Effective Use of Concentrated Sludge for Paddy Rice
Chemical Characteristics and Safety of Concentrated Sludge
Concentrated sludge discharged out of wastewater treatment facilities has a water content of about 97%, and acts like a liquid. The level of potassium it contains is lower than that of composted livestock manure (Table 1). The pH of concentrated sludge may be 12 or more. The addition of 0.5% (W/V) cal-cium oxide, can control the number of coliform bacteria in be sludge to below than the minimum limit for detection. The nitrogen content of the sludge falls just after it is treated with calcium oxide, and then remains stable for some time. Therefore, since the coliform bacteria, will have been destroyed, the con-centrated sludge can safely be used in three days or more after the calcium oxide treatment.
Use of Sludge Injector
How to Apply the Concentrated Sludge Treated with Calcium Oxide
The sludge injector was developed from a bulldozer designed for use in marshes, with harrows at the back. It was developed by the Hiroshima Prefecture Agricultural Development Corporation. It is able to inject concentrated sludge into soil through its tires while it is moving. The concentrated sludge is loaded into a sealed container positioned on the front of the sludge injector (Fig. 1). This makes it possi-ble to inject the concentrated sludge into soil without the sludge coming into contact with the air, so there is not an odor problem from the sludge (Fig. 2 and Fig. 3).
Effect of Concentrated Sludge on Rice
Concentrated sludge treated with calcium oxice was applied to the rice varieties "Koshihikari" and "Hi-tomebore", it was applied twice a year in spring and fall at a rate of 80 m3/ha/year. It was also applied to the variety of "8-tan Nishiki No.1" in spring once a year, (40 m3/ha/year). In addition, a top dressing of compound fertilizer was applied at the ear formation stage, provided a leaf-color check. The yield of rice was equal to, or greater than, that from customary practices. Showed it was necessary. The quality of brown rice grown with concentrated sludge was almost equal to that of grown by customary practices.
Effect of the Concentrated Sludge on Soil
In place of a basal application of fertilizer, concentrated sludge treated with calcium oxide was applied twice a year, in spring and fall, at a rate of 80 m3/ha/year over two years. The soil was then tested for heavy metals. Tests showed that the level of cadmium in soil had not risen. As for the level of zinc in the soil, it is expected to take about 30 years to reach 120 ppm, which is the maximum permitted level of zinc in soil (Table 2). No accumulation of heavy metals in brown rice was seen.
Concentrated sludge treated with calcium oxide can be safely applied to farmland, using a sludge in-jector, without damaging the yield or quality of paddy rice.
Index of Images
Figure 1 The Sludge Injector
Table 1 Influence of Alkali-Treatment (PH³12) on the Number of Bacteria and Nitrogen Content in Concentrated Sludge
Figure 2 Field after Injection of Sludge
Figure 3 Sludge Injector Operating in Fallow Paddy Field (Rear View).
Table 2 Influence of the Concentrated Sludge Treated with Calcium Oxide Used for the Farmland on the Level of Cadmium and Zinc in Arable Soil and Brown Rice (MG/KG)
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