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Composting of three phase olive mill solid waste using different bulking agents
International Biodeterioration & Biodegradation 91 (2014) 66e73
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Composting of three phase olive mill solid waste using different bulking agents Abu Khayer Md Muktadirul Bari Chowdhury a, Michail K. Michailides a, Christos S. Akratos a, *, Athanasia G. Tekerlekopoulou a, Stavros Pavlou b, c, Dimitrios V. Vayenas a, b a b c
Department of Environmental and Natural Resources Management, University of Patras, G. Seferi 2, GR-30100 Agrinio, Greece Institute of Chemical Engineering Sciences, FORTH, Stadiou Str., Platani, GR-26504 Patras, Greece Department of Chemical Engineering, University of Patras, GR-26504 Patras, Greece
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 January 2014 Received in revised form 13 March 2014 Accepted 13 March 2014 Available online 12 April 2014
Pilot-scale experiments were carried out to produce good quality, low-cost composting technology from three phase olive mill solid waste (olive pomace, OP) using rice husk (RH) and olive leaves (OL) as bulking agents. A series of parallel experiments was carried out to examine the effect of: (a) initial moisture content, (b) water addition during the composting process, and (c) material ratios. To monitor the composting process and evaluate compost quality, physicochemical parameters (temperature, moisture content, pH, electrical conductivity, organic matter, volatile solids, total organic carbon, total nitrogen, total phosphorus, potassium, sodium, and water soluble phenols) were measured at different phases of the composting period. Experimental results showed that even after short composting periods, the quality of the final product remained high. To achieve higher quality of the final product, OP should be used in higher quantities than the other two materials (OL and RH). A full-scale compost unit was designed based on the experimental results. For a typical small-sized olive mill, processing 30 tonnes of olives per day for a 90 days operation period, a total area of about 500 m2 is needed to compost the mill’s entire waste production. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Three phase olive mill solid waste Rice husk Olive leaves Physicochemical parameters Phytotoxicity test
1. Introduction The olive oil extraction industry represents a substantial share of the economies of Mediterranean countries but leads to serious environmental problems by producing huge amount of wastes (byproducts) within a short production period. These by-products are olive pomace (OP) and olive mill wastewater (OMW) (for threephase systems, 3P), and two phase olive mill waste (TPOMW) (for two-phase systems, 2P) (Roig et al., 2006). 3P olive mills separate oil from by-products (e.g. OP and OMW) using a three-phase centrifuge (decanter) and 2P olive mills separate oil from TPOMW, which is a mixture of wastewater and OP, using a twophase centrifuge. 2P olive mills require smaller amounts of water for oil separation, thus producing smaller quantities of wastes compared to 3P systems. The production rate of olive oil is about 1.4e1.8 million tonnes per year in the Mediterranean resulting in 30 million tonnes of by-products (Barbera et al., 2013). A small * Corresponding author. Tel.: þ30 26410 74209; fax: þ30 26410 74176. E-mail address: [email protected] (C.S. Akratos). http://dx.doi.org/10.1016/j.ibiod.2014.03.012 0964-8305/Ó 2014 Elsevier Ltd. All rights reserved.
portion of these wastes can be used as raw materials in different industries as they contain valuable natural resources. Greece has about 2300 small-scale, rural, agro-industrial units that extract olive oil (Michailides et al., 2011). These are generally 3P systems and their by-products include olive mill residual solids (olive pomace and leaves) and OMW. Olive mills produce significant quantities of solid wastes with outputs of 0.35 tonnes of olive pomace and 0.05 tonnes of leaves per tonne of olives (Niaounakis and Halvadakis, 2004). The huge quantities of OP and OL produced within the short oil extraction season cause serious management problems in terms of volume and space. The soils of most Mediterranean countries have low organic matter content (<1%) which has negative impacts on agriculture (Alburquerque et al., 2007). Frequent application of composted organic residues increases soil fertility, mainly by improving aggregate stability and decreasing soil bulk density. Organic amendments play a positive role in climate change abatement by soil carbon sequestration. Recurrent use of composted materials enhances soil organic nitrogen content by up to 90% (Diacono and Montmurro, 2010). According to Toscano et al. (2013), composted
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olive mill by-products applied to olive groves increased olive production by 9% and olive oil production by 166e180 kg ha1. To replenish soil organic matter content and benefit eco-friendly crop production the application of OP compost could be a good solution. Several experiments have been carried out on composting olive mill by-products using OMW from 3P mills and olive humid husk (OHH) from 2P mills and adding other agricultural by-products as bulking agents. These agents include wool waste, wheat straw, olive leaves, wood chips, by-products from rice harvesting, sesame bark, sewage sludge, poultry manure, and sheep manure (Muktadirul Bari Chowdhury et al., 2013). Previous studies produced mature compost with C/N ratios ranging from 11.5 (Sellami et al., 2008) to 53.5 (Alfano et al., 2008) depending on initial carbon and nitrogen content of the materials. Compost maturity is evaluated using the germination index (GI) method. GI experiments were introduced by Zucconi et al. (1981) and include the quantification of seed germination and growth when a solution of compost extract is applied to growth media (Komilis and Tziouvaras, 2009). Lepidium sativum (salad cress) seeds are most commonly used for this experiment. Previous research has recorded GI values ranging from 45% (Alfano et al., 2008) to 195% (Michailides et al., 2011). Rice husk (RH) is an agro-industrial by-product with significant production in Greece and especially in western Greece. While rice husk is highly resistant to biodegradation, it could be used to improve compost porosity. The main objective of this work was to perform comparative pilot-scale experiments for composting OP, OL and RH, and define the best material ratios, optimum humidity and minimum composting duration. Furthermore, optimum experimental results were scaled up in order to design a full-scale composting unit for an olive mill receiving 30 m3 of olives per day, which is the typical capacity of small olive mill units in Greece. To observe the composting process, physicochemical parameters (i.e., temperature, moisture content, pH, electrical conductivity, organic matter, total C, total N, total P, K, Na, and water soluble phenols) were assessed at different stages of the experiments. To ascertain the quality of the end product, phytotoxicity tests were carried out using L. sativum seeds. 2. Materials and methods 2.1. Composting materials and process Olive pomace (OP), olive leaves (OL), rice husk (RH), water and OMW were used for this study. The OP, OL and OMW were collected
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from a three-phase olive mill located in Amfilochia, western Greece. To improve the physical condition of olive pomace for composting, olive leaves and rice husk were added as bulking agents. Water and olive mill wastewater were used as humidifying agents during the composting period. In this study, six trapezoidal bins with dimensions 1.26 m long, 0.68 m wide and 0.73 m deep and a total volume of 0.62 m3 were employed. The study was carried out at the facilities of the Department of Environmental and Natural Resources Management, University of Patras, Agrinio, in a closed area to maintain controlled temperature conditions. Three experiments were performed with different experimental set-ups (Table 1). The three experiments were conducted in different time periods using six parallel composting bins for each experiment. In each experiment different composting materials were used to determine optimal mixing ratios, and two of the six bins had the same contents to examine the effect of other operational parameters. The main objectives of the first experiment were to: (a) examine the effects of different OP and OL ratios on compost quality, (b) examine the effect of ambient temperature and compare data with a previous industrial-scale experiment (Michailides et al., 2011), and (c) control total composting duration using the compost cooling process. The second experiment was performed to examine the optimum mixing ratio of RH, while compost duration was again controlled as in the first experiment. Finally, the third experiment was performed to examine the effect of prolonged composting (not controlled by cooling) on final compost quality. In the first experiment, two identical series were set-up: Bins 1e3 and 4e6 were filled with OP and OL using different ratios (Table 1). In this experiment water was applied as a wetting agent. In the second and third experiments one bin (Bin 7 and Bin 13 respectively) was used as reference to the first experiment, while bins 8, 10, 14 and 16 were used to examine the effect of OMW as a wetting agent. Bins 9e12 and 15e18 were used to examine the effect of RH as a bulking agent. In all experiments aeration was achieved by mechanical turning which took place daily for the first three days, once every four days during the thermophillic phase, and once a week during the maturation phase. In the first two experiments compost moisture content was kept constant at about 60e65%, whereas in the third experiment compost moisture content was above 45%. According to Gajalakshmi and Abbasi (2008), moisture contents between 45 and 60% are ideal for the composting process. In the first two experiments the compost mixture was kept in the bins for 60 days. It was then removed from the bins and stored in a covered place to
Table 1 Description of experimental set-ups. Bin
1st exp
2nd exp
3rd exp
Bin1 Bin2 Bin3 Bin4 Bin5 Bin6 Bin7 Bin8 Bin9 Bin10 Bin11 Bin12 Bin13 Bin14 Bin15 Bin16 Bin17 Bin18
Composting materials ratios (per volume) Olive pomace
Olive leaves
Rice husk
2 1 1 2 1 1 2 2 1 1 1.5 0.5 2 2 1 1 1.5 0.5
1 1 2 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0 1 1 0.5 1.5 0 0 1 1 0.5 1.5
Moistening agent
Initial mass (kg)
Initial vol. (L)
Final mass (kg)
Final vol. (L)
Water Water Water Water Water Water Water OMW Water OMW Water Water Water OMW Water OMW Water Water
154.19 163.92 162.50 168.80 158.60 145.30 93.53 91.13 69.92 62.55 74.51 49.25 214.82 218.08 179.77 172.08 193.90 151.94
294 294 294 294 294 294 252 252 252 252 252 252 406 406 406 406 406 406
132.73 130.96 106.68 140.00 125.00 102.00 122.97 122.67 91.16 88.32 101.76 73.50 111.32 115.80 82.24 81.73 93.88 69.09
210 238 238 208 239 242 210 210 176 176 196 198 195 195 173 172 177 160
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mature. In the third experiment the thermophillic phase was longer in duration and the compost mixture was kept in the bins for 63e 93 days depending on when the thermophillic phase ended in each bin. The compost was then moved into a covered area for maturation (up to 120 days).
three experiments that contained the same materials at identical ratios in order to statistically assess the effect of different experimental conditions.
2.2. Physicochemical analysis
3.1. Temperature evolution
During the experiments compost temperature was monitored daily using a temperature probe inserted into the middle of each bin at a depth of 0.25 m. The ambient temperature was also monitored. Compost samples were taken from each bin on a weekly basis. Each sample was divided into two parts. One part was analysed immediately for moisture content, pH, electrical conductivity (EC), water soluble phenols (WSPH), oxygen consumption, and volatile solids. Moisture content was determined by drying the sample at 105 C for 24 h in an oven. To measure volatile solids, an oven dried sample was taken and burned at 600 C for 4 h (FCQAO, 1996). pH, EC and WSPH were measured in aqueous extract (using 1:10 w/v). The second sample portion was air dried, ground, sieved (0.5 mm), and used to determine total Kjeldahl nitrogen (TKN), total organic carbon (TOC), total phosphorus (TP), potassium (K), sodium (Na), and phytotoxicity. TKN was measured following the method described by Bremner and Mulvaney (1982), and TP was measured following the method described by Murphy and Riley (1962). Absorbance for TP was measured using a Boeco S-20 spectrophotometer. Na and K were measured using a flame photometer (Model FP902). Static respiration activity was measured using manometric respirometers (Komilis and Tziouvaras, 2009). Water soluble phenols were measured in water extracts (1:10 w/v) following the Folin-Ciocalteu method (Waterman and Mole, 1994).
Temperature evolution indicates the bio-chemical activities of compost (Tuomela et al., 2000; Ryckeboer et al., 2003). Fig. 1 presents time series charts of compost and ambient temperature for the three experiments. Experiment 1 (Bins 1e6) initially showed a characteristic rise in temperature during the composting process (Fig. 1). Here the thermophillic phase (over 45 C) commenced after day 5. The thermophillic phase was quickly obtained due to the generation of heat resulting from aerobic microbial activities on readily available organic materials. The same trend was also observed in industrial-scale compost piles using the same materials (Michailides et al., 2011; Gigliotti et al., 2012). Although the compost piles of Michailides et al. (2011) were established in an open area and those of the present study in a closed area, their temperature patterns did not show great variations. Thus ambient temperature does not greatly affect compost temperature. Maximum temperature was observed at day 7 in Bin 1 (61 C), at day 13 in Bin 2 (approximately 63 C), and at day 11 in Bins 4e6 (over 60 C). Temperature remained over 55 C for at least three days indicating that the compost remains free of pathogenic organisms (de Bertoldi et al., 1983). In the second experiment (Bins 7e12), initial moisture content was very high and water was added to minimize the composting process duration. Bins 7 and 8 reached maximum temperature (39 C) at day 43 and 44, respectively. Only Bins 9 and 10 reached the thermophillic phase (46 C) at day 9. It should be noted that initial moisture contents were very high (around 65%) in this experiment (Table 2). The temperature pattern recorded in the third experiment was almost the same as that observed in the industrial-scale application described by Michailides et al. (2011). It took almost four days to reach the thermophillic phase in all bins, a result comparable to experiment 1 data. Bins 13 and 14 had the longest thermophillic phases (92 days and 93 days, respectively). The shortest thermophillic phase was observed in Bin 18 (62 days), while the remaining bins (15, 16 and 17) had thermophillic phases of 75, 75 and 83 days, respectively. In all experiments, the use of different composting materials in different ratios did not affect temperature evolution as the temperature in all bins of the same experiment had a high crosscorrelation coefficient ranging from 0.785 to 0.979. On the contrary, excessive water addition significantly affected temperature evolution as correlation coefficients of Bin 1 with Bins 7 and 8 were low (0.007 and 0.116, respectively).
2.3. Phytotoxicity Cress (Lepidium sativum) seeds were used for the phytotoxicity test. Phytotoxicity was estimated using the germination index (GI) as described by Zucconi et al. (1981). Phytotoxicity was determined by comparing the root development of each seed that sprouted in the compost and in deionized water (the control sample). For this test, 10 g of powdered compost were mixed with 100 ml of deionized water and the solution was then stirred for 2 h. After stirring, the solution was allowed to rest for half an hour and was then diluted with distilled water to three different concentrations (25%, 50% and 100%) with clear supernatant. The control concentration (0%) comprised only distilled water. 5 ml of each compost solution was added to a petri dish containing five pieces of blotting paper onto which 25 cress seeds were evenly placed. All the concentrations of each compost sample, including the control, were replicated three times. Petri dishes were incubated under light for 48 h at 25 C. After the incubation period, root length was measured in each sprouted seed and GI was estimated. 2.4. Statistical analysis All statistical analyses were performed using SPSS 17.0 software for Windows. Cross correlation coefficients were calculated to statistically evaluate the differences in compost temperature, OM and TKN content, and C/N ratio. These four parameters were chosen as temperature is the most important operational parameter controlling the compost process, while OM, TKN and C/N ratio are important indicators of compost maturity. Cross-correlation coefficients were calculated for all bins in the same experiment to assess the effect of the different composting materials. Furthermore, cross correlation coefficients were calculated for bins in all
3. Results and discussion
3.2. Compost moisture content Moisture is one of the major physical factors that regulate indirectly compost temperature by affecting its microbial activity. In experiments 1, 2 and 3, compost moisture contents were around 60, 65% and 45%, respectively. Due to these high moisture contents the first two experiments had thermophillic phases shorter than in the third experiment. Moisture contents in excess of 60% mean pore spaces in the compost pile will be filled with water rather than air, leading to anaerobic conditions (Das and Keener, 1997). In experiment 1, 30 L of water were added from day 23 to day 33 (three times) and in experiment 2, 80 L of water were added in the first three days. In experiment 3, 41e59 L of water (the quantity varied per bin) were added from day 10 to day 106 (nine times including
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Fig. 1. Compost and ambient temperature time series charts for: (a) Bins 1e6 (first experiment) containing only OP and OL in different mixing ratios, (b) Bins 7e12 (second experiment) containing OP, OL and RH in different mixing ratios, (c) Bins 13e18 (third experiment) containing only OP and OL in the same mixing ratios as those used in the second experiment, but with prolonged composting duration.
once in the mesophillic phase). Therefore, moisture content alone does not cool the composting process, but rather the addition of water on critical days (thermophillic phase) when compost temperature is rising. Compost moisture contents of around 40e65% were used in most olive mill waste composting studies reviewed by Muktadirul Bari Chowdhury et al. (2013). 3.3. Compost attributes Table 2 presents physicochemical attributes of the compost at different stages, such as the initial stage (day 0), end of the thermophillic phase, and end of the maturation phase (day 120). The experiments presented in this paper and Michailides et al. (2011), did not show great variations in quality characteristics (Table 2), although they do have different composting durations and temperature patterns. In experiment 1, the initial pH range was 6.74e7.58 increasing to 7.58 to 8.87 at the end of the thermophillic phase. pH values then decreased and stabilized from 7.69 to 7.89 at the end of the maturation phase. Similar pH trends
were also observed in experiments 2 and 3. These pH ranges may benefit microbial activities by creating favourable environmental conditions. Several studies report that pH increases during the thermophillic phase and later decreases to near neutral (Hachicha et al., 2006; Michailides et al., 2011). The pH values of matured composts range from 6.83 to 7.79 in the present research work and are compatible with most cultivated crops (Lasaridi et al., 2006). The pH values of the present work are in line with various olive mill waste composts mentioned in the review of Muktadirul Bari Chowdhury et al. (2013). Electrical conductivity (EC) values increased during the composting process in all experiments (Table 2). Initial EC values ranged from 388 to 570 mS/cm in the first experiment, from 336 to 663 mS/ cm in the second experiment, and from 445 to 619 mS/cm in the third experiment. Final EC values (mature compost) ranged from 654 to 750 mS/cm, 548 to 861 mS/cm, and 636 to 1050 mS/cm in the first, second and third experiments, respectively. These EC values indicate that rice husk increased soluble salt concentration. The
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Table 2 Physicochemical properties of compost at different composting stages. Bin
Moist (%)
pH
One day B1 60.4 6.74 B2 60.5 7.04 B3 62.3 6.86 B4 60.5 7.08 B5 59.2 6.76 B6 74.6 7.58 B7 59.9 5.38 B8 59.1 5.07 B9 65.8 5.94 B10 62.1 4.82 B11 63.2 5.49 B12 64.8 5.92 B13 56.4 6.4 B14 52.9 6.46 B15 59.7 6.76 B16 61.3 6.72 B17 56.1 6.44 B18 57.7 7 End of thermophilic phase B1 57.1 7.58 B2 65.8 7.91 B3 63.6 7.82 B4 54.1 8.87 B5 58.0 8.79 B6 62.3 8.8 B7 50.9 8.12 B8 51.1 8.11 B9 53.8 7.74 B10 54.1 8.4 B11 50.2 7.96 B12 51.7 7.91 B13 47.6 7.3 B14 47.1 7.28 B15 48.1 7.48 B16 48.8 7.45 B17 46.4 7.46 B18 47.4 7.58 End of maturation phase B1 56.1 B2 51.0 B3 59.0 B4 48.8 7.79 B5 56.5 7.89 B6 61.0 7.69 B7 51.5 6.83 B8 50.2 7.17 B9 53.4 7.37 B10 54.8 7.14 B11 48.7 6.9 B12 52.6 7.43 B13 45.5 7.56 B14 46.6 7.4 B15 46.6 7.42 B16 47.0 7.39 B17 50.4 7.51 B18 47.8 7.54
EC (mS/cm)
VS (%)
OM (%)
TOC (%)
TKN (%)
C/N
P (%)
388 470 386 450 570 507 501 663 427 589 440 336 533 523 445 548 556 619
99.3 96.4 94.3 99.3 99.0 99.0 97.4 97.1 92.5 93.9 95.8 89.4 96.7 96.3 92.3 92.1 94.0 88.7
91.0 90.3 90.6 90.2 89.8 87.7 95.4 95.6 89.5 90.3 93.3 87.1 94.3 94.3 89.1 90.4 92.8 85.7
52.8 52.4 52.6 52.3 52.1 50.9 55.3 55.4 51.9 52.4 54.1 50.5 54.7 54.7 51.7 52.4 53.8 49.7
2.4 2.5 2.1 2.6 2.5 2.7 1.9 2.0 1.7 1.8 2.0 1.4 1.9 2.0 2.0 2.0 1.9 1.9
22.4 20.5 24.9 19.9 20.9 19.2 28.6 27.3 31.1 29.4 26.5 35.0 28.5 26.8 25.6 26.7 28.3 25.7
0.11 0.11 0.10 0.11 0.11 0.09 0.10 0.10 0.12 0.11 0.11 0.10
412 521 623 442 410 500 554 652 611 640 508 547 675 657 825 735 819 1007
92.8 89.2 88.3 94.4 93.9 90.6 97.0 96.7 89.6 89.7 94.4 85.5 93.3 94.0 90.0 90.4 90.4 86.1
91.6 87.8 82.3 90.7 86.6 85.6 93.6 93.8 86.5 86.7 91.9 83.6 87.9 87.9 83.0 83.9 84.8 80.7
53.1 50.9 47.7 52.6 50.3 49.7 54.3 54.4 50.2 50.3 53.3 48.5 51.0 51.0 48.2 48.7 49.2 46.8
3.2 2.9 3.1 3.2 3.2 3.1 2.5 2.8 2.5 2.5 2.5 1.9 3.9 3.9 3.6 3.7 3.9 3.2
16.6 17.5 15.3 16.5 15.7 16.3 21.5 19.1 20.5 20.2 21.1 25.7 12.9 13.0 13.3 13.3 12.5 14.7
0.14 0.14 0.14 0.13 0.13 0.12 0.20 0.19 0.19 0.17 0.18 0.19
654 702 750 789 726 861 837 548 710 670 636 761 837 770 1050
93.5 90.6 89.2 94.7 91.8 93.9 96.7 95.7 89.0 89.0 94.1 85.0 92.3 92.4 86.0 87.7 90.4 83.4
90.0 87.8 82.8 88.5 87.2 84.0 91.6 91.4 83.7 84.0 89.1 81.9 87.1 87.1 80.2 80.3 83.9 76.5
52.2 50.9 48.0 51.3 50.6 48.7 53.1 53.0 48.6 48.7 51.7 47.5 50.5 50.5 46.5 46.6 48.6 44.4
3.1 2.9 2.8 3.2 3.2 2.9 3.6 3.7 3.5 3.5 3.6 2.8 3.9 4.1 3.4 3.4 4.0 3.2
17.1 17.5 17.3 16.1 15.8 16.7 14.7 14.4 13.8 13.8 14.4 17.1 12.8 12.3 13.7 13.5 12.1 13.7
0.17 0.16 0.15 0.16 0.15 0.13 0.19 0.17 0.17 0.17 0.17 0.16
Na (%)
K (%)
Phenols (%)
0.06 0.07 0.07 0.10 0.07 0.07 0.07 0.08 0.07 0.07 0.07 0.07
0.023 0.026 0.017 0.024 0.022 0.011 0.020 0.020 0.016 0.016 0.018 0.017
1.22 1.14 1.20 1.04 1.15 1.04 3.27 3.45 3.01 3.12 2.85 2.34 3.13 2.70 1.97 2.94 1.27 1.99
0.09 0.11 0.09 0.11 0.08 0.09 0.15 0.11 0.10 0.10 0.13 0.13
0.035 0.039 0.020 0.024 0.026 0.015 0.040 0.035 0.029 0.030 0.034 0.027
2.52 3.41 1.84 2.18 2.02 1.35 0.25 0.19 0.23 0.21 0.22 0.24
0.046 0.047 0.029 0.030 0.030 0.018 0.037 0.030 0.024 0.027 0.024 0.023
0.58 0.44 0.53 0.27 0.44 0.16 1.1 1.04 0.53 0.97 0.62 0.69 0.21 0.2 0.17 0.18 0.21 0.2
0.15 0.13 0.12 0.12 0.10 0.10 0.11 0.09 0.09 0.08 0.08 0.09
MC: Moisture Content, EC: Electrical Conductivity, VS: Volatile Solid, TOC: Total Organic Carbon, TKN: Total Kjeldhal Nitrogen, C/N: Carbon Nitrogen ratio, P: Phosphorus, Na: Sodium, K: Potassium, WSPH: Water Soluble Phenols, GI: Germination Index.
highest EC values were obtained at the end of the composting process and this is attributed to the decrease of total mass, organic matter mineralization, and water evaporation (Baeta-Hall et al., 2005; Gigliotti et al., 2012), hence leading to increased relative soluble salt concentration. It is notable that the EC values of the mature composts in this study were low and far below the Greek standard upper limits of 4S/cm (Lasaridi et al., 2006). Therefore this compost is most probably suitable for application on a wide variety of agricultural crops. During the composting process, a small amount of organic matter loss was observed in all the experiments resulting in a
decrease in VS and TOC throughout the process especially during the maturation phase (Table 2). Organic matter content decreased during the composting process with ranges of 87.7e91 to 84e 90%, 87.1e95.6 to 81.9e91.6% and 85.7e94.3 to 78e87.1% in experiments one, two and three, respectively. The reduction of organic matter is due to mineralization of organic compounds. Total organic carbon (TOC) also decreased and ranged from 50.9e 52.8 to 48e52.2%, 50.5e55.4 to 47.5e53.1% and from 49.7e54.7 to 44.4e50.5% in experiments one, two and three, respectively. The variations of initial organic matter values between the three experiments are due to the different organic matter contents of the
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initial products used for composting (Table 1). These values of organic matter content and TOC are comparable to those reported by Michailides et al. (2011) where organic matter was 83.7% and TOC 48.5%. Organic matter and TOC decreased more in experiment 3 than in experiments 1 and 2. A significant change of organic matter and TOC content was recorded between the bins in experiments 2 and 3 but not in experiment 1. In all experiments the bins that contained the same materials with the same mixing ratios had high correlation coefficients ranging from 0.814 to 0.991. Furthermore, all bins of the same experiment also recorded high correlation coefficients ranging from 0.861 to 0.994, with the exception of Bins 3 and 6 which have somewhat lower correlation coefficients (from 0.004 to 0.590) than the other bins (1, 2, 4 and 5) of the first experiment. This indicates that the excessive use of OL leads to a final product with significantly different OM content. In experiments 1 and 2, OP was slightly fresher and took less time to compost (due to water addition) than that of experiment 3, and this may explain the reduced mineralization of organic matter observed in experiments 1 and 2. However, the second experiment showed slightly more mineralization than the first experiment. It was also observed that these changes were more pronounced when rice husk were used in the second and third experiments. It is possible that the rice husk improved microbial activity by enhancing aeration (porosity). Table 2 shows that TKN values increased during the composting process. In general, the trend of increasing organic nitrogen could be attributed to a concentration effect caused by compost’s volume and mass reduction. The highest value (4.1%) of TKN was found in Bin 14 and the lowest value (2.8%) of TKN was found in Bins 3 and 12. Higher values of TKN were observed in experiments 2 and 3 due to higher mineralization of composting materials and the longer composting period compared to experiment 1. The bins where higher ratios of rice husk were used showed comparatively lower nitrogen concentrations than the bins with less or no rice husk. This may be due to the chemical properties of the rice husk that contains high quantities of cellulose and silica and is more resistant to biodegradation (Low and Lee, 1997; Champagne, 2004). Furthermore, the bins having OMW as the wetting agent showed higher TKN than those with water. OMW may be responsible for increased nitrogen concentration though Aviani et al. (2010) did not find any significant differences of nitrogen content between composts which had water or OMW as wetting agents. Therefore, it could be inferred that specific ratios of rice husk (OP: OL: RH ¼ 1.50:1:0.5, by volume) can benefit OP and OL mineralization by increasing oxygen content, microbial activities, and ultimately nitrogen concentration in mature compost. The values of TKN observed in this study are comparable to those of Altieri and Esposito (2010), Sellami et al. (2008) and Sánchez-Arias et al. (2008). According to Italian legislation, if a mature compost contains more than 3% nitrogen it can be used as a nitrogenous fertilizer. The C/N ratio, i.e. the ratio of TOC to TKN, indicates the availability of nitrogen for the process of biological decomposition of compost. A decrease in this ratio with the progress of composting time has been widely mentioned as an index of compost stability and maturity. The C/N ratios of this research are presented in Table 2 and all the mature composts showed values close to 15, close to the limit of 20 established by Bernal et al. (1998). A decreased C/N ratio was more pronounced in the second and third experiments, because these used older olive pomace that took longer to compost than the first experiment. The greatest changes in C/N ratio were in Bins 9, 10 and 12 of experiment 2, however changes in C/N ratio were significant in Bins 13, 14 and 17 of experiment 3. C/N ratio did not show great
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variations between the experiments and the compost with the lowest quality (maximum C/N values) corresponds to high OL ratios. It should be noted that the moisture content was higher in experiment 2, hence bins with small quantities of, or lacking, rice husk showed less mineralization due to reduced aerobic conditions. On the contrary, experiment three had optimum moisture content (around 45%) which helps increase mineralization, especially in bin 17 which has the lowest ratio of RH. Therefore, it may be concluded that rice husk, as well as optimum moisture content, promotes the decrease of the C/N ratio. Many researchers (Sellami et al., 2008; Komilis and Tziouvaras, 2009) do not consider the C/N ratio an absolute indicator of compost maturity as it largely depends on the composition of initial composting materials. Therefore, it is not wise to use the C/N ratio alone as an indicator of compost maturity. For this reason, static respirometric tests were performed in this study to determine oxygen consumption which is an indicator of compost maturity and stability (Komilis and Tziouvaras, 2009). EU recommendations state that oxygen consumption in static respirometric tests should not exceed 10 g/kg dry matter after four days (Komilis and Tziouvaras, 2009). In this study, the oxygen consumption measured after four days was much lower (3.27e 5.93 g/kg dry matter) than the EU suggested limit. Although some significant differences were observed for OM, TKN and the C/N ratio they appear not to be affected by the use of different composting materials in different ratios or by the different operational parameters, because the correlation coefficients for all bins show extremely high values (from 0.718 to 0.998 for TKN and from 0.795 to 0.996 for the C/N ratio). Water soluble phenol (WSPH) is an important indicator of compost maturity of olive mill wastes and is related to the phytotoxic properties of these wastes (Ait Baddi et al., 2004; Alburquerque et al., 2006). The present study showed that WSPH decreased in all bins during composting. Especially in experiment three, WSPH was reduced by approximately 90% at maturation (Table 2). First two experiments showed lower degradation of WSPH due to high moisture content which reduces microbial activities. In experiment three, it was noticed that in the first 45 days the reduction rate of WSPH was higher in bins where rice husk was used. After day 45, a similar trend was observed in all the bins. It is possible that the aerobic microbes were positively influenced by the presence of RH and this enhanced WSPH reduction. It should be noted that when lignocellulosic material degrades WSPH are released leading to their high concentrations in the aquatic phase which later decreased due to microbial activity (Tortosa et al., 2012). The mature composts produced in all bins were rich in nutrients (Table 2). In the second experiment, P, K, Na concentrations were 0.13e0.17%, 0.018e0.047% and 0.10e0.15% respectively. In the third experiment, P, K, Na concentrations were 0.16e0.19%, 0.023e 0.037% and 0.08e0.11%, respectively. These concentrations are comparable or higher than those reported in Muktadirul Bari Chowdhury et al. (2013). Due to presence of high nutrient content, and optimum level of porosity (data not shown) these composts could be used to replenish soils exhausted by intensive cultivation as well as soils suitable for greenhouses (Hachicha et al., 2006). 3.4. Phytotoxicity test Results for the seed bioassay test were used to assess any changes in compost phytotoxicity at the start of the composting process, at the end of thermophillic phase, and at the end of maturation phase (Fig. 2). The GI values obtained from the initial stage of experiments two and three demonstrate that the initial
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Fig. 2. Germination index (GI) fluctuations with different compost contents of the three experiments at different stages of composting: (a) initial stage, (b) end of thermophillic stage, and (c) end of maturation stage.
composting materials were phytotoxic in nature. During the composting process a clear decreasing trend in phytotoxicity was noticed (GI values of 101.2e122.26% and 109.98e119.54% for second and third experiments, respectively). Gigliotti et al. (2012) also reported that the escalated phytotoxicity of this material decreased remarkably after aerobic treatment. The present study also showed similar results except in experiment one, where no phytotoxicity was detected at the beginning of the composting process. According to Lasaridi et al. (2006), composts with GI values below 80% are phytotoxic. The GI values indicate that the mature compost of the present study is not phytotoxic and therefore could be applied as a high quality organic soil amender. Results of the first experiment show that the optimum OP:OL ratio was 2:1 by volume, as increased OL levels lead to low GI and thus to phytotoxic effects of the final compost. Whilst observation of the optimal OL ratio was straightforward, for the other parameters (OP: RH ratio and composting duration) the optimal values were not so easy to define as the quality of the final composts from experiments 2 and 3 had small differences. No great variations of GI were observed either in different composting durations or in different OP loads. There is only one exception where GI values were below 100% and this
corresponds to the experiment with the increased OL ratio. No significant impact of rice husk or OMW on the phytotoxicity of the mature composts was observed. 4. Full-scale scenario Olive mill waste can be successfully composted with a final product that is a very good quality soil amendment, and an excellent choice for organic farming. Therefore, olive mill owners could benefit economically by composting their OP and OL. The main issue for an olive mill owner is the area requirement for composting the entire season’s production of OP and OL. To assess this issue, a full-scale scenario for a three-phase olive mill located in Amfilochia, western Greece, was developed. This olive mill can process 30 tonnes of olives per day and its typical operating period lasts 90 days (from October to the end of December), resulting in 2700 tonnes of processed olives. According to Niaounakis and Halvadakis (2004), OP and OL correspond to 35% and 5% of olive mill waste, respectively. This leads to a total seasonal production of OP and OL of 945 and 135 tonnes, respectively.
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The area required for composting in the full-scale scenario was calculated using the following expression:
Required area m2 ¼
OP production ðkgÞ kg Number of composting per year compost365d OP load m 2 duration d
As OP and OL are already stored in an area within the olive mill, there was no need to compost the whole production at once, rather OP mass could be composted in batches throughout the year. As expected, the minimum required area corresponds to maximum OP load and minimum compost duration is approximately 500 m2. The total area required is extremely low and is available in the majority of olive mills. Therefore, olive mill owners have an excellent alternative method for treating their waste by-products with a possible economic benefit. 5. Conclusion This study used parallel composting experiments to examine the effect of compost duration on the quality of the final product, and the use of rice husk as a bulking agent in composting olive mill waste. Mature compost from olive mill waste appears to have excellent characteristics as it is free from phytotoxicity and contains high organic matter and nutrient content (N, P, K, Na) rendering it a high quality soil amender. In general, olive mill waste can be composted alone or mixed with other agricultural by-products (e.g., rice husk) to produce high quality soil amendment. The main factors affecting compost quality are: (a) moisture and water addition during composting which affects the temperature pattern and total composting period; (b) the proportion of olive leaves in the mixture that should not exceed one third of the initial compost volume to ensure high quality compost; (c) the use of bulking agents which increase porosity (e.g., rice husk) by increasing aeration that is crucial for microbial functions. Olive mill waste composting could have a positive economic impact on olive mill owners as they can compost their entire season’s waste production in an area of about 500 m2 for a typical small-scale plant, and produce a high quality soil amendment which could then be sold. Acknowledgement The first author gratefully acknowledges a postgraduate scholarship from the Hellenic State Scholarships Foundation. References Ait Baddi, G., Alburquerque, J.A., Gonzalez, J., Cegarra, J., Hafidi, M., 2004. Chemical and spectroscopic analyses of organic matter transformations during composting of olive mill wastes. Int. Biodeterior. Biodegradation 54, 39e44. Alburquerque, J.A., Gonzalvez, J., Garcıa, D., Cegarra, J., 2006. Measuring detoxification and maturity in compost made from “alperujo”, the solid by-product of extracting olive oil by the twophase centrifugation system. Chemosphere 64, 470e477. Alburquerque, J.A., Gonzalvez, J., Garcıa, D., Cegarra, J., 2007. Effects of a compost made from the solid by-product (“alperujo”) of the two-phase centrifugation system for olive oil extraction and cotton gin waste on growth and nutrient content of ryegrass (Olium perenne L.). Bioresour. Technol. 98, 940e945. Alfano, G., Belli, C., Lustrato, G., Ranalli, G., 2008. Pile composting of two-phase centrifuged olive husk residues: technical solutions and quality of cured compost. Bioresour. Technol. 99, 4694e4701. Altieri, R., Esposito, A., 2010. Evaluation of the fertilizing effect of olive mill waste compost in short-term crops. Int. Biodeterior. Biodegrad. 64, 124e128. Aviani, I., Laor, Y., Medina, S.H., Krassnovsky, A., Raviv, M., 2010. Co-composting of solid and liquid olive mill wastes: management aspects and the horticultural value of the resulting composts. Bioresour. Technol. 101, 6699e6706.
Baeta-Hall, L., Sàagua, C.M., Bartolomeu, M.L., Anselmo, A.M., Rosa, M.F., 2005. Biodegradation of olive oil husks in composting aerated piles. Bioresour. Technol. 96, 69e78. Barbera, A.C., Maucieri, C., Cavallaro, V., Ioppolo, A., Spagna, G., 2013. Effects of spreading olive mill wastewater on soil properties and crops, a review. Agric. Water Manag. 119, 43e53. Bremner, J.M., Mulvaney, C.S., 1982. Nitrogen-total, principles of Kjeldahl methods (pp. 599e616). In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis Chemical and Mineralogical Properties, second ed., p. 1159 Agronomy No 9 (Part 2) ASA Inc., Madison Wisconsin, USA. Bernal, M.P., Paredes, C., Sanchez-Monedero, M.A., Cegarra, J., 1998. Maturity and stability parameters of compost prepared with a wide range of organic wastes. Bioresour. Technol. 63, 91e99. Champagne, E.T., 2004. Rice: Chemistry and Technology.. In: The American Association of Cereal Chemists Minnesota, USA, Eagan. de Bertoldi, M., Vallini, G., Pera, A., 1983. The biology of composting: a review. Waste Manag. Res. 1, 157e176. Das, K., Keener, H.M., 1997. Moisture effect on compaction and permeability in composts. J. Environ. Eng. 123, 275e281. Diacono, M., Montmurro, F., 2010. Long-term effects of organic amendments on soil fertility, A review. Agronomy Sustain. Dev. 30, 401e422. FCQAO, 1996. Methods Book for the Analysis of Compost. Abfall Now e.v.. Publishing House, Stuttgart, Germany. Gajalakshmi, S., Abbasi, S.A., 2008. Solid waste management by composting: state of the art. Crit. Rev. Environ. Sci. Technol. 38, 311e400. Gigliotti, G., Proietti, P., Said-Pullicino, D., Nasini, L., Pezzolla, D., Rosati, L., Porceddu, P.R., 2012. Co-composting of olive husks with high moisture contents: organic matter dynamics and compost quality. Int. Biodeterior. Biodegrad. 67, 8e14. Hachicha, S., Chtourou, M., Medhioub, K., Ammar, E., 2006. Compost of poultry manure and olive mill wastes as an alternative fertilizer. Agronomy Sustain. Dev. 26, 135e142. Komilis, D.P., Tziouvaras, I.S., 2009. A statistical analysis to assess the maturity and stability of six composts. Waste Manag. 29, 1504e1513. Lasaridi, K., Protopapa, I., Kotsou, M., Pilidis, G., Manios, T., Kyriacou, A., 2006. Quality assessment of composts in the Greek market: the need for standards and quality assurance. J. Environ. Manage 80, 58e65. Low, K.S., Lee, C.K., 1997. Quaternized rice husk as sorbent for reactive dyes. Bioresour. Technol. 61, 121e125. Michailides, M., Christou, G., Akratos, C.S., Tekerlekopoulou, A.G., Vayenas, D.V., 2011. Composting of olive leaves and pomace from a three-phase olive mill plant. Int. Biodeterior. Biodegrad. 65, 560e564. Muktadirul Bari Chowdhury, A.K.M., Akratos, C.S., Vayenas, D.V., Pavlou, S., 2013. Olive mill waste composting: a review. Int. Biodeterior. Biodegrad. 85, 108e119. Murphy, J., Riley, J.R., 1962. A modified single solution method for determination of phosphate in natural waters. Anal. Chim. Acta 27, 31e36. Niaounakis, M., Halvadakis, C.P., 2004. Olive Mill Waste Management e Literature Review and Patent Survey. Typothito, George Dardanos Publications, Athens. Roig, A., Cayuela, M.L., Sánchez-Monedero, M.A., 2006. An overview on olive mill wastes and their valorisation methods. Waste Manag. 26, 960e969. Ryckeboer, J., Mergaert, J., Vaes, K., Klammer, S., De Clercw, D., Coosemans, J., Insam, H., Swings, J., 2003. A survey of bacteria and fungi occurring during composting and self-heating processes. Ann. Microbiol. 53, 349e410. Sánchez-Arias, V., Fernández, F.J., Villaseñor, J., Rodríguez, L., 2008. Enhancing the co-composting of olive mill wastes and sewage sludge by the addition of an industrial waste. Bioresour. Technol. 99, 6346e6353. Sellami, F., Jarboui, R., Hachicha, S., Medhioub, K., Ammar, E., 2008. Co-composting of oil exhausted olive-cake, poultry manure and industrial residues of agro-food activity for soil amendment. Bioresour. Technol. 99, 1177e1188. Toscano, p., Casacchia, T., Diacono, M., Montemurro, F., 2013. Composted olive mill by-products: compost charactrisation and application on olive orchards. J. Agric. Sci. Technol. 15, 627e638. Tortosa, G., Alburquerque, J.A., Ait-Baddi, G., Cegarra, J., 2012. The production of commercial organic amendments and fertilisers by composting of two-phase olive mill waste (“alperujo”). J. Clean. Prod. 26, 48e55. Tuomela, M., Vikman, A., Hatakka, M., Itavaara, M., 2000. Biodegradation of lignin in a compost environment: a review. Bioresour. Technol. 72, 169e183. Waterman, P.G., Mole, S., 1994. Analysis of phenolic plant metabolites. In: Lawton, J.H., Likens, G.E. (Eds.), Methods in Ecology. Blackwell Scientific Publications, Oxford. Zucconi, F., Forte, M., Monaco, A., De Bertoldi, M., 1981. Biological evaluation of compost maturity. BioCycle 22, 27e29.
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International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod
Composting of three phase olive mill solid waste using different bulking agents Abu Khayer Md Muktadirul Bari Chowdhury a, Michail K. Michailides a, Christos S. Akratos a, *, Athanasia G. Tekerlekopoulou a, Stavros Pavlou b, c, Dimitrios V. Vayenas a, b a b c
Department of Environmental and Natural Resources Management, University of Patras, G. Seferi 2, GR-30100 Agrinio, Greece Institute of Chemical Engineering Sciences, FORTH, Stadiou Str., Platani, GR-26504 Patras, Greece Department of Chemical Engineering, University of Patras, GR-26504 Patras, Greece
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 January 2014 Received in revised form 13 March 2014 Accepted 13 March 2014 Available online 12 April 2014
Pilot-scale experiments were carried out to produce good quality, low-cost composting technology from three phase olive mill solid waste (olive pomace, OP) using rice husk (RH) and olive leaves (OL) as bulking agents. A series of parallel experiments was carried out to examine the effect of: (a) initial moisture content, (b) water addition during the composting process, and (c) material ratios. To monitor the composting process and evaluate compost quality, physicochemical parameters (temperature, moisture content, pH, electrical conductivity, organic matter, volatile solids, total organic carbon, total nitrogen, total phosphorus, potassium, sodium, and water soluble phenols) were measured at different phases of the composting period. Experimental results showed that even after short composting periods, the quality of the final product remained high. To achieve higher quality of the final product, OP should be used in higher quantities than the other two materials (OL and RH). A full-scale compost unit was designed based on the experimental results. For a typical small-sized olive mill, processing 30 tonnes of olives per day for a 90 days operation period, a total area of about 500 m2 is needed to compost the mill’s entire waste production. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Three phase olive mill solid waste Rice husk Olive leaves Physicochemical parameters Phytotoxicity test
1. Introduction The olive oil extraction industry represents a substantial share of the economies of Mediterranean countries but leads to serious environmental problems by producing huge amount of wastes (byproducts) within a short production period. These by-products are olive pomace (OP) and olive mill wastewater (OMW) (for threephase systems, 3P), and two phase olive mill waste (TPOMW) (for two-phase systems, 2P) (Roig et al., 2006). 3P olive mills separate oil from by-products (e.g. OP and OMW) using a three-phase centrifuge (decanter) and 2P olive mills separate oil from TPOMW, which is a mixture of wastewater and OP, using a twophase centrifuge. 2P olive mills require smaller amounts of water for oil separation, thus producing smaller quantities of wastes compared to 3P systems. The production rate of olive oil is about 1.4e1.8 million tonnes per year in the Mediterranean resulting in 30 million tonnes of by-products (Barbera et al., 2013). A small * Corresponding author. Tel.: þ30 26410 74209; fax: þ30 26410 74176. E-mail address: [email protected] (C.S. Akratos). http://dx.doi.org/10.1016/j.ibiod.2014.03.012 0964-8305/Ó 2014 Elsevier Ltd. All rights reserved.
portion of these wastes can be used as raw materials in different industries as they contain valuable natural resources. Greece has about 2300 small-scale, rural, agro-industrial units that extract olive oil (Michailides et al., 2011). These are generally 3P systems and their by-products include olive mill residual solids (olive pomace and leaves) and OMW. Olive mills produce significant quantities of solid wastes with outputs of 0.35 tonnes of olive pomace and 0.05 tonnes of leaves per tonne of olives (Niaounakis and Halvadakis, 2004). The huge quantities of OP and OL produced within the short oil extraction season cause serious management problems in terms of volume and space. The soils of most Mediterranean countries have low organic matter content (<1%) which has negative impacts on agriculture (Alburquerque et al., 2007). Frequent application of composted organic residues increases soil fertility, mainly by improving aggregate stability and decreasing soil bulk density. Organic amendments play a positive role in climate change abatement by soil carbon sequestration. Recurrent use of composted materials enhances soil organic nitrogen content by up to 90% (Diacono and Montmurro, 2010). According to Toscano et al. (2013), composted
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olive mill by-products applied to olive groves increased olive production by 9% and olive oil production by 166e180 kg ha1. To replenish soil organic matter content and benefit eco-friendly crop production the application of OP compost could be a good solution. Several experiments have been carried out on composting olive mill by-products using OMW from 3P mills and olive humid husk (OHH) from 2P mills and adding other agricultural by-products as bulking agents. These agents include wool waste, wheat straw, olive leaves, wood chips, by-products from rice harvesting, sesame bark, sewage sludge, poultry manure, and sheep manure (Muktadirul Bari Chowdhury et al., 2013). Previous studies produced mature compost with C/N ratios ranging from 11.5 (Sellami et al., 2008) to 53.5 (Alfano et al., 2008) depending on initial carbon and nitrogen content of the materials. Compost maturity is evaluated using the germination index (GI) method. GI experiments were introduced by Zucconi et al. (1981) and include the quantification of seed germination and growth when a solution of compost extract is applied to growth media (Komilis and Tziouvaras, 2009). Lepidium sativum (salad cress) seeds are most commonly used for this experiment. Previous research has recorded GI values ranging from 45% (Alfano et al., 2008) to 195% (Michailides et al., 2011). Rice husk (RH) is an agro-industrial by-product with significant production in Greece and especially in western Greece. While rice husk is highly resistant to biodegradation, it could be used to improve compost porosity. The main objective of this work was to perform comparative pilot-scale experiments for composting OP, OL and RH, and define the best material ratios, optimum humidity and minimum composting duration. Furthermore, optimum experimental results were scaled up in order to design a full-scale composting unit for an olive mill receiving 30 m3 of olives per day, which is the typical capacity of small olive mill units in Greece. To observe the composting process, physicochemical parameters (i.e., temperature, moisture content, pH, electrical conductivity, organic matter, total C, total N, total P, K, Na, and water soluble phenols) were assessed at different stages of the experiments. To ascertain the quality of the end product, phytotoxicity tests were carried out using L. sativum seeds. 2. Materials and methods 2.1. Composting materials and process Olive pomace (OP), olive leaves (OL), rice husk (RH), water and OMW were used for this study. The OP, OL and OMW were collected
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from a three-phase olive mill located in Amfilochia, western Greece. To improve the physical condition of olive pomace for composting, olive leaves and rice husk were added as bulking agents. Water and olive mill wastewater were used as humidifying agents during the composting period. In this study, six trapezoidal bins with dimensions 1.26 m long, 0.68 m wide and 0.73 m deep and a total volume of 0.62 m3 were employed. The study was carried out at the facilities of the Department of Environmental and Natural Resources Management, University of Patras, Agrinio, in a closed area to maintain controlled temperature conditions. Three experiments were performed with different experimental set-ups (Table 1). The three experiments were conducted in different time periods using six parallel composting bins for each experiment. In each experiment different composting materials were used to determine optimal mixing ratios, and two of the six bins had the same contents to examine the effect of other operational parameters. The main objectives of the first experiment were to: (a) examine the effects of different OP and OL ratios on compost quality, (b) examine the effect of ambient temperature and compare data with a previous industrial-scale experiment (Michailides et al., 2011), and (c) control total composting duration using the compost cooling process. The second experiment was performed to examine the optimum mixing ratio of RH, while compost duration was again controlled as in the first experiment. Finally, the third experiment was performed to examine the effect of prolonged composting (not controlled by cooling) on final compost quality. In the first experiment, two identical series were set-up: Bins 1e3 and 4e6 were filled with OP and OL using different ratios (Table 1). In this experiment water was applied as a wetting agent. In the second and third experiments one bin (Bin 7 and Bin 13 respectively) was used as reference to the first experiment, while bins 8, 10, 14 and 16 were used to examine the effect of OMW as a wetting agent. Bins 9e12 and 15e18 were used to examine the effect of RH as a bulking agent. In all experiments aeration was achieved by mechanical turning which took place daily for the first three days, once every four days during the thermophillic phase, and once a week during the maturation phase. In the first two experiments compost moisture content was kept constant at about 60e65%, whereas in the third experiment compost moisture content was above 45%. According to Gajalakshmi and Abbasi (2008), moisture contents between 45 and 60% are ideal for the composting process. In the first two experiments the compost mixture was kept in the bins for 60 days. It was then removed from the bins and stored in a covered place to
Table 1 Description of experimental set-ups. Bin
1st exp
2nd exp
3rd exp
Bin1 Bin2 Bin3 Bin4 Bin5 Bin6 Bin7 Bin8 Bin9 Bin10 Bin11 Bin12 Bin13 Bin14 Bin15 Bin16 Bin17 Bin18
Composting materials ratios (per volume) Olive pomace
Olive leaves
Rice husk
2 1 1 2 1 1 2 2 1 1 1.5 0.5 2 2 1 1 1.5 0.5
1 1 2 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0 1 1 0.5 1.5 0 0 1 1 0.5 1.5
Moistening agent
Initial mass (kg)
Initial vol. (L)
Final mass (kg)
Final vol. (L)
Water Water Water Water Water Water Water OMW Water OMW Water Water Water OMW Water OMW Water Water
154.19 163.92 162.50 168.80 158.60 145.30 93.53 91.13 69.92 62.55 74.51 49.25 214.82 218.08 179.77 172.08 193.90 151.94
294 294 294 294 294 294 252 252 252 252 252 252 406 406 406 406 406 406
132.73 130.96 106.68 140.00 125.00 102.00 122.97 122.67 91.16 88.32 101.76 73.50 111.32 115.80 82.24 81.73 93.88 69.09
210 238 238 208 239 242 210 210 176 176 196 198 195 195 173 172 177 160
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mature. In the third experiment the thermophillic phase was longer in duration and the compost mixture was kept in the bins for 63e 93 days depending on when the thermophillic phase ended in each bin. The compost was then moved into a covered area for maturation (up to 120 days).
three experiments that contained the same materials at identical ratios in order to statistically assess the effect of different experimental conditions.
2.2. Physicochemical analysis
3.1. Temperature evolution
During the experiments compost temperature was monitored daily using a temperature probe inserted into the middle of each bin at a depth of 0.25 m. The ambient temperature was also monitored. Compost samples were taken from each bin on a weekly basis. Each sample was divided into two parts. One part was analysed immediately for moisture content, pH, electrical conductivity (EC), water soluble phenols (WSPH), oxygen consumption, and volatile solids. Moisture content was determined by drying the sample at 105 C for 24 h in an oven. To measure volatile solids, an oven dried sample was taken and burned at 600 C for 4 h (FCQAO, 1996). pH, EC and WSPH were measured in aqueous extract (using 1:10 w/v). The second sample portion was air dried, ground, sieved (0.5 mm), and used to determine total Kjeldahl nitrogen (TKN), total organic carbon (TOC), total phosphorus (TP), potassium (K), sodium (Na), and phytotoxicity. TKN was measured following the method described by Bremner and Mulvaney (1982), and TP was measured following the method described by Murphy and Riley (1962). Absorbance for TP was measured using a Boeco S-20 spectrophotometer. Na and K were measured using a flame photometer (Model FP902). Static respiration activity was measured using manometric respirometers (Komilis and Tziouvaras, 2009). Water soluble phenols were measured in water extracts (1:10 w/v) following the Folin-Ciocalteu method (Waterman and Mole, 1994).
Temperature evolution indicates the bio-chemical activities of compost (Tuomela et al., 2000; Ryckeboer et al., 2003). Fig. 1 presents time series charts of compost and ambient temperature for the three experiments. Experiment 1 (Bins 1e6) initially showed a characteristic rise in temperature during the composting process (Fig. 1). Here the thermophillic phase (over 45 C) commenced after day 5. The thermophillic phase was quickly obtained due to the generation of heat resulting from aerobic microbial activities on readily available organic materials. The same trend was also observed in industrial-scale compost piles using the same materials (Michailides et al., 2011; Gigliotti et al., 2012). Although the compost piles of Michailides et al. (2011) were established in an open area and those of the present study in a closed area, their temperature patterns did not show great variations. Thus ambient temperature does not greatly affect compost temperature. Maximum temperature was observed at day 7 in Bin 1 (61 C), at day 13 in Bin 2 (approximately 63 C), and at day 11 in Bins 4e6 (over 60 C). Temperature remained over 55 C for at least three days indicating that the compost remains free of pathogenic organisms (de Bertoldi et al., 1983). In the second experiment (Bins 7e12), initial moisture content was very high and water was added to minimize the composting process duration. Bins 7 and 8 reached maximum temperature (39 C) at day 43 and 44, respectively. Only Bins 9 and 10 reached the thermophillic phase (46 C) at day 9. It should be noted that initial moisture contents were very high (around 65%) in this experiment (Table 2). The temperature pattern recorded in the third experiment was almost the same as that observed in the industrial-scale application described by Michailides et al. (2011). It took almost four days to reach the thermophillic phase in all bins, a result comparable to experiment 1 data. Bins 13 and 14 had the longest thermophillic phases (92 days and 93 days, respectively). The shortest thermophillic phase was observed in Bin 18 (62 days), while the remaining bins (15, 16 and 17) had thermophillic phases of 75, 75 and 83 days, respectively. In all experiments, the use of different composting materials in different ratios did not affect temperature evolution as the temperature in all bins of the same experiment had a high crosscorrelation coefficient ranging from 0.785 to 0.979. On the contrary, excessive water addition significantly affected temperature evolution as correlation coefficients of Bin 1 with Bins 7 and 8 were low (0.007 and 0.116, respectively).
2.3. Phytotoxicity Cress (Lepidium sativum) seeds were used for the phytotoxicity test. Phytotoxicity was estimated using the germination index (GI) as described by Zucconi et al. (1981). Phytotoxicity was determined by comparing the root development of each seed that sprouted in the compost and in deionized water (the control sample). For this test, 10 g of powdered compost were mixed with 100 ml of deionized water and the solution was then stirred for 2 h. After stirring, the solution was allowed to rest for half an hour and was then diluted with distilled water to three different concentrations (25%, 50% and 100%) with clear supernatant. The control concentration (0%) comprised only distilled water. 5 ml of each compost solution was added to a petri dish containing five pieces of blotting paper onto which 25 cress seeds were evenly placed. All the concentrations of each compost sample, including the control, were replicated three times. Petri dishes were incubated under light for 48 h at 25 C. After the incubation period, root length was measured in each sprouted seed and GI was estimated. 2.4. Statistical analysis All statistical analyses were performed using SPSS 17.0 software for Windows. Cross correlation coefficients were calculated to statistically evaluate the differences in compost temperature, OM and TKN content, and C/N ratio. These four parameters were chosen as temperature is the most important operational parameter controlling the compost process, while OM, TKN and C/N ratio are important indicators of compost maturity. Cross-correlation coefficients were calculated for all bins in the same experiment to assess the effect of the different composting materials. Furthermore, cross correlation coefficients were calculated for bins in all
3. Results and discussion
3.2. Compost moisture content Moisture is one of the major physical factors that regulate indirectly compost temperature by affecting its microbial activity. In experiments 1, 2 and 3, compost moisture contents were around 60, 65% and 45%, respectively. Due to these high moisture contents the first two experiments had thermophillic phases shorter than in the third experiment. Moisture contents in excess of 60% mean pore spaces in the compost pile will be filled with water rather than air, leading to anaerobic conditions (Das and Keener, 1997). In experiment 1, 30 L of water were added from day 23 to day 33 (three times) and in experiment 2, 80 L of water were added in the first three days. In experiment 3, 41e59 L of water (the quantity varied per bin) were added from day 10 to day 106 (nine times including
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Fig. 1. Compost and ambient temperature time series charts for: (a) Bins 1e6 (first experiment) containing only OP and OL in different mixing ratios, (b) Bins 7e12 (second experiment) containing OP, OL and RH in different mixing ratios, (c) Bins 13e18 (third experiment) containing only OP and OL in the same mixing ratios as those used in the second experiment, but with prolonged composting duration.
once in the mesophillic phase). Therefore, moisture content alone does not cool the composting process, but rather the addition of water on critical days (thermophillic phase) when compost temperature is rising. Compost moisture contents of around 40e65% were used in most olive mill waste composting studies reviewed by Muktadirul Bari Chowdhury et al. (2013). 3.3. Compost attributes Table 2 presents physicochemical attributes of the compost at different stages, such as the initial stage (day 0), end of the thermophillic phase, and end of the maturation phase (day 120). The experiments presented in this paper and Michailides et al. (2011), did not show great variations in quality characteristics (Table 2), although they do have different composting durations and temperature patterns. In experiment 1, the initial pH range was 6.74e7.58 increasing to 7.58 to 8.87 at the end of the thermophillic phase. pH values then decreased and stabilized from 7.69 to 7.89 at the end of the maturation phase. Similar pH trends
were also observed in experiments 2 and 3. These pH ranges may benefit microbial activities by creating favourable environmental conditions. Several studies report that pH increases during the thermophillic phase and later decreases to near neutral (Hachicha et al., 2006; Michailides et al., 2011). The pH values of matured composts range from 6.83 to 7.79 in the present research work and are compatible with most cultivated crops (Lasaridi et al., 2006). The pH values of the present work are in line with various olive mill waste composts mentioned in the review of Muktadirul Bari Chowdhury et al. (2013). Electrical conductivity (EC) values increased during the composting process in all experiments (Table 2). Initial EC values ranged from 388 to 570 mS/cm in the first experiment, from 336 to 663 mS/ cm in the second experiment, and from 445 to 619 mS/cm in the third experiment. Final EC values (mature compost) ranged from 654 to 750 mS/cm, 548 to 861 mS/cm, and 636 to 1050 mS/cm in the first, second and third experiments, respectively. These EC values indicate that rice husk increased soluble salt concentration. The
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Table 2 Physicochemical properties of compost at different composting stages. Bin
Moist (%)
pH
One day B1 60.4 6.74 B2 60.5 7.04 B3 62.3 6.86 B4 60.5 7.08 B5 59.2 6.76 B6 74.6 7.58 B7 59.9 5.38 B8 59.1 5.07 B9 65.8 5.94 B10 62.1 4.82 B11 63.2 5.49 B12 64.8 5.92 B13 56.4 6.4 B14 52.9 6.46 B15 59.7 6.76 B16 61.3 6.72 B17 56.1 6.44 B18 57.7 7 End of thermophilic phase B1 57.1 7.58 B2 65.8 7.91 B3 63.6 7.82 B4 54.1 8.87 B5 58.0 8.79 B6 62.3 8.8 B7 50.9 8.12 B8 51.1 8.11 B9 53.8 7.74 B10 54.1 8.4 B11 50.2 7.96 B12 51.7 7.91 B13 47.6 7.3 B14 47.1 7.28 B15 48.1 7.48 B16 48.8 7.45 B17 46.4 7.46 B18 47.4 7.58 End of maturation phase B1 56.1 B2 51.0 B3 59.0 B4 48.8 7.79 B5 56.5 7.89 B6 61.0 7.69 B7 51.5 6.83 B8 50.2 7.17 B9 53.4 7.37 B10 54.8 7.14 B11 48.7 6.9 B12 52.6 7.43 B13 45.5 7.56 B14 46.6 7.4 B15 46.6 7.42 B16 47.0 7.39 B17 50.4 7.51 B18 47.8 7.54
EC (mS/cm)
VS (%)
OM (%)
TOC (%)
TKN (%)
C/N
P (%)
388 470 386 450 570 507 501 663 427 589 440 336 533 523 445 548 556 619
99.3 96.4 94.3 99.3 99.0 99.0 97.4 97.1 92.5 93.9 95.8 89.4 96.7 96.3 92.3 92.1 94.0 88.7
91.0 90.3 90.6 90.2 89.8 87.7 95.4 95.6 89.5 90.3 93.3 87.1 94.3 94.3 89.1 90.4 92.8 85.7
52.8 52.4 52.6 52.3 52.1 50.9 55.3 55.4 51.9 52.4 54.1 50.5 54.7 54.7 51.7 52.4 53.8 49.7
2.4 2.5 2.1 2.6 2.5 2.7 1.9 2.0 1.7 1.8 2.0 1.4 1.9 2.0 2.0 2.0 1.9 1.9
22.4 20.5 24.9 19.9 20.9 19.2 28.6 27.3 31.1 29.4 26.5 35.0 28.5 26.8 25.6 26.7 28.3 25.7
0.11 0.11 0.10 0.11 0.11 0.09 0.10 0.10 0.12 0.11 0.11 0.10
412 521 623 442 410 500 554 652 611 640 508 547 675 657 825 735 819 1007
92.8 89.2 88.3 94.4 93.9 90.6 97.0 96.7 89.6 89.7 94.4 85.5 93.3 94.0 90.0 90.4 90.4 86.1
91.6 87.8 82.3 90.7 86.6 85.6 93.6 93.8 86.5 86.7 91.9 83.6 87.9 87.9 83.0 83.9 84.8 80.7
53.1 50.9 47.7 52.6 50.3 49.7 54.3 54.4 50.2 50.3 53.3 48.5 51.0 51.0 48.2 48.7 49.2 46.8
3.2 2.9 3.1 3.2 3.2 3.1 2.5 2.8 2.5 2.5 2.5 1.9 3.9 3.9 3.6 3.7 3.9 3.2
16.6 17.5 15.3 16.5 15.7 16.3 21.5 19.1 20.5 20.2 21.1 25.7 12.9 13.0 13.3 13.3 12.5 14.7
0.14 0.14 0.14 0.13 0.13 0.12 0.20 0.19 0.19 0.17 0.18 0.19
654 702 750 789 726 861 837 548 710 670 636 761 837 770 1050
93.5 90.6 89.2 94.7 91.8 93.9 96.7 95.7 89.0 89.0 94.1 85.0 92.3 92.4 86.0 87.7 90.4 83.4
90.0 87.8 82.8 88.5 87.2 84.0 91.6 91.4 83.7 84.0 89.1 81.9 87.1 87.1 80.2 80.3 83.9 76.5
52.2 50.9 48.0 51.3 50.6 48.7 53.1 53.0 48.6 48.7 51.7 47.5 50.5 50.5 46.5 46.6 48.6 44.4
3.1 2.9 2.8 3.2 3.2 2.9 3.6 3.7 3.5 3.5 3.6 2.8 3.9 4.1 3.4 3.4 4.0 3.2
17.1 17.5 17.3 16.1 15.8 16.7 14.7 14.4 13.8 13.8 14.4 17.1 12.8 12.3 13.7 13.5 12.1 13.7
0.17 0.16 0.15 0.16 0.15 0.13 0.19 0.17 0.17 0.17 0.17 0.16
Na (%)
K (%)
Phenols (%)
0.06 0.07 0.07 0.10 0.07 0.07 0.07 0.08 0.07 0.07 0.07 0.07
0.023 0.026 0.017 0.024 0.022 0.011 0.020 0.020 0.016 0.016 0.018 0.017
1.22 1.14 1.20 1.04 1.15 1.04 3.27 3.45 3.01 3.12 2.85 2.34 3.13 2.70 1.97 2.94 1.27 1.99
0.09 0.11 0.09 0.11 0.08 0.09 0.15 0.11 0.10 0.10 0.13 0.13
0.035 0.039 0.020 0.024 0.026 0.015 0.040 0.035 0.029 0.030 0.034 0.027
2.52 3.41 1.84 2.18 2.02 1.35 0.25 0.19 0.23 0.21 0.22 0.24
0.046 0.047 0.029 0.030 0.030 0.018 0.037 0.030 0.024 0.027 0.024 0.023
0.58 0.44 0.53 0.27 0.44 0.16 1.1 1.04 0.53 0.97 0.62 0.69 0.21 0.2 0.17 0.18 0.21 0.2
0.15 0.13 0.12 0.12 0.10 0.10 0.11 0.09 0.09 0.08 0.08 0.09
MC: Moisture Content, EC: Electrical Conductivity, VS: Volatile Solid, TOC: Total Organic Carbon, TKN: Total Kjeldhal Nitrogen, C/N: Carbon Nitrogen ratio, P: Phosphorus, Na: Sodium, K: Potassium, WSPH: Water Soluble Phenols, GI: Germination Index.
highest EC values were obtained at the end of the composting process and this is attributed to the decrease of total mass, organic matter mineralization, and water evaporation (Baeta-Hall et al., 2005; Gigliotti et al., 2012), hence leading to increased relative soluble salt concentration. It is notable that the EC values of the mature composts in this study were low and far below the Greek standard upper limits of 4S/cm (Lasaridi et al., 2006). Therefore this compost is most probably suitable for application on a wide variety of agricultural crops. During the composting process, a small amount of organic matter loss was observed in all the experiments resulting in a
decrease in VS and TOC throughout the process especially during the maturation phase (Table 2). Organic matter content decreased during the composting process with ranges of 87.7e91 to 84e 90%, 87.1e95.6 to 81.9e91.6% and 85.7e94.3 to 78e87.1% in experiments one, two and three, respectively. The reduction of organic matter is due to mineralization of organic compounds. Total organic carbon (TOC) also decreased and ranged from 50.9e 52.8 to 48e52.2%, 50.5e55.4 to 47.5e53.1% and from 49.7e54.7 to 44.4e50.5% in experiments one, two and three, respectively. The variations of initial organic matter values between the three experiments are due to the different organic matter contents of the
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initial products used for composting (Table 1). These values of organic matter content and TOC are comparable to those reported by Michailides et al. (2011) where organic matter was 83.7% and TOC 48.5%. Organic matter and TOC decreased more in experiment 3 than in experiments 1 and 2. A significant change of organic matter and TOC content was recorded between the bins in experiments 2 and 3 but not in experiment 1. In all experiments the bins that contained the same materials with the same mixing ratios had high correlation coefficients ranging from 0.814 to 0.991. Furthermore, all bins of the same experiment also recorded high correlation coefficients ranging from 0.861 to 0.994, with the exception of Bins 3 and 6 which have somewhat lower correlation coefficients (from 0.004 to 0.590) than the other bins (1, 2, 4 and 5) of the first experiment. This indicates that the excessive use of OL leads to a final product with significantly different OM content. In experiments 1 and 2, OP was slightly fresher and took less time to compost (due to water addition) than that of experiment 3, and this may explain the reduced mineralization of organic matter observed in experiments 1 and 2. However, the second experiment showed slightly more mineralization than the first experiment. It was also observed that these changes were more pronounced when rice husk were used in the second and third experiments. It is possible that the rice husk improved microbial activity by enhancing aeration (porosity). Table 2 shows that TKN values increased during the composting process. In general, the trend of increasing organic nitrogen could be attributed to a concentration effect caused by compost’s volume and mass reduction. The highest value (4.1%) of TKN was found in Bin 14 and the lowest value (2.8%) of TKN was found in Bins 3 and 12. Higher values of TKN were observed in experiments 2 and 3 due to higher mineralization of composting materials and the longer composting period compared to experiment 1. The bins where higher ratios of rice husk were used showed comparatively lower nitrogen concentrations than the bins with less or no rice husk. This may be due to the chemical properties of the rice husk that contains high quantities of cellulose and silica and is more resistant to biodegradation (Low and Lee, 1997; Champagne, 2004). Furthermore, the bins having OMW as the wetting agent showed higher TKN than those with water. OMW may be responsible for increased nitrogen concentration though Aviani et al. (2010) did not find any significant differences of nitrogen content between composts which had water or OMW as wetting agents. Therefore, it could be inferred that specific ratios of rice husk (OP: OL: RH ¼ 1.50:1:0.5, by volume) can benefit OP and OL mineralization by increasing oxygen content, microbial activities, and ultimately nitrogen concentration in mature compost. The values of TKN observed in this study are comparable to those of Altieri and Esposito (2010), Sellami et al. (2008) and Sánchez-Arias et al. (2008). According to Italian legislation, if a mature compost contains more than 3% nitrogen it can be used as a nitrogenous fertilizer. The C/N ratio, i.e. the ratio of TOC to TKN, indicates the availability of nitrogen for the process of biological decomposition of compost. A decrease in this ratio with the progress of composting time has been widely mentioned as an index of compost stability and maturity. The C/N ratios of this research are presented in Table 2 and all the mature composts showed values close to 15, close to the limit of 20 established by Bernal et al. (1998). A decreased C/N ratio was more pronounced in the second and third experiments, because these used older olive pomace that took longer to compost than the first experiment. The greatest changes in C/N ratio were in Bins 9, 10 and 12 of experiment 2, however changes in C/N ratio were significant in Bins 13, 14 and 17 of experiment 3. C/N ratio did not show great
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variations between the experiments and the compost with the lowest quality (maximum C/N values) corresponds to high OL ratios. It should be noted that the moisture content was higher in experiment 2, hence bins with small quantities of, or lacking, rice husk showed less mineralization due to reduced aerobic conditions. On the contrary, experiment three had optimum moisture content (around 45%) which helps increase mineralization, especially in bin 17 which has the lowest ratio of RH. Therefore, it may be concluded that rice husk, as well as optimum moisture content, promotes the decrease of the C/N ratio. Many researchers (Sellami et al., 2008; Komilis and Tziouvaras, 2009) do not consider the C/N ratio an absolute indicator of compost maturity as it largely depends on the composition of initial composting materials. Therefore, it is not wise to use the C/N ratio alone as an indicator of compost maturity. For this reason, static respirometric tests were performed in this study to determine oxygen consumption which is an indicator of compost maturity and stability (Komilis and Tziouvaras, 2009). EU recommendations state that oxygen consumption in static respirometric tests should not exceed 10 g/kg dry matter after four days (Komilis and Tziouvaras, 2009). In this study, the oxygen consumption measured after four days was much lower (3.27e 5.93 g/kg dry matter) than the EU suggested limit. Although some significant differences were observed for OM, TKN and the C/N ratio they appear not to be affected by the use of different composting materials in different ratios or by the different operational parameters, because the correlation coefficients for all bins show extremely high values (from 0.718 to 0.998 for TKN and from 0.795 to 0.996 for the C/N ratio). Water soluble phenol (WSPH) is an important indicator of compost maturity of olive mill wastes and is related to the phytotoxic properties of these wastes (Ait Baddi et al., 2004; Alburquerque et al., 2006). The present study showed that WSPH decreased in all bins during composting. Especially in experiment three, WSPH was reduced by approximately 90% at maturation (Table 2). First two experiments showed lower degradation of WSPH due to high moisture content which reduces microbial activities. In experiment three, it was noticed that in the first 45 days the reduction rate of WSPH was higher in bins where rice husk was used. After day 45, a similar trend was observed in all the bins. It is possible that the aerobic microbes were positively influenced by the presence of RH and this enhanced WSPH reduction. It should be noted that when lignocellulosic material degrades WSPH are released leading to their high concentrations in the aquatic phase which later decreased due to microbial activity (Tortosa et al., 2012). The mature composts produced in all bins were rich in nutrients (Table 2). In the second experiment, P, K, Na concentrations were 0.13e0.17%, 0.018e0.047% and 0.10e0.15% respectively. In the third experiment, P, K, Na concentrations were 0.16e0.19%, 0.023e 0.037% and 0.08e0.11%, respectively. These concentrations are comparable or higher than those reported in Muktadirul Bari Chowdhury et al. (2013). Due to presence of high nutrient content, and optimum level of porosity (data not shown) these composts could be used to replenish soils exhausted by intensive cultivation as well as soils suitable for greenhouses (Hachicha et al., 2006). 3.4. Phytotoxicity test Results for the seed bioassay test were used to assess any changes in compost phytotoxicity at the start of the composting process, at the end of thermophillic phase, and at the end of maturation phase (Fig. 2). The GI values obtained from the initial stage of experiments two and three demonstrate that the initial
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Fig. 2. Germination index (GI) fluctuations with different compost contents of the three experiments at different stages of composting: (a) initial stage, (b) end of thermophillic stage, and (c) end of maturation stage.
composting materials were phytotoxic in nature. During the composting process a clear decreasing trend in phytotoxicity was noticed (GI values of 101.2e122.26% and 109.98e119.54% for second and third experiments, respectively). Gigliotti et al. (2012) also reported that the escalated phytotoxicity of this material decreased remarkably after aerobic treatment. The present study also showed similar results except in experiment one, where no phytotoxicity was detected at the beginning of the composting process. According to Lasaridi et al. (2006), composts with GI values below 80% are phytotoxic. The GI values indicate that the mature compost of the present study is not phytotoxic and therefore could be applied as a high quality organic soil amender. Results of the first experiment show that the optimum OP:OL ratio was 2:1 by volume, as increased OL levels lead to low GI and thus to phytotoxic effects of the final compost. Whilst observation of the optimal OL ratio was straightforward, for the other parameters (OP: RH ratio and composting duration) the optimal values were not so easy to define as the quality of the final composts from experiments 2 and 3 had small differences. No great variations of GI were observed either in different composting durations or in different OP loads. There is only one exception where GI values were below 100% and this
corresponds to the experiment with the increased OL ratio. No significant impact of rice husk or OMW on the phytotoxicity of the mature composts was observed. 4. Full-scale scenario Olive mill waste can be successfully composted with a final product that is a very good quality soil amendment, and an excellent choice for organic farming. Therefore, olive mill owners could benefit economically by composting their OP and OL. The main issue for an olive mill owner is the area requirement for composting the entire season’s production of OP and OL. To assess this issue, a full-scale scenario for a three-phase olive mill located in Amfilochia, western Greece, was developed. This olive mill can process 30 tonnes of olives per day and its typical operating period lasts 90 days (from October to the end of December), resulting in 2700 tonnes of processed olives. According to Niaounakis and Halvadakis (2004), OP and OL correspond to 35% and 5% of olive mill waste, respectively. This leads to a total seasonal production of OP and OL of 945 and 135 tonnes, respectively.
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The area required for composting in the full-scale scenario was calculated using the following expression:
Required area m2 ¼
OP production ðkgÞ kg Number of composting per year compost365d OP load m 2 duration d
As OP and OL are already stored in an area within the olive mill, there was no need to compost the whole production at once, rather OP mass could be composted in batches throughout the year. As expected, the minimum required area corresponds to maximum OP load and minimum compost duration is approximately 500 m2. The total area required is extremely low and is available in the majority of olive mills. Therefore, olive mill owners have an excellent alternative method for treating their waste by-products with a possible economic benefit. 5. Conclusion This study used parallel composting experiments to examine the effect of compost duration on the quality of the final product, and the use of rice husk as a bulking agent in composting olive mill waste. Mature compost from olive mill waste appears to have excellent characteristics as it is free from phytotoxicity and contains high organic matter and nutrient content (N, P, K, Na) rendering it a high quality soil amender. In general, olive mill waste can be composted alone or mixed with other agricultural by-products (e.g., rice husk) to produce high quality soil amendment. The main factors affecting compost quality are: (a) moisture and water addition during composting which affects the temperature pattern and total composting period; (b) the proportion of olive leaves in the mixture that should not exceed one third of the initial compost volume to ensure high quality compost; (c) the use of bulking agents which increase porosity (e.g., rice husk) by increasing aeration that is crucial for microbial functions. Olive mill waste composting could have a positive economic impact on olive mill owners as they can compost their entire season’s waste production in an area of about 500 m2 for a typical small-scale plant, and produce a high quality soil amendment which could then be sold. Acknowledgement The first author gratefully acknowledges a postgraduate scholarship from the Hellenic State Scholarships Foundation. References Ait Baddi, G., Alburquerque, J.A., Gonzalez, J., Cegarra, J., Hafidi, M., 2004. Chemical and spectroscopic analyses of organic matter transformations during composting of olive mill wastes. Int. Biodeterior. Biodegradation 54, 39e44. Alburquerque, J.A., Gonzalvez, J., Garcıa, D., Cegarra, J., 2006. Measuring detoxification and maturity in compost made from “alperujo”, the solid by-product of extracting olive oil by the twophase centrifugation system. Chemosphere 64, 470e477. Alburquerque, J.A., Gonzalvez, J., Garcıa, D., Cegarra, J., 2007. Effects of a compost made from the solid by-product (“alperujo”) of the two-phase centrifugation system for olive oil extraction and cotton gin waste on growth and nutrient content of ryegrass (Olium perenne L.). Bioresour. Technol. 98, 940e945. Alfano, G., Belli, C., Lustrato, G., Ranalli, G., 2008. Pile composting of two-phase centrifuged olive husk residues: technical solutions and quality of cured compost. Bioresour. Technol. 99, 4694e4701. Altieri, R., Esposito, A., 2010. Evaluation of the fertilizing effect of olive mill waste compost in short-term crops. Int. Biodeterior. Biodegrad. 64, 124e128. Aviani, I., Laor, Y., Medina, S.H., Krassnovsky, A., Raviv, M., 2010. Co-composting of solid and liquid olive mill wastes: management aspects and the horticultural value of the resulting composts. Bioresour. Technol. 101, 6699e6706.
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