The way in which sewage treatment produces sludge is illustrated in figure 1. Production of sewage sludge in a sewage treatment plant. The options for dealing with sewage sludge include application to agricultural land, incineration, land reclamation, landfill, forestry and sea disposal. Nutrients are being added to the soil and are slowly released as the organic matter is degraded. Organic matter is added to the soil improving soil structure and increasing the ability of a soil to hold water and nutrients.
Plant growth is enhanced compared to unamended soils.
However, sewage sludge also contains components that are considered to be harmful to the environment. Only heavy metals will be considered here. Table 1 displayes some of the heavy metals of most environmental concern and their typical concentration range in sludge. Heavy metals in sludge arise from domestic, industrial and road runoff input to the sewage system. The dominant source of most metals is the domestic water discharge.