Coagulation using Aluminum-based Product Chemistries

As a rule, a majority of the particulate debris found in untreated wastewater is negatively charged and must be neutralized before solids separation and particle agglomeration can take place.  Coagulation is the chemical process that can promote this.

Through coagulation, the aim is to neutralize any electrostatic repulsive charges that are present so particle agglomeration can take place.  When an inorganic aluminum-based coagulant is added to a waste stream, the metal-based Al³⁺ ion is hydrolyzed to promote the formation a positively charged aluminum hydroxide precipitate Al(OH)_3(s) and release of H⁺ hydrogen ions.  It is this positively charged “pinfloc solid” that is responsible for the charge neutralization that takes place within the process of coagulation.

As is the case for most inorganic coagulant reactions, the subsequent release of H⁺ hydrogen ions, or the “consumption of alkalinity” in the water, is the reason why a drop in treated water pH is so commonly observed.  Alum for example, Aquasan’s AQ-8924, is a material that is not pre-hydrolyzed and by default will consume 3 OH^– ions from the water when it is added. Once the aluminum hydroxide precipitate starts to form, the resultant release of 3 H+ ions can acidify the water to a point where municipal pH discharge targets are not being met and some form of alkalinity adjustment (typically caustic soda) may be required.  The diagram below provides a summary of the generic reaction that takes place when using aluminum based coagulant chemistries.

In addition to alum, there are several aluminum-based coagulants on the market that are “pre-hydrolyzed”, which means, they will have little to no effect on the water as it is being treated.  From within the Aquasan product line, one of many pre-hydrolyzed coagulants that are available for use is AQ-8921.

Pre-hydrolyzed aluminum-based coagulants are formulated to contain a certain amount of hydroxide within their chemical structure.  As a result, when added, these products will consume less alkalinity within the water and have a less pronounced effect on treated water pH.

Advantages of using a “pre-hydrolyzed” product chemistry (AQ-8921) over other aluminum-based coagulants?

When evaluating a waste treatment application, it is important that the management team takes into consideration the total overall costs of an application as opposed to the unit price of a specific product on its own.  AQ-8921 is one of our highly concentrated aluminum-based chemistries that comes in pre-hydrolyzed form.  Given its high concentration, this product is more expensive on a $/kg basis when compared to simple commodity inorganics (alum, ferric chloride, ferric sulfate, etc.) but, when applied properly it can provide a number of operational benefits.

The following are a list of advantages that can be realized when AQ-8921 is used in a WWT application.

• AQ-8921 contains 3 times more aluminum content when compared to alum which can significantly reduce the coagulant demand and the application rates that are being applied.
• It can promote significantly reduce and, in some cases, eliminate the need for alkalinity supplementation or pH adjustment to meet operational and or discharge targets.
• A reduction in the volume of sludge being generated.
• AQ-8921 is not considered a hazardous material (unlike alum)
• Improved reaction kinetics when applied in cold water conditions (much more reactive than alum).

Taking into consideration all the variables that are at play, it is not uncommon to discover that a higher priced “pre-hydrolyzed” coagulant chemistry can in fact be better fit for a waste treatment application.  Under the right conditions and when applied properly, AQ-8921 is a product that can have significant and positive impact on the total costs or potential savings that borne by a WWT facility.

 How do I choose the best coagulant for my treatment?

 Trust the AQUASAN technical team. We have a great deal of technical expertise in wastewater treatment.  Taking into full consideration the characteristics of the water being treated and the treatment process(es) that are in place, we will choose the most relevant chemistry for your application and ensure the program promotes the lowest overall operating cost for the facility.

Our goal is to where possible, provide creative, innovative, effective and easy-to-use treatment solutions that will meet your operational objectives and ensure your treated water quality targets are being maintained.