Decontaminated biochar from sewage sludge as an efficient soil fertilizer and conditioner

Sewage sludge is by-products of wastewater treatment plants (WWTP). Annual generation of sludge in the 27 European Union member countries to was estimated to 13.5 million tons dry solids in 2020. Sewage sludge is composed of organic C, is rich source of P, N which account for up to 4% and 9% of the dry matter, and of other nutrients required for crop production. Phosphate rock, the main P resource for fertilizers in agronomy have been rapidly depleted. Recycling of P, N and C from sludge is critical for a sustainable world.  Sewage sludge amendments improved soil properties and enhanced crop productivity. However, direct sludge land application is confronted with ever-increasing economic, public and regulatory pressures due to its high-water content, odour, pathogens, toxic metals and metalloids, and organic contaminants. To address these challenges, a number of sludge treatment technologies are being developed which can be categorized based on their primary aims to (i) dewaterability enhancement; (ii) solids reduction and sludge stabilisation; (iii) removal of toxic metals. The sludge dewaterability technologies include anaerobic digestion, hydrolysis, ultrasound, enzymatic lysis, acidification, alkaline hydrolysis, alkaline-thermal, thermal-H2O2, microwave alkaline treatments, and hydrodynamic cavitation. The purpose of solid reduction and sludge stabilisation is better sludge management. Sludge incineration is conventional, energy consumptive solution which removes organic and biological contaminants but concentrates toxic metals. The most common methods used at WWTP are based on cell lysis-cryptic growth i.e., after oxidation of sludge with ozone or other oxidants. Sludge pyrolysis is a new promising method which removes pathogens, thermally decomposes/ removes organic pollutants and produces dry biochar. Although some C is lost during processing, the biochar is excellent soil conditioner. Methods of toxic metals removal from sludge include dissolution with organic and mineral acids, bio-leaching, and chelator extraction coupled with other means of lysing cellular matter to release ionic forms of metals. EDTA is an excellent chelator, which has a wide range of coordination ability and can form stable water-soluble chelates with almost all heavy metals. Currently there are no available methods for removal of toxic metals, metalloids and organic contaminants from sewage sludge and retain plant-accessible P, N and C pool.      The objective of this project is therefore to develop a new method for the treatment of sewage sludge that is compatible with the usual processes used in WWTP and allows the production of biochar that is free of toxic metals, metalloids and organic contaminants and can be used as a soil fertilizer and conditioner in agriculture.  To meet the objective hydrodynamic cavitation and EDTA-enhanced Fenton-like oxidation will be used to improve sludge dewaterability and solids reduction. EDTA heavy metal chelation/ removal and EDTA recycling will be innovatively coupled with common sludge an/aerobic processes at waste water treatment plants: anaerobic digestion and aerobic cell-lysis/ cryptic growth. No corrosion of equipment is expected, since EDTA does not chelate metal atoms in the free metallic state but coordinate only with ionized metals. Pyrolysis will be used to remove organic pollutants and remaining EDTA. The environmental safety, P, N, K concentration and phyto-availability, and soil humus-like properties of novel biochar will be investigated. The compatibility of EDTA processes with an/aerobic sludge processes will be examined, as well as synergies and antagonisms.