Phenomenon of Floating Floc

Abstract

Some water treatment plants have a problem with particles rising to the surface of the water instead of settling out in the sedimentation tank. These floating particulates then contaminate the effluent water. This problem likely stems from the supersaturated condition of the water as it enters the facility. As the gas goes out of the solution the bubbles can get attached to sediment particles and bring them to the surface instead of letting the particulate matter settle to the bottom of the sedimentation tank. This problem can be alleviated by increasing the rate of gas transfer so that the solution can equilibrate in the grit chamber so there will not be many bubbles still forming once the water reaches the sedimentation tank. One way to increase the gas transfer rate is to add even more air bubbles to the solution to increase the area of air/water interface, a property on which the gas transfer rate is defined. The water can be aerated by making small holes around the influent pipes. The difference in pressure will cause the air to be sucked into the water pipe at a rate dependent on factors such as the velocity of the water, the size and number of holes and how far above the end of the pipe the holes are made. A program was written to calculate the flow rate of the air into the orifice as a function of the water flow rate, pipe size, location of the holes and total area of orifice. The program is used to find the optimum set up for a given water flow rate.

Introduction and Objectives

A major concern at the Tamara water treatment plant in Honduras is the phenomenon of floating floc particles. The floating particulates then leave the plant with the cleaned effluent thus polluting it again. The other AguaClara treatment facilities also have problems with rising sludge, but to a much lesser degree. This project was made to devise a way to keep the solid particulates from rising to the surface of the sedimentation tank in adherence to AguaClara's mission of sustainable water treatment. Since the solution can not require any electricity, the fundamentals of fluid dynamics were utilized. Aeration is used in the lab in order to equilibrate solutions rapidly but these systems use air pumps which in turn use electricity. This project had the task of constructing a sustainable method of pumping large quantities of air into flowing water. The set up could then be used for any treatment plant that has problems with floating floc particles.

Procedure

Several fundamental properties of fluid dynamics were used in order to devise a solution to the problem of floating floc particles. The concept of aeration was employed to prevent the solids in the solution from rising up the water column in the sedimentation tank. Infusing the water with many air bubbles increases the area of air/water interface thus increasing the gas transfer rate. This means that the gasses infused into the supersaturated water will form into bubbles and leave the solution faster. The goal is for all of the excess gas to have left the water by the time the solution gets to the sedimentation tank. Once the water has equilibrated with the atmosphere all the excess gas will have been removed so there will no longer be gas bubbles forming and bringing floc particles up to the water surface. Without the influence of gas bubbles the particles will settle to the bottom of the sedimentation tank. The rapid influx of air bubbles will also cause more turbulence in the water, helping dislodge any gas pockets that have attached to the suspended solids.

Vertical, full free flowing water pipes have below atmospheric pressure on the inside thus when there is a hole in the side of the pipe exposing it to air at atmospheric pressure a vacuum will form as the air enters the orifice since molecules move from areas of high pressure to those of low pressure. Once the pressure inside the pipe equals that of outside the pipe there will no longer be any mass transfer. Another constraint on the amount of air that will be sucked into the pipe is that of head loss. As the flow of air combines with that of the influent water the total flow rate, density and viscosity of the solution changes increasing the amount of head loss. The amount of head loss incurred between the orifice and the open end of the pipe can not exceed the actual distance between said orifice and pipe exit. Once either condition is met the system will be at equilibrium and there will be no net change in flow rate.

Calculating the flow of air through the orifice can be done via an iterative process. For any given influent flow rate, influent pipe diameter, orifice diameter and distance of the orifice from the outlet the flow rate of air through the hole can be projected. The value can not be solved for directly because there are too many dependent variables throughout the equations. This dependence can be seen in the list of equations used to model a vertical pipe full of free falling solution that has a hole in the side that is open to the atmosphere. Thus an iterative "guess and check" algorithm was made to solve the system of equations. There are two variations of the program. One calculates the flow rate of air entering the pipe for many different orifice areas until the total orifice area allows so much air in that the pressure inside the pipe equals that of outside the pipe. The other version varies the distance between the outlet of the pipe and the location of the holes until the headloss through that section of pipe equals the distance between the hole and the outlet. This second program takes the orifice size as an input. Both programs can be found QaOrificeDiffL.xmcd .

Principals used:
Orifice flow
Friction
Reynolds number
Blended density
Blended viscosity
Flow rate
Headloss
Static pressure
Bernoulli's principal

Equations:
Basic Equations
Floating Floc Research

Calculating performance Index
Floating Floc Research
Determining the properties of the mixed solution
Floating Floc Research

Subscript x:
a = air
w = water
wa = air water mix

Variables:
Q.x = Flow rate of x
K.or = Orifice coefficient
d.hole = Orifice diameter
D.pipe = Pipe diameter
L.pipe = Distance between the orifice and the outlet
P1 = Pressure at orifice
P2 = Pressure at outlet
V1 = Velocity of mix at orifice
V2 = Velocity of mix at outlet
h.l = Headloss
f.p = Friction factor in pipe
Re.p = Reynolds number
h.under = Outlet's depth below the surface
ρ.x = Density of x
μ.x = Dynamic Viscosity of x
x.x = Mass fraction
VBN.x = Viscosity blending number
ν.x = Kinematic viscosity
PI = Performance index
g = Gravity