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Sustainability as Fertilizer
Biosolids have long been used as fertilizer. Dry biosolids from municipal sludge contain about 4 to 7 percent nitrogen and 4 to 6 percent APA* (expressed as P2O5). These are valuable plant nutrients. Biosolids also contain important plant micronutrients, including iron, copper, and zinc.
Beneficial reuse of biosolids as fertilizer conserves several natural resources, making it a highly sustainable process compared to manufacturing chemical fertilizers from irreplaceable resources.
*Available Phosphoric Acid
Synthetic nitrogen fertilizers are manufactured in a three-step process:
Synthetic ammonia made from fossil fuel is the primary building block for almost all nitrogen fertilizers.
Synthetic phosphate fertilizers are made from phosphate rock. The phosphate rock is mined and treated with sulfuric acid to make the phosphate soluble and available to plants.
In sum, beneficial reuse of dry biosolids conserves precious natural gas, coal, phosphate and sulfur resources for future generations.
Sustainability as Renewable Fuel
In more recent times, biosolids are increasingly being used as renewable fuel, notably in cement kilns. The fuel value of dry biosolids can be quite high, and is attracting attention as a carbon-neutral substitute for coal in these energy-conscious times. The net energy balance depends on the fuel value of dry sludge and feed dryness.
The amount of fuel energy obtained from dried solids is greater than the amount of energy used to prepare it. Thus, using dry biosolids as fuel is “energy positive” and carbon neutral.