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Sevinç Arcak, Ayten Karaca, Koray Haktanır Soil Science Department, Faculty of Agriculture, Ankara University, Ankara ABSTRACT Wastewater sludge has for many years posed disposal problems. Its content of N, P and organic matter makes it a very valuable fertilizer and soil amendment material. Wastewater sludges often contain relatively high concentrations of certain heavy metals compared with the amounts found unpolluted soils. However, amounts in wastewater sludge are frequently in proportion to the degree of industrialization of the area that the municipal wastewater treatment plant serves. When sludge from plant treating industrial wastewater is applied to land, the ability of soil to bind heavy metals and accumulate them in plant rooting zones raises a serious question as to the influence of the more soluble metals on biological life and activities in soils. In our country where environmental awareness is developing and rapid industrialization occurs, wastewater treatment facilities is growing up in number resulting in a huge amount of sewage sludge of secondary wastes. This increase in secondary wastes direct researchers towards studies with regard to the problems, attributes and utilization possibilities of the material. The purpose of this work was to determine the effect of the continuos applications of digested municipal sludge originating from ASKI (Ankara Central Wastewater Treatment Plant) to soil. Within the context of the study, physical and chemical properties by setting incubation trials were determined. Following the incubation periods of 1, 7, 14, 28, 42, 56, 70, 105 and 140 days of soil samples which were mixed with certain rates of sludge (control, 20, 40, 80, and 160 t ha-1) samples were analyzed for the determination of electrical conductivity, pH, ortho-P, NH4+-N, NO3--N, extractable Fe, Zn, Cu, Mn, Co, Ni, and Cd. High NH4+-N value obtained at the first two weeks of the incubation brought about increase in pH values. The application of sewage sludge significantly increased the NH4+-N, available Cd and Zn the differences being greater for the highest rates of sewage sludge applied. INTRODUCTION Sewage sludges consist of multi-element organic wastes that are used commonly as manures (Otabbang et al., 1997). Sludges applications to most agricultural soils are regulated by various guidelines and regulations, and these are usually based on the heavy metal concentrations in the sludges (Alloway and Jackson, 1991). Waste application on agricultural lands, however, has caused concerns. It is often argued that heavy metals such as cadmium (Cd), nickel (Ni) or lead (Pb) in sludge, when applied to soils, may enter the food chain through plants or animals, contaminate surface and ground water, and thus cause health hazards (Hue et al., 1994). In reality, metal concentrations in sewage sludge vary widely (Sommers, 1977), depending on several factors, including (1) sludge origin (e.g., industrial wastes usually contain high levels of heavy metals than residential wastes), and (2) sludge pretreatment processes (e.g., raw v.s. anaerobically treated). In addition, when applied to soils, the bioavailability of sludge-borne metals is further influenced by soil properties (e.g., pH, sesquioxide content, organic matter) as well as sludge application rate (Sommers et al., 1987; Logan and Chaney, 1983). This explains the lack of metal accumulation in plants grown on certain sludge-amended soils and the beneficial effects of sludge on soil fertility and plant nutrition (Hue, 1988; Corey et al., 1987; Chang et al., 1978). Given this background, a better understanding of the basic chemistry of wastes and their interactions with soil will help sludge managers make intelligent decisions on the use of sludges on lands (Hue et al., 1994). Thus, the objectives of this study were (1) to determine the chemical composition of sewage sludge from Ankara Central Wastewater Treatment Plant and (2) to evaluate sludge/soil interactions. MATERIALS and METHODS Soil and Sludge Description: The soil used was a clay taken from Sincan-Tatlar village near Ankara. The sewage sludge used in this experiment from a domestic source on Ankara, was digested and dried before being used. Sewage sludge and soil were air dried and sieved (>2 mm) before analysis. Experimental Design: The soil was amended with sewage sludge. Sewage sludge was applied at four rates: 20, 40, 80, and 160 t ha-1. Control is established (soil) and five treatments were carried out. For each treatment three replications were done. Each pot consisted of 300g of coarsely sieved soil with various amendments. The water content of the soil was adjusted to 70% of field capacity. The pots were placed in an incubator at 28 °C. Throughout the incubation period, water losses exceeding 10% of the initial values were compensated for by addition of distilled water. Sampling and Analysis: Samples were taken at 1, 7, 14, 28, 42, 56, 70, 105, and 140 days of the incubation period for the following analyses. For each sample, pH and electrical conductivity (EC) were measured in a 1:2.5 water extract (Richards, 1954); organic material by using modified Walkley-Black Method (Jackson, 1962); grain size distribution by Bouyoucos (1951); soil cation exchange capacity (CEC) was determined by saturation with ammonium acetate at pH 7 (Chapman, 1965); soil available P was extracted with sodium bicarbonate (Olsen et al., 1954) and the content determined by the Murphy and Riley method (Murphy and Riley, 1962). Total nitrogen was assessed using the Kjeldahl method, as specified by Bremner (1965). For NH4+-N and NO3--N, moist soil was placed in 150 ml Erlenmeyer flasks with 100ml of 2M KCl. Flasks were placed on a rotary shaker for 30 min and contents filtered through whatman filter paper No. 42. Soil extracts were used for the measurement of NH4+-N and NO3--N by steam distillation using MgO and Devarda's alloy (Bremner, 1965). Soil samples were extracted for available Fe, Cu, Zn, Mn, Co, Ni, and Cd in DTPA solution (0.005M DTPA+0.005M CaCl2+0.1M TEA (triethanolamine) pH 7.3), (Lindsay and Norvell, 1978). All the solutions of Fe, Zn, Cu, Mn, Co, Ni, and Cd were analyzed by atomic absorption spectrophotometer (AAS) with flame or graphite furnace when required. RESULTS and DISCUSSION The physical and chemical characteristics of the sludge and soil are shown in Table 1. The sewage sludge was slightly alkaline (pH 7.08). The soil had a pH of 7.90. Sewage sludge and soil gave the moderate values of conductivity (2.10 and 0.49 dSm-1, respectively). Digested sludge possessed a relatively high content of organic carbon (14.2 %) and the soil had a lower content (0.70 %), as expected. In terms of basic nutrients, such as total nitrogen and extractable phosphate, the sewage sludge had the moderate levels. Table 1 list the extractable contents of various heavy metals. In general, sludge contained lower levels of extractability metals.  The effects of adding waste materials to the soil properties are illustrated in Table 2, 3, 4, 5, and 6. During the incubation period, pH is slowly decreased and the differences are not significant at the higher rates. The values of EC were raised after addition of the wastes and higher levels were obtained with higher application rates (Table 2). Till the 7th day of the incubation there was no statistically significant increase both in the control and the increased sludge-added pots. However, as for the 14th day of incubation, depending on doses, considerable increase was observed in EC values of the soils (P<0.05).  Tables 3, 4, and 5 list the extractable metal contents of the clayey soil after applications of various sludge levels. Application of sludge, especially at higher application rates, generally raised the extractable concentrations of heavy metals of the clayey soil. During the incubation period, depending on increased sludge doses, available Cd increased in the sludge-added soils in comperation to the control (P<0.05, Table 3).  It was observed that the extractability of Cu increased with the addition of sewage sludge (P<0.05, Table 4). The extractable Zn content increased when the sludges were added to the soil, because the Zn content was higher than that of the soil (Table 4). At the beginning of the experiment (sample time 1) the samples were not humified. This Zn fraction decreased during the experiment in soil presumably due to the insolubilization with time of the Zn released in the degradation of organic matter (Hernandez et al., 1990). Until the 56th day extractable Mn and Ni increased depending on the increased sludge doses (P<0.05, Table 5). At the first day of incubation, extractable Co increased depending on the increased doses (P<0.05, Table 5), however, differences between the control and the doses were found statistically insignificant in the other incubation times. Amounts of NH4+-N and NO3--N during the incubation of the soil mixed with sewage sludge are shown in Table 6. Depending on the incubation period and the increased doses, there was considerable changes in NH4+-N of the sludge-added soils. The highest NH4+-N amounts was determined in soils being applied the highest sludge of 160 t ha-1 (P<0.05). NH4+-N of the control samples reached their peak level at the 14th day of the incubation, then, gradually decreased depending on the incubation time. NH4+-N levels in the samples being applied different doses of sludges continuously decreased due to NH4+ oxidation till the 70th day of the incubation. Sludge application at the 80 t ha-1 level retarded nitrification. This may be due to the high concentration of ammonia which accumulated in the soil (Justice and Smith, 1962). Yoneyama and Yoshida (1978) have reported that the high amount of ammonia appears to retard the oxidation of ammonia to nitrite, and a similar amount than that may depress the oxidation of nitrite to nitrate; thus resulting in an accumulation of nitrite in the soil. As decreased in this, the application of a large amount of sewage sludges may cause the accumulation of ammonia, nitrite or nitrate in high concentration, and the excessive inorganic nitrogen may be reduced to a suitable extent by a simultaneous application of organic matter low in nitrogen content. Depending on doses and time NO3--N increased in the soils being added sewage sludge until the 28th day (P<0.05). NO3--N showing decrease till the 105th day of incubation increased at the 105th day, then decreased again at the last day of the incubation.  In general, it can be said that sewage sludge applied at 20 t ha-1 did not a significantly increase the soluble compounds released at any time, excluding obviously NO3--N (Dalmau et al., 1989). The application of sewage sludge significantly increased the NH4+-N, extractable Cd and Zn the differences being greater for the highest rates of sewage sludge applied. ACKNOWLEDGEMENTS This project has been supported by TUBITAK through contract No: TOGTAG-1712. REFERENCES Alloway, B.J., Jackson, A.P. (1991). The behaviour of heavy metals in sewage sludge-amended soils. The Sci. of the Total Environ. 100: 151-176. Bouyoucos, G.J. (1951). A Calibration of the Hydrometer for Making Mechanical Analysis of Soils. Agronomy Journal 43: 9. Bremner, J.M. (1965).Total Nitrogen. In Methods of Soil Analysis 2; (C.a. Black, Ed),. American Society of Agronomy, Madison, Wis., 1145-1178. Chang, A.C., Page, A.L., Lund, L.J., Pratt, P.R., Bradford, G.R. (1978). Land application of a sewage sludge-a field demonstration, final report for regional wastewater solid program. Dept. Of Soil and Environ. Sci. Univ. of California, Riverside, CA. Chapman, H.D. (1965). Methods of soil analysis Part 2. Chemical microbiological properties. Ed. C.A. Black. Amer. Soc. Of Agron. Inc. Publ. Agron. Series no: 9, Medison, Wis, USA. 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