USE OF SEWAGE SLUDGE IN REAL FARMS - A. Rigueiro-Rodríguez^[3], R. Mosquera-Losada^[4] and L. Andrade-Couce^[5]

ABSTRACT

The use of sewage sludge as a fertlizer is a good practice for ecological elimination of this residue. The applied doses are restricted by law because of heavy metal contents of this disposal. Knowledge about pasture production and its establishment as a result of different doses application of sludge in real farms will help to understand the behaviour of this organic fertlizer on a big scale in comparison to small plot experiments. Five sludge doses where evaluated in different farms close to the city of Lugo. The results found revealed that pasture production is raised by the increment of sewage sludge doses up to 100 t ha^-1, after this value the variable was reduced and the Lolium establisment is related to OM content of the soil.

INTRODUCTION

Sewage sludge disposal is an important problem in Europe because of the quantity of this residual material produced. The use of this material as fertlizer is the more economic way of elimination. The agricultural use of this disposal is increased as transport to the sea and rubbish tip is reduced following the EU normative of 1999 (Bontoux et al., 1998).

Sewage sludge has a relatively important percentage of nitrogen, phosphorous and potash. The main problem of this organic fertlizer is the heavy metal content which is regulated by the EU and state normative in the European Union (86/278/EEC). The study of the different doses used in farms and the relationships with the production, establishment of different sown species and the modification of different soil parameters, will help the farmer to rationalize the use of this residue.

MATERIALS AND METHODS

The study was conducted in Lugo (Northwest Spain). Six farms sown in autumn 1997 were chosen in order to examine pasture production, botanical composition and soil conditions. Plots between 3 000 and 10 000 m² were fertlized with different doses of sewage sludge (36 000, 50 000, 71 700, 98 000, 137 000 and 140 000 kg ha^-1), sown with a mixture of 12.5 kg ha^-1 of Lolium perenne, 12.5 kg ha^-1 of Lolium multiflorum and 2 kg ha^-1 of Trifolium repens. No liming was applied.

Two cuts were made in the months of March and May. To date four random pasture samples of 0.09 m² have been taken from each plot. Samples were transported to the laboratory and placed at 4ºC. Each sample was distributed in different species, dried (24 h x 80ºC) and weighted. Samples from soils at 25 cm were randomly taken. Soil samples were transported to the laboratory and pH in water and MO determinations were made (Walkley, 1935).

RESULTS

Sewage sludge mean composition is shown in Table 1. The heavy metal contents of the used sludge are within the limits recommended by state legislation. The main nutrients are within the usual range for Lugo. The comparison of these dates with sludge from the Cataluña region showed a higher fertlizer capacity of Lugo sewage sludge with respect to phosphorous as it was half in Cataluña sludge. The same conclusion is obtained for CaO content. However, the potash and nitrogen mean is lower from Lugo sludge. Zn, Ni and Pb content are very similar to those found by Smith (1996) and Martinez-Farré (1995) in England and Cataluña, respectively. Cu and Hg content in the sludge is lower than those found in the bibliography. The content of Chromium is higher in UK sludge and the Cadmium content is very high in Cataluña sludge. Slurry is the fertlizer generally used in the area, the comparison of this organic fertlizer showed a higher phosphorous and potash content than sewage sludge, a lower heavy metal content with the exception of Nickel and chromium which are very close to sewage sludge concentration

The quantity of nutrients and heavy metals applied with each sewage sludge dose is presented in Table 2. Nitrogen is an element with important fertlizer behaviour in the sludge as the content of quickly released nitrogen is between 82 kg/ha and 318 kg/ha for lower and higher doses, respectively. Potash and phosphorous are lower than the recommended values for annual grassland fertlization, which are usually 100 and 120 kg ha^-1, respectively (Xunta, 1989). This could make a fertlizer complement necessary with these nutrients. The liming capacity of sewage sludge can be important at higher doses and in Galicia because of the natural acidity of its soils. All the values of heavy metals are within the limits established by legislation with the exception of Zinc and Cadmium for higher doses. The higher quantity allowed annually by law (126 012 kg/ha) with this sludge, if we consider Zinc content and 105 000 kg per ha of sludge, if we considered Cadmium content.

Table 1. N (%), P₂O₅ (ppm), K₂O (ppm), C(%), CaO (g/kg), Zn, Cu, Pb, Ni,Cr, Hg and Cd (ppm) content of sewage sludge in the present experiment and other bibliography sources (L: mean of two years composition in Lugo, C: in Cataluña and U: in UK) compared with slurry percentages for the same parameters (Labrador, 1996).

Pasture production and sewage sludge application relationship is shown in Figure 1. The dose of sewage applied explained an 83 percent of variance of the variable pasture production. They were related by a quadratic regression. Pasture production increased with the doses of sewage sludge applied up to a value of approximately 100 000 kg/ha, after this limit the grass production was reduced.

Table 2. Kilograms per hectare of nitrogen (N), phosphorous (P₂O₅), potash (K₂O), calcium CaO, Zn, Cu, Pb, Ni, Cr, Hg and Cd applied with the different doses of sewage sludge tested.

Figure 1. Pasture production and sewage sludge dose relationship.

The percentage of gramineous and legume sown is presented in Table 3. The sown species were around 100 percent in those farms fertlized with more than 71 700 kg/ha with the exception of farms which had received 140 000 kg/ha of sewage sludge where the sown grass content was around 80 percent. The grass of the farm fertlized with 36 000 kg/ha of sewage sludge had only a 37 percent of Lolium sp but a 51 percent of Dactylis which is a species with lower nutrient requirements. The percentage of legumes was very low in all the studied farms, however, lower doses had around 2 percent of Trifolium.

The soil characteristics are presented in Table 4. The OM content in the soil was related to Lolium sp percentage in the pasture (r² =0.93). The percentage of this species increased with the OM content in the soil up to a value of approximately six was reached, after this value the composition of pasture was exclusively Lolium. The sown species establishment was very dependent on OM content in the soil. However, the relationship between doses of sewage sludge and OM content in the soil was not found due to different OM content of the different farms prior to starting the experiment.

Table 3. Percentage of sown species (Lp:Lolium sp, Tr: Trifolium repens) and unsown species (Dg: Dactylis glomerata, Erod: Erodium moschatum, Agr: Agrostis sp) in first (1c) and second (2c) cut.

Table 4. pH and OM (%) content of different soils fertilized with different sludge doses.

In conclusion, pasture production is raised by the increment of sewage sludge doses up to 100 t ha^-1, after this value, this variable was reduced. The OM content in the soil was very much related to the establishment of Lolium up to a value of six, when the percentage of this species was 100 percent of pasture.

ACKNOWLEDGEMENTS

Thanks are expressed to Julio Torre Rodríguez for sampling and processing and Aurora López Veigas for laboratory analyses and to Mª de las Mercedes Fernández Couto Gómez and J.M. Cainzos Calvo from GESTAGÜA S.A. for field assistance. This experiment was financed by XUNTA de Galicia.

REFERENCES

Bontoux, L., Vega, M. & Papameletiou, D. 1998. Tratamiento de las aguas residuales urbanas en Europa: el problema de los lodos. Instituto de prospectiva tecnológica (Eds) IPTS, abril, report nº 23. Comisión Europea.

Martinez-Farré, F.X. 1995. Posibles usos de los residuos urbanos en agricultura:abono, enmienda orgánica y sustrato de cultivo. In “Gestión y utilización de residuos urbanos para la agricultura”. Fundación la Caixa (Eds).

Smith, S.R. 1995. Agricultural recycling of sewage sludge and the environment. Smith, S.R. (Eds) CAB International.

Walkley, A. 1935. An examination of methods for determining organic carbon and nitrogen in soils. J. Agr. Sci., 25, 316-325

Xunta. 1989. Producción de leite en Galicia. Manual técnico. Consellería de Agricultura. Xunta de Galicia:171 pp