anaerobic digestion of the barcelona central food market organic wastes: experimental study

Bioresource Technology 39 (1992) 39-48

Anaerobic Digestion of the Barcelona Central Food Market Organic Wastes: Experimental Study Joan Mata-Alvarez, P. Llabr6s

Department d'Enginyeria Qufmica, Universitat de Barcelona, 08028 Barcelona, Spain

Franco Cecchi & Paolo Pavan

Dipartimento Scienze Ambientali, Universith di Venezia, 30123 Venezia, Italy

(Received 6 August 1990; accepted 25 November 1990)

Abstract HS-OFMSW

Experiments carried out to obtain data for a pre- k liminary design of an anaerobic digestion plant treating the organic wastes coming from a large MS-OFMSW food market are described. Four hydraulic retention times (8, 12, 14 and 20 days) were tested in OFMSW 3-litre laboratory digesters. The methane fields were high (around 0"478 m 3 CH4/kg VS added). Q Kinetic analysis, using a first-order model, resulted S in a kinetic constant of 3.1 day- ~ and an ultimate methane yield of 0.489 CH4/kg VS added. The S o biodegradability of the soluble and non-soluble fractions of the Volatile Solids (VS) is discussed. SBVS

Key words: Municipal solid wastes, refuse, kinetic, biogas, methanization, anaerobic treatment, yields.

SC-OFMSW

SMP

SS-OFMSW

NOTATION STS SVS

B Specific methane production (m 3 SVSre m

CH4/kg TVS) TA B 0 Feed ultimate methane yield (m 3 TBVS

CH4/kg TVS) B0~ Ultimate methane yield of the TS

non-soluble fraction of the feed TVS (m 3 CH4/kg TVS) TWSre m Ultimate methane yield of the t soluble fraction of the feed (m 3 V CH4/kg TVS) VFA Continuous stirred tank reactor VMPR Hydraulic retention time (days)

Bos

CSTR HRT

39 Bioresource Technology 0960-8524/92/S03.50 © 1991 Elsevier Science Great Britain

Hand sorted organic fraction municipal solid waste First-order kinetic constant (day- 1) Mechanically sorted organic fraction municipal solid waste Organic fraction municipal solid waste Flow rate (m3/day) Digester concentration (kg TVS/ m 3 ) Substrate concentration (kg TVS/ m 3 ) Soluble biodegradable volatile solids (%) Separately collected organic fraction municipal solid waste Specific methane production (m 3 CHJkg TVS) Source-sorted organic fraction municipal solid waste Soluble total solids (%) Soluble volatile solids (%) Removal of SVS (%) Total alkalinity (mg CaCO3/liter ) Total biodegradable volatile solids (%) Total solids (%) Total volatile solids (%) Removal of TVS (%) Time (day- l) Digester volume (m 3) Volatile fatty acids (mg/litre) Volumetric methane production rate (m 3 CH4/m 3 digester day)

Publishers Ltd, England. Printed in

40 J. Mata-Alvarez, P., Llabrds, F. Cecchi, P. Pavan

INTRODUCTION

Mercabarna SA is a large food market serving Barcelona and its surroundings. Every day around 2600 metric tonnes of fruit and vegetables, and about 300 metric tonnes of fish are sold. This activity generates around 100 metric tonnes per day of wastes, 60% o f them being putrescibles (Table 1).

These wastes are presently sent to a sanitary landfill. However, this is progressively more expensive and hence other solutions were considered (Mata-Alvarez et al., 1990a). One of the most promising was to digest the organic fraction in an anaerobic reactor. The main advantage of this process is the production of biogas, which can be used to produce electricity for own consumption. A valuable effluent is also obtained, which eventually can be used as an excellent soil con- ditioner, given the origin of these wastes.

Another possible solution was the co-digestion of this organic fraction with sewage sludge in a nearby wastewater treatment plant. This inte- grated approach has the additional advantage of using an existing digester and allows the wastewater treatment plant to be autonomous as far as energy is concerned (Cecchi & Traverso, 1985; Mata-Alvarez & Cecchi, 1989a). However, trans- portation and administrative difficulties made this approach inadvisable.

The anaerobic digestion of putrescible wastes has been studied extensively in relation to the organic fraction of municipal solid waste (OFMSW) (Cecchi et aL, 1988b). The results

Table 1. Different fractions of the wastes of Mercabarna food market

Organic fraction and paper/cardboard 67-0" Putrescibles 60.3 Paper/cardboard 6"7

Fuel fraction 25"5 Palets (wood) 4"6 Boxes 16"4 Small fragments (wood) 4.5

Plastics 5.7 Bottles, bags 0.09 Polystyrene 0'62

Landfilling fraction 1.8

Glass 0"18 Metals 0.42 Others non-organic 0' 56

aFigures in metric tonnes/day.

obtained with this type of substrate are very satisfactory and there is a growing interest in this tech- nique as a means of treating OFMSW. These satisfactory results were a decisive factor in con- sidering this approach for putrescible waste abate- ment. However, as has been recently shown (Mata-Alvarez et al., 1990b), the yield and kinetics of the biological reactions involved are very dependent upon waste composition.

This paper describes experiments carried out to obtain experimental information (reaction yields and kinetics) necessary to carry out a preliminary design of an anaerobic digestion plant to treat this specific waste.

METHODS

Experimental device Experiments were carried out in a series of four 3-1itre working-volume glass reactors, the contents of which were completely stirred. The mix- ing device was a stainless steel stirrer with four arms rotating at 100 rpm. The shape of the reactor was elongated and four sample ports were provided at different heights on the digester side wall. Analysis carried out on samples from different ports assured the homogeneity of the reactor contents. Temperature was within the opt- imum mesophilic range (35 + 0"5°C). A diagram of one of the reactors is shown in Fig. 1. The digester was fed once a day with a substrate diluted to approximately 4% TS (see substrate preparation section). Gas production was measured by an impulse-counter gas meter (Mata-Alvarez et al., 1986).

Substrate preparation First of all, a substrate was prepared which approximately represented the putrescible fraction of the Mercabarna wastes. For a laboratory experiment, direct collection and sorting was not possible because of the large amount of waste required to obtain a representative sample of the right composition. Moreover, the fruit and vegetables sold are not quite the same all through the year. All these factors led to the conclusion that the best approach would be to prepare a waste sample, taking into account the amounts of food sold throughout the year. The assumption of pro- portions of wastes/food sold is fairly approximate, because a significant amount of waste is food not sold for market reasons.

Anaerobic digestion of organic wastes 41

Table 2 shows the amounts in tonnes sold during 1987. About 50 kg of a representative substrate was prepared. After shredding and homo- genizing, it was stored in 10 kg vessels at 2°C.

Table 3 shows the mean results of analysis of the substrate. The soluble fraction of organic matter is higher than other municipal solid wastes (Mata-Alvarez et al., 1990b) and therefore a high biodegradability can be expected (Mata-Alvarez et al., 1990b). The substrate is well balanced in respect of nutrients, so that no extra additions were necessary. Analyses of volatile fatty acids (VFA) were performed on the soluble fraction of the shredded waste. These were carried out after homogenization and before storage.

Analysis VFA was monitored using a Shimadzu GC-9A gas chromatograph (Shimadzu, Japan), according to the method reported by Cecchi et al. (1988a). Analysis of Total Alkalinity (TA) (pH end point 3-8) was carried out on the liquid phase of the samples after 10 min centrifugation (3000 rpm). Gas composition was measured in the same chromatograph using a Porapak Q column, 1/8 in. diameter and 3 m long. Conditions were: oven temperature 37°C and detector (thermal con- ductivity) temperature 100°C. Other parameters were measured according to Standard Methods (1985).

RESULTS AND DISCUSSION

Four experiments were carried out simultaneously at the four Hydraulic Retention Times (HRT) presented in Table 4. Previously diluted substrate was used so as to achieve a TS concentration of 3-96% (this dilution was adopted after analysis of the diluted waste). The use of the undiluted substrate was discarded because the minimum HRT required to achieve an Organic Loading Rate (OLR) above 6 kg TVS/m 3 day (limit found for a similar type of waste (Cecchi et al., 1986)) was too large (over 20 days). Moreover, as mentioned above, this waste was presumably more biodegradable (Mata-Alvarez et al., 1990b), which meant a larger and a faster VFA production and which stressed the validity of this OLR limit. For all these reasons and also because of the avail- ability of substrate (50 kg), the maximum OLR tested was below 3 kg TVS/m 3 day.

( i )

I II (2)

I Z Z ~

= (3)

F ~

F - - ~

- - ( 4 )

Fig. 1. Scheme of the elongated digesters used to treat the organic fraction of the wastes coming from a large food market. (1) Feed inlet; (2)biogas outlet; (3) sample port; (4) effluent outlet.

Table 2. Components of the substrate prepared to feed the digesters

Item Metric tonnes % of total commercial" commercial

Fruit Oranges 76 515 23'4 Apples 34 920 10'7 Peaches 30 205 9.3 Bananas 27 906 8.6 Melon 27 888 8"5 Tangerines 25 986 8'0 Pears 18 928 5.8 Water melon 18 018 5-5 Grapes 17 683 5-4 Lemons 14 654 4-5 Strawberries 7 537 2"3

Total 300 240 92-0

Vegetables Potatoes 69 136 22-0 Tomatoes 61 553 19-6 Lettuce 39 902 12-7 Onions 19 896 6-3 Beans 17 306 5.5 Pepper 14 614 4.7 Carrots 14 229 4"5 Artichokes 11 471 3"7 Chards 6 275 2.0

Total 254 382 81.0

Seafood Mussels 14 232 30.3 Small hake 5 339 11-4 'Rape' 4 714 10.0 Hake 4 406 9.4 Cod 3 318 7.1 Sardines 3 067 6'5

Total 35 076 74.7

aComposition followed the tonnes commercialized in 1987, so as to achieve a representative sample of the putrescible wastes.

42 J. Mata-Alvarez, P. Llabr~s, F. Cecchi, P. Pavan

Table 3. Mean results of the analysis performed on the prepared substrate previous to dilution

Total Solids (TS, g/litre) 121.1 Total Volatile Solids (TVS, g/litre) 102"6 Soluble Total Solids (STS, g/litre) 76"5 Soluble Volatile Solids (SVS, g/litre) 61"1 Total Carbon (TC, % TS) 46.4 Total Nitrogen (N, % TS) 2.8 Total Phosphorus (P, % TS) 0.28 Total Alkalinity (TA, mg CaCO3/litre ) 2980 Total VFA 1446

Acetic 692 Propionic 199 Iso-butyric 104 Butyric 175 Iso-valeric 45 Valeric 88 Iso-caproic 70 Caproic 43 Heptanoic 30

Table 4. Yields obtained in the digestion of the putrescible fraction of Mercabarna wastes at the four conditions tested a

Reactor results HR T (days)

20 16 14 12

OLR (kg SW/m 3 day) 1.68 2.10 2.40 2.8 Biogas production (litre/day) 3.84 4.8 5"52 6.4 CH4(% ) 63 63 62.5 62"5 SMP (m 3 CHa/kgSV ) 0.480 0.480 0.479 0.476 TVS removal (%)h 89.7 8 9 . 7 9 0 . 7 90.1

Reactor TS (%) 1 0.7 1.1 1"05 Reactor TVS (%) 0.4 0.35 0.4 0"35 Reactor STS (%) 0"55 0 . 6 5 0.55 0"6 Reactor SVS (%) 0.04 0.09 0"05 0"11

TS removal (%) 74.7 8 2 . 3 72"2 73.5 TVS removal (%) 88-1 8 9 . 6 88"1 89"6 STS removal (%) 78-1 7 4 . 1 78-1 76-1 SVS removal (%) 98-0 9 5 . 5 97-5 94-5

aDiluted feed composition

Feed TS 3"96 (%) Feed TVS 3"36 (%) Feed STS 2.51 (%) Feed SVS 2.01 (%)

bComputed from biogas production.

Start-up Digesters were loaded to 60% of the total volume with the effluent of an industrial sewage sludge digester and 10% with substrate, but diluted at 0.5% TS concentration. Once the digester was filled, a volume equal to the quantity added each day, was removed.

The H R T for each digester was as indicated in Table 4, but the substrate did not achieve the stationary concentrat ion until the 5th week.

The substrate concentrations used were:

1 st week 0"5% 2nd week 1% 3rd week 2% 4th week 3% 5th week 4%

Gas product ion increased steadily day by day in all the digesters, except for number 4. Probably due to the larger load of this digester some problems of acidification occurred and the start- up procedure was repeated after the second week, allowing 10 days at each concentrat ion before changing to the next.

After 4 0 - 5 0 days, all the digesters reached the final gas yield levels. However, the experiment was continued for nearly three months to assure that steady-state conditions were reached. Figure 2 shows the gas product ion during the start-up period for the four digesters. As can be seen, the maximum gas product ion was achieved with the digester working at the higher OLR. In fact, this behaviour could be expected because at the experimental HRT, Specific Methane Production (SMP) should be nearly constant (as it is, see steady-state results below) provided there is neither bacteria washout nor digester overloading (Mata-Alvarez et aL, 1990b). As a consequence, and taking into account that the volumetric methane product ion rate (VMPR) is:

V M P R = OLR x SMP ( 1 )

the biogas product ion should be proportional to the OLR.

Steady-state results Table 4 presents the most significant results obtained during one mon th of steady digester performance. The experimental values of this table were obtained by averaging more than 10 samplings under steady-state conditions. Figure 3 reports an example of the reactor moni tor ing parameters, where the frequency of the analysis is indicated. As can be seen, gas produc t ion reached more than 2 m3/m 3 day, which is high for a con- t inuous stirred tank reactor (CSTR) type digester loaded at l ow-med ium level. These high rates of gas product ion are a consequence of the easily digestible substrate fed to the digesters. This becomes more evident f rom the data for specific biogas product ion (per kg of VS fed) which is constant a round 0.762 m 3 biogas/kg VS fed. In comparison with other values repor ted in the literature, this yield is one of the highest, com-


,7 o

© E -

0 v

m

6 -

5 -

4 -

3 -

2 -

0 ~ I I F I I I I r

0 20 40 60 80

Time (days)

Fig. 2. Gas production evolution during start-up, zx, Digester operating at 8 days HRT; <>, digester operating at 12 days HRT; +, digester operating at 14 days HRT; D, digester operating at 20 days HRT.

parable with the biogas yields obtained with the anaerobic digestion of the OFMSW separately collected (SC-OFMSW) in restaurants, food markets, etc. (Mata-Alvarez & Cecchi, 1989b; Mata-Alvarez et al., 1990b). The Specific Methane Production (SMP) and biogas composition, are fairly constant, an indication of the absence of digester stress.

As a result of this high biodegradability, the removal of solids, especially the volatile solids, is also high. If computed from the biogas production, which is less subject to experimental errors, the value is around 90%. This removal is even higher if the soluble fraction is considered (around 97%).

These results can be compared with those obtained in a previous study, treating source- sorted OFMSW in a pilot plant at Treviso, Italy (Cecchi et al., 1986). Both substrates, undiluted, have a large TS content (20% for the SS-OFMSW, 12% for Mercabarna waste (MB-waste)) with a percentage of soluble solids notably higher for MB-waste (61% compared to 35%). Con- sequently, it is not surprising that specific methane

production is 30% higher for MB-wastes as compared with that of SS-OFMSW (Cecchi et al., 1986). These differences are to be attributed to their different origins: SS-OFMSW are domestic wastes and they contain a significant percentage of poorly or non-degradable material (peel, bones, etc.). On the other hand, MB-wastes contain only eatable compounds and, as a consequence, a larger percentage of biodegradable components. Another possible comparison of these results can be carried out using the reported performances of the co-digestion process of sewage sludge and the SC-OFMSW commented on above (Mata- Alvarez & Cecchi, 1989b). When 80% of TS of the mixed substrate came from the SC- OFMSW, the SMP was 0"403 m 3 CHa/kg VS, but extrapolating to a 100% SC-OFMSW using the results reported for different mixtures in previous work (Mata-Alvarez & Cecchi, 1989b), a specific methane yield of 0.450 m 3 CHa/kg VS arises (r2=0-98) which is of the same order as that obtained here. SC-OFMSW has characteristics which are very similar to the MB-wastes (for instance the percentage of soluble VS is around


f f l I--

~7~

0-3 . . . . . . . . . 1

0 ' 1 0

3 5 7 9 11 13 15 Days

0.081

006: 0.04 ~ - ~ - - ~ 0.02 !

0 3 5

1.2

7 Days

9 11 13 15

1-1 7_0

1;0 cn

0.9

0.8 3 5 7 9 11 13 15 Days

064 t 0-Sq

> ~ 0.3~

i

0.2 ~ 1 3 5 7 9 11 13 15 Days

Fig. 3. Some monitored parameters in the effluent of the digester working at H R T = 2 0 days. These data were obtained at the stationary period.

64%, very close to the 60% of MB-wastes). Thus, the coincidence of these yields is quite reasonable.

Taking into account the composition given in Table 3 for MB-wastes and that, according to Gijzen et al. (1987), insoluble compounds com- prise (TVS basis percentages): cellulose (32%), hemicelluloses (15%) and lignins (15%) and, according to Ghosh and Henry (1985), these compounds, in mesophilic conditions, are removed up to 32, 86 and 0%, respectively, the distribution of TVS compounds appearing in Fig. 4(a) can be estimated. The percentage of compounds of group A (methanol and VFA having less than 3 carbons, that is, compounds directly utilized by methanogenic bacteria) and group B (ethanol and VFA having more than 3 carbons), can vary depending upon the external temperature (Cecchi et al., 1990a). Figure 4(b) and (c) also present the same distribution, but applied to SS-OFMSW and Mechanically Sorted

A (6"7°/o)

E (~5.7 0 / o ) ~ °1o

D (24.7°/o)

~ C(45-5 /o)

(a)

A(4.9°/o) o ~ ( 3 ' 7 ° 1 ° )

E ( 2 5 " 3 ' ~ / / / / ~ (21.6o/o)

D(44.5°/o) (b)

A 15 C(6.10/o)

E(52.20/o)

D(36.8o/0)

(c) Fig. 4. Percentage of different types of compounds in (a) Mercabarna wastes; (b) source-sorted organic fraction of municipal solid waste; (c) mechanically sorted organic fraction of municipal solid waste. The symbols are: A: Acetate and compounds directly utilized by methanogenic bacteria; B: ethanol and Volatile Fatty Acids (VFA) having more than 3 carbons; C: monomeric, soluble organic matter; D: complex (polymeric) organic matter; E: non-biodegradable organic matter.

OFMSW (MS-OFMSW), respectively (Mata- Alvarez et al., 1990b). As can be seen, MS- OFMSW has the largest non-biodegradable TVS fraction, whereas MB-waste has the smallest.


K i n e t i c s t u d y

A first-order model was selected to perform a kinetic study. The reasons were threefold: first, it is simple and easy to apply for design purposes; second, it fits well for this type of wastes (Mata & Cecchi, 1989); and, third, a more comprehensive model such as the step-diffusional (Cecchi et al., 1990b), could not be applied because, at laboratory scale, it is very difficult to measure the biogas production rates after feeding.

The basic equation is:

dS - k S (2 )

dt

where k is the first-order constant and S the biodegradable substrate concentration. Taking into account the existing relation between S and methane production (Chen & Hashimoto, 1978):

Bo - B S

B0 So (3)

and integrating, the following equation is obtained:

B 0 - B - - - exp( - kt) (4)

B0

where B is the methane production per kg of VS fed, and B 0 the same parameter but measured after an infinite digestion time (it is called the ultimate methane yield).

A mass balance applied to a continuous digester of volume V, with a feed flow rate Q, gives:

QS o - QS - V k S = 0 (5)

Taking into account eqn (3) and that HRT = V/Q, eqn (5) leads, after rearrangement, to the following expression:

1 1 1 - - - + (6 ) SMP B 0 B0(HRT)k

where SMP is the specific methane production rate, named firstly B.

A regression of 1/SMP versus 1/(HRT) gives B0 and k. If the data of Table 4 are used to perform this linear regression the following parameters are obtained:

B0 = 0.486 m 3 CH4/kg VS

k-- 4"1 day -1

However, this linear regression is subject to large errors, and a small variation in SMP leads to sub- stantial differences in the parameters estimated. Therefore, a non-linear regression using commercial software was performed. The following results were obtained:

B 0 = 0.489 m 3 CH4/kg VS

k = 3-1 day-

which are more reliable and, as a consequence, they were adopted to perform the preliminary design calculations.

Again, these results can be compared with those obtained with other but similar wastes. Table 5 shows different values of B 0 estimated for a number of organic fraction municipal solid waste (OFMSW) differently selected. B 0, as a measure of biodegradability, is very dependent upon the form of collection of these wastes, because this affects the composition. Thus, mechanically sorted OFMSW (MS-OFMSW) has the largest cellulosic content, and MB-wastes the lowest. The observed low B 0 value for the MS- OFMSW coming from Treviso (Table 5), is pos- sibly a consequence of the composting process which this MS-OFMSW has undergone. Other data concerning MS-OFMSW are referred to a non-composted, fresh waste.

On the other hand, the value of the kinetic constant k (3"1 day -~) for MB-wastes is also in the range of the high biodegradable wastes and thus it is comparable with the k value estimated through extrapolation for SC-OFMSW (2.7 day -~) and that of the SS-OFMSW (3.0 day- ~ ).

The ratio SMP/B0 is a measure of the biodegradation achieved (Standard Methods, 1985) and can be estimated from eqn (6):

SMP 1 (7)

Bo (1 + 1/(k × HRT))

Using the value of B 0 and the values of the SMP of Table 4, the percentage of the biodegradation achieved in the experiments carried out can be computed. Table 6 shows the resulting values, with a mean around 98%. (In fact, a slight decrease is observed with decreasing HRT.) Table 6 shows other interesting parameters estimated through the value of B 0 such as the soluble fractions removed and effluent concentrations of total and soluble biodegradable volatile solids, which can be used for a preliminary plant design. These parameters are important, for instance, for an

46 J. Mata-Alvarez , P. Llabr~s, F. Cecchi, P. P a v a n

evaluation of the organic load which will be sent to the wastewater treatment plant, after the anaerobic digestion process. To do this, the following hypothesis must be made:

(TVSrem)exp -(8WSrem)exp (8)

(TVSrem)max (8WSrem)max

(TVSrem)ex p and (SVSrem)ex p are experimental results concerning the removal of TVS and SVS, respectively (see Table 4) and (TWSrem)ma x c a n be estimated using B 0 and mean biogas composition, converting the biogas production to equivalent TVS:

(Bo) x (BMW) (TVSrern)max- X 100 (9)

22.411 × (% CH4)/100

where BMW is the Biogas Molecular Weight. The TVS and SVS removal experimental values considered are those deriving from the homogeneous balance data obtained through the reactor effluent analysis.

Using the experimental SVS removals (Table 4) and eqn (8), four maximum SVS removals result. Some of them are over 100% and this is due to the intrinsic experimental error on the effluent solids analysis. However, the mean value, around 99.3% can be considered as a representative value. Using a reverse procedure, a B 0 for the soluble fraction can be estimated using this maximum SVS removal. Assuming that the biogas composition coming from the soluble fraction is the same as

that coming from the overall VS (Zehnder, 1990), the resulting B0s is 0.530 m 3 CH4/kg SVS. Using the percentage of SVS on TVS in the substrate (around 60%, Table 4), a maximum removal for the non-soluble fraction can also be estimated. The resulting value is 79.8% which, using a similar procedure, yields a Box for the non-soluble fraction of 0.427 m 3 C H 4 / k g IVS. As can be seen, both soluble and non-soluble fractions are highly biodegradable as compared with other wastes (see Table 5).

Table 5. Values of the ultimate methane yield and first- order kinetic constant for differently sorted organic fractions of municipal solid waste (OFMSW)

Item Bo ~ k"

MS-OFMSW from Valorga b 0"301 MS-OFMSW from Dranco' 0"321 HS-OFMSW from U. Louvain ~ 0.397 MS-OFMSW from WSL e 0"381 8C-OFMSW from Ternif 0.445 SS-OFMSW from Treviso g 0.401 MS-OFMSW from Treviso h 0" 158 MB-wastes (this study) 0"489

(a) 0.6 (a) 0.6 (a) 3.0 (a) 0.6 (b) 2.7

3.0 0.4 3.1

aValues estimated in Mata-Alvarez et al. (1990b) with data from the references given below. (a) Estimated assuming a value for the kinetic constant k; (b) estimated through extrapolation. bValgora ( 1985). 'De Baere and Vestraete (1984). dpauss et al. (1984). eLe Roux and Wakerley (1978). fMata-Alvarez and Cecchi (1989b). SCecchi et al. (1986). hCecchi etal. (1989).

Table 6. Different parameters regarding the biodegradability of the substrate estimated from the four experiments carried out; these parameters are useful for a preliminary design of the anaerobic digestion plant

Parameter HR T (days)

(1) Biodegradation (SMP/B0) (%)

(2) Maximum TVS removal (%) (3) TVS removal (%)

Ratio ( 3 )/(2)

(4) SVS removal (%) (5) Maximum SVS removal (%)

Mean maximum SVS removal (%) (6) TBVS substrate fed (%) (7) TBVS effluent (%) (8) SBVS substrate fed (%) (9) SBVS effluent (%)

4

20 16 14 12 I i

98"16 98"16 97"99 97"38

-, 91"5 88-1 89-6 88"1 89"6

0-96 0"98 0"96 0-98

98"0 95"5 97"5 94"5 101"8 97"6 101"3 96"5

-, 99"3 • . 3"07

0"11 0-06 0-11 0-06 2"00 •

0"03 0"08 0-04 0-10

(1), (2), Computed from B0; (3), (4), experimental (Table 4);(5 ), computed assuming the same ratio as (2)/( 1)(see eqn (8) in text); (6), computed from (2); (7), computed from (6) and the experimental TVS removals; (8), computed from maximum SVS removal; (9), computed from (8) and experimental SVS.


Using the maximum TVS removal, the total biodegradable volatile solids of substrate (TBVS) can be estimated. That is:

TVS x (TWSrem)ma x TBVS = (10)

100

TBVS in the effluent can be then estimated through the experimental values of TVS removals (see Table 6). A similar procedure allows estima- tion of the soluble biodegradable volatile solids (SBVS) in the effluent. As can be seen in Table 6, some of the SBVS in the reactor effluent are higher than the TBVS. This is of course errone- ous, and is due to the above-mentioned experimental error affecting this analysis, specially at low values of solids concentrations. However, the biodegradable solids in the effluent are low enough in all cases.

As a final remark, the non-biodegradable solids fraction estimated from the analytical values and data from the literature (Fig. 4), and presented in Fig. 4 are in reasonable agreement with the biodegradability estimations performed using the experimental values (Table 6).

CONCLUSIONS

The experiments have confirmed the expected high yields from biomethanization of the organic fraction of food wastes from a vegetable market.

The first-order model, used to obtain kinetic information of the biodegradation process, fits very well to the experimental data and allows an estimate of the ultimate methane yield B 0 of the waste. This yield is very high compared with other similar municipal solid wastes. After some assumptions, the ultimate methane yields of the soluble and non-soluble fraction have also been estimated. Both values are also very high, con- firming the significant biodegradability of these wastes.

Using the values of the ultimate methane yield, the biodegradable volatile solids content has been evaluated, giving a value of 91% of the total volatile solids.

Finally the biodegradable content of the effluent leaving the digester has been evaluated. The resulting value (around 0.05%) is very low, which assures a low consumption in the necessary wastewater treatment plant, once the process has been scaled up.

ACKNOWLEDGEMENT

The authors acknowledge the permission of MERCABARNA SA to publish the paper.

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anaerobic digestion of the barcelona central food market organic wastes: experimental study

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