Fibre-optic on-line flocculant dose control in water treatment operations

Abstract

Control systems for economically and ecologically optimized flocculant dosing require rapid knowledge of the state of flocculation from in-situ measurements. In this respect techniques which detect flocculant-induced changes in the early stage of floc growth are more promising than conventional measurements based on secondary parameters like sedimentation rate or supernatant turbidity. A microcomputer-controlled fibre-optic flocculation sensor (FOFS) installed in a small representative bypass flow is particularly suitable. The FOFS technique is based on the root mean square data analysis of fluctuating light blockage signals from a laser-diode/detector assembly connected with a flow-through cell operating in a sheath flow mode (self-cleaning). A linear relation between the FOFS signals, F, and the corresponding floc size has been established in bentonite suspensions for different states of flocculation induced by changing the dose of FeCl 3. In a compact flocculation plant different floc sizes, adjusted by the dose of a polymeric flocculant, could be detected with high resolution and rapid response in a small bypass flow sampled close to the dosing point. Moreover, a close correlation between F and the corresponding effluent turbidity, τ, of the finished water was found. The lag-time between both signals of ∼ 15 min was due to the average residence time in the parallel plate clarifier. Contrary to τ, the FOFS signal should be well suited as the controlled variable of a direct closed-loop flocculant dose control system. An adaptive feedforward control system, by rapidly optimizing a small bypass flow, is suggested for flocculation plants with rapidly changing influent qualities. Examples from the first installation in a typical wastewater treatment plant in the ceramics industry are discussed. The results, presented as original records, document a performance level of the treatment process in terms of minimum polymeric flocculant demand with optimum quality of the finished water which cannot be established by conventional manual dosing.