System Overview
The San Francisco Water Department operates the 144 mgd Harry W. Tracy Filter Plant, in San Bruno, Calif. It serves residents of the City of San Francisco and San Francisco County as well as communities in the northern Peninsula. Raw water comes from the San Andreas Reservoir, located just west of the plant. Raw influent is primarily made up of water originating in the Hetch Hetchy Reservoir in Yosemite National Park pumped over from Crystal Springs Reservoir as well as local runoff and water transferred from Pilarcitos Reservoir and small quantities of local watershed. The facility uses turbidity measurement to adjust ferric chloride and polymer feed rates. There is little raw water turbidity variation during the year, and the influent turbidity typically is very low (1.0 to 2.0 NTU).
Water is pumped from the plant’s raw water pump station and directed to the headworks where it can be distributed to four individual ozone contactors for disinfection (the configuration of contactors in service is adjusted and governed by flow rate). The water then enters one or two influent channels where raw water chemicals (i.e., a polymer coagulant aid and ferric chloride as coagulant) are dispersed at average dosage rates of 3.6 mg/L and 1.0 mg/L, respectively.
From there, the water is dispersed into channels and proceeds to a flocculation area consisting of a series of baffles before it is directed to the plant’s 10 dual media filters. Each filter has approximately 1,850 square feet of surface area and is rated to handle six gallons per square foot, or approximately 16 mgd.
The Need to Look Closer
Water quality at the Harry W. Tracy Filter Plant is high enough that measurements are close to the minimum sensitivity of most turbidimeters. The plant’s filtered water turbidity typically averages .03 to .04 NTU. Filter backwash operations often are based on headloss (8-foot headloss) and maximum runtimes of 72 hours rather than turbidity spikes because of the high quality water.
However, treated water turbidity of less than 0.1 NTU does not necessarily indicate that the plant’s filter operations are being optimized. When turbidities are below 0.1 NTU, a turbidimeter can no longer be used to evaluate control strategies or detect small changes in effluent characteristics that could signal filter breakthrough. As a result, plant management decided to evaluate on-line particle counting as an additional tool to complement its turbidimeters.
Through a light-blocking technique, laser-driven water particle counters determine both the concentration and the size distribution of particles present. In contrast to turbidimeters, particle counters can indicate coagulant and filter performance by detecting individual particles in the 0 to 100 micron size range.
In September 1999, the Tracy Filter Plant installed a water particle counter (USFilter Stranco Products, Bradley, Ill.) to monitor combined filter effluent. By detecting small rises in particle levels, the unit provides plant operators an advanced warning system for filter breakthrough. This water particle counter has been on loan for trial testing after the department evaluated a number of other units, including one that had been installed previously but taken off line due to software problems.
In the past, one of the major drawbacks to particle counters was that they provided operators with only relative particle counts, not real numbers. Using light extinction technology combined with advanced electronic and opto-electronic components, the USFilter unit provides an absolute particle count traceable to a recognized standard (JIS B 9925-1997) and with true zero count at normal flows. The manufacturer loaned the plant two of these units to test for trend purposes. The two units ran side-by-side for three months and their readings remained within 5 percent of each other.
Control Strategies Using Particle Counting
With the water particle counter installed to monitor combined filter effluent, the goal is to identify when particles are greater than 2 microns and particle quantities reach 12 particles/mL. Once this point is reached, operators may adjust chemical dosage to optimize treatment, depending on the characteristics of the water.
When the plant encounters a raw water turbidity change, operators often try different chemical dosages to determine the optimum, and here the particle counter assists in decision making. For example, if operators increase the rate of ferric chloride, notice an increase in particle size and then increase the polymer accordingly and do not notice a change in particle count, the operator may leave the ferric chloride at the higher dosage rate but bring the polymer dosage down to the original level. Other times, monitoring particles helps save polymer by optimizing dosage rates since particle counts typically will increase when polymer is either overdosed or underfed.
Accurate particle measurement also has fine-tuned filtration by detecting fluctuations in performance that were missed by turbidimeters. For example, when a filter bed fills with solids causing shear forces that break down flocculated particles, it can experience a turbidity breakthrough. The particle counter, by detecting small rises in the particle level, provides an advance warning system, allowing operators to initiate backwash procedures prior to breakthrough.
The particle counter’s ability to determine particle size also helps the plant monitor for the presence of Giardia and Cryptosporidium cysts. Most liquid-borne particles are in the 3 micron size range, but Giardia cysts are 4—10 µm and Cryptosporidium cysts are 2—7 µm. Particle counting for particles greater than 2 microns provides better assurance of Cryptosporidium and Giardia removal in the range of 2 to 10 microns.
Continuous Fine-Tuning
The water particle counter’s Windows-based software provides a graphic interface and a data plot function that depicts real-time data over a selected time span and is updated every minute. Modifying counting and sizing parameters is a straightforward procedure. The keypad interface of the unit enables the operator to select particle counts, and a local display shows real-time particle data. The datalogging and graphing features of the particle counter support continuous fine-tuning of filter operations by allowing operators to track performance over designated intervals, take "snapshots" of real-time conditions and make system adjustments either at the unit’s local display or host computer.
A unique and important capability of this counter is that it also can be operated without the software and advanced telemetry features. With the previously-installed unit at the plant, communications would sometimes become lost between the particle counter and the host computer. During those periods, operators could not see what the unit was doing. However, with the new water particle counter, if the plant’s host computer is down or signal is lost, operators have the advantage of viewing complete current particle data on the unit’s local display.
Expanding Capabilities
Based on the successful performance of its online water particle counter, management is planning to fine-tune filter performance even further with the installation of 14 additional particle counters to monitor raw water influent as well as individual filter and combined filtered effluent. Operators will be able to monitor real-time data from selected sensors over selectable time spans and automatically calculate and display log removal ratios for selected particle size ranges between two sensors.
Water particle counting is rapidly becoming a necessary technology for optimizing the treatment process. At San Francisco’s Harry W. Tracy Filter Plant, effluent quality now is more consistent because operators can detect slight changes in particle counts. With the addition of monitoring points at individual filters and raw water influent, consistency and efficiency will be even further improved.
Download: Here
