Both the treatment of wastewater and drinking water must include coagulation and flocculation.
Since ancient times, including the period of the Egyptians and Romans, coagulating chemicals have been used to clarify the water in potable water treatment.
Chemically improving primary treatment to lower suspended particles and organic loads from primary clarifiers is an example used in overburdened wastewater treatment facilities.
To prevent pathogen contamination, several water utilities are committed to continuing to produce treated water with turbidities of less than 0.1 NTU.
What are coagulants?
Curdling and clotting are two synonyms for the term “coagulation,” which is used to describe the process by which a liquid transforms into a solid state.
It is a chemical process in which the chemical composition of a liquid changes, causing particles to congregate and eventually solidify.
Coagulants are chemicals used to remove suspended solids from water to meet standards of drinking water requirements and consist of positive charge molecules that neutralize suspended solids in wastewater to precipitate them.
What are the types of coagulants?
The two main types of regularly used coagulants are those based on aluminum and those based on iron.
Aluminum sulfate, aluminum chloride and sodium aluminate are some of the aluminum coagulants.
Iron coagulants include ferric sulfate, ferric chloride, and ferric chloride sulfate.
Hydrated lime and magnesium carbonate are two more substances that are utilized as coagulants.
The main reason why aluminum and iron coagulants are useful is that they can create multi-charged polynuclear complexes with improved adsorption properties.
Utilizing inorganic coagulants has additional benefits, such as low unit cost, wide availability and the potential to remove some of the organic precursors that may interact with chlorine to create disinfection byproducts.
The pH of the system may be able to influence the type of complexes that form.
They have a few significant disadvantages:
They produce vast amounts of metal-rich floc that must be disposed of properly to protect the environment, which can significantly increase disposal costs.
They require pH management since they can dramatically change the pH of the water, which is necessary for proper coagulation. They also need feed and storage equipment that is corrosion-resistant.
Ferrous sulfate, ferric sulfate, and aluminum sulfate are all extremely acidic substances that deplete alkalinity and lower pH.
On the other side, sodium aluminate will increase pH and enhance alkalinity.
The most commonly used inorganic coagulants
Aluminum sulfate
The most popular aluminum coagulant is aluminum sulfate. It is available as a solution as well as in a variety of solid forms such as block, kibbled, or ground.
The reaction between the alkalinity of the water’s natural state and the acidic coagulant results in the formation of an aluminum hydroxide floc, which typically contains calcium bicarbonate.
The removal of turbidity and color, as the maintenance of acceptable minimum amounts of dissolved residual aluminum clarified water, depends on pH control during the coagulation process.
Ferric chloride
An effective flocculant and coagulant is ferric chloride. It is a flexible water treatment product and a substitute for ferric sulfate.
In general, especially in cold water, it encourages rapid sedimentation.
However, because chloride can make water more corrosive, it is the less common option.
Ferric sulfate
A form of iron coagulant known as ferric sulfate can produce a denser floc than aluminum sulfate and is frequently used in conjunction with chlorine.
Ferric sulfate has several benefits over alum, such as flock particles of ferric hydroxides having a larger density and being easier to remove by sedimentation.
However, there are limitations as well because it is challenging to dissolve and forms a much heavier hydroxide sludge.
Sodium aluminate
Aluminum oxide and sodium oxide are combined to create sodium aluminate.
Sodium aluminate typically makes up 70–80% of this chemical’s solid forms whereas only around 30% of its liquid forms do.
Due to the low molecular weight of AI, sodium aluminate solutions minimize chemical sludge generation compared to iron. Additionally, aluminates increase the water’s alkalinity, negating the need for lime or hydroxides.
Organic coagulants
The removal of the majority of suspended solids has been demonstrated to be very successful using both polyamine and poly-DADMAC coagulants.
Tannates work very well with fats and oils.
They provide several benefits, including:
Allow for relatively low charge densities to more efficiently neutralize less charged suspended particles. without using metals or hydroxides, create longer polymer chains that will improve micro flocs production.
having the ability to remove some of the organic precursors that could interact with chlorine to create disinfection byproducts
Create a low volume of flocs.
Non-corrosive liquid forms that are ready for immediate usage.
pH has no effect and just a minor or insignificant impact.
They have a few limitations:
A higher unit price.
In cases of significant charge demand, greater dosages are required.
It’s not always easy for low-density floc to settle.
How do coagulants work?
Positively charged molecules make up coagulants, which neutralize particle electrical charge by weakening the magnetic forces that hold colloids apart.
Iron or aluminum precipitates are created when inorganic coagulants have been introduced to the wastewater and the cationic metal ion has neutralized the colloid particles, allowing them to absorb pollutants and purify the water.
This procedure, known as the “sweep-floc” mechanism, although efficient, increases the volume of sludge that must be treated and removed.
Similar steps are followed when using organic chemical coagulants, although often an amine (NH4+) group linked to the coagulant molecule provides a positive charge rather than a metal.
What are flocculants?
After coagulation (“Charge Neutralization”), a subsequent procedure known as flocculation is required.
Small, neutral particles evolve into larger particles in this manner.
The substances known as flocculants encourage the clumping of small particles into floc, which can
easily be removed from the water.
They are always made of polymers.
The flocculation procedure is a gentle mixing stage that transforms tiny suspended particles known as micro-flocs into large, observable pin-flocs.
As pin-flocs collide more frequently, they grow increasingly bigger, or “macro-flocs.”
The flocculants help with this by including hydrogen bonding, improved van der Waal forces, and entanglement amongst the particles.
Best Practices for Storage and Management
The chemistry of coagulants varies, and many—but not all—of them have a low viscosity and are simple to pump using regular chemical metering pumps.
Some coagulants have a significantly higher viscosity, necessitating the use of specialized pumps to assure correct dosing.
The properties of the product should be examined in light of feed and chemical storage requirements.
Important factors to take into account include product properties including viscosity, pour and freeze point temperatures, chemical compatibility with a storage container, as well as feed and transportation equipment.
Some of this information can frequently be found in product safety data sheets (SDS), but the end-user must collaborate with the product provider to make sure these elements are assessed before initiating a program.
For example, Emulsion flocculants are, by nature, a mixture of water, oil, and polymer that need to be stored carefully and mixed frequently to prevent oil and polymer separation and maintain a homogeneous product.
These compounds may gel if exposed to moisture from the outside and may block feed equipment if improperly handled and stored.
Without the proper make-down and feed equipment, the product is used inefficiently, frequently at higher than necessary use rates, and is wasted, denying the customer the full value of their purchase.
Coagulation and flocculation agent inventory control is crucial to prevent product runouts that could reduce water flow through the system being treated, temporarily halt all water treatment, or, in the worst case, stop production of whatever is being created.
A sensible strategy to prevent this is to routinely check the inventory of any product, although occasionally this is not done as well as it should be.
Tank level sensors are a component of inventory management when using tanks of liquid coagulants and flocculants during program setup.
For years, this has been done with success.
References
[1] Brain Campbell, Oct 15, 2021, what is coagulation for water treatment? (Online)available at https://www.wwdmag.com/what-articles/what-coagulation-water-treatment
[2] ENVA, the role of coagulants in wastewater treatment, (online) available at https://enva.com/case-studies/coagulants-in-wastewater-treatment
[3] IWA, Coagulation, and Flocculation in Water and Wastewater Treatment,(online), available at https://www.iwapublishing.com/news/coagulation-and-flocculation-water-and-wastewater-treatment
[4] James Greenwood, Feb 2, 2022, How are coagulants and flocculants used in water and wastewater treatment?, WCS group,(online)available at https://www.wcs-group.co.uk/wcs-blog/coagulants-flocculants-wastewater-treatment
[5] Troy Adams August 16, 2021, flocculation and flocculants solution, (online) available at https://globalroadtechnology.com/flocculation-flocculant-solutions-overview/
[6] Water Treatment Chemical Storage & Management Best Practices, (online) available at: https://www.wwdmag.com/chemicals/chemical-coagulation-flocculation/article/10940133/water-treatment-chemical-storage-management-best-practices
