Controlled-Release Fertilizer from Biosolids-Energy and Greenhouse Gas Comparison with other Biosolids Management Options

Nitrogen-based fertilizers for agriculture require energy-intensive production of ammonia, which may then be converted to urea or other fertilizer products. The nitrogen mineralizes more quickly than the rate of plant uptake, resulting in nitrate loss as leachate, runoff, and volatilized as nitrogen gas and nitrous oxide. Biosolids products mineralize more slowly, so a higher percentage of their nitrogen is plant-available, but, since biosolids products have relatively low initial nitrogen content, they are applied as soil conditioners, requiring supplemental nitrogen. The VitAg process treats biosolids in a high temperature reaction of ammonia and sulfuric acid to produce a controlled-release nitrogen release fertilizer that is also rich in organic matter. The end product meets Class A pathogen reduction requirements and provides 14-17% controlled- release nitrogen.The paper describes the VitAg process and presents a methodology for tracking energy consumption through the fertilizer lifecycle, beginning with ammonia production and following through to plant nitrogen uptake and losses. Inputs are normalized on the basis of tons of plant available nitrogen, as measured by plant tissue analysis. Nitrogen uptake and loss entries are based on literature searches of field trials including the WERF study Estimating Plant Available Nutrients in Biosolids, field trials for rice, citrus, and field crops using a process similar to VitAg, and on other published sources. It is concluded from this literature review that biosolids-based fertilizers including VitAg provide approximately 1.7 times as much plant-available nitrogen (PAN) as inorganic fertilizers.As a result, the VitAg process uses approximately 31 percent less energy to deliver the same quantity PAN than inorganic fertilizers.Greenhouse gas emissions are affected by energy consumption, nitrous oxide volatilization at the point of application, and carbon sequestration offsets. Greenhouse gas emissions are tracked as equivalent tonnages of carbon dioxide per ton of PAN. The results show that the net greenhouse gas emissions associated with the production and application of the VitAg product are about 40 percent less than with an inorganic fertilizer delivering an equivalent amount of plant-available nitrogen.Energy consumption and greenhouse gas emissions are compared for the following Class A biosolids management alternatives:Class A lime stabilization with heat stage for pathogen reduction:Thermal drying to produce either a soil conditioner or a fertilizer ingredient:Aerated static pile composting: andThe VitAg process.Because of the high energy and greenhouse gas cost of ammonia production, the ‘bottom line’ results of comparing options a), b), and c) is highly sensitive to the assumed percentage of plant availability of nitrogen. Percentage of plant availability is highly variable, determined by crop, climate, and other specifics. When considering the lower amount of ammonia required to deliver a specific amount of plant-available nitrogen the VitAg process has lower net energy and greenhouse impacts per dry ton of biosolids processed than the alternatives.