Water Treatment: Sedimentation

SEDIMENTATION
Purpose

Sedimentation is a treatment process in which the velocity of the water is lowered below the suspension velocity and the suspended particles settle out of the water due to gravity. The process is also known as settling or clarification.

Most water treatment plants include sedimentation in their treatment processes. However, sedimentation may not be necessary in low turbidity water of less than 10 NTU. In this case, coagulation and flocculation are used to produce pinpoint (very small) floc which is removed from the water in the filters.

Types of Basins

1. Rectangular basin

Rectangular basins are the simplest design, allowing water to flow horizontally through a long tank. This type of basin is usually found in large-scale water treatment plants.
Advantages :
1. predictability
2. cost-effectiveness
3. low maintenance
4. least likely to short-circuit, especially if the length is at least twice the width

1. Double-deck rectangular basin
   Double-deck rectangular basins are essentially two rectangular sedimentation basins stacked one atop the other. This type of basin conserves land area, but has higher operation and maintenance costs than a one-level rectangular basin.

1. Clarifiers
   Square or circular sedimentation basins with horizontal flow are often known as clarifiers. This type of basin is likely to have short-circuiting problems.

In a CLARIFIER, water typically enters the basin from the center rather than from one end and flows out to outlets located around the edges of the basin.

SEDIMENTATION BASINS HAVE FOUR ZONES :

Inlet zone
Settling zone
Sludge zone
Outlet zone

Each zone should provide a smooth transition between the zone before and the zone after. In addition, each zone has its own unique purpose.

RECTANGULAR SEDIMENTATION BASIN

INLET ZONE

Primary Purposes of Inlet Zone:
To distribute the water.
The incoming flow in a sedimentation basin must be evenly distributed across the width of the basin to prevent short-circuiting. Short-circuiting is a problematic circumstance in which water bypasses the normal flow path through the basin and reaches the outlet in less than the normal detention time.
To control the water’s velocity.

If the water velocity is greater than 0.5 ft/sec, then floc in the water will break up due to agitation of the water. Breakup of floc in the sedimentation basin will make settling much less efficient.

Inlet devices acts to prevent turbulence of water.
Two types of inlets
Stilling wall (also called perforated baffle wall)
- spans the entire basin from top to bottom and from side to side. Water leaves the inlet and enters the settling zone of the sedimentation basin by flowing through the holes evenly spaced across the stilling wall.

Channel or Flume
- allows water to enter the basin by first flowing through the
holes evenly spaced across the bottom of the channel and then
by flowing under the baffle in front of the channel. The
combination of channel and baffle serves to evenly distribute
the incoming water.

SETTLING ZONE

After passing through the inlet zone, water enters the settling zone where water velocity is greatly reduced. This is where the bulk of floc settling occurs and this zone will make up the largest volume of the sedimentation basin. For optimal performance, the settling zone requires a slow, even flow of water.

The settling zone is simply a large expanse of open water.

OUTLET ZONE

The outlet zone controls the water flowing out of the sedimentation basin - both the amount of water leaving the basin and the location in the basin from which the out flowing water is drawn. Like the inlet zone, the outlet zone is designed to prevent short-circuiting of water in the basin. In addition, a good outlet will ensure that only well-settled water leaves the basin and enters the filter. The outlet can also be used to control the water level in the basin.

Outlets are designed to ensure that the water flowing out of the sedimentation basin has the minimum amount of floc suspended in it. The best quality water is usually found at the very top of the sedimentation basin, so outlets are usually designed to skim this water off the sedimentation basin.

A typical outlet zone begins with a baffle in front of the effluent. This baffle prevents floating material from escaping the sedimentation basin and clogging the filters. After the baffle comes the effluent structure, which usually consists of a launder, weirs, and effluent piping. A typical effluent structure is shown below:

Components of the effluent structure :

1. effluent launder
   • a trough which collects the water flowing out of the
     sedimentation basin and directs it to the effluent piping. The sides
     of a launder typically have weirs attached.

2.Weirs
- are walls preventing water from flowing uncontrolled into the launder. The weirs serve to skim the water evenly off the tank.

A weir usually has notches, holes, or slits along its length. These holes allow water to flow into the weir. The most common type of hole is the V-shaped notch shown on the picture which allows only the top inch or so of water to flow out of the sedimentation basin. Conversely, the weir may have slits cut vertically along its length, an arrangement which allows for more variation of operational water level in the sedimentation basin.

Then the launder channels the water to the outlet, or effluent pipe
Effluent pipe
- This pipe carries water away from the sedimentation basin and to the next step in the treatment process.

SLUDGE ZONE

The sludge zone is found across the bottom of the sedimentation basin where the sludge collects temporarily.  Velocity in this zone should be very slow to prevent resuspension of sludge.  
A drain at the bottom of the basin allows the sludge to be easily removed from the tank.  The tank bottom should slope toward the drains to further facilitate sludge removal. 
In some plants, sludge removal is achieved continuously using automated equipment.  In other plants, sludge must be removed manually.  If removed manually, the basin should be cleaned at least twice per year, or more often if excessive sludge buildup occurs.  It is best to clean the sedimentation basin when water demand is low. Many plants have at least two sedimentation basins so that water can continue to be treated while one basin is being cleaned, maintained, and inspected.  

If sludge is not removed from the sedimentation basin often enough, the effective (useable) volume of the tank will decrease, reducing the efficiency of sedimentation.  In addition, the sludge built up on the bottom of the tank may become septic, meaning that it has begun to decay anaerobically.  Septic sludge may result in taste and odor problems or may float to the top of the water and become scum.  Sludge may also become resuspended in the water and be carried over to the filters.  

SLUDGE DISPOSAL

The sludge which is found in the bottom of a sedimentation tank is primarily composed of water. The solids in the sludge are mainly excess coagulant, such as alum. Alum sludge has a solids concentration of only about 1% when automatically removed from the basin, or about 2% if manually removed. (The greater solids concentration of manually removed sludge is due to a small amount of gravity
thickening).
Thickening of Sludge
Most of the other alternatives require transporting sludge away from the treatment plant. Sludge is typically dried before it is trucked away into landfill since the greater volume of wet sludge makes it much more expensive to transport. This drying process is known as dewatering or thickening.
Common Devices used to thicken the sludge:
Lagoons
- are small-volume storage ponds; the simplest device used to thicken sludge. These lagoons are filled with sludge and the solids are allowed to settled due to gravity to the bottom of the lagoon while the clear water is pumped off the top.
After a few months, gravity and evaporation will have reduced the sludge to a 30-50% solid state. The sludge can then be covered with soil and left on site, or may be trucked to a landfill off-site.

A drying bed is similar in design to a sand filter, with a layer of sand underlain by a layer of gravel. The sludge is applied to the top of the sand and the water percolates down through the sand and gravel and is drained away. When the sludge is sufficiently dry, it is carefully removed from the top of the sand and is trucked to a landfill.
DISCRETE PARTICLE SETTLING

Discrete particle settling occurs when particles are discrete and do not interfere with one another as they settle. For this type of settling ,the movement of particle in water is determined by a balance of downward gravitational force, an upward buoyancy force, and an upward drag force.

OTHER DESIGN CONSIDERATIONS FOR RECTANGULAR SEDIMENTATION BASIN:

Typical detention times for basin w/ horizontal flow : 1.5 – 4.0 hours
Horizontal mean flow velocity: 0.3 – 1.1 m/min
Overflow rate: 1.25 – 2.5 m/h
Length to depth ratio: greater than 15
Length to width ratio: greater than 4

Sources: Introduction to Wastewater Treatment Lesson 8