Comparing Air and Steam Stripping

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High concentrations of ammonia and other volatile organic compounds (VOCs) in wastewater pose a significant risk to the environment. That’s why so many industries have turned to wastewater stripping techniques to remove these harmful substances from their water streams.

There are two primary types of wastewater stripping processes — air stripping and steam stripping. Though both removal methods are designed to accomplish the same general task, they have several fundamental differences between them.

What Does Wastewater Stripping Look Like?

Some water contains high levels of ammonia or ammonia-forming nitrogen compounds. Wastewater stripping, also known as ammonia stripping, lowers the ammonia content in a wastewater stream through desorption or releasing a substance through the water’s surface. This method is relatively straightforward and doesn’t generate backwash or ammonia regeneration. Wastewater fluctuations and toxic compounds do not disrupt ammonia stripping as they may in biological removal systems.

Ammonia can present itself in wastewater in one of two forms — ammonium ions or ammonia gas. These substances’ concentrations depend on the wastewater’s temperature and pH. In most cases, removing nitrogen from wastewater in the form of ammonia is easier and less expensive than converting it to nitrate-nitrogen before stripping it.

Ammonia stripping requires a high pH level — applying lime or caustic sodium hydroxide raises the water’s pH level. In response, the wastewater’s pH level increases, converting the ammonium hydroxide ions to ammonia gas.

The wastewater stripping process is most effective with wastewater containing ammonia contents between 20 and 100 milligrams per liter. It involves using one of two methods — air stripping and steam stripping.

Ammonia Stripping Pros and Cons

The advantages and disadvantages of ammonia stripping compared to other ammonia removal processes include the following.

Pluses

When doing ammonia stripping, consider the following assets:

  • It is a relatively simple process. Wastewater fluctuation does not affect operation if the pH and air temperature remain stable.
  • It is a mechanical procedure and does not create backwash or regeneration.
  • The operation remains unaffected by toxic compounds that can disrupt a biological system.
  • It is a controlled process designed for specific ammonia removals.

Minuses

Ammonia stripping has some potential downsides:

  • You need to re-pump water to the stripping tower, which requires higher maintenance and power.
  • In some cases, you can’t remove scale formation hydraulically. Pilot testing in locations can determine this.
  • You can only perform ammonia stripping in freezing conditions with heated air. Foggy and icy conditions diminish the ammonia removal process.
  • Low levels of ammonia discharge into the atmosphere during the stripping process. These emissions may be unacceptable in specific locations due to regulations and quality concerns.
  • Ammonia stripping doesn’t remove nitrite and organic nitrogen.
  • When ammonia reacts with sulfur dioxide, it can cause air pollution.
  • It is a noisy operation, which can be disruptive.

Air Stripping

Air stripping involves moving air through contaminated wastewater, including surface water or groundwater, in an above-ground treatment system. This process removes volatile organic compounds, exposing the contaminated wastewater to uncontaminated airflow and releasing the compounds into the atmosphere. This wastewater stripping method is a common pump-and-treat remedy to remove contaminants from groundwater.

VOCs evaporate easily, so passing air through contaminated water speeds up the vaporization process. Once the air and chemical vapors accumulate, the system either removes them from the water or vents them into the air, depending on VOC levels.

Air stripping typically occurs in a packed column tower containing three primary parts. They comprise a manifold or spray nozzle on the top of the column, which distributes the water over the packing, a blower to move the air opposite the water current and a sump that collects decontaminated water at the base.

You can also air strip with a low-profile sieve tray aeration system. These air strippers contain baffles and weirs within the trays that channel the contaminated water along the box two or more times. Air collected at the bottom of the system then travels through the water in a countercurrent direction to eliminate the VOCs.

The air and vapor collect at the top of the tank for treatment and release. The treated water flows to the bottom of the tank, where it collects for testing to ensure it meets cleanup requirements. It may require several treatments to reach the appropriate level.

Once the water meets the standards, you can discharge or reuse it. There are various ways to do this, depending on the site:

  • Pump it into a nearby stream or river or underground using injection walls and trenches.
  • Distribute it over the ground using a sprinkler system so it seeps into the soil.
  • Release it into the public sewer system.
  • Reuse the water.

Flowing water through an air stripper or aeration tank can take a few minutes — the tank’s size and flow rate will determine how long it takes. Air stripping can take several months or years if there are high contaminant levels or you need to treat a high volume of water.

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Steam Stripping

Steam stripping is a wastewater distillation process that removes substances like benzene, xylenes and chlorinated hydrocarbons from the water using high temperatures that eliminate heavy soluble organic compounds. Industrial facilities, such as petrochemical plants and petroleum refineries, often use this process.

Steam stripping uses heat to reduce a pollutant’s partial pressure, generating a positive mass transport between the water and gas through steam injection. It is most effective for reducing VOCs with lower boiling points than water and those with limited solubility.

In a standard steam stripping column, liquid enters through the top of the tower while the steam input moves through the bottom. The water travels into the column and flows downward and through the packing in a countercurrent motion to the steam. The packing then increases contact between both streams. Once the vapor leaves the stripping tower, it either collects for purification or evaporates into the atmosphere.

Air Stripping vs. Stream Stripping

Though air stripping and steam stripping accomplish the same general task, these processes are decidedly different, each with unique advantages and disadvantages. Knowing their differences can influence your choice for your particular applications.

Air Stripping Pros and Cons

The air stripping process has many advantages.

  • Application versatility: Numerous industries rely on air stripping, including organic and inorganic chemistry, pharmaceuticals, glass engraving, graphics and viscose production.
  • Cost efficiency: This wastewater stripping method is highly cost-effective. The desired purification yield and effluent concentration influence the cost, determining the number of towers that must go in the series.
  • Size variation: Air strippers are available in numerous sizes, meaning you can choose one that best suits your space.
  • Capabilities: Air strippers are effective at a broad range of airflow rates and can achieve high removal rates at low-pressure drops.

Though air stripping has any benefits, it also has its disadvantages:

  • Susceptible to pollution
  • Quick to foul
  • Must remove and wash bed packing with acid
  • Usually requires a very tall column for optimal efficiency, which can boost construction costs

Steam Stripping Pros and Cons

Like air stripping, steam stripping also has several benefits.

  • High removal rates: Steam strippers achieve high removal rates and low effluent concentrations.
  • Minimal emissions: The steam stripping process reduces air emissions and load incineration.
  • Economical solution: This VOC removal technique is highly affordable at specific feed concentrations.
  • Easy solute recovery: Steam strippers can recover solutes efficiently.

The cons of the steam stripping process are as follows:

  • Subject to fouling
  • It uses electricity in the process and has a high energy consumption.
  • The lime residue left on the tower can pollute the stripping tower.
  • Not useful for stripping glycols, glycerine, phenol, ethylene, propylene and ionic compounds
  • Cannot strip inorganic compounds aside from those in gaseous, dissolved form

Learn More About Air and Stream Stripping Services at MACH Engineering Today

Wastewater stripping techniques are vital for removing harmful substances from the water we use daily. While air stripping and steam stripping achieve the same general task, they are ideal for different use cases.

You can secure high-quality air or steam stripping technologies that last at MACH Engineering. Our design and manufacturing company has spent over 25 years creating innovative wastewater treatment applications, odor control systems and air and water emission control vessels that customers trust worldwide.

We pride ourselves on catering to our customers with customized products geared to their unique needs while exemplifying our quality engineering techniques. When you partner with MACH Engineering, we’ll deliver the most flexible and practical designs that keep up with industry standards, whether you work in chemical processing, pharmaceuticals, metal finishing or anything in between. Contact us today for assistance on a current or upcoming project!

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