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There is a big difference between a trend depicting "in control" of effluent residual and a trend depicting "no control". Good control can simply be determined by the fact that the process variable will track the setpoint (in this case the residual measured). A lack of understanding of fundamental considerations results in higher operational and chemical cost.
A trend depicting a small deviation from setpoint demonstrates excellent process tuning. But in wastewater effluent facilities not having good control of residual results in chemical waste, and therefore, gaining effluent residual control is more important than having excellent process tuning. The immediate result of gaining control of effluent residual is chemical savings. When there is no effective control of effluent residual, operational activities include time spent daily verifying the effluent residual will not go out of compliance, and taking multiple samples daily to determine the actual value of the effluent residual. Also, the operator is more likely to set the system to manual control, and set the chemical feed rate for enough chemical feed to guarantee the effluent residual will not go out of compliance. The result of these activities will show up in the effluent residual trend depicting a process variable not tracking the setpoint. Worse, a residual trend indicating the residual measured at full scale does not represent the actual value of the residual measured; it's higher. In contrast, when the effluent residual is under control, the time spent verifying if the residual is in compliance is now a matter of checking the influent and effluent residual trends for consistency; and a dosage adjustment is now based on the knowledge of actual process disturbances. In wastewater facilities with no effective control of effluent residual, addressing the important factors and implementing fundamental considerations result in gaining control of effluent residual and reduction of operational time and chemical consumption. Upgrading the programmable logic controller and software is most likely not required. Implementation and initial tuning can be completed in a few days. Additional tuning time may be required, but the overall implementation cost will most likely prove to be affordable with immediate cost savings in chemicals. Before residual control can be gained a few important factors need to be addressed. Click here for a checklist of important factors on gaining automatic control of effluent residual.
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