The environmental effects of olive-mill liquid wastes on soil are known since antiquity. The Roman author Varro (I, 55) had observed that where the amurca — the watery residue obtained when the oil is drained from olive fruits — flowed from the olive presses onto the fields, the ground became barren. Theophrastus10 (IV, 16) (see Fig. 3.1) wrote that pouring olive oil over the roots could kill trees, young trees being more susceptible to this treatment than mature ones — see also Chapter 10: "Uses", section: "Use as herbicide/pesticide".
The uncontrolled disposal of OMWW on the land has the drawback of dispersing in the environment substances that are foul smelling and possibly pathogenic. In fact, higher application rates result in anomalous fermentations of the dispersed organic substances, which lead to changing the environmental conditions for microorganisms, the soil-air and the air-water balance and, therefore, to reduction of the soil fertility. However, if one could optimize the use of these wastes, they could be proved beneficial, as soil amendments, to the physical, chemical, and biological properties of the soil — see Chapter 8: "Biological processes'', section: "Irrigation of agricultural land/Land spreading''.
10Theophrastus (c.372-c.287 B.C.), Greek philosopher born in the island of Lesvos; Aristotle's successor as head of the Peripatetics. The school flourished under his leadership. He wrote on many subjects, but many of his treatises are lost. He did much to popularize science. His works on plants are perhaps the most important of his technical writings. His History of Plants and Enquiry into Plants presented the first thorough treatment of the science of botany and remained the definitive works on the subject through the Middle Ages. Also extant are portions of his History of Physics; nine scientific treatises including On Stones, On Fire, and On Winds.
Fig. 3.1. Theophrastus.
The porosity corresponds to the volume of the soil occupied by water and air. Through the pores the soil exchanges water and air with the environment. These exchanges are indispensable for the development of the fauna and the microflora of the soil as well for the respiration of the roots.
Cox L. et al. (1997) studied the effect of OMWW on soil porosity in clay soil columns. Soil columns were hand packed with the unamended clay soil and with the same soil, which had been treated for three years with two different doses of OMWW (low dose: 300ml/m2 a year and high dose: 600ml/m2 a year). OMWW amendment resulted in an increase in the organic carbon content of the soils and a reduction in soil porosity, the later confirmed by mercury intrusion poro-simetry (MIP) and scanning electron microscopy (SEM) studies. MIP and SEM data showed that the reduction in porosity is basically due to a reduction in larger pores (radius > 1 mm) and an important increase in finer pores (radius <0.1 mm) (Cox L. et al., 1996). Similar results were reported by Zenjari B. and Nejmeddine A. (2001). These changes in porosity are attributed to the combined effect of the suspended and soluble organic matter and salts in OMWW and the solubilization-insolubilization of the soil carbonate minerals promoted by OMWW.
In the open field the application of OMWW on soil initially brings about a reduction in the microporosity (pores < 50 mm) in the surface layers of the soil. At the end of the winter with the resumption of the microbial activity the micro-porosity increases significantly compared to a non-treated soil (Pagliai M., 1996). The temporary decrease of the microporosity is not harmful neither for the microorganisms nor for the roots because of the reduction of their metabolic activity during winter. The macroporosity (pores > 50 mm) increases proportionally with the quantity of applied OMWW. However, excessive doses (more than 200m3/ha) can cause structural damage accompanied by a decrease of the porosity, particularly in clay soils (Pagliai M. et al., 1993); in France and Italy the use of such quantities is forbidden (Italian law 574/1996; Le Verge S., 2004).
The aggregates of the soil have the tendency to disintegrate under the impact of the rain droplets forming a crust on the surface that obstructs the oxygenation of the soil and causes erosion. The application of OMWW contributes to the stabilization of the soil's aggregates, thanks to the binding action of certain organic components, in particular polysaccharides. The stabilizing effect remains for several months till the degradation of the organic compounds (Pagliai M. 1996; Le Verge S., 2004). It appears, therefore, that the application of OMWW could increase the stability of aggregates, prevent erosion phenomena, and the formation of surface crusts due to rain action, improve oxygenation of the surface profile of the soil in which root growth and microbial activity occur (cultivation layer), and contribute to a better hydraulic retention of the land due to its increased microporosity (Mellouli H.J. 1996; Colucci R. et al., 2002; Le Verge S., 2004). A laboratory study has shown that a surface layer of a sandy soil incorporated with OMWW is more effective in reducing evaporation losses (~30%) than a surface on which OMWW is applied as a mulch (25 g/m2), while the application of a straw mulch (450 g/m2) was effective only during the initial stage of the evaporation (Mellouli H.J., 1998; Mellouli H.J. et al., 2000).
Olive cake contains 94% organic matter and, therefore, can be highly beneficial to agricultural soil. However, said waste contains oil that may increase soil hydrophobicity and decrease water retention and infiltration rate. Abu-Zreig M. and Al-Widyan M. (2002) investigated the impact of olive cake on water retention, saturated and unsaturated hydraulic conductivity, and capillary rise of three soils: loam, clay loam, and dune sand and under laboratory conditions. Application of the waste resulted in an increase in water retention and saturated hydraulic conductivity, but caused a decrease in capillary rise and unsaturated hydraulic conductivity for all soils tested. The increase in water retention has been observed at all levels of pressure potential and was significantly different at 3 bars or higher. The highest increase in saturated hydraulic conductivity occurred at 4% application rate at which about 300%, 200%, and only 12% increase was observed for loam, clay loam, and dune sand, respectively. Application of olive cake caused a significant decrease in the capillary rise ranging from 11.5% for dunes to 70% for clay loam soil.
There are several studies on the chemical characteristics of OMWW (Della Monica M. et al., 1979; Potenz D. et al., 1985b; Senette C. et al., 1991; Marsilio V. et al., 1989; Levi-Minzi R. et al., 1992; Saviozzi A. et al., 1993; Proietti P. et al., 1995), and its humification index (Alianiello F., 1997; Alianiello F. et al., 1998).
The application of OMWW at a moderate dose does not affect the acidity of the soil. Levi-Minzi R. et al. (1992) studied the evolution of acidity of an alkaline soil treated with various amounts of OMWW (80, 160, and 320m3/ha) for a period of 135 days. Because of its acidic character (pH = 5) OMWW had a temporary acidifying action shortly after their application; during the next fifteen days, the treated soil recovered its original acidity. Similar evolution patterns of the acidity are found in several other studies on various types of alkaline soil (Della Monica M. et al., 1978; Potenz D. et al., 1985b; Morisot A. et al., 1986; Monpezat G. de et al., 1999). This slight acidification is considered to be beneficial for the alkaline soils because it renders phosphorus and other elements more assimilable by the olive trees (Le Verge S., 2004).
The application of OMWW on acidic soils can cause acidification of the ground (Le Verge S., 2004). A study carried by Marsilio V. et al. (1989) showed that a dose exceeding 160m3/ha causes only a minimum acidification of the soil (0.03 units of pH) during the first 100 days; a distinct increase in the pH of the treated soil was observed after this period. As a measure of precaution Monpezat G. de et al. (1999) recommends the neutralization of OMWW with lime before its application on acidic soils.
OMWW contains many acids, minerals, and organics that could destroy the cation exchange capacity of the soil. Higher levels of soil salinity due to potassium and sodium replacement of soil cations were detected in an alkaline soil after pollution with OMWW. The pH was practically unchanged and soil C/N ratio was increased (Paredes M.J. et al., 1986).
Sierra J. et al. (2001) studied the characterization and evolution of a soil affected by OMWW on a location used for 10 years as an uncontrolled OMWW disposal site. The study area included several evaporation ponds built on land without an impervious layer. The soil is formed by sedimentary materials (calcareous crusts and conglomerates). Once the disposal site was closed, the sediment remaining on the soil surface was removed. The use of a calcareous soil as a medium for OMWW disposal allowed the neutralization of the waste pH when passing through soil. The acidity of OMWW was compensated by soil carbonate alkalinity. The carbonates at the same time became bicarbonates and moved and accumulated in deeper horizons. An increase in salinity and in soluble phenolic compound contents was detected. The enrichment diminished in deeper layers, due to OMWW soil retention. Changes in electrical conductivity and phenolic compound content were observed down to 110-125 cm, where the OMWW flux was restrained by the sedimentary rock, which is more compact. Once the sediment remaining on the surface was removed, the salinity decreased quickly by rainfall leaching and biological activity, in time led to an effective decrease in electrical conductivity and phenolic compounds, although residual levels can be important even two years later. This similar evolution of conductivity and phenolic compounds is in accordance with the results obtained by Levi-Minzi R. et al. (1992), in an experiment undertaken for agricultural soils treated with OMWW and by Sierra J. et al. (2000), with leaching columns under laboratory conditions.
The application of OMWW at a moderate rate does not affect the salinity of the soil (Le Verge S., 2004). The application of an excessive dose (320m3/ha) on a clay soil caused only a temporary increase of the salinity (Levi-Minzi R. et al., 1992). An experiment with an application dose of 200m3/ha showed that the salinity increased slightly after 2.5 months (0.360 compared to 0.240 of a control soil) (Morisot A. et al., 1986).
A series of incubation experiments were performed in order to study the effects of OMWW in a calcareous soil (Perez D.J. and Gallardo-Lara F., 1987, 1989; Gallardo-Lara F. et al., 2000). The first incubation experiment studied the effects of OMWW on nitrogen transformation in a calcareous soil (Perez D.J. and Gallardo-Lara F., 1987). The application of this wastewater was shown to decrease NO^ formation in comparison with control assays during approximately the first half of the experimental period (6 weeks). Results were similar although were marked when OMWW plus ammoniacal nitrogen was applied as opposed to ammoniacal nitrogen alone. The incorporation of OMWW during the initial phases of study also reduced soil N-NH4+ levels both when residue only treatments were compared with controls and when OMWW plus ammoniacal nitrogen treatments were compared with ammoniacal nitrogen only. The second incubation experiment studied the effects of OMWW on sulfur transformation in a calcareous soil (Perez D.J. and Gallardo-Lara F., 1989). In addition to raw OMWW, other preparations were tested including OMWW devoid of organic matter and deionized OMWW. The addition of OMWW to soil inhibits the formation of S-SO42~ when OMWW plus elemental sulfur is compared to a treatment consisting of elemental sulfur applied alone. No such effect, however, was seen when the treatment with OMWW only is compared with control soils. Of the three types of OMWW tested, the least effective inhibitor of S-SO42~ formation was OMWW in which all organic matter has been eliminated, while the deionized effluent yielded lowest levels of S-SO42~. The exclusive application of OMWW on calcareous soils may raise S-SO42~ levels in the middle run; however, when a sulfur deficient soil is fertilized with elemental sulfur, concurrent application of OMWW is unadvisable, given that it may interfere with soil S-SO42~ formation. A pot experiment using calcareous soil was performed in a growth chamber to examine the effects of OMWW on the availability and post harvest soil extracta-bility of K, Mg, and Mn (Gallardo-Lara F. et al., 2000). The experiment included 6 treatments: two rates of OMWW, two mineral fertilizer treatments containing K (which supplied K in amounts equivalent to the K supplied by the OMWW treatments), a K-free mineral fertilizer treatment, and a control. The pots were sown with rye-grass as the test plant, harvesting 3 times at intervals of one month. OMWW has demonstrated a considerable capacity for supplying K that can be assimilated by the plant, tending in fact to surpass the mineral potassium fertilizer tested. The application of OMWW tends to reduce the concentration of Mg in the plant, similarly to the effect of adding mineral potassium fertilizer. An enhancement of Mn availability takes place in the soil amended with OMWW, which on occasion has produced Mn concentrations in plant that could be considered phytotoxic or at least excessive. After harvesting the amount of exchangeable K in soil with added industrial wastewater was increased. However, these increases are lower than those in soil treated with mineral potassium fertilizer. The levels of exchangeable, carbonate-bound, organic-bound, and residual Mg in soil were higher in treatments incorporating OMWW than in those with added mineral K, with the opposite tendency occurring in the amount of Fe-Mn oxides-bound Mg in soil. Treatments based on OMWW, especially in high doses, increased the amount of exchangeable and carbonate-bound Mn in soil, in comparison with treatments adding mineral fertilizers with or without K. In contrast, the addition of industrial wastewater caused a drop in the amount of Fe-Mn oxides-bound and organic-bound Mn in soil.
OMWW contains on average about 6% of organic matter and 0.4% of mineral salts suspended or dissolved in an aqueous medium. The organic matter of OMWW contains compounds that are easily biodegradable by the microorganisms of the soil. The degradation of the organic matter produces volatiles substances that are foul smelling and possibly pathogenic. Mineralization of the organic matter produce higher contents of NO3~-N in soil and increased NO3_-N uptake by plants. OMWW contains also phenols that are assumed to be responsible for phytotoxicity and their bioconversion is very important for humic acid biosynthesis. OMWW has a high and unbalanced ratio of C/N and is often necessary to add other materials to optimize the C/N ratio (e.g. ~35) in order to cause more rapid microbial degradation in the soil and, minimize competition with agricultural crops for the nitrogen contained in the solution circulating in the soil (Paredes M.J. et al., 1986) — see also Chapter 2 "Characterization of olive processing waste'', section: "Antimicrobial activity of OMWW'' and Chapter 10: "Uses", section: "Use as fertilizer/soil conditioner''.
Riffaldi R. et al. (1993) evaluated the changes in organic and inorganic compounds of soil amended with two doses of sludge obtained from OMWW during a 40-day incubation period. Differences between the amounts of organic components of the amended soil and those of the control, although related to doses and sampling time, disappeared at the end of the experimental period. On the contrary, the inorganic anion content was still different for the various processes, which suggest, especially for NO3~ and SO42~, a transient inhibition in the soil-sludge system.
Zenjari B. and Nejmeddine A. (2001) reported the effect of successive OMWW treatments on the chemical properties of clay soil profiles. The study showed that the clay soil has a very effective absorption/adsorption capacity. Over 99% of nutrients and 99% of phenols were removed after the first infiltration with OMWW. On the contrary, after the second infiltration the soil capacity to absorb/adsorb the anions was exhausted, while the phenol concentration was increased in the leachates which can present a risk of contamination of the groundwater.
The application of 2POMW to the soil is considered to have similar effects, although the available literature is still limited. The main organic constituents of 2POMW are lignin, hemicellulose, and cellulose — see Table 2.13. The high lignin content of 2POMW and the degree of binding of this component to other organic constituents in lignocellulosic materials may hinder the ability of microorganisms and their enzymes to degrade 2POMW, if used as a composting substrate (Alburquerque J.A. et al., 2004).
The use, therefore, of OMWW and/or 2POMW as a soil amendment requires knowledge of the effects that its application may produce on the status of the mineral nutrients in the plant-soil system — see Chapter 8: "Biological processes", section: "Irrigation of agricultural land/Land spreading'' and Chapter 10: "Uses", section: "Use as fertilizer/soil conditioner''.
Although some research has been done to the effects of the addition of OMWW on soil characteristics, such as soil hydraulic properties or soil composition, information on the effect of these amendments on other compounds that are retained by the soil, such as pesticides or heavy metals, is scarce.
The discharge of OMWW in soils causes the release of heavy metals retained by them. This effect was simulated by leaching homogeneous soil columns with OMWW after passing solutions of Cu or Zn through the columns. Previous addition of a compost made from olive-mill sludge and plant refuse to the soil caused a significant reduction of the release of retained metal by OMWW. Previous addition of concentrated sugarbeet vinasse caused much less significant effects (Madrid L. and Diaz-Barrientos E., 1998b).
The effect of OMWW on the solubilization of some heavy metals present in a river's sediment was studied by equilibrating the sediment with solutions of various concentrations of the residue at various pH values (Bejarano and Madrid, 1992a,b). It was shown that at a given pH OMWW caused a nearly linear increase in dissolved lead (Pb) from the sediment as the OMWW concentration increased, and the lower the pH, the higher were the amounts released. Iron (Fe) and copper (Cu) were mobilized by OMWW at the higher pH values tested, but in more acid conditions the solubility of these two metals seems to be lower than in the absence of OMWW. For high OMWW concentrations, the concentrations of Fe and Cu tend to be a pH-independent value, which can correspond to an equilibrium distribution of metal-organic matter complexes between the two phases. OMWW does not show any mobilizing effect on manganese (Mn) or zinc (Zn) from the sediment, and in the case of Mn the sediment even removes part of the metal originally present in OMWW solutions. A later study by the same authors examined also the effect of OMWW on the solubilization of more heavy metals (Ni, Cd, Zn, Cu, Mn, Pb, and Fe) present in a sediment from Agrio river (Seville, Spain) at different pH values (Bejarano M. and Madrid L., 1996a-d). Metal solubilized by OMWW in solutions was compared with data from different fractions of metal speciation of the sediment. The data shows that the dominant effect is pH for all metals with the exception of Fe and Mn. Within a given pH, it is shown that the presence of OMWW causes mobilization of most metals studied at pH 5 except Cd and Zn and this effect is progressively less marked as pH decreases, so that at pH 4 mobilization is detected for Ni, Cu, Mn, and Pb, and at pH 3 is only noticeable for Ni and Mn. The joint effect of pH and of the presence of OMWW is the release of amounts of metals which are comparable to those metal fractions attributed to exchangeable and bound to carbonates.
The discharge of OMWW can affect sorption, degradation, and movement of pesticides in soil. Cox L. et al. (1996, 1997) studied the effect of OMWW on soil porosity and on leaching of the herbicides clopyralid (3,6-dichloropicolinic acid) and metamitron (4-amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one) in clay soil columns. Organic amendments used to enrich soils of low organic matter content can affect sorption and movement of pesticides in soils. Clopyralid moved more rapidly than metamitron in the unamended soil due to greater sorption and degradation of metamitron. Total amounts of clopyralid leached from the OMWW amended soils were significantly reduced (75 and 25% for the lower and higher dose, respectively) when compared with the unamended soil (100%), whereas metamitron did not leach at all from the amended soils. Sorption and degradation studies with soil slurry suggested this reduction may be mainly due to an increase in sorption and dehydration processes in amended soils, as a consequence of the increase in the organic carbon content. However, the decrease in mobility produced by OMWW amendment is greater than suggested from the sorption and degradation increases. The reduction in large size conducting pores and the increase in the small non-conducting pores, induced by OMWW amendment, produce an increase in the residence time of the herbicides in the immobile water phase, enhancing diffusion, sorption, and degradation processes, thereby retarding mobility. The retarding effect was more pronounced for metamitron than for clopyralid due to the higher sorptivity and degradability of the former herbicide. These results suggest the possible use of OMWW or similar wastewater amendment in reducing contamination of groundwater by pesticide drainage.
Albarran A. et al. (2004) investigated the effects of the addition of exhausted 2POMW on the sorption, degradation, and leaching of the herbicide simazine [2-chloro-4,6-bis(ethylamino)-1,3,5-triazine] in a sandy loam soil. Simazine is a non-selective herbicide commonly used in olive-growing areas of Mediterranean regions at application rates close to 2 kg/ha. The soil was amended in the laboratory with exhausted 2POMW at two different rates (5 and 10% w/w). The results were compared with those of a previous study, where crude 2POMW was applied to the same soil (Albarran et al., 2003). The addition of exhausted 2POMW increased the extent and strength of sorption of simazine, reduced herbicide biodegradation, and retarded the vertical movement of the herbicide through the soil and reduced the amount of herbicide available for leaching compared to the untreated soil. Therefore, amendment with exhausted 2POMW may be useful to prolong the residence time of the herbicide in the topsoil and to reduce the risk of groundwater contamination as a result of simazine leaching losses. Interestingly, the results were quantitatively different from those obtained for the crude 2POMW, illustrating the importance of the specific characteristics of the organic amendment in determining its effect on pesticide behavior.
Effects on Soil Biological Properties
In nature OMWW is metabolized by microorganisms, insects, larvae, and earthworms present in the soil, to give a mixture of complex aromatic molecules known as humic or fulvic compounds or, more generally, as humic acids or humic extracts — see Fig. 3.2.
There are several studies on the effects of OMWW on the microflora of the soil (Paredes M.J. et al., 1986; Moreno E. et al., 1987, 1990; Lombardo N. et al., 1988; Flouri et al., 1990; Marsilio V. et al., 1989; Briccoli-Bati C. et al., 1990; Picci G. and Pera A., 1993) and the invertebrate community (Senette C. et al., 1991; Cicolani B. et al., 1992).
Marsilio V. et al. (1989) showed the beneficial influence a controlled disposal of OMWW can have on the populations of microorganisms; in a soil treated with 160m3/ha of OMWW, the number of microorganisms per gram of earth is multiplied 2.5 times after 15 days and 2.3 times after 100 days with reference to an untreated soil. This increase of the microflora and/or microfauna is accompanied by an accentuation of the respiration activity by more than 100%. The application of OMWW has a positive effect on the populations of mushrooms, actinobacteria, N2-fixing bacteria, and cellulololytic bacteria. A negative effect has been recorded on the nitrite and nitrate bacteria after 15 days of the application. However, the
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