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Free air space (FAS) is a physical parameter that can play an important role in composting processes to maintain favourable aerobic conditions. Aiming to predict the FAS of initial composting mixtures, specific materials proportions ranged from 0 to 1 were tested for a case study comprising industrial potato peel, which is characterized by low air void volume, thus requiring additional components for its composting. The characterization and prediction of FAS for initial mixtures involving potato peel, grass clippings and rice husks (set A) or sawdust (set B) was accomplished by means of an augmented simplex-centroid mixture design approach. The experimental data were fitted to second order Scheffé polynomials. Synergistic or antagonistic effects of mixture proportions in the FAS response were identified from the surface and response trace plots in the FAS response. Moreover, a good agreement was achieved between the model predictions and supplementary experimental data. Moreover, theoretical and empirical approaches for estimating FAS available in literature were compared with the predictions generated by the mixture design approach. This study demonstrated that the mixture design methodology can be a valuable tool to predict the initial FAS of composting mixtures, specifically in making adjustments to improve composting processes containing primarily potato peel.

Industrial eggshell waste (ES) is classified as an animal by-product not intended to human consumption. For reducing pathogen spreading risk due to soil incorporation of ES, sanitation by composting is a pre-treatment option. This work aims to evaluate eggshell waste recycling in self-heating composting reactors and investigate ES effect on process evolution and end product quality. Potato peel, grass clippings and rice husks were the starting organic materials considered. The incorporation of 30% (w/w) ES in a composting mixture did not affect mixture biodegradability, nor its capacity to reach sanitizing temperatures. After 25 days of composting, ES addition caused a nitrogen loss of about 10 g N kg(-1) of initial volatile solids, thus reducing nitrogen nutritional potential of the finished compost. This study showed that a composting mixture with a significant proportion of ES (30% w/w) may be converted into calcium-rich marketable compost to neutralize soil acidity and/or calcium deficiencies.

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The main objective of this study was to evaluate the feasibility of recycling large quantities of industrial eggshell waste through turned windrows composting in order to obtain a value-added soil improver. For that, four different formulations were tested to produce stable composts with adequate properties to be further used for agronomic applications such as amendment in soils with acid characteristics and/or with low calcium concentration. The eggshell waste is mainly an inorganic animal by-product, and thus its co-composting was conducted with farm wastes (horse and chicken manures) and grass clipping. The windrows were aerated periodically and monitored during 50 days regarding temperature, moisture, pH, conductivity, and carbon/nitrogen ratio (C/N). The thermophilic phase lasted for 10-20 days. At the end, moisture, pH, electrical conductivity, organic matter, C/N ratio, and toxic heavy metals (Cd, Cr, Pb, Cu, Ni, Zn) were determined. The microbiological assessment involved total mesophilic heterotrophic bacteria, total coliform bacteria, thermotolerant coliform bacteria, E.coli and spores of sulphite-reducing clostridia. The assessment of maturity based on germination index and stability by using Dewar selfheating test and respirometry showed that all the composts were mature and stable, with adequate properties for agronomic applications. This study evidenced that large quantities (up to 30 % in weight) of eggshell waste may be converted into calcium-rich marketable compost by thermophilic windrows composting.

Composting is one of the technologies recommended for pre-treating industrial eggshells (ES) before its application in soils, for calcium recycling. However, due to the high inorganic content of ES, a mixture of biodegradable materials is required to assure a successful procedure. In this study, an adequate organic blend composition containing potato peel (PP), grass clippings (GC) and wheat straw (WS) was determined by applying the simplex-centroid mixture design method to achieve a desired moisture content, carbon: nitrogen ratio and free air space for effective composting of ES. A blend of 56% PP, 37% GC and 7% WS was selected and tested in a self heating reactor, where 10% (w/w) of ES was incorporated. After 29 days of reactor operation, a dry matter reduction of 46% was achieved and thermophilic temperatures were maintained during 15 days, indicating that the blend selected by statistical approach was adequate for composting of ES.