Use of Coagulants for Waste Water Treatment
Coagulants are Formulated to Enhance Liquid-Solid Separation
Coagulation and flocculation have 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.
Chemical coagulants used in wastewater treatment fall into two main families: organic and inorganic.
What are the common coagulants?
Organic Coagulants VS Inorganic Coagulants
Organic Coagulants
Organic coagulants are generally used for solid & liquid separation and sludge generation. Organic formulations are based on the following chemistries:
- PolyAMINEs and PolyDADMACs – The most widely used organic coagulants, which are cationic in nature and function by charge neutralization alone. Cationic coagulants neutralize the negative charge of colloids and form a spongy mass called microflocs.
- Melamine Formaldehydes and Tannins – These coagulate the colloidal material in the water, and absorb organic materials such as oil and grease.
The main advantages of organic coagulants include; lower dosage, lower volume of sludge produced and no effect on the pH.
Inorganic Coagulants
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.
Once added to water, the inorganic coagulants form aluminum or iron precipitates. These absorb impurities in the water as they fall, serving to clean the water. This process is known as the ‘sweep-floc’ mechanism. However, a downside of metal hydroxide precipitate sweep-floc is that they add to the overall sludge volume that must be treated and removed.
Examples of inorganic coagulants are as below:
- Aluminum Sulfate (Alum) – One of the most commonly used water treatment chemicals in the world. Alum is manufactured as a liquid, from which the crystalline form is dehydrated.
- Aluminum Chloride – A second choice to Alum as it is more expensive, hazardous and corrosive.
- Polyaluminum Chloride (PAC) & Aluminum Chlorohydrate (ACH)
- Ferric Sulfate & Ferrous Sulfate – Ferric sulfate is the more commonly used, but ferrous sulfate is typically used in applications where a reducing agent or excess soluble iron ions are required. Iron coagulants work similarly to aluminum coagulants but the cost may vary based on the local source of supply.
- Ferric Chloride – The least expensive inorganic coagulant, as it is generated as a waste material from steel making operations. However, it’s by far the most corrosive and hazardous inorganic coagulant, and its use is limited to facilities equipped to handle it safely.
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.
