Practical Applicability of Germination Index Assessed by Logistic Models

Sewage sludge management is a major challenge in environmental protection. Composting is an organic waste treatment method that is cost effective and leads to resource recovery. Composting is considered an environmentally and agriculturally friendly method of sewage sludge utilisation. The objective of this study was to evaluate maturity of three composts prepared on the basis of sewage sludge mixed with structure-forming waste materials, such as pine bark, sawdust and wheat straw. The germination index (GI) was used to assess the maturity and phytotoxicity of composts at particular composting stages (initial, mesophilic, thermophilic, cooling, maturation). Cress seeds were used to determine the GI. The logistic model, which belongs to a broad class of generalized linear models, was used to analyze experimental data. Using this model the interesting probabilities (from the point of view of the experimenter) for the occurrence of a specific root length were determined. In addition, a model was constructed providing a dependence of probability on temperature.This work indicates a marked dependence between root length produced by cress seeds and the temperature of the composting process, which was closely related to the GI values. The longest plant roots, similarly as the highest GI values, were found at the lower temperature, which took place at the beginning and at the end of the composting process. Our findings suggest that the practical applicability of GI in the evaluation of compost maturity is limited. Additionally, the role of additional wastes being structure-forming agents in composted mixtures with sewage sludge was stressed as a sorption matrix for harmful substances released from sewage sludge.