Anaerobic digestion of municipal solid organic waste: Valorga full-scale plant in Tilburg, The Netherlands

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  • ~ Pergamon

    PU: S0273-1223(97)00555-6

    Waf. Sci. Tech. Vol. 36, No. 6-7, pp. 457-462,1997. 1997 fAWQ. Published by Elsevier Science Ltd

    All rights reserved. Printed in Great Britain.0273-1223/97 $17'00 + 0'00


    Helene Fruteau de Laclos, Serge Desbois andClaude Saint-Joly

    Steinmiiller Valorga Sarl, Pare du Millenaire, 1300 rue Albert Einstein, BP 51,34935 Montpellier Cedex 09, France


    Anaerobic digestion has been up to now essentially applied to wastewater. However, anaerobic treatment oforganic solid waste fits in well with the new requirements for waste management. The Valorga full-scaleplant in Tilburg (the Netherlands) is designed to process 52,000 tons per year of organic municipal solidwaste separately collected. The Valorga digestion process is a semi-continuous, high-solid, one-step, plugflow type process. The main characteristic is the complete absence of any mechanical equipment inside thereactors. The waste to be treated consists of food and garden waste. The characterization of the waste streamrevealed a seasonal fluctuation in quantity and quality, that was correlated with seasonal garden wasteproduction. The methane yield is varying from 210 to 290 m3 STP per Mg of volatile solids. It is related towaste composition. During the slack winter period the waste contains proportionally more food waste that ismore biodegradable than lignocellulosic garden waste. The biological process was stable based on volatileacidity, alkalinity and ammonia measurements in the effluent. The organic residue, after dewatering andstorage under aerobic conditions, can be considered as soil conditioner. 1997 IAWQ. Published byElsevier Science Ltd


    Anaerobic digestion; biogas production; full-scale process; municipal solid waste.


    Anaerobic digestion has been up to now essentially applied at full-scale for the treatment of liquid waste.Nevertheless it is becoming more and more attractive for the treatment of organic solid waste as it producesenergy and an organic residue that can be used as soil conditioner. Concerning municipal solid waste(MSW), up to 60% consists of organic materials. As the environmental constraints for the treatment of MSWare getting more and more strict, anaerobic digestion of the organic fraction of MSW fits in well with thenew requirements for waste management: gases and odours emission, energy saving, recycling. In order toprovide an optimal separation of the organic fractions of MSW -and thus an optimal quality of the organicresidue- several countries have developed sorting at source and separate collection of organics. In Tilburg(the Netherlands) the organic waste separately collected from 300,000 inhabitants are treated by the Valorgaanaerobic digestion process. The plant, designed for processing 52,000 tons of waste per year, has been inoperation since 1994. The process diagram is shown in Figure 1.


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    ---.. To gastrea1ment






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    "'Compressor andpressurIzed gas





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    . ~ t'II II ' , II ,' , 1, 111 ,

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  • Anaerobic digestion of municipal solid organic waste



    The so-called "Vegetable-Garden-Fruit" (VGF) fraction of the MSW consists of food and garden waste. It issorted at the source and separately collected once a week. It is obvious that the quantity of garden waste isvarying over a year period in Europe and that this variation will be of importance for plant operation andperformances. To characterize the waste stream variation, daily analysis of total solids (TS) and volatilesolids (VS) content of the crushed waste was carried out, in addition to quantity registration.The results areexpressed as weekly moving averages of 3 weeks in order to see better the trends. They are depicted inFigure 2 for a period of 60 weeks.

    ~ 2000 70Q) TS .-.Q) ~ 60 ~t 1600 '-"50 .....6 51200 -='g 40 0

    0~ 800 Waste delivered 30 C/)> >.... ~- 20 ]Q)-0Q) 400 -..... 10 C/){/J~ ~~ 0 0

    17 27 37 47 5 15 25

    week of the year

    Figure 2. Fluctuation of waste quantity and quality (TS and VS).

    The quantities of waste delivered to the plant are varying between 400 and 1,100 tons per week. Thisvariation is due to production of garden waste. The TS and VS contents are varying in a large range, namelybetween 37-55% and 32-65%, respectively. The high TS and low VS content corresponds with the peak ofwaste production, i.e., with production of garden waste. That means that garden waste is associated to alarge extent with a dry and mineral fraction that was identified to be sand. This feature is characteristic forthe local conditions in the Netherlands. All these variations were reproducible from one year to another.



    The incoming waste is discharged in a platform hall. It is first screened in a rotating screen. The coarsefraction is sent to a hand-picking station for a negative-way sorting (Le., picking out the good organicmaterial). A magnetic separator allows residual iron metals to be removed. The waste is then crushed toreduce particle size below 80 mm.

    Anaerobic di&estion

    After crushing, the waste is intensively mixed with part of the excess process water and heated by steaminjection. The diluent added is calculated in order to keep the TS content of the mixture fed at around 30 %.This mixture is pumped into the reactors with a piston pump.

  • 460 H. FRlITEAU DE LACLOS tt al.

    The digestion is carried out in two digesters of 3,300 m3 total volume each, und~r me.sophilic co~ditions ~t400C. The two digesters are operating with the same loading rate. The Valorga dIgestIOn proces.s IS a serrucontinuous. high-solid, one-step, plug-flow type process. The rea~tors are. ve~ical cylinders With ~ lateralplug-flow type path of the fermenting matter. They contain ~ vertIcal median lOner wall on approximately2/3 of their diameter. The orifices for introducing and extractmg the matter are located at. the bottom of thereactor on either side of the inner wall. The wall forces the fermenting matter to follow a clfcular pathway inorder to go around it, and to cover the whole surface of the reactor. The height ?f the ve~tical rea~tor~ allowsto simply extract the digested matter by gravity. The mixing system is pneumatic. The blOgas which IS storedin a non-rigid tank is compressed and injected with high pressure at the bottom of t.h~ reactor through anetwork of injectors. The mixing occurs approximately every quarter of an hour and It IS fully automated.The biogas used for mixing is within a closed circuit.

    An important characteristic of the process is the complete absence of any mechanical part inside the reactor.This allows the process to operate in high-solid conditions without any hindrance to the matter circulationand without maintenance of mechanical devices.

    The biogas is piped to a purification unit together with the biogas extracted from the landfill. The methane isinjected into the grid.

    Post treatment

    The digested material that is extracted from the digesters by gravity, directly enters the screw presses fordewatering. The liquid sludge from the presses is then conducted to a hydrocyclone to remove fine heavyparticles, and to a flocculation-filtration unit to remove suspended solids. The filtered effluent is dischargedto the nearby wastewater treatment plant. The solid fractions from the presses and the band-filter are mixedand stored for complete stabilization under aerobic conditions for four weeks in a closed hall. The finalorganic residue is considered as compost.

    In order to prevent any odour emission, the air from the product treatment units and the composting hall ispurified in a biofilter.


    The amounts of waste fed into the digesters are measured using an integrating scale in the conveyor belt.The TS and VS content of the incoming waste are followed as described earlier. The quantities of diluentadded are measured using sensors located in the storage tank. The biogas production rate of the two reactorsis measured on line by a differential pressure flowmeter. The biogas composition -CH4 and CO2 content- isdetermined every two hours by infrared spectrometer. These data are automatically integrated and recordedat the central supervisor plant.

    The retention time and the methane yield over a period of 60 weeks normal operation are illustrated byFigure 3.

    The retention time is expressed as weekly moving averages of 3 weeks. It is varying over a year periodaccording to the quantities of waste to be treated: from 20 days during the peak period to 55 days during theslack period. Actually the retention time is not proportional to the quantity of waste fed but depends also onth~ TS content of the waste and thus on the quantity of diluent added for keeping the final TS content of therruxture at 30 %. It has to be noted that the digesters design must take into account the peak period in orderto guarantee the processing efficiency whatever the period of the year.

    The meth~e ~ield is exp~essed as weekly moving averages of 3 weeks. It can be clearly seen that themethane yield IS also varymg over the year period. During the slack period in winter the average methaneyield is 290 m3 STP per Mg .V~. Du.ring. the peak period in summer, the average m~thane yield is 210m3STP per Mg VS. These vanatlons 10 yield appear to fit well with the variations in the retention time.

  • Anaerobic digestion of municipal solid organic waste 461

    However, they are a~tu~l~y more likely related to the variation in waste composition. Food and garden wasteare known to be slgmflcantly different in their composition and thus in their ability to anaerobicallybiodegrade (Owens et ai., 1992; Chynoweth et aI., 1993). Food waste is rich in easily degradablecomponents such as starch, proteins, lipids, while garden waste contains more recalcitrant materials such aslignocellulose. During the winter period, the VGF contains proportionally a much higher content of foodwaste which is biodegradable (within the considered degradation times) and the methane yield is higher. Themethane yield is thus correlated with the ratio of food and garden waste.


    -..-Methane yield Retention time




    40 -.~

    30 ~E-t



    week of the year

    Figure 3. Fluctuation of methane yield (Y CH4) and retention time (RT).

    Alkalinity, ammonia and volatile fatty acids from each digester were monitored from the start-up of the plantin order to assess the stability of the biological process as well as the impact of process water recycling.Since it is impossible to sample digested material in a representative way due to its heterogeneity, and sincethe compounds of interest are in the liquid phase, sampling and analysis are carried out on the liquid effluentfrom the screw presses. Alkalinity and total volatile acidity are measured on the plant site as described byAnderson et ai. (1992). Individual volatile fatty acids (VFA) are analysed from time to time by gaschromatography in the Steinmiiller Valorga laboratory as described by Jouany (1982) in order to confirm theresults of the previous method.

    Evolution of alkalinity and ammonia concentrations is shown in Figure 4. They first slowly increase untilthey reach an equilibrium value after 18 months of operation. The increase is due to production but also dueto the process-water recycling for diluting the waste. The alkalinity was very high with about 18 gil CaC03at equilibrium, that ensures a considerable buffering capacity. As far as a possible imbalance of the processis concerned, the system would be able to face large increases in VFA without exhibiting an inhibitionrelated to unionised VFA. The ammonia concentration stabilized at about 3gll N. Fluctuations observed inammonia concentration can be the result of several parameters. Variations in TS content lead to differentwaste/diluent ratios, variations in food/garden waste ratio lead to different N contents in the waste fed, andvariations in waste quantity lead to different retention times. All these factors can strongly influence theammonia concentration in the effluent. Despite the high pH value between 7.6 and 8.0 (data not shown), andthus relatively high free ammonia concentrations, no significant inhibitory effect by ammonia was observed.From the start-up, the residual volatile fatty acids concentrations remains below 1.7 gil Hac, mainly aceticacid, which is a low level for such a high-solid content system.

  • 462 H. FRUTEAU DE LACLOS et al.











    3 Z~2.5~

    2 ~1.5


    14/03/94 Date since start-up



    Figure 4. Evolution of alkalinity and ammonia concentrations since the start-up.

    The digested residue contains the mineral fraction and the recalcitrant organic material such as lignin andlignocellulose. These materials are good humus precursors. After dewatering and storage under aerobicconditions, the residue can be considered as compost or soil conditioner. Because the VOP fraction wassource-sorted and separately collected, the heavy metal content of the final product complies with thestandards on clean compost . The sanitary quality complies with the requirements of the European Uniondecision on ecological label attribution to soil conditioner, referring to the absence of Salmonella in 25 g andE. Coli less than 1000 mpn/g (Official Journal of European Communities, 1994).


    These results of the Valorga plant in Tilburg clearly demonstrate that anaerobic digestion can be consideredas a reliable industrial process for the treatment of organic solid waste. Despite the annual variation in wastequantity and quality the biological process remains well balanced. A key factor for reliability of the Valorgaprocess is the complete absence of any mechanical device inside the reactors, which avoids abrasion andmaintenance problems which are of great importance at full scale.


    Anderson. G.K. and Yang G. (1992). Determination of bicarbonate and total volatile acid concentration in anaerobic digestersusing a simple titration. Water Environ. Research, 64 (1), 53-59.

    Chynoweth, D.P., Turick, C.E., Owens, I.M., Ierger, D.E., Peck, M.W. (1993). Biochemical methane potential of biomass andwaste feed stocks. Biomass and Bioenergy, 5 (I), 95-111.

    Iouany, I.P. (1982). Volatile fatty acid and alcohol determination in digestive contents, silage juices, bacterial cultures andanaerobic fermentor contents. Sciences des aliments, 2, 131-144.

    Official Iournal of European Communities (1994). N" L364 Commission decision 94/923/CE on ecological criteria for attributionof community ecological label to soil conditioner.

    Owens, I.~. a?d Chyn~weth, D.P. (.1992). Biochemical methane potential of MSW components. Proc. Int. Symp. AnaerobicdigestIOn ofsoltd waste, Vemce, Italy. Cecchi, F. Mata-Alvarez, I., Pohland, F.G. (Ed.), pp 29-42.


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