Phosphorus recovery from sewage sludge by high-temperature thermochemical process (KUBOTA process)

Abstract

With the increase in world population causing a rise in food demand, demand for nutrients such as phosphorus, potash, and nitrogen is on an upward trend. To maintain sustainable agricultural production, it is essential in many areas of the world to ensure stable procurement and to utilize these nutrients more efficiently. It is known that phosphorus is concentrated in sewage sludge through the cycle of consumption of food, human living, and sewage treatment systems. Hence, discussions have been held and studies done on ways of recovering phosphorus from sewage sludge and how to utilize it as a fertilizer. On the other hand, in addition to phosphorus, sewage sludge also includes micropollutants and heavy metals. How to separate heavy metals from phosphorus and decompose organic matter are important from the viewpoint of effectively and safely using phosphorus and preventing groundwater contamination. With the above background, we have developed a thermochemical process (melting process) under high temperature (around 1300 °C) that recovers 90% of phosphorus from sewage sludge and converts it into slag. Organic components are utilized as fuel in the furnace and decomposed. Heavy metals are vaporized in the furnace and collected by a gas treatment system. More than 90% of the phosphorus in the slag is citric acid-soluble phosphorus, indicating that plants can utilize the recovered phosphorus in the slag. We have also confirmed the effectiveness and safety of the slag as a fertilizer with a plant cultivation test. In this chapter, we describe the performance of a technology developed to recover phosphorus from sewage sludge based on the results of laboratory and pilot plant tests. We also describe the properties of the slag as a fertilizer based on the results of chemical analysis and the plant cultivation test.