4. Results and discussion
The results are summarized in Table 1. The extent of vermicomposting has been reported in terms of vemricast generated per animal, per day, as it gives a more realistic picture than the statistic of ‘percent convertion’ which is a function of earthworm density employed. The statistic of vermicast generated per worm per day can be used to estimate the extent of convertion of the substrate to vermicast had the number of animals been larger or smaller.
The rate of vermicomposting was at its slowest during the first 20 days possibly because of the lack of familiarity with the new feed of the earthworms who had, till then, been fed cow-dung. During the next 80 days the extent of vermicast production increased by about 35%. In subsequent 100 days there was further increase, though at much slower rate. The overall trend lines (Figure 9) reveal that the vermicompost generated per animal would have risen further had the experiment been continued. It would have perhaps peaked before the earthworms entered their old age and began to die.
All through the 6-month long uninterrupted experiment the HEVSTOW system, including the APSU and the SEVS machines flawlessly functioned to their designed potential. It is thus shown that the system can be, as designed, used in the simultaneous and efficient vermicomposting of different substrates with different species of earthworms.
Vermicomposting is essentially an aerobic decomposition process during which the microorganisms present in the earthworm gut decompose the organic carbon contained in the feed. An estimated 50±10% of the total solids present in a feed are converted to carbon dioxide in the course of the feed’s vermicomposting. The CO2 is then emitted and is thus lost to the vermireactor. Hence each unit mass of vermicast generated represents the bioconvertion of about twice as much feed mass.
The approximately 40 mg of vermicompost being generated per earthworm per day in the vermireactors by the 180th day represent the bioconvertion of about 80 mg of the substrate per worm per day. In other words about 80 * 20 * 50 mg, or 80 g, of the substrate was getting vermicomposted in 20-day pulses by the 50 earthworms in each reactor. As the feed mas was 200 g per module, this represents 40% of it getting vermicomposted in 20 days. By employing a larger number of earthworms this could be easily enhanced to about 60% of vermicomposting occurring over 20 days, or the entire feed getting vermicomposted in about 35 days. This leads to a rate of vermicomposting that is several times higher than the 90-120 days required by conventional systems (Edwards et al., 2011; Banupriya, 2018).
Moreover, in the controlled experiment, the earthworm offshoot born in the vermireactor were removed once every 20 days so that only the vermicast generated by the 50 ‘parant’ worms could be quantified. But in subsequent HEVSTOW operation, this removal was not done. It soon built up an increasingly dense earthworm population in each module, causing 100% vermicomposting within 20 days. By engaging larger number of vertically stacked modules in HEVSTOW, a very high space efficiency can also be achieved. Hence with its defining virtues, HEVSTOW enables high vermicomposting ability in terms of space as well as time.