Introduction and Objectives
The Four Communities plant was designed with shallower sedimentation tanks than those of Ojojona and Tamara. To determine the effects of the design changes the plant effluent turbidity was compared to the turbidity coming out of a tube settler at the end of the flocculator. The tube settler was designed to act as a model sedimentation tank. If the sedimentation tanks were working efficiently, the plant effluent turbidity should have been equal to or less than the turbidity coming out of the tube settler.
Additionally, flocs were observed to be settling out in the channel carrying water from the flocculator to the sedimentation tanks and very few were reaching the second tank. To determine what effect, if any, the uneven distribution of flocs had on the two tanks, the effluent turbidities from both tanks were compared. Samples were taken from within the effluent launders of both tanks.
Methods
To compare ideal sedimenation with actual sedimentation in the plant, a tube settler was placed at the end of the flocculation tank (Figure 2). The tube settler design (See Methods Section) mimicked the design of the lamella in the sedimentation tanks except that the tube settler was a much simpler system. The effluent from the tube settler was compared to the effluent from the sedimentation tanks. Turbidity measurements were taken in twenty minute or half hour intervals over the course of several hours and averaged when the influent, effluent, and alum dose were relatively constant.
Results and Discussion
The results of each test of the effluent from both the tube settler and the sedimentation tanks are shown below in figure 3. The effluent turbidity is compared to the incoming raw water turbidity.
The effluent turbidity from the sedimentation tanks and the tube settlers were fairly similar at all incoming turbidities although the tube settlers generally performed better. The largest difference, 1 NTU between the two was at a raw water turbidity of 135 NTU. Most of the tests began in the morning and the incoming plant turbidity was decreasing throughout the day. This decrease was measured first in the tube settlers biasing the tube settler measurements lower than the sedimentation tank measurements.
There was very little difference between the two sedimentation tanks. In the other trials, one tank does not perform consistently better or worse.
The tests were taken when the plant was performing well- the effluent close to or below 5 NTU. The test would be more informative when the plant was not working well but flocs were forming. In this case the test could show whether the flocculator or the sedimentation tanks were performing poorly.