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Journal of Environmental Science and Health, PartA: Toxic/Hazardous Substances and EnvironmentalEngineeringPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/lesa20

Strategies of management for the whole treatment ofleachates generated in a landfill and in a compostingplantJuan García-Lópeza, Carlos Radb & Milagros Navarrob

a Department of Civil Engineering, University of Burgos, EPS-La Milanera, Burgos, Spainb Composting Research Group (UBUCOMP), University of Burgos, EPS-La Milanera, Burgos,SpainPublished online: 19 Aug 2014.

To cite this article: Juan García-López, Carlos Rad & Milagros Navarro (2014) Strategies of management for the wholetreatment of leachates generated in a landfill and in a composting plant, Journal of Environmental Science and Health, PartA: Toxic/Hazardous Substances and Environmental Engineering, 49:13, 1520-1530, DOI: 10.1080/10934529.2014.938526

To link to this article: http://dx.doi.org/10.1080/10934529.2014.938526

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Strategies of management for the whole treatment of leachatesgenerated in a landfill and in a composting plant

JUANGARC�IA-L�OPEZ1, CARLOS RAD2 and MILAGROS NAVARRO2

1Department of Civil Engineering, University of Burgos, EPS-La Milanera, Burgos, Spain2Composting Research Group (UBUCOMP), University of Burgos, EPS-La Milanera, Burgos, Spain

This study compares the leachates generated in the treatment of Municipal Solid Wastes (MSW) of similar origin but managed in twodifferent ways: (a) sorting and composting in a Treatment Plant in Aranda de Duero (Burgos, Spain), and (b) direct dumping in alandfill in Aranda de Duero (Burgos, Spain) with no prior treatment. Two different leachates were considered for the former: thosegenerated in the fermentation shed (P1) and those generated in the composting tunnels (P2); another leachate was collected from thelandfill (P3). Physical and chemical properties, including heavy metal contents, were seasonally monitored in the different leachates.This study allowed us to conclude that the sampling season had a significant effect on Pb, Cd, Ni, Mg and total-N contents (P <

0.01). Similarly, leachates P1, P2 and P3 exhibited significant overall differences for most of the measured parameters except for Cd,Cu, Pb, K, Fe, C-inorg and C-org contents (P < 0.01). This study concludes with the feasibility of a whole treatment for both leach-ates using ultrafiltration in a membrane bioreactor (MBR).

Keywords: Leachate, landfill, composting plant, MBR, ultrafiltration, heavy metals.

Introduction

The need to achieve an overall protection of the environ-ment has increased over the past few years. The almostexponential population growth, changes in social andresource-use habits, higher output and consumption, ourincreasingly opulent lifestyles and the ongoing industrialand technological progress over the past 20 or 30 years,have been accompanied by an equally rapid increase in thegeneration of municipal and industrial solid waste world-wide.[1,2] The most common way of eliminating municipalsolid wastes (MSW) is to dump them in a landfill site aftertreatment. Another alternative, although less widely used,is incineration.Landfill sites experience at least five waste decomposi-

tion stages, each of which results in the generation of dif-ferent compounds and emissions.[3]:

1. Aerobic: The main products of this stage are water andcarbon dioxide, the latter of which is either released as

a gas or absorbed by the water to form carbonic acid,which acidifies the leachate.

2. Acidogenic: Carbon dioxide, hydrogen, ammonia andorganic acids.

3. Acetogenic: Acetic acid and its derivatives, carbondioxide and hydrogen.

4. Methanogenic: The typical landfill gas composition:approximately 60% methane and 40% carbon dioxide.

5. Aerobic: Carbon dioxide and water.

Leachate generated from landfills may have a long-termenvironmental impact for several centuries if proper man-agement is overlooked.[4] Therefore, there is an urgent taskto find an efficient technology to dispose of landfill leach-ates; otherwise, it will cause an important environmentalrisk since leachate leakage may contaminate ground andsurface waters. Several biological and physicochemicaltechniques are used for treating MSW-leachate, such asmodified sequencing batch reactors, anaerobic sludgeblanket, coagulation, flotation, chemical precipitation,adsorption, etc. Biological methods can remove individualpollutants efficiently using mixed biological populations;and usually, the physicochemical techniques are adoptedsubsequently as refining processes. However, with the con-tinuously stringent discharge standards in surface waters,the use of conventional biological methods combined withphysicochemical treatments are no longer adequate topurify stabilized leachates from old landfill sites, whichhave a poor biodegradability.[5]

Address correspondence to Juan García-L�opez, Department ofCivil Engineering, University of Burgos, EPS-La Milanera,Villadiego s/n, 09001 Burgos, Spain; E-mail: juangl@ubu.esReceived March 2, 2014.Color versions of one or more figures in this article can be foundonline at www.tandfonline.com/lesa.

Journal of Environmental Science and Health, Part A (2014) 49, 1520–1530Copyright © Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934529.2014.938526

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Two main types of facilities are commonly used in Spainin the centralized treatment schemes of MSWs: a) manualor mechanical sorting and composting, and b) sorting, bio-methanization and composting. Both types of facilitiesallow recovering recyclable materials, such as plastic,glass, or light packaging, and a biostabilized material(compost), which can be used as an organic soil condi-tioner after appropriate refining process.Composting is the biological decomposition of the

organic components of waste, usually under aerobic condi-tions. This process reduces the volume of the waste andresults in marked morphological and chemical changes inthe organic material. [6] The main types of composting sys-tems include turned or aerated windrows, closed-reactorand vessel-type systems.[7] During composting, organic mat-ter is metabolically transformed into CO2 and H2O and theenergy is used to synthesise microbiological biomass.[8]

At the same time, a parallel process of humification is car-ried out by specific microorganisms with the most recalci-trant organic materials, which lead to the formation of astabilised product with similar properties to soil humus, withbeneficial effects for soil microbial activity and plant growth.Metabolic heat is also generated during the organic decom-position of biodegradable matter.[9] This process maydestroy pathogens, parasites, worms and weed seeds, all ofwhich represent a risk when this product is applied in thefield.[10] Numerous studies regarding specific compostingconditions have discussed specific aspects related to temper-ature, moisture content, aeration, pH and C/N ratio duringthe process.[11] Similarly, the use of a large variety of solidwastes in composting processes, including sewage sludge’s,crop wastes,[12,13] animal slurries,[10,14,15] and industrial bio-wastes,[16,17] has been investigated.A significant amount of leachate is produced in both,

waste treatment facilities and landfill sites. The amount ofleachate generated and the decomposition, stabilisationand extraction of contaminants from the waste matrix willdepend on various factors, including the composition ofthe waste, its degree of compaction and its ability toabsorb a wide variety of contaminants. Landfill leachatecomposition is often characterized by its high ammoniaconcentration, as compared with conventional single passleaching.[18] Leachate recirculation during the first phaseof waste decomposition leads to the accumulation of fer-mentation products, which primarily consist of volatileorganic acids and alcohols because of the imbalance of thegrowth rates between fast-growing acidogenic bacteriaand slow-growing methanogens. Consequently, methano-genesis may be delayed or inhibited.[19,20]

Leachate produced in a landfill has a dark brown col-our, normally has a very strong smell and contains highlevels of contaminants (for example, a COD of 5,000 mgL¡1 in comparison with typical values of 100–200 mg L¡1

normally found in municipal wastewaters). This highlyloaded leachate also contains a mixture of contaminantssuch as: (i) Inorganic species (ammonium, calcium,

magnesium, sodium, potassium, iron, sulphates, chloridesand heavy metals such as cadmium, chromium, copper,lead, nickel and zinc), (ii) Organic compounds (dissolvedorganic matter, volatile fatty acids, humic and fulvic-likesubstances, and xenobiotic organic compounds generallypresent in concentrations of less than 1 mg L¡1), and (iii)pathogenic microorganisms.[3,21,22]

Although the source of and the problems derived fromleachates are well known, there is little information regard-ing the differences between leachates generated in a MSW-landfill site and those produced in a MSW-TreatmentPlant. The importance of the leachate generated by cen-tralized facilities is sometimes neglected, giving priority tothe whole treatment of MSW, the appropriate deposit ofrejected materials and the amount of recycled components.Furthermore, the leachate generated in a landfill is usuallytreated directly in a wastewater treatment plant, whichcauses consequent problems in the performance of the bio-logical reactor and the accumulation of their metal contentinto the sludge.The aim of this study was to compare the composition of

a leachate generated in a MSW-Treatment Plant (aerobicprocess) with that produced in an adjacent sealed MSW-landfill site (anaerobic process) and to study the feasibilityto treat both leachates using a biological process in aMembrane Biological Reactor (MBR). Furthermore, theeffect of seasonal variations (spring, summer, autumn andwinter) on the physicochemical properties of the leachateswas also studied.

Materials and methods

Description of the MSW-treatment plant in Aranda de

Duero (Burgos, Spain)

The Municipal Solid Waste Treatment Plant (MSW-TP)processes urban wastes produced in a total of 161 small vil-lages around the main town of Aranda de Duero (Burgos,Spain), 41� 41’ 53’’ N 03� 43’ 02’’ W, approximately29,000 tonnes year¡1. After a partial segregation at source,mixed MSWs were collected and transported using with-drawal trucks form different transfer places to the MSW-TP, according with a centralized scheme of MSW-manage-ment (Fig. 1).MSW enters the facility in MSW-collection trucks, and

thereafter it is manually sorted and the organic fraction sep-arated on a conveyor belt by the use of a rotary trommel of5 mm mesh. This fraction is subsequently introduced intocomposting tunnels, where it performs a 15 days activedecomposition step under controlled conditions of humid-ity, temperature and aeration. After this time it is sent to acovered fermentation shed for another four weeks of aero-bic fermentation in windrows, where it is weakly turnedand moisturized. The resulting biostabilized material issieved and refined (<2 mm) and stored until to be used as

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an organic amendment. The final product could be classi-fied as Class B compost (without restrictions in their agro-nomic use), according to the Spanish national rule fororganic fertilizers (RD 824/2005).[23] In the same area islocated an old MSW-landfill that was operating until5 years ago, time at which the MSW-TP was built and theold landfill was sealed and the area restored. Leachates pro-duced in the landfill were collected in a well, from whichthey are sucked, transported and treated in an externalfacility according with their toxicity levels.In the MSW-TP, the runoff network is separated from

the leachate network and channelled into different storageponds (“leachates pond” and “runoff pond”). The leach-ates from the pre-treatment and fermentation sheds arechannelled directly to the leachates pond, whereas theleachates from the composting tunnels are channelled intoan open concrete tank (“leachates pool”) that overflowsinto the “leachates pond.” This leachate is periodicallyused to irrigate compost windrows in the fermentationshed, using a system of motorised valves and sprayers,thus allowing it to be recirculated. Composting tunnelshowever are irrigated only with natural water stored in a350-m3 concrete tank, filled from a collection well locatedon the outskirts of the facilities.Another leachate is also generated in the biofiltration

process, which purifies the exhaust air generated in the aer-ation of the biological processes that take place inside thecomposting tunnels. This leachate is channelled and mixedin the “leachates pond.”

Description of the closed and sealed landfill site located

in Aranda de Duero (Burgos)

The landfill site occupies around 35,000 m2 of a total sur-face of 62,435 m2 located 500 m apart the MSW-TP. Itbegan to operate in 1997, was closed in 2007 and the

sealing and restoration of the area was completed in 2009.Post-closure maintenance in accordance with Directive99/31/EC has been performed since 1 December 2009. Itholds around 200,000 tonnes of wastes, comprisingapproximately 19,000 tonnes year¡1 of MSW and 2,500tonnes/year of hazardous and non-hazardous industrialwastes (HIW and NHIW). No selective paper/cardboardor light-packaging (LP) recovery system was in place whenthe landfill site was in operation. Actually, it generates amean amount of 0.45 m3 day¡1 of leachate.

Experimental procedure

The main advantage of this study is that the leachates pro-duced in the landfill site (reducing environment) and thosefrom the MSW-TP (oxidising environment) are producedfrom wastes generated in the same geographical area, bythe same inhabitants, and with the same consumption hab-its. The main drawback is that the landfill site also con-tains industrial waste, although in much lower quantities,as the selective collection of paper/cardboard and packag-ing had not been implemented at its operational time.Three different leachate samples were taken: “leachate

pool” (P1) [produced in the composting tunnels], “leach-ates pond” (P2) [produced in the fermentation shed andthe leachates pool overflow] and “sealed landfill site” (P3).Four samples of each leachate were collected in May 2011(P1.1, P2.1, P3.1), August 2011 (P1.2, P2.2, P3.2), October2011 (P1.3, P2.3, P3.3) and December 2011 (P1.4, P2.4,P3.4), in order to assess the seasonal variation in the compo-sition of leachates. All samples were collected in 5-L poly-ethylene buckets and frozen at ¡20�C in the laboratoryuntil physical and chemical analyses could be perform.In addition, a leachate treatment was introduced com-

prising: a chemical treatment with coagulant, in some casesa decanter and a MBR with an ultrafiltration device using20–50 nm nanopore membranes. Different running proce-dures were applied in order to assess its usefulness for awhole treatment of both leachates: (a) 100% P2 withoutdecanting, (b) mixture of 75% P2 and 25% P3 leachates,without settling, (c) mixture at 50% of P2 and P3 leachates,without settling, (d) 100% leachate P1 and decantation, and(e) mixture at 50% of P1 and P3 with decantation. Differentliquid samples were taken each three days at the entrance ofthe storing tank, at the output of physicochemical treat-ment, into the biological reactor, and at the entrance and atthe exit of the ultrafiltration device.

Analytical methodology

The following parameters were analysed for leachates P1,P2 and P3: pH, electrical conductivity (EC), dry mass(DM) at 60 �C, chemical oxygen demand (COD), bio-chemical oxygen demand (BOD), total solids (TS), solublesolids (SS), volatile solids (VS), C-total, C-inorg, total

Fig. 1. Aerial view of the MSW-treatment plant of Aranda deDuero (Burgos) displaying the different facilities.

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organic carbon (TOC), N-total, soluble N-NH4C and N-

(NO3¡ CN-NO2

¡), total organic nitrogen (N-org), reac-tive PO4

3¡, total contents of Cd, Cr, Cu, Mn, Ni, Pb, Zn,Na, K, Ca, Mg, Fe. All samples were determined intriplicate.Briefly, TS were determined by drying leachate samples

in a pre-weighted ceramic plate in an oven at 105�C for1 h. SS were determined using a glass-fibre filter and VSby incineration in an oven at 550�C. Biochemical oxygendemand (BOD5) was measured using a BOD Track instru-ment (Hach), whereas chemical oxygen demand (COD)was determined after digestion with K2Cr2O7 at a refluxfor 2 h in a closed system, and the subsequent back-titra-tion. N-Total was determined by the Kjeldahl methodafter digestion at 320�C for 3 h. Soluble N-NH4

C, N-(NO3

¡CNO2¡) and PO4

3¡were determined using a SKA-LAR SANC flow autoanalyzer. N-org was determinedsubsequently by difference between N-Total and the inor-ganic soluble forms of N. C-Total and C-inorg were deter-mined by dry combustion in a LECO TruSpec analyser,and total organic carbon (TOC) was determined as the dif-ference between them. Trace metals and elements weredetermined by flame atomic absorption spectroscopy(F-AAS, Perkin Elmer model 3100) after microwave-assisted digestion in 70% nitric acid (Millestone EthosOne).

Statistical analysis

All data were checked for the assumption of the normalityand the equality of variances by the Kolmogoroff’s andLevene’s tests, respectively. Thereafter, they were

subjected to an analysis of the variance ANOVA in whichleachate origin and sampling season were the fixed effects.Post-hoc analyses using the Bonferroni test were intro-duced to differentiate between means. Bivariate Pearson’scorrelation coefficients between physical and chemicalproperties were also obtained. All the statistical analyseswere conducted using the software package SPSS 18.5.

Results and discussion

The analytical results obtained for the analysis of leachatesP1 (leachates pool), P2 (leachates pond) and P3 (landfillsite) can be found in Tables 1, 2 and 3, respectively. Thesecond digit assigned indicates the season of the year whenthe sample was collected: 1 (spring), 2 (summer), 3(autumn), 4 (winter). e.g., P.1.4 is the leachate from theleachate pool collected in winter and P.3.2 is the landfillsite leachate collected in summer.

Metal variation of the leachates

The subsequent discussion is sub-divided into heavy metals(Cd, Cr, Cu, Mn, Ni, Pb and Zn), mayor elements (Na, K,Ca, Mg and Fe) and other parameters such as TOC, SS,VS, N-total, N-NH4

C and N-org. We also compare theconcentration of each parameter and/or element by order-ing them from highest to lowest value for each leachateand grouping them by season. Example: Cr (P3 > P2 >

P1) indicates that the concentration of Cr is higher in P3than in P2 and, in turn, higher in P2 than in P1. Theseresults are displayed in Figure 2.

Table 1. Physical and chemical parameters.

Parameters P1.1 P2.1 P3.1 P1.2 P2.2 P3.2

pH 7.7 § 0.2 7.6 § 0.2 8.1 § 0.2 7.7 § 0.2 6.9 § 0.1 8.0 § 0.2EC (dS m¡1) 11.7 § 0.2 8.6 § 0.2 54.9 § 1.1 23.8 § 0.5 36.0 § 0.7 61.2 § 1.2DM (%) a 60�C 0.702 § 0.090 0.425 § 0.014 4.593 § 0.616 1.542 § 0.150 3.098 § 0.079 5.353 § 0.251Density (g m¡1) 0.998 § 0.020 0.998 § 0.020 1.023 § 0.020 0.998 § 0.020 1.001 § 0.020 1.026 § 0.021COD (mg L¡1) 4097 § 1439 11585 § 998 16812 § 0 12042 § 1485 40758 § 2008 24589 § 1747BOD (mg L¡1) 200 § 4 1260 § 25 600 § 12 300 § 6 700 § 14 440 § 9COD/BOD 20 9 28 40 58 56TS (mg L¡1) 7005 § 886 4236 § 132 46842 § 6581 15496 § 1539 31249 § 739 54775 § 2662SS (mg L¡1) 1123 § 173 2059 § 204 597 § 36 1349§ 154 10171 § 1788 1020 § 52VS (mg L¡1) 1562 § 245 1998 § 218 27854 § 15 5150§ 1262 14124 § 241 31695 § 1885C total (mg L¡1) 2354 § 1 5400 § 1 11950 § 4 3477§ 2 7685 § 1 12820 § 2C inorg (mg L¡1) 1620 § 0 1136 § 0 684 § 0 2200§ 1 1008 § 15 694 § 0TOC (mg L¡1) 735 § 0 4265 § 0 11266 § 0 1277§ 2 6677 § 16 12127 § 0N total (mg L¡1) 851 § 0 1445 § 1 6805 § 3 1494§ 1 2853 § 0 7165 § 1N org (mg L¡1) 181 § 2 680 § 27 783 § 133 218 § 47 591 § 2 416 § 172N-NH4

C(mg L¡1) 670 § 19 765 § 35 6022 § 52 1276§ 80 2262 § 14 6749 § 160N-NO3

¡ CNO2¡ (mg L¡1) n.d. n.d. n.d. n.d. n.d. n.d.

PO43¡ (mg L¡1) 26 § 1 83 § 2 83 § 2 18§ 0 82 § 2 85 § 12

Analytical results for leachates P1, P2 and P3 for samples collected in spring and summer (n.d. under de limit of detection).

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Heavy metals (Cd, Cr, Cu, Mn, Ni, Pb, Zn)

The results obtained are as follows:

� Spring: Cd (P3 > P2 � P1), Cr (P3 > P2 > P1), Cu (P3> P1 > P2), Mn (P2 > P1 > P3), Ni (P3 > P1 � P2), Pb(P2 > P3 > P1), Zn (P2 > P1 � P3).

� Summer: Cd (P2 > P3 � P1), Cr (P3 > P2 > P1), Cu (P3> P1 > P2), Mn (P2 > P1 > P3), Ni (P2 > P3 > P1), Pb(P2 > P1 > P3), Zn (P2 > P1 � P3).

� Autumn: Cd (P2 > P1 � P3), Cr (P3 > P2 > P1), Cu (P3> P2 > P1), Mn (P2 > P1 > P3), Ni (P3 > P2 > P1), Pb(P1 > P2 > P3), Zn (P2 > P1 > P3).

� Winter: Cd (P3 > > P2 > P1), Cr (P3 > P2 > P1), Cu(P3 > P2 > P1), Mn (P2 > P1 > P3), Ni (P2 > P3 >

P1), Pb (P3 > P2 > P1), Zn (P2 > P1 > P3).� Mean: Cd (P3 > P2 > P1), Cr (P3 > P2 > P1), Cu (P3 >P2 > P1), Mn (P2 > P1 > P3), Ni (P3 > P2 > P1), Pb(P2 > P3 > P1), Zn (P2 > P1 > P3).

The origin and composition of MSWs that produce bothleachates are similar, which lead us to hypothesize a

similar concentration of heavy metals in the leachates pro-duced in the MSW-Treatment Plant (MSW-TP) and theMSW-landfill site. Furthermore, as discussed above, theimplementation of separate collection, the absence of IWin MSW, etc., the composition of MSW derived to thelandfill site is likely to have a higher metal concentrationthan those of the MSW delivered to the MSW-TP. Themain difference between both would be the way in whichthe waste that generates the leachate is “stored” and thedifferent chemical and biological processes that occur inthe MSW-landfill site and in the MSW-TP. Thus, thewaste in the MSW-TP is produced in an oxidising environ-ment (active composting phase: fermentation in tunnels;passive phase: fermentation shed), whereas the waste inthe landfill site is produced in a reducing environment(lack of oxygen in the mass of residue stored in the landfill:anaerobiosis).However, in contrast to our initial expectations, the con-

centration of Mn, Pb and Zn in leachates P1 and P2 washigher than in leachate P3. Although, the concentration ofthe remaining heavy metals (Cd, Cr, Cu and Ni) washigher in P3 than in P2, and higher in P2 than in P1.

Table 2. Physical and chemical parameters.

Parameters P1.3 P2.3 P3.3 P1.4 P2.4 P3.4

pH 7.4 § 0.1 7.6 § 0.2 7.7 § 0.2 7.4 § 0.1 7.4 § 0.1 8.0 § 0.2EC (dS m¡1) 19.9 § 0.4 24.1 § 0.5 59.7 § 1.2 20.4 § 0.4 18.2 § 0.4 65.8 § 1.3DM (%) a 60�C 1.150 § 0.019 1.805 § 0.012 5.125 § 0.358 1.586 § 0.023 1.807 § 0.208 2.864 § 0.238Density (g m¡1) 0.999 § 0.020 1.001 § 0.020 1.013 § 0.020 0.999 § 0.020 1.001 § 0.020 1.025 § 0.021COD (mg L¡1) 26154 § 523 25066 § 501 16982 § 340 21420 § 428 37560 § 751 9538 § 191BOD (mg L¡1) 4230 § 85 7740§ 155 880 § 18 3715 § 74 5813§ 116 715 § 14COD/BOD 6 3 19 6 6 13TS (mg L¡1) 11524 § 182 18182 § 150 52525 § 3607 15908 § 252 18034 § 2057 29339 § 2452SS (mg L¡1) 1203 § 233 4234§ 304 577 § 52 1755 § 542 11862 § 11515 933 § 13VS (mg L¡1) 2144 § 48 6034§ 99 29048 § 3181 6559 § 63 10059 § 879 5265 § 2210C total (mg L¡1) 4591 § 0 6695§ 2 12320 § 1 4145 § 232 7891§ 33 11550 § 1C inorg (mg L¡1) 1685 § 1 431 § 1 706 § 1 1625 § 1 1631§ 1 4871 § 1TOC (mg L¡1) 2906 § 0 6264§ 1 11614 § 1 2520 § 263 6260§ 26 6679 § 0N total (mg L¡1) 1181 § 0 1612§ 1 6680 § 0 1933 § 14 3213§ 1 9255 § 0N org (mg L¡1) 444 § 12 630 § 29 410 § 132 799 § 907 1625§ 53 8008 § 44N-NH4

C(mg L¡1) 737 § 45 982 § 33 6270 § 269 1134 § 63 1588§ 36 1247 § 90N-NO3

¡ CNO2¡ (mg L¡1) n.d. n.d. n.d. 0 § 0 3 § 0 9 § 1

PO43¡ (mg L¡1) 22 § 1 87 § 0 84 § 2 3 § 0 12 § 1 124 § 1

Analytical results for leachates P1, P2 and P3 for samples collected in autumn and winter (n.d. under de limit of detection).

Table 3. TS, VS and VS/TS values for leachates P1, P2 and P3 in spring, summer, autumn, winter and the mean values obtained.

Spring Summer Autumn Winter Mean

P1 P2 P3 P1 P2 P3 P1 P2 P3 P1 P2 P3 P1 P2 P3

TS 7005 4236 46842 15496 31249 54775 11524 18182 52525 15908 18034 29339 12483 17925 45870VS 1562 1998 27854 5150 14124 31695 2114 6034 29048 6559 10059 5265 3846 8054 23466VS/TS 22% 47% 59% 33% 45% 58% 18% 33% 55% 41% 56% 18% 31% 45% 51%

P3 > P2 > P1 P3 > P2 > P1 P3 > P2 > P1 P2 > P1 > P3 P3 > P2 > P1

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Fig. 2. Heavy metal contents of Cd, Cr, Cu, Mn, Ni and Pb, in leachates P1, P2 and P3 in spring, summer, autumn, winter and themean values.

Fig. 3.Heavy metal contents of Zn, and other metallic elements Na, K, Ca, Mg and Fe in leachates P1, P2 and P3 in spring, summer,autumn, winter and the mean values.

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The most likely explanation for the higher concentrationof Mn, Pb and Zn found in leachates P1 and P2 would bethe different chemical reactions that occur in these wastesin the two environments under study, where pH plays akey role. Thus, inside the landfill site, heavy metals arelinked to the organic matter (OM) present in the waste,which makes their subsequent separation and leachingmuch slow and/or difficult. In contrast, in the compostingprocess, leaching of Mn, Pb and Zn is greater despite theirapparently lower initial concentration. Thus would explainwhy leachate P2 presents a 2731%, 425% and 22% higherconcentration of Mn, Zn and Pb, respectively, than leach-ate P3, and why leachate P1 exhibits a 579% and 28%higher concentration of Mn and Zn, respectively, thanleachate P3.

Other elements (Na,K,Ca,Mg, Fe) can be found in Figure 3.

The results obtained are as follows:

� Spring: Na (P3> P1 > P2), K (P3 > P1 > P2), Ca (P2 >P1 > P3), Mg (P1 > P2 > P3) and Fe (P3 > P2 > P1).

� Summer: Na (P3 > P2 > P1), K (P2 > P1 > P3), Ca (P2> P1 > P3), Mg (P2 > P1 > P3) and Fe (P2 > P3 > P1).

� Autumn: Na (P3 > P2 > P1), K (P2 > P3 > P1), Ca (P2> P1 > P3), Mg (P2 > P1 > P3) and Fe (P3 � P2 > P1).

� Winter: Na (P3 > P1 > P2), K (P3 > P1 > P2), Ca (P2> P1 > P3), Mg (P1 > P2 > P3) and Fe (P2 > P1 > P3).

� Mean: Na (P3 > P1 � P2), K (P3 > P2 � P1), Ca (P2 >

P1 > P3), Mg (P1 > P2 > P3) and Fe (P2 > P3 > P1).

The values for Na and K agree well with the EC values.The main difference between the EC for leachate P3 (60.41dS m¡1) and those for leachates P1 and P2 (18.95 and21.73 dS m¡1, respectively) arises due to the differentmean Na and K concentration in P3 with respect to P1(241% and 34% higher for the former, respectively) and P2(245% and 33% higher for the former, respectively). It maybe the case that Na and K form more stable bonds to OMparticles in the composting tunnels and fermentation shedsthan in the sealed landfill site.The opposite is found for Ca and Mg, both of which are

present in higher concentrations in leachates P1 (612% and565%, respectively) and P2 (2226% and 511%, respec-tively) than in leachate P3. This can be explained on thebasis of the different mean pH values for leachate P3 (7.9)and leachates P1 and P2 (7.6 and 7.4, respectively). Caand Mg are strongly leached from the landfill site due toanaerobic decomposition of the waste mass. In contrast, inthe composting tunnels and fermentation shed, Ca andMg are more strongly bound to, and stabilised by, OMparticles and some proteins and are therefore leached to alesser extent. Finally, the Fe concentration in all threeleachates varies markedly with season and not displayed aclear pattern of variation with the origin of the leachate.

Evolution of the chemical properties of the leachates

Other parameters (TOC, SS, VS, N-total, N-NH4C,

N-organic) can be found in Figure 4. The results obtainedare as follows:

Fig. 4. TOC, N-total, N-NH4C, SS, VS and TS in leachates P1, P2 and P3 in spring, summer, autumn, winter and the mean.

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� Spring: TOC (P3> P2> P1), SS (P2> P1> P3), VS (P3> P2 > P1), N-total (P3 > P2 > P1), N-NH4

C (P3 > P2> P1) and N-organic (P3 > P2 > P1).

� Summer: TOC (P3 > P2 > P1), SS (P2 > P1 > P3), VS(P3 > P2 > P1), N-total (P3 > P2 > P1), N-NH4

C (P3> P2 > P1) and N-organic (P2 > P3 > P1).

� Autumn: TOC (P3 > P2 > P1), SS (P2 > P1 > P3), VS(P3 > P2 > P1), N-total (P3 > P2 > P1), N-NH4

C (P3> P2 > P1) and N-organic (P2 > P1 > P3).

� Winter: TOC (P3 > P2 > P1), SS (P2 > P1 > P3), VS(P2 > P3 > P1), N-total (P3 > P2 > P1), N-NH4

C (P2> P3 > P1) and N-organic (P3 > P2 > P1).

� Mean: TOC (P3 > P2 > P1), SS (P2 > P1 > P3), VS (P3> P2 > P1), N-total (P3 > P2 > P1), N-NH4

C (P3 > P2> P1) and N-organic (P3 > P2 > P1).

The fact that mean values of COT, N-Total, N-NH4C

and N-org for leachate P3 are much higher than those forleachates P1 and P2 is easily explained. Thus, P3 exhibits78% higher TOC, 228% higher N-total, 173% higher N-NH4

C and 262% higher N-org values than P2. Further-more, the TS value is higher for P3 than for P2, and for P2than for P1. As can be seen from Table 3, a similar situa-tion is found for VS (except for the winter sample), thusagreeing with the potential biogas production capacity ofanaerobic digestion.The higher TOC content for P3 is due to the washout

of recalcitrant OM, which requires more time to bedecomposed by anaerobic than for aerobic processes.The majority of these volatile solids are converted intoCH4, CO2, NH3, etc. with time, whereas in bio-oxida-tive processes part of the carbon remains un-volatilizedand is converted into humus. A similar situation isfound for nitrification. Thus, as most nitrogen is pres-ent in the ammonium form, a longer period of time isrequired for some of this nitrogen to be converted intonitrate. Furthermore, a significant amount of the Npresent would be lost by volatilization during theprocess.

The reason why the SS value is lower for leachate P3than for leachates P1 and P2 is probably related to thesample collection method. Thus, sample P3 is collectedusing a small ladle whereas, due to their restricted access,samples P1 and P2 are obtained using a submersible pump.The TS and VS/TS ratio values can be found in Table 3.The results obtained were analysed using an ANOVA

with two fixed factors, namely leachate type (three levels,P1, P2 and P3) and season of year when the sample wasobtained (four levels, four seasons). Although the differen-ces obtained for Cd, Cu, Pb, K, Fe, C-inorg, C-org werenot significant for any leachate type, statistically signifi-cant differences with leachate type were obtained for Cr,Mn, Ni, Zn, Na, Ca, Mg, TS, SS, VS, C-total, TOC, N-total, N-NH4

C and PO43¡ (P < 0.01). The season when

the samples were collected was only significant for Pb, Cd,Ni, Mg and N-total (P < 0.01).The overall bivariate Pearson’s correlations coefficients

between heavy metals and the other elements in leachatesP1, P2 and P3 can be found in Table 4. The degree ofbivariate correlation using the Pearson coefficient indicatesthe degree of correlation between two variables.The main finding of this part of the study was that the

presence of metals such as Cd, Cr, Zn and Mn in the leach-ate was strongly correlated with the of other metals such asPb, Na, Mn, Ca and Fe, which suppose similar leachingmechanisms besides the different processes affecting theevolution of MSWs in the MSW-TC and in the landfill.Thus, the Cd content in the leachates was highly correlatedwith the content of Pb (89%) with a significance of 99%.Similarly, Cr was correlated with Na (89%), Zn with Mn,Ca and Fe (83%, 78% and 74%, respectively) and, finallythe Mn content of the leachates was correlated to Ca andFe (74% and 70%, respectively).

Use of MBR in leachates depuration

Zhao et al.[24] reported that aged refuse reactors have longbeen used for the cost-effective treatment of landfill

Table 4. Bivariate correlations between heavy metals and other metallic elements in leachates P1, P2 and P3.

[Cu] [Ni] [Cd] [Pb] [Cr] [Zn] [Mn] [Fe] [Ca] [Mg] [K] [Na]

[Cu] 1[Ni] 0.247 1[Cd] ¡0.138 0.675** 1[Pb] ¡0.157 0.596** 0.943** 1[Cr] 0.331* 0.641** 0.311 0.170 1[Zn] ¡0.195 0.424** 0.224 0.251 ¡0.253 1[Mn] ¡0.328 0.113 0.075 0.136 ¡0.445** 0.910** 1[Fe] ¡0.128 0.381* 0.047 0.029 ¡0.030 0.858** 0.836** 1[Ca] ¡0.268 0.302 0.295 0.379* ¡0.445** 0.884** 0.861** 0.682** 1[Mg] ¡0.198 0.229 0.723** 0.761** ¡0.377* 0.319 0.279 ¡0.003 0.531** 1[K] 0.324 0.652** 0.332* 0.292 0.400* 0.037 ¡0.268 ¡0.077 0.042 0.149 1[Na] 0.464** 0.738** 0.337* 0.209 0.946** ¡0.218 ¡0.496** ¡0.067 ¡0.369* ¡0.266 0.613** 1

*95% probability. **99% probability.

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leachate. Moreover, the chemical oxygen demand (COD),biological oxygen demand, and ammonia–nitrogen (NH3–N) content can be reduced, thereby making the effluentclear and odorless.[25] Xie et al.[26] constructed aged refusebioreactors to simulate the landfill leachate degradationprocess. Their results show that the aged refuse bioreactorcould effectively remove leachate pollutants at a hydraulicloading rate of 20 m3 d¡1. However, the aged refuse biore-actor can often be blocked during operation, which coulddeteriorate the effluent quality.Recently, MBR technology, an advanced biological

treatment process that replaces the traditional second-ary clarifier of an activated sludge process by a mem-brane separation unit, has emerged as a promisingalternative. The MBR technology has been applied tothe treatment of contaminants in both municipal andindustrial wastewater.[27,28] MBR technology providesbiological treatment with membrane separation.[29,30]

The system consists of an aerated water-filled tankcontaining activated sludge and multiple capillary-formmembrane tubes. The pores of ultrafiltration mem-branes are approximately 20–50 nm in diameter, whicheffectively retains microorganisms, macromolecules andsuspended solids. Microorganisms use the contaminantsas nutrients for growth and metabolism.Results from the reatment of the leachates can be found

in Tables 5 and 6 for the different leachates and their mix-tures. The global performance of the biological process,with and without decantation, and the performance in themembranes were analyzed. The global performance wasvery high with an important decrease in the TOC contentsin all of the cases. In general, this decrease in the contents

of N-total and N-NH4C in leachate was not as good as the

organic content elimination, but level of depuration wasimportant in same cases (82% and 83%, respectively). Thereduction in the heavy metal content of leachates was alsoimportant. Maximum reductions were reached in Ni, Crand Mn contents (98%), Zn (97%), Pb (71%), and Cd andCu (58%). The better performance was obtained with thecontent of Fe and Ca which reached 99% and 94% of theinitial concentration, respectively. In general, the decreasewas lower for alkaline metals such as Na, K and Mgcontents.The membrane yield in the ultrafiltration process was

very high with the TOC, reaching the maximum 100%,and with the concentration of heavy metals Ni, Mn, Zn,Cu, Cr, Cd and Pb in which reductions of 99%, 97%, 97%,95%, 89%, 85%, 57% were reached for these metals,respectively. With the elements Fe, Ca, Mg, K and Na themaximum amounts in metal reduction were 100%, 93%,37%, 31% and 15%, respectively.

Conclusions

The leachate from the landfill site exhibited a higher TScontent than those from the MSW-TP. Furthermore, thepercentage of VS with respect to TS was also higher forthe landfill site. A similar situation was found for TOC,N-total and N-NH4

C. No significant differences in thecontents of Cd, Cu, Pb, K, Fe, C-inorg or C-org werefound between leachates of different origin. The samplingtime significantly affected the concentration of Pb, Cd, Ni,Mg and N-total in leachates (P < 0.01).

Table 6. Performance of the ultrafiltrationdevice operating with the MBR effluent at different ratios of leachates.

Parameters Heavy metals Elements

Leachate mixtures TOC N total N-NH4C Cd Cu Ni Pb Zn Cr Mn Fe Na K Ca Mg

LE100P2-LP100P2 81% 49% 19% 57% 66% 99% 42% 97% 86% 97% 93% 3% 13% 92% 37%LE75P2P3-LP75P2P3 83% 33% 4% 36% 15% 86% 57% 92% 87% 92% 98% 5% 19% 87% 28%LE50P2P3-LP50P2P3 71% 22% 3% 45% 95% 19% 39% 87% 82% 86% 97% 15% 31% 73% 33%LE100P1-LP100P1 97% 40% 21% 37% 75% 72% 23% 90% 89% 94% 99% 4% 9% 88% 34%LE50P1P3-LP50P1P3 100% 39% 28% 85% 51% 76% 55% 93% 78% 95% 100% 4% 11% 93% 37%

Table 5.Global performance of the MBR using different mixtures of leachates.

Parameters Heavy metals Elements

Leachate mixtures TOC N total N-NH4C Cd Cu Ni Pb Zn Cr Mn Fe Na K Ca Mg

LE100P2-LP100P2 80% 51% 30% 58% 58% 98% 46% 97% 81% 98% 93% 18% 29% 91% 21%LE75P2P3-LP75P2P3 91% 57% 35% 52% 40% 92% 71% 96% 94% 96% 95% 27% 16% 94% 42%LE50P2P3-LP50P2P3 79% 11% 13% 42% 21% 26% 21% 88% 70% 88% 83% 20% 19% 84% 36%LE100P1-LP100P1 98% 38% 8% 23% 28% 42% 13% 77% 69% 77% 96% 0% 23% 78% 23%LE50P1P3-LP50P1P3 100% 82% 83% 45% 47% 86% 59% 88% 98% 68% 99% 78% 58% 73% 13%

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The Mn, Zn and, to a lesser extent, Pb present in MSWswere found to leach more readily in an oxidising (compost-ing process) than in a reducing environment (anaerobicdigestion). The presence of certain metals, such as Cd, Cr,Zn and Mn, in the leachate has been found to be stronglycorrelated with the presence of other metals such as Pb,Na, Mn, Ca and Fe. Usually, a combinations of physical,chemical and biological methods for landfill leachate treat-ment, is more efficient than using one of these methodsseparately.In general, the use of a MBR with ultrafiltration is a

good tool to treat in a similar way, leachates from MSW-TP and landfill. Removal efficiency values from 20% toover of 90% in chemical oxygen demand (COD) wereachieved according to leachate characteristics (origin andage), process type and process operational aspects. Nitrifi-cation is generally readily achievable, with >95% removalof ammonia reported through the exclusive application ofbiological techniques to the treatment of both young andold leachates.

Funding

This work was partially financed by VALORIZA, thecommercial firm managing the MSW treatment centre ofAranda de Duero (Burgos, Spain).

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