Fertilizer value of composts obtained with addition of coffee grounds and maize.

Management of catering waste, including coffee grounds, is becoming an important economic issue. Coffee grounds contain both organic compounds and mineral compounds, including macronutrients and micronutrients. The purpose of the study was to determine the fertilizer value of composts obtained using coffee grounds. The compost was prepared from vegetable raw material and coffee grounds with the addition of biocarbon or effluent obtained during yeast production. The experiment included 4 treatments: M – control - maize straw; M+CG – maize straw + coffee grounds; M+CG+BC – maize straw + coffee grounds + willow biochar; M+CG+E – maize straw + coffee grounds + yeast effluent. The amount of waste materials introduced into the composted maize biomass was limited not only due to their physical parameters, but also because of the humidity of the feedstock. The ratios of feedstocks used in individual treatments by weight of the dry matter were: M+CG – 1:0.89; M+CG+BC – 1:0.89:0.1; M+CG+E – 1:0.89:0.07. After mixing the materials, moisture of the mixture was equilibrated to 60% by weight. Aeration of the biomass was performed in cycles, 6 times a day; the air was flowing through the bioreactor in the amount of 15 dm³ min^-1 for 60 min; the biomass was manually shifted every 10 days. The content of macronutrients and trace elements was determined in the product with regard to the admissible values for organic fertilizers. Composts with a very high proportion of coffee grounds meet the criteria regarding the minimum NPK and organic matter content in the fertilizer aspect. Coffee grounds are characterized by a low content of heavy metals, which in practice can cause their dilution in biological processing products. Utilization of the effluent from a yeast factory for irrigation of composted biomass is not possible on an industrial scale. A small addition of this waste leads to significant salinity of the compost. [ABSTRACT FROM AUTHOR]