As a product of wastewater treatment process, biosolids can be beneficial for a number of purposes when managed and treated properly. A municipality must be careful in distinguishing between biosolids and sewage sludge.
A material can only be called and referred to as biosolids if it fulfilled the requirements of treatment and management. There is a set of approved biosolids management guidelines that must be followed for sewage sludge to turn into biosolids that could be applied on farms, forestlands, and ranches.
Sewage sludge is mainly a mix of water and organic materials. These are what biosolids are made of; they are a by-product of sewage treatment processes. But since most wastewater comes from household kitchens, laundries, and bathrooms, they may contain in small or large quantities macronutrients (nitrogen, phosphorus, potassium, and Sulphur) and micronutrients (copper, zinc, calcium, magnesium, iron, boron, molybdenum, and manganese).
Biosolids may also contain traces of synthetic organic compounds and metals such as arsenic, cadmium, chromium, lead, mercury, nickel, and selenium. The presence of these contaminants limits the extent to which biosolids can be applied. All applications are, of course, regulated by appropriate authorities in each state.
Treatment processes create a stabilized product, which is almost free of pathogens and other harmful contaminants thus allowing the beneficial use of biosolids as organic fertilizer. Typically, once treated, biosolids contain between 15% and 90% solids.
In many parts of the world like in Australia, 75% of the 327,000 dry tons of biosolids were applied to agricultural land while around 19% were used for landscaping and land rehabilitation. The remaining 6% is either stockpiled, landfilled, or discharged to the ocean.
Uses of biosolids
In many parts of the Unites States, as well as in other countries and cities, biosolids are mainly used for land application in agriculture (vine, cereal, pasture, and olive), co-generation and energy recovery, road base, land application in forestry operations, land rehabilitation (landfill capping), landscaping and topsoil, composting, and oil from sludge is being used for experiments.
Depending on the regulations, biosolids could also be used for bricks and construction material, vitrification (glass manufacture), biofuel, and as fuel substitute (cement works).
Remember that not all biosolids can be used for land application. They are graded according to their chemical composition and the amounts of pathogens remaining after the treatment. Only the highest grade of biosolids, Class A and Class A EQ, are allowed to be applied in soil that would be used for crop production.