Introduction and Objectives
The Four Communities plant has two sedimentation tanks designed for an upflow velocity of 70 m/day. The tanks are shallower than those of previously constructed Aguaclara plants. Like the other plants, the tanks use lamella to increase their efficiency. The sedimentation tanks were designed to accommodate a floc blanket. A floc blanket is suspension of flocs that are too large to rise to the top of the tank and two small to settle out. A floc blanket should act as a filter trapping flocs as they rise to the top of the sedimentation tanks. In the laboratory floc blankets have been show to greatly increase the water quality.
The Four Communities plant was designed with a plant flow rate of 380 liters per min but the available head in the conduction line sets a plant flow rate of only 190 -270 liters per minute. At these flow rates no effects of a floc blanket were observed and the flow may not have been great enough to keep flocs in suspension. One tank was shut off to increase the upflow velocity through the sedimentation tanks and attempt to form a floc blanket.
Methods
There were several initial challenges to shutting off one sedimentation tank. When all the flow passed through one tank, the head loss through the original effluent launders was high enough to flood the plant. New effluent launders were created to run one tank. They were designed with a head loss of four centimeters. The effluent launder holes were increased from 5/16 in to 7/16 in. DESIGN PROCESS. In the first three attempts to run one tank the effluent turbidity increased from below 10 NTU above 20 NTU. At this point it was unacceptable to send the effluent to the distribution line and the water was sent to waste. This procedure could only be maintained for 2-3 hours during the day without draining the water storage tank for the communities. No improvement in effluent quality was observed in these trials. The plant was recently cleaned before the second and third of the three trials. During these trials the incoming turbidity was above 100 NTU. In the third trial the after an hour it was hard to visually distinguish between the water at the end of the flocculator and the end of the sedimentation tank closest to the flocculator. The turbidity measured at the end of the flocculator was 119.1 NTU and in the back of the sedimentation tank the turbidity was 102.4 NTU. However, close to the effluent channel the turbidity measured in the sedimentation tank was 27.12 NTU indicating that most of the flocs were rising in the back of the tank.
In order to shut off the plant for a longer period of time, the fourth trial was run at night when the town was not consuming water. The sedimentation tank used was cleaned at 9 pm before the plant was started. The plant was successfully running by 12:00 pm with an influent turbidity of 40 NTU and effluent turbidity of 4 NTU. It should be noted that the three hours in between were spent cleaning the sedimenation tank and filling the plant.
After 36 hours, the lamella were removed from the tank to see if there was floc blanket beneath them.
After the lamella were replaced, the plant continued to run with only one sedimentation tank for the next four days. The incoming turbidity was consistently between 20 and 40 NTU.
Results and Disscusion
There was no initial period of time with unpotable water leaving the tank as had been expected from the first three trials. The plant was run for 36 hours with no significant changes in effluent quality (figure ###), although the plant was not monitored the second night. No evidence of a floc blanket was seen in the plant sludge judge. Additionally, water was siphoned from different heights in the sedimentation tank. The turbidity was consistently below 10 NTU.
Other than the initial disturbance of removing the lamella from the tank the water was fairly clear. It was possible to see to the top of the plates resting in the bottom of the sedimentation tanks. Flocs that had settled in the channel entering the sedimentation tanks were swept into the tank in an attempt to see where they were going. It was not possible to see what happened to these flocs. However, in general it seemed as though more flocs were rising in the middle of the sedimentation tanks as opposed to the sides. There was no measured improvement in effluent quality.