3 Metal concentrations in wastewater sludges are often expre

3.) Metal concentrations in wastewater sludges are often expressed in terms of grams of metal per kilogram of total dry solids. A wet sludge has a solids concentration of 200,000 mg/L, and 8000 mg/L of these solids are zinc. a. What is the concentration of zinc as g Zn g dry solids? b. If this sludge is to be applied to farmland and spread on pastures used by cows, explain how you might monitor the project with regard to the potential environmental or health effects caused by the presence of the zinc.

Solution

a. The concentration of Zn is (8000/200000) *100 = 4% or 0.04g Zn/g solid waste

b. The solid waste spread on the pasture may bioaccumulate in the grass of the pasture (subjected to the bioremediation abilities of the grass) and enter the food chain. The entry of the heavy metal in the food chain would lead to its accumulation in the cow\'s viscera.

The heavy metals are toxic to the plants and animals. The bioaccumulation of zinc would directly affect the primary productivity of the grasses. Although cows can egest extra zinc with the dung, but high Zn intake is known to reduce somatic cell count in the lactating females and it also induces the immune response in the calves.

The zinc egested by the cows or the amount not taken up by the grasses would only contaminate the soil and make the soil unfit to support the growth of the vegetation.