ELSEVIER PIl:SO960-8524(97)00102-8

Bioresourcr Terhnolog) 63 (1998) 29-35 0 1998 Elsevier Science Ltd. All rights reserved

Printed in &eat Britain 0960-8524198 $19.00

SOLID-STATE ANAEROBIC DIGESTION OF UNSORTED MUNICIPAL SOLID WASTE IN A PILOT-PLANT SCALE

DIGESTER

J. Rodriguez-Iglesias, L. Castrillh, E. Marafih & H. Sastre*

Chemical Engineering and Environmental i”ecknolog): Department, High Technical School of industrial Engineering Campus of C’ijon, Universiq of Oviedo, Asturias, Spain

(Received I8 March 1995; revised version received 10 June 1997; accepted 7 July 1997)

Abstract An anaerobic digestion of municipal solid waste (MSW) was carried out in a pilot-scale reactor to study anaerobic fermentation as it happens in a sanitary landfill. The reactor was not inoculated and the MSW used was taken from the COGERSA r’andfill site in Asturias, Spain. The experiments were petformed in an opaque PVC reactor at 36 + 1°C. Water was added in the same proportion as the da+ rainfall at the landfill site. The pH of the leachates was low (5.5-6.1 approximately) from days 40 to 170 when it began to increase to neutral. 2.25 litres of the leachates produced (previously treated with NaOH and NaHCOJ until day 300) were recirculL!ted from day 120 to the end of the aperiment. Leachate chemical oxygen demand decreased with time, reaching levels of approx. 2000 mgll by the end of this study, Volatile fatty acids presented a similar behavioul: The main metals found in the leachates were Fe, Mn and Zn. The methane percentage reached a maximum value of 66%. 0 1997 Elsevier Science Ltd. All r&hts reserved.

Key words: MSW, landfill, anaerobic treatment, leachate, gas (biogas).

INTRODUCTION

Among the different alternatives for the elimination or treatment of municipal waste, sanitary landfilling is probably the most widely used, owing to its economic and technical advantages. In sanitary landfills, anaerobic conditions are reached within the bulk of the landfilled waste, resulting in the slow, progressive decomposition of the organic material present. A mixture of methane and carbon dioxide, known as ‘landfill gas’, is produced by the anaerobic conditions (Price & Cheremisinoff, 1981; Owens &

*Author to whom correspondence should be addressed.

Chynoweth, 1993). At the same time, the infiltration of rain, together with the moisture in the waste and that produced by biological decomposition, produces leachates. The major environmental problem associ- ated with landfills is related to the discharge of this leachate into the environment (Andreottola & Cannas, 1992; Berrueta & Castrillbn, 1992; Clement, 199.5; Ehrig, 1983; Lema et al., 1988).

If the anaerobic degradation process reaches the methanogenic phase as soon as possible after the refuse has been landfilled, more organic material is transferred into the gaseous and less into the liquid phase (leachates), so that more profitable energy is available, and less energy and costs are required for treatment of the leachates.

Laboratory and pilot-plant studies have been carried out in order to observe this mechanism under controlled conditions (Attal et al., 1992; Blakey et al., 1995; Chugh et al., 1995; Ehrig, 1996; O’Keefe et al., 1993; Pohland et al., 1993; Yuen et al., 1995).

Asturias is a region whose population is slightly over one million and it produces approx. 360000 t MSW by year.

The purpose of this work was to study the anaerobic decomposition process of MSW, whilst also attempting to reproduce the behaviour of a sanitary landfill under controlled conditions. With this aim in mind, the work was divided up into various stages. Various amounts of MSW with similar characteristics were introduced into the reactor at different times, thus forming different cells or layers, analagous to the usual operating- mode of a sanitary landfill. Thus, 48.5 kg of MSW were introduced into the first cell, 66 kg into the second on top of the previously digested residue in the first cell and 59 kg on top of the other two layers. This paper describes the work carried out in the pilot plant digester in the first cell.

29

30 J. Rodriguez-Iglesias et al.

METHODS The pilot plant was constructed from opaque PVC in order to prevent possible interactions of a photo-

Pilot plant The study was conducted by constructing a pilot plant shown in Fig. 1 (Rodriguez-Iglesias, 1995).

chemical nature. The diameter and height were 0.5 and 2.85 m, respectively, and the possibility existed of adding another unit. Refuse depth was 1.5 m. The

Gas exit I=

Leachates recirculation circuit

Temperatun probes

Container for water

Liquid entrance

External water iacket

Temperature probes

% Closed container for recirculation water II

I]

Closed container for leachates

Fig. 1. Frontal view of the pilot plant.

Digestion of unsorted MSW 31

water distribution device and gas collection equip- ment were set up at the top and at the bottom where there was a system for collecting the leach- ates. In the gas collection system there was a septum in order to collect samples of gas for analysis. The leachates were collected in a closed container, and a circuit enabled their recirculation by pumping. Another circuit introduced water into the plant, to simulate precipitations. Six temperature probes were located inside the MSW to obtain a profile of temperatures throughout the reactor.

The operating temperature of the reactor, 36& l”C, was maintained constant by means of an external water jacket through which water from a thermostatic bath circulated.

Chemical analyses The parameters analysed for the leachates were chemical oxygen demand (COD), suspended solids (SS), volatile suspended solids (VSS): volatile fatty acids (VFA): total alkalinity (TA.C), Kjeldahl nitrogen, ammoniacal nitrogen, phosphates, metals content and gas volume as well as gas (CH4, C02) and waste composition. The Standard Methods were employed when applicable (APHA, 1989).

The metals were determined by atomic absorp- tion. Samples of homogenized leachate were treated with mixtures of HClO, and HN03, in a proportion of l/10 by volume, until a colourless residue was obtained. The equipment used was a Perkin Elmer 3110 spectrophotometer.

The VFA were determined by gas chromato- graphy. The leachates were previously homogenized and filtered and then treated with H3P04 (10%). The equipment used was a Perkin Elmer 8600, equipped with a Flame Ionization Detector (FID) and a l/8‘ column, packed with 20% Carbowax- 20M+2% H3P04 (85%) in Chromosorb WAW 100/120.

The gas composition was also determined by gas chromatography, by means of a Hewlett-Packard chromatograph 5840, with a thermal conductivity detector (TCD) and a Porapak Q 80/100 column.

MSW characterization was also determined following the process described in Parcel et al. (1994) and APHA (1989).

Experimental procedure The first step of the experimental process consisted of collecting the MSW to be introduced into the pilot plant. The sample chosen came from the Central Deposit of Asturias (COGERSA landfill) in ‘La Zoreda’ Valley, taken at the front of the landfill, where the waste was fragmented and compacted by a Caterpillar machine 826 C for sanitary landfill. These wastes had come from a previous transfer station without recycling. A total of 300 kg of MSW was obtained, of which a representative sample of 48.5 kg was introduced into the pilot plant. This

sample was similar in composition to that of the landfill from which it had been taken.

Water was introduced every day from the top in order to simulate rainfall over the sanitary landfill. The quantity of water to be introduced was calculated according to the meteorological informa- tion from the previous month supplied by the Meteorological Centre of the Iron and Steel Works, CSI Planos, AvilCs, Asturias, located near the landfill. The leachates collected at the bottom were recirculated from day 120; each day 2.25 1 was pumped back over the waste.

In order to accelerate the process, the leachates were neutralized before their recirculation until day 300, by adding a solution of NaOH (6 M) and NaHCO, (6 M).

RESULTS AND DISCUSSION

Characterization of municipal solid waste The composition of MSW, shown in Table 1, shows a percentage of organic matter of approx. 52% with 19% paper fraction. These two fractions can be completely anaerobically degraded (Owens & Chynoweth, 1993; Ehrig, 1996). The results of the MSW organic fraction analysis, obtained using a 25 mm sieve, are shown in Table 2. A high density in the Organic Fraction (OF) was observed in the MSW, partly due to its origin, since the sample taken from the front of the landfill had undergone several crushing processes before being buried. It contained 94.0% of organic substance (dry waste) but a high level of moisture (63.8%) since Asturias is a very rainy area of Northern Spain. The existence of a total nitrogen quantity perceptibly greater than the ammonium nitrogen, indicated the presence of a high level of proteins in the MSW. With regard to metals, the most important were Al, Fe and Zn, which represented more than 92% of the total metals; the less abundant ones were Cd and Cu. The appearance of metals was mainly due to the previous contact existing between the organic fraction and the metallic fraction in the transfer station, as well as during transport.

Leachates Initial leachate analysis for COD and VFA indicated high concentrations for both parameters with most

Table 1. Municipal solid waste composition in the Asturias landfill

Municipal solid waste composition

Weight percentage

Organic fraction Paper Glass Plastics Metals Others

52 19

;

1:

32 J. Rodriguez-Iglesias et al.

Table 2. Composition of the organic fraction of municipal solid waste

PH Conductivity (mS) Apparent density (pm’) Real density (g/cm- )

_ . I I

Porosity (%) Moisture (%) Organic matter (dry waste %) N-NH: (D.W. mgig) N-Org (D.W. mg/g) Total nitrogen (D.W. mg/g) COD (D.W. mg Odg) Phosphates (D.W. mgig) Fe P.W. shd Zn PW. &g) Pb (D.W. a’g) Cu P.W. .dd AI P.W. a’g) Mn P.W. &g> Ni (D.W. pgig) Cd (D.W. a/g> Cr PW. cldg)

4.4 8.7

1.2~0.03 1.5*2.5x 1o--4

24.4 & 1.96 63.8kO.2

94 + 0.03 3.7kO.16

23.1+ 0.37 26.8 &- 0.33 803 *0.14 2.7 * 0.054

37.8 * 0.28 118A1.27 2.5 * 0.03 0.6 f 0.02

413.4k21.3 7.5kO.11 1.6kO.27 0.4+0.0316

35.2 f 1.42

of the COD being attributed to lhe volatile organic acids.

The initial leachate COD was very high, and subsequently decreased progressively until day 150, to values of approx. 40000 mg/l, then remained virtually stable until day 264. The COD concentra- tions then decreased markedly until day 375, when their levels were approx. 2000 mg OJl. The results obtained are shown in Fig. 2.

During the same period the concentration of VFA showed similar behaviour, fmally decreasing to levels essentially below detection. Of these VFA the most abundant was butyric acid, which initially had a value of 19 g/l and whose concentration subse- quently decreased until day 300. Acetic acid started off at 8 g/l and subsequently decreased until day 340 [see Fig. 3(a)]. Isovaleric acid and valeric acid

1 00.000

80.000

;‘“i , , ,^‘“1,

0 100 200 300 400 Time (Days)

Fig. 2. Variation of COD with time.

showed continuous fluctuations until day 320, then fell below detection levels [see Fig. 3(b)].

Time trends of organic compound concentrations clearly showed that passing from the acid phase to the methanogenic phase led to a notable decrease in concentrations.

The pH (Fig. 4) remained stable at approx. 6-7 during the first 40 days and then suddenly fell to pH 5.5-6.1, and remained relatively stable again for approximately 170 days, after which the pH increased until day 300 to values of approx. 6.5. During this period the pH was essentially controlled by the volatile organic acid concentrations. The pH then began to increase to a level of 7.5 on day 327. At this point, the methanogenic step in the waste bed was stable and robust. On day 350 the volatile acid concentrations, in general, were below the analytical detection limit and the leachate pH was

0 0 100 200 300

(a) Time (Days)

6

0 100 200 300 400

(b) Time (Days)

Isovaleric acid - I

Fig. 3. (a) Variation of acetic, propionic and butyric acids with time. (b) Variation of isovaleric and valeric acids

with time

Digestion of unsorted MS W 33

8

7s

% 695

6

0 100 200 300 400 Time (Days)

Fig. 4. pH variation in the leachate with time.

mainly controlled by the bicarbonate buffering system. It is well known that a low pH: environment caused by vigorous acid production in a landfill ecosystem inhibits the growth of methanogenic bacteria. For this reason, the leachate was neutral- ized before recirculation, whilst the pH remained at low levels. The leachate was recirculated: to promote an optimum state of moisture; to induce a water flux to provide a mechanism for the effective transfer of microbes, substrates 2nd nutrients throughout the refuse mass; and to dilute local high concentrations of inhibitors (Yuen et al., 1995). Similar behaviour was noted in the literature (Pohland et al., 1993).

Alkalinity (Fig. 5) was high at first with values of approx. 14000 mg/l, decreasing later to approx. 5000 mg/l. In general, the anaerobic fermentation of

16

6

0 100 200 300 400 Time (Days)

Fig. 5. Alkalinity variation with time.

a concentrated waste produces more alkalinity than does a dilute waste.

The result of the increase in pH and the trans- formation of sulphate to sulphide by sulphate- reducing bacteria (sulphate is a major component of many waste types: demolition waste, incinerator slag, fly ashes) produced changes in the leachate metal concentrations. Iron, zinc and manganese were the most representative metals present in the leachate. During the initial phases, there was a high degree of metal solubilization due to low pH values, caused by the high production of organic volatile acids, when the increase in pH causes a lesser degree of solubili- zation. The variation of metals as a function of time is shown in Table 3.

Ammoniacal nitrogen and total nitrogen were high at first, approx. 1500 and 2900 mg/l, respec- tively. These values decreased to values of approx. 600 mg/l; from day 150 onwards the decrease of total nitrogen to values near that of ammoniacal nitrogen implied rapid protein degradation. The concentra- tion of ammonia was high in leachates following hydrolysis and fermentation of the protein fraction of the biodegradable substrate. At the onset of the methanogenic phase this concentration tended to decrease slowly (see Fig. 6).

Suspended solids and volatile suspended solids were present at low concentrations with similar behaviour to that found in previous studies (Berrueta & CastrUn, 1992; Castrill6n & Berrueta, 1991).

Landfill gas Figure 7 shows the variation of gas composition with time. In the first 10 days, the process experienced an aerobic period, where the air that entered with the MSW was consumed, producing a large quantity of COZ and, in practice, no methane. The process then began an acid period dominated by CO,. When the amount of VFAs increased, these would react with the bicarbonate and produce CO2 and this changed the landfill gas. From day 100 onwards, there was a continuous increase of methane to 45% on day 216. On day 235, the methane percentage was greater than that of C02, and subsequently the percentage of methane increased to maximum levels of approx. 66% by volume.

CONCLUSIONS

During this work, anaerobic degradation of MSW was studied. During this process, various phases were observed. The first phase was characterized by the initial production of CO,. This was followed by the acid formation phase characterized by anaerobic conditions with high volatile acid production and high COD (approximately until day 150), and a decrease in the pH. The following phase observed was a mixture of acid and methanogenic phases,

34 J. Rodriguez-lglesias et al.

Period

Table 3. Metals in leachates: average values (mg/I)

Acid phase (days 34-150)

Acid methanogenic phase

Methanogenic

(days 150-265) phase

(days 265-343)

Maturation phase

(days 343-390)

147.8 + 26.9 119.76k23.18 192.2k56.0 0.10~0.05

29.3k9.4 0.06$0.02 0.10&0.08

1.28+1.31 0.10 + 0.08

0.32*0.22 0.83kO.48 0.83kO.32 0.75kO.30 0.54kO.14 0.59-to.14 0.54kO.09

12.48k3.62 12.14k2.62 7.84k2.12 0.74kO.27 0.82+0.20 0.51* 0.22 0.34kO.16 0.54-to.13 0.30*0.08 0.07*0.03 0.06+0.03 0.05& 0.01 0.5820.25 0.36*0.13 0.82kO.46 0.48+0.18

2.000

1.800

s 1.600

4 1.400 0

*g 1.200

5 z

1.000

$ 800

iI 600

i 400

200

0. 0 100 200 300 400

Time (Days) Fig. 6. Nitrogen composition of leachates.

100.000 REFERENCES

80.000

.$j 60.000

3 :

; 40.000

8 u

20.000

0 0 100 200 300 400

Time (Days)

Fig. 7. Composition of biogas (% volume).

Andreottola, G. & Cannas, P. (1992). Chemical and biological characteristics of landfill leachate. In Landfilling of waste leachate, ed. T. M. Christensen, R. Cossu & R. Stegman, pp. 65-87. Elsevier Applied Science, Oxford.

APHA, AWWA, WPCF (1989). Standard methods for the examination of water and wastewater, 17th edn. The Public Health Association, Washington, D.C.

Attal, A., Akunna, J., Camacho, P., Salmon, P. & Paris, Y. (1992). Anaerobic degradation of municipal wastes in landfill. Wat. Sci. Technol., 25, 243-253.

Berrueta, J. & Castrilkjn, L. (1992). Anaerobic treatment of leachates in UASB reactors J. Chem. Technoi. Riotechnol., 54, 33-37.

Blakey, N., Archer, D. & Reynolds, P. (1995). Bioreactor landfill: a microbiological review. Proc. Sardinia 95. Fifth International Landfill Symposium, Cagliari, Italy, 1, 97-116.

Castrillbn, L. & Berrueta, J. (1991). Tratamiento anaerobio de lixiviados de vertederos de residuos s6lidos urbanos Tecnologia de1 &a, 77, 70-79.

characterized by virtually constant values in the COD, of approx. 40000 mg 0,/l. The next step, methane fermentation, was then characterized by a decrease in both volatile acid and COD concentra- tions and an increase in the pH as well as in the methane percentage. Finally, the maturation phase was characterized by a substantial reduction in the organic content of the leachate. The variations of COD and the pH of the leachate were strongly related to the variation of the fatty volatile acids. Most of the metal concentrations in the methano- genie and maturation steps were found to be lower than those in the acidogenic step.

ACKNOWLEDGEMENTS

This work was financially supported by COGERSA, S.A. (Company for Solid Waste Management in Asturias) and the ‘II Research Program of Asturias (FICYT, Foundation for the Promotion of Research in Applied Science and Technology, Asturias, Spain)‘, contract PA-MAS94-03. We also wish to express our gratitude to Mr Santiago FernBndez, manager of COGERSA.

Digestion of unsorted MSW 35

Chugh, S., Clarke, W., Nopharatana, A., Pullammanap- pallil, P. & Rudolph, V. (1995). Degradation of unsorted MSW by sequential batch anaerobic reactor . Proc. Sardinia 95 Fifth International Lanafill Symposium, Cagliari, Italy, 1, 67-77.

Clement, B. (1995). Physico-chemical characterization of 25 French landfill leachates. Proc. Sardinia 95 Fifth International Landfill Symposium, Cagliari, Italy, 1, 315-325.

Ehrig, H. J. (1983). Quality and quantity of sanitary landfill leachate. Wat. Management Res., 11, 53-68.

Ehrig, H. J. (1996). Prediction of gas production from laboratory scale tests, ed. T. H. Christensen, R. Cossu & R. Stegmann. E & FN Spon, London.

Lema, J. M., Mendez, R. & Blazquez, R. (1988). Charac- teristics of landfill leachates and alternatives for their treatment. A review. Water; Air Soil Pollut., 40, 223-250.

O’Keefe, D. M., Chynoweth, D. P., Barrdoll, A. W., Nordshdt, R. A., Owens, J. M. & Sifontes, J. (1993). Sequential batch anaerobic cornposting of municipal solid waste (MSW) and yard waste. Wat. Sci. Technol., 27, 77-86.

Owens, J. & Chynoweth, D. (1993). Biochemical methane potential of municipal solid waste (MSW) components Wat. Sci. Technol., 27, 1-14.

Pohland. F., Cross, W. & King, L. (1993). Codisposal of disposable diapers with shredded municipal refuse in simulated landfills. Wat. Sci. Technol., 27, 209-223.

Parcel, O., Aguilar, F., de Leon, J., Revilla, J.&Z Diz, J. (1994). Caracterizacion fisicoquimica de 10s residuos solidos urbanos de la ciudad de Cordoba. Retema. Noviembre-Diciembre, 66-71.

Price, E. C. & Cheremisinoff, P.N. (1981). Biogas. Produc- tion and utilization. Ann Arbor Science, Ann Arbor, Michigan.

Rodriguez-Iglesias, J. (1995). Digestion anaerobia de residuos solidos urbanos en planta piloto. Trabajo de Investigation. Chemical Engineering and Environment Technology Department, Oviedo University.

Yuen, S. T. S., Styles, J. R. & McMahon, T. A. (1995). An active landfill management by leachate recirculation: a review and an outline of a full-scale project . Proc. Sardinia 95. Fifih International Landfill Symposium, Cagliati, Italy, 1, 403-418.