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The potential use of olive mill sludge in solidification process
Resources, Conservation and Recycling 40 (2004) 129–139
The potential use of olive mill sludge in solidification process N. Hytiris a , I.E. Kapellakis a,c,∗ , R. La Roij de b , K.P. Tsagarakis c a
School of the Built and Natural Env., Glasgow Caledonian University, Glasgow, G4 0BA UK b Powerbetter, Leeds, LS268Jy/Mega-tech Tech Engineering Consultancy, Dordrecht, G3316GG The Netherlands c NAGREF, National Agricultural Research Foundation, Regional Foundation of Crete, P.O. Box. 2229, 71307 Iraklio, Greece Received 2 July 2002; accepted 27 January 2003
Abstract The industrialisation of agriculture in the second half of the last century has been accompanied by an increase in production of organic wastes. Olive mill wastewater (OMW), a by-product of olive oil processing, is one such waste. It is produced in large quantities in the Mediterranean Region, an area that accounts for 95% of the total olive oil production worldwide. In recent years both physicochemical and biological treatment methods for OMW have been employed. Olive mill sludge (OMS), a by-product of the biological treatment methods, poses a major environmental threat, yet little research has been undertaken in its treatment and recycling. The aim of this study was to investigate the potential of using OMS as an additive for the development of construction materials. An attempt was made to solidify (fixate and stabilise) this sludge with cement, mixed with an improver/additive containing a mixture of natural and synthetic zeolites, alkaline elements, and oxides. The results to date appear to be very promising. © 2003 Elsevier B.V. All rights reserved. Keywords: Olive mill sludge; Recycling; Solidification; Stabilisation; Zeolites
1. Introduction The industrialisation of agriculture in the second half of the last century has led to an increase in production of organic wastes. Olive mill wastewater (OMW), a by-product of olive oil processing, is one such waste produced mainly in areas with a semi-arid climate. It has ∗
Corresponding author. Tel.: +30-6974-63-7518. E-mail address: [email protected] (I.E. Kapellakis).
0921-3449/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-3449(03)00038-7
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N. Hytiris et al. / Resources, Conservation and Recycling 40 (2004) 129–139
Fig. 1. OMS production.
greatly contributed to environmental degradation for the following reasons: (a) the conversion from classic-type mills (3.25 kg of OMW per kg of olive oil produced) to centrifuge-type mills (5 kg of OMW per kg of olive oil produced), (b) a huge increase in olive oil production, and (c) despite the publication of over 540 studies on OMW treatment methods worldwide (Kapellakis et al., 2002), an economic and viable treatment method has yet to be found. Large amounts of OMW are produced in the Mediterranean region, an area that accounts for 95% of the total olive oil production worldwide. This type of wastewater poses a great environmental hazard if not treated properly due to its very high organic load (ca. 100 times higher than that of domestic origin). In recent years a number of OMW treatment methods have been employed, which can be divided into physico-chemical and biological methods. The former are generally very expensive and/or insufficient to completely solve the problem, however, the biological methods have the added benefit of by-products which can be recycled. A current cost-effective practice of OMW management is storage in evaporation ponds (Herold et al., 2000; Ammar and Ben Rouina, 1999; Garci-Ortiz et al., 1999). This can be considered as a biological-natural method due to the high evaporation rates prevailing during the summer in the countries where olive trees are cultivated, but large quantities of unstabilised sludge are produced, which settle at the bottom of the lagoons, accounting for 30% of the volume occupied by the OMW (Fig. 1). The aim of this study was to investigate the potential of recycling olive mill sludge (OMS) by developing construction materials. OMS was mixed with a small amount of cement containing a cement improver called ‘Powercem’. This improver comprises a mixture of natural zeolites, synthetic zeolites, oxides and small amounts of alkaline and earth alkaline elements. 2. State of the art 2.1. Olive mill sludge management Although the majority of the sludge produced in evaporation ponds is disposed of in landfill sites, the remainder is used either in agriculture after natural dewatering due to
N. Hytiris et al. / Resources, Conservation and Recycling 40 (2004) 129–139
131
Fig. 2. Disposal of OMS.
its high fertilising value or as a heat source because of its oil content. A survey of olive mill owners in Crete, Greece, revealed that only a few actually use the sludge for their own benefit. 28% reported that sludge was collected by farmers and after being digested used as a fertiliser. A further 16% reported that farmers were interested in using sludge as a heat source, either for the production of ‘raki’, a traditional Cretan alcoholic drink, or for domestic heating. However, 56% of owners reported that sludge was not recycled and consequently it was disposed of in landfill sites (Fig. 2), or was used to reinforce the inner sides of OMW evaporation ponds in order to avoid accidental leakages (Kapellakis et al., 2002). Limited research has been carried out into the utilisation of OMS, as research is mainly oriented towards OMW treatment. The majority of the studies which refer to OMS focus on composting (Paredes et al., 2000). Sahini (1997) refers to a method of co-composting OMS with other solid by-products of olive processing (olive cake and leaves) for use in agriculture. Cegarra et al. (1996) studied the characteristics of OMS composts and examined their effects on the yield and nutritional status of different crops. Paredes et al. (1996) tried to ascertain the most suitable conditions for degrading the OMS through composting and concluded that co-composting with cotton waste appeared to be better, as it generated a more humified organic matter at the end of the process. In a later study, Paredes et al. (1999) classified OMW and OMS samples from different mills in an attempt to find correlations which would make it possible to estimate the composition of these wastes from easily determined parameters. It was found that both groups of wastes showed notable contents of organic matter and substantial quantities of plant nutrients in comparison to those found in manures and city refuse composts. In the case of OMS, only the total organic carbon and total nitrogen concentrations could be calculated by reference to the ash content, a method that is more straightforward and less costly than those methods usually used for nutrient analysis. Recently, the evolution of parameters describing the stabilisation and humification of OMS when co-composted with different agricultural wastes was studied, in order to evaluate the requirements for an adequate composting. It was concluded that composting can be an environment-friendly alternative to OMS disposal (Paredes et al., 2002).
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2.2. Solidification Solidification of waste has existed since Roman times (van Winden et al., 1991) and is practised mainly in countries with high population densities, where large quantities of waste are produced and shortage of space creates handling difficulties. In most cases it is practised in countries where the availability of natural aggregates is limited, such as in the Netherlands (Delsman, 1991; Stoelhorst, 1991). It should be noted that most of the studies carried out on stabilisation/solidification techniques involved the use of municipal and industrial solid waste, fly ash, etc. However, the use of solidified OMS in construction industry has never been investigated before.
3. Objectives Objective of this project was to produce a construction material by mixing OMS with cement containing a special cement improver/additive, consisting of artificial and natural zeolites and lime at specific proportions (called commercially ‘Powercem’). Cement has always been an attractive medium to use since the stabilisation/solidification process is brought about by a so-called ‘cold-process’, which consumes very little energy. During the 1990s, engineers were interested in the application of cement as a cheap and effective medium for immobilisation/solidification processes. However, using cement alone was found to be unsatisfactory as binding processes fail in acidic environment (pH < 7) or the organic content of the medium is too high. Furthermore, inadmissible fulvic and substances can negatively influence the final result and the long term physically and chemically characteristics of the end product. Powercem is the ‘ideal medium’ for use in processes, where cement alone, fails. Powercem is the brand name of a newly developed stabilisation and immobilisation product. It is a very fine grain sized additive, which consists of alkaline and earth alkaline elements, complemented with complex compounds, which are determined per type of application and related to treated base material. Eventual organic and/or chemical/toxic contaminants can be treated effectively. This fine sized grain mixture, in addition with water, causes a complex chemical reaction. This chemical reaction is different and complex in art form related to the type of Powercem product and base material, which has to be treated.
4. Materials and methods In order to examine the stabilisation/solidification effect of Powercem on OMS, two random representative OMS samples with different moisture levels were collected from two evaporation lagoons in Crete, Greece, and labelled as Sample 1 and 2 (Fig. 3). Prior to stabilisation and solidification process, tests on the physical and chemical properties of the two samples were carried out. These tests included determination of pH, oxide composition, phenol concentration, loss of ignition, sieve analysis, cationic and anionic substances (salts), water content and pH. The methodology and the techniques used for the analysis of the samples are presented in Table 1.
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Fig. 3. Magnification of the untreated OMS after its collection.
Table 1 Methodology used for the analysis of the samples Analysis Compressive strength Sieve analysis E-modulus Moisture content Density Microscope analysis pH measurement Fraction <63 m Phenol Dry matter
Technique Gravimetric Traverse
Electron microscopy Potentiometric analysis HPLC Dry matter at 105 ◦ C
Method NEN 5968 Standard RAW pr. 2 ULTRASONIC NEN 5934 NEN 5967 SEM/EDXA NEN 5750 st. RAW test 2 INTRON NEN 5747
The samples were processed together with cement and Powercem without any addition of aggregates, according to the procedures shown in Fig. 4. Mixing the samples with differing proportions of aggregates such as gravel, sand and demolition residues can provide alternatives to the first mix and can be used as a fill or as environmentally safe secondary materials. In the treated samples the tests undertaken were chemical (leaching behaviour of aromatics, and of organic and inorganic substances according to the Dutch norm1 ) and physical (unconfined compressive strength; E-stat./E-dyn.; permeability). After being stabilised, sample 1 presented higher percentage of dry matter than that of sample 2, as it had been collected from an area with lower precipitation levels and 1 The Dutch NEN norm 5920 is especially developed in order to determine the degree of destructive pollution caused by fine substances of organic origin.
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Fig. 4. Procedure followed for the preparation of the two samples.
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Fig. 5. Product 1 modified with Powercem after 14 days curing. Volume 1 dm3 .
higher evaporation rates. This is the explanation for the different consistency showed by sample 2. From the sample with the lowest moisture content (Sample 1) two small sized cylinders and one proctor cylinder were prepared. Products behaved as elastoplastic for the first 5 days. After 7 days they started to gain stiffness and from then onwards their strength increased (Fig. 5). Three small sized cylinders and one proctor cylinder were prepared from the second sample with the highest moisture content. Like before, the products behaved as an elastoplastic material for the first 5 days. After 7 days they started to gain additional stiffness and from then onwards, their strength increased. 5. Results and discussion The results obtained in this study show that OMS when used in solidification/stabilisation processes can form a construction material with favourable characteristics (Table 2). The results for the chemical composition of both OMS samples are presented in Table 3, while the composition of granulometric fractions is presented in Fig. 6. Table 2 Mechanical and physical properties of both OMS samples 14 days after stabilisation/solidification treatment Analysis
Unit
Sample 1
Phenola
mg/l % w/w
<0.75 92.6 11.72 0.76 49.24 1490.00 1380.00
Dry matter pH (CaCl2 ) Compressive strength E-modulus Wet density Dry density a
Based upon oven dry material.
N/mm2 N/mm2 kg/m3 kg/m3
Sample 2 61.8
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Table 3 The oxides composition of OMS samples Element
MgO
Al2 O3
SiO2
SO3
Cl2 O
K2 O
CaO
Fe2 O3
Total
% w/w
4.1
9.4
33.8
6.2
0.5
2.0
40.8
3.2
100
Fig. 6. Sieve analyses of OMS prior to stabilisation/solidification.
The first sample of OMS was successfully treated, without any addition of aggregates. After treatment, a solid material was formed within 15 days, having a compressive strength of almost 1 Mpa and an elastic modulus of almost 50 MPa. It is worth considering that the mixture contained only 3.6 g of cement additive, which corresponds to less than 0.1% of the OMS by weight. To get significant benefits from the additive, proportions should not be reduced. Consequently, an increase in the proportion of Powercem should lead to increased compressive strength. This, however, will have to be established in future work during optimisation. The second sample of the OMS had a totally different consistency and due to its high moisture content behaved like ‘slurry’. Hence, optimisation tests will be necessary in order to obtain a more homogeneous mix with constructive characteristics. Therefore, the moisture content of the sludge must be taken into consideration. As stated previously, sludge is produced after the evaporation of the liquid phase of the OMW. Consequently, the optimum period for sludge collection is between August and October. This period is determined by the evaporation rates occurring in the lagoons and the precipitation levels. The OMS particles performed low cohesion and showed an asymmetric net-shape, but in general were irregular. After being treated, particles were bound due to the chemical composition of Powercem and they formed crystals (Fig. 7). Concentration of phenols in settled OMS ranges from 460 to 1080 mg/l, depending on factors such as the olive oil
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Fig. 7. A microscopic view of the OMS shown in Fig. 3(a) before the Powercem addition, (b) with the addition of Powercem, where the crystalline structure is evident.
production process, olive variety, climatic conditions, and the harvesting period. Although phenols pose one of the most serious problems in OMW and OMS treatment, as the acidity and the presence of phytotoxic compounds can induce negative effects on cultures, when OMS was mixed with Powercem and cement, even in high proportions, phenol concentration dropped to less than 0.75 mg/l, which corresponds to a reduction by a factor of almost 1000. Furthermore, the pH value of the treated material increased with the addition of Powercem. The pH value of untreated sludge was less than 5, but when was mixed, it was increased to 11.72. Powercem binds the water/cement ratio strongly and secures the segregation of water and cement during diffusion (mixing process). This results in a chemical equilibrium, which activates the growth and number of crystals within the matrix (Fig. 7). It eliminates harmful substances in general and activates covalent binding properties. If effective and long lasting, cold processes are usually the most suitable and cheapest solutions for the environment.
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5.1. Economic considerations Apart from the technological considerations, it is important to ascertain whether the use of treated OMS with cement and cement additive (such as Powercem) is economically viable. The quantity of Powercem to be used and consequently the cost of the resultant mixture depend on the desired strength of the mixture. When the formed material is to be used for the enlargement of parking areas, roads, water purification plants, foundations, etc., the preliminary results show that the cost of treating the sludge ranges from 10 to 40 ? per m3 of OMS, depending strongly on the country where is being processed. Infrastructure constructed with cement, Powercem and OMS can be up to 20% cheaper in cost when compared with traditional construction methods. However, more detailed study is required in order to achieve cost optimisation. 5.2. Practical applications The findings of this study show that the use of OMS as a construction material is technically justified. Depending on the economic considerations, it appears that the use of OMS is possible in the following two fields. Sample 2 (sludge with high moisture content) could be used as non-formed material in landfill or it could be crushed, pulverised and used as drainage material. Sample 1 (sludge with low moisture content) could be used as formed material in the enlargement of parking areas, roads, water purification plants, foundations etc. In previous applications (Queretaro highway, Mexico; Philips Electronics building, Germany; streets at Bosa, Colombia; etc.) was revealed that addition of Powercem in concrete prevents exploding during fire, increases the water-impermeability and it is highly shock-resistant (resists to earthquakes) (Ferguson and Gibbons, 2002).
6. Conclusion Powercem is a specially selected blend of natural and synthetic zeolites with small amounts of alkaline elements and oxides, which counteracts the harmful effect of the cement alone mix so that finally a durable long needle crystalline structure occurs. The pH value rises with the addition of Powercem even when the medium is extremely acidic. Powercem binds the water cement proportion strongly and secures the segregation of water and cement during diffusion (mixing process). This results in a chemical equilibrium, which activates the growth and number of crystals within the matrix. Powercem eliminates harmful substances in general and activates covalent binding properties. Cold processes if effective and long lasting are the cheapest and most environmental friendly solutions. Sample 1 was treated successfully without any further addition of aggregates. A formed construction material can be obtained with economic value, which can be used for the enlargement of parking areas, roads, water purification plants, foundations etc. Optimisation of sample 2 is preferable in order to obtain a homogeneous mix with construction characteristics. Non-formed material can be used in landfill, or as a drainage material instead of natural gravel. Preliminary results show that the cost of treating the OMS represents a cost-effective solution. In addition, further reduction could be achieved after optimisation,
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as the purchase price of the product had not been taken into consideration. The cost effectiveness of this solution will be enhanced if environmental benefits of reducing wastes by recycling are considered.
Acknowledgements This study was financed by I.O.T. Ltd., UK. Part of this paper was presented on the ‘IWA Regional Symposium on Water Recycling in Mediterranean Region’ in Iraklio, Greece, 26–29 September 2002.
References Ammar E, Ben Rouina B. Potential horticultural utilization of olive oil processing wastewater. Acta Hort 1999;474:741–4. Cegarra J, Paredes C, Bernal A, Roig MP, Garcia D. Use of olive mill wastewater compost for crop production. Int Biodeterioration Biodegradation 1996;38(3–4):193–203. Delsman G. Waste policy related to the national environmental policy plan. In: Proceedings of the International Conference on Environmental Implications of Construction with Waste Materials, 10–14 November, Maastricht, The Netherlands, 1991. p. 37–48. Garci-Ortiz A, Beltran G, Gonzalez P, Ordonez R, Giraldez JV. Vegetation water (alpechin) application effects on soils and plants. Acta Hort 1999;474:749–52. Herold D, Neskakis A, Wagner L. Experimental olive oil wastewater treatment plant in Crete. 1st World Water Congress of the IWA, Paris, 3–7 July, 2000. Ferguson D, Gibbons J. Personal communication. International Overseas Limited (IOT). 2002. Kapellakis IE, Tsagarakis KP, Avramaki Ch, Crowther JM, Hytiris N. Potential for olive mill wastewater reuse: The case of the Messara basin in Crete, Greece. In: IWA Regional Symposium on Water Recycling in Mediterranean Region, Iraklio, Greece, 26–29 September 2002, Vol. I: 515–524. Paredes C, Bernal MP, Roig A, Cegarra J, Sanchez-Monedero MA. Influence of the bulking agent on the degradation of olive-mill wastewater sludge during composting. Int Biodeterioration Biodegradation 1996;38(3–4):205–10. Paredes C, Cegarra J, Roig A, Sanchez-Monedero MA, Bernal MP. Characterization of olive mill wastewater (alpechin) and its sludge for agricultural purposes. Bioresour Technol 1999;67:111–5. Paredes C, Roig A, Bernal MP, Sanchez-Monedero MA, Cegarra J. Evolution of organic matter and nitrogen during co-composting of olive mill wastewater with solid organic wastes. Biol Fertil Soils 2000;32:222–7. Paredes C, Bernal MP, Cegarra J, Roig A. Bio-degradation of olive mill wastewater sludge by its co-composting with agricultural wastes. Bioresource Technology 2002;85(1):1–8. Sahini K. Olive mill waste, treatment methods. Elia and Elaiolado 1997;2:43–7 (in Greek). Stoelhorst D. The use of waste materials in civil engineering: AVI slag can replace gravel in concrete production. In: Proceedings of the International Conference on Environmental Implications of Construction with Waste Materials, 10–14 November, Maastricht, The Netherlands, 1991. p. 71–80. van Winden R, Van der Zwan JT, Zeilmaker J. Applications of waste materials at infrastructural works. In: Proceedings of the International Conference on Environmental Implications of Construction with Waste Materials, 10–14 November, Maastricht, The Netherlands, 1991. p. 91–106.
The potential use of olive mill sludge in solidification process N. Hytiris a , I.E. Kapellakis a,c,∗ , R. La Roij de b , K.P. Tsagarakis c a
School of the Built and Natural Env., Glasgow Caledonian University, Glasgow, G4 0BA UK b Powerbetter, Leeds, LS268Jy/Mega-tech Tech Engineering Consultancy, Dordrecht, G3316GG The Netherlands c NAGREF, National Agricultural Research Foundation, Regional Foundation of Crete, P.O. Box. 2229, 71307 Iraklio, Greece Received 2 July 2002; accepted 27 January 2003
Abstract The industrialisation of agriculture in the second half of the last century has been accompanied by an increase in production of organic wastes. Olive mill wastewater (OMW), a by-product of olive oil processing, is one such waste. It is produced in large quantities in the Mediterranean Region, an area that accounts for 95% of the total olive oil production worldwide. In recent years both physicochemical and biological treatment methods for OMW have been employed. Olive mill sludge (OMS), a by-product of the biological treatment methods, poses a major environmental threat, yet little research has been undertaken in its treatment and recycling. The aim of this study was to investigate the potential of using OMS as an additive for the development of construction materials. An attempt was made to solidify (fixate and stabilise) this sludge with cement, mixed with an improver/additive containing a mixture of natural and synthetic zeolites, alkaline elements, and oxides. The results to date appear to be very promising. © 2003 Elsevier B.V. All rights reserved. Keywords: Olive mill sludge; Recycling; Solidification; Stabilisation; Zeolites
1. Introduction The industrialisation of agriculture in the second half of the last century has led to an increase in production of organic wastes. Olive mill wastewater (OMW), a by-product of olive oil processing, is one such waste produced mainly in areas with a semi-arid climate. It has ∗
Corresponding author. Tel.: +30-6974-63-7518. E-mail address: [email protected] (I.E. Kapellakis).
0921-3449/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-3449(03)00038-7
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N. Hytiris et al. / Resources, Conservation and Recycling 40 (2004) 129–139
Fig. 1. OMS production.
greatly contributed to environmental degradation for the following reasons: (a) the conversion from classic-type mills (3.25 kg of OMW per kg of olive oil produced) to centrifuge-type mills (5 kg of OMW per kg of olive oil produced), (b) a huge increase in olive oil production, and (c) despite the publication of over 540 studies on OMW treatment methods worldwide (Kapellakis et al., 2002), an economic and viable treatment method has yet to be found. Large amounts of OMW are produced in the Mediterranean region, an area that accounts for 95% of the total olive oil production worldwide. This type of wastewater poses a great environmental hazard if not treated properly due to its very high organic load (ca. 100 times higher than that of domestic origin). In recent years a number of OMW treatment methods have been employed, which can be divided into physico-chemical and biological methods. The former are generally very expensive and/or insufficient to completely solve the problem, however, the biological methods have the added benefit of by-products which can be recycled. A current cost-effective practice of OMW management is storage in evaporation ponds (Herold et al., 2000; Ammar and Ben Rouina, 1999; Garci-Ortiz et al., 1999). This can be considered as a biological-natural method due to the high evaporation rates prevailing during the summer in the countries where olive trees are cultivated, but large quantities of unstabilised sludge are produced, which settle at the bottom of the lagoons, accounting for 30% of the volume occupied by the OMW (Fig. 1). The aim of this study was to investigate the potential of recycling olive mill sludge (OMS) by developing construction materials. OMS was mixed with a small amount of cement containing a cement improver called ‘Powercem’. This improver comprises a mixture of natural zeolites, synthetic zeolites, oxides and small amounts of alkaline and earth alkaline elements. 2. State of the art 2.1. Olive mill sludge management Although the majority of the sludge produced in evaporation ponds is disposed of in landfill sites, the remainder is used either in agriculture after natural dewatering due to
N. Hytiris et al. / Resources, Conservation and Recycling 40 (2004) 129–139
131
Fig. 2. Disposal of OMS.
its high fertilising value or as a heat source because of its oil content. A survey of olive mill owners in Crete, Greece, revealed that only a few actually use the sludge for their own benefit. 28% reported that sludge was collected by farmers and after being digested used as a fertiliser. A further 16% reported that farmers were interested in using sludge as a heat source, either for the production of ‘raki’, a traditional Cretan alcoholic drink, or for domestic heating. However, 56% of owners reported that sludge was not recycled and consequently it was disposed of in landfill sites (Fig. 2), or was used to reinforce the inner sides of OMW evaporation ponds in order to avoid accidental leakages (Kapellakis et al., 2002). Limited research has been carried out into the utilisation of OMS, as research is mainly oriented towards OMW treatment. The majority of the studies which refer to OMS focus on composting (Paredes et al., 2000). Sahini (1997) refers to a method of co-composting OMS with other solid by-products of olive processing (olive cake and leaves) for use in agriculture. Cegarra et al. (1996) studied the characteristics of OMS composts and examined their effects on the yield and nutritional status of different crops. Paredes et al. (1996) tried to ascertain the most suitable conditions for degrading the OMS through composting and concluded that co-composting with cotton waste appeared to be better, as it generated a more humified organic matter at the end of the process. In a later study, Paredes et al. (1999) classified OMW and OMS samples from different mills in an attempt to find correlations which would make it possible to estimate the composition of these wastes from easily determined parameters. It was found that both groups of wastes showed notable contents of organic matter and substantial quantities of plant nutrients in comparison to those found in manures and city refuse composts. In the case of OMS, only the total organic carbon and total nitrogen concentrations could be calculated by reference to the ash content, a method that is more straightforward and less costly than those methods usually used for nutrient analysis. Recently, the evolution of parameters describing the stabilisation and humification of OMS when co-composted with different agricultural wastes was studied, in order to evaluate the requirements for an adequate composting. It was concluded that composting can be an environment-friendly alternative to OMS disposal (Paredes et al., 2002).
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2.2. Solidification Solidification of waste has existed since Roman times (van Winden et al., 1991) and is practised mainly in countries with high population densities, where large quantities of waste are produced and shortage of space creates handling difficulties. In most cases it is practised in countries where the availability of natural aggregates is limited, such as in the Netherlands (Delsman, 1991; Stoelhorst, 1991). It should be noted that most of the studies carried out on stabilisation/solidification techniques involved the use of municipal and industrial solid waste, fly ash, etc. However, the use of solidified OMS in construction industry has never been investigated before.
3. Objectives Objective of this project was to produce a construction material by mixing OMS with cement containing a special cement improver/additive, consisting of artificial and natural zeolites and lime at specific proportions (called commercially ‘Powercem’). Cement has always been an attractive medium to use since the stabilisation/solidification process is brought about by a so-called ‘cold-process’, which consumes very little energy. During the 1990s, engineers were interested in the application of cement as a cheap and effective medium for immobilisation/solidification processes. However, using cement alone was found to be unsatisfactory as binding processes fail in acidic environment (pH < 7) or the organic content of the medium is too high. Furthermore, inadmissible fulvic and substances can negatively influence the final result and the long term physically and chemically characteristics of the end product. Powercem is the ‘ideal medium’ for use in processes, where cement alone, fails. Powercem is the brand name of a newly developed stabilisation and immobilisation product. It is a very fine grain sized additive, which consists of alkaline and earth alkaline elements, complemented with complex compounds, which are determined per type of application and related to treated base material. Eventual organic and/or chemical/toxic contaminants can be treated effectively. This fine sized grain mixture, in addition with water, causes a complex chemical reaction. This chemical reaction is different and complex in art form related to the type of Powercem product and base material, which has to be treated.
4. Materials and methods In order to examine the stabilisation/solidification effect of Powercem on OMS, two random representative OMS samples with different moisture levels were collected from two evaporation lagoons in Crete, Greece, and labelled as Sample 1 and 2 (Fig. 3). Prior to stabilisation and solidification process, tests on the physical and chemical properties of the two samples were carried out. These tests included determination of pH, oxide composition, phenol concentration, loss of ignition, sieve analysis, cationic and anionic substances (salts), water content and pH. The methodology and the techniques used for the analysis of the samples are presented in Table 1.
N. Hytiris et al. / Resources, Conservation and Recycling 40 (2004) 129–139
133
Fig. 3. Magnification of the untreated OMS after its collection.
Table 1 Methodology used for the analysis of the samples Analysis Compressive strength Sieve analysis E-modulus Moisture content Density Microscope analysis pH measurement Fraction <63 m Phenol Dry matter
Technique Gravimetric Traverse
Electron microscopy Potentiometric analysis HPLC Dry matter at 105 ◦ C
Method NEN 5968 Standard RAW pr. 2 ULTRASONIC NEN 5934 NEN 5967 SEM/EDXA NEN 5750 st. RAW test 2 INTRON NEN 5747
The samples were processed together with cement and Powercem without any addition of aggregates, according to the procedures shown in Fig. 4. Mixing the samples with differing proportions of aggregates such as gravel, sand and demolition residues can provide alternatives to the first mix and can be used as a fill or as environmentally safe secondary materials. In the treated samples the tests undertaken were chemical (leaching behaviour of aromatics, and of organic and inorganic substances according to the Dutch norm1 ) and physical (unconfined compressive strength; E-stat./E-dyn.; permeability). After being stabilised, sample 1 presented higher percentage of dry matter than that of sample 2, as it had been collected from an area with lower precipitation levels and 1 The Dutch NEN norm 5920 is especially developed in order to determine the degree of destructive pollution caused by fine substances of organic origin.
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Fig. 4. Procedure followed for the preparation of the two samples.
N. Hytiris et al. / Resources, Conservation and Recycling 40 (2004) 129–139
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Fig. 5. Product 1 modified with Powercem after 14 days curing. Volume 1 dm3 .
higher evaporation rates. This is the explanation for the different consistency showed by sample 2. From the sample with the lowest moisture content (Sample 1) two small sized cylinders and one proctor cylinder were prepared. Products behaved as elastoplastic for the first 5 days. After 7 days they started to gain stiffness and from then onwards their strength increased (Fig. 5). Three small sized cylinders and one proctor cylinder were prepared from the second sample with the highest moisture content. Like before, the products behaved as an elastoplastic material for the first 5 days. After 7 days they started to gain additional stiffness and from then onwards, their strength increased. 5. Results and discussion The results obtained in this study show that OMS when used in solidification/stabilisation processes can form a construction material with favourable characteristics (Table 2). The results for the chemical composition of both OMS samples are presented in Table 3, while the composition of granulometric fractions is presented in Fig. 6. Table 2 Mechanical and physical properties of both OMS samples 14 days after stabilisation/solidification treatment Analysis
Unit
Sample 1
Phenola
mg/l % w/w
<0.75 92.6 11.72 0.76 49.24 1490.00 1380.00
Dry matter pH (CaCl2 ) Compressive strength E-modulus Wet density Dry density a
Based upon oven dry material.
N/mm2 N/mm2 kg/m3 kg/m3
Sample 2 61.8
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Table 3 The oxides composition of OMS samples Element
MgO
Al2 O3
SiO2
SO3
Cl2 O
K2 O
CaO
Fe2 O3
Total
% w/w
4.1
9.4
33.8
6.2
0.5
2.0
40.8
3.2
100
Fig. 6. Sieve analyses of OMS prior to stabilisation/solidification.
The first sample of OMS was successfully treated, without any addition of aggregates. After treatment, a solid material was formed within 15 days, having a compressive strength of almost 1 Mpa and an elastic modulus of almost 50 MPa. It is worth considering that the mixture contained only 3.6 g of cement additive, which corresponds to less than 0.1% of the OMS by weight. To get significant benefits from the additive, proportions should not be reduced. Consequently, an increase in the proportion of Powercem should lead to increased compressive strength. This, however, will have to be established in future work during optimisation. The second sample of the OMS had a totally different consistency and due to its high moisture content behaved like ‘slurry’. Hence, optimisation tests will be necessary in order to obtain a more homogeneous mix with constructive characteristics. Therefore, the moisture content of the sludge must be taken into consideration. As stated previously, sludge is produced after the evaporation of the liquid phase of the OMW. Consequently, the optimum period for sludge collection is between August and October. This period is determined by the evaporation rates occurring in the lagoons and the precipitation levels. The OMS particles performed low cohesion and showed an asymmetric net-shape, but in general were irregular. After being treated, particles were bound due to the chemical composition of Powercem and they formed crystals (Fig. 7). Concentration of phenols in settled OMS ranges from 460 to 1080 mg/l, depending on factors such as the olive oil
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Fig. 7. A microscopic view of the OMS shown in Fig. 3(a) before the Powercem addition, (b) with the addition of Powercem, where the crystalline structure is evident.
production process, olive variety, climatic conditions, and the harvesting period. Although phenols pose one of the most serious problems in OMW and OMS treatment, as the acidity and the presence of phytotoxic compounds can induce negative effects on cultures, when OMS was mixed with Powercem and cement, even in high proportions, phenol concentration dropped to less than 0.75 mg/l, which corresponds to a reduction by a factor of almost 1000. Furthermore, the pH value of the treated material increased with the addition of Powercem. The pH value of untreated sludge was less than 5, but when was mixed, it was increased to 11.72. Powercem binds the water/cement ratio strongly and secures the segregation of water and cement during diffusion (mixing process). This results in a chemical equilibrium, which activates the growth and number of crystals within the matrix (Fig. 7). It eliminates harmful substances in general and activates covalent binding properties. If effective and long lasting, cold processes are usually the most suitable and cheapest solutions for the environment.
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5.1. Economic considerations Apart from the technological considerations, it is important to ascertain whether the use of treated OMS with cement and cement additive (such as Powercem) is economically viable. The quantity of Powercem to be used and consequently the cost of the resultant mixture depend on the desired strength of the mixture. When the formed material is to be used for the enlargement of parking areas, roads, water purification plants, foundations, etc., the preliminary results show that the cost of treating the sludge ranges from 10 to 40 ? per m3 of OMS, depending strongly on the country where is being processed. Infrastructure constructed with cement, Powercem and OMS can be up to 20% cheaper in cost when compared with traditional construction methods. However, more detailed study is required in order to achieve cost optimisation. 5.2. Practical applications The findings of this study show that the use of OMS as a construction material is technically justified. Depending on the economic considerations, it appears that the use of OMS is possible in the following two fields. Sample 2 (sludge with high moisture content) could be used as non-formed material in landfill or it could be crushed, pulverised and used as drainage material. Sample 1 (sludge with low moisture content) could be used as formed material in the enlargement of parking areas, roads, water purification plants, foundations etc. In previous applications (Queretaro highway, Mexico; Philips Electronics building, Germany; streets at Bosa, Colombia; etc.) was revealed that addition of Powercem in concrete prevents exploding during fire, increases the water-impermeability and it is highly shock-resistant (resists to earthquakes) (Ferguson and Gibbons, 2002).
6. Conclusion Powercem is a specially selected blend of natural and synthetic zeolites with small amounts of alkaline elements and oxides, which counteracts the harmful effect of the cement alone mix so that finally a durable long needle crystalline structure occurs. The pH value rises with the addition of Powercem even when the medium is extremely acidic. Powercem binds the water cement proportion strongly and secures the segregation of water and cement during diffusion (mixing process). This results in a chemical equilibrium, which activates the growth and number of crystals within the matrix. Powercem eliminates harmful substances in general and activates covalent binding properties. Cold processes if effective and long lasting are the cheapest and most environmental friendly solutions. Sample 1 was treated successfully without any further addition of aggregates. A formed construction material can be obtained with economic value, which can be used for the enlargement of parking areas, roads, water purification plants, foundations etc. Optimisation of sample 2 is preferable in order to obtain a homogeneous mix with construction characteristics. Non-formed material can be used in landfill, or as a drainage material instead of natural gravel. Preliminary results show that the cost of treating the OMS represents a cost-effective solution. In addition, further reduction could be achieved after optimisation,
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as the purchase price of the product had not been taken into consideration. The cost effectiveness of this solution will be enhanced if environmental benefits of reducing wastes by recycling are considered.
Acknowledgements This study was financed by I.O.T. Ltd., UK. Part of this paper was presented on the ‘IWA Regional Symposium on Water Recycling in Mediterranean Region’ in Iraklio, Greece, 26–29 September 2002.
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