On this basis, the sludges have been tentatively classified into low, medium, 

 and high-risk categories as related to their possible detrimental effects if placed 

 on the land (Table 4). Although the concentrations of potentially toxic elements 

 in the "low-risk" sludges are not of major concern, certain of the elements may 

 be accumulated in the soil. Therefore, care should be exercised that excessive 

 amounts of these sludges are not applied to soils. An average of four to six tons 

 (dry) per acre per year would probably be acceptable. The "medium-risk" 

 sludges should probably not be applied to land at rates exceeding one or two 

 tons (dry) per acre per year. Until additional information on the fate of heavy 

 metals added to land is available, the "high-risk" sludges should probably not be 

 added to land. If no other disposal technique is available, the sludge should be 

 spread very thinly over the land, adding no more than one or two tons (dry) of 

 sludge, to any one area within a ten-year period. 



As a fertilizer material, none of the sludges would be particularly good. All 

 are low in potassium and usage of the sludges as a fertilizer would, therefore, 

 require additions of this element. Whether additional phosphorus would be 

 required, will depend upon the amount of sludge applied, the amount of 

 phosphorus in the soil, and the phosphorus requirements of the plants to be 

 grown. Whether additional nitrogen needs to be apphed is more dependent upon 

 the C/N ratio than on the total nitrogen content of the material. If organic 

 materials have a C/N ratio of greater than 15 or 20 to 1, additional nitrogen may 

 be needed to prevent temporary nitrogen deficiencies in the plants. Sludge from 

 secondary treatment plants using activated sludge and primary plants having an 

 anaerobic digester have a satisfactory C/N ratio. (Table 5). However, most of the 

 primary sludges that were not digested had a rather high C/N ratio. Thus, when 

 using these sludges as fertilizer , additional nitrogen may be needed. 



While all the elements listed in Table 4, except sodium, barium and 

 strontium, are required by plants, several may become toxic if present in too 

 large quantities. This list would include manganese, boron, copper, zinc, and 

 molybdenum. In most circumstances, manganese in soil is sufficiently insoluble 

 that plants would be unlikely to absorb toxic quantities. Furthermore, boron, 

 zinc, and molybdenum are probably not present in quantities large enough to 

 cause plant nutritional problems if applied to soils at the suggested rates. The 

 potential for copper toxicity depends on the sludge copper content in excess of 

 200 ppm. While sodium is not highly toxic to plants, high sodium levels can alter 

 the physical and chemical properties of soils. In addition, sludge containing high 

 sodium levels should not be used on food crops where they will be consumed by 

 persons suffering hypertension. 



12 



