Valorization of biorefinery residues for sustainable fertilizer production: a comprehensive review

Abstract

The management of biowaste and agricultural solid waste is gaining attention due to rising landfill disposal costs and the need for locally available agricultural feedstocks. The biorefinery concept aims to achieve zero waste through valorizing residues as fertilizers. Despite containing NPK macronutrients, residues may not promote plant growth due to limited nutrient availability and phytotoxic compounds. The production of valuable organic, mineral-organic, or mineral fertilizers with confirmed agronomic properties as marketable biorefinery products remains understudied. This comprehensive review broadens our understanding of fertilizer production in biorefineries, which complements the energy (thermal, biogas, biodiesel) and chemical compounds (e.g., succinic acid, propanediol, protein concentrates) that are also generated within biorefineries. It is among the first reviews to investigate the importance of valorizing biorefinery residues as fertilizers, emphasizing methods leading to commercial products and the rationale behind this process. The findings confirm that directly applying unprocessed residues to the soil does not fully exploit their value as by-products. This study contributes to the practical analysis of barriers (legal, chemical, biological, technological) and opportunities (rising prices and reduced global availability of mineral fertilizers) related to fertilizer production in the biorefining process.

Conversion of Sewage Sludge into Biofuels via Different Pathways and Their Use in Agriculture: A Comprehensive Review

“…Fertilization with sewage sludge causes an increase in annual growth and in the number of new shoots. It is generally assumed that sewage sludge is safe when it is obtained from treatment plants and has undergone various processes (i.e., biochemical stabilisation, inactivation of pathogenic organisms, thickening, and dewatering) [32,33,51].…”

“…Drugs can be released into the environment through several ways, but mainly with sewage entering the sewage treatment plant and, after treatment, sewage sludge. Soil quality and fertility are preserved because of microbial activities, and sewage sludge can enhance these activities due to its high level of organic substances [32][33][34]. The use of sewage sludge with low heavy metal contents in the soil can positively impact organic carbon, microbial action, and biomass.…”

“…The content of contaminants in EU organic-mineral fertiliser must not exceed established threshold values [32,33,60]. For example, for cadmium (Cd), this value is 3 mg/kg dry weight, whereas the total P 2 O 5 content is <5% (m/m) in organic-mineral fertiliser.…”

“…However, these prescriptions do not apply to fruit trees and to soils intended for growing fruits and vegetables that have direct contact with the soil and are consumed raw. The ban applies for 10 months preceding the harvest of these plants and during their harvest [32,33].…”

“…The establishment of biorefineries is a promising solution for combining waste processing with the production of value-added compounds for the implementation of closed-loop production models with practically zero waste production [32][33][34]. Different biorefinery technologies, such as hydrothermal pretreatment (HTP) and anaerobic digestion (AD), allow the efficient processing of biowaste to produce valued products such as fertilisers or biofuels.…”