Lesson 9:

Filtration

 

 

Objective

In this lesson we will answer the following questions:

 

 

Reading Assignment

Read the online lecture.

 

 

Lecture

Introduction

In the conventional water treatment process, filtration usually follows coagulation, flocculation and sedimentation.

 

 

At present, filtration is not always used in small water systems, however, recent regulatory requirements under the USEPA Interim Enhanced Surface Water Treatment rules may make water filtering necessary at most water supply systems. Water filtration is a physical process of separating suspended and colloidal particles from water by passing water through a granular material.

The process of filtration involves straining, settling, biological action, and adsorption. As floc passes into the filter, the spaces between the filter grains become clogged, reducing this opening and increasing removal. Some material is removed merely because it settles on a media grain.

One of the most important processes is adsorption of the floc onto the surface of individual filter grains. This helps collect the floc and reduces the size of the openings between the filter media grains. In addition to removing silt and sediment, floc, algae, insect larvae, and any other large elements, filtration also contributes to the removal of bacteria and protozoa such as Giardia lamblia and Cryptosporidium. Some filtration processes are also used for iron and manganese removal.

 

 

Factors Affecting Filtration

There are certain factors that are known to affect the process of filtration, with the first being the chemical characteristics of the water being treated. Others include the nature of the suspended particles in the water, the types of treatment preceding filtration (coagulation/flocculation/sedimentation) and the type of filter and operation being used.

 

 

The Filtration Process

Regardless of the type of filter, however, filtration involves the processes of straining (where particles are captured in the small spaces between filter media grains), sedimentation (where the particles land on top of the grains and stay there), and adsorption (where a chemical attraction occurs between the particles and the surface of the media grains).

A granular media filter, generally, consists of a rectangular concrete structure with 4-feet-deep media formed of sand or a combination of sand, garnet, anthracite (crushed hard coal), and activated carbon. The media are supported by a layer of gravel. Under the gravel is a drain system for the drainage of filter effluent, called filtrate.

A small amount of cationic polymer is applied to the filter influent for micro flocculation. Polymer and turbidity particles form a very fine floc that accumulates on the top of the filter media and forms a straining mat (schmutzdecke) that removes the turbidity. Turbidity is removed by two mechanisms, straining and adsorption. Adsorption is acquiring the turbidity particles on the surface of micro floc. Most of the turbidity is removed in the top few inches of media.

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.  Straining occurs when you remove spaghetti from water by pouring the water and spaghetti into a strainer. The picture below shows an example 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.  

 

In the past, straining has been assumed to be very important in the filtration process.  However, in many cases, the pores between sand particles in the filter are much larger than the particles captured by the filter.  It has been suggested that small particles become wedged between sand grains as filtration occurs, making the pore spaces smaller and allowing the filter to strain out yet smaller particles.  However, a clean filter will produce clean water before any of this pore size-reduction has occurred.  Therefore, it is now believed that straining is not an important part of most filtration processes. 

The second, and in many cases the most important mechanism of filtration, is adsorption.  Adsorption is the gathering of gas, liquid, or dissolved solids onto the surface of another material, as shown 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.

The third mechanism of filtration is biological action, which 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. 

The action of straining suspended particles plays a minor role during filtration. Absorption plays the most important role of all the actions that occur.

Filtration is either accomplished through gravity filters or pressure driven filters. Let's take a look at those now.

 

 

Gravity Filters

Gravity filters operate without any added pressure. The water flows into the top of the filter and drains down through the filter media due to gravitational forces. Gravity filtration uses different media formats, depending upon the type of filter being used. Gravity filters can utilize strictly sand in single media filters, sand and anthracite in dual media filters, sand, anthracite and carbon in multi-media filters, or activated carbon alone. Gravity filters are capable of a filtering 2-10 gpm/ft2, however, due to regulations within some states, the normal maximum filtration rate is 6 gpm/ft2. The filtration rate of the filter is based on the flow going over the filter (gpm) and the surface area of the filter media (ft2).

 

Above shows a single media filter, but the set up is the same for multi-medium filters.

 

Gravity filters can be broken into the following classifications:

 

 

Slow Sand Filters

Slow sand filtration is well suited for small water systems, up to 5,000 people, because it does not require constant operator attention. It is a proven, effective filtration process with relatively low construction costs and low operating costs. The filtration rate is generally in the range of 45-150 gpd/ft2, or 0.015-0.15 gpm/ft2, which is a very low filtration rate.

 

 

Slow sand filters have 3.5 ft of sand layered over 1 ft of graded gravel.

The area above the top of the sand layer is flooded with water to a depth of 3 to 5 feet, and the water is allowed to trickle down through the sand. An overflow device prevents excessive water depth. The filtration occurs due to straining, adsorption and biological predation. A sticky mat of biological matter, called a "schmutzdecke" forms on the sand surface, where particles are trapped and organic matter is biologically degraded. Slow sand filters rely on this cake filtration at the surface of the filter for straining out particles. As the surface cake develops, it assumes the dominant role in filtration rather than the granula media (sand).

After the filter run, which could be several days or even weeks, the filter is taken out of service and cleaned. This type of filter is not backwashed. Cleaning occurs by scraping the top inch of sand and schmutzdecke off of the filter. It may take several days to re-establish the schmutzdecke, so filtered water must be wasted until it is re-established. This process can be repeated until the sand depth reaches 2 ft.

These filters are effectively used for direct filtration of source water with very low turbidity, such as pristine mountain streams or reservoirs.

 

Slow sand filters require a large land area, large quantities of filter media, and manual labor for cleaning. Water with high turbidity levels can quickly clog the fine sand, so low turbidity waters (<10 NTU) utilize slow sand filtration.

 

 

Rapid Sand Filters

The rapid sand filter, which is similar in some ways to the slow sand filter, is one of the most widely used filtration units. The sand used in this process is more coarse than the sand used in a slow sand filter, which allows a higher flow rate.

During the filtration process water passes downward through a coarse sand bed, suspending particulates in the upper several inches of the filter bed. The suspended particles consist of the coagulated matter remaining in the water after sedimentation, as well as a small amount of uncoagulated suspended matter. This trapped material will increase the filter's head loss, requiring it to be backwashed. The water used for backwashing should not exceed 4% of the total water produced.

Usually 2 to 3 feet deep, the filter media are supported by approximately 1 foot of gravel. The media may be fine sand or a combination of sand, anthracite coal, and coal.

Unlike the slow sand filters, surface loading in these filters is 2- 4 gpm/ft2, and there is backwashing after the filter run. Sand depth, in these filters, is 2 to 3 feet. Filtration takes place in the top few inches of the medium.

 

 

 

High-Rate Filters

High-rate filters use more than just sand as the filter media; they utilize anthracite coal, or granular activated carbon, as well. Some include a layer of garnet sand below the silica sand.

The type of media used is all based on the source water and how turbid it is . High-rate filters operate up to 4 times faster than a conventional rapid sand filter, ranging from 3-8 gpm/ft2.

Granulated Activated Carbon (GAC) filters have a layer of activated carbon on top of anthracite or sand. All this media is stacked on the filter underdrain, which collects the filtered water while preventing the filter media from passing through.

Activated carbon adsorbs various contaminants, such as tastes and odor-causing organics, THMs, and synthetic organics. GAC or anthracite coal are normally the top layer in a mixed media filter. These filters have the problem of losing some carbon during the backwashing; therefore backwashing is properly controlled to prevent the excessive loss of GAC. Commonly, backwashing causes 1-6% GAC loss per year. When the filter is backwashed, the different layers will stay stratified due to their individual densities. There may be a little combining of layers during backwash, but not much.

 

High-rate filters and rapid sand filters should both have a Unit Filter Run Volume (UFRV) between 5,000-10,000 gal/ft2. A UFRV beyond that increases the chance of a major floc breakthrough. During a breakthrough, a crack, or break in a filter bed allows passage of floc or particulate matter through the filter.

 

High-rate filters use more backwash water than rapid sand filters, but should not exceed 6% of the total water produced.

 

 

Deep-Bed, Monomedium Filters

The deep-bed, monomedium filter uses activated carbon (if used as a polishing filter) or anthracite coal as the media, instead of sand. This single layer of media is 4-6 ft deep and is more coarse than the sand or anthracite used in conventional filtesr. This type of filter allows for the highest filtration rates and is suited for direction filtration, which means the sedimentation process can be eliminated.

 

 

Biologically Active Filters

Biologically active filters use non-pathogenic microbes that are growing on the sand or carbon media to break down organic matter. This type of filter produces water that is free of poor tastes and odors, has a low chlorine demand, and doesn't produce microbial growth in the distribution system. If not properly operatored, however, they can introduce pathogens or harmful by-products in the water system. This type of filtration process is not widely used in the U.S.

 

 

Operational Parameters

Gravity filters are designed to operate within certain parameters, those include:

 

 

Gravity Filters: Operational Problems

While using gravity filters for filtration, you may run into some operational problems. Let's take a look at a few of those problems and what you would need to do to correct it.

Filter Operating Problem Corrective Actions
Short filter run due to turbidity

Check coagulation/flocculation process for:

  • Coagulant dose
  • Flash-mixing
  • Flocculator mixing energy
  • Filter aid dose
Rapid fluctuations in filter flow rate

Determine and correct the root cause:

  • Change in plant/filter flow rate
  • Valve malfunction
  • Instrumentation malfunction
Ineffective backwashing, causing turbidity and head loss problems

Optimize filter backwash sequence:

  • Test multiple options and record results
Mudball formation within the media

Ensure adequate backwash flow rate and agitation:

  • Test filter expansion during a backwash cycle to ensure >20%
  • Ensure proper operation of air scour and surface wash systems
Filter bed shrinkage

Optimize filter backwash sequence:

  • Test multiple options and record results
Gravel displacement/sand boil formation

Slow the rate of backwash valve operaton:

  • Propgram the valve controller to reduce the opening speed
  • Media must be removed and gravel redistributed
Air binding

Maintain a minimum head of about 5 ft over filter media:

  • Take filter out of service and backwash before head loss reaches 4.5 ft
  • Backwash if filter has become air bound
Media loss Raise the wash water trough level

 

Now that we have covered the different types of gravity filters that can be used in the treatment process, let's take a look at the pressure driven filters that can be utilized.

 

Part 2: Pressure Driven Filters