Filtration removes suspended and colloidal matter from water passing through a suitable medium, producing water of sparkling clarity. Filtration through sand beds has purified water for thousands of years; today’s municipal standard is the dual media filter, using crushed anthracite coal and sand atop a gravel subfill — sometimes called a rapid sand filter.
Dual media sand-anthracite filters primarily remove turbidity and suspended solids as small as 10 to 20 microns, offering efficient particle removal at relatively high filtration rates. This section covers WETS’ two basic filter types: pressure filters and gravity filters.
On biofiltration: traditional sand, anthracite, and/or activated carbon media can maintain a biofilm capable of degrading organic matter and reducing taste and odor issues. Traditional gravity filters likely carry some degree of biological activity by default; the term “biofilter” typically applies where nutrients such as nitrogen and phosphorus are deliberately used to enhance biological activity. Biofilters have been used in Europe since the 1970s but have seen limited U.S. application and aren’t specifically addressed here.
Much of the objectionable material in water is colloidal in size and won’t be removed by sedimentation or standard filtration alone — coagulant chemicals are used to cause these fine particles to collect into larger, filterable clumps, or floc, using positively charged metal ions to attract the negatively charged colloidal particles that would otherwise repel one another (nearly all suspended particles in natural water carry a negative charge).
Coagulation is affected by factors including pH, salt concentration, temperature, coagulant type, and turbidity, and coagulant aids (flocculants) such as activated silica can improve floc durability or settling speed. Because effective coagulation depends on so many variables, jar testing or field pilot testing is often necessary to determine the best approach for a given water.
To summarize: chemicals using positively charged metal ions to attract negatively charged colloidal particles are called coagulants — common examples include aluminum sulfate (alum), ferrous sulfate (copperas), ferric sulfate, and ferric chloride. Chemicals used alongside a coagulant to improve floc durability or settling are called coagulant aids or flocculants, including anionic polymer, non-ionic polymer, sodium silicate, bentonite, and calcium carbonate.
Natural and man-made organic pollutants can cause color, taste, and odor issues tied to varying amounts of suspended and dissolved material, much of it colloidal — making coagulant use necessary for effective filtration. These issues are typically more common in surface water than well water.
WETS has designed numerous systems for iron and manganese removal, addressing the same oxidation and staining issues discussed above, including the conversion of soluble ferrous bicarbonate to insoluble ferric hydroxide on air exposure. While iron precipitates are generally straightforward to capture in dual media filters, manganese can be more troublesome, particularly at lower pH — for which WETS often uses alternate media such as Greensand and DMI 65, typically evaluated through pilot testing against dual media.
Arsenic is removed to some extent in iron and manganese treatment due to its affinity for insoluble iron oxides, with iron oxide-coated sand serving as an effective adsorbent for arsenic and other metals and metalloids. TCEQ recognizes co-precipitation as an alternative treatment process, subject to more rigorous pilot testing requirements. WETS improvements for arsenic removal may include lowering pH (from 7.5 to 6.5) and increasing ferric chloride concentration; because arsenic levels can be elevated at well startup and taper off over the following hours, tank blending may also help address peak concentrations. Peroxidation is typically used to convert arsenite (As III) to arsenate (As V) before lowering pH, with conversion occurring within about a minute in the pH range of 6.3 to 8.3.
Filtration is the primary treatment process for removing microbial contaminants such as Cryptosporidium and Giardia, typically using granular activated carbon (GAC) media, which offers more attachment sites than sand-anthracite media and therefore tends to host a higher concentration of microbes.
WETS filters have also been applied to radium/radionuclide removal (radium isn’t stripped from water but can be removed to some extent by aeration, flocculation, and filtration, with iron/manganese precipitation also pulling some radium along) and THM removal (where coagulation and filtration target precursor removal, and WETS has successfully installed a number of systems to address THM contamination).
WETS’ two basic filter types are the pressure filter — an ASME pressure vessel typically of low-carbon steel — and the gravity filter, a domed ambient-pressure vessel typically of fiberglass with a body flange several feet from the top.
Modern filters are sometimes called rapid sand filters, distinguishing them from the original slow sand filters, which used no coagulant or backwash, relied on much finer sand, and depended on a surface layer of sediment and bacterial growth for effectiveness — operating at only around 0.1 GPM/ft², versus 3 to 5 GPM/ft² for rapid sand filters. Slow sand filters required periodic removal and washing of the top sand layer and are no longer in use.
Pressure filters are cylindrical; gravity filters may be cylindrical or rectangular, with very large-capacity units often built as long rectangular vessels. WETS gravity filters have historically been relatively small in capacity and typically cylindrical.
This discussion focuses mainly on dual media filters using crushed anthracite and sand; other types include mono-media (sand), multi-media (garnet, anthracite, and sand), birm, and alternate media such as activated carbon. “Effective size” refers to the sieve size in millimeters that permits only 10% of the media to pass; “uniformity coefficient” expresses the ratio between the sieve size passing 60% of the media (by weight) and the size passing only 10%. A typical WETS dual media filter uses 12 inches of anthracite atop 18 inches of sand.
Typical quartz sand specification: effective size 0.35–0.45 mm; uniformity <1.5; specific gravity 2.60–2.65. Finer sand produces shorter filter runs, with most suspended material collecting in the upper few inches of the bed.
Anthracite coal, used alongside or in place of sand, offers longer filter runs (through deeper floc penetration), higher filter rates at equivalent filtrate purity, lower backwash velocities, greater resistance to coating and buildup, and insolubility in acid or alkali. Typical specification: effective size 0.75–0.85 mm; uniformity <1.6; specific gravity 1.50–1.55.
The gravel subfill serves two purposes: acting as a barrier between filter media and underdrain (preventing media loss through underdrain openings) and aiding distribution of the backwash flow. The top “barrier media” layer supports the filter media directly, while lower “dispersion media” layers assist backwash distribution. Only minor gravel displacement should occur during backwash; excessive displacement requires the filter media to be removed and the gravel regraded or replaced.
The underdrain uniformly collects filtered water to maintain a constant filtration rate and directs it to the filter outlet, while — even more critically — evenly distributing backwash water to keep the gravel subfill undisturbed and the media evenly expanded. The ceramic tile Leopold Compound Duplex Tile Filter Bottom is common industry-wide, as are perforated pipe underdrains.
Filter loading rate, expressed in GPM/ft², typically ranges from 2 to 4 GPM/ft², with 4 to 6 GPM/ft² increasingly common and even higher rates possible for debottlenecking — though TCEQ caps filtration rates for iron and manganese removal at 5 GPM/ft² (30 TAC 290.42(b)(2)(A)). Earlier WETS designs typically used around 4 GPM/ft², with 5 GPM/ft² more typical in recent years; WETS designs gravity and pressure filters to similar rates.
Common coagulants include aluminum sulfate (alum), ferrous sulfate (copperas), ferric sulfate, and ferric chloride. Coagulant aids (flocculants) — typically inorganic materials that accelerate coagulation and flocculation, producing quick-forming, dense, rapidly settling flocs — are especially useful for low-turbidity waters and include anionic polymer, non-ionic polymer, sodium silicate, bentonite, and calcium carbonate. Periodic jar testing helps maintain near-optimal coagulant feed rates.
As filter media becomes plugged with solids and precipitates, headloss on the filter gauge rises, signaling the need to reverse flow and flush the media. Backwash expands the bed 30 to 50%, carrying off lighter floc and foreign matter while allowing sand particles to scour each other; it’s typically triggered by a headloss limit (roughly 4 psi), elapsed operating time, or filter effluent turbidity. Backwash water must be distributed uniformly across the filter’s full horizontal cross-section and must itself be clean, filtered water — sourced from parallel filter units or storage.
For municipal systems, backwashing is typically manual, though some systems are fully automated on a set schedule (often weekly). Ineffective backwash leads to permanent media fouling requiring eventual replacement, which has driven development of several enhancement techniques:
Combined air-water backwash requires a media-retaining system on the backwash collection troughs to separate air and water flow and minimize media loss.
Typical nominal backwash rate is 15 GPM/ft², lower when surface washing is used; rate can be optimized via core sampling before and after backwash. A rate that’s too low won’t properly clean or expand the bed, while too high a rate disrupts it and reduces performance. Typical backwash duration is 8 to 10 minutes.
Sub-optimal backwash can produce mudballs — roughly one-inch spherical aggregates that form near the sand surface and gradually sink if unaddressed. Surface washing helps prevent their formation. Monitoring pressure trends in backwash supply lines is also important, since rising pressure signals reduced or plugged passages in the underdrain.
Filter media is designed with rough, jagged particles to minimize void fraction and maximize floc collection sites; the scouring action of backwash gradually smooths and rounds these particles over time, reducing performance and signaling the need for replacement. Typical media loss runs 0.5–1 inch per year, and periodic bed depth measurement — with topping off as needed — helps maintain filtration capacity and avoid shorter filter runs or contaminant breakthrough.
Wash-water troughs, placed above the filter media, collect wash-water, floc, and suspended matter during backwash and should sit slightly above the bed’s maximum expansion during washing.
Surface washing, supplemental to conventional backwashing, helps prevent mudball formation, cracking, particle growth, and cementation, while extending filter run length — quickly dispersing accumulated material on and just below the media surface, allowing faster bed expansion and a shorter backwash cycle. The rotary surface wash has emerged as the preferred method: self-propelled, water-driven rotating pipe arms just above the bed surface are fitted with high-velocity nozzles that both rotate the arms (like a lawn sprinkler) and direct water backward and downward to loosen adhered material and break up incipient mudballs, allowing backwash to carry the disintegrated matter out. Subsurface agitators are also available, delivering supplemental scouring — typically at the sand/anthracite interface of dual media filters — from vendors including Leopold (part of Xylem) and Roberts (Style SW).
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.
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