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 the Wastewater Stripping Process Look Like?
Wastewater stripping, also known as ammonia stripping, is the process of lowering the ammonia content in a wastewater stream through desorption, or the release of a substance through the water’s surface. This stripping 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. The concentrations of these substances are dependent on the wastewater’s temperate and pH. Ammonia stripping requires a high pH level, which can be achieved by applying lime or caustic to the water. In response, the wastewater’s pH level increases between 10.8 and 11.5 standard units, converting the ammonium hydroxide ions to ammonia gas.
The wastewater stripping process is most effective with wastewater containing ammonia contents between 10 and 100 milligrams per liter (mg/l). It involves the use of a stripping tower, of which there are two types — air stripping and steam stripping.
Air stripping is the process of moving air through contaminated wastewater, including surface water or groundwater, in an above-ground treatment system. This process is used to remove volatile organic compounds, exposing the contaminated wastewater to uncontaminated airflow and releasing the compounds into the atmosphere. This wastewater stripping method is commonly used as a pump-and-treat remedy to remove contaminants from groundwater.
VOCs evaporate easily, so when air passes through contaminated water, it speeds up the vaporization process. Once the air and chemical vapors accumulate, they are either removed from the water or vented into the air, depending on VOC levels.
Air stripping typically takes place 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 of the tower.
You can also carry out the air stripping process through a low-profile sieve tray aeration system. These air strippers contain baffles and weirs within the trays that channel the contaminated water along the length of 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.
Steam stripping is a wastewater distillation process that removes VOCs from the water using high temperatures that enable them to eliminate heavy soluble organic compounds. The steam stripping process removes substances like benzene, xylenes and chlorinated hydrocarbons. It is often used in industrial facilities, such as petrochemical plants and petroleum refineries.
Steam stripping operates on the principle of using 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 are used to accomplish the same general task, these processes are decidedly different, each with its own unique advantages and disadvantages. Knowing the differences between them can influence which you choose for your particular applications.
Air Stripping Pros and Cons
The air stripping process has many advantages, including:
- Application versatility: Air stripping is used in numerous industries, including organic and inorganic chemistry, pharmaceuticals, glass engraving, graphics, viscose production, and more, making it a highly versatile process.
- Cost efficiency: This wastewater stripping method is highly cost-effective. Implementation costs are influenced by the desired purification yield and effluent concentration, which determines 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.
- VOC removal: The air stripping process is proven to eliminate 99% of VOCs.
- 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 pros, such as:
- High removal rates: Steam strippers achieve high removal rates and low effluent concentrations. Like air strippers, they can also eliminate 99% of VOCs.
- Minimal emissions: The steam stripping process reduces air emissions and load incineration.
- Economical solution: This VOC removal technique is considered highly economical at certain feed concentrations.
- Easy solute recovery: Steam strippers are able to recover solutes simply and efficiently.
The cons of the steam stripping process are as follows:
- Subject to fouling
- Consumes large amounts of energy
- Risk of lime residue
- 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
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 come to 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 for assistance on a current or upcoming project today!