In recent years, reed bed systems (RBSs) have been widely considered as a valid technology for sludge treatment. In this study are presented results about sludge stabilization occurring within beds in four RBSs, situated in Tuscany (Italy). The results showed that stabilization of the sludge over time occurred in all RBSs, as shown by the low content of water-soluble carbon and dehydrogenase activity, which measures indirectly the overall microbial metabolism, and by the re-synthesis of humic-like matter highlighted by the pyrolytic indices of mineralization and humification. Results about heavy metal fractionation, an appropriate technique to estimate the heavy metal bioavailability and sludge biotoxicity, showed that the process of sludge stabilization occurring in RBSs retains metals in fractions related to the stabilized organic matter, making metals less bioavailable. Moreover, the concentrations of various toxic organic compounds were below the limit of concentration suggested by the European Union's Working Document on Sludge, for land application. The effectiveness of the stabilization processes in RBs was hence clearly proven by the results that measured mineralization and humification processes, and by the low levels of bioavailable heavy metals and toxic organic compounds in stabilized sludges.

INTRODUCTION

Safe and cost-effective management of sewage sludge is still a worldwide environmental challenge. A common goal in sludge treatment is to reduce toxicity, decrease volume, and convert sludge into useful resources. In recent years, reed bed systems (RBSs) have been widely considered as a valid technology for sludge treatment. However, even though this technology is not widely used in Mediterranean countries, significant experience in the Tuscany region has been gained since 2004.

The RBS is a combination of a traditional sludge drying bed and constructed wetland: Phragmites australis is directly planted in the drying beds, where sludge is frequently applied. This technology involves low construction costs, minimal daily maintenance, water content reduction, and good stabilization of biosolids.

The RBS technology in stabilizing sewage sludges has been proven not only in its effectiveness in dewatering (Stefanakis & Tsihrintzis 2011; Iannelli et al. 2013) and reducing the pollutant content (Peruzzi et al. 2011b; Matamoros et al. 2012) and pathogen content (Nielsen 2007), but also in improving the quality of organic matter in sludges (Uggetti et al. 2010; Peruzzi et al. 2011a; Kołecka & Obarska-Pempkowiak 2013; Nielsen et al. 2014). The stabilization process, resulting from the synergic action of plants, organic matter and microorganisms, is a combination of mineralization and the humification process of organic matter. The organic matter stabilization process occurring in the basins can be successfully studied following parameters usually applied to soil and other environmental matrices.

In this paper, results about sludge stabilization from four different RBSs in Italy after several years of functioning are reported.

MATERIALS AND METHODS

Reed bed systems

Stabilization of sludges was investigated in four urban wastewater treatment plants: Acque S.p.A., RBS 1 La Fontina, RBS 2 Oratoio, RBS 3 Colle di Compito and RBS 4 Pittini, all situated in the Tuscany region (Italy).

In this paper, results from the setup of the plants (2005 for RBS 1 and RBS 2; 2006 for RBS 3 and RBS 4) are reported. For RBS 1, all results for the entire period of operation of the plant have been presented (2005–2012, 72 months), while RBS 2 is still operating (2005–2013, 96 months). Both RBS 3 and RBS 4 have been operating since 2006 (2006–2013, 90 months). All operational details about each RBS are reported in Table 1.

Table 1

Reed bed system plants and loading programs

 RBS 1 (La Fontina) RBS 2 (Oratoio) RBS 3 (Colle di Compito) RBS 4 (Pittini) 
Population equivalent 15,000 10,000 4,000 5,000 
Basin area (m21,210 (11 beds) 375 (5 beds) 225 (5 beds) 252 (6 beds) 
Bed area (m2110 75 45 42 
Loading rate (kg dw/m2 yr) 38 45 67 67 
Sludge (% dw) 1.5 1.5 
Loading rate (m3/m2 yr) 3.80 4.50 4.40 4.70 
Loading/resting Autumn–Winter–Spring (days) 1/14–20 1/14–20 1/14–20 1/14–20 
Loading/resting Summer (days) 1/7–10 1/7–10 1/7–10 1/7–10 
Starting period 2005 2005 2006 2006 
Ending period 2011 Active Active Active 
Resting period 2012    
 RBS 1 (La Fontina) RBS 2 (Oratoio) RBS 3 (Colle di Compito) RBS 4 (Pittini) 
Population equivalent 15,000 10,000 4,000 5,000 
Basin area (m21,210 (11 beds) 375 (5 beds) 225 (5 beds) 252 (6 beds) 
Bed area (m2110 75 45 42 
Loading rate (kg dw/m2 yr) 38 45 67 67 
Sludge (% dw) 1.5 1.5 
Loading rate (m3/m2 yr) 3.80 4.50 4.40 4.70 
Loading/resting Autumn–Winter–Spring (days) 1/14–20 1/14–20 1/14–20 1/14–20 
Loading/resting Summer (days) 1/7–10 1/7–10 1/7–10 1/7–10 
Starting period 2005 2005 2006 2006 
Ending period 2011 Active Active Active 
Resting period 2012    

Methods

Total organic carbon (TOC) was determined by RC-412 multiphase carbon (LECO Corporation, St Joseph, MI, USA) on 50–70 mg of dried sample. The RC-412 employed a state-of-the-art furnace control system, which allowed the temperature of the furnace to be stepped and subjected to ramping. Different sources of carbon (organic and inorganic carbon) were differentiated by the temperature at which they oxidize, decompose or turn into volatiles. Water and carbon dioxide released from the minerals were detected by means of infrared absorption cells.

Total nitrogen (TN) was determined by FP-528 protein/nitrogen (LECO Corporation, St Joseph, MI, USA). An encapsulated sample (60–80 mg) was placed into the loading head of the FP-528, where it was sealed and purged of any atmospheric gases that had entered during sample loading. The sample was then dropped into a hot furnace and flushed with pure oxygen for very rapid combustion. By-products of combustion (CO2, H2O, NOx, and N2) passed through the furnace filter and thermoelectric cooler for subsequent collection in a ballast apparatus. These collected gases in the ballast were equilibrated, and a small aliquot was then used for further conversion of the gases. The remaining aliquot was reduced and then was measured by the thermal conductivity cell for nitrogen.

Water soluble carbon (WSC) was determined according to the method of Yeomans & Bremner (1988) by dichromate oxidation on aqueous extract (1:10, w/v, 1 hour at 60 °C) (Garcia et al. 1991). Dehydrogenase activity (DHase) (EC 1.1.1.1) was determined by the reduction of 2-p-iodo-nitrophenyl-phenyltetrazolium chloride (INT) to iodo-nitrophenyl formazan (INTF) using 0.2 g of sludge at 60% of field capacity, exposed to 0.2 ml of 0.4% INT in distilled water for 20 hours at 22 °C in darkness. The INTF formed was extracted with 5 ml of a mixture of 1:1.5 tetrachloroethylene/acetone by shaking vigorously for 1 minute. INTF was measured spectrophotometrically at 490 nm. DHase activity was expressed as mg INTF/kg dw h (dw: dry weight) (Masciandaro et al. (2000).

For the pyrolysis gas chromatography (Py-GC) analysis, the dried sludge (about 0.2 g) was put into pyrolysis microtubes in a CDS Pyroprobe 190 (Oxford, PA, USA) and pyrolysis was carried out at 800 °C for 10 seconds, with a heat gradient of 10 °C/ms. The probe was directly coupled to a Carlo Erba (Milan, Italy) 6000 gas chromatograph with a flame ionization detector. The pyrograms obtained were quantified by normalizing the areas of the characteristic seven peaks. Identification of pyrogram fragments (acetic acid, acetonitrile, benzene, toluene, furfural, pyrrole, and phenol) in the samples was carried out on the basis of the relative retention times compared with standard spectra (Ceccanti et al. 2007; Macci et al. 2012).

The heavy metal fractionation was determined using the Mocko & Waclawek method (2004). The concentrations of different metal fractions were transformed from mg/kg to mEq/kg and then the concentration for each fraction was summed to give the total for the metal.

Linear alkylbenzene sulfonates (LAS), nonylphenols (the sum of nonylphenol, NP; nonylphenol ethoxylate with the one ethoxy group, NP1EO; and nonylphenol ethoxylates with two ethoxy groups, NP2EO (NPEs)) and di-2-ethylhexyl-phthalate (DEHP) were simultaneously extracted from air-dried samples with methanol by microwave-assisted extraction (5 ml of methanol, 10 minutes and 250 W microwave irradiation; Villar et al. 2007). Chromatographic analysis was performed on an Agilent 1100 series high performance liquid chromatograph with an ultraviolet diode array (wavelength excitation 1⁄4 226 nm) and fluorescence detectors (wavelength excitation 1⁄4 226 nm; wavelength emission 1⁄4 301 nm). Organic compounds separation was carried out using a ZORBAX Eclipse XDB-C8 (150 × 3 mm i.d., 5 μm). A mobile phase of acetonitrile and 10 mM ammonium acetate solution was used at different gradients (30:70 for 8 minutes, 65:35 for 7 minutes, 100:0 for 10 minutes and 70:30 for 5 minutes). A column equipped with a thermostat at 28 °C and a flow rate of 1 ml/minute were adopted (Fountoulakis et al. 2005; Santos et al. 2007; Pakou et al. 2009).

Sampling

Sludge sampling was carried out every 3 months for the first year of operation, then every 6 months for the rest of the experimental period. For each bed of each RBS, five subsamples were taken, which were mixed in order to obtain a representative sample of each bed. The samples were collected near the gravel layer. The plant material was removed from samples. About 20 days before the sampling, the sludge applications were stopped.

Statistical analysis

All results reported in the text are the means of determinations for each bed in each RBS. One-way analysis of variance was carried out to compare the means between times of sampling, in order to follow the progress of sludge stabilization. Where significant F-values were obtained, differences between individual means were tested using Tukey-HSD (honest significant difference) tests at 0.05 significance level, using the Statistica 7.0 software (StatSoft, Inc., USA).

RESULTS AND DISCUSSION

Sludge stabilization: mineralization and humification processes

The sludges were stabilized and transformed within the RBSs by sludge–plant–microorganism interactions (Peruzzi et al. 2009). The two main concomitant processes taking place are mineralization and humification of organic matter.

In general, the content of TOC and TN, the main nutrients present in sludges, slowly decreased over time, following a similar trend in all four RBSs. During the first phase of operation (24–36 months), the TOC and TN concentrations were generally higher than concentrations found during the last period (72–96 months) (Figure 1). These results were in agreement with the findings of Gagnon et al. (2013). This overall constant level of organic matter mineralization was more clearly highlighted by the trend of two sensitive parameters to biological stabilization status, DHase and WSC: the latter represents a readily biodegradable substrate for microorganisms, while the former, occurring intracellularly in all living microbial cells, represents the overall microbial activity (Masciandaro et al. 2000) (Figure 2). As a consequence of mineralization processes occurring in the basins, WSC and DHase activity dramatically decreased in all RBSs during the first months of operation, and immediately after reached significantly low and stable concentrations, even though fresh organic matter was continuously added to the basins.

Figure 1

TOC (%C) and TN (%N) in stabilized sludges. Means followed by the same letter(s) (lowercase for TOC and uppercase for TN) are not significantly different according to Tukey's test at P < 0.05 over time.

Figure 1

TOC (%C) and TN (%N) in stabilized sludges. Means followed by the same letter(s) (lowercase for TOC and uppercase for TN) are not significantly different according to Tukey's test at P < 0.05 over time.

Figure 2

WSC (mg C/kg dw) and DHase (mg INTF/kg dw h) in stabilized sludges. Means followed by the same letter(s) (lowercase for WSC and uppercase for DHase) are not significantly different according to Tukey's test at P < 0.05 over time.

Figure 2

WSC (mg C/kg dw) and DHase (mg INTF/kg dw h) in stabilized sludges. Means followed by the same letter(s) (lowercase for WSC and uppercase for DHase) are not significantly different according to Tukey's test at P < 0.05 over time.

The occurrence of stabilization is also evident in the significant changes in the chemical structural properties of organic matter, measured by the Py-GC technique (Ceccanti et al. 2007; Macci et al. 2012) (Figure 3). The index of mineralization (O/N) expresses the ratio between pyrrole (a heterocyclic aromatic organic compound derived from nitrogenous compounds, humified organic matter, and microbial cells) (Song & Farwell 2008) and furfural (a pyrolytic product coming from degradation of polysaccharides). The higher the ratio, the higher the extent of mineralization of organic matter, meaning that a high concentration of labile organic compounds remains (Ceccanti et al. 2007; Macci et al. 2012). The mineralization index generally decreased over time in all RBSs, even though with some fluctuations. Lower values of O/N were noted during the final phases, meaning that the rate of the mineralization process slowly decreased over time, as the sludge organic matter becomes more stabilized. As the mineralization process slowed, the humification process tended to be the dominant stabilization process for sludge organic matter. The index of humification B/E3 (benzene to toluene, the former deriving basically from stable condensed aromatic structures, the latter deriving from pseudo-stable aromatic structures with short aliphatic chains) increases when organic matter is becoming more mature. The humification index significantly increased over time, especially during the final phases in all RBSs, reaching significant values that demonstrated the re-synthesis of humic-like matter (Song & Farwell 2004, 2008; Macci et al. 2012).

Figure 3

Pyrolytic indices of mineralization (O/N) and humification (B/E3) in stabilized sludges. Means followed by the same letter(s) (lowercase for B/E3 and uppercase for O/N) are not significantly different according to Tukey's test at P < 0.05 over time.

Figure 3

Pyrolytic indices of mineralization (O/N) and humification (B/E3) in stabilized sludges. Means followed by the same letter(s) (lowercase for B/E3 and uppercase for O/N) are not significantly different according to Tukey's test at P < 0.05 over time.

Heavy metal and toxic organic compounds

The total heavy metal content (Table 2) remained below the level established by law for the reuse of sewage sludge in agriculture (Italian regulation D. Lgs. 99/92 for disposal on agricultural lands: Cu 1,000, Ni 300, Cd 20, Pb 750, Zn 2,500 mg/kg dw). Similar concentrations of heavy metals were reported by Fytili & Zabaniotou (2008) for sewage sludge content in European countries.

Table 2

Total heavy metal content of the sludge (mg/kg dw)

Months Cr Cu Ni Cd Pb Zn Months Cr Cu Ni Cd Pb Zn 
RBS1 RBS 2 
29 362 67 2.7 91 917 40 467 55 <5 121 1357 
24 60 550 55 <1.5 60 1760 24 205 635 55 <1 55 2170 
30 123 573 52 2.53 86 1836 30 384 360 26 1.24 39 915 
36 24 556 32 1.62 99 1538 36 102 608 26 1.33 70 1462 
42 37 578 33 69 1437 42 136 529 26 <1 39 1126 
48 55 549 34 43 1705 48 159 563 30 <1 30 1532 
54 14 390 34 1.91 78 1209 54 243 585 36 1.57 53 1529 
60 38 611 47 1.52 66 1853 60 116 824 40 1.39 47 2323 
72 32 492 46 1.67 91 1765 72 89 691 45 1.46 53 1619 
       84 39 480 53 2.82 67 1256 
              96 94 652 57 <2 113 1449 
Months Cr Cu Ni Cd Pb Zn Months Cr Cu Ni Cd Pb Zn 
RBS 3 RBS 4 
42 310 28 <2 81 661 41 333 27 <2 129 416 
18 58 515 58 <1.5 55 975 18 60 535 45 <1 35 845 
24 139 583 50 1.95 79 1222 24 115 537 35 1.21 36 834 
30 21 525 31 1.07 106 944 30 41 656 38 0.78 109 990 
36 30 519 25 <1 48 939 36 36 479 21 <1 36 1352 
42 24 466 27 <1 29 1090 42 42 367 23 <1 28 849 
48 12 524 33 1.20 50 994 48 39 395 29 <1 28 1003 
54 27 636 37 1.49 48 1703 54 38 671 35 0.58 27 1332 
66 73 683 54 1.73 84 1094 66 48 573 43 1.20 49 1159 
78 29 380 36 3.16 65 896 78 21 361 26 2.08 50 730 
90 40 476 22 <2 77 601 90 40 472 23 <2 53 629 
Months Cr Cu Ni Cd Pb Zn Months Cr Cu Ni Cd Pb Zn 
RBS1 RBS 2 
29 362 67 2.7 91 917 40 467 55 <5 121 1357 
24 60 550 55 <1.5 60 1760 24 205 635 55 <1 55 2170 
30 123 573 52 2.53 86 1836 30 384 360 26 1.24 39 915 
36 24 556 32 1.62 99 1538 36 102 608 26 1.33 70 1462 
42 37 578 33 69 1437 42 136 529 26 <1 39 1126 
48 55 549 34 43 1705 48 159 563 30 <1 30 1532 
54 14 390 34 1.91 78 1209 54 243 585 36 1.57 53 1529 
60 38 611 47 1.52 66 1853 60 116 824 40 1.39 47 2323 
72 32 492 46 1.67 91 1765 72 89 691 45 1.46 53 1619 
       84 39 480 53 2.82 67 1256 
              96 94 652 57 <2 113 1449 
Months Cr Cu Ni Cd Pb Zn Months Cr Cu Ni Cd Pb Zn 
RBS 3 RBS 4 
42 310 28 <2 81 661 41 333 27 <2 129 416 
18 58 515 58 <1.5 55 975 18 60 535 45 <1 35 845 
24 139 583 50 1.95 79 1222 24 115 537 35 1.21 36 834 
30 21 525 31 1.07 106 944 30 41 656 38 0.78 109 990 
36 30 519 25 <1 48 939 36 36 479 21 <1 36 1352 
42 24 466 27 <1 29 1090 42 42 367 23 <1 28 849 
48 12 524 33 1.20 50 994 48 39 395 29 <1 28 1003 
54 27 636 37 1.49 48 1703 54 38 671 35 0.58 27 1332 
66 73 683 54 1.73 84 1094 66 48 573 43 1.20 49 1159 
78 29 380 36 3.16 65 896 78 21 361 26 2.08 50 730 
90 40 476 22 <2 77 601 90 40 472 23 <2 53 629 

The procedure for fractionation differentiated the sludge heavy metals into four fractions with different degree of bioavailability:

  1. Exchangeable fraction associated with carbonated phase (Fraction 1). This is the most mobile fraction potentially toxic for plants. Metals are adsorbed on the sludge components and Fe and Mn hydroxides.

  2. Reducible fraction (Fraction 2). Heavy metals are strongly bound to Fe and Mn oxides, but they are thermodynamically unstable in anoxic and acidic conditions.

  3. Oxidizable fraction bound to organic matter (Fraction 3). Heavy metals complexed by humic substances become soluble when organic matter is degraded in oxidizing conditions. This fraction is not considered to be bioavailable and mobile.

  4. Residual fraction (Residual Fraction). Heavy metals are included in crystalline structures in the residual solids. They are considered to be not extractable and in an inert form.

In this paper are reported results for Fraction 1 and Fraction 3 (Figure 4). Fraction 1 content significantly declined over time, as noticed previously in other papers (Kołecka & Obarska-Pempkowiak 2013; Peruzzi et al. 2014), in particular during the first phases, as the stabilization proceeded in RBSs. Other authors found higher content of metals associated with Fraction 1 in sewage sludge stabilized with traditional methods (thickening, heat treatments, anaerobic digestion, etc.) (Walter et al. 2006; Fuentes et al. 2008). Conversely, Weng et al. (2014) reported similar results in sludge dewatered by heat treatment. As the bioavailable fraction dropped, the Fraction 3 bound to organic matter significantly rose, as a consequence of the humification process (Peruzzi et al. 2014). This increase is particularly evident during the last phases of operation.

Figure 4

Heavy metal fractionation in RBSs, Fraction 1 (%) and Fraction 3 (%) in stabilized sludges. Means followed by the same letter(s) (lowercase for Fraction 1 and uppercase for Fraction 3) are not significantly different according to Tukey's test at P < 0.05 over time.

Figure 4

Heavy metal fractionation in RBSs, Fraction 1 (%) and Fraction 3 (%) in stabilized sludges. Means followed by the same letter(s) (lowercase for Fraction 1 and uppercase for Fraction 3) are not significantly different according to Tukey's test at P < 0.05 over time.

Results for toxic organic compounds are reported in Figure 5. The concentration found in the RBSs reflects the concentration of toxic organic compounds present in sewage sludges: RBS 2 showed, in fact, higher values of LAS, NPEs and DEHP with respect to other RBSs.

Figure 5

Toxic organic compounds (LAS sewage sludge: RBS 1 464 mg/kg dw, RBS 2 2359 mg/kg dw, RBS 3 693 mg/kg dw, RBS 4 511 mg/kg dw; NPEs sewage sludge: RBS 1 63 mg/Kg dw, RBS 2 132 mg/kg dw, RBS 3 41 mg/kg dw, RBS 4 74 mg/kg dw; DEHP sewage sludge: RBS 1 11 mg/kg dw, RBS 2 38 mg/kg dw, RBS 3 35 mg/kg dw, RBS 4 7 mg/kg dw).

Figure 5

Toxic organic compounds (LAS sewage sludge: RBS 1 464 mg/kg dw, RBS 2 2359 mg/kg dw, RBS 3 693 mg/kg dw, RBS 4 511 mg/kg dw; NPEs sewage sludge: RBS 1 63 mg/Kg dw, RBS 2 132 mg/kg dw, RBS 3 41 mg/kg dw, RBS 4 74 mg/kg dw; DEHP sewage sludge: RBS 1 11 mg/kg dw, RBS 2 38 mg/kg dw, RBS 3 35 mg/kg dw, RBS 4 7 mg/kg dw).

In all RBSs, the concentration of LAS in the sewage sludge was lowered to the limit of concentration suggested by the European Union's Working Document on Sludge (2000) for land application (2,600 mg/kg dw). Because of their relatively high biodegradability in the aerobic environment of RBSs, LAS were immediately degraded in all RBSs during the stabilization process in the basin.

For NPEs, the concentrations found in sewage sludge, even though similar to results found by previous authors (Roig et al. 2012), remained above the limit of 50 mg/kg dw suggested for land application. The concentrations remained high during the first 2–4 years of operation, confirming the resistance of NPEs to biodegradation (González et al. 2010; Ömeroğlu et al. 2015), then tended to significantly decrease in all RBSs, thus demonstrating the presence of aerobic conditions needed to degrade NPEs (Chang et al. 2007). The behaviour of DEHP was quite similar, even though a small nucleus seemed to remain persistent in RBS 2. DEHP is a hydrophobic contaminant with lipophilic properties (Aparicio et al. 2009), which needs aerobic conditions to be successfully degraded. The concentrations found in sewage sludge were similar to results found by other authors (Pakou et al. 2009) and were lower than the limit suggested for land application (100 mg/kg dw). Similar observations about the persistence, occurrence and degradation of toxic organic compounds in RBSs for sludge treatment were reported by Nielsen & Willoughby (2005) and Matamoros et al. (2012).

CONCLUSIONS

The stabilization of sludge organic matter was clearly proven by a long-term monitoring of parameters expressing mineralization and humification processes. In particular:

  • A long period of monitoring allowed the complex dynamics of organic matter stabilization occurring in RBSs to be followed.

  • The content of TOC and TN slowly decreased over time as a consequence of the mineralization process.

  • The mineralization process was clearly demonstrated by the dramatic decrease of water-soluble carbon and dehydrogenase activity found in all four RBSs.

  • The mineralization process was predominant during the first period of operation, while the humification process became predominant during the last phases, as highlighted by chemical structural characterization of sludge organic matter.

  • The degradation of toxic organic compounds was a consequence of the mineralization process.

  • The bioavailability of heavy metals in sludges was greatly affected by the humification process occurring in RBSs.

  • In all RBSs, similar patterns of organic mater mineralization and humification processes were noticed.

ACKNOWLEDGEMENT

This work was supported by ACQUE S.p.A. (Pisa, Italy).

REFERENCES

REFERENCES
Ceccanti
B.
Masciandaro
G.
Macci
C.
2007
Pyrolysis-gas chromatography to evaluate the organic matter quality in a mulched soil
.
Soil and Tillage Research
97
,
71
78
.
Chang
B.
Chiang
B.
Yuan
S.
2007
Biodegradation of nonylphenol in soil
.
Chemosphere
66
(
10
),
1857
1862
.
Fuentes
A.
Lloréns
M.
Sáez
J.
Isabel Aguilar
M.
Ortuño
J. F.
Meseguer
V. F.
2008
Comparative study of six different sludges by sequential speciation of heavy metals
.
Bioresource Technology
99
(
3
),
517
525
.
Fytili
D.
Zabaniotou
A.
2008
Utilization of sewage sludge in EU application of old and new methods: a review
.
Renewable and Sustainable Energy Reviews
12
(
1
),
116
140
.
Gagnon
V.
Chazarenc
F.
Comeau
Y.
Brisson
J.
2013
Effect of plant species on sludge dewatering and fate of pollutants in sludge treatment wetland
.
Ecological Engineering
61
,
593
600
.
Garcia
C.
Hernandez
T.
Costa
F.
1991
Study on water extract of sewage sludge compost
.
Soil Science and Plant Nutrition
37
,
399
408
.
González
M. M.
Martín
J.
Santos
J. L.
Aparicio
I.
Alonso
E.
2010
Occurrence and risk assessment of nonylphenol and nonylphenol ethoxylates in sewage sludge from different conventional treatment processes
.
The Science of the Total Environment
408
(
3
),
563
570
.
Iannelli
R.
Nielsen
S.
Peruzzi
E.
Piras
F.
Støvring
M.
Masciandaro
G.
2013
Short-term performance analysis of sludge treatment reed beds
.
Water Science and Technology
68
,
1520
1528
.
Macci
C.
Doni
S.
Peruzzi
E.
Ceccanti
B.
Masciandaro
G.
2012
Pyrolysis-gas chromatography to evaluate the organic matter quality of different degraded soil ecosystems
. In:
Advanced Gas Chromatography – Progress in Agricultural, Biomedical and Industrial Applications
(
Mustafa Ali Mohd, ed.
).
In Tech
.
Rijeka, Croatia
pp.
283
306
.
Matamoros
V.
Nguyen
L. X.
Arias
C. A.
Nielsen
S.
Laugen
M. M.
Brix
H.
2012
Musk fragrances, DEHP and heavy metals in a 20 years old sludge treatment reed bed system
.
Water Research
46
(
12
),
3889
3896
.
Nielsen
S.
Willoughby
N.
2005
Sludge treatment and drying reed bed systems in Denmark
.
Water and Environment Journal
19
(
4
),
296
305
.
Nielsen
S.
Peruzzi
E.
Macci
C.
Doni
S.
Masciandaro
G.
2014
Stabilisation and mineralisation of sludge in reed bed systems after 10–20 years of operation
.
Water Science and Technology
69
(
3
),
539
545
.
Pakou
C.
Kornaros
M.
Stamatelatou
K.
Lyberatos
G.
2009
On the fate of LAS, NPEOs and DEHP in municipal sewage sludge during composting
.
Bioresource Technology
100
(
4
),
1634
1642
.
Peruzzi
E.
Macci
C.
Doni
S.
Peruzzi
P.
Aiello
M.
Masciandaro
G.
Ceccanti
B.
2009
Phragmites australis for sewage sludge stabilization
.
Desalination
246
(
1–3
),
110
119
.
Peruzzi
E.
Masciandaro
G.
Macci
C.
Doni
S.
Ravelo
S. G. M.
Peruzzi
P.
Ceccanti
B.
2011a
Heavy metal fractionation and organic matter stabilization in sewage sludge treatment wetlands
.
Ecological Engineering
37
,
771
778
.
Peruzzi
E.
Masciandaro
G.
Macci
C.
Doni
S.
Ceccanti
B.
2011b
Pollutant monitoring in sludge treatment wetlands
.
Water Science and Technology
64
(
7
),
1558
1565
.
Peruzzi
E.
Macci
C.
Doni
S.
Volpi
M.
Masciandaro
G.
2014
Organic matter and pollutants monitoring in reed bed systems for sludge stabilization: a case study
.
Environmental Science and Pollution Research
22
(
4
),
2447
2454
.
Roig
N.
Sierra
J.
Nadal
M.
Martí
E.
Navalón-Madrigal
P.
Schuhmacher
M.
Domingo
J. L.
2012
Relationship between pollutant content and ecotoxicity of sewage sludges from Spanish wastewater treatment plants
.
The Science of the Total Environment
425
(C)
,
99
109
.
Uggetti
E.
Ferrer
I.
Llorens
E.
García
J.
2010
Sludge treatment wetlands: A review on the state of the art
.
Bioresource Technology
101
(
9
),
2905
2912
.
Weng
H. -X.
Ma
X. -W.
Fu
F. -X.
Zhang
J. -J.
Liu
Z.
Tian
L. -X.
Liu
C.
2014
Transformation of heavy metal speciation during sludge drying: mechanistic insights
.
Journal of Hazardous Materials
265
,
96
103
.
Working Document on Sludge 3rd draft
2000
(
accessed 9 October 2014
).
Yeomans
J. C.
Bremner
J. M.
1988
A rapid and precise method for routine determination of organic carbon in soil
.
Communications in Soil Science and Plant Analysis
19
,
1467
1476
.