Agronomy

ABSTRACT

Phosphorus is a depletable resource and essential macro‐nutritional element in agriculture. Nowadays phosphorus is almost exclusively produced by the mining of phosphate rock. Europe imports phosphate rock mostly from Morocco where mines are mainly located in annexed Western Sahara. Regional political instability and strong new phosphorous markets in Africa and Asia could lead to relative scarcity of phosphorus. In 2014 phosphate rock was placed on the EU critical raw material list. EU also adopted the concept of circular economy addressing mounting resource‐related challenges. Phosphorus is potentially recyclable and re‐usable by direct application of sludge on agricultural land and as P mineral struvite extracted and recrystallized from ash after sludge incineration.  Widespread contamination of sludge with toxic metals and further concentration of metals in sludge ash are the main, jet unsolved problem in environmentally sustainable phosphorus reclamation. Decontamination of sludge and sludge products (digestate, pyrolysate, ash, struvite), prior to their use as phosphorus fertilizers in agriculture, by extraction of toxic metals using strong chelator ethylenediamine tetraacetate (EDTA) has gained much attention due to high metal removal efficiency. However, EDTA is not biodegradable and persist in the environment. High cost of EDTA, generation of vast volumes of waste waters, and leaching of EDTA chelates with toxic metals from sludge and sludge products applied on agricultural land hampered the development of feasible decontamination method.  In this project we are introducing novel EDTA‐based method for sludge and sludge products decontamination. The method will be developed considering sustainability concepts to reduce the unintended environmental and economic footprint: EDTA will be recycled, no wastewater will be generated, and toxic emissions to the environment will be prevented. The apparent EDTA non‐biodegradability will cease being an issue. The hydrodynamic cavitation process will be coupled to decontamination method. It will be optimised for efficient sludge disaggregation and enhanced removal of contaminants from sludge and sludge products. Next to toxic metals the linear alkyl benzene sulphonates (LAS) are the most ubiquitous and hazardous organic pollutants in sludge. The feasibility of novel decontamination method to remove LAS and LAS soluble lipophilic xenobiotics simultaneously with toxic metals will be investigated. Pyrene, a typical 4‐ring polyromantic hydrocarbon (PAH) will serve as a model for lipophilic compounds. In addition, the application of bulking material and CaO as oxygen releasing compound to the slurry before dewatering of decontaminated sludge will be tested as means for enhanced sludge composting. The agronomical properties of decontaminated and composted sludge and ash‐derived struvite as phosphorus fertilizers will be examined in experimental beds with maize (Zea mays). Maize growth and fitness will be assessed, and phosphorus uptake put in relation to uptake from commercial fertilizer. Beds will be designed as lysimeters; all leachates will be collected and analysed for potential toxic emissions. The functioning of soil fertilised with decontaminated sludge and struvite will be assessed using microbial and enzymes activity as bioindicators. Potential sludge and struvite phytotoxicity will be assessed by measuring plant stress proteins.  The Life cycle assessment (LCA) will be used to analyse economic, social and environmental viability of novel decontamination method. 

THE PHASES OF THE PROJECT AND THEIR REALIZATION

1. Hydrodynamic cavitation in processes with sewage sludge and sludge products 
2. Removal of toxic metals from sewage sludge and sludge products.
3. Removal of toxic metals, LAS and PAHs from sewage sludge.
4. Soil fertilisation and effect on soil functioning.
5. Life Cycle Assessment   

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