End-product quality of composts produced under tropical and temperate climates using different raw materials: A meta-analysis

Compost Quality and Markets Are Pivotal for Sustainability in Circular Food-Nutrient Systems: A Case Study of Sri Lanka

Sustainable management of municipal solid waste (MSW) is a critical issue around the world, especially in South Asia where waste generation is expected to double by 2050. Closing the food-nutrient cycle through composting biodegradable MSW has the potential to meet human needs, including sanitation and food security, while protecting the environment. We use an interdisciplinary case study approach including systems thinking to assess Sri Lanka's national MSW composting system, which primarily receives residential and commercial food waste. We embed quantitative compost quality analysis and interviews at 20 composting facilities within a broader qualitative assessment informed by ~60 stakeholders in total. This approach yields insights on how institutional, economic, social, and biophysical aspects of the system are interrelated, and how challenges and solutions can create undesirable and desirable cascading effects, respectively. Such dynamics can create risks of composting facility failure and unintended consequences, diminishing the chances of achieving a sustainable circular food–nutrient system. Compost quality, which was variable, plays a pivotal role within the system—a function of program design and implementation, as well as a determinant of value capture in a circular economy. We make several recommendations to inform future efforts to sustainably manage biodegradable MSW using composting, drawing on our case study of Sri Lanka and prior case studies from other nations. Key among these is the need for increased emphasis on compost product quality and markets in policy and program design and implementation. Targeted measures are needed to improve waste separation, boost compost quality, effectively use compost standards, encourage compost market development, ringfence the revenues generated at municipal compost plants, and identify efficient modes of compost distribution. Such measures require adequate space and infrastructure for composting, resource investment, local expertise to guide effective system management, strong links with the agriculture sector, and continued political support.

Role of psychrotrophic fungal strains in accelerating and enhancing the maturity of pig manure composting under low-temperature conditions

This study investigatedthe effects of applying psychrotrophic cellulose-degrading fungion cellulase production, fungal community structure, and maturity of pig manure (PM) compost under low-temperature conditions. Three psychrotrophic fungal strains were isolated and identified, and after the cold-active cellulase production conditions were optimized, they were inoculated into PM compost. While the control (CK) compost temperature failed to reachthe thermophilic stage, the inoculated compost temperature reached it within 3 days and was maintained for up to 17 days. Fungal inoculants improved fungal community structure at the end of composting, as suggested by network analysis. Principal component analysis revealed that the germination index (GI), total phosphorus (TP), total potassium (TK), and total nitrogen (TN) were the most influential physicochemical parameters affecting compost maturity. The results of the compost products reflected the suitability of the compost as a fertilizer. This study indicated that newly identified strains positively impacted composting at low temperatures.

Comparison of the effects of different maturity composts on soil nutrient, plant growth and heavy metal mobility in the contaminated soil

Numerous studies on the remediation of metal-contaminated soil by compost focus on the remediation efficiency of compost, however, they have not delved into the issue of nutrients and metal speciation. In this study, the application of municipal solid wastes primary compost (MSW-C), secondary compost (MSW-SC) and the aging compost (MSW-AC) has been conducted in heavy metal contaminated soil remediation. Eight different treatments were carried out to compare the effect of three different maturity composts and their addition ratio (i.e. 0, 25%, 50%) on the changes of physical properties, nutrient content and metal morphology distribution of soil. The enhancement of Sedum aizoon growth was also compared. The results showed that the treatments applied with composts increased the nutrient, organic carbon and the cation exchange capacity. In overall, the most effective treatment method was to use MSW-AC to improve soil physicochemical properties and reduce the heavy metals immobilization, and the addition of 25% MSW-AC showed significant promotion on plant biomass accumulation and root growth. The ability of compost to improve the conditions of the contaminated soil and increase the plant stress resistance was demonstrated by analyzing the root membrane lipid peroxidation, which was lower in the soil treatments with compost, especially with MSW-AC. Based on the compost maturity indexes, soil properties, and efficiency of metal activity reduction, the treatment of 25% MSW-AC is suggested for efficient soil remediation.

Evaluation of mixing ratio and frequency of turning in the co-composting of biowaste with sugarcane filter cake and star grass

This work studied the effect of mixing ratio (MR) and turning frequency (TF) in biowaste composting (BW) with sugarcane filter cake (SFC) and star grass (SG), both on process performance (temperature, static respiration index, total organic carbon, total nitrogen and total phosphorus) and on product quality (pH, cation exchange capacity, electrical conductivity, organic matter, nutrients, stability, maturity, total coliforms and faecal coliforms), through the Principal Components Analysis (PCA). The Pearson correlation coefficients were calculated for all the quality parameters. A joint effect of mixing ratio and turning frequency was demonstrated, highlighting the importance of studying the operational parameters simultaneously. The results of the PCA showed that the best operating conditions and therefore higher product quality is achieved with a TF of twice a week and MR between 20 and 30% of SFC or SG. Additionally, it was found that a frequency of one turn per week generates the lowest product quality, regardless the co-substrate and the MR. The best treatment corresponded to BW composting with MR of 20% SFC and TF of two turnings per week. The obtained results allow to optimize the operation in composting facilities.

The evolution of compost stability and maturity during the full-scale treatment of the organic fraction of municipal solid waste

Composting is the method most commonly applied worldwide for the recovery of the source sorted organic waste. The process aims at stabilizing the organic matter, so as to produce a material with soil improver properties, referred to as compost. The effective recovery of the organic waste fraction via composting implies compost safe use on soil. In this view, the assessment of compost characteristics, depending on both the quality of the input material and the process operation, is fundamental. At full scale, the process monitoring usually relies on parameters enabling the indirect control of its evolution, whereas the biological stability and maturity are usually evaluated on the final product. Aim of this work was in assessing both biological stability and maturity during the composting process of the organic fraction of municipal solid waste performed at industrial scale. Representative samples were collected over time in a composting facility operating in the South of Italy and analysed by the dynamic respirometric index, the content of humic substances as well as by their phytotoxicity. Results showed the key role of stability and maturity parameters in the monitoring of composting processes. Experimental outcomes further addressed wider considerations on the operational procedures for a sustainable compost production process.

Assessment of the use of organic composts derived from municipal solid waste for the adsorption of Pb, Zn and Cd

Waste management is a continuous global need. To minimize problems arising from municipal solid waste (MSW) disposal, composting has emerged as a simple alternative for the organic fraction of the waste. The composting process generates organic composts with a high metal retention capacity for potentially toxic elements (PTE). Thus, our objective was to examine how different composting methods (windrow composting, wire mesh composting bin, and passively aerated static pile composting) affect the final product, and how the characteristics of the generated composts influence their adsorption capacity for the lead (Pb), zinc (Zn) and cadmium (Cd) elements from mining waste. Therefore, the physical and chemical properties of Brazilian composts were investigated, as well as their adsorption capacities, through batch equilibrium tests with Pb, Zn and Cd in single-element solutions. All composts revealed promising adsorption characteristics, including a near-neutral pH (6.4-7.7); a negative ΔpH (-0.4 to -1.0); oxidizing conditions (Eh between +267.67 and + 347.00 mV); a considerable presence of organic matter (193.92-418.70 g kg^-1); a substantial (albeit very varied) cation exchange capacity (29.00-75.00 cmol_c kg^-1); and significant porosity (pore volume between 0.01113 and 0.05400 cm³ g^-1). These results showed that the composts share similar intrinsic characteristics, indicating that the different composting methods influenced subtly the physical and chemical properties of the final products. Overall, the removal selectivity follows the order Pb > Cd > Zn, with the removal percentage ranging from 94.0 to 99.6% for Pb, 55.4-89.8% for Cd and 22.1-64.0% for Zn. Thus, the joint assessment of the characterization and adsorption results shows evidence that composts, a low-cost organic material produced from waste, may be promising as alternative reactive materials for remediation of soils contaminated by Pb, Zn and Cd.

Fate of nutrients and heavy metals during two-stage digestion and aerobic post-treatment of municipal organic waste

The biogas technology is a promising approach for the recovery of energy and fertilizer from municipal organic waste (MOW). However, only scarce information on the development of initial nutrient and heavy metal loads during processing is available. Therefore, this study investigates properties of source-separated MOW during treatment in a semi-industrial scale two-stage biogas plant and subsequent digestate composting including impurities removal. Data from 15 batch experiments was investigated by material and substance flow analysis. Results of this study have shown that about 40% of nutrients contained in the MOW inflow are mineralized during anaerobic and subsequent aerobic treatment. A higher nutrient release was observed during the anaerobic treatment step. Additionally, impurities removal causes a significant reduction of final nutrient content. Heavy metal analysis confirmed a high heterogeneity of contamination levels. However, digestion and composting do not seem to significantly impact on total heavy metal loads in the substrate flow.

Effect of drying, composting and subsequent impurity removal by sieving on the properties of digestates from municipal organic waste

The application of organic soil amendments is a common measure to prevent structural degradation of agricultural soils and to maintain and improve long-term soil fertility. Solid residues from anaerobic digestion of municipal organic waste (MOW) are rich in nutrients and organic matter and have a promising potential to be used as soil amendment. However, no study has related amendment properties of MOW digestate of one origin to different treatment procedures. We therefore investigated the impact of drying, composting and sieving on final digestate properties and specifically nutrient availability and heavy metal and carbon elution. Samples were provided by a semi-industrial two-stage biogas plant with dry fermentation of MOW. Results confirm that in comparison to drying, composting of MOW digestates leads to a significant increase of K, P, Mg, Ca, Cd and Cr in the digestates. Sieving of composted digestates showed that heavy metals are not evenly distributed and that heavy metal concentration in the digestate increases with decreasing mesh sizes (highest concentrations in the fractions <1 mm). Although the element concentration is higher in composted batches, the water-extractability of nutrients, heavy metals and carbon is significantly lower from composted over dried digestates. A significant correlation was found between the dissolution of Zn, Ni, Ca and Mg and pH of eluate as well as dissolved organic carbon (DOC) release (R > 0.7, p<0.05). Results confirm that the extent of carbon elution depends on the degradation rate of digestates. DOC may therefore be a good measure to evaluate digestate stability and to decide on treatment measures.