126
M. Rajin et al.
The data plotted in Fig. 3.3 also showed that there was further decrease in the
pH reading for food waste without lipase; which was 3.98–3.86 from day 10 to day
20, then a small increase in pH was observed from day 20 to day 40. On the other
hand, the pH for food waste with lipase showed a slight rise from day 20 to day 40.
The subsequent rise of pH of food waste and food waste with lipase was due to the
decomposition of nitrogen-containing organic matter which led to the accumulation
of NH 3 which dissolved in moisture to form alkaline NH
4+ . Moreover, the organic
acids formed were decomposed to form gaseous carbon dioxide and water. The
further decomposition of fatty acids into smaller molecular acids to evaporate will
also result in an increase of pH. Furthermore, mineralisation of proteins, amino acids
and peptides contributes to the release of ammonium or volatile ammonia which
results in the increase of pH (Lin 2008).
It was found that the changes in pH within day 10 to day 40 were less significant as
compared to the first 10 days of digestion for both samples. This was due to the hardly
degradable fractions of the organic wastes (Petric and Mustafi´ c 2015). In comparison,
both samples showed identical pH at the end of the digestion process. However, from
the graph, it is shown that the food waste with lipase achieved the constant and lowest
pH on day 30, which was 10 days earlier as compared to the control. This may be due
to the action of lipase which sped up the reaction by lowering the activation energy
(Salwanee et al. 2013).
3.4 Effect of Lipase Addition on Electrical Conductivity
Figure 3.4 displays the electrical conductivity profile for the food wastes with and
without lipase. Electrical conductivity (EC) indicates the total salt content in an
anaerobic digestate which shows whether the salt content may affect the quality of
digestate to be used as a fertiliser. EC measures the total soluble salts in the food
waste digestate. The higher the EC, the higher the nutrient content of the digestate.
However, EC exceeding 4 dS/m (4 mS/cm) will cause a negative impact on plant
growth (Lin 2008). It has been suggested that the suitable electrical conductivity for
safe plant growth is 2.5 mS/cm (Himanen and Hänninen 2011). Digestate with low
EC can be used directly as fertiliser, whereas digestate with high value of EC must
be mixed with soil or other materials with low ECs before it can be used for growing
crops (Ishak et al. 2014).
As shown by the plot in Fig. 3.4, the initial EC for both samples were 6.26
mS/cm. After 10 days, The EC rose to a maximum value of 8.16 mS/cm and 8.26
mS/cm for food waste without lipase and food waste with lipase, respectively. The
increasing trend of EC at the earlier stage was caused by the presence of large
quantities of mineral salts such as phosphate and ammonium in the food waste, which
were released during the decomposition of organic substance (Lin 2008; Chan et al.
2016). The basic ions released after degradation increased the electrical conductivity
(Fang and Wong 1999).
M. Rajin et al.
The data plotted in Fig. 3.3 also showed that there was further decrease in the
pH reading for food waste without lipase; which was 3.98–3.86 from day 10 to day
20, then a small increase in pH was observed from day 20 to day 40. On the other
hand, the pH for food waste with lipase showed a slight rise from day 20 to day 40.
The subsequent rise of pH of food waste and food waste with lipase was due to the
decomposition of nitrogen-containing organic matter which led to the accumulation
of NH 3 which dissolved in moisture to form alkaline NH
4+ . Moreover, the organic
acids formed were decomposed to form gaseous carbon dioxide and water. The
further decomposition of fatty acids into smaller molecular acids to evaporate will
also result in an increase of pH. Furthermore, mineralisation of proteins, amino acids
and peptides contributes to the release of ammonium or volatile ammonia which
results in the increase of pH (Lin 2008).
It was found that the changes in pH within day 10 to day 40 were less significant as
compared to the first 10 days of digestion for both samples. This was due to the hardly
degradable fractions of the organic wastes (Petric and Mustafi´ c 2015). In comparison,
both samples showed identical pH at the end of the digestion process. However, from
the graph, it is shown that the food waste with lipase achieved the constant and lowest
pH on day 30, which was 10 days earlier as compared to the control. This may be due
to the action of lipase which sped up the reaction by lowering the activation energy
(Salwanee et al. 2013).
3.4 Effect of Lipase Addition on Electrical Conductivity
Figure 3.4 displays the electrical conductivity profile for the food wastes with and
without lipase. Electrical conductivity (EC) indicates the total salt content in an
anaerobic digestate which shows whether the salt content may affect the quality of
digestate to be used as a fertiliser. EC measures the total soluble salts in the food
waste digestate. The higher the EC, the higher the nutrient content of the digestate.
However, EC exceeding 4 dS/m (4 mS/cm) will cause a negative impact on plant
growth (Lin 2008). It has been suggested that the suitable electrical conductivity for
safe plant growth is 2.5 mS/cm (Himanen and Hänninen 2011). Digestate with low
EC can be used directly as fertiliser, whereas digestate with high value of EC must
be mixed with soil or other materials with low ECs before it can be used for growing
crops (Ishak et al. 2014).
As shown by the plot in Fig. 3.4, the initial EC for both samples were 6.26
mS/cm. After 10 days, The EC rose to a maximum value of 8.16 mS/cm and 8.26
mS/cm for food waste without lipase and food waste with lipase, respectively. The
increasing trend of EC at the earlier stage was caused by the presence of large
quantities of mineral salts such as phosphate and ammonium in the food waste, which
were released during the decomposition of organic substance (Lin 2008; Chan et al.
2016). The basic ions released after degradation increased the electrical conductivity
(Fang and Wong 1999).