What Are Aerobic Wastewater Treatment Systems and How Do They Work?

Typically used as a secondary wastewater treatment method after the initial larger contaminants have been settled and/or filtered out, biological wastewater treatment systems can be efficient and economical technologies for breaking down and removing organic contaminants from heavily organic-laden wastes, such as those produced in the food and beverage, chemical manufacturing, oil and gas, and municipal industries.

Anaerobic and aerobic systems are two of the main types of biological wastewater treatment, but this article will focus on “what aerobic wastewater treatment systems are and how they work.”

What is aerobic wastewater treatment?

Aerobic wastewater treatment systems use oxygen-feeding bacteria, protozoa, and other specialty microbes to clean water (as opposed to anaerobic systems that do not need oxygen). These systems optimize the naturally occurring process of microbial decomposition to break down industrial wastewater contaminants so they can be removed.

The organic contaminants these microorganisms decompose are often measured in biological oxygen demand, or BOD, which refers to the amount of dissolved oxygen needed by aerobic organisms to break down organic matter into smaller molecules. High levels of BOD indicate an elevated concentration of biodegradable material present in the wastewater and can be caused by the introduction of pollutants such as industrial discharges, domestic fecal wastes, or fertilizer runoff.

How do aerobic wastewater treatment systems work?

Because these organisms require oxygen, aerobic systems require some means of supplying oxygen to the biomass by adding wastewater treatment ponds (which work by creating a large surface area for introducing air to the wastewater) and/or by incorporating some type of mechanical aeration device to introduce oxygen into the biomass.  

Depending on the chemical makeup of the wastewater in relation to the effluent requirements, a biological wastewater treatment system might be composed of several different processes and numerous types of microorganisms. They will also require specific operational procedures that will vary depending on the environment needed to keep biomass growth rates optimal for the specific microbial populations. For example, it is often required to monitor and adjust aeration to maintain a consistent dissolved oxygen level to keep the system’s bacteria multiplying at the appropriate rate to meet discharge requirements.

In addition to dissolved oxygen, biological systems often need to be balanced for flow, load, pH, temperature, and nutrients. Balancing a combination of system factors is where the biological treatment process can become very complex. Below are examples of some common types of aerobic biological wastewater treatment systems, including a brief description of how they function within an industrial wastewater treatment regimen to give you an idea of the types of technologies and systems that might benefit your industrial facility.

Activated sludge

Used widely used in municipal applications, activated sludge processes occur when wastewaters from the primary treatment phase enter an aeration tank. After aeration in the presence of suspended (freely floating) aerobic microorganisms, the organic material is broken down and consumed, forming biological solids which flocculate into larger clumps, or flocs. The suspended flocs enter a settling tank and are removed from the wastewater by sedimentation. Recycling settled solids to the aeration tank controls levels of suspended solids, while excess solids are wasted as sludge. Activated sludge treatment systems typically have larger space requirements and generate large amounts of sludge, with associated disposal costs, but capital and maintenance costs are relatively low, compared to other options.

Fixed-bed bioreactors, or FBBRs

These systems consist of multiple-chambered tanks in which the chambers are packed tight with porous ceramic, porous foam, and/or plastic media. Wastewater then passes through the immobilized bed of media. The media is engineered to have a high enough surface area to encourage a robust biofilm formation with long solids lifespan, resulting in low sludge formation and lowest sludge disposal costs. A well-engineered fixed-bed bioreactor will allow wastewater to flow through the system without channeling or plugging. Chambers can be aerobic and still have anoxic zones to achieve aerobic carbonaceous removal and full anoxic denitrification at the same time. More advanced biological processes can be facilitated with these systems (for example, nitrification, denitrification, desalination, sulfide-reduction, and anammox), by having unique bacterial populations colonize the biofilm media in separate tank chambers, which can be uniquely configured to treat your facility’s specific wastewater constituents.

Moving bed bioreactors, or MBBRs

MBBRs typically consist of aeration tanks filled with small moving polyethylene biofilm carriers held within the vessel by media retention sieves. Today the plastic biofilm carriers come from many vendors in many sizes and shapes, are typically half- to one-inch diameter cylinders or cubes and are designed to be suspended with their immobilized biofilm throughout the bioreactor by aeration or mechanical mixing.

Because of the suspended moving bio-film carriers, MBBRs allow high BOD wastewaters to be treated in a smaller area with no plugging. MBBRs are typically followed by a secondary clarifier, but no sludge is recycled to the process; excess sludge settles, and a slurry removed by vacuum truck, or settled solids are filter pressed and disposed as a solid waste.

Membrane bioreactors, or MBRs

MBRs are advanced biological wastewater treatment technologies that combine conventional suspended growth activated sludge with membrane filtration, rather than sedimentation, to separate and recycle the suspended solids. As a result, MBRs operate with much higher mixed liquor suspended solids (MLSS) and longer solids residence times (SRTs), producing a significantly smaller footprint with a much higher quality effluent compared to conventional activated sludge.

MBRs primarily target BOD and total suspended solids (TSS). MBR system design varies depending on the nature of the wastewater and the treatment goals, but a typical MBR might consist of aerobic (or anaerobic) treatment tanks, an aeration system, mixers, a membrane tank, a clean-in-place system, and either a hollow fiber or flat sheet ultrafiltration membrane. As a result of its many parts and cleaning processes, MBRs are known for high capital, high operating, and high maintenance costs.