Floating Flocs

Overview

Rising flocs in the sedimentation tanks have been a problem from the very beginning at the plant in Tamara. The plant in Ojojona is also having this issue but to a lesser degree. Initially the rising of the floc was thought to be caused by alum overdosing but the problem persisted even after the dosage was changed. The speed with which the floc particles rise in the tank suggests that air bubbles are lifting them to the surface. Some water treatment facilities purposely make the flocs rise to the surface as a way to remove particles in the water but since the Tamara and Ojojona plants are designed to have the flocs settle out at the bottom of the sedimentation tanks the layer of particles at the surface of the tank pollutes the effluent.

Gas bubbles can form in a solution of water when the total dissolved gas pressure is greater than the local solution pressure. Very high rates of mixing can cause the pressure to drop below that of the atmosphere, creating a negative local gauge pressure. This decrease in pressure causes more gas to enter as the total dissolved gas pressure is again greater than the local water pressure. There is a lot of water turbulence at the entrance of the Tamara water treatment plant because of influent's high velocity. The churning in the grit chamber could behave like a very high rate rapid mixer and thus significantly increase the amount of gas in the water. Another possible reason for the high oxygen content of the water is the transmission line leading from the mountain stream in Tamara to the treatment plant. There are some breaks in the line that allow air into the transmission line; this air-water combination then goes through regions of high pressure causing the air to be infused into the water. Different methods of reducing this influx of gas into the water or to take it out at the plant are being researched. The hope is that decreasing the amount of gas in the water at the beginning of the water treatment process will solve the problem of rising floc in the sedimentation tanks.

A procedure is needed that will either prevent supersaturation of air in the transmission line or a method to remove dissolved oxygen prior to sedimentation to increase the effectiveness of the water treatment plant.

Objectives

Floating Floc Team Detailed Task List
Floating Floc Team Meeting Minutes

Research Areas

One method used in the lab to get gases out of water is to aerate the water before it enters the system. This process requires a large amount of air to be pumped into the system, causing many little bubbles. The addition of more small bubbles to the system increases the rate of gas transfer. The gas in the water will then rise to the surface more rapidly. The contact time between the air and the water required to allow all or most of the gas to rise out of the water would thus be decreased.

The method of aeration for gas removal would require a high flow rate of air to be injected into the water. Pumps for getting air into the water are impractical to use in the Honduras towns that have AguaClara designed water treatment are not sustainable. Instead, the properties of gases and liquids can be used to create a sustainable method for infusing the water with small bubbles.

Henry's Law states:
At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Henry's Law can be utilized to pump air into the beginning of the system. A small hole in the pipe headed to the grit chamber at a point where the water is in free fall would create a pressure difference between the inside of the pipe and the atmosphere thus causing an influx of air. Henry's Law can then be applied to calculate the flow rate of air into the water. The density and velocity of the water after passing this hole can then be calculated. A time estimate for the amount of contact time between the atmosphere and water that is needed for all or most of the gas to leave the water can be calculated from those values.

A model of this process is being derived for the AguaClara systems.

Experimental Methods and Results

[Floating Floc Phenomenon Research]
This page discusses the research into the floating floc phenomenon that is being conducted.

[Quiz]
This quiz checks that you have a basic understanding of the principles behind this research.

Additional Information

Floating Floc Team [Relevant Literature]
Floating Floc Team Annotated Bibliography