By Sean Murnan
Food security continues to be a major issue in the world, as the global population grows and certain resources become increasingly scarce. The agricultural industry is heavily reliant on fertilizers containing nitrogen and phosphorus to ensure healthy crop yields that feed the world.
However, there are growing concerns about the availability of these fertilizers and the environmental impact from their production. Researchers have estimated that we will hit global peak phosphorus production in 2030, while phosphorus demand will continue to rise . This means that costs, as well as environmental burdens for phosphorus production will increase alongside demand for food. There is also concern for the overall impact of fertilizer manufacturing on the environment. A review of life cycle assessment studies conducted on various synthetic fertilizers showed the production process for several nitrogen-based fertilizers released 1.5-6 kg CO2eq for every kg of fertilizer produced, along with emissions that caused significant eutrophication and acidification concerns .
These issues have led to increased use of alternative nutrient sources within the agricultural industry. One of the alternatives that has been used for many years are biosolids from sewage sludge. Biosolids are made up of organic material and nutrients that are removed from wastewater as part of the treatment process. The nutrient content of biosolids varies depending on many factors, but typically contains significant amounts of nitrogen and phosphorus as they are found in large concentrations in wastewater . Other nutrients commonly found in biosolids include calcium, potassium, magnesium, and sulfur; alongside micronutrients such as copper and iron. Due to the nature of the source of biosolids, the EPA has set strict guidelines for their use in order to avoid crop contamination with pathogens and pollutants. These guidelines lay out the procedure for application of biosolids to agricultural fields including required pretreatments, testing responsibilities, pollutant thresholds, required recordkeeping and proper permitting .
Another nutrient source more frequently being utilized is digestate from anaerobic digesters. Anaerobic digestion is a process where microorganisms are used to breakdown organic material in an oxygen-deprived environment. The primary byproduct from anaerobic digestion is a biogas with high methane content that can be used for energy generation.
Digesters in the US are increasing in popularity, both among municipal wastewater treatment facilities and the agricultural community. Farms can use the manure produced from livestock as feed for digesters to produce biogas for energy, giving them an additional source of income. Manure has long been used as a fertilizer itself, but there are some benefits to processing it in a digester before field application. Aside from the income from energy generation, research has shown that a higher percentage of nitrogen in digestate comes in a readily available form (such as ammonium) compared to livestock manure slurries . The digestate will also contain a large quantity of organic material which can boost crop yields by providing a source of other important nutrients, including phosphates and potash . The anaerobic digestion process is also effective at destroying pathogens that can cause disease issues for both livestock and plant crops. This makes it a more hygienic nutrient source than raw manure slurries, and allows for additional uses of the digestate. Solids separation from digestate will yield a fibrous material that has many applications, including bedding for farm animals, material for composting or pelletized fertilizer; and can even be used to produce fiberboard for construction .
One of the most important technologies being developed in the wastewater industry is centered around the ability to produce crystalline struvite from wastewater sources. Struvite is a mineral that forms when magnesium, ammonium and phosphate are present in wastewater under slightly alkaline conditions . Typically struvite would be a problem at treatment plants, where it can cause issues with scaling of pipes and other equipment. However, its benefit as a phosphorus-rich fertilizer has led to researchers looking at ways to harvest struvite from the waste stream before it can cause any problems. Systems that remove struvite will have a significant economic impact on wastewater treatment plants by providing them a valuable commodity to sell—removing nutrients that are required to be controlled by regulations, and reducing O&M costs caused by things like scaling and sludge handling. Beyond the wastewater industry, this technology will play an important role in the agricultural industry by providing a renewable source of phosphorus as the world’s supplies dwindle.
Alternative sources for nutrient fertilizers will be integral in the continued push for sustainable agricultural practices, and wastewater can become an unlikely partner in providing food for the world. In my next post I will further look at ways in which wastewater can be recycled for beneficial uses.
|||D. Cordell, “The story of phosphorus: Global food security and food for thought,” Global Environmental Change, vol. 19, no. 2, p. 292–305, 2009.|
|||M. Skowroñska, “Life cycle assessment of fertilizers: a review,” International Agrophysics, vol. 28, no. 1, pp. 101-110, 2014.|
|||D. Sullivan, C. Cogger and A. Bary, “Fertilizing with Biosolids,” Pacific Northwest Extension, June 2007.|
|||Office of Wastewater Management, “A Plain English Guide to the EPA Part 503 Biosolids Rule,” The United States Environmental Protection Agency, 1994.|
|||Waste & Rescource Action Program, “Using quality anaerobic digestate to benefit crops,” 2012.|
|||Waste & Rescource Action Program, “Field experiments for quality digestate and compost in agriculture,” Banbury, England, 2016.|
|||D. M. Kirk and M. C. Gould, “Uses of Solids and By-Products of Anaerobic Digestion,” Michigan University Extension, 17 June 2016.|
|||M. Rahman, “Production of slow release crystal fertilizer from wastewaters through struvite crystallization – A review,” Arabian Journal of Chemistry, 2014.|