Studies on the Application of Power Spectrum Analysis to Primary Clarifier Dynamics
Authors: Sakata N. , Cordoba-Molina J. F. , Silveston P. L. 
Published: March 2019

Appreciable damping of the short period (5 to 10 minute) or “high” frequency fluctuations in suspended solids and suspended organic matter in sewage occurs in the primary clarifier of a waste treatment plant. The unit acts as “frequency” filter for the remainder of the plant. This project was undertaken to investigate the mechanism for damping and to develop, if possible, a quantitative model for clarification dynamics. The currently promulgated plug flow model for a clarifier, of course, predicts no damping. Dynamic models of clarifiers would also be useful to investigate the response of these units and entire treatment plants to sharp surges of flows such as occur in storms.

Investigations were carried out using a 4 ft. diameter scale model of a center feed circular clarifier. Overflow rates used were in the range employed in full scale units. In place of a waste, a silica power was used. Settling of the powder was non flocculating and approached ideal settling. Residence time distributions were obtained under conditions where the feed was more dense and less dense than the mean contents of the clarifier. Response was initiated by step changes in flow and in suspended solids loading. Changes in both directions (high to low and vice versa) were used but flow and loading were not changed together. The clarifier response was measured by determining the suspended solids concentration in the overflow as a function of time. The measurement was made gravimetrically. Plow measurements were not made since with an overflow weir, basin capacitance for flow is negligible.

Time constants observed for flow changes were of the order of the nominal holding time in the clarifier, whereas for load changes the time constants were larger than the holding time. Again a simple plug flow model predicts time constants less than the holding time. The source of the “longer” time constants appears to be mixing caused by sensity currents induced by flow or the change of loading itself. Tracer studies showed appreciable differences in the residence time distribution for cases where a dense feed entered a less dense vessel and vice-versa.

Experimental data are well represented assuming a cascade model of well stirred stages in which settling is described by a first order time process. The first order process was established by settling measurements made in a quiesent column. The model is arbitrary although some mixing is evidently occurring. The data are equally well represented by a simple resistance - capacitance system.

An attempt was made to measure the transfer function of a full scale primary clarifier in the Kitchener, Ontario, waste treatment plant using time sequence analysis. Suspended solids in the feed and overflow were sampled and measured for total carbon. Although some correlation of feed and overslow was observed, the noise level was so high that the attempt had to be abandoned. Flow disturbances during the sampling period were too small to account for the noise. It is believed to come from changing settling properties of the sewage solids with time.