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
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
- 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.
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
- 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.
Aquaculture: concentration of sludge at the outlet of a drum filter
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.
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.
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?
- 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
Design and costs
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
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!