Weighing the Advantages of Different Biological Wastewater Treatment Technologies
Biological wastewater treatment is a tried-and-true technology that has been around for well over 100 years. Considered an economical treatment option for streams with organic wastes, biological treatment is widely used by municipalities and industrial facilities alike for removal of contaminants such as nitrogen, phosphorus, bacteria, biochemical oxygen demand (BOD), and total suspended solids (TSS).
The basic biological wastewater treatment process works by harnessing a biomass comprised of bacteria, protozoa, or other microorganisms to break down organic wastes. Biological wastewater treatment systems essentially provide an environment to optimize this natural degradation process and simplify separation, although exactly how they do so varies from one technology to the next. Here, we’ll look at popular biological wastewater treatment technologies, how they work, and how their advantages and disadvantages affect their suitability for different applications.
Activated Sludge (AS)
Activated Sludge (AS) is a biological wastewater treatment technology where raw wastewater is mixed in large tanks with suspended biomass known as activated sludge. AS technology has existed for over a century and is an economic choice for treating large volumes of low- or residential-strength wastewater.
How it works
A typical activated sludge system consists of one or more large bioreactor tanks, a secondary clarifier or settling tank, and piping and peripherals. Aerobic systems are equipped with an aerator mechanism in the bioreactor, which delivers oxygen to the activated sludge. During the treatment process, influent wastewater is piped into the bioreactor tank and mixed with activated sludge, while the aerator injects either atmospheric air or pure oxygen into the stream. As the activated sludge degrades materials in the wastewater, masses of biological material agglomerate into clumps called flocs. The mixture is then pumped to a clarifier, and the flocs are allowed to settle to the bottom of the tank as a layer called a sludge blanket. The liquid effluent is decanted and, in most cases, routed for final polishing, while the sludge blanket exits the bottom of the unit and is diverted back to the aeration tank as return activated sludge (RAS) to be used for future treatment cycles. Any excess sludge is periodically removed and dewatered for disposal.
Advantages & disadvantages
When matched to the right application, activated sludge systems offer the following advantages:
- Low capital cost
- Low operational costs
- Simple operation
Despite their benefits, AS systems aren’t right for every situation. Compared to other biological WWTS, the main disadvantages of activated sludge systems include:
- Large footprint
- Sensitivity to variations in wastewater
- Not suitable for high-strength wastewater or high BOD/COD loads
Common applications
Activated sludge systems are best used for wastewaters with with low to moderate organic loads, and at facilities that have space to accommodate their large footprint. For this reason, activated sludge systems are mostly used at centralized treatment facilities and municipal water treatment plants.
Sequencing Batch Reactor (SBR)
A Sequencing Batch Reactor (SBR) is a biological WWTS that performs equalization, biological treatment, and clarification all in the same tank. SBRs are functionally the same as conventional activated sludge systems, but they have a streamlined set of components. For this reason, SBRs usually cost about the same or less than an AS system and occupy a smaller footprint.
How it works
An SBR system typically consists of a bioreactor tank with a mixer and aerator (if aerobic), a decanter, and automated control system. The influent wastewater stream is added to the batch bioreactor tank and mixed with the activated sludge inside for a period sufficient for degradation of most nutrients and organic wastes. Once the reaction is complete, the biomass is allowed to settle out, and the treated effluent is decanted for discharge or final polishing.
Advantages & disadvantages
Compared to other biological WWTS, SBR systems offer the following advantages:
- Smaller footprint than conventional AS
- Low capital cost
- Operational flexibility
- High quality effluent (10mg/L BOD and TSS)
The main disadvantages of SBR systems versus other biological WWTS are:
- Moderate maintenance complexity
- Moderate energy consumption
- Requires skilled operators
- Does not support continuous flow
Common applications
SBR systems are well-suited to a variety of industrial facilities who need to treat waste streams with low to moderate organic loads while conserving space. Common applications for SBR systems include municipal wastewater treatment, as well treatment of industrial wastewaters generated by refineries, petrochemical plants, tanneries, pharmaceutical manufacturers, pulp and paper mills, and textile mills.
Fixed Bed Bioreactors (FBBR)
Fixed Bed Bioreactors (FBBR) are a type of biological wastewater treatment system where wastewater is passed over an attached biofilm layer within the reactor tank. This differs from conventional AS because the biomass is fixed to the surface of media within the tank instead of floating freely. This design characteristic allows FBBR systems to hold more biomass, thus allowing FBBR systems to treat higher BOD wastewaters than conventional AS. Additionally, FBBR can house aerobic, anaerobic, and/or anoxic zones within the same unit, allowing for more complete removal of contaminants in complex wastewater streams.
How it works
An FBBR system consists of a bioreactor tank with multiple interior chambers, as well as pumps and piping to direct influent and effluent flow. For aerobic systems, the bioreactor also includes aeration diffusers to supply oxygen to the tank. The inside of the bioreactor is packed with porous foam, ceramic, or plastic media that contribute ample surface area for the biomass to colonize in a thin layer known as a biofilm. During a treatment cycle, wastewater is pumped into the bioreactor and circulated through the media. As the influent stream flows over the biofilm, the bacterial cultures consume the organic wastes present in the stream, breaking them down and separating them out from the effluent.
Advantages & disadvantages
Fixed bed bioreactor systems offer the following advantages over other biological WWTS:
- Low sludge waste & low solid waste disposal costs
- Smaller footprint than conventional AS
- Energy efficient
- Low operational cost
- Flexible configuration options to target specific wastewater constituents
Despite their benefits, FBBR systems have the following disadvantages compared to other biological WWTS:
- High capital costs
- Sensitive to variations in wastewater
- Media is prone to clogging
Common applications
FBBR systems are good for complex industrial wastewaters with high BOD where other treatment processes are not practical. Common applications including treating wastewaters from the chemical, petrochemical, food and beverage, and meat processing industries.
Membrane Bioreactor (MBR)
A Membrane Bioreactor (MBR) is a biological wastewater treatment system that combines conventional activated sludge with membrane filtration. As a result, the benefits of MBR systems include high removal efficiency and a compact footprint.
How it works
The main components of an MBR system include a bioreactor tank, aeration system (for aerobic units only), mixers, and a clean-in-place (CIP) system, and chemical pumps for balancing pH and nutrient balance. MBR systems are designed for microfiltration (MF) or ultrafiltration (UF), and typically use flat sheet or hollow fiber membrane elements. In a submerged membrane configuration, the membranes are placed within the bioreactor unit. Alternatively, in a side-stream configuration, the membranes are housed within a separate membrane tank.
While other biological treatment systems, like SBR, can operate without a separate primary clarification step, wastewater will typically need to be pretreated ahead of MBR to remove any large debris that could lead to membrane fouling or damage. Pretreated wastewater is pumped into the bioreactor tank, where it is mixed with biomass and aerated. The membrane retains the biomass, while allowing the effluent stream to move through the membranes’ pores and out of the unit.
Advantages & disadvantages
MBR systems offer the following advantages relative to other biological WWTS:
- Up to 50% smaller footprint than conventional activated sludge
- High-quality effluent suitable for reuse (acceptable for irrigation, land application, use in cooling towers, etc.)
- Suitable for streams high in organic matter
- Able to remove a wide variety of contaminants
Despite their benefits, MBR systems have a few disadvantages compared to other biological WWTS:
- High capital costs
- High operating costs
- High maintenance costs due to membrane cleaning and replacement
Common applications
MBRs offer high removal efficiency in a compact footprint, making them an appealing choice for industrial use even though they may have higher CAPEX and OPEX than conventional AS. Common applications for MBR systems include municipal wastewater treatment, as well as industrial wastewater applications for dairy, bakery and other food and beverage facilities, or where water reuse and/or surface water discharge is desirable.
Moving Bed Bioreactor (MBBR)
A Moving Bed Bioreactor (MBBR) is a type of biological wastewater treatment system where wastewater is mixed with small plastic units known as biofilm carriers. Much like the media in a fixed bed bioreactor, the carriers in an MBBR serve to maximize surface area within the bioreactor so that the unit can house as much biomass as possible. As a result, MBBR systems are suitable for treating streams with high BOD loads.
How it works
An MBBR system consists of a bioreactor tank, a secondary clarifier or settling tank, piping, and peripherals. The bioreactor contains carriers, which are small, cylindrical plastic units with openings and/or ridged surfaces that microorganisms colonize to form a biofilm. Aerobic bioreactors are typically large, open-topped basins outfitted with an aerator component. Anaerobic systems are typically sealed tanks with an up-flow design where wastewater is piped in from below.
During the treatment process, wastewater is pumped into the reactor tank and mixed with the carriers inside by an aerator that also serves to introduce oxygen into reactor. The biofilm on the carriers digests the wastes in the water, and because the carriers can move freely, they are more resistant to clogging than FBBR media. Treated effluent flows from the bioreactor on to secondary clarification while the carriers are retained within the bioreactor by sieves or screens.
Advantages & disadvantages
Moving bed bioreactor systems offer the following advantages over other biological WWTS:
- Smaller footprint than conventional AS
- Low energy use
- Suitability for medium-to-high BOD loads
- Low sludge waste & low solid waste disposal costs
- Able to treat larger volumes of wastewater than MBR
- Resistance to shock loads and system upset
Despite their benefits, MBBRs have the following disadvantages compared to other biological WWTS:
- Lower quality effluent
- Clogging and damage to carriers
- Periodic media replacement needed
Common applications
MBBRs are suitable for treating larger volume wastewater streams with moderate to high organic content. They can be particularly effective for reducing BOD upstream of other treatment technologies. These qualities make them a good choice for treating municipal sewage, as well as industrial wastewaters from meat processing, meat packing, other food and beverage applications, petrochemical plants and refineries.
The bottom line
When considering biological wastewater treatment solutions, it’s important to understand what technologies are out there, and how they compare to one another. But because every project has its own unique set of priorities and constraints, talking to a water treatment professional is the best way to see how different wastewater treatment technologies may suit your needs. At SAMCO, we are dedicated to delivering tailored wastewater treatment solutions, so whether you’d like to talk to an engineer or get a quote, we’re here to help.
For more articles on water and wastewater treatment, head on over to our blog. Some that might be of interest to you include:
- Biological vs. Chemical Wastewater Treatment Which Is Better for Your Industrial Facility?
- What Is Anaerobic Wastewater Treatment and How Does It Work?
- What Are Aerobic Wastewater Treatment Systems and How Do They Work?
- Common Problems with Biological Wastewater Treatment Systems and How to Avoid Them
How can SAMCO help?
SAMCO has over 40 years’ experience in the design and manufacture of water treatment systems for a range of applications within the power industry. Our separation and wastewater treatment solutions have helped our clients achieve goals like water conservation, process optimization, and regulatory compliance.
If you’re interested in learning about the best water treatment technologies for energy generation applications, we invite you to contact us. Our engineers will walk you through the process for developing a water treatment solution to meet your specific objectives. You can also request a quote to help you develop a realistic budget.