Process air discharged from a waste management facility may contain ammonia, volatile organic compounds, and other substances that must be controlled before the gases are released into the atmosphere. Depending on the type of waste being processed, the technology used, and operating conditions, the composition and concentrations of pollutants can vary significantly. Therefore, the selection of an air purification system should begin with an assessment of the gas stream’s characteristics, rather than with the selection of a specific piece of equipment.
One of the solutions used in such systems is a biofilter with a scrubber, in which the process air first passes through a wet cleaning stage and then through a biological bed. The scrubber may use water or a suitably selected chemical reagent solution. The differences between these variants relate to the mechanism of contaminant removal, the method of process control, and operational requirements.
Is a water scrubber sufficient to reduce ammonia concentrations? When is it appropriate to use an acid scrubber? We explain which parameters to consider when choosing a solution and why the configuration of the entire process system determines the effectiveness of the treatment.
Why is a pre-wash used before the biofilter?
A scrubber upstream of the biofilter is used to pre-treat or condition the process air before it is directed to the biological bed. Depending on the system configuration, the unit can reduce the concentration of specific gaseous pollutants, remove some particulate matter, and regulate the humidity of the air stream.
A biofilter utilizes the activity of microorganisms colonizing a specially prepared bed. Substances present in the passing air can be adsorbed by the moist biological layer and then undergo microbial transformation.
However, the operating conditions of a biofilter depend on the composition of the inlet gas, the pollutant load, humidity, temperature, and the properties of the filter media. Not every process air stream should be directed directly to biofiltration without prior analysis.
A scrubber can serve as a stage for preparing air for further treatment. This is particularly important in the case of gas streams containing ammonia (NH3), whose concentration and charge can affect the operating conditions of the biological bed.
Is a scrubber an essential component of every biofiltration system?
No. The biofilter can operate without a pre-scrubber if the air characteristics and the specified operating conditions allow for this arrangement.
The decision to use an additional treatment stage should be based on an analysis of the gas composition, the expected outlet parameters, and the biofilter’s operating capacity.
In some systems, proper air treatment and humidification may be sufficient. In others, it will be appropriate to use a scrubber designed to reduce the concentration of specific compounds.
The mere presence of a biofilter does not, therefore, determine the need to install a scrubber. Similarly, the use of a scrubber does not automatically guarantee that the required treatment parameters will be achieved.
How does a water scrubber work?
A water scrubber is a device in which pollutants contained in a gas stream are transferred to the water with which it comes into contact. The process is based on absorption, that is, the transfer of specific substances from the gas phase to the liquid phase.
Process air flows through the unit, where it comes into contact with water supplied in a specifically designed manner. Depending on the design, this contact may occur within a packing bed, through liquid atomization, or in another configuration that increases the mass transfer surface area.
During contact, some of the gaseous pollutants are absorbed into the liquid. The air, which has been purified to a certain extent, leaves the scrubber and can be directed to the next stage, such as a biofilter.
What determines the effectiveness of a water scrubber?
The efficiency of absorption depends primarily on the properties of the substance being removed and the conditions under which the gas comes into contact with the liquid.
Among other things, the following are important:
- the concentration and load of pollutants in the incoming air;
- the solubility of individual compounds in water;
- temperature of gases and liquids;
- pH of the coolant;
- the intensity of contact between the gas and water;
- contact time and mass transfer area;
- the method of replacing, replenishing, or purifying the circulating fluid.
The concentration of substances already present in the water is also important. As these substances accumulate, the conditions for further absorption may change.
Therefore, the evaluation of a water scrubber’s performance should not be limited to air parameters. It is also necessary to take into account the properties of the circulating fluid and how it is managed.
Does a water scrubber remove ammonia?
Yes. Ammonia can be absorbed into water, and the course of this process depends on physicochemical conditions, including pH, temperature, and the concentration of the substance in both phases.
Ammonia is a compound that is highly soluble in water; however, solubility alone does not determine the effectiveness of a particular device.
In an aqueous solution, an equilibrium is established between dissolved ammonia (NH3) and the ammonium ion (NH4+). The proportions of these two forms depend, among other factors, on pH and temperature.
As the properties of the working fluid change, the conditions for the transfer of ammonia from the gas phase to the liquid phase may also change.
In practice, a water scrubber may be a suitable solution for specific process conditions, but its suitability requires an analysis of the expected degree of ammonia removal and the variability of the system’s load.
How does an acid or chemical scrubber work?
An acid scrubber is a type of chemical scrubber in which a gas is brought into contact with a solution of controlled acidic pH to remove pollutants. In the case of ammonia, not only does its absorption into the liquid play a significant role, but so does the reaction that leads to the formation of ammonium ions.
A chemical scrubber is a broader term. It refers to devices that use appropriately selected reagents to remove specific gas components. An acid scrubber is one type of such system.
Therefore, not all chemical slurries should be equated with acid slurries.
What is the chemical process for removing ammonia?
The ammonia present in the process air comes into contact with the acidic solution circulating in the scrubber.
Once it enters the liquid phase, NH3 reacts with hydrogen ions to form the ammonium ion NH4+.
The process can be represented by a simplified equation:
NH3 + H+ ⇌ NH4+
The formation of ammonium ions reduces the proportion of un ionized NH3 in the liquid. Under appropriate conditions, this promotes the further transfer of ammonia from the gas phase into the solution.
If sulfuric acid is used as a reagent, ammonium compounds, including ammonium sulfate, may form in the solution.
The simplified reaction equation can be written as follows:
2 NH3 + H2SO4 → (NH4)2SO4
However, the actual composition of the process fluid depends on the type of reagent, the composition of the gas, the concentration of impurities, and the way the process is conducted.
What is the role of reagent dosing?
In the acid scrubber, the reagent is used to maintain conditions that promote the binding of ammonia in the liquid.
While the unit is operating, the incoming ammonia reacts with the acidic solution, which may cause changes in its pH. Therefore, the system may require controlled acid dosing and continuous pH monitoring.
Depending on the system's design, the dosing system can be integrated with measuring equipment and control systems.
However, this does not mean that maintaining a specific pH is the only condition necessary for the scrubber to function properly. Gas flow, liquid parameters, phase contact conditions, and contaminant load also remain important factors.
Does an acid scrubber always remove ammonia more effectively than a water scrubber?
One cannot accept such a rule without knowing the circumstances of the case.
The use of an acidic solution allows for the chemical binding of ammonia, which can be beneficial under certain load conditions. However, this does not mean that every acid scrubber will be more effective than any water scrubber.
The result is determined by the design parameters, the ammonia concentration and load, the gas-liquid contact conditions, and the required level of treatment.
The assessment should focus on specific technological options, not just the names of devices.
Water-Based vs. Chemical Paint Thinners – Key Differences
The fundamental difference between a water scrubber and a chemical scrubber lies in the method used to remove pollutants. In a water scrubber, absorption by water plays the primary role, whereas in a chemical scrubber, the process is aided by a carefully selected chemical reaction.
This does not mean, however, that no chemical reactions occur in the water wash. Even in pure water, dissolved ammonia contributes to the acid-base equilibrium. The difference lies primarily in the way the process is conducted and the deliberate use of reagents.
Criterion
Water Washer
Chemical/acid rinse
Main mechanism
Absorption of pollutants in water
Chemically Assisted Absorption
The Role of pH
It affects the chemical balance and absorption conditions
It is one of the key parameters monitored during the process
NH3 Removal
Possible depending on the parameters of the gas and liquid
It also utilizes the NH3 bond in its ionic form
Variable ammonia load
Requires an assessment of absorption capacity and fluid circulation parameters
Requires adjustments to the system's operation, including control of reagent dosing
Reagent Dosing
Generally, it does not require the addition of acid to bind NH3
In the acid variant, the appropriate reagent is to be added
Process Control
Gas parameters, liquid flow, pH, temperature, and water quality
Gas and liquid parameters, pH, reagent dosing, and reaction control
Operational Requirements
Circulating Water Management, Water Quality Control, and Equipment Maintenance
In addition, handling of reagents, the dosing system, and post-reaction liquid management
Process fluid
Water containing absorbed contaminants
A solution containing the reaction products and other components absorbed from the gas
Selecting a Solution
Depending on the desired effect and the properties of the gas stream
Depends on the gas composition, the desired effect, and the feasibility of the chemical process
The table illustrates technological differences rather than ranking solutions. At a particular plant, a water-based scrubber may meet the process requirements, while at another, a chemical-based option will be necessary.
When comparing devices, it is also important to consider operating costs, water, reagent, and energy consumption, process liquid management, and operational and safety requirements.
How does pH affect ammonia removal?
pH affects the ratio between un ionized ammonia (NH3) and the ammonium ion (NH4+) in an aqueous solution. Under more acidic conditions, the proportion of the ammonium form increases, which may promote the binding of ammonia transferred from the gas phase.
This relationship is of fundamental importance for the design and operation of scrubbers intended for the removal of NH3.
NH3 and NH4+ Equilibrium
When ammonia is dissolved in water, an equilibrium is established:
NH3 + H2O ⇌ NH4+ + OH−
Under conditions favorable for the protonation of NH3, the proportion of NH4+ ions increases. As the pH rises, however, the proportion of the nonionized form of ammonia increases.
A change in this equilibrium affects the ability of ammonia to transfer between the gas and liquid phases.
In an acid scrubber, maintaining the proper pH can limit the concentration of free NH3 in the solution, thereby promoting further absorption.
Is there a universal pH value for an acid wash?
A single pH value should not be considered a universal operating parameter for all acid scrubbers.
The appropriate range depends on the reagent used, the ammonia concentration, the gas parameters, the instrument configuration, and the requirements for the product or process fluid.
Specific settings should be based on design calculations, process characteristics, and technical documentation.
Why isn't pH alone sufficient to evaluate the process?
A pH measurement indicates the acidity or alkalinity of a liquid, but does not directly determine the total amount of ammonia absorbed by the scrubber.
To evaluate the system's performance, information may also be needed on the NH3 concentration in the incoming and treated air, gas flow rate, reagent consumption, and the properties of the circulating fluid.
Only by compiling this data can we evaluate the process and determine whether the device is operating as intended.
What factors should influence the selection of a scrubber upstream of a biofilter?
The selection of a scrubber should be based on the characteristics of the process air, the required purification performance, and the parameters of the entire process system. It is not enough to simply compare the stated capabilities of two units without considering the actual installation conditions.
The analysis should cover several related areas.
Composition of the inlet air
The first step is to identify which substances are present in the process air and which of them need to be controlled.
In waste management facilities, these may include, among others, ammonia, sulfur compounds, volatile organic compounds, and other substances related to the characteristics of the waste being processed.
Not all compounds will be removed with the same efficiency in the same system. Therefore, the choice of scrubber should be based on the actual composition of the gas.
Concentration and load of pollutants
Concentration refers to the amount of a given substance per unit volume of gas. Pollutant load additionally takes into account the volume of air flowing through per unit of time.
Two systems may have similar ammonia concentrations but differ in their total ammonia load due to different gas flow rates.
This has a direct impact on the selection of the unit size, the volume of circulating fluid, and the reagent requirement.
Variability in process conditions
In waste treatment facilities, the composition and volume of process air may vary depending on the type of waste received, the intensity of biological processes, and the plant’s operational organization.
Therefore, the analysis should not be based solely on a single measurement result.
It is important to determine the range of variation in the parameters and to verify whether the designed system will be able to operate under the anticipated conditions.
Required purification effect
Another step is to specify the expected air quality parameters after purification.
A distinction must be made between requirements for the entire system and parameters assigned to a single stage.
If the scrubber is used in conjunction with a biofilter, its purpose may be to prepare the gas for further treatment, rather than to remove all contaminants on its own.
Characteristics of a Biofilter
When selecting a scrubber, the characteristics of the subsequent treatment stage should also be taken into account.
Factors such as the type of bed, the allowable load, humidity and temperature requirements, and the conditions under which biological processes are carried out may be significant.
The scrubber-biofilter system should be analyzed as a whole, because the parameters of the gas leaving the first stage affect the operating conditions of the second stage.
Operational Capacity of the Plant
A chemical washer requires proper management of reagents, control of dosing, and handling of the process fluid.
In a water scrubber, it is also necessary to ensure proper operation, monitoring, and water management.
When selecting a technology, it is therefore important to consider staff availability, maintenance requirements, reagent storage capacity, and existing infrastructure.
Process Liquid Management
Every wet scrubber requires a procedure for handling the liquid containing the removed contaminants.
Depending on the technology, it may be necessary to periodically discharge, treat, replace, or further manage it.
The solution should be tailored to the characteristics of the liquid and the formal requirements in effect at the facility in question.
Water or chemical washer—the short answer
A water scrubber removes specific pollutants primarily by absorbing them into water. A chemical scrubber additionally utilizes a reaction with a suitably selected reagent. In the case of an acid scrubber, ammonia can be bound in the form of ammonium ions, which affects the conditions for its subsequent absorption. This does not mean, however, that an acid scrubber is always the better solution. The choice should be based on the composition and concentration of pollutants in the air, the required purification effect, the characteristics of the biofilter, and the plant’s operational capabilities. The entire process system should be evaluated, including water and reagent consumption, control methods, and the management of process liquids.
Can the byproduct of the air purification process be something useful?
In certain process configurations, the liquid produced during the chemical removal of ammonia may contain compounds that are potentially suitable for further utilization. However, this is not a universal outcome of chemical scrubber operation.
For example, ammonium sulfate may be formed during the absorption of ammonia in a sulfuric acid solution. This compound has industrial applications, including in fertilizer production.
However, this does not mean that every liquid from an acid leach can be used as a commercial product or fertilizer.
What determines whether a liquid can be used?
The following are particularly important:
- chemical composition of the liquid after the process;
- the concentration of the compounds formed;
- the presence of airborne contaminants;
- stability of parameters during operation;
- technical capabilities for further processing;
- Legal requirements regarding the potential use of the product.
In some cases, further treatment may require additional processing steps. In other cases, the liquid will require treatment appropriate for wastewater or waste.
Therefore, the possibility of recovering usable material should be assessed on a case-by-case basis, rather than assumed as a design premise without proper testing.
The scrubber and biofilter as components of the overall air purification system
A process air purification system should be designed as a network of interconnected devices whose functions are determined by the characteristics of the gas stream and the desired end result. A scrubber and a biofilter can perform various, complementary functions within the system.
In waste management facilities, air is extracted from specific process areas and directed to a treatment system. Depending on the configuration, it may pass through dust removal equipment, scrubbers, conditioning systems, and biological filters.
Each of these elements affects the parameters of the beam directed to the next stage.
Why shouldn't you choose a rinse tank independently of the biofilter?
The scrubber alters the properties of the gas, which is then directed to the biofilter. It can reduce the concentration of certain pollutants and affect the humidity and temperature of the air.
These changes should be consistent with the requirements of the subsequent process.
If the equipment is selected independently, there is a risk that the operating parameters of the individual stages will not be properly matched. Therefore, the analysis should cover the entire process line, rather than just the efficiency of a single scrubber.
What is the importance of automation and monitoring?
In process air purification systems, automation can be responsible for flow control, pump operation, reagent dosing, and monitoring of liquid parameters.
For a chemical scrubber, pH control is one of the key factors; however, measurements of flow, temperature, and other process parameters may also be important throughout the entire system.
Monitoring allows you to observe the operating conditions of equipment and identify deviations. However, it does not replace the proper selection of technology or periodic inspections of the system's condition.
How does AK NOVA approach the design of environmental systems?
AK NOVA carries out waste management projects, combining design, environmental consulting, the selection of technological solutions, and construction.
When designing an air purification system, it is important to consider its role within the entire facility. This includes the characteristics of the processes that generate pollutants, environmental requirements, technical capabilities, and operating conditions.
The selection of a scrubber upstream of the biofilter should therefore be part of a broader technical analysis, rather than a decision based solely on the type of equipment.
If you are planning to build or modernize a waste management facility and need an assessment of a specific air purification system, please contact the AK NOVA team. An analysis of the technical specifications allows us to determine the requirements that the designed system must meet.
FAQ – Frequently Asked Questions
What is the difference between a water rinse and a chemical rinse?
A water scrubber primarily relies on the absorption of pollutants in water. A chemical scrubber enhances this process through a reaction with a carefully selected reagent. The differences lie in the mechanism of substance removal, the control method, and operational requirements.
How is ammonia removed from process air?
Ammonia can be removed by absorption in a liquid. In an acid scrubber, a reaction also occurs that leads to the formation of ammonium ions. The choice of method depends on the concentration and charge of NH3, the desired purification effect, and the system configuration.
Why is a pre-wash used before the biofilter?
A scrubber can reduce the concentration of certain pollutants and prepare the air for further biological treatment. Its use should be based on the characteristics of the gas and the requirements of the biofilter.
Can a biofilter operate without a scrubber?
Yes. A biofilter can operate without a scrubber if the air parameters and process conditions allow for direct biological treatment. The need for a scrubber requires a case-by-case assessment.
How does pH affect the effectiveness of a chemical scrubber?
pH affects the equilibrium between ammonia (NH3) and the ammonium ion (NH4+) in solution. Under acidic conditions, the proportion of the ammonium form increases, which can promote the binding of ammonia in the liquid. The appropriate pH range depends on the specific process.
Do "acid scrubber" and "chemical scrubber" mean the same thing?
No. An acid scrubber is one type of chemical scrubber. It uses an acidic solution to remove specific pollutants, such as ammonia. Other chemical scrubbers may use different reagents and reaction mechanisms.
How do you choose a scrubber for a waste management system?
The selection should take into account the composition and flow of the air, the concentration and load of pollutants, the required treatment effect, the characteristics of the subsequent process stage, and the plant’s operational capabilities.
What information is needed before selecting an air purification system?
The basis consists of information on the sources of process air, its flow, composition, pollutant concentrations, and variations in parameters. It is also necessary to define environmental requirements, existing infrastructure, and the conditions for the discharge and treatment of process liquids.