Extraction of heavy metals and phosphorus from sewage sludge ashes

Phosphorus is a crucial element for food and fodder production, and generally for ecosystem stability. Phosphate fossil resources are its common source, but industry waste can be also a valuable alterative in agriculture applications. Sewage sludge, which comes from municipal wastewater treatment plants, is an important source of phosphorus, which is listed as a critical element for EU countries. Therefore, it is very important to implement new effective methods of phosphorus recovery from these type of waste into the fertilizer industry. It is available in large quantities in wastewater treatment plants, with biological stage based on activated sludge. However, municipal sewage sludge (MSS) contains significant amounts of various pollution types, including parasites, eggs, antibiotics, microplastic and heavy metals. Thermal conversion removes all organic matter, toxic organic compounds and all parasites, but increases concentrations of heavy metals. Different extraction methods of ash from thermal treatment of municipal sewage sludge enables the use of an effective, cheap, and simple technology of phosphorus recovery from sewage sludge ash through an easy soluble form of heavy metal removal. The aim of this work was to find a method for easy soluble forms of heavy metals removal to create an ash phosphorus fertilizer. It is therefore necessary to identify the amount of heavy metals and bioavailable phosphorus to determine the possibility of using sewage sludge ashes (SSA) as agriculture fertilizer. Experimental extraction with sulfur and acetic acid in different dilutions (0.05, 0.005, 0.0005 mol/ dm3 of H2SO4 and 0.001, 0.01, 0.02, 0.04, 0.06, 0.08 and 0.1 mol/dm3 of CH3COOH) was introduced for bioavailable phosphorus and easy soluble forms of heavy metal removal to achieve safe levels of metal concentrations that fulfill requirements for mineral fertilizers. The obtained results show strong potential for phosphorus elution; however, a significant difference was observed in molybdenum removal compared to other heavy metals in acidic solutions.