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What are Biosolids?
Biosolids are the nutrient-rich organic byproduct of the wastewater treatment process which can be beneficially reused.

How are Biosolids Treated to Allow Beneficial Reuse? 
Wastewater solids separated during treatment of municipal wastewater must be treated and stabilized to meet specific requirements established by State and Federal laws and regulations to be classified as biosolids. There are two different classifications of biosolids, Class A and B biosolids. The Town of Cary biosolids management program includes state of the art biosolids treatment systems, called heat dryer systems, to produce a pathogen free, pellet product that can be beneficially reused in agricultural applications. The heat dryer systems produce biosolids that meet State and Federal pathogen reduction requirements for Class A Exceptional Quality biosolids. Class A Exceptional Quality is the highest quality regulated classification for biosolids. Class A biosolids are free of pathogens, can be distributed to the public, and can be beneficially reused without site restrictions. Producing a sustainable Class A product provides the Town the high level of flexibility for beneficial reuse of its biosolids.

How are Biosolids Reused?  
Biosolids are a valuable source of nutrients and micronutrients for enriching soils used to grow plants.  Through recycling biosolids, nutrients captured in the WRF are returned to the soil where they can improve plant growth. The primary nutrients found in biosolids include nitrogen, phosphorus, zinc, potassium, copper, and calcium. The Town’s biosolids provide a cost-effective supplement or alternative to traditional chemical fertilizers.

What Quantity of Biosolids does Cary Recycle?
The Town of Cary is currently producing approximately 3,200 tons of biosolids per year at its South Cary regional dryer system. All of these solids are beneficially reused in agricultural applications.

What Substances are in Biosolids?
Biosolids are generally rich in organics, nutrients, and micronutrients, and low in heavy metals. Enviro Gems is registered with NC Department of Agriculture and Consumer Services as a 5-4-0 nitrogen, phosphorus, potassium) fertilizer. The pellets often achieve a higher fertilizer value of 7-6-0 (nitrogen, phosphorus, potassium). In addition to being a good source of nutrients (nitrogen and phosphorus) the biosolids are a good source of micronutrients for enhancing plant growth.  Some supplemental nutrients include zinc, potassium, copper, and calcium. To meet Class A Exceptional Quality standards, the biosolids must have low heavy metals content. The Town monitors its biosolids metal concentrations to ensure biosolids are below regulated metals limits.

Is the Town of Cary’s Biosolids Program Sustainable?
Sustainability in biosolids management is achieved by multiple means, including reducing solids handling energy demands, minimizing volume of biosolids produced, beneficially reusing the end product, and minimizing recycle of nutrients to the liquid treatment process at the WRFs. The Town of Cary’s regional dryer program is a sustainable solution for the Town. Results from an end-use evaluation completed as part of the 2013 Town of Cary Residuals and Biosolids Master Plan supports that the existing heat drying program is the low cost long-term management option for the Town. In addition to being the low-cost solution, the heat drying program offers the following additional sustainability benefits:

  • Heat drying is considered a robust, sustainable long-term solution in that it has a lower risk for being negatively impacted by more restrictive regulatory trends and increased restrictions on land application programs.
  • Heat drying produces a high quality, Class A Exceptional Quality product that increases flexibility and opportunities for beneficial reuse.
  • Heat drying achieves significant reduction in biosolids volume generated from the WRFs, by achieving greater than 90% removal of water from the final product.  By achieving such a large volume reduction, the Town recognizes a significant reduction in transportation costs and has additional flexibility to cost-effectively transport product to locations outside of the Triangle area.
  • Heat drying significantly decreases dependence on available, dedicated land for direct land application.
  • The majority of the nutrients treated at the WRFs are recovered in the final pellet products, which, in turn, are beneficially reused as a fertilizer amendment.