Land disposal of sludges Land disposal Digested sludge represents the residue after most of the easily metabolized substances have been consumed. Although any organic matter deserves consideration as fertilizer, digested sludge has little value and is generally used only as a soil conditioner. For example, plowing in sludge may improve the characteristics of clay-like soils that drain poorly and have low concentrations of nutrients.
Undigested sludge contains more nutrients and valuable water.
Direct application as irrigation water makes a great deal of sense, but there are some strong arguments against this practice. The logistics may be bad if the land to be irrigated is remote from the treatment plant. But the overriding consideration is safety. While there are acceptable methods for protecting the persons who handle sludge, there is no possibility of securing approval from regulatory agencies to irrigate food crops with fluids that are likely to contain pathogens. Even if the streams have been sterilized, there could still be a health hazard from heavy metals or poisons that reach the sewage treatment plant.