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.