Vermicomposting two-phase olive mill waste: effect on growth and reproduction of earthworm (Eisenia fetida)

Abstract

Olive oil production is the main agro-industrial activity in the Mediterranean countries as well as in Greece. The operation of the olive oil mills generates significant quantities of liquid waste and solid residues with a high organic load, the nature of which depends on the applied extraction system. In recent years, vermicomposting has become a popular technique for organic waste management, fulfilling the principles of circular economy. An experiment was conducted in order to determine the effect of Two-Phase Olive Mill Waste (TPOMW) on the reproduction rate, mortality and growth of the earthworms Eisena fetida in a laboratory test. The treatments were TPOMW-0 (Manure 370gr & TPOMW 0gr), TPOMW-33 (Manure 246 gr & TPOMW 124gr), TPOMW- 50 (Manure 185gr & TPOMW 185gr) and TPOMW-75 (Manure 124gr & TPOMW 246gr). There were used 4 vessels per treatment. There were added 10 earthworms/vessels, approximately 4 months old. For the phytotoxicity test, it was tested the germination rate of 10 seeds of Lactuca sativa. Extract of each vessel (1:10 compost extract w/v) was mixed with distilled water to create 3 different solutions of different compost proportions (25%, 50%, 75%). With similar method the pH and Electrical Conductivity (EC) values were also measured, but the extract had 1:2 w/v rate. Approximately every 10 days, the vessels were weighed, so that the moisture lost during these days could be measured, and water equal to the weight lost by each vessel was added. The whole experiment lasted about 70 days, at which time the weight of the earthworms in each container was measured again to calculate the rate of weight increase, and also the cocoons in each vessel were counted. The results of the study were tested for their significance on the JMP statistical analysis program. It resulted, that the TPOMW-0 treatment, was the most effective at promoting earthworm reproduction and weight increase. However, the TPOMW-33 treatment stands out as the overall best treatment among the three TPOMW- containing treatments when considering the favorable results in terms of reproduction, growth, and reduced phytotoxicity. The high nutritional content of TPOMW, which explains the significant increase in earthworm biomass seen in the TPOMW-75 containers, is responsible for the rise in EC. A slight variation in electrical conductivity (EC) between the TPOMW-33 and TPOMW-75 treatments suggested a lesser effect on phytotoxicity. The 25% vermicompost extract solution demonstrated the best results in terms of seed germination