Impact of suspended solids and organic matter on chlorine disinfection efficiency of greywater
RESULTS AND DISCUSSION Batch chlorination experiment
As expected, presence of TSS and organic matter reduced chlorine efficiency of removing FC and was more pronounced for the lower initial chlorine concentrations used such as 0.5 and 1 mg/l. For example, disinfection efficiency at TSS and BOD5concentrations of 15 and 50 mg/l (respectively) was 2 and 1.5 order of magnitude higher at chlorine dose of 6 mg/l than at doses of 0.5 and 1 mg/l, respectively. The reduction in the efficiency of chlorine disinfection when TSS increased, was much more pronounced than the efficiency reduction when BOD5concentrations were higher (Fig.
3). Moreover, the negative effect of TSS started already at low concentrations and increased consistently with increasing TSS concentrations across all the concentrations range tested. On the other hand, the negative effect of BOD5 on FC inactivation was observed only when its concentration was higher than 50 mg/l.
The total residual chlorine, measured 0.5 h after its application ranged between 0.21 and 1.86 mg/l (Table 2- phase 1); mostly within the acceptable levels for reuse (minimum of 0.5 or 1 mg/l, dependent on intended use; Halperin and Aloni, 2003, WHO 2006). In addition, 3-log inactivation of FC was obtained on average, while FC was not detected at 60% of the samples.
Table 2. Performance of the chlorination disinfection systems- batch and flow-through Phase 1 Phase 2 FC Log inactivation Average 3.0 (±1.45) 3.0 (±0.6)
Range
The relationship between FC inactivation and TSS, BOD, log FC concentration of the treated GW (before disinfection) and the measured total residual chlorine were described by a MLR model (Eq.
1).
Figure 2. Flow-through chamber
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(1) FCinactivation=E1·[BOD5]+E2·[TSS]+E3·[log FC raw]+E4·[residual chlorine]
Where:
FCinactivation in log(cfu/100ml); BOD5 TSS and residual chlorine in mg/l; log FC raw in log(cfu/100ml); E1,E2,E3 and E4are coefficients estimating the explanatory variables (Table 3).
All coefficients were established based on the batch disinfection experiments, and the model was found to be statistically significant (p<0.0001 and R2= 0.76; Figure 4a). Moreover, all explanatory variables were statistically significant indicating that that these chosen variables have significant influence on the efficiency of chlorine disinfection (at the examined ranges).
Other combinations of these variables (including interactions between them) were tested, however they did not increase the fit of the model. Therefore, the simplest model is presented. To compare the effects of the various explanatory variables on chlorination efficiency the effect size test, which is used for assessing the variables effect on a suggested model, was applied. In this test the p-value was transformed to the LogWorth (-log10(p-value)), assuming that larger effects lead to more significant p-values and larger LogWorth (Table 3).
Figure 3. Log Inactivation of fecal coliforms (FC) as a function of total suspended solids (TSS) (a); and biochemical oxygen demand (BOD5) concentrations in the GW (b)
The model indicates that initial microbial concentration was the most significant parameter (having positive effect) followed by TSS and then BOD5concentrations, both resulted in a reduction of FC inactivation. These results are consistent with established theory and demonstrate the negative effect of TSS and BOD5 on chlorine disinfection. Most likely some of the BOD5and TSS increased the chlorine demand as they were oxidized, thus reducing the active chlorine concentration and consequently the FC inactivation. In addition, as aforementioned, TSS and BOD5 may affect chlorination efficiency by increasing the bacteria resistance due to stabilization of the microbial cell membranes (Virto et al., 2005) or due to bacteria attachment to suspended solids (Bohrerova and Linden, 2006; Winward et al., 2008).
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Table 3.THE MLR model coefficients
Explanatory Variable Coefficient Estimate p-value LogWorth
Dissolved BOD (mg/l) ȕ1 -0.016 <0.0001 5.43
TSS(mg/l) ȕ2 -0.013 <0.0001 10.8
Log FC raw (log(CFU/100ml)) ȕ3 0.831 <0.0001 22.8 Residual chlorine (mg/l) ȕ4 0.644 <0.0001 2.83
Figure 4.Predictedvsmeasured FC inactivation (a) batch setup (b) flow-through chamber Flow-through chlorine tablet chamber
FC inactivation of 3-logs on average was obtained for all samples (including the ones with elevated TSS and BOD concentrations) and in over 90% of the samples counts were less than 10 cfu/100ml.
Similarly to the batch experiment, the effect of TSS was more pronounced in reducing chlorine disinfection efficiency as compared with the one of BOD5(Fig. 5 maps). As explained above, these results are consistent with previous findings that suggest that coliforms in GW were associated to particles and were sheltered by them, thus were resistant to disinfection whereas organic matter affected chlorine demand but not resistance.
The total residual chlorine in the flow-through system, measured 0.5 h following chlorine application ranged between 0.5 to 5.8 mg/l (Table 2- phase 2). Similar to the batch experiment, residual chlorine range was within the acceptable levels for unlimited wastewater reuse (Halperin and Aloni, 2003; WHO, 2006).
The MLR model, developed based on the batch-phase experiments, was verified on the flow-through disinfection chamber results and was found to be statistically significant with (p<0.0001;
R2= 0.6; Fig. 4 b). Although the quality of treated GW samples from the two phases was quite different, and so were the means of chlorination, the model fitted well and explained most of the variability in the measured FC inactivation. This correlation suggest that it would be possible to predict the required residual concentration needed in the flow through reactor given the initial FC concentration, BOD5, TSS and the required final FC concentration of the water to be disinfected, as described in Eq. 2 below. This approach is valuable not only from an operational standpoint, but also from a research perspective.
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(2) [ݎ݁ݏ݅݀ݑ݈ܽ ݄݈ܿݎ݅݊݁] =ிೌೡೌା.ଵή[ைఱ]ା.ଵଷή[்ௌௌ]ି଼ଷଵή[୪୭ ி ௪]
.ସସ
Controlling the residual chlorine concentration in a flow-through disinfection reactor for a certain GW quality would require manipulation of the amount of chlorine (e.g. number of chlorine tablets) and the contact time such as by manipulation of the flow rate.
Figure 5.FC removal in the flow-trough disinfection reactors as a function of TSS and BOD5 in GW, for three different scenarios: a. increase of TSS (BOD5 relatively constant); b. Increase of BOD5(TSS relatively constant); c. combined increase of TSS and BOD.
Total chlorine residual ranged from 0.5 to 5.8 mg/l (average 1.3 mg/l). Percentage of FC removal ranging from 98 to 100 % with line interval of 0.1.
CONCLUSIONS
This study indicates that the efficiency of chlorine disinfection of treated GW decreases as a result of increasing TSS and BOD5 concentrations. Both the batch experiment and the flow-through setup have shown that dissolved organic matter affect the efficiency of chlorination significantly less than TSS, as expresses by much lower LogWorth value. Furthermore, the effect of TSS was continuous starting from low concentrations, while the effect of BOD5 became significant only above a certain threshold concentration. A multiple linear regression model was developed in a batch setup and verified successfully against results of treated GW that were disinfected by a flow-through chlorine disinfection chamber. It is therefore postulated that the total chlorine residual required for on site flow-through chambers can be evaluated based on the model in order to achieve the needed disinfection efficiency.
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