Changes in the HRT of the reactor system caused variation of the anaerobic and aerobic react times. It also may affect the loading rate imposed to the system if the substrate concentration is maintained. These conditions will affect the microbial activity within the biogranules and may influence the performance of the reactor system. The details of the experimental conditions of the reactor system are shown in Table 5.

The microbial activity was measured based on the OUR of a complete one cycle operation.

The OUR was measured several times before each of the stages ended and showed that most of the external substrate was consumed more or less within the first 30 minutes of each aerobic reaction phase. Figures 12 and 13 show the profiles of the OUR throughout the experiment from Stage I to Stage VI.

Stage	Days covered	Phase (hours)	HRT (hrs)	OLR (kg COD/ m^day)
1st	2nd
Anaerobic	Aerobic	Anaerobic	Aerobic
I	49	1.42	1.42	1.42	1.42	6	2.5
II	43	2.92	2.92	2.92	2.92	12	1.3
III	51	5.92	5.92	5.92	5.92	24	0.6
IV	43	5.92	5.92	5.92	5.92	24	0.8
V	46	8.92	2.92	8.92	2.92	24	0.8
VI	46	2.92	8.92	2.92	8.92	24	0.8

OLR=x — Tdd, where X = COD concentration of the influent (mg/L); Vadd= Volume of influent added in each cycle operation (mL); Vtotal = Total working volume of the experiment (mL); T = Hydraulic retention time (hour). Table 5. Details of experimental conditions of the reactor system

The OUR profile (Figure 12) shows that the initial measurement of the OUR was reduced as the HRT increased (Stage I to Stage III). This is due to the reduction in the OLR as the HRT increased. Less oxygen is required as the organic load concentration is reduced. After a sharp increase of OUR at the beginning of each cycle in all stages, the OUR measurement was consistently low until the end of the cycle. The low value of the OUR indicates that most of the external substrates have been consumed. It also means that the microorganisms in the reactor system are under starvation phase. At this phase, no further degradation was observed even though the HRT was extended. During the starvation phase, endogenous respiration will take place, except at the beginning of the second phase of aerobic reaction where there was a short increase in the OUR. This increase is caused by the mineralization of

amines, the byproduct of dye degradation during the second anaerobic reaction phase. As the duration of anaerobic reaction phase increased, the short pulse increased as shown in Figure 13 (a and b) of Stage IV and V, respectively. Stage IV and Stage V were operated with the same HRT and organic loading but were different in the anaerobic and aerobic reaction phase ratio.

500 400 300 200 100 0

0 0.5 1 1.5 2 2.5 3

The changes in the HRT will also affect the biomass accumulated within the reactor system. The HRT was increased from 6 hours in Stage I to 24 hours in Stage Ш, without the addition of any substrate. This resulted in the reduction of OLR supplemented into the reactor system from 2.5 to 0.6 kg COD/m3 day. The HRT for Stage Ш to VI was kept constant i. e. 24 hours, but the duration of anaerobic and aerobic react phases was varied. From Stage III onwards, the OLR was increased to 0.8 kg COD/m3 day by increasing the concentration of the carbon sources in the synthetic textile dyeing wastewater.

250

250

200

150

100

50

0

140

120

100

80

60

40

20

0

6 8 10 Time (hours)

Figure 13. OUR profile of (a) Stage IV (Aerobic phase 11.84 hours), (b) Stage V (Aerobic phase 5.84 hours), (c) Stage VI (Aerobic phase 17.84 hours)

Table 6 shows the oxidation-reduction potential (ORP) values measured during the second phase of the anaerobic and aerobic reactions during the experiments. The ORP profile of all the stage corresponded very well with the dissolved oxygen. As the anaerobic react phase increased, more of negative values of the ORP were recorded. During the aerobic phase the ORP varies between +98 to +177 mV.

The biomass profile at steady state with stepwise increment of HRT (Stage I to Ш) and variation of react phases (Stage IV to VI) are shown in Table 8. As shown in Table 7, it is apparent that the biomass concentration (MLSS) in the reactor decreased and the VSS in the effluent were also reduced with the increase in the HRT (Stage I to III). The reduction of the biomass concentration in the reactor may be due to the lower value of OLR applied in the reactor system as the HRT increased.

Stage	Anaerobic React Phase	Aerobic React Phase
I	-124 ± 27	125 ± 19
II	-219 ± 33	129 ± 24
III	-358 ± 29	174 ± 34
IV	-355 ± 51	151 ± 17
V	-407 ± 21	112 ± 21
VI	-225 ± 28	177 ± 15

Table 6. Oxidation Reduction Potential

React Phase	Stage
	I	II	III	IV	V	VI
Anaerobic	2.8	5.8	11.8	11.8	17.8	5.8
(hours) Aerobic (hours)	2.8	5.8	11.8	11.8	5.8	17.8
MLSS (g/L)	35.3 ± 1.6	28.7 ± 0.6	25.2 ± 1.8	30.5 ± 3.4	31.6 ± 3.7	23.3 ±0.8
MLVSS (g/L)	31.9 ± 1.8	24.5 ± 2.2	18.5 ± 2.2	26.0 ± 3.4	22.4 ± 2.0	20.2 ± 0.8
VSS/SS	0.90	0.85	0.73	0.85	0.71	0.87
Effluent (VSS	0.34 ± 0.16	0.31 ± 0.11	0.26 ± 0.19	0.34 ± 0.11	0.33 ± 0.10	0.55 ± 0.22
g/L) SRT (day)	27.6 ± 13.4	42.4 ± 10.2	78.9 ± 23.9	70.1 ± 23.9	72.5 ± 23.3	41.6 ± 18.4

Table 7. Biomass concentrations at different stages of the experiment

When the OLR was increased to 0.8 kg COD/m3-day, there was an improvement in the biomass concentration where the biomass concentration have increased to 30.5 ± 3.4 g/L and 31.6 ± 3.7 g/L in Stage IV and Stage V as compared to 25.2 ± 1.8 g/L of biomass concentration in Stage III which run at the same HRT (24 hours) but with OLR 0.6 kg COD/m3-day. The increase in OLR has caused an increment in the biomass concentration in the reactor. A

slight increase in the biomass concentration was also observed along with the longer period of the anaerobic phase (Stage V), i. e. 18 hours.

The ratio of the volatile biomass (MLVSS) to total biomass (MLSS) reduced from Stage I to Stage III mainly due to decrease in the OLR as the HRT increased from 6 to 24 hours, whereas the MLVSS/MLSS ratio of the Stage III and Stage IV with 12 hours aerobic reaction phase was observed higher with the ratio of 0.73 and 0.85, respectively. The increment may be due to the increase of the OLR from 0.6 to 0.8 kg COD/m3 day (Stage III to Stage IV). Increase in the OLR means more carbon sources were supplied to the microorganisms in the reactor. When more food is available, more growth will take place and this is indicated by the increase in the MLVSS/MLSS ratio.

However, when the anaerobic period of the HRT is extended, the MLVSS/MLSS ratio decreased (0.71). Decrease in MLVSS/MLSS ratio may indicate an increase of inorganic accumulation within the granulation biomass. When the duration of aeration phase was increased up to 18 hours, the biomass started to reduce again (Stage VI) and increase of VSS in the effluent was once again observed. This may give an indication that too long of aerobic reaction phase is not suitable for granular biomass system. Prolong of aeration time may result in instability of the reactor performance. The profile of biomass concentration of the reactor system is given in Figure 14.

Figure 14. Profile of biomass concentration at different stages of the experiment. (•) MLSS, (□) MLVSS. Stage I: anaerobic (2.8 h): aerobic (2.8 h); Stage II: anaerobic (5.8 h): aerobic (5.8 h); Stage III and Stage IV: anaerobic (11.8 h): aerobic (11.8 h); Stage V: anaerobic (17.8 h): aerobic (5.8 h); Stage V: anaerobic (5.8 h): aerobic (17.8 h)

The SRT of the reactor system increased from 27.6 ± 13.4 to 78.9 ± 30.8 d when the length of the HRT increased from 6 to 24 hours (Stage I to Stage III). With HRT of 24 hours, increase of anaerobic reaction phase up to 18 hours (Stage IV to Stage V) has slightly increased the SRT from 70.1 ± 23.9 to 72.5 ± 23.3 d. The SRT value changes in each stage of the experiment. According to Wijffels and Tramper (1995), the favorable sludge age for high removal efficiency for COD and nitrification process is more than 4 days. Based on the SRT obtained, this biogranular system is capable of the simultaneous degradation of nitrification process and COD removal. Since the treatment goal is to remove recalcitrant dyeing compound, the SRT value of all stages evaluated in this experiment was in the acceptable range from degradation of xenobiotic compounds (Grady et al. 1999).

React Phase	Stage
I	II	III	IV	V	VI
Anaerobic	2.8	5.8	11.8	11.8	17.8	5.8
(hours)
Aerobic (hours)	2.8	5.8	11.8	11.8	5.8	17.8
SVI (mL/g)	13.1 ± 0.4	18.8 ± 1.5	21.4 ± 1.6	16.8 ± 1.3	15.5 ± 1.3	24.8 ± 0.9
SV (m/h)	41.3 ± 3.1	35.1 ± 0.8	24.5 ± 1.1	28.4 ± 1.3	33.4 ± 2.5	21.3 ± 0.5

Table 8. Physical properties of the biogranules at different stages of the react phase

The SVI value of the biogranules was used to evaluate the biogranules settling ability. It is anticipated that bigger biogranules will have higher settling velocity and hence, reduce the SVI value, indicating good settling ability. The SVI value improved when the anaerobic react phase was prolonged in Stage V indicating such reaction pattern will help to develop granules with better settling profile. According to Panswad et al. (2001a), inert biomass increased as the anoxic/anaerobic condition was prolonged. It is possible that the accumulation of inert particles within the biogranules increased and resulted in improved SVI properties. Table 8 showed the physical properties of the biogranules at different stages of the react phase.

Figure 15 shows the profile of SVI of the reactor system. The SVI value in Stage V was reduced from 16.8 ± 1.3 mL/g (in Stage IV) to 15.5 ± 1.3 mL/g. This is expected to be due to the accumulation of more inert solids within the biogranules as shown with low levels of MLVSS/MLSS ratio in Stage V (0.71). Despite changes in HRT that caused decrease in the size of biogranules, the SVI values of the whole experiments were good except for Stage VI. During Stage VI, the prolonged of the aerobic phase (i. e. 17.8 hours), which was operated at high superficial air velocity (2.5 cm/s), cause the biogranules to rupture. At this stage, the size of biogranules becomes smaller causing the settleability of the particles to reduce and was demonstrated with increase in SVI value.

Hydraulic retention time is an important parameter that control the contact time between the biomass and the wastewater in a reactor system. The HRT of a system must be long enough for the degradation process to take place. However, in the application of biogranules in the treatment system, the HRT should not be too long as it may cause the disintegration of the granules. According to Tay et al. (2002) and Wang et al. (2005), a short

HRT is favorable for rapid granulation process, while too long HRTs may lead to granulation system failure due to high biomass lost (Pan et al., 2004). An optimum HRT of biogranulation systems will be able to stabilize the reactor performance with good biomass retention and high removal performance. According to Pan et al. (2004), the optimum HRT for aerobic granulation systems ranging from 2 to 12 hours where stable aerobic granules with good settleability and microbial activities. However, the optimum HRT for treating different types of wastewater may vary depending on the type of wastewater and the targeted degradation compound.