Use of Coagulants for Waste Water Treatment
Coagulants are Formulated to Enhance Liquid-Solid Separation
Coagulation in Wastewater Treatment has been used to clarify water since ancient times – as early as 2000BC, when the Egyptians used almonds to clarify river water. There is also evidence to suggest that the Romans were using alum as a coagulant at around 77AD. Today, coagulation and flocculation are still essential components of treatment processes, e.g. for reducing water turbidity. Wastewater treatment operations also require coagulation for chemical phosphorus removal and for reducing suspended solids.
What Is The Purpose Of Coagulation In Wastewater Treatment?
Coagulation plays a vital role in the wastewater treatment process, allowing for solids removal and dewatering, water clarification, lime softening, and sludge thickening. With the help of other specialized chemicals and mechanical filtration methods, coagulants help companies maintain a consistent and reliable source of clean water to support their industrial processes.
With the help of other specialized chemicals and mechanical filtration methods, coagulants help companies maintain a consistent and reliable source of clean water to support their industrial processes. Rather than having to pay another company for costly water treatment, coagulants enable efficient, on-site water treatment, providing:
- Long-term process sustainability
- High water reclamation with near closed-loop systems
- Significant cost and efficiency savings
Chemical coagulants used in wastewater treatment fall into two main families: organic and inorganic. A Primer on Wastewater Treatment from EPA can be found here.
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What is Coagulation?
Coagulation is a somewhat simple chemical process that involves bringing insoluble materials together by manipulating the charges of particles, by adding iron or aluminum salts, such as aluminum sulfate or ferric sulfate, to a wastewater stream. The primary purpose of using a coagulant besides removing vary fine particles from suspension is that this process results also in less turbidity of the water, i.e. clearer water.
With coagulants’ positive charge, the negatively charged particles in the water are neutralized. This causes the suspended solids in the water to bind together into larger flocs. These larger flocs begin to settle at the base of the water supply. The larger the size of the particles, the quicker the floc settles.
Coagulation helps to remove a number of different pollutants that cause your water to become dirty or toxic, including:
- Organic compounds and certain dissolved organic materials, commonly referred to as Natural Organic Matter (NOM) or Dissolved Organic Carbon (DOC)
- Suspended inorganic precipitates like iron and some metals
- Certain viruses and bacteria
Through coagulation, industrial water supplies are put into the perfect chemical state for easy mechanical filtration. Once the flocs settle at the bottom of your clarifier, equipment like a filter press can then take those larger clumps of aggregated particles and remove them, delivering clean water back into your system.
When used together, coagulants, clarifiers and filter presses offer maximum water reclamation of over 95 percent. With so little water actually discharged with the solids, you can create a nearly closed-loop process.
What are the common coagulants?
Organic Coagulants VS Inorganic Coagulants
Organic Coagulation
Organic coagulants are best used for solid-liquid separation. They are also good options to use when trying to reduce sludge generation. Being organic in nature, these coagulants offer the added benefits of working at lower doses and having no effect on the pH of your water.
Organic coagulants are typically based on the following formulations:
- PolyAMINEs and PolyDADMACs – These cationic coagulants work by charge neutralization alone and are the most widely used organic coagulants. PolyAMINEs and PolyDADMACs neutralize the negative charge of colloids in your water, forming a spongy mass called a “microfloc.” Since they only coagulate through charge neutralization, they don’t offer any advantages in regard to the sweep-floc mechanism (explained later with inorganic coagulants).
- Melamine Formaldehydes and Tannins – These natural coagulants work somewhat similarly to inorganic coagulants in that they both coagulate colloidal material in the water and also contribute their own precipitated floc. This sweep-floc precipitate can absorb organic materials such as oil and grease while coagulating unwanted particles together in your water. Since the precipitate dewaters everything to low moisture concentration, these coagulants are great for operations that generate hazardous sludge, such as what’s found in oil refineries.
The main advantages of organic coagulants include; lower dosage, lower volume of sludge produced and no effect on the pH.
Inorganic Coagulant
Inorganic coagulants are typically cheaper than their organic counterparts, making them a cost-effective solution for a broad range of water treatment applications. They are especially effective when used on raw water with low turbidity.
When added to water, inorganic coagulants form aluminum or iron precipitates. These help to clean the water by absorbing impurities in the water as they fall. This process is known as the “sweep-floc” mechanism. However, this can add to the overall sludge volume that must be treated and removed, so it’s not the right choice in every scenario.
The main types of inorganic coagulants include:
- Aluminum Sulfate (Alum) – As one of the most common water treatment chemicals used in industrial processes, alum is the go-to coagulant choice for many. Manufactured as a liquid, alum’s crystalline form is created when the liquid is dehydrated. It should be noted that alum is mildly hazardous and has similar health effects/corrosion characteristics as diluted sulfuric acid.
- Aluminum Chloride – This coagulant works similarly to alum, but it’s more expensive, hazardous and corrosive. As such, it’s usually only picked as a second choice in processes where alum could not be used.
- Polyaluminum Chloride (PAC) and Aluminum Chlorohydrate (ACH) – These inorganic coagulants are best used for more basic water supplies.
- Ferric Sulfate and Ferrous Sulfate – While ferric sulfate is more commonly used, both iron coagulants work similarly to aluminum coagulants. Ferrous sulfate is typically a good choice in applications where you need a reducing agent or excess soluble iron ions are required.
- Ferric Chloride – Since it is generated as a waste material from steel making operations, ferric chloride is the least expensive inorganic coagulant. However, it’s only used in facilities that can handle its reputation as the most corrosive and hazardous inorganic coagulant.
Once you have the right coagulant, you add these chemicals to your dirty water and mix rapidly. That way, the coagulant is quickly and easily circulated throughout the water.
The residuals or by-products of these coagulants generally do not pose a water quality issue, so long as they were applied properly and with the right dosage. This is why having a water treatment expert is key. Professionals who are experienced with wastewater treatment can even set up the coagulation process so that the coagulant chemicals are removed with the floc during filtration.
Coagulant Products
Organic coagulants consist in liquid chemicals which have a high coagulant efficiency mainly focused on its polymer structure and on the strength of the cationic charges.
Inorganic coagulants are both cost-effective and applicable for a broad variety of water and wastewater. Inorganic coagulants are particularly effective on raw water with low turbidity and will often treat this type of water when organic coagulants cannot.
