Onion waste recycling by vermicomposting: nutrients recovery and agronomical assessment

Abstract

Vermicomposting is an efficient technology to treat organic wastes and recover nutrients for agricultural purposes. The aim of this work was to assess the feasibility of vermicomposting to treat onion waste and the resulting vermicompost as an agricultural input. Onion waste from the food industry was mixed with cow dump and vermicomposted for 154 days at the pilot scale. The evolution of temperature, moisture, pH, electrical conductivity and total nitrogen was monitored during the vermicomposting process. Finally, the vermicompost was characterized to assess its potential use in agriculture. The vermicomposting process was carried out at moisture around 85% and maximum temperature of 33 °C. The pH decreased throughout the process, while the electrical conductivity increased. Total nitrogen concentration decreased during the first days but then was constant. The final vermicompost was a mature material (C/N = 13), with considerable carbon content (15.7%), lightly basic and with low electrical conductivity. Nutrient equilibria were 1:8:3 with appreciable content of other nutrients such as Ca, Mg and S. Toxic elements were in low concentration and below the legal threshold. The vermicomposting of onion waste with cow dump is a feasible technology to treat this waste and produce a valuable agricultural input to close the nutrient circle.