Waste Management 30 (2010) 1828–1833

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Waste Management

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Anaerobic digestion of pressed off leachate from the organic fractionof municipal solid waste

Satoto E. Nayono a,b, Josef Winter b,*, Claudia Gallert b

a Department of Civil Engineering, Yogyakarta State University, Campus UNY Karangmalang Yogyakarta 55281, Indonesiab Institute of Biology for Engineers and Biotechnology of Wastewater, University of Karlsruhe, Am Fasanengarten, 76131 Karlsruhe, Germany

a r t i c l e i n f o

Article history:Received 25 February 2009Accepted 14 September 2009Available online 13 October 2009

0956-053X/$ - see front matter � 2009 Elsevier Ltd.doi:10.1016/j.wasman.2009.09.019

* Corresponding author. Address: Institut für Ingenlogie des Abwassers, Universität Karlsruhe (TH), AmOG, 76131 Karlsruhe, Germany. Tel.: +49 (0) 721 6087704.

E-mail address: josef.winter@iba.uka.de (J. Winter

a b s t r a c t

A highly polluted liquid (‘‘press water”) was obtained from the pressing facility for the organic fraction ofmunicipal solid waste in a composting plant. Methane productivity of the squeezed-off leachate wasinvestigated in batch assays. To assess the technical feasibility of ‘‘press water” as a substrate for anaer-obic digestion, a laboratory-scale glass column reactor was operated semi-continuously at 37 �C.

A high methane productivity of 270 m�3 CH4 ton�1 CODadded or 490 m�3 CH4 ton�1 VSadded wasachieved in the batch experiment. The semi-continuously run laboratory-scale reactor was initially oper-ated at an organic loading rate of 10.7 kg COD m�3 d�1. The loading was increased to finally 27.7 kgCOD m�3 d�1, corresponding to a reduction of the hydraulic retention time from initially 20 to finally7.7 days. During the digestion, a stable elimination of organic material (measured as COD elimination)of approximately 60% was achieved. Linearly with the increment of the OLR, the volumetric methane pro-duction of the reactor increased from 2.6 m3 m�3

reactor d�1 to 7.1 m3 m�3reactor d�1.

The results indicated that ‘‘press water” from the organic fraction of municipal solid waste was a suit-able substrate for anaerobic digestion which gave a high biogas yield even at very high loading rates.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The introduction of the European Landfill Directive (EC, 1999)has stimulated European Union Member States to develop sustain-able solid waste management strategies, including collection, pre-treatment and final treatment methods. According to the EuropeanLandfill Directive, it is compulsory for the member states to reducethe amount of biodegradable solid waste that is deposited on san-itary landfills. The target is that by the year 2020 only less than 35%of the total biodegradable solid waste that was produced in 1995will be deposited on sanitary landfills.

Separation of municipal waste into a recyclable fraction, resid-ual waste fraction and a source-sorted organic fraction (OFMSW)is a common practice of waste management in German cities in or-der to meet the obligations of the Landfill Directive. In 2006 around8.45 million tons of OFMSW were collected. These organic wastesconsisted of 4.15 million tons of source-sorted organic householdresidues and 4.3 million tons of compostable solid wastes fromgardens and parks (Statistisches Bundesamt, 2008a). There exist1742 biological treatment plants and 45 mechanical–biological

All rights reserved.

ieurbiologie und Biotechno-Fasanengarten, Geb. 50.31, 4.

2297; fax: +49 (0) 721 608

).

treatment plants throughout Germany, including compostingplants and anaerobic digesters (Statistisches Bundesamt, 2008b).Germany is categorized as an advanced composting country sinceit has installed a wide range of composting plants from simplewindrow systems to highly sophisticated technical processes. Sev-eral technologies and methodologies have been applied in order tooptimize the composting process and to improve the quality ofcompost. Already in 1995 around 28% of the composting plantsin Germany were categorized as technically advanced (Gruneklee,1997). In 2006, a total number of 485 OFMSW treatment plantsparticipated in the State Commission for Delivery Terms and Qual-ity Assurance (Ger.: RAL-Reichsausschuß für Lieferbedingungen undGütesicherung) of compost, fermentation products and humus(Ger.: RAL-Gütesicherungen für Kompost, Gärprodukte und AS-Hu-mus). These plants altogether treated 7.8 million tons of biodegrad-able waste. The majority of this amount (approx. 5.9 million tons)was treated in composting plants and generated compost predom-inantly from source-sorted OFMSW as well as garden and parkwastes (BGK, 2007).

One important parameter of OFMSW for a successful compost-ing process is its moisture content since the microbial decomposi-tion of organic matter mainly occurs in the thin liquid film aroundthe surface of the particles (Krogmann and Körner, 2000). To sup-port growth and activity of microorganisms that are involved inthe composting process, OFMSW should have a moisture contentwithin the range of 40–60%. A moisture content below 40% will

S.E. Nayono et al. / Waste Management 30 (2010) 1828–1833 1829

severely inhibit the microbial activity, whereas a moisture contentabove 60% will lead to anaerobiosis and may cause emission of badodor. Each fraction of OFMSW has a different moisture content.Previous research (e.g. Rodriguez Iglesias et al., 2000; Hansenet al., 2003; Nordberg and Edström, 2005; Bolzonella et al., 2005)reported that raw OFMSW had a relatively high moisture contentof more than 60%, which was too high for composting. For compostproduction the respective OFMSW must either be mixed withstructured support material (which must be sieved off after com-posting) or dewatered by pressing off surplus water to reach 55%or less moisture content. The leachate from pressing will later becalled press water, which has a high content of suspended and sol-ubilised organic material.

Aerobic degradation of compounds from the liquid fraction ofOFMSW to CO2 and water normally requires less time but an aer-ation system, that consumes much energy. Bad odor and greenhouse gasses may be emitted if the system is not covered for off-gas treatment. Anaerobic degradation to CO2 and methane is notquite as fast but reactors are closed, require less space and shouldbe thermostated. A surplus of energy is available from biogas(Baldasano and Soriano, 2000; Hartman and Ahring, 2006). Fur-thermore, aerobic treatment generates more sludge than anaerobicdigestion. Minimization of mass and volume by intensifying anaer-obic degradation processes before deposition reduces landfill-gasgeneration and odor emissions (Fricke et al., 2005). Therefore, withthe high concentration of organic material in OFMSW anaerobicdigestion of press water is a preferred treatment option.

The main goal of this study was to characterize the press waterand its biogas productivity in order to assess the suitability of presswater as a substrate of anaerobic digestion for recovery of its en-ergy potential and to reduce handling problems.

2. Materials and methods

2.1. Press water sample and inoculum

Press water samples were obtained from a municipal compost-ing plant. In this composting plant, the source-sorted OFMSW fromseven municipalities is prepared for compost production. A press-ing method with mash-separator technique is employed to reducethe moisture content of the OFMSW. A general overview of the pro-cesses involved in the composting plant is presented in Fig. 1.Using the mash separation for the <80 mm fraction, from one tonof delivered OFMSW typically 700 kg of solid phase and 300 kg of

Press water

S

Mash-sep(pressin

Biogas plant(possibility)

SorteWeighing and interim storage

Composwindrow

Fig. 1. Schematic overview of a composting plan

press water are generated. The daily production of press water inthis composting plant is approximately 40 m3.

The anaerobic sludge inoculum was obtained from the effluentof a full-scale wet anaerobic digestion plant treating source-sortedOFMSW from a city in Germany. Before using the digester effluentas inoculum for batch assays and the semi-continuous reactor, theanaerobic sludge was sieved to remove coarse material such asleaves, branches, bones and nutshells.

2.2. Experimental set-up

Methane production of press water was investigated in tripli-cate assays in Schott-bottles of one liter volume. The test was per-formed by adding 2.5 mL press water to 247.5 mL of inoculum,making the total volume of the assay 250 mL (corresponding toan addition of 0.53 g of chemical oxygen demand, COD, or 0.29 gof volatile solids, VS). In control assays methane production fromthe inoculum alone (no press water added) and from the inoculumplus glucose as a positive control was determined. Cumulativemethane production of the assays was measured 2–3 times a day.

The laboratory-scale semi-continuous reactor consisted of athermostated glass column with a liquid working volume of 10 L.Organic matter degradation at decreasing hydraulic retention time(HRT) and increasing organic loading rate (OLR) was investigatedby measuring biogas production, COD and VS elimination. Theglass column reactor was inoculated with a total VS-amount of125.4 g, using the same inoculum as for batch assays, and wasoperated at a temperature of 37 �C, which was maintained by awarm water jacket. To obtain a homogeneous suspension, liquidand/or biogas from the top of the reactor was withdrawn by apump and re-circulated through the bottom of the reactor. Biogasproduction was measured continuously with a Ritter wet gas me-ter (Hanau, Germany).

The feeding of the reactor was done manually twice a day. In thefirst period (intermittent feeding period) the reactor was fed 5 daysper week and received no feeding during weekends, whereas in thesecond period the reactor was fed twice a day for 7 days per week.Every day pH, COD and VFA in the effluent as well as biogas pro-duction and its composition were analysed before addition of freshsubstrate.

2.3. Analytical methods

Standard procedures according to DEV (1983) were employedto determine total solids (TS), volatile solids (VS), ammonia and to-tal Kjeldahl nitrogen (TKN). COD was measured according to Wolf

olid phase

aratorg)

Compostablewaster

< 100 mm

Shredder

Star-sieve< 80 mm

Landfill/ incinerator

Uncompostable/ inorganic waste

tings

t equipped with mash-separator technique.

Table 2Heavy metal concentration in press water – comparison of inhibitory and toxicityconcentrations for anaerobic digestion.

Parameters Press water (mg L�1) Inhibitory (mg L�1)a Toxic (mg L�1)a

Total Soluble

Iron 1249 291.0 n.a. n.a.Zinc 59.6 42.0 150–400 250–600Nickel 96.4 13.4 10–300 30–1000Cobalt 22.2 12.8 n.a n.aCopper 29.4 15.2 40–250 170–300Cadmium 1.9 1.3 - 20–600Lead 15.0 15.0 300–340 340Chromium 13.1 9.8 100–300 200–500Manganese 202.6 134.0 n.a. n.a.

a Konzeli-Katsiri and Kartsonas (1986).

1830 S.E. Nayono et al. / Waste Management 30 (2010) 1828–1833

and Nordmann (1977). Biogas composition (methane and carbondioxide) and VFA were analysed by gas chromatography accordingto Gallert and Winter (1997). The sand content of the press waterwas analysed by washing with water (up-flow velocity approx.0.01 m s�1). The heavy metals content was analysed using a flameatomic absorption spectrometer (Varian-Spectra AA 220FS, Mul-grave, Australia).

3. Results and discussion

3.1. Characteristics of press water

The parameters of the composition of press water are presentedin Table 1. Approximately half of the total COD was soluble, as wasfound earlier for another source of OFMSW (Gallert and Winter,1997). This may indicate that hydrolysis must have started alreadyduring collection, weighing and interim storage and may have pro-ceeded with high hydrolysis rates after the pressing procedure dueto the small particle size in the suspension, obtained by the appliedmash-separator technique. Palmowski and Müller (2000) reportedthat size reduction of materials with high fibre content will im-prove degradability up to 50% and biogas productivity by 20%.The authors assumed that size reduction did not only increase sur-face areas for biodegradation in a more easy and rapid way but alsosupported hydrolysis of suspended solid compounds in the longterm. In line with the high soluble COD content of press waterthere was an accelerated acidification process, indicated by thepresence of relatively high concentrations of total VFA(9.51 g L�1) with acetic acid as the predominant organic acid.

The sand content of press water was analyzed using a gentlewashing method since, due to the consistency and the grayish darkcolor of the press water, sedimentation in Imhoff cones did notlead to a clearly visible layering. The sand content is an importantparameter since the sand might settle in the less turbulent zones ofanaerobic digesters, independently of the agitation system. This re-duces the working volume and the nominal HRT of the reactor.Even if fluidization could be maintained properly, sand wouldcause abrasion of pipe bends or moving mechanical equipmentsuch as pump impellers, which consequently would increase main-tenance costs.

Table 2 presents some important heavy metal concentrations inthe press water. Many heavy metal ions are essential for anaerobicdigestion as modulators of enzymes which are required for properenergy metabolism of organisms that drive anaerobic reaction se-quences (Oleszkiewicz and Sharma, 1990). Takashima and Speece(1989) investigated heavy metals in cells of ten methanogenic

Table 1Main characteristics of press water.

Parameter Unit Value

pH – 4.3Density ton m�3 1.02Chemical oxygen demand g L�1 213.4Soluble COD g L�1 100.1Total solids g L�1 168.4Volatile solids g L�1 117.7Ashes g L�1 50.7Total Kjeldahl nitrogen g L�1 4.10TKNsoluble g L�1 1.52Ammonia nitrogen g L�1 0.72Acetic acid g L�1 8.56Propionic acid g L�1 0.16Butyric acid g L�1 0.21Valeric acid g L�1 0.58Sand sediment Wet volume mL L�1 3.0

Dry weight g L�1 4.40Volatile fraction g L�1 0.05

strains. They showed the presence of the following heavy metals(in falling concentration): Fe � Zn P Ni > Co = Mo > Cu. A properdosage of heavy metals is required for anaerobic processes. Nickelions at a concentration of 5 mg L�1 for instance stimulated meth-ane production by Methanobacterium thermoautotrophicum (Oles-zkiewicz and Sharma, 1990) and tungstate was required byMethanobacterium wolfei (Winter et al., 1984). Although the pres-ence of heavy metals in organic matter may cause stimulationfor anaerobic digestion, it was also observed that heavy metals inhigher concentration may cause inhibition or even exert toxic ef-fects. Aquino and Stuckey (2007) collected data from several pub-lications and concluded that the action of heavy metals asnutrients or toxicants was affected by many factors, such as thetotal metal concentration, the environmental conditions (pH andredox potential), the kinetics of precipitation, complexation andadsorption. Stronach et al. (1986) considered already that onlythe soluble part of metals was bioavailable and thus relevant forthe anaerobic bacteria. From Table 2, it can be seen that almostall of the essential metals (except for molybdenum, which wasnot measured) were available in the press water. With the excep-tion of iron and nickel, the heavy metal concentrations (both, totaland soluble) were relatively low and far from inhibitory or toxicamounts.

3.2. Potential methane production of press water

Results of methane production from press water in batch exper-iments are presented in Fig. 2. The maximum methane productionrate was achieved during the first two days of the digestion (ca.

Fig. 2. Methane production from press water. Curves represent methane produc-tion from press water only and were obtained by subtracting methane productionin assays with and without press water addition.

S.E. Nayono et al. / Waste Management 30 (2010) 1828–1833 1831

180 m3 CH4 ton�1 VSadded d�1). About 90% of the total methane wasreleased in the first four days. After seven days of digestion therewas no significant methane production any more. Therefore, itwas decided that after two weeks of digestion methane productionof press water must have reached its final state. The maximumnet methane production of press water was 270 m3 CH4 ton�1

CODadded, corresponding to 490 m3 CH4 ton�1 VSadded.

3.3. Performance of the semi-continuous reactor

3.3.1. OLR, biogas production and residual volatile fatty acidsA laboratory-scale semi-continuous reactor was operated for

about five months. According to Lissens et al. (2001), a reactorcould be categorized as a wet anaerobic digester with completemixing if the solid content was less than 15% (w/v). Although theraw press water had a TS content of 17% (w/v), immediately afterfeeding to the reactor twice a day and mixing for 1 min the reactorcontent had a maximum solid content of 11%. Thus, the digesteroperated as a wet process.

Biogas and methane production for increasing OLRs to morethan 25 kg VS m�3 d�1 during the reactor experiment are shownin Fig. 3. The average biogas yield and its methane content for eachHRT is listed in Table 3. Initially the reactor was fed with an OLR of10.7 kg COD m�3 d�1. The OLR was increased stepwise to finally27.7 kg COD m�3 d�1 (from 5.9 kg VS m�3 d�1 to finally 15.3 kgVS m�3 d�1). Each increment was performed when the reactorreached steady-state conditions, as judged by a constant COD elim-ination, methane content of the biogas, pH of the digestate andconcentration of residual VFA in the effluent. The stepwise increaseof the OLR required an increasing press water addition from0.5 L d�1 to 1.3 L d�1, which corresponded to a reduction of theHRT from 20 to 7.7 days. Until day 98 feeding of press water wasonly from Monday to Friday (no feeding at the weekends), but fromday 98 onwards the reactor was fed 7 days per week.

Fig. 3. OLR and daily volumetric biogas and methane production.

Table 3Average biogas yield and methane content at each HRT.

HRT (days) OLR [COD] (kg m�3 d�1) OLR [VS] (kg m�3 d�1) Biogas prod

20.0 10.7 5.9 81.516.7 12.8 7.1 80.814.3 14.9 8.2 76.812.5 17.1 9.4 76.711.1 19.2 10.6 77.810.0 21.3 11.8 75.78.7 24.5 13.5 76.37.7 27.7 15.3 80.3

a PW = press water.

The volumetric biogas production of the reactor increased line-arly with the increment of the OLR. The specific biogas or methaneproduction was relatively stable at values between 647 and 696 m3

biogas ton�1 VS d�1, equivalent to (438–450 m3 CH4 ton�1 VS d�1).The methane yield of the reactor reached 89.6–91.8% of the maxi-mum potential methane production of press water after prolongedincubation (490 m3-CH4 ton�1 VSadded; Fig. 2). This indicated thatthe initially inoculated population contained all those organismsin sufficient amounts that were required for efficient press waterbiodegradation.

On day 119 and day 130 there were aeration accidents in thereactor. After clogging of the gas outlet tube by massive productionof foam the upper rubber stopper was lifted off. The recirculation-pump pumped air from the top of the open reactor through thepress water content for 6–10 h. After the reactor was repaired,the OLR was decreased to 10.7 kg COD m�3 d�1 by lowering thefeeding rate. It was increased back to 24.4 kg COD m�3 d�1 in largeincrements. After only 3–4 days, the biogas production reached itshigh value from before the disturbance.

Fig. 4 presents acetate and propionate concentrations in theeffluent of the press water bioreactor. In the first week, propionateincreased to more than 2.5 g L�1. However, this high propionateconcentration seemed not to inhibit methane production or toinfluence the overall anaerobic process. Within a few days the pro-pionate decreased to a non-measurable concentration, indicatingthat the propionate-degraders within the group of acetogenic bac-teria had adapted their activity to the new situation. n-Butyratewas not measurable at any time. It is either not an intermediateor its acetogenic conversion to acetate and hydrogen proceedsmuch faster at any time than its generation (Gallert and Winter,2005).

The concentration of propionate and of acetate increased sud-denly at each stepwise increase of the OLR (Fig. 4). This indicatedthat more propionate and acetate was produced than could be

uction (m3 m�3 PWa d�1) Biogas production (m3 ton�1 VS d�1) CH4 (%)

696 64.6691 65.8656 67.4656 65.8665 66.8647 67.7652 67.9686 67.6

Fig. 4. OLR and residual volatile fatty acids in the effluent.

1832 S.E. Nayono et al. / Waste Management 30 (2010) 1828–1833

degraded by the bacteria of the consortium for a short while, but afast recovery within a few days was possible. These two bottle neckreactions may have been caused by limiting activities of syntrophicpropionate-degraders and aceticlastic methanogens.

Another sudden increase of both acetate and propionate occu-red after accidental oxygen intrusion into the reactor at day 119.The concentration of acetate increased to more than 2 g L�1 andthat of propionate to more than 1.5 g L�1. However, by reducingthe OLR for 2 days, these concentrations decreased to their normallow level within 14 days. Biogas and methane production de-creased immediately, but recovered fast (Fig. 3).

Fig. 5. OLR and COD elimination efficiency.

Fig. 6. Total solids and volatile solids elimination at increasing OLR.

Table 4Energy balance, reactor volume design and potential energy recovery.

Parameter Unit Valu

Reactor volume design and potential energy recovery:Press water production m3 d�1 40Designed HRT Days 10Active reactor volume m3 400Daily methane production m3 d�1 2050Energy recovered kWh d�1 8968Potential benefit €/year 589,1

Energy balance in the composting plant (per ton OFMSW delivered):Energy recovered from press water kWh 67.3Energy for composting kWh 24.5Energy for AD processes (pre-treatment and pumping) kWh 26.9Energy for AD heating kWh 5.0Surplus energy kWh 10.8

3.3.2. Removal efficiency of organic compounds in press waterThe removal efficiency of organic compounds was measured

daily by determining the elimination of COD. When steady-stateconditions at each HRT were reached, based on stable values forpH, low residual fatty acids, stable biogas production and CODelimination, TS and VS of the reactor effluent were also measured(see Fig. 5).

In the first weeks of the operation, the reactor apparentlyreached a relatively high COD elimination of more than 75%. Thishigh COD elimination was probably due to a high inoculum-sub-strate ratio. During the time of intermittent feeding from Mondayto Friday, the COD elimination varied from 60% to 70%. The highestCOD elimination was measured on every Monday since there wasno feeding during the weekend. When feeding was supplied for se-ven days a week the COD elimination reached stable values be-tween 60% and 65%.

In Fig. 6 the relationship between solids elimination (TS and VS)and the OLR is presented. Assuming that a VS elimination of 50–60% is considered close to the optimum for anaerobic degradationof press water, it can be concluded that the OLR of the reactorshould be within the range of 13.5–22.5 kg COD m�3 d�1 (7.5–12.4 kg VS m�3 d�1). This high OLR values for a still acceptableCOD removal support the conclusion of Hartman and Ahring(2006) that a high-solids anaerobic process appeared to be moreefficient when the reactor was operated at an OLR higher than6 kg VS m�3 d�1.

3.4. Potential energy recovery from anaerobic digestion of press water

Table 4 presents the energy recovery from anaerobic digestionof 40 m3 press water per day that are generated in a compostingplant with a pressing facility. Based on experience, one ton of deliv-ered OFMSW resulted in 0.7 ton of solid state waste and 0.3 ton ofpress water. A HRT of 10 days was suggested for anaerobic diges-tion to prevent massive foaming, which occurred at an OLR higherthan 21.3 kg COD m�3 d�1 and to stabilize the organic matter re-moval efficiency. Finally, at a HRT of 10 days reserve capacitiesfor shock loading or for treatment of an increased amount of presswater in future are available. A rough energy balance with energygain from biogas and energy requirement for substrate pre-treat-ment and maintenance of anaerobic digestion is also presentedin Table 4. Overall, about 15% of the energy of the biogas from presswater may be obtained as surplus energy.

4. Conclusions

Part of the water content of the wet organic fraction of munici-pal solid wastes was pressed off as ‘‘press water” to reduce or avoid

e Remarks

– 1 m3 CH4 = 31.46 MJ (at 37 �C)– 1 MJ = 0.278 kWh– Generator efficiency = 50%– 1 kWh = 0.19 Euro

96

35 kWh pro ton OFMSW input (Hartman and Ahring, 2006)40% of energy produced (Murphy and McKeogh, 2004)10% of energy produced-as electricity (Murphy and McKeogh, 2004)

S.E. Nayono et al. / Waste Management 30 (2010) 1828–1833 1833

addition of structural material for composting of the solid residues.The press water had a high potential for methane production. Itwas fed to a CSTR laboratory column reactor for 5 months. A stablemaximal OLR of 27.7 kg m�3 d�1 (15.3 kg VS m�3 d�1) could bereached, which is a relatively high loading compared to otheranaerobic digesters treating OFMSW. More than 387 m3 biogaswere generated per ton of COD added. The methane content ofthe biogas was around 65% and COD elimination was decreasingfrom 70% at an OLR of 17 kg. m�3 d�1 to 60% at an OLRof >25 kg m�3 d�1.

The separation of the surplus moisture from the OFMSW im-proves the composting process and reduces carbon dioxide emis-sion, since a significant part of the biodegradable organiccompounds is soluble and is separated with the press water. Thebiogas from anaerobic digestion of press water can displace fossilfuel and due to greenhouse gas savings provide an environmentaladvantage.

Acknowledgements

Satoto E. Nayono was a recipient of a PhD-Grant from Bundes-ministerium für Bildung und Forschung, Bonn within the IPSWaTprogramme. We also thank DFG for financial support.

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