Settling

Or How to separate what floats from what sinks

Definition of settling in wastewater treatment

Definition of settling in wastewater treatment

In water treatment, settling (décantation en français) is an operation that removes suspended particles from the water to be treated. It is a physical process that consists to separate particles of higher density than water from the liquid in which they are found. These particles are recovered at the bottom of the tank. In a wastewater treatment plant, we speak of primary mud in the case of a primary decantation upstream of the biological treatment. Tertiary mud is all the mud collected from tertiary treatment works, such as clarificator

Settling is free. Gravity is responsible for everything.

Nicolas MEUDAL

Importance of settling for water purification

Settling is a crucial process for water purification. It effectively removes impurities such as sediment, suspended solids and organic debris, thereby improving water quality and preventing contamination. Among the 3 solid/liquid separation methods, settling is the simplest and least expensive. So it is a waste to skip it. By using settling, we can ensure that we have clean and safe water to use, which is crucial for human health and the environment. By combining it with other water purification methods, we can produce more pure and safe water.

Standard applications of settling

Settling is used for many water purification purposes such as sewage, drinking water production and aquaculture:
  • In the wastewater treatment field, they are used as primary treatment to separate suspended particles, as tertiary treatment to concentrate sludge and improve the quality of treated water before being discharged.
  • In drinking water purification, settling is used to protect the particle filtration by removing suspended particles that could clog the filters.
  • It is also used in aquaculture and fish farming to concentrate sludge and improve the water quality of culture ponds.
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Settling applications

Wastewater treatment: Using settling to remove suspended solids

An important component of this treatment is the use of settling to remove suspended solids. The primary treatment of wastewater with a buffer tank is recommended as it provides many advantages:

  • The primary ponds reduce the amount of suspended solids and pollutants incorporated into them.
  • This can help to reduce the flow rate on the biological system, which makes the purifying micro-organisms’ life easier.
  • In addition, it can reduce the equipment size installed, including biological aeration, which can result in cost savings.
  • And finally, it saves electricity because the buffer avoids peak flows and already eliminates part of the carbon pollution.

Drinking water production: use of settling to remove impurities and protect the downstream filter

To maintain the quality of the water, it is important to remove impurities before filtering. This is where settling comes in. It is a process of physical separation of sludge and impurities suspended in water. It involves floating the suspended particles to separate them from the clear water. The process has several steps:
  • Water is slowly added to a settling tank, allowing suspended particles to settle to the bottom.
  • Then the clear water is pumped from the top of the tank, leaving impurities at the bottom. This process is important because it protects the downstream filter and increases its efficiency and profitability.
Actually, producing drinking water and using it to wash a filter does not make sense. The more a filter is washed, the more the production unit loses efficiency and profitability. Therefore, to remove TSS, lamella clarifiers are necessary upstream of the filtration process to guarantee the quality of the water produced. They remove the largest impurities, reducing the load on the downstream filter and increasing its service life.

Aquaculture: concentration of sludge at the outlet of a drum filter

Aquaculture is the cultivation of aquatic organisms such as fish, shellfish and even plants. Because 70% of the planet’s surface is covered by water, humans have understood its importance as a resource. For this reason, aquaculture is the most exploited resource for water use and offers unique opportunities for sludge treatment at the drum filter outlet. The sludge produced by drum filtration systems is very liquid, which makes it difficult to concentrate. This represents a major issue for aquaculture operators, as carrying liquid sludge is expensive and requires proper treatment to preserve environmental quality. This is the purpose of a lamella clarifier. This type of equipment increases the concentration of the sludge at the outlet of the drum filter, which improves dewatering.

Separation of heavy particles and oils from industrial effluents

Separation of heavy particles and oils from industrial effluents is a very important step. Industrial effluents can contain substances that are harmful to the environment and human health, such as heavy metals, chemicals and oils. It is therefore important to treat them before releasing them into the environment.

Many processes use active ingredients that are best reused. For example, chopper juices contain soapy water, oils and heavy particles. Our lamella clarifiers are designed to recycle these three streams separately. This will minimize the amount of waste produced and maximize the resources reused. Hence the importance of having a separate collection of floats to optimize the treatment of industrial effluents.

Design and operating costs

Primary clarifiers and clarifier digesters

Both models of clarifiers play an important role in wastewater treatment, as they participate in removing pollutants and organic residues, which can improve the quality of the water that is discharged into the environment.

  • Primary clarifiers are used to separate solids and grease from wastewater. They operate by slowly circulating wastewater through a compartment where large waste and grease settle to the bottom. The sludge is then removed and sent to the clarifiers for further treatment.
  • Clarifier digesters are used to decompose organic matter into simpler substances through a controlled fermentation process. Biological reactors use bacteria to separate the remaining organic matter in the sludge.
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The most efficient clarifiers are those that have been used for years in Germany.

They should be systematically included in each treatment plant. However, the sludge does not necessarily have to be stored for a long time, although it must be able to remove a good part of the carbon pollution and smooth the flow to avoid hydraulic and organic load jolts.

Generally, multi-chamber clarifiers are considered much more efficient than single clarifiers because they can achieve greater phase separation, which can reduce the amount of sludge produced and improve the quality of the treated water.

Clarificcator

Their primary goal is to separate suspended solids from the water and produce clear water that can be safely used or discharged. Clarificcator can be designed in different ways, but their basic functionality is to slowly circulate the water to be treated through a settling surface, allowing solids to settle to the bottom of the clarificcator. They are typically ring-shaped structures, supplied from the center. They allow sludge thickening (typically after the biological part of a treatment, to separate the sludge from the treated water). This is the most common tertiary treatment structure, but it requires a lot of floor space. In addition, a clarifier is rarely equipped with a float recovery system. A lamella clarifier can achieve the same performance as a clarificcator on a surface 10 times smaller. However, it is important to note that not all wastewater pollutants can be removed by clarificcator and that they must be used in combination with other treatments to achieve fully treated water. Finally, it is important to choose the appropriate clarificcator for the site requirements, such as size, treatment capacity and characteristics of the water to be treated.

Lamella clarifiers

Lamella clarifiers are commonly used in the water industry.

Unlike clarificcator, which require a large footprint, lamella clarifiers can be installed in small spaces without compromising their performance. They work with inclined plates to separate the effluent into three streams: supernatant, float and sludge.

The supernatant and floats are sent to an additional treatment system, while the sludge is removed for further treatment. Lamella clarifiers are provided with a specific float recovery chute which allows a more efficient separation and a reduction of the amount of waste sent to the final treatment system. It is the most compact of the settling structures. Lamella clarifiers can be used to treat a wide range of industrial and municipal effluents and are particularly useful for applications where space is limited.

When is settling not recommended?

Mud storage

Low sedimentation of an effluent can present a major difficulty for its proper treatment.
  • If the wastewater does not separate sufficiently into clear water and solid sediment, it may be difficult to treat it effectively and reduce its suspended solids load. Before considering a settlement facility, it is recommended to conduct a settlement test to determine the performance of the effluent and to determine the measures needed to improve its settlement.
  • If coagulation flocculation is scheduled upstream, a jar test is performed to test the optimal dosages of coagulants and flocculants to improve effluent sedimentation. The performance of the entire treatment system can be significantly improved by working to optimize effluent settling.
  • However, different approaches may need to be tested to find the one that works best for each specific effluent.

Some factors that can affect the efficiency of settling

Settling is a major process in water treatment, but it can be affected by various factors that can influence its efficiency.

  • One of the main factors is the high flow rate, which can lead to a loss of efficiency due to insufficient residence time for settling of suspended particles.
  • In addition, low concentrations of suspended solids (SS) can also affect settling efficiency, as it may be more difficult to separate them from the clear water. Temperature is also an important factor to consider, as temperature variations can affect the solubility of coagulants and flocculants and thus the formation of flakes.
  • Finally, pH can also impact settling efficiency by influencing the surface charge of suspended particles and their ability to stick together.

To ensure effective settling, it is essential to control these factors and adjust dosages accordingly.

Limitations on processing capacity

Settling can be a very effective process for separating clear water from suspended solids (SS), but there can be limitations in terms of treatment capacity. Indeed, although a lamella clarifier can have an efficiency of 80 to 90% for TSS reduction, there may be situations where the remaining TSS level is still high, above 30 mg/L. In these cases, it may be difficult to meet water quality requirements without additional treatment steps. For this reason, we recommend combining a drum filter with a lamella clarifier in such cases. This system can help to significantly reduce the remaining TSS levels, allowing additional separation of the finer particles, thus improving water quality. In this way, it is possible to guarantee water quality that meets regulatory requirements, while minimizing the cost and environmental consequences of treatment.

Design and costs

Mud storage

To improve the decantability of particles to be separated and thus to improve efficiency, it is possible to add coagulant and flocculant chemicals. The heavier a molecule is, the better its decantation. Let’s consider sand, for example, it will decant very easily! On the other hand, very light particles such as very aerated mud, for example, will settle very slowly. In these cases, these famous coagulants and flocculants are used to increase the mass of particles and thus their decantability. Some processes use microsand in addition to the coagulants and flocculants action.

In a sewage treatment plant, the most commonly used coagulant is ferric chloride: FeCl3. It is an orange-red liquid, very effective and cheaper than other coagulants that can be found.

Regarding flocculants, there are many different types of them. Each type of mud has a flocculant with which it reacts the most effectively. It is necessary to test several products to identify the ideal candidate, by carrying out jar tests.

In fact, in the wastewater treatment plant operating budget, coagulants and flocculants constitute a major expense category.

Coagulants and flocculants

Another important design parameter is the mud storage time. This has a direct impact on OPEX (operational expenditure) and CAPEX (capital expenditure). The larger the mud storage capacity of a settling tank, the more mud it will be stored in, and the lower the discharge frequency will be. In fact, the more you invest initially (CAPEX), the more you reduce OPEXs afterward. As in any project, it is better to have a long-term vision and estimate the overall costs. Some settling equipment, such as lamellar settling tanks, are not designed to store mud, while others such as the Imhoff tank are specifically designed to store and concentrate the mud. Usually, all structures used to store mud are made of concrete. The concentration of mud depends on its retention time. However, this increase in concentration is not linear. After a certain period of time, usually 6 months, the concentration does no longer vary. However, designing generously a settling system for mud storage can increase the mud concentration by 50%! This reduces the mud volume and operating costs! Finally, after a certain plant size, the mud volume produced is so large that other extraction technologies such as belt filters or centrifugation have to be considered.

Other parameters

Finally, several other elements come into consideration. For example: BOD5 load, peak flow rate, buffer volume, sedimentation volume, impact of FeCl3 and denitrification… All these parameters influence the calculation of a settling system. To go further, we have created an online lamella clarifier design form, and another to determine your mud index and Hazen speed!

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