Online Flocculation Analyzer for Coagulant Control

Treatment plants are faced with the challenge of stricter regulations and higher standards for finished water quality. New developments in control strategies and instrument design have overcome some of the previous limitations of monitoring and controlling the coagulation process. An online floc analyzer provides valuable information on the dynamics of particle aggregation subsequent to coagulation. The analyzer can be used online as part of a control strategy to optimize coagulant dosing. The instrument uses fiber optics to measure floc particles in a flowing suspension. Plant and lab data have shown that optimizing coagulation will form floc that is large and easily settled. Fractal analysis has suggested that diffusion and collision of colloidal particles limit particle aggregation. So, proper mixing is a major factor influencing floc formation. Streaming current instruments and jar testing have been the traditional tools for adjusting and controlling coagulants. The new floc analyzer can be used in conjunction with the traditional approaches to provide a more complete system for dose control. While streaming current provides information on the charge or zeta potential of the colloidal particles, it doesn't measure how well the process is performing. Historically the SCD setpoint must be determined empirically by the operator. The floc analyzer can assist in providing feedback to determine the proper setpoint for the SCD. This technique can be accomplished manually or automatically in a multi-sensor "expert system" approach. Other parameters that are important to measure are flow, pH, UV254, turbidity and particle counts. The floc analyzer has been used successfully in the lab to characterize the dynamics of floc growth. Experiments with the instrument have indicated that flocs form faster with PACl than with alum. The experiments have also shown that the flocculation rate increased with Al concentration. The experiments were verified with a particle counter. In online applications, a process stream can be sampled at a convenient point and passed directly through the flow cell of the floc analyzer, often by gravity flow. This provides a simple means of monitoring the state of aggregation of particles. Once the optimum reading is established, departures from this condition are immediately apparent by changes in the instrument's reading or output. The output is used to activate an alarm, or, for control purposes to adjust the streaming current setpoint or change the dosing rate of the chemical feed pump. Instrument Background The instrument is based on the Photometric Dispersion Analyzer or “fluctuating turbidity” technique developed by Gregory (1984) 3 . As floc particles pass through the light beam, the fiber optic detects fluctuations in transmitted light. These fluctuations follow a Poisson statistical distribution, so that the standard deviation about the mean is the square root of the mean value. For example, if a given suspension which contains on average 1000 particles/ml the actual number of particle will vary randomly. Ninety-five percent (95%) of samples examined should have a particle number within two standard deviations of the mean, i.e. 1000 ± 64. The smaller the average number of particles per unit volume, the more noticeable the variations become. The floc analyzer illuminates the flowing suspension by a narrow beam of light extended from a fiber optic so that a small sample volume is examined of the order of 1mm. The number of particles in the light beam is continuously changing since the suspension is flowing. These fluctuations in the intensity of transmitted light are carried through a fiber optic to a sensitive photodiode. The light intensity is converted to a voltage proportional to the variations. (Fig. 1) The output voltage has a large dc component corresponding to the average transmitted light intensity (related to the turbidity of the suspension) and a much smaller fluctuating (ac) component due to the random variations in particle number and size.