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Biodegradation of organic compounds during cocomposting of olive oil mill waste and municipal solid waste with added rock phosphate a




Farid Barje , Loubna El Fels , Houda El Hajjouji , Peter Winterton & Mohamed Hafidi



Laboratoire Ecologie and Environnement (Unité associée au CNRST, URAC 32; Unité associée au CNERS), Département de Biologie, Faculté des Sciences Semlalia, Université Cadi Ayyad, Marrakech, Morocco b

Département de Langues & Gestion, Université Paul Sabatier (Toulouse III), Toulouse, France Accepted author version posted online: 17 Apr 2013.Published online: 08 May 2013.

To cite this article: Farid Barje, Loubna El Fels, Houda El Hajjouji, Peter Winterton & Mohamed Hafidi (2013) Biodegradation of organic compounds during co-composting of olive oil mill waste and municipal solid waste with added rock phosphate, Environmental Technology, 34:21, 2965-2975, DOI: 10.1080/09593330.2013.796009 To link to this article:

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Environmental Technology, 2013 Vol. 34, No. 21, 2965–2975,

Biodegradation of organic compounds during co-composting of olive oil mill waste and municipal solid waste with added rock phosphate Farid Barjea , Loubna El Felsa , Houda El Hajjoujia , Peter Wintertonb and Mohamed Hafidia∗ a Laboratoire

Ecologie and Environnement (Unité associée au CNRST, URAC 32; Unité associée au CNERS), Département de Biologie, Faculté des Sciences Semlalia, Université Cadi Ayyad, Marrakech, Morocco; b Département de Langues & Gestion, Université Paul Sabatier (Toulouse III), Toulouse, France

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(Received 8 April 2012; final version received 28 March 2013 ) Liquid and solid olive oil mill waste was treated by composting in a mixture with the organic part of municipal solid waste and rock phosphate. The transformations that occurred during the process were evaluated by physical, chemical and spectroscopic − analyses. After five months of composting, the final compost presented a C/N ratio under 20, an NH+ 4 /NO3 ratio under 1 and a pH around neutral. A high level of organic matter decomposition paralleled a notable abatement of phenols and lipids. The results show the effective dissolution of mineral elements during composting. This transformation was followed by Fourier transform infrared which showed a decrease in the absorption bands of aliphatic bonds (2925 and 2855 cm−1 ) and carbonyls of carboxylic origin (1740 cm−1 ). In addition to the increase in humic substances and the improvement of germination indices, the parameters studied confirm the stability and the maturity of the composts. The absence of phytotoxicity opens the way to agricultural spreading. Keywords: composting; olive oil mill waste; municipal solid waste; rock phosphate; organic compounds; mineral elements

1. Introduction The world production of olive mill effluents is estimated at 30 million tonnes/year on average. Their high toxicity has made finding depollution strategies for this agro-food waste a priority that is both national and international. For more than half a century, attempts to deal with the effluent have been of many kinds.[1] As yet, irrespective of the country considered worldwide, no economically viable processes have been developed for full-scale treatment. Indeed, the effluents contain compounds that impede conventional biological treatment (activated sludge, anaerobic digestion).[2] The high levels of organic compounds and minerals as well as water in the effluent make its moderate and controlled use as fertilizer a possible means for its depollution and recycling. However, this strategy is still a subject of much debate. Direct spreading of OMW (olive oil mill waste) at strictly controlled rates is, in other Mediterranean countries, still being experimented.[3,4] However, in spite of its potential as a fertilizer, olive waste contains high levels of toxic phenolics, decreasing its biodegradablility and classifying the waste as a highly polluting refractory substance.[5] During the grinding of the olive pulp, the enzymatic hydrolysis of esters produces a considerable quantity (3–8 g/L) of free phenolics such as gallic acid, veratric acid, cinnamic acid, vanillic acid, caffeic acid and tyrosol. However, the list continues with other problem molecules

such as oleuropein, luteoline-7 glucoside, 1-caffeoylglucose, apigenin, quercetin, cyanidin, etc.[6] The antimicrobial action and the phytotoxic effect, mainly attributed to the liquid olive mill effluent (olive water), partially or totally inhibit seed germination and the growth of plants and microorganisms,[7–9]. These effluents can damage membranes and denature cell proteins. They hinder enzyme activity and promote the precipitation of nutritional proteins.[10,11] The free fatty acids that are also present exacerbate the inhibitory action of the phenolics. In addition, neutralizing the acid pH of the effluent is a further constraint.[12] Several studies have been undertaken concerning the aerobic treatment and pretreatment of olive water using microorganisms such as basidiomycetes,[13] Pleurotus ostreatus [14] and Aspergillus niger [15] owing to their great ability to degrade phenolics. Toxicity tests have indicated a decrease in the toxicity of effluents from olive oil mills after treatment by these organisms. Aerobic microorganisms are well known to degrade organic compounds through oxidation using the oxygen in the air – they can also function with pure oxygen – and they are able to degrade most of the organic compounds present for their nutrition and reproduction.[16,17] Other authors have worked with mixed suspensions of microorganisms and have reported very high abatements in terms of chemical oxygen demand (COD) and polyphenols.[18,19] However, the abatements achieved are dependent on the

∗ Corresponding author. Email: hafi[email protected]; hafi[email protected] This article was originally published with erroneous pagination. This version has been corrected. Please see Erratum ( 10.1080/09593330.2013.869393).

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F. Barje et al.

strains selected. Some strains of bacteria and fungi have also been tested for the decolouration of olive mill effluent and the reduction of its toxicity; in particular, we can mention Baccilus pumilus [20]; Coriolus versicolor and Funalia troggi [21]; Phanerochaete chrysosporium [22] and Pleurotus ostreatus.[23] However, extrapolation of the results obtained to the industrial scale is limited by the investments required and the processes used have not really proved appropriate for the situation encountered in the field where many small mills are scattered over the whole territory. To overcome this problem, the solution of composting would be much more suitable to ensure the harmlessness of the final product with respect to the toxic compounds and with respect to its microbiological properties when fully mature.[24] Concerning organic municipal waste, to reduce its size and volume, composting of the biodegradable fraction has become a widely accepted approach compared with other disposal methods. Besides, to resolve the problem of phosphorus fixation in agricultural soils, soluble forms of phosphorus are expensive to apply. Thus, there is an increasing interest in using organic amendments and rock phosphate to increase plant growth and enhance plant phosphorus uptake. Extensive research has focused on understanding the composting process. However, few studies have investigated mineral dissolution during composting. It is in this context that the aim of the present work was to study the feasibility of composting mixtures of OMW with the organic part of municipal solid waste (MSW) supplemented with Moroccan rock phosphate. However, the evolution of the physicochemical parameters and characteristics of organic matter in decomposition are insufficient to estimate the degree of stability and maturity of composts, which considerably affect the development of plants.[25] A more relevant approach could be to proceed to a biological evaluation by germination and growth tests, to characterize the organic matter evolution. The study of seed germination in water-soluble extracts can thus be regarded as a sensitive indicator of phytotoxicity, stability and maturity of composts. In this objective, the maturity of the substratum was determined by monitoring the evolution of the water-soluble fraction and the description of possible phytotoxicity at different stages of composting. During the different stages of the composting process, the changes occurring in various parameters were monitored: physical-chemical parameters (e.g. tempera+ ture, C/N ratio, NO− 3 /NH4 ratio) and the organic fractions (phenols, lipids, lignins, humic substances). Besides, in most previous studies, organic matter evolution and the effects on phosphate dissolution were never analysed. The routine use of Fourier transform infrared (FT-IR) spectroscopy was tested to evaluate the degree of organic matter transformation and its relationship with mineral dissolution.

2. Materials and methods 2.1. Composting tests Composting trials were carried out with the organic matter piled into a windrow built on a slab. The mixtures contained pomace, i.e. the solid waste from olive oil mills (having a C/N ratio of 87 and containing 35% water), the organic part of MSW (mean C/N 12, water 67%) and liquid effluents from olive oil mills (C/N 15, water 84%). A mineral supplement was added in the form of crushed rock phosphate (30% P2 O5 , 70% limestone) to composts C2 and C3 to evaluate the effect of adding the mineral on the biodegradation of the organic compounds and on the overall composting process. The MSW was added to speed up the composting process and to dilute the toxic OMW water substances in the mixture. Also it increased the pore space while maintaining bulk density to allow sufficient oxygen to spread into the composting mixture. The mixtures were adjusted to have a C/N ratio of about 30; globally a C/N ratio of 30–35 in the raw materials is recommended as optimum for composting. Rock phosphate amendment was applied to the composting process in order to adjust the pH level and provide sufficient mineral nutrients. The initial composition of the three composted mixtures was as follows:

Pile No

Total initial weight (kg)

C1 C2 C3

1500 1400 1200

Pomace (kg)

Organic MSW (kg)

Olive water (litres)

Crushed rock phosphate (kg)

1140 1006 708

160 180 180

200 200 300

0 14 12

Optimization of the various factors is related to the composting conditions, including the level of humidity and how often the piles are aired by turning. The windrows of compost were aerated by mechanical turning for five months, (once a week for the first month then once a fortnight for the next four months). Aeration regulates the composting processes, achieving desirable temperature levels by removing heat and attaining the oxygen concentrations necessary for the aerobic microorganisms.[26] It also homogenizes the composting material while re-establishing porosity and increasing the active surface area. The temperature was measured (HI 8751 electronic thermometer recording range −40◦ to +150◦ C) at various points and depths in the windrow. The water content was also monitored each week to maintain it between 50% and 60%, especially during the first month when temperatures peaked at over 60◦ C. This has important implications for the physical properties and microbial activities of the compost. Each time the compost

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had been thoroughly mixed, a homogeneous sample of 4 kg was taken at different points and different levels from the windrow using the method of quartering. The samples were stored at −18◦ C until required for biochemical and physical-chemical analyses. 2.2. Physical–chemical analyses The pH and the electrical conductivity were measured on an aqueous extract of the compost (1 g/10 ml) as stipulated in French standard AFNOR NF-T 90-015.[27] Assaying the water-soluble carbon consisted of a cold oxidation by potassium bichromate in acid medium (H2 SO4 ); the excess bichromate was titrated by a solution of Mohr salt, in the presence of orthophosphoric acid (5 ml) and a few drops of the indicator (diphenylamine).[28] Total Kjeldahl nitrogen was assayed by steam distillation according to standard AFNOR T90-110. Similarly, ammonium ion content was assayed by alkaline distillation, and nitrates after reduction by Devarda alloying as reported in [28]. The mineral elements were determined by inductively coupled plasma atomic emission spectrometry – after digestion at 150◦ C of the organic matter (0.5 g) finely crushed in acid medium, by addition of HClO4 (35%) and HNO3 (65%). The level of ash was calculated after calcination in an oven at 600◦ C for 6 h. The proportion of decomposition was calculated using the following formula [29]: Decomposition(%) = 100 − 100 ×

Ashi × (100 − Ashf ) Ashf × (100 − Ashi )

where Ashi is the initial level of ash and Ashf is the final level. The polyphenols were extracted with methanol and purified by ethyl acetate following the Folin–Ciocalteu method.[30,31] Total lipids were extracted with methanol/chloroform (1/2 v/v) and assayed using the method of Folch.[32] The levels of lignin were determined on fresh 1 g samples using the method of the American National Standards Institute and American Society for Testing and Materials.[33] The proportion is given after acid hydrolysis of the carbohydrates according to the standard. After sulphuric acid digestion, the filtered sample was incinerated at 430◦ C to determine the percentage of organic matter and lignin. 2.3. Extraction and assay of humic substances A 15 g sample of fresh compost was defatted at 4◦ C using 120 ml chloroform/methanol (2:1). This operation was repeated three times, at which point the extract was clear. After filtration of the pooled sample, the residue was evaporated to dryness under nitrogen to eliminate the


solvents.[29] Then, the samples were extracted with 60 ml distilled water to remove the simple organic molecules soluble in water (sugars, proteins, etc.).[34] After shaking for 2 h, and centrifugation for 25 min at 5000 rpm, the supernatant was recovered. This step was repeated three times and the supernatants pooled to determine the level of total soluble carbon. The same protocol was applied to extract the humic substances but using 0.1 M NaOH instead of water. Again the extraction was repeated three times and the third extract obtained was clear. The humic acids (HAs) were separated from the total humic substances by acidification with 3 M HCl to reach a pH of 1. At this pH, the HAs form a precipitate while the fulvic acids (FAs) remain in solution. After being left to settle for 24 h at 4◦ C, centrifugation at 4000 rpm for 20 min left the FA in solution while the HA were recovered in the pellet. The pellet was then taken up again in a known volume of 0.1 M NaOH. The HA solution thus obtained was dialysed using a 1000 Da MWCO Spectra/Por membrane, and the mass of HA finally obtained was determined by weighing after lyophilization. 2.4. Germination test The test was based on the germination of seeds of a few significant plant species. Fifty seeds of oat (Avena sativa), lucerne (Medicago sativa) and tomato (Solanum lycopersicum) were germinated in water-soluble extracts of composts (1 g/100 ml) in the dark at ambient temperature (25◦ C) for 72 h. Three replicates were made for each sample. Phytotoxicity was assessed using the germination index (GI) which measures both germination and root growth,[35–37] and compares them with the values obtained in distilled water using the formula below: GI% =

(NGext × LR ext ) × 100, (NGwater × LR water )

where NGext and NGwater are the number of seeds germinated in the aqueous extract and in distilled water and LR ext and LR water are the lengths of the roots in the extract and in water. 2.5. Fourier transform infrared spectroscopy A quantity of 1.5 mg of homogenized composting material was compressed under vacuum with 250 mg of KBr. The pellets obtained were analysed with a Perkin–Elmer series 1600 FTIR spectrophotometer covering a frequency range of 4000–400 cm−1 . 2.6. Statistical analyses Means were compared with ANOVA post-hoc Tukey test. The correlations were studied with Pearson’s bilateral test. The linear adjustment was made at the 95% confidence interval.


F. Barje et al. Table 1. Compost C1


Stage (months)


Conductivity (ms/cm)

Water %

Ash %


− NH+ 4 /NO3

0 1 3 5 0 1 3 5 0 1 3 5

5.49 6.11 6.06 7.08 5.77 6.76 7.34 7.62 4.90 6.04 6.68 7.98

3.02 3.00 1.29 1.21 4.26 4.57 2.59 2.20 5.20 4.97 6.29 5.38

54.35 43.61 42.87 33.62 55.4 41.14 32.55 32.14 54.86 44.86 37.62 35.3

3.9 4.0 5.6 6.6 12.3 13.2 16.3 16.9 14.0 15.3 19.7 21.9

32.18 27.26 18.84 15.09 35.20 34.62 22.58 19.02 33.25 26.96 23.91 14.94

2.88 3.08 2.69 0.37 2.65 2.62 2.73 0.36 2.24 2.63 2.75 0.88

3. Results and discussion 3.1. Physical–chemical characterization Analysis of the various physical–chemical parameters (Table 1) at the different stages of composting provided information on the stabilization and the maturation or curing of the waste. The three trials all showed acceptable composting, with similar temperature patterns revealing a sequence of three stages in the composting (Figure 1) which depend on the microbiological activity, under the influence of physical–chemical and biological factors.[37] In the initial stage, mesophilic microorganisms are responsible for the decomposition of organic matter, and the heat generated increases the compost temperature. As the temperature rises in the thermophilic stage, reaching over 56◦ C and peaking at 62◦ C, the thermophilic microorganisms start to dominate, while the biodegradation of organic matter reaches the highest rate.[38,39] This in turn depends on the availability of nutrient, supporting high microbial activity.[40] Phase I

Phase II

Phase III


Temperature (°C)

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Physical–chemical parameters during the different stages of composting.












Time (months)

Figure 1. Temperature of the composts: pile C1 (◦), pile C2 (•), pile C3 () and ambient temperature () versus time. Phase I: mesophilic stage; Phase II: thermophilic stage; Phase III: cooling stage.

The decrease in the abundance of biodegradable substances brings about a decrease in microbial metabolism. When the rate of heat generation is lower than that of heat loss, a fall in temperatures occurs, leading to the cooling stage, when mesophilic microorganisms move the compost towards maturity.[41] The pH range of the initial mixtures was 4.9–5.5, i.e. slightly acid during the mesophilic stage; this is most likely due to the relatively high levels of organic acids, which reduce the pH. The pH then rises rapidly to become alkaline as the thermophilic stage begins. The organic acids are rapidly decomposed so the pH increases. The mineralization of organic nitrogen through microbial activities also contributed to this pH shift as ammonium was released. At the later maturation stage, the pH rose steadily to reach values near neutrality (7.08–7.98), indicating substrate alkalinization through reduction of the organic acids and release of exchangeable bases. Moreover, the increasing quantities of humic substances generated present a buffering effect, and could help to reduce the pH to neutral. The level of nitrogen showed a steady increase with respect to carbon, as seen through a remarkable fall in the C/N ratio from 35.2 to 14.9 due to carbon loss without nitrogen loss. This ratio is considered to act as a maturity index for composts; in the present study the ratio varied between 15 and 22 for the three trials, in agreement with ratios usually indicative of an acceptable degree of compost maturity.[42,43]

3.2. Changes occurring in the mineral fraction The mineralization of the organic matter leads to the formation of mineral forms of nitrogen, monitored by means − of the NH+ 4 /NO3 ratio, which presented low values during the maturation stage (0.37–0.88), providing information on the oxidation state of the substrates. The ammonium produced in the composting mixture can be transformed into oxidized forms via a nitrification reaction, especially after the temperature drops to less than 40◦ C at the maturing

Environmental Technology Table 2.

Evolution of the mineral elements in the water-soluble fraction during composting. Stages

Composts C1


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(months) 0 1 3 5 0 1 3 5 0 1 3 5





(mg/g dw) 0.753 0.537 0.374 0.365 2.144 0.707 1.451 2.307 1.940 1.190 1.609 2.731

10.91 8.74 5.71 6.90 7.17 7.75 4.76 4.63 8.51 7.71 9.47 8.47

0.37 0.31 0.71 0.55 4.61 4.38 2.60 2.46 5.42 4.94 6.16 5.51




(μg/g dw) 0.33 0.27 0.23 0.32 0.64 0.68 0.55 0.74 0.58 0.51 0.98 1.36

stage. Many factors can affect ammonium generation such as temperature, pH, aeration, nitrogen content (seen through the C/N ratio of the mixture) and microbial activity. The appearance of appreciable quantities of nitrates in the sub− strate, leading to a decrease of the NH+ 4 /NO3 ratio, can be used as an indicator of maturity when its value is lower than 01.[39,42,43] A decrease of conductivity was noted in pile C1, probably related to an increase in adsorption or in binding by the organic and mineral colloids. However, the difference with respect to C2 and C3 appears to be due to the rock phosphate addition. Water-soluble phosphorus levels were higher during the initial stages of composting (Table 1). In C1, the remarkable reduction in water-soluble phosphorus during the stabilization stage was perhaps due to the increase in the insoluble forms or forms adsorbed with organic substances. However, the increase in phosphorus, in C2 and C3 tests, indicates a clear dissolution of rock phosphate. The organic acids produced during composting favouring the dissolution of phosphates by forming complexes with the cations (e.g. Ca2+ , Fe3+ and Al3+ ) account for the fixing and precipitation of phosphorus.[44] Equally, phosphorus compounds may be dissolved by carbonic acid formed as a result of organic matter biodegradation. Depending on several factors, such as pH, cationic saturation, cationic valences, the type of metallic ion and degree of dissociation, increasing levels of HA during composting can form both soluble and insoluble complexes with metals, and account for phosphorus mobilization, by displacement of chemical balances towards desorption, or phosphorus immobilization, as indicated by the reduction of available phosphorus in test C1.[45] At low pH values, the humic substances are able to attract cations, and such electrostatic attraction leads to cation exchange reactions. At high pH values, when the humicOH groups are also dissociated, chelation reactions become important.

.127 .011 .008 .013 .092 .018 .063 .041 .150 .076 .128 .093

0.52 0.72 2.28 1.79 1.20 1.19 1.34 1.28 0.91 1.06 2.06 2.13

3.6 4.5 10.2 11.8 13.7 9.3 26.3 27.5 30.4 48.9 60.3 67.2

2.95 1.03 0.99 1.08 1.37 0.90 0.25 0.78 0.63 0.14 0.87 1.24

(K + Na)/(Ca + Mg) (ratio)

3.3 2.9 1.8 1.2 5.3 4.4 0.8 0.6 1.8 8.3 6.9 3.2

24.45 32.49 27.22 22.31 16.50 17.34 12.02 9.11 19.10 21.76 14.11 9.63

For all tests, the water-soluble fraction contained high concentrations of cations; they were essentially the monovalent cations Na and K, which form soluble salts with the mineral anions and with organic acids. They exist in equilibrium with the exchangeable form owing to the complexing power of composts.[46] During composting, the water-soluble fraction of the C1 test without added rock phosphate showed a decrease in potassium and an increase in the amount of sodium (Table 2). This is related to the behaviour of the two elements towards the acid sites of organic compounds, which occur in their ionized forms owing to the rise in pH. Further increases in the levels of sodium ions on addition of mineral rock phosphate (C2 and C3) causes parallel changes for K and Na, by saturation of the sites available for potassium. Concerning bivalent cations, the calcium release was found to be greater than that of Mg, which presented a lower concentration in the water-soluble solutions. These metals exist in solution with the balance state as ionic exchangeable forms with sorbent complexes or as organometallic complexes and seldom as salts.[47] However, the comparison between the alkaline (Na, K) and alkaline-earth (Ca, Mg) elements shows a decreasing ratio during composting (Table 2). This is thought to be related to adsorption by increasing levels of organic complexes.[48] It is weaker for metals with a smaller atomic radius and higher electronegativity, which allows a progressive mobilization of alkaline-earth metals. The water-soluble fraction contains lower concentrations of transition metals (Table 2), represented by heavy cations (Cu, Fe, Mn, Zn), which are in the form of organometallic complexes.[49] These complexes result from the formation of coordination or covalent bonds between the metal ions and the ligands. These cations are not exchangeable ionic forms and their release or dissolution in the aqueous phase, particularly in the form of salts, is negligible.[50] The chelated metal cannot be replaced by for instance K or Na, which are not capable of occupying


F. Barje et al. Lignin

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Reduction of organic compounds and water-soluble carbon (%)


Water-soluble C


stage, to reach an asymptote at the maximal level during the maturation stage (Figure 2).[51] The biodegradation coefficient, k (Table 3) was relatively high for phenols (k = 0.676 month−1 ), followed by lipids (k = 0.414 month−1 ). These two rapidly metabolizable fractions presented a level of abatement that was close to maximal C0 (100.5% and 91.3%, respectively) during the maturation stage. The high biodegradation of these two fractions shows kinetics almost identical to those already published for OMW.[52] Monitoring water-soluble carbon showed a steady decrease as composting progressed (Table 4). The strong decrease during the stabilization stage (62.8–82.8%) is largely related to the intense bacterial activity.[37] As this fraction contained the sugars, organic acids, proteins and other easily degraded organic compounds, it showed quite a severe decrease.[53] The phenolic fraction also showed high abatement during the thermophilic stage (54.7–69.7%), indicating high activity of the microorganisms able to use phenols as an energy source.[37,54, 55] Similarly, the high proportion of lipid abatement of 50.7–52.1% during the thermophilic stage is also due to strong microbial activity.[37,53] The biodegradation of lignin (a complex aromatic polymer) was not as great as that of the other fractions (Table 4). The resistance of lignin to microbial decomposition leads to its accumulation








5 Time (months)

Figure 2. Reduction of (•) lipids, (◦) phenols, () water-soluble C, () lignin with respect to the levels originally present in the mixtures before composting.

the position of any transition metal in a chelate. This kind of reaction will no doubt affect the cation exchange capacity of the composts. 3.3. Changes occurring in the organic fraction The biodegradation kinetics of the organic fractions presents an exponential pattern during the stabilization Table 3.

Biodegradation of organic compounds with respect to their initial content during composting (L% = C0 × (1 − e−kt )).

Reduction L (%)

t-value df = 10

Threshold p










Lipids Phenols Water-soluble C Lignin

8.7 11.1 3.4 3.8

3.9 2.9 2.1 2.1

0.000ˆ 0.000ˆ 0.007ˆ 0.004ˆ

0.003ˆ 0.019∗ 0.060 0.063

100.5 91.3 112 69.6

0.414 0.676 0.218 0.284

Note: L%, reduction (%); C0, maximum value of biodegradation (%); k, biodegradation coefficient (month−1 ); t, time (months). Threshold significance: ˆp < 0.01. Threshold significance: ∗ p < 0.05. Table 4.

Composts C1



Organic compounds (% dry wt.) and the HA/FA ratio at the different stages of composting.

Stage (months) 0 1 3 5 0 1 3 5 0 1 3 5

Water-soluble carbon






Ratio HA/FA

14.40 13.04 21.23 22.50 16.22 9.34 8.53 11.46 11.22 7.13 15.93 26.50

1.87 1.19 1.68 1.77 3.26 2.94 1.82 1.80 2.84 0.75 1.14 1.28

7.70 10.96 12.64 12.71 3.89 2.56 4.86 6.37 3.95 9.51 13.97 20.70

(%) 1.000 0.848 0.462 0.455 0.886 0.857 0.452 0.283 1.243 1.142 0.843 0.683

29.3 22.5 6.7 4.2 22.7 15.1 9 8.2 26.4 22.2 7.7 4.5

0.423 0.820 0.122 0.087 1.166 0.613 0.128 0.109 0.918 1.050 0.573 0.142

59.93 53.92 42.22 40.15 51.88 49.95 43.21 42.76 51.83 48.02 39.84 38.72

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in organic substrates.[52,56] However, during the thermophilic stage, lignin abatement was high, at between 55.8 and 66.3% for the three composting trials with 10.4–25.2% during the maturation stage.[37,39,56] The biodegradation of lignin is dependent on the specific microorganisms present, particularly the thermophilic actinomycetes, while other fungi proliferate in the substrate during the cooling stage. The group of basidiomycetes is reported to be the most active and the related ascomycetes and deuteromycetes also play a significant role.[56] The quantification of humic substances revealed that they were relatively abundant from the initial stages (Table 4). However, the notable decrease in humic and FAs during the first month of composting (from 1.4% to 6.9% for HAs and 0.3–2.1% for FAs) could be simply due to the metabolization of the labile components in their structure. When HAs are expressed with respect to FAs, by the ratio HA/FA, very low values were found in the initial stages, but they increased steadily during the maturation stage. The increase in this ratio, which can be used as an index of compost maturity [57,58] suggests that polymerization mainly occurs in the HA fraction and much less in the FA. The fulvic fraction is more aliphatic and less aromatic than the humic fraction and indeed FA are reported to be precursors of the HA.[49,59]




Fourier transform infrared spectroscopy

The study of the stabilization and the maturation of composts by FT-IR showed spectra (Figure 3) which have the same general pattern over time but with differences in the intensity of certain absorption bands, depending on the stage of biodegradation during the process of composting. The linear adjustment of the adsorption ratios as a function of the atomic C/N ratio (Table 5) indicates decreasing regression plots with respect to the peak at 1517 cm−1 due to absorption by unsaturated C=C aromatic bonds. Regression was greater for absorption peaks of aliphatic structures C−H (2925 cm−1 , 2855 cm−1 ) and the O−H bonds of alcohols and phenols (3423 cm−1 ), followed by the regression of the 1740 cm−1 peak of the C=O bonds of carboxyl, ketone, aldehyde and ester functions. It was the aliphatic structures and the organic acids that showed the greatest abatement during composting.[60] However, other structures were also decreased but to a lesser extent: in decreasing order of magnitude, we can mention the C−O bonds of polysaccharides (1055 cm−1 ), aromatic (C=C) and amides I (C=O) (1640 cm−1 ), the P−O−R bonds of phosphodiesters (alkyl) and −SO3 R of sulphates (1160 cm−1 ), C−H of aromatics and especially aliphatic acids (1458 cm−1 ), C−O−C and C=S bonds (1247 cm−1 ), then at 1317 cm−1 there is the peak of C−N stretching characteristic of primary and secondary aromatic amines. The spectra revealed a steady decrease in the easily biodegradable aliphatic structures, in particular the carboxylic groups of the organic acids.


Wavenumber (cm—1)

Figure 3.

FT-IR spectra of the organic matter during composting.


F. Barje et al.

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Table 5. Linear adjustment of the ratios of FT-IR absorption as a function of the C/N atomic ratio of the organic matter: Y = b0 + b1. C/N. FT-IR ratios







2925/1517 3423/1517 2855/1517 1740/1517 1055/1517 1640/1517 1160/1517 1458/1517 1247/1517 1317/1517

.628 .562 .605 .745 .438 .567 .625 .575 .718 .600

10 10 10 7 10 10 10 10 10 10

160.90 120.84 150.31 20.42 7.81 13.09 16.69 13.54 25.41 14.98

.002 .005 .003 .003 .019 .005 .002 .004 .001 .003

−10.6653 −.6280 −.9909 −.4130 .7202 .6702 .5070 .6199 .5358 .5961

.1336 .0964 .0892 .0613 .0319 .0284 .0280 .0224 .0216 .0159

Note: Tests significant (Sigf) to p < 0.05; correlation coefficient (Rsq).

During the biodegradation of the organic matter there was a comparatively smaller decrease in the absorption of bonds characteristic of aromatic compounds which are refractory to biodegradation. Studies of the biodegradation of organic matter and OMW indicate similar behaviour, through reduction of the aliphatic structures and increase in the quantities of aromatic structures.[52–54,61] The regressions of the FT-IR absorption ratios were significantly correlated to the C/N ratio and show the relatively strong persistence of the aromatic compounds, amides and molecules that can become integrated into heterocyclic compounds, finally leading to relatively stable organic compounds which only undergo very slight changes during the maturation stage.[62–64]


Phytotoxicity testing

The extracts of the initial mixtures showed very low GIs (Table 6). This was the case, for instance for the tomato with Table 6.

GI of some relevant plant species. GI%

Compost C1



Stage (months)

Oats (Avena sativa)

Lucerne (Medicago sativa)

Tomato (Solanum lycopersicum)

0 1 3 5 0 1 3 5 0 1 3 5

29.90 44.13 87.39 91.68 41.79 94.60 49.45 97.24 32.02 84.79 67.33 113.96

48.46 73.17 97.61 95.59 63.85 84.10 77.36 97.47 31.77 54.93 48.51 73.01

31.00 41.11 45.11 90.87 35.52 49.06 40.56 57.73 34.67 27.00 29.59 52.17

mixtures C1 and C2, whereas for C3 almost all the species tested presented a very low GI. This could be related to the slightly acid pH, to the high electrical conductivity related to a high concentration of ions and to the presence of watersoluble organic substances with an inhibitory action.[65] A great variety of compounds can have an inhibitory or retarding effect on germination. We can mention in particular short-chain organic acids and phenols as widely known inhibitors.[66,67] The depressive effect of the mixtures of waste had different effects on different species. For example, it was noted that the tomato was the most sensitive of the seeds tested here.[68] This can be considered in relation with a possible difference in sensitivity towards the substances extracted and/or the physical-chemical characteristics such as the pH or the ionic concentration.[69] During the stabilization stage, there was a steady increase in the GI (Table 6). However, for oats, the GI was still quite low after one month of composting in C1 and for tomato in C2. With lucerne, however, the GI rose from the first month of composting. Unexpectedly, there was a similar depression of the GI for oats and lucerne after three months in trial C3. This could be due to the release of harmful degradation products during the stabilization stage, inhibiting germination. Overall our findings corroborate the variable nature of the sensitivity towards phytotoxic compounds and germination inhibitors depending on the species considered.[70] During the maturation stage, the GI was relatively high. This increase could possibly be related to the strong reduction in phytotoxic substances after five months of composting.[71] We can mention the high levels of abatement of compounds such as lipids and phenols (Table 4) susceptible of having a phytotoxic effect.[72,73] Thus, the abatement of organic compounds brings about a global response whose effect is seen through a reduction of phytotoxicity and a concomitant improvement of the GI during composting.[74] 4. Conclusion The parameters studied here provide information on the stabilization of waste and the maturity of composts. The trials run in the present study all showed acceptable levels of transformation, all with similar temperature patterns characteristic of the three stages in the composting process. During the thermophilic stage, the highest temperatures were reached and the biodegradation of organic matter was most intense. The notable decrease of the C/N and − NH+ 4 /NO3 ratios confirmed the state of maturity of the composts obtained. Besides, the results showed that the effective dissolution of rock phosphate mixed with organic matter during composting makes it possible to increase the water-soluble phosphorus content. Meanwhile, the increase in the levels of humic substances mainly affected the HAs which led to an increase in the HA/FA ratio during composting. However, spectroscopic studies using FT-IR revealed greater reductions in

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the levels of the aliphatics, amides and carboxyls than in the more refractory aromatic compounds. In addition, the stabilization was followed by a phytotoxicity test which showed GIs that were initially very low at under 20% but that rose to over 90% at the final stages of biodegradation, after five months of composting. Consequently, the decrease in phytotoxicity during composting confirmed the stabilization state and the maturity reached. This appears to be an appropriate method for the disposal and reuse of both the toxic OMW and the biodegradable part of solid municipal waste. Similarly, the efficiency of inorganic phosphate fertilizers might be enhanced if the rock phosphate is added to the organic waste, opening the way for their use in soil amendment suitable for organic agriculture. References [1] Casa R, D’Annibale A, Pieruccetti F, Stazi SR, Giovannozzi Sermanni G, Lo Cascio B. Reduction of the phenolic components in olive-mill wastewater by an enzymatic treatment and its impact on durum wheat (Triticum durum Desf.) germinability. Chemosphere. 2003;50(8):959–966. [2] Sayadi S, Ellouz R. Role of lignin peroxidase from Phanerochaete chrysosporium in the decolorization of olive mill wastewaters. Appl Environ Microbiol. 1995;61(3): 1098–1103. [3] Rinaldi M, Rana G, Introna M. Olive-mill wastewater spreading in southern Italy: effects on a durum wheat crop. Field Crops Res. 2003;84(3):319–326. [4] Mekki A, Dhouib A, Sayadi S. Changes in microbial and soil properties following amendment with treated and untreated olive mill wastewater. Microbiol Res. 2006;161(2):93–101. [5] Fakharedine N, El Hajjouji H, Ait Baddi G, Revel JC, Hafidi M. Chemical and spectroscopic analysis of organic matter transformation during aerobic digestion of olive mill wastewater (OMWW). Process Biochem. 2006;41:398–404. [6] Ait Baddi G, Cegarra J, Merlina G, Revel JC, Hafidi M. Qualitative and quantitative evolution of polyphenolic compounds during the composting of an olive-mill waste-wheat straw mixture. J Hazard Mater. 2009;165:1119–1123. [7] Perez J, De La Rubia T, Moreno J, Martinez J. Phenolic content and antibacterial activity of olive oil wastewaters. Environ Toxicol Chem. 1992;11:489–495. [8] Carla A, Marco SL, Joao C, Antonio LC, Anjos MR, Pais C. Microbiological and physicochemical characterization of olive mill wastewaters from a continuous olive mill in northeastern Portugal. Bioresour Technol. 2008;99:7215–7223. [9] Yangui T, Dhouib A, Rhouma A, Sayadi S. Potential of hydroxytyrosol-rich composition from olive mill wastewater as a natural disinfectant and its effect on seeds vigour response. Food Chem. 2009;117:1–8. [10] Ranalli A. L’effluent des huiles d’olives: propositions en vue de son utilisation et son épuration. Références aux normes italiennes en la matière. Olivae. 1991;39:18–34. [11] Hattenschwiler S, Vitousek PM. The role of polyphenols in terrestrial ecosystem nutrient cycling. Trends Ecol Evol. 2000;15:238–243. [12] Capasso R, Evidente A, Schiro L, Orru G, Marcialis MA, Cristinzio G. Antibacterial polyphenols from olive oil mill waste waters. J Appl Microbiol. 1995;79:393–398. [13] Dias Albino A, Bezerra M, Nazare PA. Activity and elution profile of laccase during biological decolorization and dephenolization of olive mill waste water. Bioresour Technol. 2004;92:7–13.


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Biodegradation of organic compounds during co-composting of olive oil mill waste and municipal solid waste with added rock phosphate.

Liquid and solid olive oil mill waste was treated by com posting in a mixture with the organic part of municipal solid waste and rock phosphate. The t...
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