An aerator performs two basic functions: stripping volatile compounds from water (typically H2S and methane) and introducing oxygen for oxidation of undesirable compounds (typically iron and manganese). Equipment serving only a stripping function is often called a VOC Stripper or Gas Stripper — there’s no significant design difference between stripping and oxidation service, so this discussion uses “aerator” for equipment serving either or both functions.
In a typical design, the aerator is elevated on a dedicated structure with treated water gravity-draining into the ground storage tank (GST), sometimes through filters — referred to as an Elevated Aerator. Some designs place the aerator at ground level with treated water pumped to the GST (a Ground Aerator). Earlier designs sometimes mounted the aerator atop a holding tank or the filters themselves, though this is infrequent today, since a dedicated elevation structure is generally more economical than engineering equipment to support the weight of a fouled aerator.
Aeration mechanically removes dissolved or entrained gases such as hydrogen sulfide, methane, carbon dioxide, and associated odors, and also oxidizes dissolved metals such as ferrous iron and manganese. It has been used for decades in treating well water and groundwater — sources that supply upwards of 90% of Texas public water systems.
WETS aerators perform the same two basic services: stripping volatile compounds and introducing oxygen for oxidation. Our designs have typically targeted stripping of H2S, methane, and carbon dioxide alongside oxidation of iron and manganese, with resulting insoluble oxides captured in downstream filters or allowed to settle in the GST. Since stripping and oxidation share largely the same design, many units perform both functions simultaneously.
Easily removed by aeration, though dispersion into the atmosphere must avoid any source of sparks or flame. Methane isn’t classified as a VOC under state or federal regulation, so the aerator vent can discharge to atmosphere without additional treatment.
Dissolved hydrogen sulfide can be removed by aeration, though alkaline sulfide requires further treatment (typically oxidation or chlorination). Proper pH control is essential: removal improves markedly at lower pH, with about 98% removed by stripping at pH 5 versus less than 10% at pH 8, and high alkalinity further limiting performance.
Even where stripping is incomplete, downstream chlorine addition can eliminate remaining sulfide by converting it to sulfates — and aeration ahead of chlorine injection significantly reduces the chlorine required (roughly 7 parts chlorine per part H2S for full chlorine oxidation). Because oxidation of sulfides by aeration alone is slow, sulfide removal in an aerator is achieved mostly through stripping; at higher pH, sulfur precipitates can also form. Ground Aerators handling H2S are often fitted with vent stacks for better dispersion and odor control.
Aeration effectively reduces excess free carbon dioxide, typically to residual concentrations of 8 to 15 ppm. Stripping CO2 raises water pH, which can lower achievable H2S removal (since CO2 strips more easily than H2S), while the resulting pH increase often aids corrosion control.
WETS has designed numerous aerators for iron and manganese removal. Because well water typically isn’t exposed to oxygen, dissolved iron and manganese oxidize on air exposure into insoluble reddish-brown particles that discolor water and affect laundry and plumbing fixtures. Many waters containing iron also contain manganese, though iron is more common.
The EPA’s National Secondary Drinking Water Standards set MCLs of 0.3 mg/L for iron and 0.05 mg/L for manganese. As secondary standards, they address aesthetic considerations — taste, color, and odor — rather than health risk, though states, including Texas, may adopt them as enforceable standards.
A December 9, 1991 letter from the Texas Department of Health notes that iron and manganese pose aesthetic rather than health-related concerns, and that public systems exceeding the established limits must manage these elements to eliminate associated discoloration — with the Department’s preferred approach being to maximize oxidation of these metals, then effectively filter the resulting precipitate. Enhanced aeration is cited as one of four recommended oxidation methods.
One ppm of dissolved oxygen oxidizes roughly seven ppm of iron or 3.5 ppm of manganese, and aeration typically dissolves about 7 mg/L of oxygen into water at ambient temperatures. Because iron and manganese removal improves at higher pH, aeration — which often raises pH by stripping CO2 — supports removal of both; between the two, iron oxidizes more readily at lower pH.
After aeration or chlorination, most soluble ferrous (Fe+2) iron oxidizes readily to insoluble ferric (Fe+3) iron for efficient filtration, while soluble manganous (Mn+2) manganese oxidizes more slowly and may need stronger oxidants or elevated pH to form insoluble manganic (Mn+4) particulates. Other natural contaminants — organics, phosphates, complexing ligands, ammonia, and hydrogen sulfide — can bind with iron and manganese or interfere with their oxidation rates. Recent research also suggests manganese exposure at or above the current standard may be linked to developmental issues in children.
While most WETS aerator projects have targeted H2S, methane, iron, and/or manganese, our aerators have also served other applications:
THM stripping typically requires greater packing height than the standard 7 feet. THMs form as byproducts of chlorination reacting with organic and inorganic matter; chlorination is usually downstream of the aerator, though upstream chlorination has helped strip these compounds in systems with pronounced THM issues. In general, chlorinating after aeration is preferred, since it lowers chlorine consumption and keeps the aerator cleaner.
WETS has completed several aerator projects targeting benzene or BTEX removal, often alongside iron and manganese treatment, typically performing pilot studies to confirm aeration can reach acceptable removal levels.
As a naturally occurring gas formed from uranium breakdown in soil and rock, radon can enter groundwater through granite or granitic formations and is considered the second-leading cause of lung cancer in the U.S. Since radon is a gas, aeration is an effective removal process, and WETS has designed and installed aerator systems for radon removal for several clients — though no federally enforced drinking water standard yet exists (proposed limits are 4,000 pCi/L for states with an indoor radon program, or 300 pCi/L without one).
Radium, formed by uranium decay, isn’t stripped from water but can be removed to some extent through aeration, flocculation, and filtration; iron and manganese precipitation has also been observed, by WETS and others, to pull some radium along with it. Where radium removal proves insufficient, WETS has sometimes recommended a downstream zeolite softener, and manganese greensand filters have shown greater effectiveness than dual media for radium removal. The EPA has set an MCL of 5 pCi/L for combined radium-226 and radium-228, and 15 pCi/L for alpha particle activity excluding radon and uranium.
High-efficiency gas removal in an aerator depends on surfaces (packing or trays) that create thin films of water, with counter-current airflow — typically supplied by one or more blowers — sweeping away released gases and supplying oxygen for oxidation. Because aeration of well water carries a contamination risk, the unit must be properly screened and protected.
Aerator designs vary in how air and water are introduced, the vessel internals, and cylindrical versus rectangular vessel shape. WETS designs have generally used positive (forced) draft, with air from a blower moving through dumped packing or slat trays, and water introduced via a branched liquid distributor (earlier designs sometimes used a distributor tray). No air sparger is typically needed, given the relatively low gas rates and pressure drop involved.
The Elevated Aerator sits on a dedicated structure with water gravity-drained to the GST or holding tank; the Ground Aerator sits at grade with water pumped onward, using a water surge volume at the base sized for roughly 6 to 7 minutes of surge time. Ground Aerators are often chosen where the facility is near a residential area and the visual profile of an elevated unit is undesirable.
WETS’ standard is a cylindrical fiberglass vessel, easier to fabricate than a rectangular one; rectangular vessels were historically more common, sometimes by client request.
WETS designs typically use a bed of dumped plastic packing, most often 3.5" Lanpac-XL; earlier designs used 2" plastic packing (originally polypropylene Koch flexi saddles, later NOR-PAC). Slat trays — once considered preferable for precipitating service — are now used less often, since Lanpac packing delivers comparable performance; trays were typically redwood, sometimes aluminum. Coke tray aerators, once used for iron and manganese removal, are now infrequent due to susceptibility to algae and slime growth.
WETS’ forced draft aerators supply air at the base via blower. Induced draft designs, where a top-mounted blower draws airflow through the chamber at slightly below atmospheric pressure, are used elsewhere in the industry but not preferred by WETS, since the induced draft blower is exposed to corrosive aerator exhaust gas.
WETS has supplied a limited number of natural draft (blower-free) aerators over the years, typically designed for 10 GPM/ft² and constructed of redwood, louvered like a cooling tower. Forced draft aerators are generally preferred over natural draft for their consistent performance across weather conditions, minimal drift, and the longer service life of fiberglass compared with redwood construction.
Forced and induced draft aeration are the primary means of water/oxygen contact, though alternatives such as ozonation and pressure aeration also exist.
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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