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Journal of Environmental Management 106 (2012) 48e55

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Composting of anaerobic sludge: An economically feasible element ofa sustainable sewage sludge management

N. Cukjati a, G.D. Zupan�ci�c b,*, M. Ro�s b, V. Grilc c

a Public Utility Company Velenje Ltd, Koro�ska 37 b, SI-3320 Velenje, Sloveniab Institute for Environmental Protection and Sensors, Beloruska 7, SI-2000 Maribor, SloveniacNational Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia

a r t i c l e i n f o

Article history:Received 21 June 2011Received in revised form31 January 2012Accepted 1 April 2012Available online 2 May 2012

Keywords:Anaerobic sludgeCompostingSewage sludgeStabilizationSustainability

* Corresponding author. Tel.: þ386 2 3335 669; faxE-mail addresses: Nevenka.cukjati@kp-velenje.si (

ios.si (G.D. Zupan�ci�c), milenkoros@gmail.com (M. Ro�s

0301-4797/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.jenvman.2012.04.001

a b s t r a c t

An investigation into the feasibility of anaerobic sludge composting, as a sustainable treatment of sewagesludge management, was carried out under actual Slovenian environmental conditions. In order todemonstrate successful composting, five pilot plant experiments were performed during the summerand winter conditions. The first three experiments were performed with pile aeration, while experi-ments 4 and 5 were carried out by pile turning. Anaerobic sludge to bulking agent ratios were set at 1e6.4:1. The composting was successful and thermophilic temperature being achieved in all cases. Inwinter conditions, the composting process was prolonged; and low ambient temperatures had a signif-icant impact in pile turning experiments. During winter, a temperature drop of 30 �C during turning ofthe material doubled the necessary time for an adequate composting process. Five scenarios wereconsidered within an economic feasibility study and in the most favourable scenario, where 60% ofcompost was commercialised and 40% was used as landfill cover. The payback period in this scenario was2.9 years. The study of compost quality showed that it can be used in variety of civil engineeringapplications, especially as a landfill cover and for recultivation of degraded areas.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Anaerobic Sludge (AS) is a potential source of organic matter,nutrients and minerals and may be useful as an agricultural soilsupplement. Freshly digested sludge is unstable under normalenvironmental conditions as it is biodegradable, has an unpleasantodour and contains various noxious or corrosive gases such as NH3and H2S. Therefore, it must be stabilized before it can be adequatelydisposed of in the natural environment. All of these problems canbe overcome by composting, which is an obvious solution to thisproblem, where all unwanted by-products can be reduced to anacceptable level (Zbytniewski and Buszewski, 2005). Tarrasón et al.(2008) showed that composting digested sludge provides a bene-ficial effect on availability of nitrogen in the soil.

There are few management alternatives for direct disposal ofdigested sludge. With a lack of other options, mechanically dehy-drated sludge can be dried to 90% with the use of biogas and uti-lised as an alternative solid fuel in various industrial kilns (Grilc

: þ386 2 3335 680.N. Cukjati), gregor.zupancic@), viktor.grilc@ki.si (V. Grilc).

All rights reserved.

et al., 2011) using various methods of energy production(Houdková et al., 2008; Stasta et al., 2006). The net calorific value ofdry sludge is approximately 10e12 MJ kg�1, with ash contentapprox. 35e45%. In Slovenia, incineration has become thepredominant alternative to land applications since 2010, whendirect landfill disposal of sludge was legally prohibited due to itsgreenhouse gas potential.

In recent years composting anaerobic sludge has been widelystudied with different types of co-substrates and with varyingbulking agents (Nakasaki et al., 2009; Himanen and Hänninen,2011). The composting process requires a structural material toprovide sufficient porosity and air permeability for the compostpile. Various conventional wood or plant processing remnants havebeen used as such a structural material and these include woodchips, rough sawdust, tree bark, straw and corn stalks. The volumeratio of sludge to bulking agent should be between 1:1 and 1:4 (Geaet al., 2007). The majority of organic material is contributed by thebulking agent, but significant biodegradation by means of naturalaerobic micro-organisms occurs in the sludge organic material(Chroni et al., 2009).

Another important factor is the presence of nitrogen. Severalauthors have reported that the optimal C/N ratio is between 25/1and 30/1 (Pakou et al., 2009). Operations at C/N ratios as low as

Table 1Main physical properties of the inlet composting process components.

Component Parameter Unit Average measured valuesa

Anaerobicsludge

DM % 23.3 � 1.93LoI % DM 56.6 � 6.22TOC % DM 31.3 � 3.47GCV MJ kg�1 DM 12.1 � 1.50

N. Cukjati et al. / Journal of Environmental Management 106 (2012) 48e55 49

10/1 have also been reported (Yañez et al., 2009), but at such lowC/N ratios the undesirable emission of ammonia can be significant(Matsumura et al., 2010). In recent years, research focused moreon biodegradable C/N ratio (Komilis et al., 2011), which was inranges between 3.9/1 and 12.7/1. Komilis et al. (2011) introducedthe biodegradable C/N ratio as a novel indicator that could beused to characterize the composting process. Their conclusionwas that initial total C/N ratio did not correlate significantly withthe degradability of the mixtures as opposed to the biodegradableC/N ratio. While total C/N ratio did not change much during theprocess, the biodegradable C/N increased from average 10 to 20.

High oxygen content in the air contributes to more rapiddegradation (Sundberg and Jönsson, 2008) and prevent formationof anaerobic zones. Highest degradation rate in the compost pilewas achieved when the air oxygen concentration was above 15%(Day and Schaw, 2001) and the quality of aeration dependsprimarily on the structure and granulation of the compostingmaterial. Finer material has generally enabled a more effectivelyaerated compost pile, but at the expense of higher powerconsumption. In the first stage of degradation, organic acids aregenerated, which decrease the pH in the compost pile. Theoptimum pH range by which micro-organisms function is 5.5e8.5(Sánchez-Monedero et al., 2001). This research found that whenthe conductivity of the compost material is higher than8 � 105 mS cm�1 it becomes a limiting factor for growth of micro-organisms and consequently for organic matter degradation.Elevated temperature of the compost material during operation isa consequence of the exothermal biodegradation of organic matter.The optimum temperature for composting, which provides path-ogenic micro-organisms to be sanitised, has been found between55 and 70 �C (Wéry et al., 2008). In the initial phases of compostingthe predominant micro-organisms are fungi and mesophilicbacteria. These contribute to the temperature increase, but aremostly sanitised in the thermophilic range. When temperaturedecreases, many of the initial mesophilic micro-organisms reap-pear, but more highly evolved organisms such as protozoa andarthropods predominate (Schuchard, 2005). For optimum com-posting operation the correct conditions are determined by particlesize distribution, material moisture and oxygen concentration.Mohee and Mudhoo (2005) have shown that the air gaps in thecompost pile can be reduced during the process from an initial76.3% to a final 40.0%. The optimum moisture content in thecompost material should be between 50% and 70% (Ahn et al.,2008).

The objective of the present study was to examine the feasibilityof the anaerobic sludge stabilization process for sustainable sludgemanagement in the Slovenian (i.e. alpine) environment. Pilot scaleand near full-scale experiments, extreme summer and winterconditions and the economic and environmental acceptability ofsuch a process were of particular concern.

N % DM 4.3 � 0.20P % DM 5.1 � 0.30K % DM 0.5 � 0.08Bulk density kg m�3 725 � 25Particle size mm <1

Wood chips/splinters

Particle size mm (30e80) � (5e10) � (1e5)Bulk density kg m�3 280e300DM % 45e60

Wood bark Particle size mm (30e1200) � (10e30) � (1e5)Bulk density kg m�3 290e310DM % 65e75

Sawdust Particle size mm 1e5Bulk density kg m�3 200e250DM % 45e55

DM-dry matter, LoI-loss on ignition, TOC-total organic carbon, GVC-gross calorificvalue.

a Statistical data are presented for a 4 month period on the basis of weeklycomposed samples.

2. Materials and methods

A pilot composting experiment, using the anaerobic sludge froma local municipal wastewater treatment plant of Velenje, a town of33,000 inhabitants in Northern Slovenia, was conducted outdoorsunder prevailing environmental conditions. Five experiments wereconducted from midsummer to midwinter. Anaerobic sludge wasacquired from a mesophilic digester, which treated primary,secondary and tertiary sludge from the municipal wastewatertreatment plant. Mixtures of fresh sawdust, wood chips and treebark in different ratios were used as structural materials. Nomicrobiotic or fertilizing additives were used to initiate the com-posting process.

2.1. Materials

The components used in this study, described in Table 1, werefresh and supplied from local sources. The composition and mostimportant physico-chemical properties of the mixtures used in theexperiments are presented in Table 2. Other research in this field(Hernández et al., 2006; Banegas et al., 2007; Grigatti et al., 2011)has mostly applied the well established sludge to bulking agentvolume ratio 1:1 to 1:4. However, the goal of the present study wasto minimize the quantity of bulking agent due to economic reasonsand therefore substantially, by increasing the ratio from 1:1 up to6.4:1 in order to assess the composting process feasibility with suchhigh ratio mixtures.

2.2. Experimental

Five pilot scale experiments were performed. Experiments 1e3were carried out with forced aeration, while in experiments 4 and 5aeration was achieved with mechanical turning of the material inthe elongated piles (windrows). In experiments 1e3 equally sizedpiles (length �width � height ¼ 3.0 � 2.5 � 0.75 m) with an initialcontent of about 5 m3 of composting mixture were used. Thecompost piles were covered with a water resistant, air permeablemembrane weighing 200 g m�2. An aeration system was installedat the base of a compost pile and consisted of 6 parallel alkateneplastic perforated tubes, 2 m long, 0.12 m in diameter, and 0.4 mapart, with 0.7 mm perforations. The capacity of the aerationsystem was 120 m3 h�1. At pile temperatures below 65 �C, theaeration system was operating alternately in on/off mode, above65 �C it operated continuously in order to cool the compost pile. Inthe experiments 1e3, the temperature was monitored online, withthe temperature probe being set 0.45 m below the surface in thecentre of the pile. The pile temperature was controlled by means ofaeration intensity. Nowatering was applied during the experiment.

Experiments 4 and 5 were nearly full-scale. The elongatedwindrows of trapezoidal cross-section had the bottom width of4.5 m, top width 3.0 m, height 2.5 m and length of 45 m; itsapproximate volumewas 420m3. No aeration systemwas installed;therefore the piles had to be aerated by mechanical turning witha landfill loader. Experiment 4was conducted in Autumn/Fall, whileexperiment 5 was carried out during winter. In winter conditions itis essential that turning was conducted as rapidly as possible to

Table 2Parameters of initial conditions in the composting experiments.

Input parameters Units Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5

AS:WC:SD:TB m3 4:3:2:1 5:1.5:1:1 3:1:1:0 1:0.5:0:0.5 1:2:0:1AS:WC:SD:TB kg 5.5:1.6:1:0.6 15.5:2.2:1:1.3 11.6:0.8:1:0 3: 0.6:0:0.4 1:0.5:0:0.5AS:WB kg 1.7:1 3.4:1 6.4:1 3:1 1:1Volume m3 w5.0 w5.8 w5.3 w420 w420Bulk density kg m�3 589 � 8.8 552 � 8,8 624 � 9.3 615 � 9.2 550 � 8.2pH e 8.4 � 0.1 8.2 � 0.1 8.3 � 0.1 8.1 � 0.1 8.3 � 0.1Conductivity mS cm�1 668 � 6.6 766 � 11.5 921 � 13.8 720 � 11,1 670 � 10.1Dry residue % 31.0 � 0.9 32.4 � 1.1 31.6 � 1.2 31.3 � 0.9 35.2 � 1.3Loss on ignition % DM 64.5 � 6.4 68.2 � 3.7 63.3 � 5.6 67.9 � 4.1 64.1 � 5.6Porosity % 56 � 2.5 52 � 3.1 48 � 2.2 54 � 2.0 58 � 4.2Ptotal % DM 2.1 � 0.4 4.8 � 0.5 2.9 � 0.3 n.d. n.d.Ktotal % DM n.d. 0.4 � 0.1 0.1 � 0.03 n.d. n.d.C:N ratio, total e 9.4:1 8.9:1 n.d. n.d. n.d.Aeration flow rate m3 m�3 h�1 2 2 1 e e

AS-anaerobic sludge, WC-wood chips, SD-saw dust, TB-tree bark, WB-wood biomass ¼ WC þ SD þ TB, DM-dry matter, n.d.-not determined.

N. Cukjati et al. / Journal of Environmental Management 106 (2012) 48e5550

prevent cooling of the material. Experiment 4 lasted for 47 days,material turning being conducted on days 7, 15, 25, 32 and 40. Inexperiment 5, which lasted for 75 days, turning was conducted ondays 13, 23, 38, 51 and 65. In experiments 4 and 5, temperature wasmeasured on a daily basis at 4 central equidistant locations alongthe windrow, 1 m below the surface.

The temperature of the freshly supplied, mechanically dehy-drated anaerobic sludge was 25e30 �C. After mixing with thebulking material the temperature decreased to 10e20 �C, in winterconditions to 1e5 �C. In the course of the experiments the objectivewas to achieve successful composting with the smallest amount ofstructural material and in the shortest time possible, for economicreasons.

2.3. Analytical methods

In the experiments 1e3 sampling and monitoring was con-ducted every second week centrally along the piles, 0.3 m apart and0.3 m below the surface of the compost pile by means of a screwsampler. In experiments 4 and 5, material sampling and tempera-ture monitoring was conducted at 4 central equidistant pointsalong the length of the compost windrow, 1 m below the surface.The samples were disintegrated to particles less than 1 mm in sizeprior to further analysis. The volumetric porosity of the compostmaterial was determined by an internal displacement method(Miheli�c, 1991). Oxygen in the compost piles was measured 20 minafter each aeration using portable meters (Sewerin SR2-DO andDräger Mini Warm). Table 3 presents all of the applied analyticalmethods in the experiments. All measurements were performed in

Table 3Standard methods used in characterization of components and products.

Parameter Standard methods

Sludge Compost mixture

Dry matter EN 12879:2005 EN 14346:2006Loss on ignition EN 12879:2005 EN 15169:2007Total organic carbon e TOC EN 13137:2001 EN 13137:2001Adsorbable organohalogens e AOX ISO 10382:2002 ISO 10382:2002Polycyclic aromatic hydrocarbons e PAH EN 15527:2008 EN 15527:2008Polychlorinated biphenyls e PCB ISO 10382:2002 EN 15308:2008Particle size ISO 3310-1:2000 EN 15415-2:2011Bulk density e EN 15103:2009N EN13342:2000 EN 15309:2007P EN 14672:2005 EN 14672:2005Light and heavy metals EN13346:2000 EN 15309:2007Leaching test EN 12457-4:2007 EN 12457-4:2007pH EN 12176:1998 EN 12506:2004Conductivity EN 27888:1993 EN 27888:1993Salmonella CN/TR 15215:2006 CN/TR 15215:2006

parallel (except gas measurements), averages and standard devia-tions were calculated with MS excel 2010, where measurementuncertainty was also considered and determined in accordancewith the Guide to the Expression of Uncertainty in Measurement(BIPM et al., 1995). For the calculation of uncertainty a coveragefactor k ¼ 2 was used, that gives a level of confidence of approxi-mately 95%.

3. Results and discussion

Recent studies of the composting of anaerobic sludge(Hernández et al., 2006; Banegas et al., 2007; Himanen andHänninen, 2011) reported no problems in reaching the requiredthermophilic temperature. Although the sludge is anaerobic inorigin, it still contains enough biodegradable organic matter tobreak down further in the presence of atmospheric oxygen. In thepresent study this process was tested in stringent environmentalconditions, especially in winter. The environmental conditions (airtemperature and precipitation) had no effect in experiments 1e3but had a significant impact in experiments 4 and 5, which wereconducted in fall and winter conditions. The best results with pileaeration (experiments 1e3) were achieved when full aeration wasapplied for one minute each hour (corresponding to 2 m3 m�3 h�1).Above 65 �C the pile was aerated continuously to compensate forthe temperature increase. The design of the aeration system did notmanage to provide sufficiently low pressure for continuous aera-tion and the pile was unequally aerated, therefore an on/off systemwas applied. The conductivity of the compost increased during theexperiments, and this can be attributed to the degradation ofcomplex organic matter into species of low molecular weight,which dissipate in the wet compost. The final parameters of theexperiments are presented in Table 4.

3.1. Experiments 1e3

Experiment 1 had the lowest ratio of anaerobic sludge (AS) towood biomass (WB) of 1.7:1 (Table 2) and was also the mostsuccessful. The thermophilic temperature was achieved in 4 days ofoperation (Fig. 1). This result is similar to that reported by Himanenand Hänninen (2011), where compost mixture 1:1 of AS and peatwas used. Banegas et al. (2007) reported that 1:1 mixture of AS andsawdust (SD) did not achieve thermophilic temperature. Theystated that this was due to high zinc concentrations in AS, as well asa low proportion of bulking agent. In their case, 1:3 ratio of AS tosawdust was successful to achieve thermophilic temperature inapprox. 20 days. Increasing the AS to WB ratio to 3.4:1 in experi-ment 2 still resulted in an efficient composting process. However,reduced porosity (Table 2) had a significant effect. Thermophilic

Table 4Final parameters of the mixtures, obtained in the experiments.

Parameters Units Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5

Duration days 31 31 86 46 74Weather conditions e Fall, occasional rain Winter, rain and snow Summer-fall Fall, occasional rain Winter, snowCompost temperature >55 �C days 10 11 25 12 7pH e 7.2 � 0.1 7.3 � 0.1 7.1 � 0.1 7.4 � 0.1 7.3 � 0.1Conductivity mS cm�1 1386 � 20.7 1771 � 26.6 1851 � 27.8 1800 � 27.0 1420 � 21.3DM increase % 8.0 � 1.1 �3.5 � 0.6 26.8 � 1.5 5.3 � 0.8 �2.5 � 0.5OM mineralization rate % 5.7 � 0.8 6.7 � 0.7 6.3 � 0.8 6.5 � 0.6 6.2 � 0.5Porosity reduction % 8.9 � 1.0 11.5 � 1.1 4.2 � 0.7 10.3 � 0.9 9.2 � 0.8Volume reduction % 4.0 � 0.9 3.4 � 0.7 13.7 � 1.3 6.2 � 0.9 3.6 � 0.6Ptotal % DM 2.8 � 0.5 5.2 � 0.4 3.0 � 0.4 n.d. n.d.Ktotal % DM n.d. 0.4 � 0.08 0.4 � 0.09 n.d. n.d.C:N, total by mass 9.2:1 8.6:1 n.d. n.d. n.d.

DM e dry matter, OM e organic matter, n.d. e not determined.

N. Cukjati et al. / Journal of Environmental Management 106 (2012) 48e55 51

temperature was achieved in 12 days and the process lasted for 31days, as demonstrated in experiment 1. A further increase of AS tothe ratio of 6.4:1 in experiment 3 drastically slowed down thecomposting process. Thermophilic temperature was achieved in 20days and the process lasted 86 days. The results are comparable tothe research of Banegas et al. (2007) where 1:3 ratio of AS and SDwas used. Much less bulking agent was used in the current exper-iments, but with a smaller proportion of SD (Table 2). It suggeststhat using tree bark (TB) and wood chips (WC) improves theporosity and overall performance. The current study estimates theratio of 1:1 to 3.4:1 of AS to WB (mostly TB and WC) as optimal forsuccessful performance of AS composting. Using ratios of up to6.4:1 the AS composting is achievable but less optimal, due tolonger process time and therefore more expenditure. This wasconfirmed by gas measurements in the compost pile. On day 72 ofthe experiment 3 the oxygen content of the air in the compost pilewas 17.6e18.1%, whereas at day 8 of experiments 1 and 2 it hadalready reached 20.1% (air oxygen concentration was 20.9%) whichshows an increased activity in the compost piles of experiment 1and 2. The average daily gaseous content of the compost pile inexperiment 1 is shown in Fig. 2. Methane was detected in the firstfive days (max. concentration 3.5 vol. %) and in the first eight daysH2S was also present (max. concentration 25.3 ppm), which impliesthat anaerobic zones are present in the compost pile. The currentstudy determines that the problem of anaerobic zones is a conse-quence of a delicate balance between the required temperature andtime of aeration. Increasing the aeration immediately decreases thepile temperature due to air-cooling effect. There are two likelyreasons for this, firstly, the compost mixture contains littledegradable material and consequently less heat can be created.Secondly, the compost pile is rather small and heat losses are

Fig. 1. Temperature profiles in Experiments 1e3.

significant, which consequently reduces the temperature. Thisneeds to be further investigated before a full-scale application isinstalled. External air temperatures did not influence the com-posting process significantly. In comparing Experiments 1 and 2 itcan be observed that in the first phases of the process air temper-atures are similar whereas compost pile temperature of experiment1 is higher. In the latter phases of the process, the air temperature ofexperiment 2 was lower, even freezing at times, whereas thecompost pile was warmer that in experiment 1. This indicates thatheat losses were less significant than heat produced in the process.This is also evident in experiment 3. Although higher air temper-atures were present, the compost pile temperature increased moreslowly. This is most likely a consequence of insufficient air supply inexperiment 3 (Table 2). The present study encountered technicalproblems during the initial stage of the process (from day 10onward) and air flowwas reduced by 50%. Taking this difficulty intoaccount, the application of a higher air flow, experiment 3may haveperformed more satisfactorily.

The process parameters were also evaluated through the drymatter (DM) and organic matter (OM) concentrations (Table 4). TheDM concentration in experiment 3 showed the highest increase.This is a consequence of the high proportion of degradable AS in thecompost mixture and of the dry summer conditions, when verylittle rainfall occurred and, as a result, some water evaporated fromthe composting mixture. In experiment 1 the decrease in DMconcentration was moderate. This was expected because theportion of AS was much smaller, more rain having fallen during theexperiment and a smaller amount of water evaporated. In winter,the DM concentration increased, due to additional rain and snow-fall. Although, the compost pile was covered with a semi-permeable membrane, the occasional melting of snow contrib-uted to an increase in the moisture in the compost and snow coveralso minimised water evaporation. OM mineralization rate wasfrom 5.7% to 6.7%, similarly as observed by other researchers.Banegas et al. (2007) observed a 7.87% and 5.57% OM mineraliza-tion rate for AS to SD mixtures 1:1 and 1:3, respectively, whereas

Fig. 2. Gaseous content in Experiment 2.

N. Cukjati et al. / Journal of Environmental Management 106 (2012) 48e5552

Hernández et al. (2006) observed an 8.45% and 15.3% OM miner-alization rate for AS to SD mixtures 1:1 and 1:3, respectively.

3.2. Experiments 4 and 5

Thermophilic temperature (55e65 �C) in experiment 4 wasalready achieved in 5 days in parts of the pile, whereas for averagetemperature it took 21 days to achieve thermophilic range. Otherauthors using pile turning windrows report similar results. Grigattiet al. (2011) reported thermophilic temperature was achieved in 15days using a mixture of sludge to bulking agent 1:4 and Ponsá et al.(2009) reported thermophilic temperature in 15 days usinga mixture of sludge to bulking agent from 1:2 to 1:3. It is clear thatwith every turning of the pile there was a significant negativeimpact on process temperature. The temperature drop during theturning process depends mostly upon the surrounding airtemperature and it is vital that the turning process be performed asquickly as possible. In experiment 4 after the first turning of thecompost on day 9 at an air temperature of 20.7 �C, the temperaturedrop was 5 �C. On day 24 the ambient temperature was 11.1 �C andthe temperature drop in the compost pile was 10 �C. The data ispresented in Fig. 3. In experiment 5 thermophilic temperature wasachieved in day 58 and persisted for 8 consecutive days. The data inFigs. 4 and 5 shows that under the winter conditions the ambienttemperature had an even more significant impact during theturning of the compost mixture. The maximum temperature dropof 30 �C was during the turning on day 22 when the ambient airtemperaturewas�3.2 �C. The composting process is more sensitiveto temperature drops in the preliminary phases of the experiment,when the composting process and corresponding microbiologicalcommunity is being set up. Turnings in the latter phases of theexperiment had less significant effect. The temperature drops ondays 37 and 50 were only 9 �C although the outside temperatureswere �8.2 �C and �5.1 �C, respectively. In further work it may beadvisable to redesign the process in such a way that turnings areapplied less frequently in the early phases of the composting. OMmineralization rate was similar as in experiments 1e3, from 6.2% to6.5% (Table 4) which corresponds with other authors mentionedpreviously (Hernández et al., 2006; Banegas et al., 2007).

3.3. Economic feasibility

The economic feasibility was determined with a dynamicmethod (European Commission, 2006), which calculates the net

Fig. 3. Temperature profi

present value (NPV) and internal rate of return (IRR). The discountrate considered is 7% and the desired payback period is 10 years.The alternative method of sludge management is incineration, themost generally applied method in Slovenia (Grilc et al., 2011) withaverage cost at 126 V per tonne of wet dehydrated sludge. There-fore, the main factor considered in the economic analysis was thedifference between the cost of sludge composting and that ofincineration. The local municipal wastewater treatment plant,which supplied the sludge for the experiments reported within thisstudy, annually produces 3280 tonnes of wet dehydrated sludge.This quantity was the initial point for calculations with regards tothe present study. Five scenarios for the sludge management havebeen studied according to specific local needs and options. Theseare presented as follows:

1. Because raw sludge is accepted from other local treatmentplants, the AS quantity is expected to increase by 9% each yearfrom 3280 tonnes to the maximum capacity of installed plantfor anaerobic digestion. This will produce 7120 tonnes of ASannually after 10 years but no revenues are assumed to beassociated with acceptance of this additional sludge. Theproduced compost is not commercialised, but is used as thefinal cover on the local landfill. The local landfill is managed bythe same operator as the wastewater treatment plant whichproduces the AS. At the maximum production of compost, thelandfill can manage this quantity of compost for some 20 years.

2. The AS quantity is expected to increase by 9% each year from3280 tonnes to the maximum capacity of 7120 tonnes. Afterfive years 60% of the compost produced is commercialised ata price of 41 V per tonne, the remainder is used as landfillcover.

3. The AS quantity is increased immediately to the capacity of6500 tonnes. The produced compost is not commercialised, butis used as local landfill cover.

4. The AS quantity is increased immediately to the capacity of6500 tonnes. After five years 60% of produced compost iscommercialised at the price of 41V per tonne, the remainder isused as landfill cover.

5. As in scenario 1, but additionally the revenues of acceptingmunicipal sludge from other treatment plants(60 V per wet tonne) is considered in the calculation.

Full-scale installation requires an asphalt or concrete platform,equipped with an aeration system or/and turning vehicle, each

le in Experiment 4.

Fig. 4. Temperature profile in Experiment 5.

N. Cukjati et al. / Journal of Environmental Management 106 (2012) 48e55 53

assuming 50% of the load, housing and gas emission treatment(biofilter) for composting with aeration and only roofing for com-posting with pile turning. The platform should be properly slopedin order to collect spontaneous leachate and precipitation. Theinvestment costs of such composting plant with capacity of7120 tonnes are 462,646 V including preparation and processingequipment. Annual costs consist of manpower costs, bulking agentcosts, energy costs, material costs, analytical costs, a 10% overhead,and amount between 250,000 V and 360,000 V, depending on thequantity of bulking agent and energy and material costs. In Fig. 5,annual expenditures and revenues with the total balance (sum ofall up to date expenditures and revenues) from all the scenarios arepresented.

This analysis shows that all scenarios except #1 are economi-cally feasible. The sludge management presented is sustainable ifthe IRR value is >7.0% (discount rate) and the NPV value is positive.

Fig. 5. Economic analysis of the

The IRR’s for the 5 scenarios, taken consecutively, are 6.8%, 17.5%,37.4%, 43.9% and 19.7% and the NPV values are �6797 V, 529,611 V,805,996 V, 1,390,230 V and 612,245 V, respectively. The paybackperiods on the investment regarding the scenarios 2 to 5 were 7.5,2.9, 2.9 and 7.0 years, respectively. Scenario 1 is just barely infea-sible; a revenue of only 1 V per tonne of municipal sludge acceptedfrom other treatment plants would render it feasible with thepayback period of 10.0 years. Also, in this analysis it was assumedthat the majority of the produced compost would be used asa landfill cover with no additional costs. This analysis does notconsider the soil costs of the landfill cover if no composting takesplace. Precise determination of this factor proved difficult andconsequently it was omitted from consideration. If this is consid-ered with a minimum cost (approx. 1 V per tonne), scenario 1 maybe economically feasible. Considering all scenarios it is clear that fora more economically viable operation of the composting plant,

foreseen composting plant.

Table 5Characteristics of the anaerobic sludge and its composted mixture.

Parameter Unit Limit values (land application) Measured values

Sewage sludgeb Sewage sludgec 2nd quality compostd Anaerobic sludge Final mixture (experiment 5)

Anaerobic sludge/CompostDry matter (DM) % e e e 23.3 � 1.93 66.1 � 4.53Loss on ignition % DM e e e 56.6 � 6.22 36.7 � 4.04TOC % DM e e e 31.3 � 3.47 21.4 � 2.37Arsenic mg kg�1 DM e e e b.d. b.d.Cadmium mg kg�1 DM 20e40 10 1.5 1.32 � 0.41 1.42 � 0.44Chromium mg kg�1 DM e 900 200 60.7 � 18.6 49.9 � 15.3Cobalt mg kg�1 DM e e e 7.38 � 1.35 6.98 � 1.23Copper mg kg�1 DM 100e1750 800 300 166 � 26.7 147 � 23.7Lead mg kg�1 DM 750e1200 900 250 63.2 � 13.8 56.1 � 12.2Mercury mg kg�1 DM 16e25 8 1.5 1.56 � 0.24 1.60 � 0.25Nickel mg kg�1 DM 300e400 200 75 7.02 � 1.20 25.3 � 4.40Zinc mg kg�1 DM 2500e4000 2500 1200 920 � 228 855 � 212AOX mg kg�1 DM e e e 1.60 � 0.14 3.30 � 0.29BTX mg kg�1 DM e e e b.d. n.d.PAH mg kg�1 DM e e 3 b.d. 0.16 � 0.06PCBa mg kg�1 DM e e 1 b.d. b.d.

Inert wastee Non-hazardous wastee

Standard leachate from anaerobic sludge & compost (landfill relevance)pH e e 6e13 8.0 � 0.1 7.6 � 0.1Total dry solids mg L�1 400 6000 803 � 45.3 660 � 42.8DOC mg L�1 50 80 284 � 18.4 31.2 � 2.88Antimony mg L�1 0.006 0.07 0.020 � 0.002 b.d.Arsenic mg L�1 0.05 0.2 0.0070 � 0.0004 b.d.Barium mg L�1 2 10 0.017 � 0.002 0.83 � 0.09Cadmium mg L�1 0.04 0.1 0.0010 � 0.0007 b.d.Chromium, tot. mg L�1 0.05 1 b.d b.d.Copper mg L�1 0.2 5 0.154 � 0.020 b.d.Nickel mg L�1 0.04 1 0.033 � 0.006 0.007 � 0.001Mercury mg L�1 0.001 0.02 b.d. b.d.Molybdenum mg L�1 0.05 1 n.d. 0.005 � 0.002Selenium mg L�1 0.01 0.05 b.d. b.d.Lead mg L�1 0.05 1 0.022 � 0.005 b.d.Zinc mg L�1 0.4 5 0.135 � 0.030 0.300 � 0.070

n.d. e not determined, b.d. e below detection limit.a Sum of PCBs No. 28, 52, 101, 118, 138, 153, 180 and 194.b Sewage sludge directive (1986).c AbfKlärV (Germany, 1992).d Ordinance RS-biodegradable waste (2008).e Ordinance RS-waste disposal (2006).

N. Cukjati et al. / Journal of Environmental Management 106 (2012) 48e5554

accepting additional sludge and increasing the processed quantityis, in terms of shortening the payback period, more viable thansludge commercialisation. Commercialisation of produced compostafter five years has no impact on the payback period if the capacityof the composting plant is 82% fulfilled. The payback period isconsiderably shorter with even higher percentages of fulfilment,regardless of compost commercialisation, which is also marketdependent and therefore speculative.

3.4. Compost quality

Environmental impact assessment of an outdoor compostingplant for anaerobic sludge stabilization must take into consider-ation both the plant emissions and the quality of the compostproduced. The first aspect mainly relates mainly to odour emissionwhich was not measured and is not considered in this study.Unpleasant odoursmay be released only duringmechanical turningof the material and air treatment is not necessary if the plant islocatedwell away from inhabited areas, as is the case in Velenje. Forthe aerated piles an air collection system, fitted with a biofilter, isplanned. Dust emissions from periodical mechanical turnover arenot considered critical when the compost is properly wetted; noiseemissions can be controlled by selection of appropriate low noiseequipment.

The compost quality depends on the content of pollutants suchas heavy metals, persistent organic pollutants (POP), pathogenicbacteria and inert matter in the mature compost. The properties ofthe standard compost leachate must also be considered. Heavymetals and POPs accumulate during the composting process andmay cause problems upon utilization. The content of heavy metalsand POPs is determined by quality of the input material, whichshould be carefully controlled in sewage sludge or in the rawwastewater, by additional treatment and reduction at the source.Pathogenic bacteria such as Salmonella, Streptococci and Coliformsmay originate from the sludge or from other composting materials,although it has not been the case in this study. If the thermophilicphase period of the composting process (T > 65 �C) has persistedfor more than a few days, the compost producedmay be consideredsanitized and free of pathogens. This was the case within thepresent experiments where Salmonella was determined to beabsent in the initial sludge as well as in the final compost mixture.

The analytical results from the sludge and compost mixture arepresented in Table 5, where limit values for sludge or compostutilisation in agriculture and for disposal of inert/non-hazardouswastes at landfills are also presented. As can be seen, no param-eter was found to be critical either for recycling as second classcompost or for disposal. The compost considered here can thereforebe used in a broad variety of civil engineering works, particularly

N. Cukjati et al. / Journal of Environmental Management 106 (2012) 48e55 55

for recultivation of degraded areas. However, it is not suitable foragricultural use due to the elevated content of cadmium, mercuryand zinc. With respect to limiting values, the compost may beconsidered to be non-hazardous waste; but the limits for inertwastes in the standard leachate are exceeded in the case of someheavy metals, for example, barium, molybdenum and zinc. Amongorganic contaminants only AOX were measured in the raw sludgeand in the final compost mixture. These substances are persistentto bacterial activity so that no degradation was observed; corre-sponding increase of their content was noted, similar as in the caseof heavy metals. The content however is very low. As yet, POPscontent in compost for land use has not been limited. In the rawsludge for use on land the limit value of 500 mg/kg DM of AOX wasproposed in the draft EU document, which seems quite high (Anon,2000).

This compost can be used as a biofilter fill, or as a final cover onlandfills with thickness of 1.2e1.5 m, which is the case of thecompost produced in this study. This is also the reason why nostability/maturity studies were performed at this point, becausefresh compost was needed. The produced unseparated compost isused as a biofilter to capture and biologically oxidise the landfill gas(methane) and other organic compounds found in the landfill gas.This is the subject of further research.

4. Conclusions

A study of composting of anaerobically digested sewage sludgewith sawdust, wood chips and tree bark as bulking agents wascarried out. It was confirmed that using higher sludge to bulkingagent ratio of up to 3:1 compared to conventional 1:3 ratio stillyields comparable results to other research in this field (Hernándezet al., 2006; Banegas et al., 2007). The organic matter mineraliza-tion rate was 5.7e6.7% and thermophilic temperature was achievedin all cases despite the occasional harsh winter conditions (min. airtemperature �11.5 �C). The results of compost quality analysis hasshown that it can be used as landfill cover or soil supplement indegraded areas after construction of, for example, buildings orroads, and gardening, though not for food production.

The current research and experience gained with this pilotproject suggests that investment in such sludge management iseconomically feasible, since it costs considerably less than incin-eration in Slovenian conditions (Grilc et al., 2011) and has accept-able environmental impacts for certain applications.

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