Waste Management 32 (2012)1503-1508Contents lists available at SciVerse ScienceDirectWasteManagement5MELSEVIERjournalhomepage:www.elsevier.com/locate/wasmanStandardizationofBODs/cODratioasabiologicalstabilityindexforMSWRaffaello Cossu, Tiziana Lai*, Annalisa SandonICEA Department,University of Padua, Lungargine Rovetta,8,35127 Padova, ItalyARTICLEINFOABSTRACTArticle history:The control of biodegradable substances is the key issue in evaluating the short and long-term emissionReceived 10 January 2012potential and environmental impact of a landfillAccepted 2 April 2012Aerobic and anaerobic indices, such as respirometric index (Rl) and biomethane potential productionAvailableonline30April2012(GB21), can be used in the estimation of the stability of solid waste samples.Previous studies showed different degrees of relationship between BODs/COD ratio compared with RI4.Keywords:Aim of this study is to standardize the parameter BODs/COD ratio and to test the methodology underBiological stability indexdifferent operating conditions (dynamic or static leaching and leaching duration, 6 and 24-h) keepingBODsconstant temperature and liquid/solid ratio (L/S-10 I/kgrs). with the introduction of a COD fractioningCODmethod. The COD fractioning is based on the differentiation between the soluble fraction (CODsol) andCOD fractioning methodthe colloidal fraction (CODcoll) using a flocculation method.The BODs/COD and the BODs/CODsol indices are both consistent and significant and can be used as sta-bility indices.The BODs/COD ratio does not seem to be influenced, for the same test duration, by the type of test, sta-tic or dynamic. In the same way the longer test duration (24-h) does not influence significantly the valuesof BODs/COD ratio. As a consequence a leaching test duration of 6-h is preferable to avoid the beginningof the hydrolysis and oxidation processes.@ 2012 Elsevier Ltd. All rights reserved.1.Introductionproduction. The respirometric index (RI), which measure in a giventime (generally 4-7 days) the consumption of oxygen, can be car-The control of biodegradable substances is the key issue in eval-ried outunder staticand dynamicconditions(Adaniet al.,2004:Cossu and Raga, 2008: Barrena et al., 2009: Sanchez Arias et al.,uating theshortand long-term emission potentialand environ-mentalimpactofalandfill(EhrigandKrumpelbeck,2o01:Cossu,2012): it is widely applied to different organic waste (Berthe2010).et al.,2007:Barrena et al.,2011).The biomethane potential pro-As a result, the biological stability has become a strategicduction (GB21) measures the production of methane in 21daysparameter in evaluating the quality of waste before, during land-(Ponsa et al., 2008: Wagland et al., 2009).filling and at the end of the aftercare phase.The control of the bio-These parameters have some limits due to one or more of thelogical stability ofwaste can be carried out eitherby removing thefollowing aspects:putrescible fractions from the waste to be landflled (source segre-- high cost of the respirometers (either under static or dynamicgation)(Sirini et al., 2010), either stabilizing the waste before land-flling (i.e, mechanical-biological pre-treatment) (Heerenklage andconditions);Stegmann,1995).Duringandafterlandfillingthebiologicalstabil-long testing time;low representativity of the parameter in presence of toxic orityof wastecanbeachieveusinginsitutechniques,accordingtodifferent concepts and relevant technologies such as semi-aerobicinhibiting substances, which can alter the oxygen consumption,landfilling.forcedaerationandPAF(Pre-treatmentAerationFlush-or in the presence of inert organics (such as plastic or paper)ing) (Matsufuji et al., 2000; Cossu et al.,2003; Heyer et al.,2005;which can dilute the biodegradable matrix resulting in lowerLee Chong et al., 2005).respiration indices.Several tests and indices have been proposed for assessing thebiological stability (Cossu et al.,2001). Among these the mostIn order to overcomethe limits of the RI and GB21other indicesused are the respirometric index and the biomethane potentialhave been proposed, such as the black index (BI), which is relatedto the blacking time of a lead acetate paper, and the BODs/COD ra-tio, on the eluate of a leaching test (Cossu et al., 2001: Cossu andRaga, 2008) They both are cheap and effective (Cossu and Raga.* Corresponding author.2008)E-mail addresses: tiziana.lai@unipd.it, lai.tiziana@gmail.com (T. Lai)0956-053x/S - see front matter 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.wasman.2012.04.001
Standardization of BOD5/COD ratio as a biological stability index for MSW Raffaello Cossu, Tiziana Lai ⇑ , Annalisa Sandon ICEA Department, University of Padua, Lungargine Rovetta, 8, 35127 Padova, Italy article info Article history: Received 10 January 2012 Accepted 2 April 2012 Available online 30 April 2012 Keywords: Biological stability index BOD5 COD COD fractioning method abstract The control of biodegradable substances is the key issue in evaluating the short and long-term emission potential and environmental impact of a landfill. Aerobic and anaerobic indices, such as respirometric index (RI) and biomethane potential production (GB21), can be used in the estimation of the stability of solid waste samples. Previous studies showed different degrees of relationship between BOD5/COD ratio compared with RI4. Aim of this study is to standardize the parameter BOD5/COD ratio and to test the methodology under different operating conditions (dynamic or static leaching and leaching duration, 6 and 24-h) keeping constant temperature and liquid/solid ratio (L/S = 10 l/kgTS), with the introduction of a COD fractioning method. The COD fractioning is based on the differentiation between the soluble fraction (CODsol) and the colloidal fraction (CODcoll) using a flocculation method. The BOD5/COD and the BOD5/CODsol indices are both consistent and significant and can be used as stability indices. The BOD5/COD ratio does not seem to be influenced, for the same test duration, by the type of test, static or dynamic. In the same way the longer test duration (24-h) does not influence significantly the values of BOD5/COD ratio. As a consequence a leaching test duration of 6-h is preferable to avoid the beginning of the hydrolysis and oxidation processes. 2012 Elsevier Ltd. All rights reserved. 1. Introduction The control of biodegradable substances is the key issue in evaluating the short and long-term emission potential and environmental impact of a landfill (Ehrig and Krümpelbeck, 2001; Cossu, 2010). As a result, the biological stability has become a strategic parameter in evaluating the quality of waste before, during land- filling and at the end of the aftercare phase. The control of the biological stability of waste can be carried out either by removing the putrescible fractions from the waste to be landfilled (source segregation) (Sirini et al., 2010), either stabilizing the waste before land- filling (i.e. mechanical–biological pre-treatment) (Heerenklage and Stegmann, 1995). During and after landfilling the biological stability of waste can be achieve using in situ techniques, according to different concepts and relevant technologies such as semi-aerobic landfilling, forced aeration and PAF (Pre-treatment Aeration Flushing) (Matsufuji et al., 2000; Cossu et al., 2003; Heyer et al., 2005; Lee Chong et al., 2005). Several tests and indices have been proposed for assessing the biological stability (Cossu et al., 2001). Among these the most used are the respirometric index and the biomethane potential production. The respirometric index (RI), which measure in a given time (generally 4–7 days) the consumption of oxygen, can be carried out under static and dynamic conditions (Adani et al., 2004; Cossu and Raga, 2008; Barrena et al., 2009; Sánchez Arias et al., 2012); it is widely applied to different organic waste (Berthe et al., 2007; Barrena et al., 2011). The biomethane potential production (GB21) measures the production of methane in 21 days (Ponsà et al., 2008; Wagland et al., 2009). These parameters have some limits due to one or more of the following aspects: – high cost of the respirometers (either under static or dynamic conditions); – long testing time; – low representativity of the parameter in presence of toxic or inhibiting substances, which can alter the oxygen consumption, or in the presence of inert organics (such as plastic or paper) which can dilute the biodegradable matrix resulting in lower respiration indices. In order to overcome the limits of the RI and GB21 other indices have been proposed, such as the black index (BI), which is related to the blacking time of a lead acetate paper, and the BOD5/COD ratio, on the eluate of a leaching test (Cossu et al., 2001; Cossu and Raga, 2008). They both are cheap and effective (Cossu and Raga, 2008). 0956-053X/$ - see front matter 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2012.04.001 ⇑ Corresponding author. E-mail addresses: tiziana.lai@unipd.it, lai.tiziana@gmail.com (T. Lai). Waste Management 32 (2012) 1503–1508 Contents lists available at SciVerse ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman
1504R.Cossu et al./Waste Management 32 (2012)1503-1508In particular, the BODs/COD ratio overcomes the above-men-The samples were collected according to the Italian referencetioned limits:method UNI 10802 (2004), A random sampling has been carriedout to constitute a primary sample of about 200 kg. A secondaryrequires standard equipment which is present in anyitsample of about 40 kg has been obtained by successive reductions;laboratory;after accurate homogenization ofthe latest therepresentative- it is representative of the presence of toxic or inhibitingsamples for laboratory analysis have been extracted.The samplessubstances;have been transferred immediately to the laboratory using a refrig-- it is not influenced by dilution effects due to the presence oferated container and maintained at a temperature of 4C untilimpurities in the sample:analysis as the freezing could influence biological activity (Pognani- can be suited either for coarse or finely shredded materials:et al., 2012).Fig. 1 shows the composition of waste samples expressed as- the testing time is 5 days, depending from the time necessaryfor theBODanalysis.percentage of total weight.Aim of this study is to standardize the parameter BODs/COD ra-2.2. Leaching teststio, introducing the coD fractioning, and to test the methodologyunder different test operating conditions (dynamic or static leach-The leaching test was performed under different conditions:ing and leaching duration) keeping constant temperature and li-quid/solid (L/S) ratio.dynamic leaching test (D), determined on about 90 g of sam-TheCODfractioningisbasedonthedifferentiationbetweentheple on the basis of the reference standard leaching test UNIEN 12547-2;soluble fraction (CODsol) and the colloidal fraction (CODco) using aflocculationmethoddevelopedbyMamaisetal.(1993)afterfilter-- static leaching test (S), performed in 51 HDPE containers oning the sample to be analyzed. This method is based on theabout 500g of sample.assumptionthataflocculation(byusingZn(OH)2atpH10.5)fol-lowed by 0.45 μm filtration, will remove the colloidal fraction, pro-Both were carried out with two different contact times, 6 andducing a filtrate containing only truly soluble organic matter24 h, the same liquid to solid ratio (L/S = 10 1/kgrs) and waste size(Mamais et al., 1993). The relation between the COD and the two(<4 mm). except for the residual solid waste that has been analyzedmentioned fractions is:both shredded, with a size <4 mm (waste A), and not shredded(waste B).The temperature of the laboratory was kept constantCODr = CODsol + CODcoll(1)at20C.All testswereperformed intriplicate.Theeluateobtainedfrom dynamic and static leaching test was analyzed, by 24 h (IRSA-where:CODr=total COD in the sampleafter 0.45μm eluatefiltra-CNR, 2003),to determine thefollowing parameters:tion,CODsol=solubleCODafterflocculationandfurtherO0.45μmfil-tration, CoDcoll=colloidal fraction which is separated by theBiochemical Oxygen Demand (BODs).determined accordingflocculation process and kept by the filter. The CoDsol is related toto the Italian standard method IRSA-CNR 29/2003 vol. 2 n.the readily biodegradable COD, Ss, defined by Ekama et al. (1986)5120 B2. A volume of the eluate to be tested is placed in aaccording to the equation (Mamais et al, 1993):Winkler bottle (volume = 300 ml). The bottle is then filledwith dilution water saturated in oxygen and containing bac-(2)CODsol = S, + Siterial inoculum and the nutrients required forbiologicalwhere: CODsol - influent total truly soluble COD, S, = influent read-growth. The bottle is stored in the dark at a temperature of20 C for 5 days. The oxygen concentration in the bottle,ily biodegradable soluble COD, S, - influent non-readily biodegrad-before and after 5 days of incubation, is measured addingablesolubleCODto the solution manganese sulfate and potassium iodide inAn estimationof S,could beobtainedby treating the sample insodiumazideand titratingtheresidueiodinewithsodiuman activated sludge system at a mean cell residence time (MCRT)thiosulphate.greater than 3 days (Ekama et al., 1986; Mamais et al., 1993)- Chemical Oxygen Demand (COD), determined according toThe ratio between BODs and the different fractions of CODthe Italian standard method IRSA-CNR 29/2003 vol. 2 n.(CODr, CODsol), and eventually between CODsol and COD, has been5130. The organic material in the eluate to be tested is chem-compared with the values of the traditional biological stabilityindices.icallyoxidized(digested)using potassium dichromate inacid solution. After 2 h of digestion the residue dichromateismeasuredbytitrationwithiron(ll)ammoniumsulfate.2. Materials and methodsThis method was applied to eluate from leaching test both be-2.1. Waste samplesfore and after the chemical-physical COD fractionating method(Mamais et al., 1993).Waste samples were representative of different waste manage-All tests were performed in triplicate.ment situations and have been collected from full-scale plantsoperating in the Northern Italy:2.3. COD fractionating method-waste A and B: residual solid waste after separate collection ofThe colloidal fraction of coD was separated by flocculation add-different materials (plastics,glass, paper, cans and putrescibleing1mlofa100g/lzincsulfatesolutiontoa100mlofeluatesam-fraction):ple. The pH of the mixed sample was adjusted to approximately- waste C: aerobically stabilized mechanical-biological pre-trea-10.5 with 6 M sodium hydroxide solution and then mixing with ated waste (MBT) undersieve (<50 mm):magnetic stirrer for approximately1 min.After settlingquiescentlywaste D: compost of anaerobic digestate of municipal solidfor a few minutes, the clear supernatant was filtered through awaste (MSW) putrescible fraction undersieve (<50 mm).0.45 μm filter membrane (Mamais et al., 1993)
In particular, the BOD5/COD ratio overcomes the above-mentioned limits: – it requires standard equipment which is present in any laboratory; – it is representative of the presence of toxic or inhibiting substances; – it is not influenced by dilution effects due to the presence of impurities in the sample; – can be suited either for coarse or finely shredded materials; – the testing time is 5 days, depending from the time necessary for the BOD analysis. Aim of this study is to standardize the parameter BOD5/COD ratio, introducing the COD fractioning, and to test the methodology under different test operating conditions (dynamic or static leaching and leaching duration) keeping constant temperature and liquid/solid (L/S) ratio. The COD fractioning is based on the differentiation between the soluble fraction (CODsol) and the colloidal fraction (CODcoll) using a flocculation method developed by Mamais et al. (1993) after filtering the sample to be analyzed. This method is based on the assumption that a flocculation (by using Zn(OH)2 at pH 10.5) followed by 0.45 lm filtration, will remove the colloidal fraction, producing a filtrate containing only truly soluble organic matter (Mamais et al., 1993). The relation between the COD and the two mentioned fractions is: CODf ¼ CODsol þ CODcoll ð1Þ where: CODf = total COD in the sample after 0.45 lm eluate filtration, CODsol = soluble COD after flocculation and further 0.45 lm filtration, CODcoll = colloidal fraction which is separated by the flocculation process and kept by the filter. The CODsol is related to the readily biodegradable COD, SS, defined by Ekama et al. (1986) according to the equation (Mamais et al., 1993): CODsol ¼ Ss þ Si ð2Þ where: CODsol = influent total truly soluble COD, Ss = influent readily biodegradable soluble COD, Si = influent non-readily biodegradable soluble COD. An estimation of Si could be obtained by treating the sample in an activated sludge system at a mean cell residence time (MCRT) greater than 3 days (Ekama et al., 1986; Mamais et al., 1993). The ratio between BOD5 and the different fractions of COD (CODf, CODsol), and eventually between CODsol and COD, has been compared with the values of the traditional biological stability indices. 2. Materials and methods 2.1. Waste samples Waste samples were representative of different waste management situations and have been collected from full-scale plants operating in the Northern Italy: – waste A and B: residual solid waste after separate collection of different materials (plastics, glass, paper, cans and putrescible fraction); – waste C: aerobically stabilized mechanical–biological pre-treated waste (MBT) undersieve (<50 mm); – waste D: compost of anaerobic digestate of municipal solid waste (MSW) putrescible fraction undersieve (<50 mm). The samples were collected according to the Italian reference method UNI 10802 (2004). A random sampling has been carried out to constitute a primary sample of about 200 kg. A secondary sample of about 40 kg has been obtained by successive reductions; after accurate homogenization of the latest the representative samples for laboratory analysis have been extracted. The samples have been transferred immediately to the laboratory using a refrigerated container and maintained at a temperature of 4 C until analysis as the freezing could influence biological activity (Pognani et al., 2012). Fig. 1 shows the composition of waste samples expressed as percentage of total weight. 2.2. Leaching tests The leaching test was performed under different conditions: – dynamic leaching test (D), determined on about 90 g of sample on the basis of the reference standard leaching test UNI EN 12547-2; – static leaching test (S), performed in 5 l HDPE containers on about 500 g of sample. Both were carried out with two different contact times, 6 and 24 h, the same liquid to solid ratio (L/S = 10 l/kgTS) and waste size (<4 mm), except for the residual solid waste that has been analyzed both shredded, with a size <4 mm (waste A), and not shredded (waste B). The temperature of the laboratory was kept constant at 20 C. All tests were performed in triplicate. The eluate obtained from dynamic and static leaching test was analyzed, by 24 h (IRSACNR, 2003), to determine the following parameters: – Biochemical Oxygen Demand (BOD5), determined according to the Italian standard method IRSA-CNR 29/2003 vol. 2 n. 5120 B2. A volume of the eluate to be tested is placed in a Winkler bottle (volume = 300 ml). The bottle is then filled with dilution water saturated in oxygen and containing bacterial inoculum and the nutrients required for biological growth. The bottle is stored in the dark at a temperature of 20 C for 5 days. The oxygen concentration in the bottle, before and after 5 days of incubation, is measured adding to the solution manganese sulfate and potassium iodide in sodium azide and titrating the residue iodine with sodium thiosulphate. – Chemical Oxygen Demand (COD), determined according to the Italian standard method IRSA-CNR 29/2003 vol. 2 n. 5130. The organic material in the eluate to be tested is chemically oxidized (digested) using potassium dichromate in acid solution. After 2 h of digestion the residue dichromate is measured by titration with iron (II) ammonium sulfate. This method was applied to eluate from leaching test both before and after the chemical–physical COD fractionating method (Mamais et al., 1993). All tests were performed in triplicate. 2.3. COD fractionating method The colloidal fraction of COD was separated by flocculation adding 1 ml of a 100 g/l zinc sulfate solution to a 100 ml of eluate sample. The pH of the mixed sample was adjusted to approximately 10.5 with 6 M sodium hydroxide solution and then mixing with a magnetic stirrer for approximately 1 min. After settling quiescently for a few minutes, the clear supernatant was filtered through a 0.45 lm filter membrane (Mamais et al., 1993). 1504 R. Cossu et al. / Waste Management 32 (2012) 1503–1508
1505R.Cossu et al./Waste Management 32 (2012) 1503-1508100%90%80%Metals70%Woodandpaper60%Glass and Inert50%Plastics40%mOthers (haz.waste, putrescible)30%Undersieve<20mm20%10%0%cDA-BFig. 1. Composition of the different waste samples (A and B= residual solid waste after separate collection; C = aerobically stabilized MBT undersieve (<50 mm): D = compostof anaerobic digestate of MSWputrescible fraction undersieve (<50mm))AB10000800080006000[/zo 8w][/zo w]6000400040002000200000D6D24S6S24D6D24S6S24COD口CODSOI口BODSCODCODsol口BOD5cD300050004000[zosw]2000[/zo aw]30002000100010000D6D24S24S6D6D2456524OCODCODsolBOD5CODCODsol口BOD5Fig.2.BOD..CODandCOD.ovalues from theleaching testunder differentoperative conditions(D6=dynamicleachingtest of 6h duration:D24=dynamic leaching test of24 h duration; S6 = static leaching test of 6 h duration: S24 = static leaching test of 24 h duration)for the different waste samples (A and B = residual solid waste after separatecollection; C = aerobically stabilized MBT undersieve (<50 mm): D = compost of anaerobic digestate of MSW putrescible fraction undersieve (<50 mm)).2.4.Stabilityindiceson solidphase-GB21fermentation test to assess thepotential biogas pro-duction under anaerobic conditions.The method consistsWaste samples were ground to a size<4 mm before analysis.of adding 4 g of solid sample in 80 ml of distilled water withThe following parameters weredetermined on the solid20ml of bacterial inoculum from an anaerobic digester of asample:wastewater treatment plant.Samples are maintained at35C temperature in a thermostatic bath (APAT, 2003).- Total Solids (TS) and Volatile Solids (VS), determined onCumulative biogas production was evaluated after 21 days;about 30 g of sample according to the Italian gravimetric Black index (Bl) a qualitative test applied to assess the stabil-standard method IRSA-CNR Q 64/85 vol. 2 n. 2;ity of the waste (Cossu et al., 1999). The method consists in- RI4 and RI7, expressed in terms of mgo2/grs, determined onplacing 500 g of sample in 1Ibottle in which the cap is fixedabout30gofsamplebymeansoftheSapromatmodelEto a lead acetate paper with a surface of approximatelyequipment (APAT, 2003).All samples have been tested both1 cm2.The bottles are maintained at 35C temperature inin the unaltered conditions (Rlu) and in wet conditions (Rlw)a thermostatic bath. The lead acetate paper in the presenceobtainedbyaddingaquantityofwatertoreach75%oftheofhydrogensulfideproduced bythesulfate-reducingbacte-maximum field capacity.equivalent to a moisture contentria blackens due to the formation of lead sulfide.The index isof50%on dry matter (UNI/TS11184,2006).the inverse of the blacking time, expressed in days-1, in
2.4. Stability indices on solid phase Waste samples were ground to a size <4 mm before analysis. The following parameters were determined on the solid sample: – Total Solids (TS) and Volatile Solids (VS), determined on about 30 g of sample according to the Italian gravimetric standard method IRSA-CNR Q 64/85 vol. 2 n. 2; – RI4 and RI7, expressed in terms of mgO2/gTS, determined on about 30 g of sample by means of the Sapromat model E equipment (APAT, 2003). All samples have been tested both in the unaltered conditions (RIu) and in wet conditions (RIw) obtained by adding a quantity of water to reach 75% of the maximum field capacity, equivalent to a moisture content of 50% on dry matter (UNI/TS 11184, 2006). – GB21 fermentation test to assess the potential biogas production under anaerobic conditions. The method consists of adding 4 g of solid sample in 80 ml of distilled water with 20 ml of bacterial inoculum from an anaerobic digester of a wastewater treatment plant. Samples are maintained at 35 C temperature in a thermostatic bath (APAT, 2003). Cumulative biogas production was evaluated after 21 days; – Black index (BI) a qualitative test applied to assess the stability of the waste (Cossu et al., 1999). The method consists in placing 500 g of sample in 1 l bottle in which the cap is fixed to a lead acetate paper with a surface of approximately 1 cm2 . The bottles are maintained at 35 C temperature in a thermostatic bath. The lead acetate paper in the presence of hydrogen sulfide produced by the sulfate-reducing bacteria blackens due to the formation of lead sulfide. The index is the inverse of the blacking time, expressed in days1 , in Fig. 1. Composition of the different waste samples (A and B = residual solid waste after separate collection; C = aerobically stabilized MBT undersieve (<50 mm); D = compost of anaerobic digestate of MSW putrescible fraction undersieve (<50 mm)). A B C D Fig. 2. BOD5, COD and CODsol values from the leaching test under different operative conditions (D6 = dynamic leaching test of 6 h duration; D24 = dynamic leaching test of 24 h duration; S6 = static leaching test of 6 h duration; S24 = static leaching test of 24 h duration) for the different waste samples (A and B = residual solid waste after separate collection; C = aerobically stabilized MBT undersieve (<50 mm); D = compost of anaerobic digestate of MSW putrescible fraction undersieve (<50 mm)). R. Cossu et al. / Waste Management 32 (2012) 1503–1508 1505
1506R.Cossu etal./WasteManagement32(2012)1503-1508AB1.01.0 0.80.80.60.60.40.40.20.20.00.056D6D24S24D6D2456$24BOD5/CODBOD5/CODsOICODsol/CODBOD5/CODBOD5/CODsolCODsol/CODcD1.01.00.80.80.60.6 0.40.40.2 0.2 0.00.0D6D2456524D6D24S6S24BOD5/CODBOD5/CODSOICODSOI/CODBOD5/CODBOD5/CODsolCODsol/CODFig. 3. BODs/COD, BODs/CODsot and CODsa/COD values from leaching test under different operative conditions (D6 = dynamic leaching test of 6 h duration; D24 = dynamicleaching test of 24 h duration; S6 = static leaching test of 6 h duration; S24 = static leaching test of 24 h duration) for the different waste samples (A and B = residual solidwaste after separate collection; C= aerobically stabilized MBT undersieve (<50 mm): D = compost of anaerobic digestate of MSW putrescible fraction undersieve (<50 mm)CODsol should represent the biologically degradable part of thisorderto haveamagnitudebeing congruentwiththe otherindices. The blacking time may vary from minutes to daysfraction and the relative influence on the groundwater quality.and is indicative of the amounts of hydrogen sulfide pro-Regarding the CODsol/COD ratio the values are not indicative ofduced by bacterial activity.the stability of the waste as it has similar values both for non-sta-bilized (AandB)andstabilizedwastes(CandD)Results obtained should be verified with waste of different char-3.Results and discussionacteristics compared to Msw such as those with a high content infats (Ruggieri et al., 2008) that have low solubility (Lalman and3.1. Leaching testBagley, 2000).BODs, COD and CODsol values from the leaching test under dif-Table 1ferent operating conditions are represented in Fig. 2 for the differ-Values of various stability indices for the different waste samples (A and B= residualent samples.solid waste after separate collection; C=aerobically stabilized MBT undersieveIn general it can be observed that under dynamic conditions the(<50 mm): D=compost of anaerobic digestate of MSW putrescible fraction undersi-eve (<50 mm)COD release is higher than the static ones, while BOD, is not toomach influenced, except for not shredded waste B that it is proba-cDUnitsA-Bbly due to a lower exchange surface. The test duration does notCharacterizationinfluence significantly the release of COD and BODs under both sta-黑器器59661TSvs10tic and dynamic conditions. Under dynamic conditions is a slightlyM423426higher BODs release observed by increasing the test duration. ThisIndicescouldbeconnectedtoabeginningofthehydrolysisprocessofthe17338.5RI,4mgoz/grsmost complex organic molecules: in this regard, lower test dura-825RL.742.2mgoz/grstionshouldbepreferred.27RIw450.223mgoz/grsIn Fig. 3 are shown the values of BODs/COD, BODs/CODsol andRIw779.738mgoz/gtsCODsoi/COD. The BODs/COD ratio does not seem to be influenced,2319GB21NI/kgrs49.4dr1 kgrs-BI0.50.180.28atthesametest duration,fromthetypeoftest staticordynamic.cDAnanomalousbehaviorwasfoundforwasteA,inthecaseofstaticNew indicesUnitsABleaching test of 6-h duration, in which the value of BODs/COD ratioBODs/CODD60.670.580.080.35is high because of the low value of coD, which is of the same orderD24BODs/COD0.620.650.150.34S6BODs/COD.0.910.620.110.33of magnitude of BODs. In general, the BODs/COD ratio is slightlyS24-BODs/COD0.660.670.110.35higher for the test of longer duration, but the difference does notD60.810.740.160.40BODs/CODsotseem particularly significant to justify the use of 24htestD240.830.240.46BODs/CODsel0.76duration.S6二BODs/CODsol0.970.840.160.43Similar results were obtained for BODs/CODsol ratio. The BODs/S240.800.760.170.41BODs/CODsolCODsol can be a useful index in combination with the BODs/COD ra-TS= total solids; VS = volatile solids: M= moisture; RI,4= respirometric index oftio.Infact, theCoDsol could represent thefraction of organicmat-unaltered sample (4 days); Rl,7 =respirometric index of unaltered sample (7 days):Rlw4 = respirometric index of wet sample (4 days): Rlw7 = respirometric index ofter that can be more easily dispersed into groundwater because ofwet sample (7 days): GB21 = fermentation test; BI = black index.itsownsize(MetcalfandEddy.2003):asaconsequence,theBODs/
order to have a magnitude being congruent with the other indices. The blacking time may vary from minutes to days and is indicative of the amounts of hydrogen sulfide produced by bacterial activity. 3. Results and discussion 3.1. Leaching test BOD5, COD and CODsol values from the leaching test under different operating conditions are represented in Fig. 2 for the different samples. In general it can be observed that under dynamic conditions the COD release is higher than the static ones, while BOD5 is not too mach influenced, except for not shredded waste B that it is probably due to a lower exchange surface. The test duration does not influence significantly the release of COD and BOD5 under both static and dynamic conditions. Under dynamic conditions is a slightly higher BOD5 release observed by increasing the test duration. This could be connected to a beginning of the hydrolysis process of the most complex organic molecules: in this regard, lower test duration should be preferred. In Fig. 3 are shown the values of BOD5/COD, BOD5/CODsol and CODsol/COD. The BOD5/COD ratio does not seem to be influenced, at the same test duration, from the type of test static or dynamic. An anomalous behavior was found for waste A, in the case of static leaching test of 6-h duration, in which the value of BOD5/COD ratio is high because of the low value of COD, which is of the same order of magnitude of BOD5. In general, the BOD5/COD ratio is slightly higher for the test of longer duration, but the difference does not seem particularly significant to justify the use of 24 h test duration. Similar results were obtained for BOD5/CODsol ratio. The BOD5/ CODsol can be a useful index in combination with the BOD5/COD ratio. In fact, the CODsol could represent the fraction of organic matter that can be more easily dispersed into groundwater because of its own size (Metcalf and Eddy, 2003): as a consequence, the BOD5/ CODsol should represent the biologically degradable part of this fraction and the relative influence on the groundwater quality. Regarding the CODsol/COD ratio the values are not indicative of the stability of the waste as it has similar values both for non-stabilized (A and B) and stabilized wastes (C and D). Results obtained should be verified with waste of different characteristics compared to MSW such as those with a high content in fats (Ruggieri et al., 2008) that have low solubility (Lalman and Bagley, 2000). A B C D Fig. 3. BOD5/COD, BOD5/CODsol and CODsol/COD values from leaching test under different operative conditions (D6 = dynamic leaching test of 6 h duration; D24 = dynamic leaching test of 24 h duration; S6 = static leaching test of 6 h duration; S24 = static leaching test of 24 h duration) for the different waste samples (A and B = residual solid waste after separate collection; C = aerobically stabilized MBT undersieve (<50 mm); D = compost of anaerobic digestate of MSW putrescible fraction undersieve (<50 mm)). Table 1 Values of various stability indices for the different waste samples (A and B = residual solid waste after separate collection; C = aerobically stabilized MBT undersieve (<50 mm); D = compost of anaerobic digestate of MSW putrescible fraction undersieve (<50 mm)). Units A–B C D Characterization TS % 58 66 74 VS % 11 10 13 M % 42 34 26 Indices RIu4 mgO2/gTS 38.5 17 3 RIu7 mgO2/gTS 42.2 25 8 RIw4 mgO2/gTS 50.2 21 27 RIw7 mgO2/gTS 79.7 34 38 GB21 Nl/kgTS 49.4 23 19 BI d1 kgTS1 0.5 0.18 0.28 New indices Units A B C D BOD5/COD D6 – 0.67 0.58 0.08 0.35 BOD5/COD D24 – 0.62 0.65 0.15 0.34 BOD5/COD S6 – 0.91 0.62 0.11 0.33 BOD5/COD S24 – 0.66 0.67 0.11 0.35 BOD5/CODsol D6 – 0.81 0.74 0.16 0.40 BOD5/CODsol D24 – 0.76 0.83 0.24 0.46 BOD5/CODsol S6 – 0.97 0.84 0.16 0.43 BOD5/CODsol S24 – 0.80 0.76 0.17 0.41 TS = total solids; VS = volatile solids; M = moisture; RIu4 = respirometric index of unaltered sample (4 days); RIu7 = respirometric index of unaltered sample (7 days); RIw4 = respirometric index of wet sample (4 days); RIw7 = respirometric index of wet sample (7 days); GB21 = fermentation test; BI = black index. 1506 R. Cossu et al. / Waste Management 32 (2012) 1503–1508
1507R.Cossu et al./Waste Management 32 (2012)1503-15085------ Rlw4+--Rlw74*GB21BIBOD5/COD_D63Q—BOD5/COD_D24艺日—BOD5/COD_S6口2BOD5/COD_S24------BOD5/CODsol_D6-----BOD5/CODsolD241---BOD5/CODsol S6量--BOD5/CODsol_S24UABcFig. 4. Normalized values (N) of different stability indices (Rlw4= respirometric index of wet sample (4 days); Rlw7 = respirometric index of wet sample (7 days):GB21 -fermentation test; BI-black index; BODs/COD and BODs/CODsol at various leaching conditions) for the different waste samples (A and B=residual solid waste afterseparate collection: C= aerobically stabilized MBT undersieve (<50 mm): D = compost of anaerobic digestate of MSW putrescible fraction undersieve (<50 mm)3.2.Stability indices on solid phaseThe Rlu4, Rlu7 indices are not reported in Fig. 4 because thewater content of the unaltered samples is probably too low to sup-In Table 1 are shown the stability indices on solid phase com-port the biological activity. For this reason these indices do notpared with the stability indices measured on eluate of leaching testshow the real biodegradability of the waste samplein different conditions.The Bl index reflects the performance of the respirometric indi-ThesamplesAandBpresentthehighervaluesofrespirometricces Rlw4 and RIw7, but the use of this index implies some disadvan-indices, typical of municipal waste with a residual content of bio-tages, in particular being essentially a qualitative index anddegradable substances (Redon et al., 2005). In reference to the val-entailing a continuous control by an operator to verifytheblacken-ues obtained on unaltered samples,particularly for the D sample,ing time of the leadacetate paper.Moreover,a lower BI value couldthe low quantity of humidity seemed to inhibit biological activity.also occurfornotstabilized wastewithalowcontent of sulfurOtherwise,the values obtained under wet conditions, that could be(Cossu et al., 2001).The BODs/COD and the BODs/CODsot ratio, as highlighted earlier.considered as a potential respiration index, seems more represen-tative and less influenced by the characteristics of the sampleareinlinewiththeindicesonsolidphase,exceptinthecaseofre-sults from static leaching test of 6-h duration, in particular forThe values obtained for Bl were in line with those measuredwith respirometric indices.waste A and B.Values obtained for respirometric index are comparable withThe BODs/COD and the BODs/CODso indices are both consistentthose obtained by van Praagh et al. (2009) while the GB21 valuesand can be both used as stability indices.seem to be lower in particular for the residual waste.4. Conclusions3.3.Comparisonamongthedifferent indicesThe findings obtained demonstrate that the BODs/COD ratio isIn Fig.4 is shown the comparison among the stability indicescomparable with the indices measured directly on the solid samplemeasured on solid phase with those measured on eluate of leach-and could be a useful index for determining the stability of waste.ing test in different conditions. The values are normalized consid-The advantages of using this index are principally that it requiresering as a unit value the stability index for sample D, calculated asstandard equipment, which is usually present in a chemical labora-follows:tory and it is not influenced by the specific characteristics of thesample (e.g.moisture, size)N, = stability index value for the i-waste/stability index valueThe BODs/COD and the BODs/CODsol indices are both consistentforthe D waste(3)and significant and can be used as stability indices; moreover, the
3.2. Stability indices on solid phase In Table 1 are shown the stability indices on solid phase compared with the stability indices measured on eluate of leaching test in different conditions. The samples A and B present the higher values of respirometric indices, typical of municipal waste with a residual content of biodegradable substances (Redon et al., 2005). In reference to the values obtained on unaltered samples, particularly for the D sample, the low quantity of humidity seemed to inhibit biological activity. Otherwise, the values obtained under wet conditions, that could be considered as a potential respiration index, seems more representative and less influenced by the characteristics of the sample. The values obtained for BI were in line with those measured with respirometric indices. Values obtained for respirometric index are comparable with those obtained by van Praagh et al. (2009) while the GB21 values seem to be lower in particular for the residual waste. 3.3. Comparison among the different indices In Fig. 4 is shown the comparison among the stability indices measured on solid phase with those measured on eluate of leaching test in different conditions. The values are normalized considering as a unit value the stability index for sample D, calculated as follows: Ni ¼ stability index value for the i-waste=stability index value for the D waste ð3Þ The RIu4, RIu7 indices are not reported in Fig. 4 because the water content of the unaltered samples is probably too low to support the biological activity. For this reason these indices do not show the real biodegradability of the waste sample. The BI index reflects the performance of the respirometric indices RIw4 and RIw7, but the use of this index implies some disadvantages, in particular being essentially a qualitative index and entailing a continuous control by an operator to verify the blackening time of the lead acetate paper. Moreover, a lower BI value could also occur for not stabilized waste with a low content of sulfur (Cossu et al., 2001). The BOD5/COD and the BOD5/CODsol ratio, as highlighted earlier, are in line with the indices on solid phase, except in the case of results from static leaching test of 6-h duration, in particular for waste A and B. The BOD5/COD and the BOD5/CODsol indices are both consistent and can be both used as stability indices. 4. Conclusions The findings obtained demonstrate that the BOD5/COD ratio is comparable with the indices measured directly on the solid sample and could be a useful index for determining the stability of waste. The advantages of using this index are principally that it requires standard equipment, which is usually present in a chemical laboratory and it is not influenced by the specific characteristics of the sample (e.g. moisture, size). The BOD5/COD and the BOD5/CODsol indices are both consistent and significant and can be used as stability indices; moreover, the Fig. 4. Normalized values (Ni) of different stability indices (RIw4 = respirometric index of wet sample (4 days); RIw7 = respirometric index of wet sample (7 days); GB21 = fermentation test; BI = black index; BOD5/COD and BOD5/CODsol at various leaching conditions) for the different waste samples (A and B = residual solid waste after separate collection; C = aerobically stabilized MBT undersieve (<50 mm); D = compost of anaerobic digestate of MSW putrescible fraction undersieve (<50 mm)). R. Cossu et al. / Waste Management 32 (2012) 1503–1508 1507
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