Water Treatment : Filtration

FILTRATION
Purpose

The purpose of filtration is to remove suspended particles from water by passing the water through a medium such as sand. As the water passes through the filter, floc and impurities get stuck in the sand and the clean water goes through. The filtered water collects in the clear well, where it is disinfected and then sent to the customers.

Filtration is usually the final step in the solids removal process which began with coagulation and advanced through flocculation and sedimentation. In the filter, up to 99.5% of the suspended solids in the water can be removed, including minerals, floc, and microorganisms.

COMMON METHODS OF FILTRATION

1. Conventional filtration
   In conventional filtration, filtration follows coagulation/flocculation and sedimentation. This type of filtration results in flexible and reliable performance, especially when treating variable or very turbid source water.
   2.Direct filtration
   Some treatment plants operate without some or all of the sediment removal processes which precede filtration. If filtration follows coagulation and flocculation, without sedimentation, it is known as direct filtration. This method can be used when raw water has low turbidity.

2. In-line filtration
   In-line filtration involves operating the filters without flocculation or sedimentation. A coagulant chemical is added to the water just before filtration and coagulation occurs in the filter. In-line filtration is often used with pressure filters, but is not as efficient with variable turbidity and bacteria levels as conventional filtration is.
   Mechanisms of Filtration

3. Straining

4. Adsorption

5. Biological action

6. Absorption
   Straining
   Straining involves passing the water through a filter in which the pores are smaller than the particles to be removed. This is the most intuitive mechanism of filtration, and one which you probably use in your daily life.
   The picture below shows an illustration of straining in a filter. As you can see, the floc cannot fit through the gaps between the sand particles, so the floc are captured. The water is able to flow through the sand, leaving the floc particles behind.

Adsorption
Adsorption is the most important mechanism of filtration in many cases. This is the gathering of gas, liquid, or dissolved solids onto the surface of another material, as show below

Coagulation takes advantage of the mechanism of adsorption when small floc particles are pulled together by van der Waal's forces. In filtration, adsorption involves particles becoming attracted to and "sticking" to the sand particles. Adsorption can remove even very small particles from water.
Biological Action

This mechanism involves any sort of breakdown of the particles in water by biological processes. This may involve decomposition of organic particles by algae, plankton, diatoms, and bacteria or it may involve microorganisms eating each other. Although biological action is an important part of filtration in slow sand filters, in most other filters the water passes through the filter too quickly for much biological action to occur.

Absorption
Absorption is the soaking up of one substance into the body of another substance. Absorption should be a very familiar concept - sponges absorb water, as do towels. In a filter, absorption involves liquids being soaked up into the sand grains, as shown below:

After the initial wetting of the sand, absorption is not very important in the filtration process.
TYPES OF FILTERS

1. Slow Sand Filter
   The slow sand filter is the oldest type of large-scale filter. In the slow sand filter, water passes first through about 36 inches of sand, then through a layer of gravel, before entering the underdrain. The sand removes particles from the water through adsorption and straining.

Unlike other filters, slow sand filters also remove a great deal of turbidity from water using biological action. A layer of dirt, debris, and microorganisms builds up on the top of the sand. This layer is known as schmutzdecke, which is German for "dirty skin."

The schmutzdecke breaks down organic particles in the water biologically, and is also very effective in straining out even very small inorganic particles from water.

Maintenance of Slow Sand Filter

Maintenance of a slow sand filter consists of raking the sand periodically and cleaning the filter by removing the top two inches of sand from the filter surface. After a few cleanings, new sand must be added to replace the removed sand.

1. Rapid Sand Filter
   The rapid sand filter differs from the slow sand filter in a variety of ways, the most important of which are the much greater filtration rate and the ability to clean automatically using backwashing. The mechanism of particle removal also differs in the two types of filters - rapid sand filters do not use biological filtration and depend primarily on adsorption and some straining.

The filter is contained within a filter box, usually made of concrete. Inside the filter box are layers of filter media (sand, anthracite, etc.) and gravel. Below the gravel, a network of pipes makes up the underdrain which collects the filtered water and evenly distributes the backwash water. Backwash troughs help distribute the influent water and are also used in backwashing (which will be discussed later).
In addition to the parts mentioned above, most rapid sand filters contain a controller, or filter control system, which regulates flow rates of water through the filter. Other parts, such as valves, a loss of head gauge, surface washers, and a backwash pump, are used while cleaning the filter.

Operation of a rapid sand filter during filtration is similar to operation of a slow sand filter. The influent flows down through the sand and support gravel and is captured by the underdrain. However, the influent water in a rapid sand filter is already relatively clear due to coagulation/flocculation and sedimentation, so rapid sand filters operate much more quickly than slow sand filters.

Filter Cleaning

When to Backwash

Rapid sand filters can be backwashed. During backwashing, the flow of water through the filter is reversed, cleaning out trapped particles.

Three factors can be used to assess when a filter needs backwashing:

Length of the filter run
- arbitrarily scheduling backwashing after 72 hours or some other length of filter operation.
Turbidity of the effluent water
Head loss within the filter
- to determine when the filter is clogged enough to need cleaning.
Head loss is a loss of pressure (also known as head) by water flowing through the filter. When water flows through a clogged filter, friction causes the water to lose energy, so that the water leaving the filter is under less pressure than the water entering the filter. Head loss is displayed on a head loss gauge. Once the head loss within the filter has reached between six and ten hours, a filter should be backwashed.

The Process of Backwashing

In order to backwash a filter, the influent valve is closed and a waste line is opened.  A backwash pump or tower forces treated water from the system back up through the filter bed.  The dirty backwash water is collected by the wash troughs and can be recycled to the beginning of the plant or can be allowed to settle in a tank, pond, or basin. 

Some points to consider in backwashing:

1. Backwashing should begin slowly. If begun too quickly, backwash water can damage the underdrain system, gravel bed, and media due to the speed of the water. Beginning backwashing too quickly will also force air bound in the filter out, further damaging the filter.
2. After a slow start, the backwash rate should be accelerated to reach around 10 to 25 gpm/ft2. The backwash water must have enough velocity and volume to agitate the sand and carry away the foreign matter which has collected there.
3. Backwashing normally takes about 10 minutes, though the time varies depending on the length of the filter run and the quantity of material to be removed. Filters should be backwashed until the backwash water is clean.
   Surface Washing

At the same time as backwashing is occurring, the surface of the filter should be additionally scoured using surface washers. Surface washers spray water over the sand at the top of the filter breaking down mudballs.

Filter Media
The filter media is the part of the filter which actually removes the particles from the water being treated. Filter media is most commonly sand, though other types of media can be used, usually in combination with sand. The gravel at the bottom of the filter is not part of the filter media, merely providing a support between the underdrains and the media and allowing an even flow of water during filtering and backwashing.
The sand used in rapid sand filters is coarser (larger) than the sand used in slow sand filters. This larger sand has larger pores which do not fill as quickly with particles out of the water. Coarse sand also costs less and is more readily available than the finer sand used in slow sand filtration.
Dual and Multi-Media Filters
In many cases, multiple types of media are layered within the filter. Typically, the layers (starting at the bottom of the filter and advancing upward) are sand and anthracite coal, or garnet, sand, and anthracite coal. The picture below shows a cross-section through a dual media filter.

The media in a dual or multi-media filter are arranged so that the water moves through media with progressively smaller pores.  The largest particles are strained out by the anthracite.  Then the sand and garnet trap the rest of the particulate matter though a combination of adhesion and straining.  Since the particles in the water are filtered out at various depths in a dual or multi-media filter, the filter does not clog as quickly as if all of the particles were all caught by the top layer.  

The media in a dual or multi-media filter must have varying density as well as varying pore size so that they will sort back into the correct layering arrangement after backwashing. Anthracite coal is a very light (low density) coal which will settle slowly, ending up as the top layer of the filter. Garnet is a very dense sand which will settle quickly to the bottom of the filter.
DESIGN CALCULATIONS IN FILTRATION PROCESS

For the rapid sand filter, the most important dimension is the surface area.  Filters must be designed so that the water flowing through is spread out over enough surface area that the filtration rate is within the recommended range.  
The clear well is a reservoir for storage of filter effluent water.  The clear well must be designed  with sufficient volume to backwash the rapid sand filter.  However, clear well has other purposes, most important of which is to allow sufficient contact time for chlorination.

Backwashing is a very important part of filter calculations.

Backwash characteristics:

1.Backwash frequency is the same as filter run time. Either term can be used to signify the number of hours between backwashing.

2.Backwash period is the length of time which backwashing lasts.

1. Backwash water is the water used to backwash the filter.

2. Backwash rate is the rate at which water is forced backwards through the filter during backwashing. This rate is similar to the filtration rate, only with water moving in the other direction through the filter. The backwash rate is typically much greater than the filtration rate.

3. Filter rise rate is the speed at which water rises up through the filter during backwashing. This is another way of measuring the backwash rate.

4. Bed expansion
   During backwashing, the water pushes the media up until it is suspended in the water. The height to which the media rises during backwashing is known as the bed expansion. For example, if the filter media is 2 feet deep, it may rise up to 3 feet deep during backwashing. This is a 50% bed expansion.
   Most of these backwash specifications merely describe the type of filter we will be considering and are not used in calculations. However, two factors - the filter rise rate and the backwash period - will be used when calculating the volume of the clear well chamber.

Specifications
A water treatment plant will typically have several filters. Each filter in our succeeding calculations will be assumed to have the following specifications:

Square tank
Basin depth: 10 ft
Media depth: 2-3 ft
Surface area: <2,100 ft2
Filtration rate: 2-10 gal/min.ft2
Flow through filter: 350 - 3,500 gpm
Backwash frequency: every 24 hours
Backwash period: 5-10 minutes
Backwash water: 1-5% of filtered water
Backwash rate: 8-20 gal/min.ft2
Filter rise rate: 12-36 in/min
Bed expansion: 50%
Backwash trough 3 ft above media
Backwash water piped to raw water intake

Sources: Introduction to Wastewater Treatment Lesson 8