Biosolids Management
Ways We Manage Biosolids
Land Application
Biosolids have been applied to agricultural land in Canada and internationally for over 40 years. The process is widely used in Ontario, the U.S. and Europe. Sometimes land application is called "recycling", because it returns to the soil the nutrients that we remove from it when we harvest and consume food.
Like animal manure, biosolids contain a number of nutrients that are beneficial for crop growth. They also contain organic matter that enhances soil and improves its ability to retain water. Biosolids reduce the need for commercial fertilizers, and can reduce the environmental impacts of commercial fertilizer production. Biosolids are also sometimes applied to forest land and tree nurseries. Application to strip mines can also help reclaim and vegetate the land. Numerous studies completed over the years have shown that biosolids land application is safe and has significant benefits.
50% of Toronto's biosolids were land applied in 2010, in various ways (land application of cake, pellets & alkaline stabilization). These are all described below.
Biosolids Cake
Land application of biosolids is regulated under Regulation 347 of the Environmental Protection Act and Regulation 267/03 under the Nutrient Management Act, 2002. The Regulations ensure that land-applied biosolids do not harm the environment or threaten the health of humans, animals and crops. They also safeguard crops by setting limits for nutrients (nitrogen, phosphorus), micro-nutrients (iron, manganese, zinc) and organic matter. For more information on land application regulations, see Nutrient Management Act.
Biosolids contain varying types and concentrations of metals. While some of these metals are necessary in small amounts for plant, animal and human health, they can pose a health risk if present in excessive quantities. For this reason, metal concentrations in biosolids and soils are strictly regulated for all land application programs. Toronto's biosolids applied to agricultural land are well within the provincial limits. For more information about metals and to view the most recent lab results for metals in our biosolids, see Biosolids Quality (Nutrients, Metals & Pathogens).
Pathogens (disease-causing organisms) are also present in raw wastewater and are one of the most frequently raised concerns with regards to land application of biosolids. Secondary treatment in the wastewater process removes the majority of these pathogens, but regulations with regards to bacteria content in biosolids, spreading restrictions and waiting periods are also followed as a precautionary measure.
Since 2004, the City has hired a professional engineering firm to provide field inspection for every agricultural land application of our biosolids. The inspector ensures that all land application complies with the Nutrient Management Act and with site specific approvals. Each field where sewage biosolids will be applied is first tested for pH, phosphorus and regulated metal concentrations. Field inspectors also ensure that protective distances (from homes, wells and surface water) have been appropriately identified before spreading begins.
Ministry of the Environment staff also have a role in enforcing the Nutrient Management Act. MOE staff conduct inspections on wastewater treatment plants where the biosolids are produced and the agricultural fields where biosolids are applied. These inspections further ensure that all land application complies with al the regulations and standards.
To find out more about land application, check out these links:
- Biosolids Quality (Nutrients, Metals & Pathogens)
- Nutrient Management Act
- Biosolids use on agricultural land: Questions and Answers
Biosolids Pellets
Thermal drying has been used since the 1920s and has become an effective method to treat biosolids. The biosolids are heated in a dryer to evaporate water content, producing a finished material that is 90% solids or greater. The finished material is dust-free, low-odour pellets 2 to 4 mm in size that can be stored for long periods of time. The high temperatures during thermal drying also reduce pathogens (harmful microorganisms) to below detection level, thus making the pellets a registered fertilizer product. Pellets can be used in bulk agricultural application, or used as a blending material to create specialty fertilizers. The reduction in mass and volume cuts down on transportation and storage costs.
A pelletizer facility is currently operating at the Ashbridges Bay Treatment Plant. In 2010, the pelletizer processed 39% of the biosolids generated at the Ashbridges Bay Treatment Plant.
Alkaline Stabilization
Treating biosolids through chemical stabilization does the opposite to composting because it decreases rather than accelerates bacterial activity. The most common form of chemical treatment consists of mixing biosolids with lime. This process, called alkaline stabilization, controls pathogen levels by increasing temperature and pH. It converts biosolids into an alkaline soil conditioner suitable for use as a low-grade fertilizer, landfill cover, or to increase the pH of acidic soil.
Incineration
Biosolids incineration is an option that has been widely used in North America and Europe for many decades. In recent years, research and development have improved incineration technologies, making them more energy efficient, even energy producing. A number of other Ontario municipalities rely almost exclusively on incineration.
Incineration reduces biosolids volume by up to 80%. The process converts organic material in the sludge into a hot combustion gas called flue gas. This gas can be used as a source of energy for various plant processes. The solid remaining at the end of the incineration process is ash, a non-hazardous waste that can be disposed in a municipal landfill or recycled in cement manufacturing.
Currently, Highland Creek Treatment Plant incinerates all of its biosolids. The emissions are tested annually and are well below the regulatory limits. None of the other City of Toronto treatment plants use incineration.
Landfill
Disposal of biosolids in landfills is a common practice in North America and overseas. Biosolids are either disposed of in "biosolids only" landfills (monofills) or in combination with other municipal waste (co-disposal). In many states in the U.S., up to 50% of biosolids produced are landfilled. Canada manages approximately 28% of its biosolids through landfill (CWWA, 2001).
Biosolids can also be used as daily covers for municipal waste. Landfill daily covers limit surface infiltration and reduce the need for soil cover. They also discourage rodents and other disease-carriers from gaining access to the waste. Biosolids with a moisture content of less than 50% make an ideal daily cover.
An advantage of landfilling is that large quantities of biosolids can be landfilled in a short period of time. However, the nutrient value and most of the fuel value of the biosolids materials is wasted.
Toronto landfilled 28% of its biosolids in 2010. We continue toward reducing this number.