </p><p>&gt;- 400 113 </p><p>lxl &gt; I--- .&lt; ._1 </p><p>:[ </p><p>Fig. 2. </p><p>200 </p><p>"~'~"n n a ! 0 200 tO0 600 800 </p><p>CUMULATIVE WEIGHT FED </p><p>/ i </p><p>Grad ient 1 I ~ =0.137 l i t reg~. I .~ </p><p>qv 1' ..7 / , /Y Day 63 </p><p>2 /~'o'o'" Grad i ent </p><p>14." = 0 '477 l i t re g - I ./.... </p><p>io </p><p>I I </p><p>1000 1200 (g ) </p><p>Cumulative gas yield vs cumulative weight of feedstock (TS) during digestion of apricot waste (loading 4.0 kg TS m -3 day-l). </p></li><li><p>252 A.G. Lane </p><p>TABLE 4 Total Solids (TS) and Ash (% of TS) Contents of Mixed Digester Liquor, Settled Supernatant Fluid and Settled Solids During Digestion of Municipal Sewage Sludge </p><p>and Apricot Waste </p><p>Apricot digestion </p><p>Day 23 Day 74 </p><p>Sewage sludge digestions (~ w/w) </p><p>TS Ash TS Ash TS Ash </p><p>Mixed liquor 3.82 33.7 3.73 23.3 2.10 41.3 Supernatant fluid 0.23 48.6 0.72 50.0 0.17 46.3 Solids 11.5 41.0 11.4 18.6 12.8 42.6 </p><p>TABLE 5 Average COD Values of Centrifuged Digestion Supernatant Fluids During Digestion </p><p>of Fruit and Vegetable Processing Wastes </p><p>Waste Loading (kg TS m-3 day -1) COD (g litre -1) </p><p>Apricot fibre 4.0 6.34 Corn cobs 4.0 4.97 Apple cake 4.0 2-26 Apple 3.5 4.51 Asparagus 4.25 10.11 Sugarbeet 4-2 3.05 Pineapple 4.0 5.35 </p><p>4. DISCUSSION </p><p>4.1. Alkalinity levels </p><p>Loading rates and percentage conversions reported here for fruit and vegetable wastes are considerably higher than those obtained by previous authors. 9'1 Hills and Roberts 9 examined the anaerobic digestion of peach and melon wastes without nitrogen or phosphorus addition or alkalinity adjustment. Digestion of melon waste was balanced up to a loading rate of 3 kg m -3 day -1 and retention time of 15 days (alkalinity, 4.06 g litre -1 ; VFA, 2.20 g litre -1 ; pH, 8.0). However, peach digestion </p></li><li><p>Anaerobic digestion of fruit and vegetable refuse 253 </p><p>was stable at a loading rate of only 1 kg m -3 day -x (retention time 15 days) and severe overloading occurred at 3 kg m -3 day -a, retention time 20 days (alkalinity, 1.08 g litre -1 ; VFA, 3.05 g litre -a ; pH, 6.2). </p><p>Failure of peach digestion appears to have been due to inadequate levels of alkalinity to balance the levels of VFA in the digestion liquors. We have found that careful attention to alkalinity levels during digestion of fruit and vegetable materials is crucial to the success of the digestion. For stable digestions, it is imperative that a satisfactory ratio be main- tained between VFA and alkalinity levels. This ratio is given by the empirical relationship that for balanced digestion, alkalinity (mg litre -1) ---0.7 VFA (mg litre -a) should not be less than 1500. n'x2 On this basis, an alkalinity of 1080 mg litre -x in the presence of 3050 mg VFA litre -a (Hills and Roberts 9) is grossly inadequate and digestion failure would be anticipated. </p><p>Inadequate alkalinity levels appear to have also been the cause of digestion failure in experiments reported by Knol et al. xo Maximum loading rates for stable digestion reported by these authors for a variety of wastes were only 0.9-1.6 kg m -3 day -x, with retention time 32 days in all cases. Values for pH as low as 5.3 were reported during digestion of apple 'pulp' at higher loadings (3 kg m -3 day -a) and stable digestion was not achieved with this material even at loading rates as low as 1.02 kg m -3 day -~ (VFA, 1.5-4.5 g litre -x ; alkalinity not given). These authors also fed unsupplemented apple 'slurry' at 3 kg m -3 day -1 with- out alkalinity adjustment and found that pH began to fall after 10 weeks. This time corresponds to 2.2 volume changes, so the alkalinity originally present in the sewage digester sludge, typically 3 g litre -x in our experience, would have been diluted to inadequate levels. </p><p>Knol et al.X reported unbalanced digestion of asparagus waste at a loading of only 1.06 kg m -3 day -1, with only 40% conversion of VS to gas. Our results show digestion of asparagus waste was stable at loadings of 4.25 kg m -3 day -~ and 89.7% conversion of volatile solids to gas was obtained (Table 2). Addition of sodium bicarbonate was not required, because the digestion naturally maintained alkalinity levels (4.6-5.6 g litre -1) adequate to balance the levels of VFA. As observed by Knol et al., x VFA levels during asparagus digestion were somewhat higher than with other wastes (Table 3). Digestions that were fed apricot pulp and pineapple pressings also maintained adequate pH and alkalinity levels without the need for addition of bicarbonate. Small additions of bicarbonate were required to maintain correct alkalinity balance and </p></li><li><p>254 A. G. Lane </p><p>pH values during digestion of the other waste materials (Table 3), The maximum loading rate (3.5 kg m -a day -...</p></li></ul>


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