Biosolids (treated sludge) are nutrient-rich organic materials that are created through the treatment of domestic wastewater generated from sewage treatment facilities. At the facility, the sewage goes through physical, chemical and biological processes to remove the solids and separate water. If necessary, the solids get treated with lime to raise the pH level to eliminate odor problems. The wastewater treatment processes sanitize wastewater solids to control pathogens and other organisms that can cause diseases. In Toronto, biosolids are treated at the Ashbridges Bay Treatment Plant (77%) and the Highland Creek Treatment Plant (23%) and generates about 195,000 tonnes every year (about 72% of this is water). Biosolids are used as sustainable agricultural fertilizer to improve and maintain productive soils and to stimulate plant growth. They have been used by farmers and gardeners to promote the growth of agricultural crops, fertilize gardens and parks, and reclaim mining sites.
In this picture, municipal wastewater enters the treatment plant in which it undergoes primary treatment where heavier solids settle out. These heavier solids are sent to either an aerobic or anaerobic digester where the organic solids are converted by micro-organisms into a stable form. When that process has been completed, biosolids get transported to a land application site or a storage facility.
The cleaner wastewater resulting from primary treatment undergoes secondary treatment where chemicals are added to precipitate out phosphorus in chemical compounds, and promote the coagulation or aggregation of other solids that still remain in wastewater. The water from secondary treatment gets subjected to chlorination (disinfection) and then gets discharged to the surface water outlet.
Quality of Biosolids
Nutrients and Phosphorus in Biosolids
Biosolids contain large amount of nutrients such as nitrogen and phosphorous, and contain trace elements such as sulfur, magnesium, calcium, copper, and zinc. Nutrients are important for crop production and growth. Plants obtain nutrients from the soil and return them to the land when it dies and decays. This natural cycle is interrupted when agricultural harvesting removes part or all of the plant from the land. Farmers can replace nutrients in the soil with biosolids.
Biosolids are mainly composed of organic matter. Application to land can help restore organic matter that is removed from the soil by tilling and harvesting. The organic matter in biosolids makes soils more absorbent, improves the circulation of air and water and provides space for plant roots. This results in better plant growth, greater resistance to unfavourable weather conditions and better soil manageability. Most cultivated soils in Ontario contain less desirable amounts of organic matter but when its added with biosolids, organic matter gets added.
Metals in Biosolids
Biosolids contain small quantities of metals such as iron, manganese, zinc, copper, boron, molybdenum and cobalt. This is linked to insoluble inorganic and organic components. As a result, metals added to soil are stable and will not easily migrate into surrounding ground and surface water. Plant uptake of metals in the soil is small. Field crops remove less than 0.4 kg/ hectare per year of metals from the soil. Uptake of metals added to the soil will vary depending upon the crop grown, soil pH, soil organic matter, clay content, soil aeration and the concentrations of nutrients and metals originally in the soil.
Uses of Biosolids
Biosolids are used in different areas: agriculture, mine reclamation, land scraping and forestry.
Biosolids are used to fertilize fields for raising crops. Agricultural use of biosolids has made significant improvements in crop growth and yield. It reduces the farmer’s production costs and replenishes the organic matter that has been depleted from the soil. The organic matter increases the soil’s ability to absorb and store moisture.
Biosolids have been used at mine sites to establish sustainable vegetation. The organic matter, inorganic matrix and nutrients in the biosolids reduce the bioavailability of toxic substances that are found in mine soils and they also regenerate the soil layer. This regeneration is very important for reclaiming abandoned mine sites with little or no topsoil.
Biosolids composted with sawdust, wood chips, yard clippings, or crop residues makes excellent mulches and topsoils for horticultural and landscaping purposes. Many professional landscapers use composted biosolids for landscaping new homes and businesses. Home gardeners also find composted biosolids to be an excellent addition to planting gardens.
Biosolids have been found to promote rapid timber growth that is allowing quicker and more efficient harvest of an important natural resource.
Throughout the world, biosolids are returned to the environment in a variety of ways. Although the city of Toronto continually pursues beneficial reuse options, we currently use a combination of methods to manage biosolids. The table below shows where our biosolids went in 2008:
2008 Biosolids Reuse/Disposal Methods Management Percentage
Incineration at Highland Creek 23%
Land Application 11%
Alkaline Stabilization 5%
Site Remediation 4%
Advantages of Biosolids
- Biosolids are rich in both organic matter and essential plant nutrients
- It can be utilized as a soil amendment and fertilizer.
- Biosolids act as a soil amendment through the contribution of organic matter.
- Increased organic matter improves soil physical properties including moisture holding capacity, aggregation, porosity and tilth.
- A reduction in fertilizer costs
- Biosolids serve as a fertilizer by providing essential micro (e.g. zinc, copper and iron) and macronutrients (e.g. nitrogen, phosphorus and potassium).
Disadvantages of Biosolids
- Biosolids can cause odor problems especially in residential areas
- Some metals in biosolids can create problems with human health and plant growth.
- The soil can have an excess of nutrients.
- Biosolids may have pathogens, which can cause diseases
Biosolids can improve and maintain productive soils and stimulate plant growth so it has been used by farmers and gardeners widely. It contains lots of nutrients such as nitrogen and phosphorous, and trace elements such as sulfur, magnesium, calcium, copper, and zinc. Biosolids can be used in a variety of areas such as agriculture, mine reclamation, land scraping, and forestry. It can improve crop growth and soil productivity in agriculture and landscaping. It can establish suitable vegetation in mine reclamation and promote timber growth in forestry.
Increased organic matter improves soil physical properties including moisture holding capacity, aggregation, porosity and tilth. Improvement of these properties facilitates transport of air, water and nutrients throughout the soil, benefiting the establishment and growth of vegetation. Biosolids serve as a fertilizer by providing essential micro (e.g. zinc, copper and iron) and macronutrients (e.g. nitrogen, phosphorus and potassium) that increase vegetation growth and productivity. The stabilized biosolids provide a slow release source of nutrients that can be utilized by plants for several years following application. Farmers and gardeners can save money by using biosolids to fertilize.
On the other hand, there are disadvantages in using biosolids. Biosolids can cause odor problems especially in residential areas. It may contain some pathogens that can cause disease. Some metals present in biosolids can create problems with human health and plant growth. For example, arsenic (a metal present in biosolids) can cause health problems in a child eating a soil mixed with biosolids. Nickel (another metal present in biosolids) can deteriorate plant growth. The soil can only store a certain amount of nutrients so the excess moves to surface water or groundwater. That can cause high nitrate concentrations in forage that can kill animals consuming the forage.
When comparing the advantages and disadvantages of biosolids, I find that there are more advantages than disadvantages. For successful utilization of biosolids, we need a combination of proper application methods, best management practices, and nutrient management planning. When biosolids are applied to land according to regulatory requirements, risks to environment and human health are negligible. Therefore, I believe that it is beneficial to use biosolids to maintain plant growth and soil productivity.