Anaerobic Digestion of Organic Waste in UMS Campus …
139
organic nitrogen may be converted to ammonia, NH 3 gas as shown in Eqs. 4 and 5
(Metcalf and Eddy 1991).
H
+
+ N H 3 → N H
+
4
(4)
N H
+
4 + B
−
→ H B + N H 3
(5)
Thus, it can be assumed that a certain amount of NH 3 gas was released as biogas
during the degasification of 15 days AD period. This occurrence also explained the
unpleasant odour released from the reactor during the degasification.
The concentration of PO 4
3− slightly increased from 509.18 mg/L in FWRL to
554.04 mg/L in FWDL. According to the past research by Frear and Dvorak (2011),
the amount of PO 4
3− should increase ~60% after the AD. Instead, in this study, it
only occurs at ~8.8%. As mentioned earlier, when nutrients such as Mg
2+ , NH 4
+
and PO 4
3− exist in equimolar ratio, they can form a directly used fertiliser called
struvite. During the 15 days period of the AD, all those ions may already present in
an equimolar ratio in the reactor which subsequently leads to struvite precipitation.
This is possible because the controlled conditions of the AD (37 °C and pH 6.8–7.2)
is the optimum condition for struvite formation. Hence, it can be assumed that some
of the PO 4
3− were precipitated as struvite in the digested liquor. This explained the
cloudy solutions and the brownish-white precipitate that appeared and settled at the
bottom of the glass bottle during sample storage after filtration.
3.4 Potential of Phosphorus (P) Recovery from Anaerobic
Digestion of Food Waste
In this study, P recovery is represented by the concentration of soluble phosphate,
PO 4
3− in the digested liquor. The percentage of P recovery was low (~8.8%), which
was calculated based on the initial PO 4
3− concentration before AD. Even though
the percentage of recovery was low, the concentrations of PO 4
3− in food waste,
both before and after AD, were significantly high. This indicates that food waste
is a naturally nutrient-rich waste and an excellent source of phosphorus. Table 2
compares P concentrations with previous studies which revealed the current study
has the highest P concentration.
One of a way to recover P from the digested liquor is through precipitation to
form struvite (magnesium ammonium phosphate, MgNH 4 PO 4 .6H 2 O.6H 2 O). Table 3
shows the potential of P recovery in a form of struvite which was calculated using
a molar ratio of Mg
2+ , NH 4
+ and PO 4
3− in the digested liquor. This suggests that
1 g of struvite can be precipitated from 1 L of digested liquor. The recovery can also
be expressed based on the weight of food waste added in the anaerobic digester. In
this case, 136 mg of struvite can be recovered from every 1 g of food waste used
for anaerobic digestion. If we consider the amount of food waste produced by the
139
organic nitrogen may be converted to ammonia, NH 3 gas as shown in Eqs. 4 and 5
(Metcalf and Eddy 1991).
H
+
+ N H 3 → N H
+
4
(4)
N H
+
4 + B
−
→ H B + N H 3
(5)
Thus, it can be assumed that a certain amount of NH 3 gas was released as biogas
during the degasification of 15 days AD period. This occurrence also explained the
unpleasant odour released from the reactor during the degasification.
The concentration of PO 4
3− slightly increased from 509.18 mg/L in FWRL to
554.04 mg/L in FWDL. According to the past research by Frear and Dvorak (2011),
the amount of PO 4
3− should increase ~60% after the AD. Instead, in this study, it
only occurs at ~8.8%. As mentioned earlier, when nutrients such as Mg
2+ , NH 4
+
and PO 4
3− exist in equimolar ratio, they can form a directly used fertiliser called
struvite. During the 15 days period of the AD, all those ions may already present in
an equimolar ratio in the reactor which subsequently leads to struvite precipitation.
This is possible because the controlled conditions of the AD (37 °C and pH 6.8–7.2)
is the optimum condition for struvite formation. Hence, it can be assumed that some
of the PO 4
3− were precipitated as struvite in the digested liquor. This explained the
cloudy solutions and the brownish-white precipitate that appeared and settled at the
bottom of the glass bottle during sample storage after filtration.
3.4 Potential of Phosphorus (P) Recovery from Anaerobic
Digestion of Food Waste
In this study, P recovery is represented by the concentration of soluble phosphate,
PO 4
3− in the digested liquor. The percentage of P recovery was low (~8.8%), which
was calculated based on the initial PO 4
3− concentration before AD. Even though
the percentage of recovery was low, the concentrations of PO 4
3− in food waste,
both before and after AD, were significantly high. This indicates that food waste
is a naturally nutrient-rich waste and an excellent source of phosphorus. Table 2
compares P concentrations with previous studies which revealed the current study
has the highest P concentration.
One of a way to recover P from the digested liquor is through precipitation to
form struvite (magnesium ammonium phosphate, MgNH 4 PO 4 .6H 2 O.6H 2 O). Table 3
shows the potential of P recovery in a form of struvite which was calculated using
a molar ratio of Mg
2+ , NH 4
+ and PO 4
3− in the digested liquor. This suggests that
1 g of struvite can be precipitated from 1 L of digested liquor. The recovery can also
be expressed based on the weight of food waste added in the anaerobic digester. In
this case, 136 mg of struvite can be recovered from every 1 g of food waste used
for anaerobic digestion. If we consider the amount of food waste produced by the