Raw water clarification is an essential operation to ensure a minimum quantity and, above all, acceptable water quality for the intended processes. The configuration, types of unit operations, choice of chemical agents, and other variables are primarily a result of the raw water quality and the final water requirements. For example, it may only be necessary to adjust the oxidation of water that already has very close to zero turbidity. On the other hand, it may be necessary to remove a specific agent, such as metals that must be insolubilized and removed. All these variables can contribute to different configurations and operational designs.
Figure 1 below provides a generic illustration of a water treatment plant that could potentially be called "traditional." Again, other steps can still be incorporated. The objective of this article is to discuss details and important tips for operating these most common unit operations.
Figure 1 – Typical water treatment plant.
Below are brief descriptions of important aspects of these unit operations and chemical dosage options. Obviously, there is much more to discuss individually for each of these parameters. The objective here is to discuss what is considered the minimum essential.
Typical Operations
1. Raw Water Pumping
This operation is necessary to transport water from the selected source to the treatment plant. It must ensure flow stability to ensure a stable water supply. Protection such as grates and filtration mesh may be required.
2. Coagulation (Rapid Mixture)
In situations involving high turbidity/suspended solids, insolubility is achieved by adding one or a combination of chemical coagulants. Coagulation works by neutralizing the charges contained in the contaminants. Strong and vigorous mixing is essential to ensure contact between the coagulant and the entire mass to be insolubilized. This is why this operation is usually known as rapid mixing or strong mixing. Rapid mixing can be achieved by a physical mixer (e.g., agitator) or by system design (e.g., baffles).
3. Flocculation (Slow Mixture)
After achieving good coagulation, it is important that the insoluble particles aggregate together to gain greater weight, thereby increasing settling capacity. At this stage, vigorous mixing is not recommended, as this can result in strong shearing of the particles, thus resolubilizing or destabilizing them. This effect prevents or at least reduces solid-liquid separation capacity. For this reason, this stage must be performed at low speed. This low speed can also be achieved by a mechanical agent (agitator) or by design (baffles).
4. Sedimentation/Settling
In many cases, the materials rendered insoluble in the coagulation and flocculation stages are denser than water. Therefore, the natural separation tendency is for these materials to settle when the water is in a semi-steady state. There are raw water sources whose contaminants have a lower density than water. In these situations, the preferred unit operation is flotation, where the materials are removed at the top.
In this settling stage, it is necessary to ensure time (to prevent flocs from being carried along with the water) and uniformity of these flocs. Small, light flocs, combined with a velocity higher than the design, can lead to the carryover of flocs that will be carried to the subsequent stages.
5. Filtration
Many define filtration as a polishing step to remove the last of the finer materials that were not large and dense enough to settle in the previous step. Different types and media can be selected for filtration, including filtration as a physical barrier. Materials such as activated carbon (removal of organics/chlorine), green sand (removal of some metals), and others can also be chosen.
Generally, they are used to reduce turbidity and suspended solids to the required levels.
6. Chlorination and Fluoridation
The dosage of chlorine/chlorinated agents can have two objectives: generating potable water and/or ensuring microbiological control in the systems served (e.g., the industrial network). In both municipal and industrial services, it may occasionally be necessary to have additional dosage points in the network due to natural chlorine consumption and loss.
The fluoride dosage option is usually only necessary for generating potable water in accordance with local legislation.
7. Alkalinity Adjustment
Adjusting the pH to the required operating range can be done with an alkaline or acidic agent at this stage. It is usually a relatively simple operation and is performed as one of the last adjustments made.
8. Water Distribution
Once the required quality and volume/quantity have been reached, the water is then distributed to its users.
Chemical Dosages
1. Oxidizing Agent
Raw waters with high residuals of soluble organic materials, iron, manganese, or other reducing contaminants may require oxidation to improve separation in subsequent steps. Common chemical options such as chlorine in various forms, bromine, hydrogen peroxide, ozone, and perhaps more sophisticated combinations (e.g., oxidizing agent and UV light) can be applied. Plants that capture water with a high soluble organic content and do not perform adequate pre-oxidation may produce finished water with very low turbidity/suspended solids. However, finished water with this high dissolved organic content may present problems such as:
- Taste and odor in the water
- Precipitation in systems such as ion exchange resins, reverse osmosis, EDI, etc.
- Incorporation into products that come into direct contact with the water
- Substrate for microbiological growth
- Generation of organic acids in boiler systems
2. Coagulant
Various chemical options are available for charge neutralization and coagulation. Inorganic (metal-based), organic, or a combination of inorganic and organic products can be applied. Choosing the best technical option is based on raw water quality, plant dynamics, cost, and other factors.
At this stage, the use of an important product determination tool, especially dosage, is essential: jar testing lab trials.
Jar testing is considered essential and often crucial to finding the correct dosage. For example, when a water source constantly changes its physical and chemical characteristics due to its hydrodynamics, it may be necessary to adjust coagulant dosage, control pH, etc. A jar test can provide the necessary and important information. This test can also be used to find a potential chemical substitute.
3. Flocculant
The use of a flocculating agent is not mandatory and will depend on the physical and chemical quality of the raw water and the plant's dynamics. Flocculants are organic polymers with varying chain, shape, and charge. They can be cationic, anionic, or even near neutral. Generally, for decantation, a higher molecular weight flocculant is preferred; conversely, for flotation, the option is usually for a lower molecular weight. However, the most appropriate way to determine the best flocculant and its dosage is through the jar test. This recommendation remains in place because if a flocculant is used, this chemical agent complements the coagulant's action.
4. Chlorine
As noted in the description of unit operation 6 (Chlorination and Fluoridation), extra dosage of chlorine, chlorinating agent or other option may be required for potable water generation, microbiological control or other functions.
5. Fluorine
The fluoride dosage option is usually only necessary for generating potable water in accordance with local legislation.
6. Alkalinity Adjustment Agent
In situations where the final water presents a pH/alkalinity in a range different from that requested, a final adjustment can be made by dosing an alkaline agent (to raise pH) or acid (to reduce pH).
Conclusion
Effective water treatment relies on understanding each operation’s unique characteristics and continuously optimizing every step of the process. At Solenis, we combine innovative chemistries, advanced monitoring and control systems, and expert support to help our partners overcome these challenges and operate more efficiently and sustainably.
Learn more about how our water treatment solutions can help you achieve your operational and sustainability goals here.