thermophilic semisolid anaerobic digestion of municipal refuses

Thermophilic Semisolid Anaerobic Digestion of Municipal Refuses

Ph. Marique. A. Gilles, F. Edeline,* and L. Joassin Unite de Microbiologie lndustrielle, Universite de Liege B23, B-4000 LIEGE-Sart Tilman, Belgium

Accepted for publication February 8, 1988

Comparison of both mesophilic (35°C) and thermophilic (55°C) anaerobic digestions of the organic fraction of municipal refuses in pilot digesters designed to process in a semisolid phase at total solids concentrations of ca. 25% shows that the average gas production is 20-25% higher in thermophilic conditions than in mesophilic conditions even for a retention time of 10 days. These results and the data recorded during long periods of experimentation indicate that the process allows to increase the net energy production and to improve the economical balance of an industrial plant.

INTRODUCTION

It is now evident that municipal solid wastes (MSW) represent an important source of organic matter to be considered for methane fermentation. Two factors have limited a greater application of anaerobic digestion to this type of substrate: long digester residence times (i.e. greater than 15 days) and the fact that the conventional completely mixed digestion process can be used for solid wastes only if the substrates are first diluted from 60 to 10% of total solids (TS) contents, so that some full-scale plants failed to demonstrate the feasibility of the process.

At the present time, anaerobic “semisolid” digestion is operating at an industrial scale in mesophilic conditions at total solids averaging 30-35%, under which the substrate can be handled and manipulated as a solid. According to the authors of the project,’ a daily mean gas production of 5 m’/m’ day has been recorded with 60% CH, content and 50% volatile solids (VS) reduction, when a loading rate of 12 kg VS/m3 day and a hydraulic retention time (HRT) of 15 days were applied.

The development of this “semisolid” process allows us to eliminate the limiting factors due to dilution, water han- dling, and sludge processing. With regards to HRT, most authors concluded that thermophilic digestion (50-65°C) makes shorter HRT and higher loading rates possible. Greater conversion efficiencies could be obtained: Cooney2 obtained an average gas production which was 50% higher under thermophilic conditions compared to

* Present address: Cebedeau, rue A. Stevart 2, B-4000 Liege, Belgium.

Biotechnology and Bioengineering, Vol. 33, Pp. 536-541 (1989) 0 1989 John Wiley & Sons, Inc.

mesophilic operation. After experiments in both mesophilic and thermophilic conditions at various HRT, Pfeffer3 demonstrated that in thermophilic digestion at 60”C, gas production was 50% higher at a retention time of 20 days and that this difference was still recorded at a HRT of 5 days. Moreover, Garber4 reported other technical benefits of thermophilic operation as improved solid-liquid separation and increased destruction of pathogens.

However, it has been suggested’ that a thermophilic system may present disadvantages including higher energy requirements and a poor process stability. On the other hand, in the case of MSW, it is clear that sufficient data are not available up to now to adequately evaluate the overall po- tential of a thermophilic “semisolid” process.

Therefore our studies were designed in order to investigate the biological efficiency of thermophilic methane production from MSW at long and short HRTs, at different temperatures, and various loading rates. The present report consists in a comparison of both mesophilic and preliminary thermophilic performances in steady state conditions, recorded when the HRT decreased from 17 to 8 days at a temperature of 55°C.

MATERIAL AND METHODS

Digester Design

The fermentation vessels used in this study were two 120-L (effective volume) stainless-steel reactors modified from a previous study by enlarging the effluent outlet part from 4 to 12.5 cm. This outlet part comprises a side arm to which was attached semiflexible rubber tubing. Figure 1 shows the basic design of the reactors. The top of each reactor contained different insertions: a feed inlet part, a thermometer, and a gas outlet through a water cooling system. Temperatures were maintained using a heating tape and fiberglass insulation, and regulated by a thermistor probe coupled to a regulator. The contents of the vessels were agitated at 140 rpm for 2 min, once per hour. The im- peller was a driven stainless-steel shaft to which four 18-cm crossbars were fixed. The mechanical stirring was arranged such that duration and frequency of mixing se-

CCC 0006-3592/89/040536-06$04.00

Figure 1. Basic design of the digesters: (a) feeding port, (b) thermistor probe, (c) draining ports, (d) stirrer, (e) effluent outlet, (f) thermometer, (g) reducer, (h) motor, and (i) water cooling system (gas outlet).

quences could be controlled either manually or automati- cally. The reactors were operated in the semicontinuous mode: they were fed once per day, and effluent of an equal volume was removed just before the feeding. Gas production was measured daily with a wet test gas meter (Con- tigea, The Netherlands).

Municipal Solid Wastes (MSWI and Microorganisms

The substrate used in these experiments consisted of the organic fraction of screened (screen opening of 50 mm) solid waste issued from the crushing and sorting line, after removing of metals, of the 100 tons/day composting plant of Tenneville, Belgium (Buhler process). The substrate samples were collected statistically each week from Octo- ber 1984 to May 1986. Each batch was dispensed in plastic bags, each providing the desired daily VS input, and stored at 4°C until feeding. Tap water plus a desirable amount of recycled liquid was added in order to adapt the influent to the desired dry matter content.

Table I gives the average composition of the substrate, which was 60% dry solids. The lignocellulosic content av-

Table I. Average chemical composition of the organic fraction

Relative content Analytical parameters ("/.I

Dry matter 60 2 5 Volatile matter (on TS) 45 2 8

C 27,5 2 3,4 N 1,0 2 0,3 H 3,4 ? 0,7 P 0,25 2 0,W

eraged 50% of the organic matter in the form of 30% lignin and 17.5% cellulose.

The mixed populations of microorganisms used in these studies were selected from the sludge of an urban sewage plant, the effluent of a mesophilic piggery waste digester and from the effluent of a thermophilic dairy waste digester and were adapted according to their ability to convert the MSW into biogas at the selected temperatures.

Analytical Procedures

Analyses were carried out on feed and effluent materials to determine levels of total solids (TS), volatile solids (VS), volatile fatty acids (VFA), and pH according to the Standard Methods6 The gas composition was determined with a Carlo Erba gas chromatograph with a silica gel column and a thermal conductivity detector. Helium served as the carrier gas for the detection of CH, and COz. All gas measurements were expressed in terms of Standard Temperature and Pressure (O"C, 760 mm Hg).

Experimental Conditions

Fermentation was performed in a semicontinuous mode. After cultures were established, the two reactors were fed once per day, five days per week, subsequent to the removal of the desired volume of the effluent. This feeding scheme was chosen with regards to the week-end intermp- tion in full scale MSW treatment plants. The dry matter contents in the reactors were adjusted to values from 20 to 25%, in order to preserve the efficiency of the mechanical stirrer. The digested sludge was pressed through a 2-mm bolting-cloth and the liquid fraction was mixed with the raw material just before feeding in a proportion defined in order to maintain the dry matter concentration in the reactor to the desired value. A reactor was considered to be in a steady-state condition for each change in VS loading rate after three weeks. After a period of assay, the HRT and loading rate were adjusted to the next level over one day, and the equilibration time and sampling were repeated. The temperatures were chosen according to the most general opinion concerning favorable conditions of digestion: 35°C for mesophilic and 55°C for thermophilic operations.

RESULTS

Starting Up Procedure

First, the two reactors were filled with the inoculum and the temperature was fixed at 35 and 55°C in the digesters which were named R, and R, respectively. Since the first day, each reactor was fed daily with 2 kg raw organic fraction of MSW, without any liquids, in order to rapidly increase the TS contents. After 10 days, the recycled liquids provided by straining the effluents were used to prepare the mixture of raw material. After two weeks, the methane contents of the gas produced in both reactors reached 35%

MARIQUE ET AL.: ANAEROBIC DIGESTION OF MUNICIPAL REFUSES 537

(v/v). After one month with a daily feeding of 2 kg MSW, we found a methane content of 65% for R, and of 60% for R, . The gas production varied from 85 to 120 L/day in R, and from 100 to 135 L/day in R,; the VFA concentrations were less than 3000 mg/L in both reactors. These values remained stable, indicating that a steady state was reached in both reactors.

Thereafter, R, and R,, were fed with a load of 4 kg MSW/day. During a period of two weeks, the gas production in R, varied from 150 to 180 L/day. After that however, it dropped radically to a value as low as 60 L/day.

At the same time, we observed that the VFA concentration in this reactor increased to 4500 mg/L, but the pH remained stable at 7.5. Five days later, the VFA content reached 7650 mg/L, the pH fell to 6.1 and a CH, content of 30% was measured, suggesting a biological failure. Conse- quently, we interrupted the feeding during three weeks, until the VFA concentration fell to 1570 mg/L.

Thereafter, R, was fed step by step and the VFA were carefully followed up during a period of two months. Then the gas production reached 153 L/day and the load was increased again from 2 to 4 kg MSW/day. The HRT aver- aged 40 days and the methane content of the gas produced in R, was 53%.

In the case of R,,, when the daily load was increased from 2 to 4 kg MSW, we did not observe any problem. After ca. two days, the gas production increased up to 300 L/day and after 40 days, the VFA concentration was found to be less than 2000 mg/L. After 60 days operation, we considered that a steady state had been reached in both reacton, with regards to the stability of the followed parameters. Then the load was increased up to 5 kg MSW/day.

The biological failure observed in R, which seems to be due to some inhibitory effect of a substance included in the load, did not imply to completely drain the reactor. A single interruption of feeding and the dilution of the fermentation liquor at 80% of its TS concentration restored efficient activity in R,. The increase of the load in R, was carried out without any activity failure. On the contrary, we observed a true activation of the phenomenon, giving rise to a VFA decrease, whereas the gas production increased. These results clearly indicate that thermophilic methanogenesis from MSW can easily be initiated and that the specific bacteria were readily efficient and available from the single startup inoculum.

Comparison of both Mesophilic and Thermophilic Semisolid Digestions

After a few weeks acclimation period, R, and R,, were considered in a steady-state condition as indicated by the stability of the gas production and the low levels of VFA. Both reactors were fed once per day, five days per week with 5 kg raw MSW, which corresponds to a loading rate of 8 kg VS/m3 day and a HRT of 16 to 17 days. In order to obtain an objective comparison of the operations in both mesophilic and thermophilic conditions, these values were maintained for eight weeks. The mean results obtained during this period are summarized in Table 11.

Table 11. tions.

Results obtained in both mesophilic and thermophilic condi-

Digester temperature Mesophilic Thermophilic (“C) 35 55

Loading rate (kg VS/m3 day) HRT (days) Gas production (L/L day)

(L/kg MSW) (L/kg VS)

CHI (%) PH N-NH4 + (mg N/L) VFA (mg HAc/L)

8.0 16.1 2.13

91.8 340

53

522 1054

1.22

8.0 17.4 3.35

112.5 417

53

944 1065

7.62

The efficiency of gas production is expressed in terms of L/L reactor/day, L/kg raw MSW infed, and L/kg VS infed. The average gas production at 55°C was 3.35 L/L day, which is 22.7% greater than at 35”C, where it aver- aged 2.73 L/L day. For all gas production parameters, the thermophilic reactor performed 22.5% better than the mesophilic one. In both conditions, the CH, contents were equivalent to a value of 53%. The gas yield in the mesophilic temperature range was 340 L/kg VS added. This parameter was substantially greater in the thermophilic temperature, averaging 417 L/kg VS added.

VFA levels were found to be the same in both conditions, with values as low as ca. 1000 mg/L. The pH was found significantly higher in the thermophilic reactor, which presumably can be related to higher levels of ammonia concentration and to a lower CO, concentration at 55°C. An interesting observation is that the pH remained neutral and stable in both reactors even at acid levels as elevated as 4500 mg/L (see the section on starting up procedure).

The measurement of VS destruction based on gravimetric determination lacked of precision due to the heterogeneity of the reactors contents and effluents. Therefore, as did Varel and co-workers,’ we utilized a method where VS destruction was calculated from the volume of CO, and CH, produced. Assuming that CO, and CH, are provided by the carbon present in the VS destroyed and also that VS fraction contains ca. 27.5% carbon (see Table I), the weight in g VS destroyed is given by mol (CO, + CH,) X

12, or 0.275 mol. By this approach, we estimated that 66 and 82% of the VS were degraded to biogas in mesophilic and thermophilic conditions, respectively, confirming that the net conversion of waste to gas and consequently, the stabilization of the degradable material, are also more efficient under thermophilic conditions.

Effect of Loading Rate and HRT on the Performances of Thermophilic Fermentation

In Table I11 are presented the mean parameters obtained when the thermophilic reactor operated at various loading rates and thus, according to our experimental design, at various HRT. The loading rates applied were the follow- ing: 8, 12.9, 16.1, and 19.3 kg VS/m3 day. When the loading rates increased, the HRT decreased from 16 to 8.3 days. The conditions defined for each step were main-

538 BIOTECHNOLOGY AND BIOENGINEERING, VOL. 33, JANUARY 1989

Table 111. ditions.

Data recorded at different thermophilic (55°C) operating con-

Loading rate (kg VS/m’ day) HRT (days) Gas production (L/L day)

(L/kg MSW) (L/kg VS)

PH

CH, (%) VFA (mg HAc/L)

8.0 11.4 3.35

112.5 417

1065 53

7.62

12.9 12.0

113.8 422

506 54

5.42

1.54

16.1 10.1

114.1 423

1400 53

6.19

7.50

19.3 8.3 6.43

90.0 333

3700 51

7.14

tained for eight weeks. When the gas yields were expressed per amount of MSW or of VS fed, mean values of ca. 113 L/kg MSW and of 420 L/kg VS were measured as long as the loading rate increased from 8 to 16.1 kg VS/m3 day and as the HRT was reduced from 17.4 to 10.1 days. Similarly, the volumetric production of the reactor increased from 3.35 to 6.79 L/L day. We could preserve steady-state conditions when the loading rate was increased to 19.3 kg VS/m3 day at a HRT of 8.3 days. However, the gas production dropped to 90 L/kg MSW or 333 L/kg VS. The volumetric gas production decreased also, but remained more than 6 L/L day. VFA concentrations which were found to be less than 1500 mg/L to a loading rate of 16.1 kg VS/m3 day, increased to reach a value of 3700 mg/L and the VS destruction dropped from 82 to 65%. Methane content remained at a stable value of 53% during the first three steps but not in the last one where it decreased to a value of 51%.

At higher loading rates giving rise to HRT lower than 8.3 days, a wide degree of variability in methane composition was recorded and VFA gradually increased assessing that an active fermentation could not be maintained at lower HRT. The process was presumably inhibited because of high loading rates and high VS contents in the fermentation sludge, leading to accumulation of solubles.

Energy Balance: Theoretical Approach

It is clear that the establishment and scaling up of a thermophilic process could not be considered if the energy balance of the system would reveal to be unfavorable. In this connection, we tried to make a direct comparison of the energy yields in both conditions, assuming that the digesters are fed at 35% dry matter, have the same insulation and that they would be sized for the treatment of 100 tons

(T)/day at the same temperatures and HRT conditions as those observed during long equilibrium periods such as in the present study. The results obtained are summarized in Table IV. We considered a gas production of 91.8 and 113 m3/T for mesophilic and thermophilic conditions, respectively.

The energy consumptions of both systems for heating purposes were calculated as follows: Q = em AT, in which Q is the energy required to heat the influent; c is a calorific coefficient (= 1); m is the volume of influent; and AT is the gradient of temperature. The temperature gradients were assumed to be 25 and 45°C in both conditions, with a mean yield of the total heating device of 0.8. On the other hand, in our calculations, heating requirements to compen- sate the heat losses in the reactors or losses due to electric- ity needs were not considered, with regards to their low percentage of energy requirements. The Q values were converted into cubic meters of biogas with a methane content of 53% and a calorific value of 8500 kcal/N m3. In any conditions, our calculations show that gas productions may largely exceed consumption, indicating clearly that thermophilic, as well as mesophilic digestion, is energy self-sufficient.

As shown in Table IV, even if the energy balance in thermophilic conditions appears to be lightly lower (89%) than in mesophilic conditions (92%) due to the heating requirements, the net energy production would be significantly higher (19% more) in thermophilic range. More- over, by determining the energy consumption, the net energy production and the energy balance in thermophilic digesters fed at identical rates, but at HRT of 15 and 10 days, respectively, the values obtained were identical, leading us to the conclusion that a lower HRT would sim- ply act to linearly reduce the volume of reactor, as well as the heat losses, although this parameter had not been taken into account in the present approach.

DISCUSSION

We have demonstrated that as well in mesophilic (35°C) as in thermophilic (55°C) conditions, semisolid, semicontinuous digestion of organic matter from MSW can be carried out at 100% refuse loading, by using only recycled liquor as liquid added. The fermentations were performed only in the presence of bacteria provided from the starting up inoculum, which revealed to be highly active and effi-

Table IV. signed to treat 100 tons/day organic fraction of refuses.

Theoretical comparison of the energetical performances of both mesophilic and thermophilic fermentations. Hypothesis of a digester de-

Annual load Effective Gas production Energy Net energy Energy Temperature Residence time of MSW volume consumption production balance

(“C) (days) (tons/year) (m’) (m3/m3 day) (m3/year) (m’ gas/year) (rn’ gas/year) (%)

35 15 26,000 2000 3.3 2,386,800 180,350 2,206,450 0.92 55 15 26,000 2000 4.0 2,951 ,OOO 3 24,640 2,626,360 0.89 55 10 26,000 1 400 5.8 2.95 1,000 324,640 2,626,360 0.89

MARIQUE ET AL.: ANAEROBIC DIGESTION OF MUNICIPAL REFUSES 539

cient to convert the specific substrate into biogas. In both operating conditions, we observed a striking long-term stability of the fermentation at high feed rates. Indeed, after the series of assays reported in this paper the two reactors were maintained for approximately one year with the same loading rates and the methane productions remained essentially identical to that obtained from the experiments reported in this article. No other sources of nutrients or chemicals were needed for pH or nitrogen control into the reactors.

In all cases, the thermophilic digester performed better than the parallel mesophilic one. A biological failure which seemed to be due to some inhibitory effect of a substance obviously present in the load, did not imply to drain completely the mesophilic reactor. However, when the load was increased step by step in the thermophilic reactor, for each step, we observed that the stability of the fermentation per- sisted, whereas the volumetric gas production increased.

Our results indicate that thermophilic methanogenesis can easily be initiated and maintained at high levels of activity even at loading rates as high as 16.1 kg VS/m3; i.e. values more elevated than previously thought possible.

The mean productions of gas observed reached: 91.8 L/ kg MSW in mesophilic conditions at a loading rate of 8 kg VS/m3 day and a 16.1 days HRT; 114.1 L/kg MSW in thermophilic conditions at a loading rate of 16.1 kg VS/m3 day and a 10.1 days HRT. The mean volumetric gas production rates were 2.73 and 6.79 L/L day, respectively, with a methane content of 53% under both conditions.

The better efficiency of thermophilic conditions has clearly been demonstrated: At identical loading rates of 8 kg VS/m3 day and HRTs of 16 to 17 days, respectively, the gas production per VS added was 22.5% more than in mesophilic conditions. This yield was maintained when the loading rate was increased to 16.1 kg VS/m3 day at a HRT of 10 days. Moreover, we found that 66 and 82% of the VS were degraded in mesophilic and thermophilic conditions respectively, which confirms that the net conversion of waste to gas, thus the stabilization of the degradable material is also more efficient under thermophilic conditions.

At 55”C, when the loading varied from 8 to 16.1 kg VS/m3 day at HRTs ranging from 16 to 10 days, the gas production per kg VS fed and the VS destruction were found to be independent on the loading rates, and the volumetric gas production of the reactors increased thus proportionally to the load, to reach 6.79 L/L day. Under higher loading rates and HRT less than 10 days, the volumetric gas production remained more than 6 L/L day, but the production per VS dropped significantly (from 420 to 333 L/kg VS).

Moreover, under these conditions, a wide variability in gas composition occurred (mean value of 51%) and the VFA concentrations gradually increased, indicating that a stable active fermentation could not be performed at loading rates higher than 19.3 kg VS/m3 day and HRT shorter than 8.4 days. We did not extensively investigate the rea- son for the decreased efficiency at the highest loading rates, but as did in the case of cattle beef manure,

one might suggest that inhibition is caused by a general overabundance of solutes such as minerals, fatty acids or glucids.

An interesting fact is that even at occasional higher acid levels, the pH remained neutral and stable. Only small changes in pH occurred. Indeed, it decreased from 7.62 at the low loading rate to 7.18 at the highest feed concentration. These data linked to the fact that the need for water is strongly limited, indicate that there are energetic and economical advantages in fermenting MSW in semisolid conditions, at a thermophilic temperature of 55”C, and a HRT of ca. 10 days.

According to the literature,’”’ there are conflicting re- ports on the energy self-sufficiency of the thermophilic digestion process. It is clear that the energy balance of any process does depend on many parameters, such as heating and insulating efficiency, gas production per kilogram of VS fed, and solid concentration in the feed and in the reactors. In our experiments, the most important fact was the feed solid concentrations, that reached values as elevated as 30%, allowing us to proportionally reduce the volume of liquids that must be heated, and to increase the volumetric production of gas.

In any case, the estimated energy requirements of a full- scale plant would be widely nine- to tenfold lower than the total energy production, allowing an energy balance reaching ca. 90%. On the other hand, at 55”C, an HRT of 10 days would allow a significant reduction of digester volumes.

Moreover, the whole gas production may be burned in an electrical power station and the hot water from cooling may be recovered for the heating purposes of the digester, providing more important energetical benefits. On the other hand, from an economical point of view, both the reduction of the digester size and high level gas production may allow a significant reduction of building and operating costs of a full-scale plant, a fact that can be easily deduced from our preliminary engineering studies: reductions varying from 15 to 44% of the treatment costs were estimated according to the general design of the systems and the energy utilization scheme.

Another advantage of thermophilic processing of organic MSW concerns pathogens removal: if digested wastes have to be used as a soil amendment, their content in pathogenic bacteria is an important factor to consider, with respect to various laws and sanitary rules. From this point of view, we have performed several experiments in “batch digesters” maintained in both temperatures conditions and fed with MSW sludges provided from our digesters (data not shown). Pre-numerated Escherichia coli, Klesbsiella Edwardis, Staphylococcus aureus, and Streptococcus fae- calis were introduced into the batches. Then, these strains were monitored and counted in samples taken at intervals varying from 5 h to 2 days, according to the experiments.

Briefly, in thermophilic conditions, we observed a com- plete inactivation of the pathogens after 24 h incubation, whereas 6 days were necessary for their elimination at 35°C. These observations confirm that, applied to our particular

540 BIOTECHNOLOGY AND BIOENGINEERING, VOL. 33, JANUARY 1989

substrate, the thermophilic range is widely more efficient for pathogenic removal. This property of thermophilic fermentation was widely studied4 in the case of sewage sludge digestion which provides a clear confirmation to our observations.

Finally, this sanitary aspect offers one more advantage leading us to the conclusion that the thermophilic conditions must be applied to the anaerobic processing of organic matter from MSW at high feed concentrations, offering a number of high benefits ranging from degradation efficiency and good energy balance to economical and sanitary improved yields.

The authors wish to thank A. Duyckaerts for diligent technical assistance and M. Comil-Merville for typing. This work was sup- ported by the “Ministere de la Rtgion wallonne pour 1’Environne- ment et I’Agriculture,” and was performed with the help of a staff provided by the Office National de I’Emploi, contract No. 30.315.

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7. V. H. Varel, H. R. Isaacson, and M. P. Bryant, Appl. Environ. Mi- crobiol., 33, 298 (1977).

8. V. H. Varel, A. G. Hashimoto, and Y. R. Chen, Appl. Environ. Mi- crobiol., 40, 217 (1980).

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MARIQUE ET AL.: ANAEROBIC DIGESTION OF MUNICIPAL REFUSES 54 1

thermophilic semisolid anaerobic digestion of municipal refuses

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