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Biological Activated Carbon (BAC) systems combine two distinct water treatment processes: physical adsorption and biological degradation. These systems use granular activated carbon (GAC) as both a filtration medium and a substrate for microbial growth. Understanding the theory behind BAC operation is essential for operators, as it helps them manage the system’s efficiency, troubleshoot potential issues, and ensure long-term reliability.

2.1 Adsorption vs. Biodegradation: Mechanisms at Work

Adsorption is the primary removal mechanism at the beginning of a BAC filter’s operational life. In this process, contaminants adhere to the surface of the GAC due to physical forces, such as van der Waals forces and electrostatic interactions. Adsorption is especially effective at removing organic molecules like volatile organic compounds (VOCs), pharmaceuticals, and pesticides, which are hydrophobic and non-biodegradable.

As the GAC becomes saturated and its adsorption sites are filled, biodegradation takes over as the dominant removal mechanism. During biodegradation, microorganisms that colonize the surface of the GAC, forming a biofilm, metabolize organic contaminants. These microorganisms convert complex organic compounds into simpler forms, such as carbon dioxide and water, through oxidation-reduction reactions.

Specific contaminants are processed differently based on their molecular structure:

• Large organic molecules (e.g., humic substances) are first adsorbed onto the GAC and then broken down by microorganisms in the biofilm.
• Small, easily degradable molecules (e.g., alcohols, fatty acids) bypass adsorption and are directly biodegraded by the microbial community.

The combination of these two mechanisms allows BAC filters to provide extended operational life compared to standard GAC systems.

2.2 Biofilm Development and Acclimation Process

The development of a robust biofilm on the GAC media is critical for the long-term success of BAC systems. Biofilm acclimation typically occurs in three stages:

1. Initial Microbial Adhesion: Microorganisms in the water attach to the GAC surface through weak, reversible bonds.
2. Irreversible Attachment and EPS Production: Once attached, microorganisms produce extracellular polymeric substances (EPS), which help them form a more permanent attachment to the GAC.
3. Biofilm Maturation: Over time, the microbial community grows and matures, forming a thick, stable biofilm capable of biodegrading a wide range of organic contaminants.

The microorganisms in the biofilm include heterotrophic bacteria, fungi, and protozoa, each contributing to the breakdown of different types of organic matter. For example:

• Bacteria primarily degrade dissolved organic carbon (DOC), such as carbohydrates and proteins.
• Fungi contribute to the breakdown of more complex organic molecules, such as lignin.

Environmental Factors Affecting Biofilm Formation

Several environmental factors influence the development and stability of the biofilm:

• Temperature: Higher temperatures typically enhance microbial activity, resulting in faster biodegradation. However, extreme temperatures can destabilize the biofilm and reduce treatment efficiency.
• pH: The biofilm microorganisms function optimally within a specific pH range (typically 6.5–8.5). Significant deviations can inhibit microbial growth and degrade system performance.
• Dissolved Oxygen (DO): Oxygen is essential for the aerobic microorganisms within the biofilm. Insufficient DO levels can lead to poor biofilm development and reduced contaminant removal efficiency.

Operators must closely monitor these factors and make adjustments as needed to maintain optimal conditions for biofilm growth.

2.3 Troubleshooting Biofilm Issues

Operators may encounter situations where biofilm formation is disrupted or impaired, leading to reduced system performance. Common biofilm-related issues include:

1. Chlorination Exposure: If chlorine residuals from upstream processes enter the BAC filter, they can kill off the biofilm’s microorganisms. Operators should ensure that chlorine is properly quenched (e.g., with sodium bisulfite) before water enters the BAC system.
2. Poor Oxygenation: Low dissolved oxygen levels can result in inadequate biofilm development, slowing down biodegradation rates. In such cases, operators can increase aeration or adjust flow rates to improve oxygen levels within the filter.
3. Biofilm Overgrowth: Excessive biofilm growth can lead to increased headloss and reduced flow capacity. Operators can manage this by adjusting the backwash frequency or performing a hydraulic bump to release excess biofilm material from the GAC.

Monitoring biofilm activity through tools like biofilm thickness sensors or regular measurement of BDOC (biodegradable dissolved organic carbon) can help operators ensure that the biofilm is functioning properly.

2.4 Case Study: Biofilm Acclimation in a Real-World BAC System

At the North City Pure Water Demonstration Facility, biofilm acclimation was critical to achieving high levels of TOC removal in a potable reuse application. During the initial 6-month acclimation period, operators monitored TOC levels and observed a gradual increase in TOC removal efficiency as the biofilm matured. By the end of the acclimation period, the BAC system was consistently achieving a 38% reduction in TOC, highlighting the importance of biofilm establishment for long-term system performance.

Biological Activated Carbon (BAC) systems are composed of a variety of components that work together to ensure effective water treatment. Each component plays a crucial role in maintaining system performance, monitoring operational parameters, and supporting routine maintenance. Understanding the configuration and function of these components is critical for operators managing BAC systems.

3.1 Overview of Filter Media and System Design

At the heart of any BAC system is the granular activated carbon (GAC) media. GAC provides a large surface area for both physical adsorption and biofilm formation, which are the two key mechanisms of contaminant removal in BAC systems. The design and configuration of the filter bed, as well as the distribution of flow, significantly impact the system’s efficiency and overall performance.

• Filter Bed Depth: The depth of the GAC media plays a significant role in determining Empty Bed Contact Time (EBCT) and the overall treatment capacity of the system. Deeper filter beds provide longer contact times, which improve both adsorption and biodegradation.
• Filter Bed Volume: Operators should regularly measure the volume of the filter bed to ensure it meets the design specifications and maintains consistent performance.

3.2 Key System Components

BAC systems consist of several critical components that must function properly to ensure the system's efficiency. Each component has a specific role in maintaining proper flow, ensuring backwash efficiency, and preventing fouling.

• Valves: Influent and effluent valves control the flow of water into and out of the filter. Operators must regularly inspect these valves to prevent leaks and ensure proper flow control. Additionally, backwash and drain valves play a crucial role in maintaining system cleanliness and resetting the filter after each operational cycle.
• Flow Meters: Flow meters monitor the rate of water flowing through the BAC system. Maintaining an accurate flow rate is critical to ensuring that the system operates within design parameters. Flow meters are often connected to the system's control interface and provide real-time data for operational adjustments.
• Pumps: In pressure-driven systems, pumps are used to maintain consistent pressure and flow through the filter. Centrifugal pumps are commonly used in these systems, and regular maintenance (such as bearing lubrication and impeller inspection) is required to prevent failures.
• Differential Pressure Sensors: These sensors monitor the pressure drop between the influent and effluent sides of the filter. As the filter media becomes clogged with solids and biofilm, the pressure drop increases. Operators can use this data to determine when backwashing is necessary or when additional maintenance is required.
• Underdrain System: The underdrain system plays a critical role in evenly distributing water during filtration and ensuring uniform backwashing. It supports the media bed and prevents clogging in the lower layers of the filter. Any blockages or damage to the underdrain can lead to uneven flow distribution, reduced filter efficiency, and operational issues.

3.3 Control Systems and Instrumentation

Modern BAC systems are equipped with advanced control systems to monitor key parameters, automate processes, and provide real-time data for operational decisions. The integration of SCADA systems allows operators to remotely monitor and control flow rates, pressure, TOC removal, and backwashing processes.

• SCADA Systems: Many large BAC systems use SCADA for centralized control and monitoring. Operators can set flow targets, monitor differential pressure, and initiate backwash sequences from a control room or mobile device. This automation reduces the need for manual intervention and increases operational efficiency.
• Automated Flow Control: Automated valves and flow meters adjust the flow rate in response to changing conditions, such as increasing headloss or changes in influent water quality. This ensures that the BAC system remains within its optimal operating range.

3.4 Gravity-Fed vs. Pressure-Driven Systems

BAC systems can be configured as either gravity-fed or pressure-driven. Each configuration has advantages depending on the facility’s design and operational needs.

• Gravity-Fed Systems: In gravity-fed systems, water flows through the filter by the force of gravity. These systems are typically simpler to operate and maintain because they do not require pumps to move water. Gravity-fed systems are commonly used in large-scale municipal facilities where high flow rates can be achieved naturally.
• Pressure-Driven Systems: Pressure-driven systems use pumps to move water through the filter media under high pressure. These systems are more compact and ideal for facilities with space limitations. However, they require more energy to operate and must be carefully monitored to ensure that pump performance does not degrade over time. Regular maintenance of the pumps and pressure sensors is essential in these configurations.

3.5 System Configurations and Performance Considerations

The configuration of the BAC system significantly impacts its performance, especially in terms of flow distribution and filtration efficiency. Two common system configurations include parallel and series filtration.

• Parallel Filtration: In parallel filtration, multiple BAC filters are operated simultaneously, splitting the flow between them. This reduces the load on each individual filter and allows for more frequent backwashing without interrupting overall system operations. Parallel configurations are often used in high-capacity treatment plants.
• Series Filtration: In series filtration, water passes through multiple BAC filters in sequence, allowing for greater contaminant removal. This configuration is beneficial when targeting high levels of organic or pathogen removal but may require more complex controls and instrumentation to ensure balanced flow through each stage.

3.6 Maintenance Requirements

Each component of the BAC system requires regular maintenance to ensure proper function and avoid system failures. Operators must develop a preventive maintenance plan that includes regular inspection, cleaning, and replacement of worn components.

• Pumps and Valves: Pumps and valves should be inspected regularly for leaks, wear, and blockages. Operators should perform routine maintenance, including bearing lubrication, seal replacement, and flow testing.
• Underdrain Cleaning: The underdrain system must be cleaned periodically to remove any blockages caused by debris or media fines. Regular inspections help ensure that water flow remains uniform during both filtration and backwashing.
• Instrumentation Calibration: Flow meters, pressure sensors, and other monitoring equipment must be calibrated regularly to ensure accurate data. Inaccurate readings can lead to operational inefficiencies and missed maintenance opportunities.

The Biological Activated Carbon (BAC) filtration process is a combination of physical filtration and biodegradation that removes contaminants from water. To ensure the system operates efficiently, operators must monitor several performance indicators, such as headloss, Total Organic Carbon (TOC) removal, turbidity, and Ultraviolet Transmittance (UVT). Understanding these indicators and how to interpret them is critical for maintaining optimal system performance.

4.1 Filtration and Biodegradation Process

During normal operation, water flows through the granular activated carbon (GAC) media, where contaminants are removed by a combination of adsorption and biodegradation. As the water passes through the filter, the following processes occur:

• Adsorption: Organic molecules adhere to the surface of the GAC, particularly during the early stages of the filter's operation when the carbon's adsorption capacity is high.
• Biodegradation: Microorganisms in the biofilm break down organic contaminants into simpler compounds, such as carbon dioxide and water. This biological activity continues even after the GAC's adsorption sites are filled, providing long-term contaminant removal.

The success of the BAC filtration process depends on the balance between adsorption and biodegradation. Operators must ensure that the filter remains clean and functional by monitoring key performance indicators.

4.2 Key Performance Indicators

Headloss

Headloss is the pressure drop across the filter bed as water flows through the GAC media. As the media becomes clogged with particulates, biofilm, and other solids, the headloss increases. Monitoring headloss is essential for determining when backwashing is necessary.

• How It's Measured: Headloss is typically measured using differential pressure sensors installed at the influent and effluent ends of the filter. These sensors provide real-time data on the pressure drop, which can be logged and analyzed to determine trends over time.
• Significance: A gradual increase in headloss is normal as the filter operates, but sudden spikes may indicate an issue, such as air binding or media fouling. Operators should establish a terminal headloss limit—the point at which the pressure drop becomes too high—and initiate a backwash when this limit is reached.
• Troubleshooting: If headloss is increasing more rapidly than expected, it could indicate that the media is fouled or that biofilm overgrowth is causing excessive flow resistance. Operators can address this by adjusting the backwash frequency or using a hydraulic bump to clear excess material.

TOC Removal

Total Organic Carbon (TOC) removal is a critical performance indicator that measures the BAC system's ability to remove organic contaminants from the water.

• How It's Measured: TOC is measured using online TOC analyzers or by collecting samples for laboratory analysis. These analyzers continuously monitor TOC levels at both the influent and effluent, allowing operators to calculate the percentage of TOC removed by the system.
• Significance: High TOC removal rates indicate that both adsorption and biodegradation are functioning well. A decline in TOC removal efficiency may signal an issue with biofilm activity, such as a disruption in microbial growth or insufficient oxygen levels.
• Troubleshooting: If TOC removal is below the target level, operators should check for issues with biofilm development (e.g., insufficient dissolved oxygen levels or exposure to disinfectants like chlorine). Adjusting flow rates or adding oxygen may help improve biodegradation.

Turbidity

Turbidity is a measure of water clarity and is used to assess the filter's ability to remove suspended solids.

• How It's Measured: Turbidity is measured using turbidimeters, which provide a continuous reading of the water's clarity at the filter effluent. High turbidity levels in the effluent may indicate that the filter media is becoming clogged or that particulates are breaking through the filter.
• Significance: Turbidity is closely monitored to ensure that water meets regulatory standards for clarity. If turbidity levels exceed acceptable limits, it may indicate that a backwash is needed or that the filter media is nearing the end of its useful life.
• Troubleshooting: High turbidity can result from inadequate backwashing, media fouling, or excessive flow rates. Operators should adjust the backwash rate or flow rate as needed to maintain acceptable turbidity levels.

Ultraviolet Transmittance (UVT)

Ultraviolet Transmittance (UVT) measures the water's ability to transmit UV light, which is essential for UV disinfection processes downstream of the BAC system.

• How It's Measured: UVT is measured using UV sensors that monitor the transmission of UV light through the water. Higher UVT values indicate that the water is clear and free of particulates, while lower values suggest higher levels of organic matter or suspended solids.
• Significance: UVT is particularly important in systems that rely on UV disinfection as part of a multi-barrier treatment approach. Low UVT can reduce the effectiveness of UV disinfection and may indicate that the BAC system is not removing enough organic matter.
• Troubleshooting: If UVT drops below the target range, operators should investigate the BAC filter for fouling or incomplete removal of organic matter. Adjusting the EBCT or performing a backwash may help restore UVT levels to the desired range.

4.3 Routine Monitoring and Data Logging

Routine monitoring of these performance indicators is essential for maintaining optimal BAC system performance. Operators should establish a data logging system to track changes in headloss, TOC removal, turbidity, and UVT over time. Analyzing these trends can help predict when maintenance or system adjustments are needed before performance declines.

• Headloss Trends: Gradually increasing headloss is normal, but operators should watch for sudden spikes that could indicate a problem, such as air binding or filter media fouling.
• TOC Removal Trends: A steady decline in TOC removal may indicate a biofilm disruption or insufficient oxygen levels. Operators can adjust operational parameters, such as flow rate or backwash frequency, to restore TOC removal efficiency.
• Turbidity and UVT Trends: Monitoring turbidity and UVT trends helps ensure that water clarity remains within acceptable limits, especially if UV disinfection is used downstream. Sudden increases in turbidity or drops in UVT may signal a need for more frequent backwashing or media replacement.

4.4 Real-World Example: Performance Indicators in Action

At a municipal water treatment plant, operators noticed a gradual increase in headloss over several weeks, accompanied by a slight decline in TOC removal efficiency. After analyzing the data, they determined that the biofilm had become too thick, leading to increased resistance to flow. By performing a hydraulic bump and adjusting the backwash frequency, the operators were able to restore normal headloss levels and improve TOC removal. This example illustrates the importance of regularly monitoring performance indicators and taking corrective actions when needed.

Understanding the mathematical calculations related to Biological Activated Carbon (BAC) systems is a crucial skill for operators preparing for the Advanced Water Treatment Operator (AWTO) License exams. Mastery of these calculations enables operators to evaluate system performance, optimize operational parameters, and troubleshoot issues effectively. This section provides key calculations related to BAC system design and performance, complete with examples, problem sets, and real-world scenarios for practice.

Operators must not only perform these calculations accurately but also understand their implications for water treatment processes. This expanded section covers essential concepts like Empty Bed Contact Time (EBCT), Total Organic Carbon (TOC) removal, backwash rates, filter loading rates, and more advanced topics related to system performance optimization.

7.1 Empty Bed Contact Time (EBCT) Calculations

Empty Bed Contact Time (EBCT) is a critical parameter for evaluating the performance of a BAC system. EBCT represents the amount of time the water is in contact with the filter media, expressed in minutes. Longer EBCTs generally result in better removal of organic contaminants, as it allows more time for adsorption and biodegradation to occur. However, excessively long EBCTs can lead to operational inefficiencies and may increase costs without substantial performance gains.

The formula for calculating EBCT is:

• EBCT (minutes) = (Filter Bed Volume (ft3))/(Flow Rate (gpm))

Where:

• Filter Bed Volume is the total volume of the GAC media in cubic feet (ft³).
• Flow Rate is the rate of water passing through the BAC system in gallons per minute (gpm).

Example Problem: A BAC filter has a media bed volume of 500 cubic feet and is operating at a flow rate of 1,250 gallons per minute. What is the EBCT?

EBCT = (500 ft3)/(1250 (gpm)) = 0.4 minutes

In this case, the EBCT is 0.4 minutes, indicating a relatively short contact time, which may require operational adjustments, such as reducing flow rate or increasing media bed depth, to ensure adequate contaminant removal.

Advanced Consideration: In some systems, a longer EBCT (5 to 20 minutes) is required to meet stringent water quality goals, such as the removal of trace contaminants or specific regulated compounds like NDMA. Operators must weigh the benefits of increasing EBCT against the potential downsides, such as lower throughput.

7.2 TOC Removal and Percent Reduction Formulas

Total Organic Carbon (TOC) removal is a key performance indicator for BAC systems. TOC measures the concentration of organic compounds in the water. Operators calculate the percent reduction of TOC between influent and effluent water to evaluate system effectiveness. Consistent TOC removal helps maintain water quality and reduce the formation of disinfection byproducts (DBPs).

The formula for calculating percent TOC reduction is:

• Percent TOC Reduction = ((〖TOC〗_influent- 〖TOC〗_effluent))/〖TOC〗_influent x 100

Where:

• TOC_{\text{influent}} is the TOC concentration in the influent water (mg/L).
• TOC_{\text{effluent}} is the TOC concentration in the treated, effluent water (mg/L).

Example Problem: If the TOC concentration entering a BAC system is 8 mg/L, and the TOC concentration in the treated water is 4 mg/L, what is the percent reduction?

Percent TOC Reduction = ((8-4))/8 x 100 = 50%

In this example, the BAC system is achieving a 50% reduction in TOC, which may or may not meet the system’s performance goals, depending on water quality standards and regulatory requirements.

Additional Example: If the TOC concentration in the effluent is reduced to 2 mg/L, the TOC removal would be:

Percent TOC Reduction = ((8-2))/8 x 100 = 75%

This higher reduction indicates better system performance, which could be achieved through operational optimizations such as increasing EBCT or improving biofilm activity within the BAC filter.

7.3 Backwash Rate and Filter Loading Rate Calculations

The backwash rate is the flow rate used to clean the BAC filter media during the backwash process. This process removes accumulated particulates and biofilm from the filter media, restoring system performance. Proper backwash rates are crucial for maintaining media integrity and operational efficiency. The backwash rate is typically expressed in gallons per minute per square foot (gpm/ft²) of filter surface area.

The formula for calculating backwash rate is:

Backwash Rate (gpm/〖ft〗^2) = (Backwash Flow Rate (gpm))/(Filter Surface Area (〖ft〗^2))

Where:

• Backwash Flow Rate is the total flow rate used during the backwash process (gpm).
• Filter Surface Area is the cross-sectional area of the filter bed (ft²).

Example Problem: A BAC system has a backwash flow rate of 3,500 gpm and a filter surface area of 250 ft². What is the backwash rate?

Backwash Rate = 3500/250 = 14 gpm/ft²

In this example, the backwash rate is 14 gpm/ft², which falls within the typical range of 12-15 gpm/ft² recommended for most BAC systems. This ensures adequate media bed expansion and effective removal of particulates.

Filter Loading Rate Calculation

The filter loading rate is the flow rate of water passing through the BAC filter per unit of filter surface area. It helps determine whether the filter is overloaded or operating within design parameters. The formula for calculating filter loading rate is:

Filter Loading Rate (gpm/〖ft〗^2) = (Flow Rate (gpm))/(Filter Surface Area (〖ft〗^2))

Example Problem: A BAC filter with a surface area of 200 ft² is operating at a flow rate of 2,000 gpm. Calculate the filter loading rate.

Filter Loading Rate = 2000/200 = 10 gpm/〖ft〗^2

In this case, the filter loading rate of 10 gpm/ft² is higher than typical recommendations for BAC systems (4-6 gpm/ft²). Operators may need to reduce the flow rate to avoid overloading the filter and compromising contaminant removal efficiency.

7.4 Case Study Problems and Advanced Scenarios

For hands-on practice, the following case study problems are designed to help operators apply the formulas and concepts discussed in this section. In these scenarios, operators will evaluate system performance based on calculated values and make recommendations for operational adjustments where necessary.

Case Study 1: EBCT Calculation

A BAC system with a media bed volume of 600 ft³ is operating at a flow rate of 1,800 gpm. Calculate the EBCT for this system. Based on the EBCT, evaluate whether the contact time is sufficient for effective organic contaminant removal.

Case Study 2: TOC Reduction

A water treatment plant using BAC treatment has an influent TOC concentration of 10 mg/L. After treatment, the effluent TOC is measured at 3 mg/L. Calculate the percent TOC reduction and determine if the system meets the plant’s performance goals of 80% TOC removal.

Case Study 3: Backwash Rate

A BAC filter requires backwashing, and the backwash flow rate is 4,500 gpm. The filter surface area is 300 ft². Calculate the backwash rate and determine if the rate is within the recommended range of 12-15 gpm/ft².

Case Study 4: Filter Loading Rate

A BAC filter with a surface area of 200 ft² is operating at a flow rate of 2,000 gpm. Calculate the filter loading rate (gpm/ft²) and determine if the loading rate is within the optimal range for the system.

Advanced Case Study 5: Multi-Step Scenario

A BAC system operating in a potable reuse application needs to achieve a target TOC removal of 85%. The influent TOC concentration is 12 mg/L, and the effluent TOC is currently at 4 mg/L. Calculate the percent reduction and determine what operational adjustments (e.g., adjusting flow rates, increasing EBCT) could help the system meet the desired removal target. Additionally, calculate the EBCT if the media bed volume is 700 ft³, and recommend any changes based on the calculated EBCT.

In preparation for the AWTO exam, operators should familiarize themselves with variations of these problems, incorporating different scenarios, such as handling fluctuations in influent water quality or dealing with biofilm overgrowth. Each calculation has direct implications for system adjustments, ensuring that operators are equipped with the knowledge to optimize BAC performance under real-world conditions.

By mastering these calculations, operators will be better prepared to manage BAC systems efficiently and achieve optimal water quality, which is crucial for passing the AWTO exams.

Operating Biological Activated Carbon (BAC) systems requires adherence to specific safety protocols to protect operators, prevent accidents, and ensure the safe handling of equipment and materials. The safety risks associated with BAC systems stem from both the physical properties of granular activated carbon (GAC) media and the biological hazards from microorganisms that thrive within the BAC system. Additionally, operators need to consider the potential dangers posed by chemical processes associated with BAC operations, such as pre-ozonation and chlorine quenching.

This section provides comprehensive guidance on the essential safety procedures and precautions necessary for the safe and efficient operation of BAC systems. A strong safety culture ensures the well-being of operators while promoting reliable system performance.

8.1 Safety Protocols for Handling GAC Media

Granular activated carbon (GAC) is a central component of BAC systems and, while chemically stable, it poses safety risks if not handled appropriately. GAC media is generally supplied in bulk, and improper handling can result in injuries due to heavy lifting, exposure to dust, or mechanical failures. Fine carbon dust, generated during media transfer, can cause respiratory issues if inhaled, while improper handling of large quantities of media can result in physical injuries.

The following safety protocols should be strictly adhered to when handling GAC:

• Personal Protective Equipment (PPE): Operators handling GAC must wear appropriate PPE to minimize exposure to carbon dust and protect themselves during media transfer operations. Recommended PPE includes:
1. Respiratory protection: To prevent inhalation of carbon dust, N95 respirators or other approved masks should be worn when handling dry GAC. In more extensive operations, powered air-purifying respirators (PAPRs) may be used.
2. Eye protection: Safety goggles or face shields should be worn to protect the eyes from airborne dust particles during media handling and transfer operations.
3. Gloves: Chemical-resistant gloves (such as nitrile or neoprene) should be used to prevent skin exposure to carbon dust or irritants.
4. Protective clothing: Long-sleeved shirts, pants, and other protective garments should be worn to minimize skin exposure to carbon dust and any residual contaminants on the GAC.
• Safe Lifting and Moving: GAC media is typically transported in bulk bags or containers weighing hundreds of pounds, making manual handling hazardous. Operators should use mechanical lifting aids, such as forklifts, pallet jacks, or hoists, to transport large amounts of GAC. When manual lifting is necessary, operators must use proper lifting techniques, such as bending at the knees, keeping the load close to the body, and working in teams to prevent strain injuries.
• Avoiding Dust Accumulation: Carbon dust generated during media handling can accumulate on surfaces or in the air, posing respiratory hazards and increasing the risk of dust explosions in confined spaces. Dust control measures should include proper ventilation, dust collection systems, and wetting the GAC media to minimize airborne dust. Operators must avoid using compressed air to clean up spills, as this can disperse dust further into the environment.
• Spill Cleanup: In the event of a GAC media spill, operators should follow established spill containment and cleanup procedures. The use of HEPA-filtered vacuums and wet cleaning methods (using minimal water) are preferred over dry sweeping, as the latter can exacerbate dust generation.

8.2 Biological Hazards and Mitigation

BAC systems rely on biological processes to remove contaminants from water, resulting in the accumulation of microorganisms in the biofilm on the GAC media. While essential to water treatment, these microorganisms may pose health risks to operators if they come into direct contact with biofilm, aerosols, or contaminated surfaces. It is critical to minimize exposure to biological contaminants, especially during maintenance activities.

The following safety precautions should be followed to mitigate biological hazards:

• Avoiding Direct Biofilm Contact: Operators should avoid direct contact with the GAC media where biofilm growth occurs. When performing maintenance tasks such as media replacement, backwash system repairs, or filter inspections, operators must wear appropriate PPE to prevent exposure to microbial contaminants. Respirators, gloves, protective clothing, and eye protection should be worn at all times during such activities.
• Airborne Microorganisms: During the backwashing process, aerosolized particles, including microorganisms from the biofilm, can become airborne. Operators working near BAC filters during backwashing or media transfers should wear respiratory protection (such as N95 respirators) and ensure the area is well-ventilated. If ventilation is insufficient, portable air filtration systems with HEPA filters should be used to reduce airborne contaminants.
• Disinfection Procedures: After completing maintenance or repair activities involving the BAC system, operators should thoroughly wash their hands, forearms, and any exposed skin using soap and water. Where contamination is suspected, work areas and equipment must be disinfected using appropriate cleaning agents, such as quaternary ammonium compounds or bleach-based disinfectants, to prevent the spread of microorganisms to other areas of the plant.
• Preventing Bioaerosol Exposure: In confined or poorly ventilated spaces, operators should be aware of the potential for bioaerosols (airborne biological particles) to accumulate. The use of engineering controls, such as local exhaust ventilation (LEV) and air scrubbers, can help reduce bioaerosol concentrations. Routine air quality testing in confined areas can identify risks of microbial exposure.

8.3 Chemical Handling and Safety

Although BAC systems primarily rely on biological filtration, they are often used in conjunction with chemical treatment processes, such as pre-ozonation or chlorine quenching, to enhance contaminant removal. Improper handling of these chemicals can result in serious injuries, chemical burns, or respiratory distress.

Operators must be trained to handle chemicals safely, including the proper use of PPE and adherence to manufacturer guidelines. Safety protocols for specific chemicals used with BAC systems include:

• Ozone Safety: Ozone is frequently used to pre-treat water before it enters the BAC system, as it increases the biodegradability of organic compounds. However, ozone is a highly reactive and toxic gas that poses serious health risks. Ozone exposure can cause respiratory distress, eye irritation, and skin burns. Therefore, ozone systems must be equipped with:
1. Ozone detectors: These devices continuously monitor ambient ozone levels and trigger alarms if concentrations exceed safe limits (typically 0.1 ppm for workplace exposure).
2. Ventilation systems: Proper ventilation is required in areas where ozone is generated or applied, to prevent the buildup of hazardous ozone concentrations.
3. PPE: Operators working near ozone systems must wear appropriate PPE, including respirators, chemical-resistant gloves, and protective clothing.
• Chlorine Quenching: In systems where chlorine is applied upstream of BAC filters, it must be quenched before entering the GAC media to avoid killing beneficial biofilm microorganisms. Operators handling chlorine must follow established chemical safety protocols, including:
1. Use of neutralizers: Sodium bisulfite is commonly used to neutralize residual chlorine before it reaches the BAC filter. Operators should ensure that dosing equipment is properly calibrated to maintain effective quenching.
2. PPE: Gloves, goggles, and respiratory protection should be worn when handling chlorine and quenching agents.
• Compressed Air Safety: Compressed air is used during the backwashing process to agitate and clean the media. High-pressure air presents risks of equipment damage, personal injury, and noise exposure. Operators must follow the manufacturer’s guidelines for compressed air systems and ensure that:
1. Air lines are inspected regularly for leaks or damage.
2. Pressure relief valves are in place and functional to prevent over-pressurization.
3. Hearing protection is worn when working in close proximity to noisy compressed air systems.

8.4 Preventing Work-Related Illness and Injuries

Aside from chemical and biological hazards, operators must be mindful of general workplace safety practices to prevent injuries and long-term health problems. The following best practices should be incorporated into daily BAC system operations:

• Proper Ergonomics: Operators must use proper ergonomic techniques when lifting, pushing, or pulling heavy equipment, such as GAC media bags or mechanical components. Poor ergonomics can lead to musculoskeletal injuries, which are common in industrial settings. Mechanical lifting aids should be used whenever possible to reduce physical strain.
• Workplace Hygiene and Housekeeping: Maintaining a clean and organized workspace is essential for preventing accidents and contamination. Regular cleanup of spills (both chemical and biological), proper disposal of used PPE, and routine disinfection of shared equipment are critical for ensuring a safe work environment.
• Training and Safety Awareness: Regular training sessions should be held for all personnel involved in BAC operations. These training sessions should cover key safety topics, including media handling, biological hazard mitigation, chemical safety, and emergency response procedures. Operators should also be trained on lockout/tagout (LOTO) procedures to ensure that electrical and mechanical systems are safely deactivated during maintenance.
• Emergency Response Preparedness: Operators must be trained in emergency response protocols for dealing with chemical spills, ozone leaks, or biological contamination incidents. Facilities must be equipped with emergency showers, eyewash stations, and first aid kits in easily accessible locations. Operators should be familiar with the location of these emergency resources and trained to use them properly.

Operators pursuing higher-level certifications, such as Advanced Water Treatment (AWT) Grade IV and V, must have a thorough understanding of advanced topics related to Biological Activated Carbon (BAC) systems. These advanced topics extend beyond the basics of BAC operation, covering detailed operational strategies, system optimization, and long-term management techniques. Mastery of these topics ensures that operators can maintain high levels of system performance and compliance with evolving regulatory requirements.

This section delves into key control parameters, ozone pre-treatment optimization, long-term filter management, media replacement strategies, and emerging contaminants. It is designed to help advanced operators optimize BAC systems to meet current and future challenges in water treatment.

9.1 Control Parameters and Their Effects on Performance

Advanced operators must have a detailed understanding of the various control parameters that influence BAC system performance. Adjusting these parameters allows operators to optimize system efficiency, reduce operational costs, and extend the lifespan of the filter media. This section explores key control parameters and their advanced operational implications.

• Empty Bed Contact Time (EBCT): As discussed in earlier sections, EBCT is one of the most critical control parameters for BAC systems, determining the time water spends in contact with the filter media. For advanced operators, optimizing EBCT is a balancing act between flow rate, media depth, and treatment goals. Adjusting EBCT can be done by increasing media depth or reducing flow rate. For instance, reducing the flow rate can increase EBCT, allowing more time for biodegradation of contaminants. However, this may reduce system throughput and require careful balancing to meet both performance and capacity goals. Advanced operators must regularly assess water quality, contaminant levels, and treatment efficiency to adjust EBCT dynamically.
• Influent Water Quality: The quality of the influent water directly impacts BAC system performance, especially when it comes to organic content, turbidity, and dissolved oxygen levels. Advanced operators should continuously monitor parameters such as Total Organic Carbon (TOC), turbidity, and dissolved oxygen to adjust operational settings in real-time. Elevated TOC levels or spikes in turbidity may necessitate an increase in backwash frequency or adjustments to flow rates to prevent excessive fouling of the filter media. Dissolved oxygen levels are crucial for maintaining a healthy biofilm in the BAC system, and adjustments may be necessary to optimize oxygen supply, especially when ozone pre-treatment is used.
• Backwash Optimization: Efficient backwashing is essential for maintaining BAC system performance, and advanced operators must be adept at fine-tuning the backwash process. Key parameters to optimize include air scour intensity, backwash flow rate, and backwash duration. Operators should adjust these parameters based on real-time performance data, ensuring that accumulated solids are effectively removed without damaging the biofilm. Monitoring the quality of backwash water is also important; advanced operators can use this data to assess the effectiveness of the backwash process and avoid over-washing, which can negatively impact the biological activity within the filter.
• Temperature Effects: Water temperature can significantly affect biological processes within a BAC system. Lower temperatures can slow microbial activity, while higher temperatures can increase biodegradation rates but may also lead to biofilm overgrowth. Advanced operators must monitor seasonal temperature variations and adjust system parameters accordingly. In some cases, adjustments to EBCT or flow rates may be necessary to compensate for temperature-related performance changes.

9.2 Optimization of Ozone Pre-Treatment

Ozone pre-treatment is commonly used in advanced BAC systems to enhance the biodegradability of organic compounds in influent water. By breaking down complex organics into smaller, more biodegradable fragments, ozone pre-treatment improves the overall efficiency of the BAC system. However, optimizing the ozone pre-treatment process requires a deep understanding of ozone dosing, contact time, and its synergy with biological processes.

• Ozone Dosing: Proper ozone dosing is essential for striking the right balance between oxidation and maintaining the biofilm’s microbial activity. Too little ozone may not sufficiently break down complex organic molecules, while too much ozone can lead to the formation of harmful byproducts such as bromate, which can exceed regulatory limits, and can also damage the biofilm by killing essential microorganisms. Advanced operators must regularly monitor ozone residuals and adjust dosing based on influent water characteristics, such as TOC levels, alkalinity, and bromide content. Operators should also consider the impact of changes in flow rate on ozone dosing and adjust accordingly to maintain system balance.
• Ozone Contact Time: In addition to dosing, the contact time between ozone and the water must be carefully managed. Increasing contact time allows for more thorough oxidation of organic contaminants, but excessive contact time can lead to the formation of byproducts like bromate. Advanced operators should continuously monitor the oxidation-reduction potential (ORP) and other key performance indicators to optimize the balance between contact time and flow rates.
• Synergy with Biological Processes: One of the primary advantages of ozone pre-treatment is its ability to enhance biological activity in the BAC filter by breaking down complex organics into more biodegradable molecules. This process improves microbial growth and contaminant removal efficiency. Advanced operators must monitor the health of the biofilm to ensure that the ozone pre-treatment is not disrupting microbial activity. Increased dissolved oxygen levels from ozone can also support biofilm development, but operators must ensure that microbial activity remains balanced and that oxygen levels do not become excessive, as this can cause biofilm overgrowth or disrupt filtration efficiency.
• Minimizing Byproducts: While ozone pre-treatment offers many benefits, it also presents challenges in terms of byproduct formation, such as bromate and aldehydes. Advanced operators must monitor for these byproducts and employ mitigation strategies, such as adjusting pH, controlling ozone dosing, and reducing contact time to minimize their formation. Additionally, advanced monitoring systems such as online bromate analyzers can provide real-time data to help operators maintain compliance with regulatory limits.

9.3 Long-Term Filter Management and Media Replacement

Long-term management of BAC systems is crucial for maintaining performance, optimizing operational efficiency, and meeting regulatory requirements. Advanced operators must be well-versed in filter bed maintenance, media replacement strategies, and biofilm management to extend the operational life of the system.

• Media Lifespan and Replacement: While BAC systems can operate for extended periods without frequent media replacement, granular activated carbon (GAC) media gradually degrades due to physical abrasion and the accumulation of fines. Over time, this reduces the filter’s ability to adsorb contaminants and leads to increased headloss and frequent backwashing. Advanced operators must monitor key performance indicators, such as TOC removal efficiency, headloss, and turbidity, to determine the optimal time for media replacement. Replacing media too late can reduce filter performance and increase operational costs due to higher energy consumption and more frequent maintenance.
• Media Regeneration: In some cases, media replacement can be delayed by regenerating the GAC through thermal reactivation. This process restores the adsorption capacity of the GAC, allowing it to continue removing contaminants. Advanced operators must understand the costs and benefits of media regeneration compared to replacement. For instance, thermal reactivation can be more cost-effective than media replacement, but it may result in a slight loss of carbon mass, reducing the overall volume of media available for filtration. Operators must also ensure that regenerated media retains sufficient biofilm to maintain biological activity within the filter.
• Extended Biofilm Management: As the biofilm matures within a BAC filter, it can lead to operational challenges such as increased headloss, uneven flow distribution, and biofilm overgrowth. Advanced operators must monitor biofilm activity and thickness using both visual inspections and online monitoring tools such as pressure sensors and flow meters. Adjusting backwash frequency, air scour intensity, and flow rates can help control biofilm growth while maintaining optimal biodegradation efficiency. In some cases, operators may need to implement biofilm control measures, such as periodic chemical cleaning or hydraulic bumps, to prevent overgrowth and maintain consistent filtration performance.
• System Upgrades: As water quality regulations evolve and new contaminants of concern emerge, BAC systems may require upgrades to meet these changing standards. Advanced operators should stay informed about emerging contaminants, such as per- and polyfluoroalkyl substances (PFAS), and be prepared to make system modifications, such as switching to new GAC media types or incorporating additional pre-treatment processes, such as advanced oxidation processes (AOP) or UV disinfection.

9.4 Emerging Contaminants and Advanced Regulatory Requirements

As new contaminants of concern emerge and regulatory frameworks evolve, advanced operators must be knowledgeable about the challenges posed by contaminants of emerging concern (CECs) and the strategies for addressing these within a BAC system. Advanced regulatory requirements increasingly demand higher levels of contaminant removal to protect public health and meet stringent water quality standards.

• Contaminants of Emerging Concern (CECs): CECs, including pharmaceuticals, personal care products, endocrine-disrupting compounds (EDCs), and PFAS, are increasingly found in water supplies due to industrial processes, consumer products, and agricultural runoff. While many of these contaminants are not yet regulated, they pose significant risks to public health and the environment. Advanced operators must stay informed of the latest research on CECs and monitor their presence in influent water. BAC systems may require modifications, such as enhanced ozone pre-treatment or switching to more specialized GAC media, to effectively remove CECs.
• Regulatory Compliance: BAC systems must meet stringent regulatory standards for the removal of organic contaminants, pathogens, and disinfection byproducts (DBPs). Advanced operators must be familiar with the specific regulatory requirements for their treatment systems and ensure that their BAC filters are optimized to meet compliance goals. This includes continuously monitoring TOC reduction, pathogen log removal, and DBP precursor removal to ensure that water quality targets are met. Operators must also be prepared to adjust system parameters or implement new treatment processes to address regulatory changes or the introduction of new contaminants.

To support the understanding of Biological Activated Carbon (BAC) systems, this glossary provides definitions for the essential terms and concepts used throughout the document. These definitions are critical for operators preparing for the Advanced Water Treatment Operator (AWTO) License exams.

Adsorption

The process by which molecules or particles adhere to the surface of a solid material, such as granular activated carbon (GAC). In BAC systems, adsorption occurs when organic contaminants are captured on the surface of the GAC media.

Assimilable Organic Carbon (AOC)

A portion of dissolved organic carbon (DOC) that can be easily consumed by microorganisms for growth. High levels of AOC in drinking water can promote bacterial regrowth in distribution systems.

Backwash

A maintenance process in which water (and sometimes air) is forced in the reverse direction through the filter media to remove trapped solids, biological material, and other particulates. Backwashing helps restore the performance of BAC filters by reducing headloss and cleaning the media.

Biofilm

A community of microorganisms that grows on surfaces, such as the granular activated carbon in a BAC system. The biofilm plays a key role in biodegrading organic contaminants in BAC systems.

Biodegradation

The breakdown of organic contaminants by microorganisms within the biofilm on BAC media. This process is a critical part of the biological treatment aspect of BAC systems, allowing for the removal of dissolved organic carbon (DOC).

Biological Activated Carbon (BAC)

A type of water filtration system that combines the adsorption capacity of granular activated carbon (GAC) with the biodegradation capabilities of microorganisms. BAC systems are widely used in water treatment and potable reuse applications.

Biodegradable Dissolved Organic Carbon (BDOC)

A portion of dissolved organic carbon that can be readily broken down by microbial activity. BAC systems are designed to remove BDOC through the action of biofilms on the GAC media.

Contaminants of Emerging Concern (CECs)

Chemical and microbial substances, such as pharmaceuticals, personal care products, and endocrine-disrupting compounds (EDCs), that are not commonly regulated but may pose risks to human health and the environment. BAC systems can be effective in removing some CECs from water supplies.

Disinfection Byproducts (DBPs)

Chemical compounds formed when disinfectants, such as chlorine or ozone, react with natural organic matter in the water. Common DBPs include trihalomethanes (THMs) and haloacetic acids (HAAs), both of which are regulated due to potential health risks.

Dissolved Organic Carbon (DOC)

The fraction of organic carbon dissolved in water. DOC can contribute to the formation of disinfection byproducts (DBPs) and promote bacterial regrowth. BAC systems remove DOC through both adsorption and biodegradation.

Empty Bed Contact Time (EBCT)

A measure of the time water spends in contact with the filter media in a BAC system, typically expressed in minutes. EBCT is calculated by dividing the volume of the filter media by the flow rate. Longer EBCTs allow for more effective contaminant removal.

Filtrate

The treated water that passes through a filter, in this case, a BAC filter. The filtrate should meet regulatory standards for water quality, including reductions in total organic carbon (TOC), turbidity, and other contaminants.

Granular Activated Carbon (GAC)

A highly porous material made from carbon-based substances that is used as a filter media in water treatment systems. In BAC systems, GAC serves as both a physical filter and a substrate for biofilm growth.

Headloss

The loss of pressure that occurs as water passes through a filter. In BAC systems, headloss increases as solids and biological material accumulate in the filter media. Monitoring headloss is critical for determining when backwashing is required.

Hydraulic Bump

A brief interruption in flow or a short backwash used to release trapped air from the filter media in BAC systems. This prevents air binding and reduces headloss.

Ozone Pre-Treatment

The use of ozone before BAC filtration to break down larger organic molecules into smaller, more biodegradable compounds. Ozone pre-treatment enhances the performance of BAC systems by improving the biodegradability of organic contaminants.

Pathogen Log Removal

A measure of the reduction in the concentration of pathogenic microorganisms, expressed as a logarithmic value. For example, a 3-log removal corresponds to a 99.9% reduction in pathogen concentration. BAC systems can contribute to pathogen removal as part of a multi-barrier water treatment approach.

Total Organic Carbon (TOC)

A measure of the total amount of organic carbon present in the water, both dissolved and particulate. TOC is a key indicator of water quality, and its removal is one of the primary goals of BAC systems.

Turbidity

A measure of the cloudiness or haziness of water caused by suspended particles. High turbidity can reduce water quality and interfere with disinfection processes. BAC systems help reduce turbidity by removing suspended solids and biological material from the water.

Ultraviolet Transmittance (UVT)

A measure of the ability of water to transmit ultraviolet (UV) light, typically expressed as a percentage. UVT is used to assess water clarity and is an important parameter in UV disinfection systems. BAC systems can improve UVT by reducing turbidity and dissolved organic carbon.