| Popis: |
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. |
