Chemosphere 84 (2011) 289–295

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Chemosphere

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PCDD/F enviromental impact from municipal solid waste bio-drying plant

E.C. Rada ⇑, M. Ragazzi, D. Zardi, L. Laiti, A. FerrariUniversity of Trento, Department of Civil and Environmental Engineering, Via Mesiano 77, I-38050 Trento, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 December 2010Received in revised form 26 March 2011Accepted 1 April 2011Available online 7 May 2011

Keywords:PCDD/FEmission factorBio-dryingExposure pathwaysDeposition

0045-6535/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.chemosphere.2011.04.019

⇑ Corresponding author. Tel.: +39 0461 282613; faxE-mail addresses: Elena.Rada@ing.unitn.it (E.C. Rad

it (M. Ragazzi), Dino.Zardi@ing.unitn.it (D. Zardi),agnese.ferrari@studenti.unitn.it (A. Ferrari).

The present work indentifies some environmental and health impacts of a municipal solid waste bio-drying plant taking into account the PCDD/F release into the atmosphere, its concentration at groundlevel and its deposition. Four scenarios are presented for the process air treatment and management: bio-filter or regenerative thermal oxidation treatment, at two different heights. A Gaussian dispersion model,AERMOD, was used in order to model the dispersion and deposition of the PCDD/F emissions into theatmosphere. Considerations on health risk, from different exposure pathways are presented using an ori-ginal approach. The case of biofilter at ground level resulted the most critical, depending on the low dis-persion of the pollutants. Suggestions on technical solutions for the optimization of the impact arepresented.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The release of dioxin (PCDD/F) from mechanical–biologicaltreatment (MBT) of municipal solid waste (MSW) is one of themost unexplored topics in the sector of MSW management, evenif this phenomenon is known since 1998 (Lahl et al., 1998). In gen-eral the role of PCDD/F in the MSW management was analyzedonly in relation with combustion processes. Anyway the PCDD/Femission into the atmosphere from MBTs is a phenomenon recog-nized also from MBT forwarders with a proposed emission factor as13.5 pgTEQ kg�1(GET, 2003). A confirmation of this phenomenoncan be found in DEFRA (DEFRA, 2004) with a proposed emissionfactor of 40 pgTEQ kg�1. As the residual MSW (the waste not sourceseparated) is collected as is, micro-pollutants like PCDD/F are notimmobilized in the waste matrix and they can have a mobility;so it is reasonable to assume that a fraction of the PCDD/F con-tained in the waste can have sufficient mobility to be stripped,for example, by an air flow blown into the waste. The enrichmentof PCDD/F in the process air during an aerobic processes applied towaste was pointed out already in 2001 (UNEP, 2001), but an expla-nation of this PCDD/F release from the bio-drying treatment wasprovided only recently (Rada et al., 2007a). The release is net (out-put higher than input) as related to values higher than the back-ground level of PCDD/F in ambient air. The measured release isgenerally low but the impact on health may be not negligible if

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: +39 0461 282672.a), Marco.Ragazzi@ing.unitn.laitil@ing.unitn.it (L. Laiti),

the dispersion is not optimized. In order to assess the role of thisrelease, in the present paper some considerations on the humanexposure from PCDD/F intake have been developed. To this con-cern, the tolerable daily intake (TDI) of dioxin established by theWorld Health Organization (WHO) and individual countries, basedon scientific knowledge was used.

In 1990, WHO proposed for the first time an acceptable daily in-take of 10 pgTEF kg�1 d�1 for 2,3,7,8-TCDD based on informationand studies available at the time (Kociba et al., 1978; Kimbroughet al., 1984). In 1998, taking into account the United State Environ-mental Protection Agency (EPA) research and the OSWER Directive9200.4.26 WHO recommended a Total Daily Intake (TDI) in therange 1–4 pgTEF kg�1 d�1 as ‘‘maximal tolerable intake on a provi-sional basis’’ but it stressed that the ultimate goal should be to re-duce human intake levels to less than 1 pgTEQ kg�1 d�1.

The Toxic Equivalency Factor (TEF) concept for polychlorinateddibenzo-p-dioxins and dibenzofurans and dioxin-like polychlori-nated biphenyls (PCBs) for human, fish, and wildlife assessmentwas introduced in 1998 (van den Berg et al., 1998), as the most‘‘plausible and feasible approach for risk assessment of halogenatedaromatic hydrocarbons with dioxin-like properties. In 2005, the To-tal Toxic Equivalency (TEQ) was introduced by WHO for abioticmatrices as soil, sediments, etc. for direct application in human riskassessment.

In 2010, after six years of research and studies developed withthe National Academy of Science (NAS), EPA has released a draft re-port entitled EPA’s Reanalysis of Key Issues Related to Dioxin Tox-icity and Response to NAS Comments and established a cancerslope factor of 1.0 � 106 (mg kg�1 d�1)�1 based on a linear dose–response model.

Table 1Emission data of PCDD/F bio-drying provided by plants in operation in Germany, Italy and Austria.

Plant t/y Air treatment m3 kg�1MSW

Concentration pgTEQ m�3FE pgTEQ kg�1

MSWNotes

Rennerod (D) 100 000 RTO 3 2 6 Personal communication from the plants manager, 2010Aßlar (D) 140 000 RTO 5.3 0.1 0.53Fusina (I) 125 000 RTO 3 1.8 5.4Oberpullendorf (A) 44 000 Biofilter 6 0.5 3 Lahl et al., 1998

290 E.C. Rada et al. / Chemosphere 84 (2011) 289–295

In this frame also the knowledge on PCDD/F releases into theatmosphere was developed in the last decades. One of the originalcontributions to this development concerned MBTs, that until the1990s were considered belonging to a PCDD/F zero emissionsector.

Bio-drying is a MBT process, an aerobic bioconversion treat-ment that can be applied to MSW as is, to MSW residual of selec-tive collection, or to contaminated organic fractions (under-sievefrom mechanical selection, etc.). The most widely used configura-tion is based on the one-stream concept: all the waste enters intothe biological stage. After an inert post-separation, the final prod-uct is aimed to be used as Refuse Derived Fuel (RDF) or disposed inauthorized plants according to the Dir. 2000/76/CE. The purpose ofthe bio-drying process is the exploitation of the exothermic biolog-ical reactions for the evaporation of the highest part of waste mois-ture with the lowest conversion of organic Carbon (into CO2).

In this paper some environmental and health impacts of a MSWbio-drying plant, taking into account the PCDD/F release into theatmosphere, the induced concentration at ground level and itsdeposition, are presented. In order to determine the health riskassociated with the PCDD/F emissions from MSW bio-dryingplants, the emission phenomenon and the emission factors areanalyzed below.

In the literature variable data regarding PCDD/F emission fac-tors or PCDD/F concentration in the process air of bio-driers are re-ported, as summarized in Table 1. These variations are caused, inaddition to the various operational strategies adopted or to differ-ent air treatment technologies, by the fact that the amount of diox-in present in the MSW may differ significantly from case to case,and the fraction of MSW can be characterized by a different mobil-ity. For example, in England the average concentration of PCDD/Fin MSW was found to be of approximately 6.3 ngTEQ kg�1

MSW, inGermany 73 ngTEQ kg�1

MSW, while in Italy approximately 20 ngTEQ

kg�1MSW (Rada et al., 2006). However these values cannot be general-

ized: variations must be expected also in the same countrydepending on the year of characterization and the level of contam-ination of MSW. Concerning concentrations measured in thebio-drying process air at the outlet, the detected values alone couldseem negligible, but when combined with the specific air flow(from a bio-drying plant generally about 3–10 m3 kg�1

MSW), theresulting release into the atmosphere may become important.Although the MBT does not have a specific EU directive, in someregulations in European countries we can find both enhanced ap-proaches (BMLFUW, 2002 and BMU, 2001), and soft approaches.For example, in Italy specific regional regulations have been estab-lished only in a few areas, but not concerning PCDD/F (D.G.R.,2003). Regarding PCDD/F, only the German regulation imposes alimit of 0.1 ngTEQ m�3, the same imposed in EU for the MSW incin-eration plant. However it must be taken into account that a mod-ern incineration plant typically adopts as air treatment the bestavailable technologies for PCDD/F prevention and reduction; in-stead a MBT plant can have a biofilter or a regenerative thermaloxidation system for process air treatment, options with not wellstudied effects on PCDD/F removal. For this reason and also takinginto account that one of the two systems generally used for theMBT air treatment releases at ground level, at low temperature

and with low velocity, it could be interesting to compare thePCDD/F impact from a MBT plant with the one from an incinerationplant; the present paper gives a contribution to this concern.

Atmospheric conditions play a crucial role in determining thefate of pollutants emitted into the atmosphere. Wind strengthand direction, turbulence intensity, thermal structure, relativehumidity, precipitation and solar radiation are all factors whichcan strongly affect the transport of pollutants and their modifica-tions. As a consequence, a precise knowledge of weather and cli-mate, occurring in the area where dispersion takes place, is amandatory requirement in order to obtain precise scenarios of airquality and fall-out mapping. For this reason advanced numericalmodels for simulation of pollutant dispersion require as an inputa series of meteorological measurements from both surface sta-tions and upper soundings. Often the sites proposed for the imple-mentation of a bio-drying plant have no specific data concerningthis aspect.

2. Materials and methods

In this paper, in order to assess the PCDD/F dispersion from aMSW bio-drying plant, the Gaussian model AERMOD was used.The AERMOD Modeling System is a software implementing the cal-culation of concentration and deposition of pollutants (emittedfrom either surface or elevated sources) at ground level, over bothsimple and slightly complex terrain. Dispersion processes arereproduced by means of suitable parameterization of atmosphericstructure and planetary boundary layer turbulence (EPA, 2004a).

AERMOD has been developed by EPA in conjunction with theAmerican Meteorological Society (AMS). This model requires a pre-processor that organizes and processes meteorological data andestimates the necessary boundary layer parameters for dispersioncalculations in AERMOD. The meteorological preprocessor thatserves to this purpose, and was used for the present calculations,is AERMET. AERMET uses available meteorological measurements,representative of the modeling domain, to compute atmosphericboundary layer parameters used by AERMOD to estimate profilesof wind, turbulence and temperature. AERMET is based on theMeteorological Processor for Regulatory Models (MPRM) (Irwinet al., 1988), but the actual processing of the meteorological datais similar to that done for the CTDMPLUS (Perry, 1992) and HPDM(Hanna and Chang, 1993) models. In particular the growth andstructure of the atmospheric boundary layer is driven by heatand momentum fluxes, which in turn depend upon surface effects(Stull, 1988). The depth of this layer, as well as the dispersion ofpollutants within it, are influenced on a local scale by surface char-acteristics, such as surface roughness, reflectivity (albedo), and theavailability of surface moisture. The surface parameters providedby AERMET are the Monin–Obukhov length, surface friction veloc-ity, surface roughness length, surface heat flux and the convectivescaling velocity (Venkatram, 1980; Panofsky and Dutton, 1984).AERMET also provides estimates of the convective and mechanicalmixed layer heights (Weil and Brower, 1983). AERMET defines thestability of the PBL by the sign of the surface heat flux (convectivefor a positive flux and stable for a negative flux). Although AER-

Table 2Main parameter for the chosen scenarios.

Bio-drying

Bio-drying plantCapacity t y�1 95 000Operating time d y�1 350Process timing h d�1 24Process air m3 kg�1

MSW10

BiofilterEmission factor pgTEQ kg�1 20Mass flow pgTEQ s�1 63Optimal volumetric flow m3 m�3 h�1 80Volume m3 1425 (15 m � 95 m � 1 m)Specific mass flow fgTEQ m�2 s�1 45Emission velocity m s�1 2.2 � 10�2

Temperature (out) �C 35

RTOConcentration pgTEQ m�3 2Mass flow pgTEQ s�1 63Emission velocity m s�1 10Temperature (out) �C 47Diameter m 1.2

E.C. Rada et al. / Chemosphere 84 (2011) 289–295 291

MOD is capable of estimating meteorological profiles with datafrom as little as one measurement height, it use as much data asthe user can provide for defining the vertical structure of theboundary layer. In addition to Planetary Boundary Layer parame-ters, AERMET passes all measurements of wind, temperature, andturbulence in the format required by AERMOD (Cimorelli et al.,2004). AERMET is designed to be run as a three-stage processand operate on three types of data: hourly surface observations,twice-daily upper air soundings, and data collected from an on-sitemeasurement program such as from an instrumented tower. Thefirst stage retrieves data and assesses data quality. The secondstage combines the available data for 24-h periods and writes thesedata to an intermediate file. The third and final stage reads themerged data file and develops the necessary boundary layerparameters for dispersion calculations by AERMOD.

The model requires, as minimum meteorological input data,hourly surface observations of wind speed and direction, ambienttemperature, opaque sky cover (in the absence of opaque sky cov-er, total sky cover). Station pressure is recommended, but not re-quired, since it is used only to calculate the density of dry air,whereas AERMET will use a default value of 1013.25 h Pa (sea levelpressure) in the absence of station pressure. Also an upper airsounding taken in the morning is required (EPA, 2004b).

In the present work hourly observations of air temperature,wind speed and direction (at 10 m above the ground level), precip-itation, relative humidity and solar radiation were provided at aweather station operated at Legnaro (Padua) by the EnvironmentalProtection Agency of Veneto region. As a case study a flat zone inthe North of Italy was chosen having about 500 000 inhabitantsproducing 268 000 tMSW y�1, and a MSW selective collection wellimplemented (about 65% of efficiency); about one third of the pro-duced MSW will be treated in the bio-drying plant, having about23% of organic fraction in the input. Bio-drying has no effect on inertin MSW, but affects moisture and volatile solid content. In the case-study, the total input of the plant is 95 071 tMSW y�1. Comparingthis value with the ones in Table 1, the studied plant can be classi-fied as of average capacity. According to an available energy model(Rada et al., 2007b) and the MSW characteristics of the case-study,the process allows extracting about 10 885 t y�1 of water, with avolatile solid consumption assessed as 14.1 g kg�1

MSW. This consump-tion can be related to the release of PCDD/F into the atmosphereassuming a level of contamination of the volatile solids (VS),according to the PCDD/F bio-drying model (Rada et al., 2007a). Inthe present paper the emission factor has been assumed20 pgTEQ kg�1, taking into account the range proposed in the litera-ture (Rada et al., 2005).

For the bio-drying plant two systems for the process air treat-ment were chosen for a comparison:

� A biofilter at ground level (height of release: 2 m) or on the roofof the plant building (height of release: 17 m).� A regenerative thermal oxidation treatment, RTO (height of

release: 25 m or 35 m).

Taking into account the above considerations, in Table 2, themost important characteristics of the bio-drying plant, of the bio-filter and of the RTO system are presented.

For the chosen scenarios a Cartesian grid square having thesource located in the center was used. For the biofilter emissionsthe considered area had the size 3 km � 3 km, with receptorsspaced 20 m, while in the case of point source, a survey area of4 km � 4 km and 40 m mesh was used.

In this paper, the behavior of PCDD/F during deposition was as-sumed as the one of passive tracers. For the deposition rate thevelocity formulas proposed by Wesely et al. (2002) were used.Through the combined results of these depositions, it can be

deduced which air treatment is less impacting from the environ-mental point of view.

For the estimation of carcinogenic risk in case of ingestion andrespectively inhalation the slope factor (slope of the linear dose–response curve) and respectively the cancer potency, the dailyamount of inhaled air (20 m3) and body weight (70 kg) were used.

In order to assess the risk related to the emitted PCDD/F, thedeposition data were compared with the standards proposed fromthe Flamisch Guide Lines (van Lieshout et al., 2001) using the EPAslope factor and unit risk.

In this paper for the impact of PCDD/F emission from the stud-ied bio-drying plant two deposition limits (DL) were adopted:

� DLTDI100%; assuming zero the background deposition, when thedeposition from the plant corresponds to this value, the plantsalone is responsible for the highest tolerable deposition accord-ing to the Flanders Guide Lines;� DLTDI_5%; assuming zero the background deposition, when the

deposition from the plant corresponds to this value, the plantsis responsible for 5% of the highest tolerable deposition accord-ing to the Flanders Guide Lines; when this deposition is belowthis limit, the impact of the plant can be considered notsignificant.

Taking into account the trend of the WHO’s vision regarding theTDI, in this paper the proposed lower value of 1 pgTEQ kg�1 d�1, wasused for safety reasons. The adopted value in Germany confirmsthat the assumption in this paper is the most stringent: the Ger-man TDI is 4 pgTEQ kg�1 d�1 with a deposition of 15 pgTEQ m�2 d�1

(LAI, 2000) similar to the Flemish less stringent one:14 pgTEQ m�2 d�1 (van Lieshout et al., 2001).

The calculation of the deposition critical value is related to con-centrations of PCDD/F in food products. The PCDD/F flow beforereaching the human body through ingestion, takes into accountthe various levels of concentration in the atmosphere, the deposi-tion in the grass and soil and also the transfer of the PCDD/F cong-eners in the chain of fish, animals farm, milk, eggs and meat. Thisapproach was used also in the Flanders in order to propose thedeposition limit.

For the risk assessments of dioxins, in this paper a linear dose–response relationship for dioxin-induced cancer was assumed; theone in a million cancer risk was calculated for an exposure of0.006 pgTCDD kg�1

body weight d�1 (Buckley-Golder et al., 1999a,b).

292 E.C. Rada et al. / Chemosphere 84 (2011) 289–295

3. Results and discussions

In Fig. 1 the PCDD/F concentrations and respectively depositiondynamics taking into account the chosen air treatment system, inthe South direction are presented.

The use of biofilter at ground level is the worst solution: takinginto account the mass flow (63 pgTEQ s�1) it results a maximumconcentration value near the source equal to 118 fgTEQ m�3; fromHovmand et al., 2007, rural area concentrations are generally low-er, being in the order of tens of fgTEQ m�3. However this value de-creases of about 2 orders of magnitude in the first 200 m, arrivingto a value of about 5 fgTEQ m�3. At about 1 km away from the bio-filter, the PCDD/F concentrations are around 0.2 fgTEQ m�3.

The PCDD/F depositions have the same dynamics moving awayfrom the source of pollution, with a peak of 14 ngTEQ m�2 y�1 at5 m away from the biofilter, value that decreases to 100 pgTEQ

m�2 y�1 at about 500 m from the plant.The solution with the biofilter on the roof of the plant building

improves the environmental impact. The values of pollution nearthe plant are reduced by two orders of magnitude: the maximumconcentration arriving at 1.2 fgTEQ m�3, while the peak of depositionis about 820 pgTEQ m�2 y�1. However, the impact at around 1 kmaway from the plant is similar for both solutions. The peak deposi-tion is anyway high. Indeed in a Belgian case-study (Nouwen et al.,2001) referred to two incinerators located in the same area, model-ing results gave a maximum deposition value of 1.2 ngTEQ m�2 y�1

near the two incinerators. These incinerators exceeded respectivelyof one and two orders of magnitude of the present EU standard(0.1 ngTEQ Nm�3).

Fig. 1. Yearly average values of induced PCDD/F con

Fig. 2. PCDD/F concentration [fgTEQ m�3] in logarithmic scale, distance (m) for

The situation improves in the case of RTO solutions. In the caseof RTO outlet placed at 25 m the values are reduced by an addi-tional order of magnitude, with maximum values of 0.1 fgTEQ m�3

and a deposition of 74 pgTEQ m�2 y�1. The PCDD/F from the stackare more sensitive to the preferred direction of the wind but thedeposition values, however, are less affected by the vertical windprofile. In the case of RTO placed at 35 m the impact is not compar-atively reduced substantially, because the situation is already goodwith heights of 25 m.

In order to facilitate the interpretation of the results, in Figs. 2and 3 the concentrations and depositions are presented in logarith-mic scale for the case of biofilter at ground level (the worst case)and RTO at 25 m (the already suitable case), in order to give anoverall information on the impact in the plant surroundings. Theclimatologic characteristics of the area do not give a prevailingdirection of the impact.

For the evaluation of the risk for human health from PCDD/Fingestion and inhalation, the PCDD/F dose and cancer potencyand slope parameters factor were used. Taking into account thePCDD/F deposition 3.4 pgTEQ m�2 d�1 for the TDI = 1 pgTEQ kg�1 d�1

(van Lieshout et al., 2001) and the slope factor of 150 pgTEQ

kg�1 d�1 adopted in the Flemmish Guide Lines, in our case the tol-erable risk results 1.5 � 10�4.

In the case of a good management of waste, in this paper a limitfor the dioxin emission in order to comply with a maximum indi-vidual risk of developing a tumor effect was set as 10�6. This is thestandard value used for human health risk evaluation in the case ofincineration. Taking into account the target risk (10�6) and in ourcase the tolerable risk, the target deposition (DLTDI_10

�6) results

centration and deposition in air at ground level.

the bio-drying plant with biofilter at 2 m (left) and RTO at 25 m (right).

Fig. 4. Isolines for the case of bio-drying with biofilter at h = 2 m (a) and 17 m (b), and for bio-drying with RTO at h = 25 m (c) and 35 m (d), depositions in pgTEQ m�2 y�1,distances in m.

Fig. 3. PCDD/F deposition [pgTEQ m�2 y�1] in logarithmic scale, distance (m) for the bio-drying plant with biofilter at 2 m (left) and RTO at 25 m (right).

E.C. Rada et al. / Chemosphere 84 (2011) 289–295 293

8 pgTEQ m�2 y�1. In Fig. 4 the isodeposition curves related toDLTDI_100%, DLTDI_5% and DLTDI_10

�6 are reported for the case of thebio-drying plant with biofilter at 2 m, 17 m and respectively forthe same bio-drying plant with RTO at 25 m and 35 m.

The isoline in red (DLTDI_100%) represents the critical value oflimit exposure, the one in green (DLTDI_5%) is the limit for relevantimpact and the blue one (DLTDI_10

�6) is the limit for the targetimpact.

Table 3Distance in meter for complying with deposition limits.

TDI 100% TDI 5% TDI � 10�6

Rmin Rmax Rmin Rmax Rmin Rmax

Biofilter 2 m 63.2 107.7 316.2 577.2 878.6 1848.3Biofiltrer 17 m 170.9 316.2 693.1 1868.2RTO 25 m 89.4 126.5 288.4 683.5RTO 35 m 282.8 611.8

294 E.C. Rada et al. / Chemosphere 84 (2011) 289–295

The worst solution is the one of bio-drying with biofilter atground level, having a PCDD/F critical environmental impact. Thecritical value of 1.24 ngTEQ m�2 y�1 (DLTDI_100%) is confirmed in anarea out of a distance of about 100 m from the source. In the caseof the more stringent DLTDI_5%, this value is not respected in an areaas far as a distance of about 500 m in the North–South directionand about 300 in West–East direction; instead the DLTDI-10

�6 isnot complied with at less than 1.5 km. The meaning of these resultsis that the population is not protected according to the WHO crite-ria required for one source of pollution.

For the second case (bio-drying with biofilter at 17 m) thePCDD/F environmental impact improves. The DLTDI_100% is alwaysrespected, the DLTDI_5% is not complied with in a small area witha maximum diameter of 300 m but for the DLTDI_10

�6 the situationdoes not change significatively, the limit exposure distance re-mains at about 1.5 km in the North–South direction and at about1 km in the West-East direction. One of the reasons is that thedeposition dynamics is not affected by the release height at dis-tance greater than 1 km.

For the last two proposed cases (bio-drying with RTO at 25 mand 35 m) the overall PCDD/F environmental impact improves sig-nificatively, and the involved population living near the plant isfurther protected. The DLTDI_100% is respected in both cases, theDLTDI_5% is not complied with in a small area (with a limit distanceof 100 m only in the case of RTO at 25 m). Even for these two casesthe DLTDI_10

�6 is not guaranteed, there is indeed still an exposedzone with a radius of up to about 600 m.

A change to the RTO height from 25 m to 35 m, gives highercapital costs, slightly higher operation costs (because of the higherenergy demand for the blowers) and a higher impact on the land-scape, but does not give a significant improvement from the envi-ronmental point of view.

For a better understanding of the PCDD/F environmental impactresulting from a bio-drying plant that treats MSW, in Table 3 theminimum and maximum critical distances for the four proposedcases are reported.

Taking into account the data resumed in Table 3, it is clear thatthe best solution for managing the problem is the prevention ofPCDD/F emissions. To this purpose the devices adopted for thetreatment of the process air of MBTs are not adequately studiedand developed in terms of PCDD/F removal.

4. Conclusions

In the present paper the PCDD/F impact analysis was provided,based on multiple exposure pathways that are resumed in the con-cept of deposition targets. The most relevant pathway for PCDD/Ffrom the bio-drying process turns out to be the deposition one.In order to limit this environmental impact, and respectively thecarcinogenic risk, it would be appropriate to regulate the produc-tion of those agricultural products that have a strong propensitytowards biomagnification of dioxins, and therefore likely to havesignificant weight in terms of intake of PCDD/F inside the humanbody. A source of uncertainty is the amount of local products thatare eaten locally. In the absence of a specific study, a generalassumption can be made, taking into account the EPA criteria.

The environmental impact from municipal solid waste bio-dry-ing treatment cannot be neglected. As demonstrated the worst caseis the one with the biofilter at ground level, which is the mostadopted in the sector of MBTs. Of course the biofilter on the roofimproves the impact in the surroundings, and it should be adoptedas compulsory, while ground level biofilters should not be autho-rized any more. A visible change can be obtained by the implemen-tation of the RTO system. This option is widely used in Germany,and has the only disadvantage of its high cost. The present studydemonstrates that the stack height for the RTO is not the core issueof the optimization: good results can already be obtained withaverage heights.

This paper demonstrates that an environmental impact assess-ment should be requested also in the case of aerobic mechanical–biological treatments, as it is currently required in the case ofincineration plants. For a detailed investigation on the PCDD/Fdynamics, generally mathematical and numerical models requiringappropriate meteorological data are widely used.

Three problems can be pointed out: (1) the present regulationdoes not require a specific study on the emissions of PCDD/F frombio-drying; (2) often there is a lack of site-specific meteorologicaldata; (3) there is not a universally-valid model for the evaluationof pollutant dispersion/deposition processes. In the first case, thelimits of the present regulations depend either on the limitedknowledge of the phenomena, or on the underestimate of its ef-fects: in Germany the measurement of PCDD/F emissions fromBMTs is required, but the imposed limit concentration is the sameas for incineration, without any attention to the emission factorand on its role in terms of the different conditions under which dis-persion may occur. Human exposure is crucial, thus the preventionthat a regulation must guarantee cannot be based only on conven-tional approaches. The second problem, related to meteorologicalanalyses, is often underestimated; the best solution could beinstalling a meteorological station in the site proposed for a newplant, thus allowing one year of measurements.

The last problem regards the limitation of the mathematicalmodel applicability. AERMOD has been originally developed for flator slightly complex terrain, and cannot simply be extended to casesof strongly complex terrain, such as within deep mountain valleys(Rotach and Zardi, 2007). The MSW treatment plants are con-structed in all the orographic conditions. For example the verticalextent and structure of both wind and temperature fields in com-plex areas are very different from those occurring over flat terrain(de Franceschi et al., 2003, 2009; de Franceschi and Zardi, 2009;Serafin and Zardi, 2010a,b); thus the model must be specificallychosen case by case.

Summing up, the present work points out the need of specificstudies related to the role of PCDD/F in MBTs. These studiesshould take into account waste contamination, dioxin removalby process air treatment, ways in which pollutants are releasedinto the atmosphere, meteorological characteristics of the area,land use in the surroundings, resident population presence andlocation.

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