Here is another great post in the series, “Using Data to Optimize Processes” by Ruby Daaman and John Cook of ADMI. This is the third installment focusing on minimizing THM formation within a water treatment plant. As always if you want to contact John personally please email him: firstname.lastname@example.org
It has been demonstrated that data can be used to build process models for both natural systems such as rivers, estuaries and groundwater movement over expansive areas, and can also be used to model man-made systems such as water treatment plants (WTP). The last article featured optimizing TOC removal in sedimentation basins. This article will address optimizing total THM formation (the same approach can be used for HAA5), within the WTP, and for the purposes of brevity, we will begin optimization modeling at the clearwell, just before pumping onto the distribution system.
TTHM Clearwell Model 1 (THMCW1)
The approach is to model the clearwell TTHMs directly. The resulting model (Figure 1) predicts TTHM in the clearwell as a function of:
TTHMACW = f (CL2DOSE, CL2CW3-FIN_R, TURBRED_FLT-SET, ALKSET, TMPDEC, RAIN)
Figure 2 shows the response surface with inputs CL2DOSE and CL2CW3-CL2FIN_R displayed. The first surface has water temperature (TMP) at its minimum value and all other inputs at their mean and the second has TMP at its maximum value and all other inputs at their mean. The shape of the surface is similar, but the TTHMs in the clearwell are shifted higher.
Sensitivity to pH and Alkalinity
As noted in Figures 3 and 4, TTHM formation in the clearwells is sensitive to a combination of coagulation pH and settled water alkalinity. Chlorine dose is the largest contributor to TTHM formation, but optimizing the coagulation process can reduce the TTHM formation. Figure 5 demonstrates the sensitivity to TTHM formation as a function of Cl2 dose and coagulation pH. It is important to note that an increase of chlorine dosing between 1.8 and 4.7 mg/L results in an approximate increase of 80 µg/L in TTHM formation with maximum water temperature (TMP) and average settled water alkalinity (ALK).