GETM

Biosolids: Recycled matter of Fertilizer value or smelly source of Pollution?

Biosolids are treated sewage sludge. The application of biosolids on agricultural lands is steadily gaining ground as a cost effective way of supplying plant nutrients to enhance the growth of crops. The fertilizing ability of biosolids is due to the nutrients phosphorus and nitrate as well as potassium, calcium and magnesium, commonly present in sewage sludge.

The treatment techniques are as specified by relevant regulatory bodies to reduce pathogen (disease causing organisms), reduce vector (flies, rodents, etc.) attraction and stabilize metals. Treatment is usually dewatering, or thickening followed by alkaline stabilization, heat treatment, composting, aerobic or anaerobic digestion or chemical conditioning.

The resulting biosolids product may be used on agricultural lands as nutrient augmenting products, enhancing soil properties with the aim of increasing crop growth. However the extent of land application depends on the conformance of the biosolids to stringent regulatory criteria, as shown in the Tables 1 and 2. Pollutant loading also influences the application rate. For example, the amount of a nitrogen-rich biosolids that is applied per hectare of agricultural land will depend on the nitrogen already present in the soil and what is required by the specific crop, in order to not apply more than the agronomic rate (what is necessary for optimal plant growth while minimizing the amount reaching groundwater). Moreover, the biosolids applied must be monitored based on the amount applied, from once a month to once a year, for pollutants, pathogen and vector attraction reduction.

The two major problems associated with land application of biosolids are unpleasant odor and the risk of non-point source of pollution through runoff. Odor aesthetically affects persons living near biosolids application sites. It is due to the emission of volatile gases during chemical and microbial degradation of organic matter in the biosolids. The main odorous substances emitted from biosolids are ammonia and sulfur compounds.

Odor emissions can occur at the biosolids treatment and storage stages. Incomplete treatment causes odors; for example, low retention time during aerobic digestion or poor aeration during composting. The solution is to ensure complete and effective treatment with frequent monitoring of treatment and indicator parameters. At the storage stage, high humidity, hot weather, precipitation (rain or snow), drop in pH and low oxygen level cause the formation of odors. It is important to minimize the storage period as well as monitor for odors continuously. Microbial degradation can be avoided short term by liming during storage. Knowing the prevailing wind direction can aid in preventing the dispersion of odors by means of storage location selection or effective barriers. Ensuring that the biosolids are kept away from any water sources is also important. Constructing structures to eliminate rain water from reaching biosolids is essential.

The application of biosolids to land poses a risk of non-point source of pollution to surface water and groundwater. Runoff events (rain or snow) can cause biosolids to reach water bodies. Groundwater under the direct influence of surface water is also vulnerable. Typical contaminants include the nutrients phosphorus and nitrates, boron, sodium and pathogens. Nutrient loading to water bodies may lead to excessive growth of algae. Pathogen present may results in sicknesses in humans such as gastrointestinal diseases. Emerging pollutants of concern include endocrine disruptors (dioxins, pesticides, etc.), pharmaceuticals (birth control pills, antibiotics, etc.) and flame retardants, and the impacts of these chemicals on human health and the environment are being investigated in wastewater discharges.

The nutrient loading to water bodies may be controlled by applying biosolids to land as per the agronomic rate of a crop (thus minimizing the nitrogen that may find its way to groundwater). Runoff as a non point source of pollution may be controlled by maintaining distances of the stored biosolids to water bodies as per regulatory requirements and constructing barriers. The slope of the land, soil permeability and proximity to a surface water or groundwater well are factored in when determining the regulatory requirements of distances to water bodies. Barriers that are constructed to provide a physical means of runoff control are roofs, side walls, dykes and runoff containment ponds.

To summarize, the benefits of land application of biosolids include recycling of a waste product (sewage sludge) and providing plant nutrients cost effectively. Issues include odor and risk of surface and groundwater pollution which may be eliminated with a case-specific biosolids management plan in place. With correct treatment and storage techniques including an effective runoff prevention design, odor problems may be eliminated and the pollution of water bodies may be prevented.

References

National Research Council (NRC) (2003). ‘Biosolids Management Program’. A Best Practice by the National Guide to Sustainable Municipal Infrastructure.

National Research Council and Federation of Canadian Municipalities (NRC and FCM) (2005). ‘Quality Management for Biosolids Program’. A Best Practice by the National Guide to Sustainable Municipal Infrastructure (Infraguide).

United States Environmental Protection Agency (2003). Environmental Regulations and Technology. Control of Pathogens and Vector Attraction in Sewage Sludge. Document #EPA 625/R-92/013.

Posted by admin on February 21st, 2007 filed in review 5 Comments »