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Cocomposting of Sludge From Olive Oil Mill Wastewater Mixed With Tree Cuttings

Posted on: Friday, 7 October 2005, 03:01 CDT

By Plaza, C; Senesi, N; Brunetti, G; Mondelli, D

The sludge residue from olive oil mill wastewater (OMW) evaporated in natural conditions in an open-air lagoon was cocomposted with tree cuttings (TC) in two mixtures, 48% of OMW sludge + 52% of TC (M1, C/N ratio, 30), and 58% of OMW sludge + 42% of TC (M2, C/N ratio, 26). The evolution and modification of properties occurring in the OMW sludge-TC mixtures at different stages of the composting process were evaluated by chemical analyses, including pH, electrical conductivity and contents of total organic C, total N, total extractable C and humified C fractions. Further, HAs were isolated from the mixtures by a conventional procedure based on alkaline extraction, acidic precipitation to pH 1, purification by repeated alkaline dissolutions and acidic precipitations, water washing, dialysis, and final freeze-drying. The HAs obtained were analyzed for elemental (C, H, N, S, O) and acidic functional group (carboxylic and phenolic) composition, and by ultraviolet/visible, Fourier transform infrared, fluorescence and electron spin resonance spectroscopies. Composting of the OMW sludge-TC mixtures resulted in an increase of pH and total N, ash and humified C contents, whereas electrical conductivity and total organic C, total extractable C and NH^sub 4^^sup +^-N contents and C/N ratio tended to diminish. At the initial stage of composting, HAs from the OMW sludge-TC mixtures were characterized by a marked aliphatic character, small O and acidic functional group contents, marked presence of proteinaceous materials and partially modified lignin moieties and polysaccharides- like structures, extended molecular heterogeneity, low organic free radical contents and small degrees of aromatic ring polycondensation, polymerization and humification. With increasing the composting time, a loss of aliphatic materials and carbohydrates, and an increase of oxygenation, acidic functional groups, S- and N-containing groups and aromaticity occurred in HA fractions. Chemical and physicochemical analyses of the two OMW sludge-TC mixtures and their HA components at the end of composting indicated that an adequate degree of maturity and stability was achieved by both end products, and especially for the one obtained from mixture M2.

Introduction

Olive oil mill extraction is an important food industry in the Mediterranean countries, which traditionally employs a three-phase centrifugation system that generates a solid by-product used for further oil extraction by use of solvents, and olive oil mill wastewater (OMW). The OMW is an aqueous, dark, foulsmelling and turbid liquid, which has a large organic load whose inappropriate disposal may lead to serious environmental pollution problems. In particular, uncontrolled application of excessive amounts of OMW to soil can generate, among others, an overload of salts and phytotoxic compounds (e.g., polyphenols and fatty acids), inhibition of soil microbial activity, alterations of soil pH, emission of bad- smelling compounds, and leaching of nutrients, which may also represent a risk for groundwater contamination (Paredes et al. 1999; Sierra et al. 2001; Paredes et al. 2002; Garcia-Gomez et al. 2003).

In the year 2002, a biannual European-Commissionsupported CRAFT Project named SOLARDIST was initiated with the aim of developing a new technology for the transformation and recycling of OMW produced in small and medium size olive oil mills. The proposed technology is based on the evaporation and condensation of about 70% of the OMW in a solar distillation unit, followed by an extended biological treatment of the condensate in a constructed wetland that could yield water suitable for irrigation use in agriculture. The residue of evaporation consists of a sludge rich in organic matter and plant nutrients which could be recycled usefully as soil amendment.

Application of organic matter that is not sufficiently mature and stable may adversely affect soil properties, and especially the content and quality of native soil organic matter pools (Senesi et al. 1996; Plaza et al. 2002 and 2003). Thus, the OMW sludge should be subjected to appropriate treatments before its application to soil, in order to transform its organic matter into stabilized forms so to enhance its potential as a soil organic amendment. Cocomposting of the OMW sludge mixed with ligno-cellulosic solid wastes, which allows a substrate to be obtained with adequate C/N ratio and physical structure, is expected to be a promising treatment for this purpose.

Composting of organic materials consists of a controlled biological transformation of organic matter operated by aerobic microorganisms which leads to an extended mineralization to carbon dioxide, ammonia and water, and to the transformation of the residual organic matter into stabilized, refractory organic materials, the so-called "humic-like" substances that chemically resemble native soil humic substances (Senesi et al. 1996; Par et al. 1998). Humic substances, and especially their humic acid (HA) and fulvic acid (FA) fractions, are the most important components of soil organic matter responsible for soil fertility and soil protection from degradation (Stevenson 1994). As a consequence, the amount and quality of the HA-like fraction in a composted material are believed to be among the most reliable indicators of the stability and maturity achieved by the organic material during and after its composting process, and of primary importance for the agronomic efficacy, environmental safety and economic value of the compost (Senesi et al. 1996).

The objective of this preliminary study was to evaluate the chemical changes during composting of two different mixtures of an OMW sludge and tree cuttings, and of the HA fractions isolated from these mixtures.

Materials and Methods

Substrates and Procedure of Composting

A sample of OMW sludge was collected in the Island of Crete (Greece) from the bottom of an open-air lagoon where OMW were allowed to evaporate in natural conditions. Tree cuttings (TC) were provided by the Tersan Puglia composting facility operating in Modugno (Bari, Italy). The main chemical properties of the OMW sludge and TC are shown in Table 1.

Two different mixtures of OMW sludge and TC were prepared using the following proportions on a dry weight basis: Ml, 48% of OMW sludge and 52% of TC; and M2, 58% of OMW sludge and 42% of TC. The amounts of OMW sludge and TC in the mixture Ml were selected in order to obtain an optimal value of the C/N ratio (i.e., 30) and an adequate physical structure for composting, whereas the amount of OMW sludge in the mixture M2 was increased with respect to TC in order to recycle as much sludge as possible (C/N ratio, 26).

TABLE 1.

Principal characteristics ( standard deviations) of the olive oil mill wastewater (OMW) sludge and tree cuttings (TC) used for preparing composting mixtures.

Mixtures Ml and M2 were composted in trapezoidal waste piles (1 m high with a 1 2 m base) at the Tersan Puglia composting facility. In the first phase of composting, the piles were turned periodically so that temperature never was > 60 C. In the second phase, when the temperature was naturally lower, the piles were turned once a day. The active phase of composting was considered completed when the temperature of the piles reached an almost constant value close to the ambient temperature, which occurred after about 80 days of composting. Pile turning was then stopped, and the mixtures allowed to stand for about one additional month in order to achieve further stabilization.

The moisture content of the piles was monitored daily during the entire active phase of composting, and corrected by adding an amount of water which allowed a moisture content ranging between 50 and 60%. After 13, 28, 56, 88 and 118 days of composting, five subsamples were collected randomly from five sites of each pile, spanning the whole profile (from the top to the bottom of the pile). A composite sample was then prepared for each pile by mixing equal amounts of the five corresponding subsamples. The composite samples are expected to be representative of the whole mixtures due to the relatively small size of the piles, relatively large amount and number of subsamples collected, and apparent uniformity of the mixtures, which was improved further by pile turning and mixing before sampling.

Analyses of Mixtures

Prior to analyses, samples of OMW sludge-TC mixtures collected from the two piles during and at the end of the composting process were air-dried and crushed to pass through a 0.5-mm sieve. The principal chemical properties of the samples were determined by conventional methods in triplicate analysis of each sample as follows: (a) ash content was measured after heating the samples for 1 night at 550 C; (b) the pH was measured at a sample:water ratio of 1:10; (c) electrical conductivity (EC) was measured on water extracts obtained at a sample:water ratio of 1:10; (d) total organic carbon (TOC) was determined by dichromate oxidation and subsequent titration with ferrous ammonium sulphate; (e) total N content was obtained by the Kjeldahl method; (f) NH^sub 4^ +-N content was determined in the solution obtained by steam distillation of the 1 M KCl extracts of samples using a sample:extractant ratio of 1:10; (g) total extractable carbon (TEC), humified carbon (HA + FA) and nonhumified carbon (NH) were dete\rmined by a procedure based on the extraction of the sample with 0.1 M NaOH and 0.1 M Na P O?, acidic precipitation to pH<2, fractionation on solid polyvinylpyrrolidone, dichromate oxidation and subsequent titration with ferrous ammonium sulphate (Ciavatta et al. 1988). The degree of humification (DH) was calculated as DH% = 100 x (HA + FA) / TEC; the humification rate (HR) as HR% = 100 x (HA + FA) / TOC; and the humification index (HI) as HI = NH / (HA + FA) (Sequi et al. 1986; Ciavatta et al. 1988).

Isolation of Humic Acids

The HAs were isolated from the mixtures Ml and M2 sampled during and at the end of the composting process using a conventional procedure (Schnitzer 1982). Briefly, air-dried and 0.5-mm sieved samples were extracted by a solution of 0.1 M Na P O and 0.5 M NaOH using a sample:extractant ratio of 1:20. The mixtures were shaken mechanically under N gas in capped plastic bottles for 24 h at room temperature (RT, 293 + 2 K). The alkaline supernatant solutions were then separated from the residues by centrifugation at 9,600 g for 30 min. The extraction procedure was repeated three times on the residues, which were finally discarded. The combined alkaline supernatants were acidified with 6 M HCl to pH 1, allowed to stand for 24 h in a refrigerator to permit coagulation of the HA fraction, and then centrifuged at 30,400 g for 20 min. The HA precipitates were purified by dissolution in a minimal volume of 0.3 M KCl and 0.2 M KOH under N gas, centrifugation at 30,400 g for 20 min to remove the residues, and acidification of the alkaline supernatants with 6 M HCl to a pH 1. The suspensions were left standing for 24 h at RT, and then centrifuged at 30,400 g for 20 min. The purifications steps were repeated three times. The precipitated HAs were then recovered with distilled water, dialyzed until free of Cl" ions, freeze-dried, and stored at RT in plastic vials placed in a desiccator containing P^sub 2^O^sub 5^.

Humic Acid Analyses

Moisture content of HAs was measured by heating overnight at 105 C, and the ash content by heating overnight at 550 C. The C, H, N, and S contents were determined in triplicate using a Fisons Instruments (Crawley, UK) elemental analyzer model EA 1108. Oxygen content was calculated by difference: 0% = 100 - (C + H + N + S)%. Total acidity was determined by the Ba(OH) method, carboxyl group content by the Ca(CH COO) method, and phenolic hydroxyl group content was calculated by difference (Perdue 1985).

The absorbances at 465 nm and 665 nm were measured on solutions of 3.0 mg of each HA in 10 mL of 0.05 M NaHCO , with pH adjusted to 8.3 with 0.02 M NaOH, using a Perkin Elmer (Norwalk, Connecticut) Lambda 15 UV/Vis spectrophotometer (Chen et al. 1977). The ratio of absorbances at 465 and 665 nm gave the E /E ratio.

The Fourier transform infrared (FT IR) spectra of HAs were recorded over the range 4000 to 400 cm" on pellets obtained by pressing under reduced pressure a mixture of 1 mg of FA and 400 mg of dried KBr, spectrometry grade. A Nicolet (Madison, Wisconsin) 5PC FT IR spectrophotometer operating with a peak resolution of 2 cm" and Omnic 1.2 software were used to obtain the spectra.

Fluorescence spectra in the emission, excitation and synchronous- scan modes were obtained on aqueous solutions of 100 mg L HA after overnight equilibration at RT, and adjustment to pH 8 with 0.05 M NaOH. Spectra were recorded using a Hitachi Model F-4500 fluorescence spectrophotometer equipped with a F4500 system software for data processing. Emission and excitation slits were set at a 5- nm band width, and a scan speed of 240 nm min"1 was selected for both monochromators. Emission spectra were recorded over the range 380 to 550 nm at a constant excitation wavelength of 360 nm. The overall relative fluorescence intensity (RFI) was expressed in arbitrary units as the unitless reciprocal to the gain used to normalize each emission spectrum (Senesi et al. 1991). Excitation spectra were obtained over a scan range of 300 to 500 nm by measuring the emission radiation at a fixed wavelength of 520 nm. Synchronous-scan excitation spectra were measured by scanning simultaneously both the excitation (varied from 300 to 550 nm) and the emission wavelengths, while maintaining a constant, optimized wavelength difference &955; = λ^sub exc^ -λ^sub em^ =18 nm (Senesi et al. 1991). exc em

The electron spin resonance (ESR) spectra were recorded at RT on solid HA samples packed in quartz ESR tubes (4 mm. o.d., 3 mm i.d.) by a Bruker ER-200D SRC ESR spectrophotometer operating at X-band frequency with 100 KHz magnetic field modulation. The magnetic field range scanned was 10 mT, and centered at about the resonance field of the free electron. A modulation amplitude of 0.63 mT, a microwave frequency of 9.52 GHz, and a microwave attenuation of 13 dB (corresponding to a microwave power of about 10 mW) were used. The absolute free radical concentration of organic free radicals was calculated using conventional procedures and equations, and expressed in spins g^sup -1^ (Senesi 1996).

Results and Discussion

Sludge-Tree Cutting Mixtures

Table 2 shows the main chemical characteristics of the two OMW sludge-TC mixtures Ml and M2 sampled after 13, 28, 56, 88 and 118 days of composting. After 13 days, the mixture Ml, with respect to the mixture M2, has lower pH, similar electrical conductivity, lower total N, NH^sub 4^^sup +^-N and ash contents and higher total organic C content and C/N ratio. As composting proceeds, the pH values in the two mixtures first increase reaching the largest values after 28 days, and then tend to slightly diminish. Further, with increasing the composting time, total N and ash contents increase in both mixtures, whereas C/N ratio and TOC content (Figure 1) decrease. Values of EC and NH^sub 4^^sup +^-N show a marked decrease after 28 days, and then decrease slightly or remain almost constant. At the end of composting, the mixture Ml shows the same pH and EC values, slightly smaller total N and NH^sub 4^ +-N contents, smaller ash content, larger TOC content and slightly greater C/N ratio, as compared to the mixture M2.

TABLE 2.

Some chemical properties ( standard deviations) of the mixtures of olive oil mill wastewater (OMW) sludge and tree cutting (TC) at different stages of composting.

FIGURE 1. Total organic C (TOC) contents of the olive oil mill wastewater (OMW) sludge-tree cutting (TC) mixtures with either 48 or 58% of OMW sludge (Ml and M2, respectively) at different stages of composting. Bars represent standard deviations.

The pH increase with composting time may be feasibly ascribed to mineralization of proteinaceous materials to alkaline ammonia, whereas the EC decrease could be attributed to loss by leaching and/ or microbial immobilization, i.e., incorporation into cell biomass, of soluble salts, and/or to formation of insoluble salts. The total N content increase and decrease of C/N ratio may be ascribed to mineralization of C-rich materials (Bernai et al. 1996; Paredes et al. 2000; Charest and Beauchamp 2002; Paredes et al. 2002). However, N-fixing bacteria may also contribute to this effect during the mesophilic phase of composting (De Bertoldi et al. 1982; Charest and Beauchamp 2002). The decrease of N-NH^sub 4^ + content may be mostly attributed to its loss by volatilization due to the increased pH of the composting mixtures, and, secondarily, to nitrification and subsequent nitrate leaching.

Despite the similar amounts initially present in the two mixtures (about 530 g kg ), the larger TOC content measured after 13 days in the mixture Ml with respect to mixture M2 suggests the occurrence of less extended mineralization processes in the Ml mixture at the initial stage of composting. This effect may be feasibly ascribed to the smaller amount of easily biodegradable compounds derived from the OMW sludge, and larger content of organic components recalcitrant to microbial degradation, such as lignin present in the TC material. The apparent ash content increase is feasibly due to organic matter mineralization and loss during composting, whereas the C/N ratio decrease reflects the loss of organic C and related enrichment of N. The decrease of C/N ratios achieved after 118 days of composting (14 for Ml and 12 for M2) indicate an advanced degree of stabilization and maturity of the residual organic matter in the end products, and especially in mixture M2 (Zucconi and De Bertoldi 1987; Senesi 1989).

After 13 days of composting, the mixture M1, in comparison to the mixture M2, has larger TEC and (HA + FA) C contents, slightly larger DH and HR values, and slightly smaller NH content and HI (Table 3). However, the relatively large (HA + FA) C contents measured in both mixtures after 13 days of composting, which results in unlikely values of DH, HR and HI for this type of material at so early stage of composting, may be ascribed to an overestimation of these parameters, possibly due to coprecipitation in the HA fraction of not or incompletely humified organic matter, such as lignin residues (Senesi 1989; Snchez-Monedero et al. 1999).

After 28 days of composting, TEC and (HA + FA) C contents and DH and HR values of both mixtures decrease markedly, whereas NH content and HI value increase. After 56 days of composting, a further, slight decrease of TEC, and (HA +FA) is measured, and then they tend to slightly increase. The values of DH and HR tend to slightly increase and those of NH content and HI to decrease since after 28 days of composting. These results suggest that composting leads to a slow stabilization of fresh organic matter by increasing the relative contents of humic-like substances in the mixtures. At the end of the composting process, i.e., after 118 days, the mixture Ml shows slightly larger TEC and (HA+FA) C contents and HI and slightly smaller DH and HR, as compared to mixture M2. These results would indicate an extended degre\e of maturity achieved by organic matter in both mixtures, especially in M2 (Sequi et al. 1986)

TABLE 3.

Total extractable C (TEC), humified C (HA + FA) (+ standard deviations) and nonhumified C (NH) contents, and humification parameters (DH, degree of humification; HR, humification rate; and HI, humification index) of the olive oil mill wastewater (OMW) sludge-tree cutting (TC) mixtures at different stages of composting

Humic Acids: Elemental and Functional Group Composition and E^sub 4^/E^sub 4^ Ratio

The elemental composition and related atomic ratios of HAs isolated from the mixtures Ml and M2 sampled during the composting process are shown in Table 4. The elemental composition of the corresponding HAs of the two series is similar, with the exception of the larger values of N and smaller values of C/N of samples M2- HAs, with respect to the corresponding samples Ml-HAs. In both series, the composting process causes an increase of N, S and O contents and C/H and O/C ratios, and a decrease of C and H contents and C/N ratio of the HAs isolated from the two mixtures. These results are in general agreement with previous findings on similar systems (Miikki et al.,1997; Snchez-Monedero et al. 2002) in indicating that, with increasing composting time, the HAs experience a relative enrichment of N-containing compounds, probably proteinaceous materials incorporated into HA, and increased oxidation and content of unsaturated structures with respect to saturated structures (smaller C/H ratios).

TABLE 4.

Elemental composition ( standard deviations) and atomic ratios of humic acids (HAs) isolated from olive oil mill wastewater (OMW) sludge-tree cutting (TC) mixtures at different stages of composting.

The COOH group contents of Ml-HAs are similar or slightly smaller and those of total acidity and phenolic OH groups are markedly smaller than the corresponding values of M2-HAs (Table 5). With increasing the composting time, the acidic functional group contents of HAs in both mixtures tend to increase, which may be related to the oxidation of methoxyl and alcoholic groups of lignin residues and/or to microbial degradation of carbohydrates (Snchez-Monedero et al. 2002).

The E^sub 4^/E^sub 4^ ratios of Ml-HAs are similar or slightly smaller than the corresponding values of M2-HAs, and tend to increase gradually with composting time in both HA series (Table 5). In agreement with Chen et al (1977), the values of E^sub 4^/E^sub 6^ ratio of HAs are correlated positively with total acidity and negatively with C content.

TABLE 5.

Acidic functional group contents ( standard deviations), E^sub 4^/ E^sub 6^ ratios, relative fluorescence intensity (RFI) and concentration of organic free radicals (OFR) of humic acids (HAs) isolated from olive oil mill wastewater (OMW) sludge-tree cutting (TC) mixtures at different stages of composting.

Humic Acids: FT IR Spectra

The FT IR spectra of HAs isolated from the two OMW sludge-TC mixtures after 13, 28, 56, 88 and 118 days of composting are shown in Figures 2 and 3, respectively for the Ml and M2 series. The main features of these spectra, and their corresponding assignments (Bellamy 1975; MacCarthy and Rice 1985; Stevenson 1994) are the following: (a) an intense broad band at about 3390 cm" of similar intensity for all HAs, which is usually attributed to O-H stretching and, secondarily, to N-H stretching of various functional groups; (b) two sharp bands at about 2930 cm^sup -1^ and 2850 cm^sup -1^ due to aliphatic C-H group stretching, whose relative intensity tends to decrease significantly with composting time, and is slightly stronger for Ml-HAs than for the corresponding M2-HAs; (c) an absorption at about 1710 cm^sup -1^ due to C=O stretching of COOH and other carbonyl groups, which is very intense and sharp in HAs isolated after 13-day composting from both mixtures, and reduces to a shoulder at later stages; (d) a band at about 1650 cm"1 generally attributed to absorptions of several groups including aromatic C=C, C=O stretching of amide groups (amide I band), quinonic C=O and/or C=O of ?-bonded conjugated ketones, which tends to become slightly more intense with composting time; (e) a peak at about 1540 cm"1 preferentially ascribed to N-H deformation and C=N stretching of amides (amide II band), whose relative intensity does not change during composting; (f ) a peak at about 1460 cm^sup -1^ attributed to aliphatic C-H, whose relative intensity tends to decrease with composting time; (g) a faint absorption at about 1420 cm^sup -1^ preferentially assigned to O-H deformation and C-O stretching of phenolic OH; (h) a weak absorption at about 1380 cm" possibly due to C-H deformation of CH^sub 2^ and CH^sub 3^ groups and/or to antisymmetric stretching of COO groups; (i) a broad band of mediumstrong intensity at about 1230 cm"1 of similar intensity for all HAs, which is generally ascribed to C-O stretching and O-H deformation of carboxyls and C-O stretching of aryl ethers; and (j) an absorption at about 1040 cm" , generally attributed to C-O stretching of polysaccharides or polysaccharide-like substances and Si-O of silicate impurities, whose relative intensity tends to decrease slightly with composting time.

FIGURE 2. Fourier transform infrared (FT IR) spectra of humic acids isolated from the olive oil mill wastewater (OMW) sludge (48%)- tree cutting (TC) (52%) mixtures sampled after 13, 28, 56, 88 and 118 days of composting (MI-13, MI-28, MI-56, MI-88 and MI-118, respectively).

FIGURE 3. Fourier transform infrared (FT IR) spectra of humic acids isolated from the olive oil mill wastewater (OMW) sludge (58%)- tree cutting (TC) (42%) mixtures sampled after 13, 28, 56, 88 and 118 days of composting (M2-13, M2-28, M2-56, M2-88 and M2-118, respectively).

A comparison of the FT IR spectra described above with typical IR spectra of HAs from soils and other sources (Stevenson and Goh 1971) indicates that at early stages of composting they are generally similar to IR spectra typical of HAs formed in systems that contain excessive moisture or are poorly aerated. This result may be attributable to the anaerobic conditions under which the HA fraction derived from the OMW sludge are formed. However, as composting proceeds, they tend to resemble progressively IR spectra typical of soil HAs.

In general, results provided by FT IR spectroscopy support and complement those of elemental and functional group analyses discussed previously in confirming that, with increasing composting time, the presence of aliphatic materials and carbohydrates decreases, whereas aromaticity and N-containing groups, probably stabilized proteinaceous materials, increase. Further, FT IR results suggest a slightly larger aliphatic character of Ml-HAs with respect to the corresponding M2-HAs.

Humic Acids: Fluorescence Spectra

The RFI values of M1-HAs are almost similar to those of the corresponding M2-HAs, and tend to increase gradually with composting time in both HA series (Table 5). The fluorescence spectra in the emission, excitation and synchronous scan modes of the HAs examined are shown in Figures 4, 5 and 6, respectively. The emission spectra of all HAs are almost featureless showing a relatively sharp increase of RFI with increasing wavelength up to about 480 nm, and then a slight increase up to about 530 nm. The fluorescence excitation spectra of all HAs are characterized by a major peak in the intermediate-wavelength region (at about 440 nm), whose relative intensity increases with increasing composting time, with respect to a shoulder extending at shorter wavelengths (at about 395 nm). The synchronous scan spectra of all HAs feature an intense peak in the long-wavelength region (at about 510 nm) and a small peak and two faint shoulders at shorter wavelengths (at about 330, 400 and 460 nm, respectively), whose relative intensity decreases with composting time.

FIGURE 4. Fluorescence emission spectra of humic acids isolated from the olive oil mill wastewater (OMW) sludge -tree cutting (TC) mixtures with either 48 or 58% of OMW sludge sampled after 13, 28, 56, 88 and 118 days of composting (MI-13, MI-28, MI-56, MI-88 and MI- 118, and M2-13, M2-28, M2-56, M2-88 and M2-118, respectively).

FIGURE 5. Fluorescence excitation spectra ot humic acids isolated from the olive oil mill wastewater (OMW) sludge-tree cutting (TC) mixtures with either 48 or 58% of OMW sludge sampled after 13, 28, 56, 88 and 118 days of composting (M1-13, M1-28, M1-56, MI-88 and M1- 118, and M2-13, M2-28, M2-56, M2-88 and M2-1 18, respectively).

FIGURE 6. Fluorescence synchronous scan spectra of humic acids isolated from the olive oil mill wastewater (OMW) sludge-tree cutting (TC) mixtures with either 48 or 58% of OMW sludge sampled after 13, 28, 56, 88 and 118 days of composting (M1-13, M1-28, M1- 56, M1-88 and M1-118, and M2-13, M2-28, M2-56, M2-88 and M2-118, respectively).

In agreement with literature data (Senesi et al. 1991), these results suggest a decrease of molecular heterogeneity and an increase of molecular size, aromatic polycondensation, level of conjugated chromophores and humification degree of HAs as composting time increases.

Humic Acids: ESR Analysis

The ESR spectra of all HA samples (not shown) exhibit a sharp and narrow single-line resonance centered at about the resonance field of the free electron. This signal is typical of HAs of any nature and origin, and is attributed to indigenous organic free radicals of semiquinone nature conjugated with an extended aromatic network (Senesi 1990). The concentration of organic free radicals (OFR), which can be calculated from the intensity of this signal, is generally related positively with the aromatic polycondensation and polymerization and humification degree of HAs (Senesi 1990).

Data in Table 5 indicate that the concentrations of OFR of M1- HAs are similar or slightly smaller than those of the corresponding M2-HAs, and increase sensibly during composting (T\able 5). These results confirm the increased aromatic polycondensation and polymerization and humification degree of HAs in the OMW sludge-TC mixtures with increasing composting time.

Conclusions

The biological transformation of OMW sludge-TC mixtures obtained by 118-days cocomposting in static piles turned and irrigated periodically results in an increase of pH and total N, ash and humified C contents, and a decrease of electrical conductivity, total organic C, total extractable C and NH^sub 4^^sup +^-N contents and C/N ratio. At the end of composting, the OMW sludge-TC mixture with 58% of OMW sludge, with respect to that with 48% of OMW sludge, feature a smaller total organic C content, larger ash content, slightly larger degree and rate of humification, and slightly smaller C/N ratio and humification index.

The molecular, structural and functional characterization by elemental and functional group analyses, and ultraviolet/visible, FT IR, fluorescence and ESR spectroscopies of HAs isolated from the OMW sludgeTC mixtures is proven to be an efficient approach for the evaluation of the changes achieved by the mixtures during composting. In particular, at the beginning of composting, HAs are characterized by a great aliphatic character, small O and acidic functional group contents, a marked presence of proteinaceous materials, partially modified lignin moieties and polysaccharides- like structures, extended molecular heterogeneity, low organic free radical contents and small degrees of aromatic ring polycondensation, polymerization and humification. With increasing the composting time, a loss of aliphatic materials and carbohydrates, and an increase in oxygenation, acidic fuctional groups, S- and N-containing groups and aromaticity occur, especially in HAs from the mixture with 58% of OMW sludge. These changes lead to the positive result that HAs in the final compost chemically and physicochemically resemble native soil HAs.

In conclusion, the cocomposting process here used represents a suitable treatment for transforming fresh organic matter in OMW sludge-TC mixtures into humified forms, i.e., humic-like substances, thus enhancing the potential of this material as a soil organic fertilizer, while minimizing its negative environmental impact. The OMW sludge-TC mixture with 58% of OMW sludge, in comparison to that containing 48% of OMW sludge, yields slightly better results in terms of HA quality and humification degree of the final product, while permitting to recycle larger amounts of OMW sludge.

Acknowledgements

This work has been conducted in the framework of the SOLARDIST- Project N.EVK1-CT-2002-30028 supported by the European Commission- CRAFT.

References

Bernal, M.P., A.F. Navarro, A. Roig, J. Cegarra and D. Garcia. 1996. Carbon and nitrogen transformation during composting of sweet sorghum bagasse. Biol. Fertil. Soils, 22: 141-148.

Bellamy, L.J. 1975. The infrared spectra of complex molecules. Chapman and Hall, London.

Ciavatta, C., L. Vittori Antisari and P. Sequi. 1988. A first approach to the characterization of the presence of humified materials in organic fertilizers. Agrochimica, 32: 510-517.

Charest, M.H. and C.J. Beauchamp. 2002. Composting of deinking paper sludge with poultry manure at three nitrogen levels using mechanical turning: behavior of physico-chemical parameters. Bioresour. Technol., 81: 7-17.

Chen, Y., N. Senesi and M. Schnitzer. 1977. Information provided on humic substances by E4/E6 ratios. Soil. Sd. Soc. Am. J., 41: 352- 358.

De Bertoldi, M., U. Citernesi and M. Griselli. 1982. Microbial population in compost process. In: The Staff of BioCyde (ed.). Composting: Theory and Practice for City, Industry and Farm. The JG Press, Pennsylvania, pp. 26-32.

Garca-Gmez, A., A. Roig and M.P. Bernal. 2003. Composting of the solid fraction of olive mill wastewater with olive leaves: organic matter degradation and biological activity. Bioresour. Technol., 86: 59-64.

MacCarthy, P. and J.A. Rice. 1985. Spectroscopic methods (other than NMR) for determining functionality in humic substances. In: Aiken, J.R., O.M. McKnight, R.L. Wershaw and P. MacCarthy (eds.). Humic Substances in Soil, Sediment and Water. Geochemistry, Isolation, and Characterization. Wiley-Interscience, New York, pp. 527-559.

Miikki, V., N. Senesi and K. Hnninen. 1997. Characterization of humic material formed by composting of domestic and industrial biowastes. Chemosphere, 34:1639-1651.

Par, T., H. Dinel, M. Schnitzer and S. Dumontet. 1998. Transformations of carbon and nitrogen during composting of animal manure and shredded paper. Biol. Fertil. Soils, 26:173-178.

Paredes, C., J. Cegarra, A. Roig, M.A. Snchez-Monedero and M.P. Bernal. 1999. Characterization of olive mill wastewater (alpechfn) and its sludge for agricultural purposes. Bioresour. Technol., 67:111-115.

Paredes, C., M.P. Bernal, J. Cegarra, and A. Roig. 2002. Biodegradation of olive mill wastewater sludge by its cocomposting with agricultural wastes. Bioresour. Technol., 85: 1-8.

Paredes, C., A. Roig, M.P. Bernai, M.A. Snchez-Monedero and J. Cegarra. 2000. Evolution of organic matter and nitrogen during cocomposting of olive mill wastewater with solid organic wastes. Biol. Fertil. Soils, 32: 222-227.

Perdue, E.M. 1985. Acidic functional groups of humic substances. In: Aiken, G.R., D.M. McKnight, R.L. Wershaw and P. MacCarthy (eds.). Humic Substances in Soil, Sediment and Water. Geochemistry, Isolation, and Characterization. Wiley-Interscience, New York, 493- 536.

Plaza, C., N. Senesi, J.C. Garcia-Gil, G. Brunetti, V. D'Orazio and A. Polo. 2002. Effects of pig slurry application on soils and soil humic acids. J. Agric. Food Chem., 50: 4867-4874.

Plaza, C., N. Senesi, A. Polo, G. Brunetti, J.C. Garcia-Gil and V. D'Orazio. 2003. Soil fulvic acid properties as a means to assess the use of pig slurry amendment. Soil Tillage Res., 74: 179-190.

Snchez-Monedero, M.A., A. Roig, J. Cegarra and M.P. Bernai. 1999. Relationships between water-soluble carbohydrate and phenol fractions and the humification indices of different organic wastes during composting. Bioresour. Technol., 70:193-201.

Snchez-Monedero, M. A., J. Cegarra, D. Garcia and A. Roig. 2002. Chemical and structural evolution of humic acids during waste composting. Biodegradation, 13: 361-371.

Schnitzer, M. 1982. Organic matter characterization. In: Page, B.L., R.H. Miller and D.R. Keeney (eds.). Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties, 2nd ed. Agronomy Monograph No. 9. Soil Science Society of America, Madison, WI, pp. 581-594.

Senesi, N. 1989. Composted materials as organic fertilizers. Sci. Total Environ., 81/82: 521-542.

Senesi, N. 1990. Application of electron spin resonance (ESR) spectroscopy in soil chemistry. In: Stewart, B.A. (ed.). Advances in Soil Science, Vol. 14. Springer-Verlag, New York, pp. 77-130.

Senesi, N. 1996. Electron spin (or paramagnetic) resonance spectroscopy. In: Sparks, D.L. (ed.). Methods of Soil Analysis: Chemical Methods. ASA, CSSA, SSSA Publ., Madison, pp. 323-356.

Senesi, N., T.M. Miano, M.R. Provenzano and G. Brunetti. 1991. Characterization, differentiation, and classification of humic substances by fluorescence spectroscopy. Soil Sci., 152: 259-271.

Senesi, N., T.M. Miano and G. Brunetti. 1996. Humic-like substances in organic amendments and effects on native soil humic substances. In: Piccolo, A. (ed.). Humic Substances in Terrestrial Ecosystems. Elsevier, New York, pp. 531-593.

Sequi, P., M. De Nobili, L. Leita and G. Cercignani. 1986. A new index of humification. Agrochimica, 30: 175-179.

Sierra, J., E. Marti, G. Montserrat, R. Cruanas and M.A. Garau. 2001. Characterisation and evolution of a soil affected by olive oil mill wastewater disposal. Sci. Total Environ., 279: 207-214.

Stevenson, F.J. 1994. Humus Chemistry: Genesis, Composition, Reactions. Wiley-Interscience, New York.

Stevenson, F.J., and K.M. Goh. 1971. Infrared spectra of humic acids and related substances. Geochim. Cosmochim. Acte, 35: 471- 483.

Zucconi, F, and M. De Bertoldi. 1987. Compost specifications for the production and characterization of compost from municipal solid waste. In: De Bertoldi, M., P. Ferranti, P. L'Hermite and F.Zucconi, (eds.). Compost: Production, Quality and Use. Elsevier, London, pp. 20-29.

C. Plaza1, N. Senesi2, G. Brunetti2 and D. Mondelli2

1. Centra de Ciencias Medioambientales, CSIC, Madrid, Spain

2. Dipartimento di Biologia e Chimica Agroforestale ed Ambientale, University of Ban, Ban, Italy

Copyright J.G. Press Inc. Summer 2005

Source: Compost Science & Utilization

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