Settling

Or How to separate what floats from what sinks

Definition of settling in wastewater treatment

Definition of settling in wastewater treatment

Water treatment uses settling. This operation is used to remove suspended particles from the water to be treated. This is a physical process that separates particles with a density heavier than water from the liquid in which they are present. These particles can be recovered at the bottom of the basin. In a wastewater treatment plant, we use the term “primary sludge”. This refers to primary settling before the biological treatment. Tertiary sludge is all the sludge collected from tertiary treatment works, such as clarifiers.

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 and organic debris. This method not only facilitates the removal of particles from the water, but also the extraction of water from the oil. This improves the quality and safety of the settled water.

Of the three solid/liquid separation methods, settling is the simplest and least expensive. It would be a waste to exclude it, since settling provides safe, healthy water. This is essential for human health and the environment. By combining it with other water purification methods, we can produce more pure and safe water.

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Common uses

Settling is used for many water purification purposes such as sewage, drinking water production and aquaculture:

  • In the wastewater treatment industry, settling is used as a primary treatment to separate suspended particles. In addition, it is used as a tertiary treatment to concentrate sludge and optimize water quality before discharge.
  • The purification of drinking water requires settling. This removes suspended particles that can clog filters.
  • In aquaculture and fish farming, its role is to concentrate sludge and improve pond water quality.

Settling applications

Wastewater treatment: to eliminate suspended solids

An important component of this treatment is the use of settling to remove suspended solids. Primary wastewater treatment with a buffer tank is recommended because it offers a number of advantages:

  • The primary ponds reduce the amount of suspended solids and pollutants incorporated into them.
  • Can smooth the flow on the biological system, making life easier for purifying microorganisms.
  • Reduced size of installed equipment, and especially biological aeration, with potential cost savings.
  • This system saves electricity by avoiding peak flows. In addition, the buffer effectively removes some of the carbon pollution.

Drinking water production: to eliminate 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 so that suspended particles settling to the bottom.
  • The clear decanted water is then pumped from the top of the tank, leaving the impurities at the bottom. This step is important, as it protects the downstream filter and increases its efficiency and cost-effectiveness.

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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.

To remove TSS, lamella clarifiers upstream of filtration guarantee the quality of the water produced. They remove large impurities, thereby reducing the load on the downstream filter and extending 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.

L’aquaculture exploite intensément l’eau comme ressource. It offers a range of unique challenges for the treatment of sludge after drum filtration.

The sludge produced by drum filtration systems is very liquid, which makes it difficult to concentrate. Transporting liquid sludge in aquaculture is expensive. Appropriate treatment is necessary to preserve environmental quality. This is the purpose of a lamella clarifier.

This system will increase the concentration of sludge after filtration, making it easier to dewater.

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 harmful to the environment. These substances include 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.

What are the different types of clarifier?

Primary clarifiers and clarifier digesters

Both types of clarifier have an important function in wastewater treatment, helping to remove pollutants and organic residues. This improves the quality of the water discharged into the environment.

  • Primary clarifiers are used to separate solids and grease from wastewater. They operate by circulating wastewater slowly through a compartment, separating coarse waste and grease at 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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Multi-chamber clarifiers

Multi-chamber clarifiers, such as those that have been in use for years in Germany, are the most efficient.

They should be systematically included in each treatment plant. Long-term sludge storage is not essential, but it is crucial to remove a large proportion of the carbon pollution. Smooth flows prevent hydraulic and organic load surges.

Multi-chamber clarifiers are generally considered more efficient than single-chamber clarifiers. They ensure better phase separation, reducing the amount of sludge produced and improving the quality of treated water.

Clariflocculator

Their primary goal is to separate suspended solids from water to produce clear, safe water. Clarifiers, including settling tanks, work by slowly circulating the water to be treated through a sedimentation surface. This allows the settled solids to settle to the bottom of the clarifier.

They are typically ring-shaped structures, supplied from the center. After biological treatment, they separate the sludge from the treated water, allowing the sludge to thicken.

This is the most common type of tertiary treatment plant, but it requires a large footprint. 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 clarifiers cannot remove all pollutants from wastewater. They must be used in association with other treatments to obtain fully treated water. It is important to choose a clarifier that is adapted to the site’s requirements, such as size and treatment capacity. The characteristics of the water to be treated must also be taken into account.

Lamella clarifiers

Lamella clarifiers are commonly used in the water industry.

Unlike clarifiers, which require a lot of floor space, lamella clarifiers require less space and are still highly efficient. They work with inclined plates to separate the effluent into three streams: supernatant, float and sludge.

The supernatant is processed elsewhere, while the sludge undergoes further treatment. Lamella clarifiers are provided with a specific channel for collecting floatables. This ensures better separation and reduces waste for final processing. It is the most compact of the settling structures. Lamella clarifiers can be used to treat a wide range of industrial and municipal effluents. It is particularly useful for applications where space is limited.

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When is settling not recommended?

Sludge storage

Low sedimentation of an effluent can present a major difficulty for its proper treatment.

  • If wastewater doesn’t separate properly into clear water and sediment, treatment becomes complicated. This complicates the process of reducing its suspended solids load. Before choosing a settling structure, it is recommended to carry out a settling test. These tests are used to assess effluent performance and identify any necessary improvement measures.
  • If flocculation coagulation is planned upstream, a jar test is performed first. This test is used to determine the right amount of coagulants and flocculants to improve effluent sedimentation. Maximizing effluent sedimentation improves overall treatment system performance.
  • 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

Limitations on processing capacity

Settling is highly effective in separating clear water from suspended solids (TSS). However, it may have processing capacity limits.

Lamella clarifiers can reach efficiencies of 80-90% by reducing TSS. However, in some circumstances, the remaining TSS level may exceed 30 mg/L. In some situations, meeting water quality standards is complex. Additional processing steps may then be required.

For this reason, we recommend combining a drum filter with a lamella clarifier in such cases. This can considerably reduce remaining TSS levels. This allows more effective separation of fine particles, improving water quality. As a result, the quality of the water is guaranteed to conform to standards. This reduces the cost and environmental impact of treatment.

Design and costs

sludge storage

To improve settling efficiency, Chemicals such as coagulants and flocculants can be added. The heavier a molecule is, the better it settling. Take sand, for example – it will settle just fine! On the other hand, very light particles, such as highly aerated sludge, will settle very gently. In these cases, the famous coagulants and flocculants are used to increase the mass of the particles and thus their settleability. Some processes use microsand in addition to coagulants and flocculants.

In a wastewater treatment plant, the coagulant most often used is ferric chloride: FeCl₃. It’s an orange-red liquid, very effective and cheaper than other coagulants on the market.

When it comes to flocculants, there are many to choose from, and each type of sludge has a flocculant with which it reacts best. You need to test several products to identify the ideal candidate, by carrying out test jars.

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

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Coagulants and flocculants

Another important dimensioning parameter is the sludge storage time. This impacts both the operating costs (OPEX) and the construction costs (CAPEX). A larger settling tank can hold more sludge. This reduces the frequency of draining required. In fact, the more you invest up front (CAPEX), the more you reduce OPEX afterwards. Like in any project, it is better to have a long term vision and to estimate the global costs.

Optimisation of Decanters for Sludge Storage

The lamella clarifier is not designed to store sludge. On the other hand, the settling digester is specifically designed for this function. Usually, all structures used to store sludge are made of concrete.

The sludge concentration depends on its residence time. However, this progression in concentration is not linear. After a certain period, usually 6 months, the concentration hardly varies. However, using a large settling tank for sludge storage can increase sludge concentration by 50%! Donc, cela diminue d’autant le volume des boues et du coût les coûts d’exploitations !

Finally, after a certain plant size, the sludge volume produced is so large that other extraction technologies such as belt filters or centrifugation have to be considered.

Other parameters

Finally, there are a number of other factors to consider. For example: BOD5 load, peak flow, buffer volume, sedimentation volume, impact of FeCl3 and denitrification… All these parameters influence the calculation of a water settling tank. For further information, we’ve created an online lamella clarifier design form, and another to determine your sludge index and Hazen speed!

Factors influencing settling

Settling processes are essential to water treatment, but a number of factors can influence their effectiveness.

  • One major factor is the high flow rate. High flow rates can reduce efficiency by reducing the residence time required for particle sedimentation.
  • In addition, low concentrations of suspended solids (TSS) can affect settling efficiency. Separating these materials from clear water can be more difficult. Temperature is an important factor in settling. Indeed, temperature variations can influence the solubility of coagulants and flocculants, thus impacting flake formation.
  • The pH affects the efficiency of settling This influences both the suspension particles’ surface charge and their ability to agglomerate.

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

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