Filtration

Introduction

Filtration can be defined as the process whereby suspended and colloidal matter are removed from the water when it passes through a suitable medium to produce a product of sparkling clarity. Filtration of water through sand beds has been used for water purification for thousands of years. The current standard filter for municipal water systems is a dual media filter using crushed anthracite coal and sand on top of a gravel subfill; these filters are sometimes referred to as rapid sand filters.

The dual media sand-anthracite filter is primarily used for the removal of turbidity and suspended solids as low as 10 to 20 microns. Dual media filters provide very efficient particle removal at relatively high filtration rates.

This overview shall address two basic types of filters, pressure filters and gravity filters. WETS has considerable experience with both types.

Note on Biofiltration: In biofiltration, the traditional media of sand, anthracite and/or activated carbon is promoted to maintain a biofilm capable of degrading organic matter and removing taste and odor-causing materials. It can be presumed that traditional media gravity filters often have some degree of biological activity; the biofilter term typically applies where nutrients such as nitrogen and phosphorous are employed to control and enhance biological activity. Biofilters have been utilized in Europe since the 1970s, but have seen only limited application in the USA, and are not specifically addressed in this manual.

Coagulation

Much of the objectionable material in water can be of colloidal size, and will not be removed by sedimentation or standard filtration. It is necessary to add a chemical to cause these extremely small particles to collect into clumps or “floc”, particles that are too large to pass through the interstices of the filter. The chemicals are referred to as coagulants.

The mechanism of coagulation involves using positively charged metal ions to attract the negatively charged colloidal particles which are otherwise repelling each other (virtually all the suspended particles in natural water have a negative charge).

Coagulation in water treatment processes is subject to difficulties and problems. Coagulant aids (flocculants) such as activated silica have been found to improve the properties of the floc (the coagulated particles) for filtration, by making the floc more durable or more rapidly settling.

There are a number of different factors affecting coagulation, such as pH, salts concentration, temperature, type of coagulant, turbidity, etc. Therefore, jar testing or field pilot testing are often necessary to determine the best means of coagulation and solids removal.

To summarize, chemicals using positively charged metal ions to attract negatively charge colloidal particles are referred to as “coagulants”; the common water treatment coagulant chemicals are aluminum sulphate (Alum), ferrous sulphate (copperas), ferric sulphate, and ferric chloride. Those chemicals (used in conjunction with a coagulant) intended to improve the floc durability or settling time are referred to as “coagulant aids” or flocculants; common coagulant aids include anionic polymer, non-ionic polymer, sodium silicate, bentonite, and calcium carbonate.

Filter Services

This section shall discuss the typical services for which WETS employs filters.

Color, Taste and Odor Removal

Organic pollutants, both natural and man-made, can pose issues related to colors, taste and odor related to varying amounts of suspended and dissolved materials. Much of this material can be of colloidal size, and the use of a coagulant is necessary to properly filter water for color, taste and odor removal.

Color, taste and odor are usually more of a problem for surface water than well water.

Iron and Manganese Removal

WETS has designed numerous systems for the removal of iron and manganese. Since well water is typically not exposed to oxygen, the dissolved iron and manganese, on exposure to air, oxidize to form insoluble reddish-brown particles. These particles discolor the water, and cause problems with laundry and plumbing fixtures. Conversion of soluble ferrous bicarbonate to insoluble ferric hydroxide on exposure to air is a common route. Many waters that contain iron also contain manganese; however, iron is more common.

The maximum contaminant levels (MCL) are 0.3 milligrams/l for iron and 0.05 mg/l for manganese. These iron and manganese standards are part of EPA’s National Secondary Drinking Water Standards. As secondary standards, they do not address a risk to human health, but are guidelines for aesthetic considerations, such as taste, color, and odor.

States can adopt these standards as enforceable standards, and Texas has done so.

Following is from a December 9, 1991 letter from the Texas Department of Health

These elements (iron and manganese) pose aesthetic concerns and are not health-related problems. Public water systems that have iron and manganese levels in excess of the limits established by the Drinking Water Standards are required to manage these elements so that the discoloration problems associated with each are eliminated. This Department’s preference in mitigating iron and manganese is to maximize the oxidation of these metals and then to effectively filter the precipitate.

While the iron and manganese precipitates can settle out in a storage tank, it is unusual that such sedimentation can produce water of the proper quality, and filtration is typically employed for the removal of the precipitates.

On a further note, while manganese is addressed in the secondary standards, which do not address health risks, there is recent research that manganese exposure at and above the current standard may be causing developmental issues in children.

After aeration or chlorination of raw water, most ferrous (Fe+2) soluble iron is easily oxidized to insoluble ferric (Fe+3) iron, allowing for efficient filtration and removal. Soluble manganous (Mn+2) ion is slower to oxidize and may require stronger oxidants or pH elevation to form insoluble manganic (Mn+4) particulates. Other natural water contaminants —such as organics, phosphates, complexing ligands, ammonia, and hydrogen sulfide—in the water supply may bind Fe and Mn or interfere with their oxidation rates.

So, while the iron is generally easily filtered in dual media filters, manganese can be more troublesome, especially at lower pH. For manganese, other filtration media such as Greensand and DMI 65 are often used. WETS recommends and typically runs pilot tests for manganese removal, comparing media such as the dual media, Greensand, and DMI 65 in this application.

Arsenic Removal

Arsenic is removed to some extent in iron and manganese treatment plants as the arsenic has some affinity for the insoluble iron oxides, and in fact, iron oxide‐coated sand has been demonstrated to be an effective adsorbent material for the removal of arsenic and other metals and metalloids from drinking water and wastewater. However, for effective arsenic removal, there are a number of other factors involved (for instance, adsorption decreases with increasing pH), so testing for the particular application is typically required for the design of an arsenic removal system.

TCEQ considers co-precipitation to be an alternative treatment process for arsenic removal, with more rigorous requirements for pilot testing, etc.

For arsenic removal improvements, WETS suggestions may include lowering the pH (from 7.5 to 6.5) and increasing ferric chloride concentration.

Also, consideration can be given to the fact that arsenic levels may be higher at the initial startup of well pump, and drop off with time over the first few hours, so high arsenic issue may possibly be addressed by tank blending.

For arsenic removal, peroxidation is typically employed to convert arsenite (As III) to arsenate (As V) before lowering pH. The conversion from arsenite to arsenate occurs within one minute in the pH range of 6.3 to 8.3.

Removal of Microbial Contaminants

Filtration is the most essential treatment process for removing microbial contaminants, including Cryptosporidium and Giardia. The filter media is typically granular activated carbon. Microbes on GAC are usually more concentrated than they are on sand–anthracite filter media because GAC provides more suitable attachment sites.

Other Filtration Services

WETS filters have been employed in other services, as follows:

Removal of radium/radionuclides

Radium is a naturally occurring radioactive metal that is formed when uranium decays in the environment. Radium is not stripped from the water, but can be removed to some extent by aeration, flocculation and filtration. It has been observed both by WETS and others in the industry that iron and manganese precipitation also pulls some radium; if insufficient radium is removed, WETS has recommended a zeolite softener downstream of the aerator for further radium removal.

EPA has established a Maximum Contaminant Level (MCL) of 5 picoCuries per liter (pCi/L) for any combination of radium-226 and radium-228 in drinking water. EPA has also established an a MCL of 15 pCi/L for alpha particle activity, excluding radon and uranium, in drinking water.

Removal of THM’s

The use of chlorine for disinfection can result in the formation of disinfection byproducts (DBPs). When chlorine is added to water with organic material, such as algae, river weeds, and decaying leaves, DBP’s are formed. DBP formation is often more an issue in warm weather climates.

Of the identified (DBPs), the two classes of compounds of most concern are the trihalomethanes (THM), and haloacetic acids (HAA). They are typically tasteless and odorless, but suspected human carcinogens. The quantity of byproducts formed is determined by several factors, including the amount and type of organic material present in water, the composition and structure of the organic matter, temperature, pH, chlorine dosage, contact time available for chlorine, and bromide concentration in the water. Therefore, the optimal strategy to minimize DBP formation will likely require bench and pilot testing and some trial and error.

The range of DBPs that can be formed as a result of the interaction of chlorine and the organic matter in chlorine is not clearly understood. Therefore, a practice has developed in which the THM’s are used as indicator chemicals for all potentially harmful compounds formed by the addition of chlorine to the water.

In addressing THM contamination, it is crucial to identify the optimum approach – as to the removal of the organic precursors, removal of the THM after formation, or a combination of the two, along with optimization of chlorination. Each situation needs to be dealt with on a case-by-case basis, so a well-executed pilot study is essential.

Where removal of precursors is a good approach for addressing a THM issue, coagulation and filtration is often utilized, and WETS has successfully installed a number of filtration systems to address THM contamination.

Filter Types and Design Alternatives

This section shall discuss the different types and design alternatives for filters.

The two basic filter types utilized by WETS are the pressure filter and gravity filter. The pressure filter is an ASME pressure vessel, typically constructed of low-carbon steel. The gravity filter is a domed ambient pressure vessel, typically fiberglass, with a body flange several feet from the top.

Modern filters are sometimes referred to as rapid sand filters. The original sand filters, referred to as slow sand filters, did not use a coagulant or backwash. The sand utilized was much finer than used today, and it was necessary for a layer of sediment and bacterial growth to be formed on the surface of the sand for the filters to be effective, and even then, the flowrate was only around 0.1GPM/ft2, as opposed to the 3 to 5 GPM/ft2 for the rapid sand filters. When slow sand filters required cleaning, the top layers of sand were removed and washed in a rotary sand washer. Slow sand filters are no longer in use.

Filter Vessel Shape

Pressure filters are cylindrical pressure vessels, but gravity filters are cylindrical or rectangular. Very large capacity filters are often long rectangular vessels. Historically, the WETS gravity filters are relatively small in capacity and have usually been cylindrical vessels.

Filter Media

This discussion shall be mostly concerned with the dual media filters utilizing crushed anthracite coal and sand. Other types of filters include mono-media (sand), multi-media (garnet, anthracite and sand, for instance), birm, and alternate media, including activated carbon.

Some definitions of terms as applied to the filter media:

The typical WETS dual media filter consists of 12 inches of anthracite atop 18 inches of sand.

Sand

The typical project uses quartz sand specified as follows:

The finer the sand, the shorter the filter runs, with most of the suspended material collected in the upper few inches of the sand bed.

Anthracite Coal

Anthracite coal has been used in conjunction with, or as a replacement for, sand for a long time. Some of the advantages attributed to anthracite include:

Longer filter run lengths (by allowing deeper penetration of the floc)
Higher filter rates for equivalent filtrate purity
Lower backwash velocities
Greater resistance than sand to coating and build-up
Insoluble in acid or alkali

The typical project uses anthracite specified as follows:

Gravel

The gravel subfill serves two purposes. First, it acts as a barrier between the filter media and the underdrain. The openings in the underdrain must be larger than the grains of filter media to avoid plugging. The layers of gravel prevent the filter media from being lost through these openings. Second, the gravel aids in the distribution of the backflow wash.

The top layer of gravel is considered the “barrier media”; the lower layers of gravel are the “dispersion media”. The barrier media provides immediate support to the filter media and separates it from the gravel.

Note that there should be only minor displacement of the gravel during the backwash. Abnormal displacement, where the gravel is washed into the filter media, is a problem for the operation. If displacement is excessive, the filter media must be removed and the gravel regraded or replaced.

Underdrain

The underdrain performs two primary functions. The underdrain uniformly collects the filtered water to maintain a constant rate of filtration across the filter bed and passes the water to the outlet of the filter. A more important function of the underdrain is to evenly distribute backwash water so that the gravel subfill is not disturbed and the filter media is evenly expanded.

The use of a ceramic tile underdrain known as the Leopold Compound Duplex Tile Filter Bottom is common in the industry. Perforated pipe underdrains are also utilized.

Filter Loading

The filter loading rate is a measure of the filter production per unit area and is typically expressed in GPM /square foot (ft2). Typical loading rates range from 2 to 4 GPM/ft2, though higher rates of 4 to 6 GPM/ft2 are becoming common, with even higher rates possible for debottlenecking.

However, also note, TCEQ has a limit of 5 GPM/ft2 for filtration rates for iron and manganese removal (30 TAC 290.42(b)(2)(A)).

For WETS designs, earlier jobs typically used rates around 4 GPM/ft2, but in more recent years 5 GPM/ft2 has been typical. There are some references in the literature to allowing higher rates for pressure filters, but generally, gravity and pressure filters are designed to similar rates, and that has been the WETS practice.

Filter Operation

This section shall discuss some operational considerations for these filters.

Chemical dosing

The common water treatment coagulant chemicals used are:

Coagulant aids (flocculants) are typically inorganic materials that improve or accelerate the process of coagulation and flocculation by producing quick-forming, dense and rapid settling flocs. They are especially useful for waters with low turbidity. Common coagulant aids include anionic polymer, non-ionic polymer, sodium silicate, bentonite, and calcium carbonate.

To maintain coagulant feed near-optimal rates, periodic jar testing is recommended.

Backwash

The filter media eventually becomes plugged with the solids/precipitates and can no longer readily pass water. This is noted by an increase in headloss on the filter gauge. When this occurs the filter flow must be reversed in order to flush the solids and precipitates out of the filter media. The backwash expands the bed 30 to 50%, which allows the lighter floc and foreign matter to be carried off, and also allows the sand particles to scour each other. A backwash is typically initiated when the design limit on headloss (approximately 4 psi normally), operating time, or filter effluent turbidity is reached.

The backwash water must be distributed in a uniform pattern and in a uniform volume across the total horizontal cross-section of the filter.

The backwash water must be clean filtered water. The water can be obtained from other parallel filter units, or as clean water from storage.

For municipal water systems, backwashingA is typically a manual operation, where an operator turns the valves. However, some systems have been fully automated; these systems typically initiate on time, perhaps once a week.

Ineffective backwash allows the filter media to become more permanently fouled/coated. Such media will require replacement to maintain performance. Therefore, there are various techniques to improve and enhance backwash.

Backwash processes include:

Water‐only backwash, which is simplest and most common, especially for smaller systems. Filter flow is reversed, with clean water pumped through media at relatively high rates. An ideal backwash flow will expand a filter bed 20‐50 percent, allowing particulates caught in voids to be carried up and out of the filter.
Surface‐wash sweeps. Surface‐wash sweep systems use rotating “sweeps” that spray water at high velocities at the surface of the media during backwash. The sweep causes the media grains to collide, which helps remove contaminants stuck to the media. See below for more on the surface wash.
Sequential air scours and water. The use of air during backwash has become more common during the last 30-40 years. Developed for the wastewater market, air scour reduces the amount of energy required to fluidize media and reduces the amount of backwash water and backwash rate (GPM/ft2). For sequential and simultaneous air‐water backwash, the air is introduced below the media and allowed to rise through the media. The rate at which air rises through the water creates high levels of energy that provide for aggressive contact between media grains. Compared with water‐only or surface‐wash techniques, the procedure significantly improves the amount of adhered contaminants removed from the media. In sequential air‐water backwash, all water flow is stopped, and only air is introduced to the media bed. After a few minutes, the air is discontinued, and normal water backwash begins.
Simultaneous air scours and water. To improve the sequential air‐water backwash process, a combined air and water backwash process was introduced. This method uses the added energy created by air and maintains water flow to carry more contaminants up and out of the filter bed. The process also allows lower backwash flow rates, and desired bed expansion is accomplished by lift supplied by the air.

To use combined air and water backwash, some form of the media‐retaining system must be installed on the backwash collection troughs. Such systems separate the flow of air and water and minimize the amount of media carried out of the filter during backwash.

The typical nominal backwash rate is 15 GPM/ft2. The backwash rate is typically lower when a surface wash is employed. A backwash rate can be optimized based on taking cores samples of the filter before and after the backwash. A proper backwash rate is critical; a too low rate will not properly clean the bed, regardless of backwash duration (as the bed will not be properly expanded); too high a rate will disrupt the bed, reducing filter performance and effectiveness. The typical backwash time is 8 to 10 minutes.

Sub-optimal backwash can lead to the formation of mudballs, spherical aggregates of solid material (typically one inch in diameter) formed by the gradual buildup of material not removed by the backwashing process. They typically form at or near the surface of the sand but will begin to sink down if the problem is not addressed. A surface wash can help prevent the buildup of mudballs, see below.

It is important to monitor pressure buildup in backwash supply lines. If there is an increasing pressure trend, passages in the underdrain are reduced or become plugged.

Filter media particles are designed to be rough and jagged, to minimize void fraction and provide more sites for floc collection. The backwash process is designed to have the sand particles scour each other; over time this action will smooth and round the particles, reducing performance. Media being worn smooth is an indication the media should be replaced.

Usually, 0.5‐1 in./yr of media loss can be expected. Filter media depth plays a crucial role in producing good effluent water quality. As media depth is lost, overall filtration capacity diminishes, leading to shorter filter runs and contaminant breakthroughs. Therefore, overall bed depth should be measured periodically and additional media installed to maintain proper depth.

The wash-water troughs are placed above the filter media to collect the wash-water, floc and suspended matter recovered during the backwash procedure. The troughs should be slightly above the maximum expansion of the filter bed when it is being washed.

Surface Wash

The benefits of surface washing filter media as supplemental to conventional backwashing have long been recognized. The prevention of mud ball formation, cracks, particle growth, and cementation, as well as an increased length of filter runs, are some of the benefits. The surface wash will quickly disperse the accumulation of material on top and in the upper few inches of the filter media. This allows for quicker expansion of the filter bed (by the backwash) and a shorter backwash cycle. Several different methods have been employed, but the rotary surface wash has established itself as the preferred method.

The basic concept is to employ self-propelled (water-driven) rotatable pipe arms situated just above the surface of the bed. The arms are supplied with high-velocity nozzles, and the velocity of the water leaving the nozzles causes the arms to rotate, in the manner of a revolving lawn sprinkler. The high-velocity water is directed backwards and downwards, and loosens material adhering to the sand, and breaks up incipient mud balls, allowing the backwash water to carry out the disintegrated foreign matter.

Vendors also market agitators that operate sub-surface. The subsurface sweep delivers supplemental scouring action, typically at the sand/anthracite interface of dual media filters. Vendors include Leopold (part of Xylem) and Roberts (Style SW).

Solutions to Your Water Treatment Needs

Whether you want a new water treatment system installed, need your current one inspected and repaired, or just want a free quote, contact the WETS LLC. With years of experience in the industry, our skilled team members are trained and knowledgeable with a variety of leading water equipment and products. Our certified water purification experts are here to help.