Settling
Or How to separate what floats from what sinks
Definition of settling in wastewater treatment
Definition of settling in wastewater treatment
Water treatment involves settling. This operation removes suspended particles from the water to be treated. It is a physical process that consists of separating 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.
Importance of settling for water purification
Decanting 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. It therefore improves the quality and safety of the decanted water.
Of the three solid/liquid separation methods, settling is the simplest and least expensive. So it’s a shame to do without it, since decantation ensures safe, healthy water. This is essential for human health and the environment. By combining this with other water purification methods, you can obtain even purer, safer water.
Common uses
Decanting is used in many water purification processes such as sanitation, drinking water production, and aquaculture:
- In the wastewater treatment industry, settling is used as a primary treatment to separate suspended particles. It is also used as a tertiary treatment to concentrate sludge and optimise water quality before discharge.
- Decanting is used to purify drinking water. 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 basin is recommended, as it allows many 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 guarantee water quality, it is important to remove impurities before filtering. This is where settling comes in.
It is a process of physical separation of mud and impurities suspended in the water. It consists of 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 settle 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.
It makes no sense to produce drinking water and use it to wash a filter. The more a filter is washed, the more the production unit loses efficiency and profitability.
To eliminate TSS, lamella decanters 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 come to understand its importance as a resource.
Aquaculture makes intensive use of water as a resource. 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, cutting 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
These two types of decanter play an important role in wastewater treatment. They help to eliminate pollutants and organic residues. This improves the quality of the water discharged into the environment.
- Primary clarifier: Separates solids and grease from wastewater by slow sedimentation. The sludge is then sent to the decanter digester for further treatment.
- Decanter digester: Uses a controlled fermentation process to break down the remaining organic matter in the sludge using bacteria.
Multi-chamber clarifiers
Multi-chamber decanters offer improved phase separation, reducing sludge production and improving the quality of treated water. They also smooth flows and minimise carbon pollution. They are more efficient than single decanters and should be included in all treatment plants.
Clariflocculator
Clarifiers separate suspended solids from water to produce clear water. They work by circulating the water through a sedimentation surface, allowing the solids to settle out. Although effective, they require a lot of space and cannot remove all pollutants. They are generally used in conjunction with other treatments to ensure complete purification.
Lamella clarifiers
Lamella decanters use inclined plates to separate effluent into three streams (supernatant, float, sludge), making them more compact than clarifiers. They are suitable for small spaces and can treat a variety of industrial and municipal effluents, with specific recovery of floatables to reduce waste.
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High Lamellar Clarifier DLHE-M
Lamella clarifiers -
Outdoor monobloc lamella clarifier DLME-L
Lamella clarifiers -
High Lamella Clarifier DLH-S
Lamella clarifiers -
High Lamella Clarifier DLH-M
Lamella clarifiers
When is settling not recommended?
Sludge storage
Sedimentation is less effective when the effluent has low settleability. If the water does not separate well from the solids, a settling test or jar test is recommended to evaluate the performance of the effluent and adjust the coagulant and flocculant dosages accordingly.
Limitations on processing capacity
Sedimentation can have limitations in terms of treatment capacity, with an efficiency of 80 to 90% for lamella settlers. If the suspended solids (TSS) concentration remains high, additional treatment steps—such as a drum filter—may be required to meet water quality standards.
Design, Cost and Maintenance Overview
| Paramètre | Description | Impact sur les Coûts |
|---|---|---|
| Coagulants et Floculants | Utilisation de coagulants comme FeCl₃ et floculants pour augmenter la masse des particules. | Augmente le coût des produits chimiques |
| Temps de Stockage des Boues | Plus le temps de stockage est long, plus la concentration des boues augmente. | Réduit la fréquence de vidange |
| Investissement Initial (CAPEX) | Investir dans un bassin de décantation largement dimensionné permet de réduire les coûts futurs. | Réduit les OPEX à long terme |
| Coût d’Exploitation (OPEX) | Réduction des coûts de vidange et d’exploitation par une plus grande capacité de stockage. | Réduit la fréquence de vidange |
| Optimisation des Décanteurs | Le décanteur lamellaire optimise l’espace tout en séparant efficacement les boues. | Optimise l’espace et réduit les coûts |
Other parameters
The chart shows the relationship between basin size (in percentage) and the associated costs for CAPEX (initial investment) and OPEX (operating expenses).
- CAPEX increases linearly with basin size, as a larger basin requires a higher investment.
- OPEX decreases as the basin size increases, since a larger basin reduces the frequency of emptying and maintenance, thereby lowering long-term operating costs.
In summary, although a larger basin requires a higher initial investment, it allows for savings on operating costs.
Several other factors must be taken into account. For example: BOD₅ load, peak flow rate, buffer volume, sedimentation volume, the impact of FeCl₃, and denitrification… All these parameters influence the design and sizing 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
Sedimentation is essential to water treatment, but various factors can affect its efficiency.
- High flow rate: Reduces efficiency by decreasing the necessary sedimentation time.
- Low TSS concentration: Makes separation more difficult.
- Temperature: Influences the solubility of coagulants and flocculants, affecting floc formation.
- pH: Affects particle charge and their ability to aggregate.
To ensure effective settling, it is essential to control these factors and adjust dosages accordingly.
FAQ
Can settling be used to separate two liquids?
Yes, decantation is not only used to separate solids from liquids. It is also applied to separate two immiscible liquids with different densities. A common example is the extraction of essential oils, such as limonene from citrus peels: the oil, being lighter, floats above the aqueous phase. A separating funnel is then used to cleanly separate the two layers.
What are the different types of settling used in industry?
Several types of decantation can be distinguished based on the nature of the particles and their concentration:
Decantation of granular particles: independent and rapid settling.
Diffuse decantation: flocculated particles, variable settling velocity.
Piston decantation: sharp separation, visible interface.
Hindered decantation: interactions between flocs slow down settling.
These types are taken into account when designing settling tanks to optimize separation efficiency.
How is settling involved in faecal sludge treatment?
In treatment plants, settling tanks are essential for separating solid fractions (sludge) from the supernatant. These tanks are structured into zones: sludge settles at the bottom, scum floats on the surface, and clarified water occupies the space in between.
Devices such as baffles improve hydraulic stability and enhance the efficiency of the process. This system helps reduce the volume of sludge that needs to be treated afterwards.
Are there any processes that combine settling and extraction?
Yes, notably counter-current washing. This industrial process uses a series of settling tanks to circulate a washing liquid in the opposite direction to the solids.
It is commonly used in hydrometallurgy to recover residual solutes from leaching sludge.
The effectiveness of this process relies on the very principle of decantation, applied here in cascade to maximize recovery efficiency (often> >95%)
In which other fields is settling applied, aside from water treatment?
Decantation is a cross-cutting process. It is used in:
Winemaking, to separate the clear wine from the lees.
The food industry, in the extraction of vegetable oils.
Chemistry, to purify solutions through phase separation.
Industrial waste treatment, to separate sediments from polluted liquids.
These applications show that decantation is both simple, cost-effective, and highly adaptable.
