Chapter 4: Results and Discussion
4.1 Characterization of Incoming Wastewater
4.1.2 Wastewater Quality
concentration at night. An opposite phenomenon happens during the day, as there is more volume of incoming wastewater resulting in increasing velocity. This gives less time for conversion and H2S in liquid to be transformed to the gas phase, and hence less H2S gas recorded. Both the cycles have 8:00 PM on one end and 8:00 AM on the other, depicting the beginning and end of the cycle. The highest point of the first and second cycles are at 4:00 AM where the H2S concentrations are 232 ppm for both cycles. The lowest point is different for both sampling events.
Figure 7: Scatterplot of average H2S concentration and average velocity. (L and N are day and night for the first sampling event. D and M are day and night for the second sampling event)
This is in agreement with our findings, where pH decreases while H2S concentration in the headspace increases (Figure 8). As discussed in the literature, pH determines the relative proportion between H2S and HS-. H2S concentration is favored at lower pH.
When the pH increases, the chances of H2S formation decreases and HS- increases.
The pH is low during the evening (12:00 PM – 10:00 PM) and quickly escalates the highest value till 4:00 AM. The pH values suddenly fall to reach the lowest 10:00 AM rises rapidly again. This trend was evident for both the first and the second sampling events. The average pH value of the wastewater was 7.07, the lowest and the highest values were 6.87 and 7.27 respectively (Table 2).
Figure 8: Comparison between H2S concentration and pH with respect to time
COD has been reported as one of the influential factors in the generation of H2S in wastewater [91]. In the second sampling event, flowrate and COD fluctuates with a difference of 2 hours as shown in Figure 9. In the first sampling event, COD increases with increasing flowrate but has a contrasting time difference compared to the second sampling event. This is contrary to the results reported by Wang et al.,
where COD decreases with increasing flowrate [92]. The authors had observed rainfall as a reason for the increase in flowrate. Hence, COD was reduced because of the dilution of wastewater. In our case, when the flowrate increases, the amount of organic matter available also increases. Thus, increasing COD with increasing flowrate. There are some outliers for COD values. The average COD value of the wastewater for both days was 279.2 ppm, the lowest and the highest values were 79.6 ppm and 546 ppm, respectively (Table 2). It was observed that flowrate and wastewater depth followed a similar pattern with average wastewater depth of 0.87 m and maximum and minimum levels were 1.13 m and 0.7 m, respectively.
Figure 9: Comparison of flowrate and COD with respect to time
TSS and TDS are two types of solids. Sharma et al. stated that solid sedimentation has a significant impact on H2S generation [93]. The average TDS level was 491.7 ppm and maximum and minimum levels were 738 ppm and 323 ppm, respectively. The average TSS level was 160.4 ppm and the maximum and minimum levels were 584 ppm and 73 ppm, respectively (Table 2).
100 150 200 250 300 350 400 450 500 550 600
2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
COD (mg/L)
Flowrate (m³/hr)
Time (Hours)
Flow rate COD
The level of dissolved oxygen in wastewater determines the amount of carbonaceous matter that can be broken down. An increase in DO level will result in lower sulfide generation by restricting the supply of food to the anaerobic bacteria.
However, a low DO level favors the generation of sulfide by enhancing the growth of anaerobic micro-organisms [94]. The typical DO level of wastewater is around 1 ppm [95]. In our case, the average DO level was 0.47 ppm and the maximum and minimum DO levels were 1.2 ppm and 0.05 ppm, respectively (Table 2).
A high temperature of wastewater is reported to increase biological activity and oxygen consumption. It also increases the sulfide generation in gravity sewers. The increase in temperature by one degree corresponds to a 7% increase in the activity of SRB until it reaches 30°C [94]. It was observed that the temperature of wastewater is higher during the night, and lower during the day. This is because water has higher specific heat and it takes time to heat, hence temperature is lower during the day. The higher specific heat also prevents rapid temperature changes, thus takes more time to cool at night [96]. The higher concentration of H2S during the night (between 12:00 AM and 6:00 AM) is in accordance with our discussion in the literature review that H2S concentration increases with increasing sewer temperature, due to enhanced microbial activity with increasing temperature [60, 62]. This pattern is similar for both sampling events. The average wastewater temperature was around 32.5°C and the maximum and minimum were 32.75°C and 31.8°C, respectively (Table 2). The differences in wastewater temperature were not huge. Regardless of the temperature of headspace air, the wastewater temperature could not reflect the impact of the ambient temperature.
Electrical conductivity (EC) has a significantly small effect on H2S in the aqueous phase [36]. According to US EPA [98], the effect of EC on H2S generation can be neglected. Average EC values were around 1,271.9 µS/cm and maximum and minimum were 1,929 µS/cm and 839.1 µS/cm, respectively (Table 2). Hvitved et al.
stated that an increase in sulfate concentrations increases the H2S generation since sulfate serves as the substrate in H2S production [91]. The results from our analysis are discussed in later sections. The average sulfate concentration was 202.2 ppm, and the maximum and minimum concentrations were 260 ppm and 153 ppm, respectively (Table 2).
H2S gas is a byproduct of the dissociation of organic sulfide compounds [99]. Hence, the amount of sulfide available is important in determining the H2S emission. The results from our analysis of sulfide are discussed in later sections. The average sulfide concentration was 0.14 ppm and the maximum and minimum concentrations were 0.37 ppm and 0.048 ppm, respectively (Table 2). Total organic carbon (TOC) could be used as a type of substrate that SRB uses for its growth. Bacterial growth will be aided by high quantities of organic materials. This results in the depletion of DO, in turn enhancing sulfide generation [94]. The average TOC was 306.3 ppm and the maximum and minimum concentrations were 430.5 ppm and 215 ppm, respectively (Table 2).
Chloride, along with other chemicals including ozone, hydrogen peroxide, permanganate, and oxygen oxidizes sulfide directly [100]. Chloride also plays a role in facilitating corrosion of steel by damaging its protective layer [101]. The average chloride level was 1098.2 ppm and the maximum and minimum concentrations were 2281.2 ppm and 167.5 ppm, respectively (Table 2).
Table 2: The maximum, minimum, and average values of wastewater characteristics
Parameter Maximum value Minimum value Average
pH 7.27 6.87 7.07
COD (ppm) 546 79.6 279.21
TOC (ppm) 430.5 215 306.3
DO (ppm) 1.2 0.05 0.47
Temperature (°C) 32.75 31.83 32.51
EC (µS/cm) 1,929 839.1 1,271.93
Sulfate (ppm) 260 153 202
TDS (ppm) 738 323 491.7
TSS (ppm) 584 73 160.4
Sulfide (ppm) 0.373 0.048 0.14
Chloride (ppm) 2281.2 167.5 1098.2