Introduction

An Accufloc streaming current monitor (SCM), now known as the Hach AF7000, was placed at a local surface water treatment plant for field testing. The facility was considering the evaluation of streaming current technology for the monitoring and confirmation of coagulant feed.

This drinking water facility practices conventional treatment for surface water with a production capacity of between 15 (winter) and 35 (summer) MGD. Treatment includes chemical injection to adjust pH and the addition of ferric chloride as the primary coagulant. After sedimentation, the water is filtered through dual media anthracite filtration and then followed by post-disinfection. The source water originates in the central Rocky Mountains and is primarily from snowmelt.

This surface water flows approximately 60 miles before being captured in an impoundment of approximately 1,000 surface acres that is located in an urban area, approximately three miles from the plant. The reservoir is considered to be a very stable raw water source, but is susceptible to flashing from the rare strong thunderstorm.

An SCM was installed in the plant to continuously sample the flash mixer effluent. The SCM signals were logged directly into the plant SCADA system, but were not wired to any alarms. The trends were displayed and updated every minute. At a glance, the operators could determine if the SCM readings were stable and in their expected range. The displayed value was expected to remain close to zero after setting it at the assumed optimal coagulation conditions, due to the typical stability of the raw water. If the readings have a strong negative deviation, it is a potential signal that the coagulant feed is lower than the previous optimum. If the deviation is positive, the coagulant feed could be higher than optimum.

Similarly, changes in the raw water quality or pH control may be interpreted by the direction and magnitude of the SCM reading deviations.

Initial Operation

When integrating any new online monitor into plant operations, operators must first understand what the measurement is and how it can be used in their day-to-day operations. Operators will then learn the critical functions, operational protocols, and maintenance requirements of the instrument to ensure the generated data is reliable. As the data is proven reliable, operators can focus on how different changes in the raw water and chemical dosages in the treatment processes can impact the response of these instruments. Once these impacts are studied, operators can also learn how to use the SCM information to:

• Better monitor their treatment processes

• Optimize these processes

• Troubleshoot unexpected events

Case Study - Detection of Chemical Feed Failure

Overall, the integration of the streaming current monitoring went well. However it was not until the staff had an unexpected chemical feed failure, that the plant fully embraced the use of SCM technology in plant operation.

One evening at approximately 7 pm, there was an abrupt increase in flash mix pH, as shown in Figure 1. There was no change in the raw water conditions (turbidity or pH) at this time. An over-feed of lime was initially suspected due to the increased sample pH (Figure 1). Only after the operator examined the SCM data trend, a coagulant feed problem became a suspect.

Conclusions

• The coagulant feed failure event at the WTP was easily and rapidly detected by the SCM technology.

• The ability to easily set zero point reflecting optimal coagulant feed provides a simple means of problem identification.

• Since the evaluation study, this water plant has acquired a second Accufloc SCM and gained a significant amount of trust and confidence in the SCM technology and the instrument.