Aeration

Introduction

The aerator is designed to perform two basic functions, the stripping of volatile compounds from the water (typically H2S and methane), and introduction of oxygen into water for oxidation of undesirable compounds (typically iron and manganese).

Equipment which is only serving a stripping function is often referred to as a VOC Stripper or Gas Stripper. There is no significant difference in design/configuration of the aerator itself in stripping versus oxidation, and this manual shall use the term “aerator” for equipment serving either or both of these stripping and oxidation functions.

In the typical aerator design, the aerator is elevated on a dedicated structure and treated water gravity drains into the ground storage tank (GST), sometimes through filters. This shall be referred to as an Elevated Aerator. There have been designs where the aerator is on the ground and the treated water is pumped to the GST (Ground Aerator).

There were designs where the aerator was mounted atop a holding tank or the filters themselves, but this is infrequently employed today, as it is generally cheaper to provide a dedicated structure for aerator elevation than to design equipment to support the weight of a fouled aerator.

Aeration Services

Aeration of water is a mechanical method of removing gases such as hydrogen sulfide, methane, carbon dioxide and odors which may be dissolved or entrained. It is also an important method of oxidizing dissolved metals such as ferrous iron and manganese. It has been used for decades for the treatment of water, in particular well water / ground water. The vast majority (upwards of 90%) of Texas public water systems have ground water sources (as opposed to surface water).

The WETS aerators perform two basic services, the stripping of volatile compounds from the water, and introduction of oxygen into water for oxidation of undesirable compounds. Typically, in the WETS designs, aerators have been designed for the stripping of H2S, methane and carbon dioxide, and for oxidation of iron and manganese, with the resulting insoluble oxides removed in downstream filters (or allowed to settle in GST). There is no significant difference in design/configuration of the aerator itself in stripping versus oxidation, and in many designs both functions are being performed.

Stripping of Dissolved Gasses

Methane is easily removed by aeration, but care must be taken to provide dispersion of the methane into the atmosphere away from any source of sparks or flame.

It should be noted that methane is not considered a VOC by state or federal regulations so the aerator vent can be discharged to atmosphere without any treating.

Hydrogen sulfide present as a dissolved gas can be removed by aeration but alkaline sulfide requires further treatment (typically oxidation or chlorination).
Proper pH control during aeration is necessary to get high percentage removal of the sulfides, with removal efficiency improving with lower pH. At a pH of 5, 98% of H2S can be removed by stripping, while at a pH of 8, less than 10% of H2S can be removed by stripping. High alkalinity also limits stripping performance.

However, even if stripping is not complete, chlorine addition downstream of the aerator can eliminate remaining sulfide by conversion to sulfates. (One advantage of aeration, in presence of downstream chlorine injection, is to significantly reduce chlorine consumption; it typically requires 7 parts chlorine per part H2S for chlorine oxidation of H2S).

In an aerator, sulfide removal is mostly by stripping H2S; the sulfides can be oxidized, but reaction is slow. So, limited additional H2S removal can occur downstream with aerated water, and, at higher pH, sulfur precipitates can form due to the oxidation of the sulfides.

Ground Aerators stripping H2S are often provided with vent stacks to allow better dispersion of H2S and avoid odor concerns.

Aeration is an excellent method to reduce excessive free carbon dioxide, with typical residual CO2 concentrations of 8 to 15 ppm.

Stripping of CO2 raises the pH of the water, which lowers possible percentage removal of H2S; the CO2 is stripped out of water much more easily than H2S. The stripping of CO2 and increase in pH of the water is often an aid in corrosion control.

Oxidation of Iron and Manganese

WETS has designed numerous aerators for 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. 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 Texas Department of Health

These elements (iron and manganese) pose aesthetic concerns and are not a health related problem.
Public water systems which 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.

The letter goes on to include enhanced aeration as one of four recommended means of oxidation.

One ppm of dissolved oxygen will oxidize seven ppm of iron or 3.5 ppm of manganese. Approximately 7 mg/l of oxygen is typically dissolved in water during aeration at ambient temperatures.

Iron and manganese removal improves with higher pH; therefore, aeration itself, which often raises pH due to stripping of CO2, improves iron and manganese removal. Between iron and manganese, iron oxidizes better at lower pH’s.

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.

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

Other Aeration Services

The vast majority of WETS Aerator projects have targeted H2S, methane, iron, and/or manganese. However, WETS aerators have been employed in other services, as follows:

The stripping of trihalohydrocarbons typically requires a greater packing height than the standard of 7 feet.

The trihalohydrocarbons are byproducts of chlorine treatment, where the chlorine reacts with organic and inorganic matter in the water. Chlorination is often downstream of the aerator. For systems with trihalohydrocarbon problems, chlorination upstream of the aerator has helped, as there is stripping of the compounds from the water in the aerator. However, in general, it is preferred to chlorinate after aeration as this lowers chlorine consumption and keeps aerator cleaner.

WETS has executed several projects using aerators for benzene or BTEX (benzene, toluene, ethylbenzene, xylenes) removal, often in conjunction with iron and manganese removal. On these projects, WETS typically. performed pilot studies to confirm aeration could remove benzene/BTEX to acceptable levels.

Radon is a naturally occurring radioactive gas formed from the breakdown of uranium in soil and rock. High levels of radon can enter groundwater flowing through granite or granitic sand and gravel formations. Radon is considered the second leading cause of lung cancer in the US. As a gas, radon can be removed by aeration/ stripping orby activated carbon filters.

There are still no federally enforced drinking water standards for radon, but the EPA has proposed regulations. In the proposed standard, the radon level in water will depend on whether the state has an indoor radon air program. States with such a program would have a standard of 4,000 picocuries/liter (pCi/l). States without a program would have a standard of 300 pCi/L.

Since radon is a gas, aeration is a good process for radon removal, and WETS has designed and installed aerators for radon removal for several clients.

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

Manganese greensand filters have been found more effective in radium removal than dual media.

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 a MCL of 15 pCi/L for alpha particle activity, excluding radon and uranium, in drinking water.

Aerator Types and Design Alternatives

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

In aerators, high efficiency gas removal is obtained by supplying surfaces (packing or trays) that provide for thin films of water. A counter-current flow of air sweeps through the water, carrying away released gasses and supplying oxygen for oxidation processes. The air is typically provided by one or more blowers.

Aeration of well water does pose a contamination risk, and the unit must be properly screened and protected from contaminants.

Aerator designs can differ in the means the air is introduced, how the water is introduced, the vessel internals and cylindrical vs rectangular vessels. WETS designs have generally been positive draft, with air introduced by a blower, with either dumped packing or slat trays.

The water is introduced by a liquid branched distributor; in early designs, a distributor tray was often used. There is no sparger for the air, given the relatively low gas rates and low pressure drop in the system.

Elevated vs Ground

The aerator is typically elevated on a dedicated structure (the Elevated Aerator), with the treated water gravity drained to the GST or a holding tank. In some designs, the aerator is on the ground (the Ground Aerator), and the treated water is pumped to the GST (or further processing/filtration). Water surge is provided at the bottom of the aerator for pump suction in the Ground Aerator. Ground Aerators are typically client preference, where the water treating facility is near the residential community and the visual of the elevated aerator is considered undesirable.

For ground aerators, vessel sizing was typically based on a water surge time of 6 to 7 minutes.

Cylindrical vs. Rectangular

The WETS standard for the aerator is a cylindrical fiberglass vessel, which is easier to fabricate than a rectangular vessel. Historically, rectangular vessels were often used, sometimes on client request.

Water-Air Contact

The WETS design will typically use a bed of dumped plastic packing for the contacting of the water and air. 3.5” Lanpac-XL is generally preferred. 2” plastic packing (first, polypropylene Koch flexi saddles, later polypropylene NOR-PAC) was used in earlier designs.

It was at one time felt that the slat trays were better for precipitating services than packing, especially when compared to the 2” saddles. However, with the 3.5” Lanpac packing, performance is considered equivalent, and slat trays are being used less frequently. Slat trays were typically redwood, but could also be fabricated from aluminum.

Coke tray aerators had been used for removal of iron and manganese. The internals typically consisted of two trays with 2.5” x 5” coke at a depth of 9”. These tray aeration systems are very susceptible to algae and slime growth and are now infrequently used.

Blower Design

The forced draft aerators designed by WETS employ blowers that supply air to the aerators at the base. Induced draft aerators, where a top mounted blower induces air flow through the chamber at slightly less than atmospheric pressure, have been utilized in the industry, but are not preferred by WETS (the induced draft blower sees the aerator exhaust gas, which is likely to be corrosive)

Natural Draft

There have been limited applications for natural draft aerators (no blower), and WETS has provided several such aerators over the years. Natural draft aerators are typically designed for 10 gpm/ft2 and constructed of redwood. They are louvered, resembling a cooling tower.

Forced draft aerators are generally used in preference to natural draft aerators because of better performance over all weather conditions, and the absence of appreciable drift. Also, the fiberglass aerator will have a much longer life expectancy than a redwood aerator.

Aerator Alternatives

The forced/induced draft aerator is one option for providing contact of the water with oxygen. There are alternatives, such as ozonation and pressure aeration.

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