Biological wastewater treatment is a crucial step in water purification, allowing organic pollutants to be broken down using microorganisms. Among the types of wastewater treatment, several biological processes are commonly used in wastewater treatment plants.

Activated sludge

What is it used for?

Activated sludge technology is a process for treating wastewater and industrial water. It is one of the most common types of wastewater treatment. This technology, developed at the beginning of the last century, remains effective today.

The designs are different from other types of biological treatment, but the model often remains the same:

We can find an activation tank which acts as a biological reactor;
A settling pond will allow the separation of solids, activated sludge and treated wastewater;
Activated sludge return equipment to transfer activated sludge to the aeration tank inlet

Operation process

The process for utilising activated sludge is quite simple. It is one of the most commonly used types of wastewater treatment. Atmospheric air is introduced into the wastewater mixture. This wastewater contains organisms capable of producing a biological floc. These flocs become the future activated sludge.

The effectiveness of the removal process depends on several factors:

The hydraulic residence time in the aeration tank. It is calculated by dividing the volume of the aeration tank by the flow rate.
The influent charge, i.e. its characteristics, such as its COD, nitrogen concentration, carbon content, etc.;
Aeration tank conditions (food/microorganism ratio) ;
Ventilation conditions ;
Temperature, etc…

After treatment, the floc settles at the bottom of the settling tank. Purified water can undergo additional treatment or return to nature if it meets the standards.

The settled suspended solids (TSS) are returned to the beginning of the aeration tank for a new treatment cycle. However, due to biological growth, an excess is formed, called residual activated sludge. This surplus can disrupt the relationship between biomass and nutrients. To avoid this, this excess is removed before putting the suspended solids back into the cycle. This step ensures optimal water purification.

The residual activated sludge is stored in separate tanks. There, they undergo treatment, either aerobic or anaerobic, before being disposed of properly.

There are various facilities that employ activated sludge technology, each with its own specific techniques.

MBBR

Before reaching the membrane, a sieve removes sand and grease to prevent blockage. Only particles smaller than 3 mm are allowed to pass through, and manual sieving is also accepted. To achieve the breakdown of impurities. MBBR only uses bacteria.

The MBBR technology relies on particles that host colonies of bacteria. These bacteria form a biofilm on these mobile particles, playing a key role in water purification.

At the end of this process, the water is cleaned, but sludge remains and needs to be treated. For this purpose, a bio-reactor with lamellae technology is used for settling. The MBBR is durable with less expensive parts, providing a cost-effective alternative to the MBR.

IFAS

The IFAS, an emerging technology, enhances treatment by adding a growth media in an activated sludge tank. This method boosts biomass and can modernize existing facilities without major construction.

The strength of IFAS is that it is the refinement of the advanced sludge method. It can therefore be easily integrated into existing installations.

In addition, adding a crib that can be either fixed or mobile provides many advantages:

This setup provides two different biological populations that act in synergy.
The mixed liquor, rich in active agents, eliminates the majority of the waste. And the biofilm with its nitrifying bacteria takes care of the nitrogen.
The biofilm processes merge anaerobic, aerobic, and anoxic zones into a single step. With IFAS systems, the additional biomass attaches to a stable surface. This eliminates the need to waste activated sludge and avoids unnecessarily increasing suspended populations.
Choosing the IFAS system is more economical than traditional activated sludge installations. Fixed-bed systems, such as IFAS, often require less space than traditional methods. By integrating IFAS into your current system, you boost its efficiency without the costs of expansion.

IFAS Technology

IFAS, or “integrated fixed film activated sludge”, is one of the most innovative types of wastewater treatment. Although inspired by traditional activated sludge methods, this technology is more efficient. The main distinction between IFAS and MBBR is the way in which activated sludge is reused. It thus offers a modern and cost-effective approach to water purification.

IFAS is a technology adapted to existing activated sludge installations in biological treatment. It has the advantage of using more compact tanks. These tanks are specially designed to accommodate media, whether dispersed or fixed. If you opt for dispersed media, consider adding additional screens for better efficiency.

This IFAS technology has many advantages over existing technologies. However, before installing an IFAS system in an existing station, it is important to make a few observations:

Check the aeration capacity of your system. Your system must withstand the increased oxygen demand with the new biomass when transitioning to IFAS.
Carefully plan the introduction of the media. Ensure that the basins are suitable to accommodate the type of media you have chosen.

MBR

MBR, or Bio Reactor Membrane, is a tricky technology compared to other biological treatment methods.

The upstream sludge buffer allows this system to:

producing less sludge and thus avoiding the need to treat them.
also resulting in a high space-saving.

The MBR system, more efficient than FBBR or MBBR, can lead to high maintenance costs. This system has bacteria on a membrane, with a concentration five times higher than other methods.

With suitable pores, the membrane filters and removes germs from the water.

To achieve these results, negative pressure is essential to circulate wastewater. This process consumes a lot of energy, increasing costs.

Additionally, the membrane must be regularly replaced and cleaned. All these operations require a qualified workforce.

FBBR

The FBBR system, or Fix Bed Biofilm Reactor, is one of the most economical types of wastewater treatment. It is more similar to MBBR than MBR, with a fixed substrate covering the walls of the vessel. Thanks to this configuration, it reduces operating and maintenance costs, making it very economically advantageous.

The biological layer converts contaminants into sedimentary matter through aerobic organisms using oxygen. This oxygen comes from a layer beneath the substrate, providing an upward flow of air. This self-regulating system is not only effective but also cost-effective in terms of component costs compared to MBR. However, as with MBBR, sludge must be treated.

SBR

SBRs (sequential batch reactors) are among the most efficient types of wastewater treatment. They use a separate pre-treatment section to mechanically retain solids. In addition, there is an aeration tank and a biological settling tank.

Small SBR wastewater treatment systems clean up wastewater. This is a few sentences explaining how the SBR technology works.

First, the wastewater goes through an initial treatment, either mechanical or chemical, to remove the solids. Following this operation, the wastewater will enter the 2nd tank. They will be processed by the SBR method.
The effectiveness of this treatment relies on controlled aeration, facilitated by specialized microorganisms. In the SBR process, there is an alternation between short aeration phases and resting periods. And it is during these stages that activated sludge forms.
After alternating cycles of rest and aeration, a long resting phase occurs. During this time, the sludge settles at the bottom of the tank, leaving purified water on the surface.
This water is then separated from the mixture, and the sludge returns to the first tank. In accordance with the standards, the purified water can either be released or receive additional treatment to ensure its purity.

Types of Wastewater Treatment

In addition to the technologies presented, it is essential to emphasize the importance of choosing a solution adapted to the characteristics of the effluent to be treated and to the local constraints. For example, MBBR stands out for its simplicity of maintenance and effectiveness in tight spaces, while MBR guarantees superior effluent quality, although it requires more specialized maintenance. IFAS, on the other hand, is an ideal option for upgrading existing infrastructure without major works. Finally, the combination of several technologies, such as the integration of MBBR and SBR, can provide optimal performance by combining the advantages of each method. These technological choices, often driven by environmental and economic objectives, enhance the sustainability and efficiency of wastewater treatment systems.

FAQ

What are the differences between free biomass and fixed biomass systems?

Free biomass systems, such as activated sludge, use microorganisms suspended in water to degrade organic matter. In contrast, fixed biomass systems, such as biofilters or biofilm reactors, grow microorganisms on solid supports (stationary or mobile). The latter offer better resistance to load variations and a smaller footprint, which makes them interesting for compact or high-flow stations.

Why integrate membrane filtration into a biological treatment?

The integration of membranes into a biological treatment – such as in membrane bioreactors (MBRs) – makes it possible to combine biological degradation with advanced physical separation. It improves effluent quality (particulate abscess, low turbidity, pathogen retention), which is particularly useful for water reuse applications or discharges to sensitive environments. In addition, it eliminates the need for a secondary clarifier.

How do hybrid technologies improve the performance of an existing wastewater treatment plant?

Hybrid technologies, such as IFAS or BAGI, can increase biological treatment capacity by adding biofilm carriers to existing tanks. This combines the advantages of free crops (flexibility) and fixed crops (high yield), without radically changing the infrastructure. It is an ideal solution for modernising an ageing installation or responding to an increase in load without major work.

Can biological treatment be adapted to complex industrial waste?

Yes, some biological processes are specifically suitable for industrial effluents with a high organic load or containing specific compounds. For example, fluidized beds and dynamic nitrification make it possible to efficiently treat highly concentrated water. In addition, the optimization of parameters (pH, temperature, COD/BOD) and the use of anaerobic processes with biogas recovery offer a solution that is both efficient and sustainable.

What are the environmental benefits of reusing treated wastewater?

The reuse of treated wastewater helps to preserve freshwater resources, especially in water-stressed areas. In agriculture, it provides nutrient-rich water, reducing the need for chemical fertilizers. It also contributes to reducing pollution of receiving environments and is part of a circular economy logic, where water becomes a reusable resource rather than a simple waste to be discarded.