(2) Biogas option
As an alternative approach to controlling water hyacinth
proliferation, this option will use water hyacinth to remove nutrients,
and produce biogas and organic fertilizer. The period of
analysis is assumed to be 15 years, corresponding to the life span of
machineries and equipment at the biogas plant. Biogas is considered
an alternative to water gas because they have similar uses,
while organic fertilizer is an alternative to chemical fertilizer. To
avoid the bioaccumulation of heavy metals in food crops to which
organic fertilizer will be applied, heavy metals are removed by
treating the digestate with passivation substances and then alkali
solution.
The biogas production from water hyacinth is composed of the
following stages: 1) collection ofwater hyacinth; 2) pretreatment of
raw materials; (3) hydrolysis and acidification; 4) anaerobic
digestion; 5) production of organic fertilizer; 6) purification and
storage of biogas; and 7) transportation and distribution of products.
The production process is shown in Fig. 1.
The biogas plant has two anaerobic digesters, whose volume is
350 m3 each. One is a high-temperature anaerobic digester, while
the other is a middle-temperature digester. The amount of total
solid needed by the biogas plant per day is calculated as follows