Coagulation, Flocculation, and Precipitation in Water and Used Water Purification

Abstract

The stringent regulations by users and stakeholders have raised the bar for the quality of water and wastewater treatment. Additionally, the requirement of the end users for the best techno-commercial process has forced the industrial players to employ new and appropriate technologies for water treatment. When it comes to pretreatment processes, there is no doubt that precipitation, coagulation, and flocculation (followed by sedimentation and/or filtration) are among the most effective steps. Though the aforementioned terms are often used interchangeably to describe the physicochemical processes involved in destabilizing the charges and agglomeration followed by the removal of the particles, there exists distinct definitions for the same as defined by Bratby (Coagulation and flocculation with an emphasis on water and waste water treatment, 1st edn. Uplands Press Ltd., Croydon, 1980). Chemical precipitation is a unit process in which undesirable dissolved ions are reacted to form insoluble salts for removal downstream. Coagulation/flocculation on the other hand is the result of the two different and independently occurring reactions which includes destabilization of surface charges (actual coagulation) and transport. The larger agglomerates formed after coagulation by transport are called flocs and hence the name “flocculation.” Coagulation is essentially a time-independent process because the term only involves the process of overcoming various forces on the surface of a particle so that the gravitational forces can take over to settle the particles. On the other hand, flocculation is a time-consuming process where the agglomerates are formed that become larger and larger. The flocculation (or transport) processes are usually the rate-determining steps, as they are much slower than the coagulation (or destabilization) processes.

This chapter aims to summarize the recent developments in the field of precipitation-coagulation-flocculation and have a deeper look at the intricate techniques that govern these unit processes. The industrial purview as well as the technologies used by WABAG to encompass these terminologies will also be discussed. Some important terms that are relevant in the grand scheme of precipitation, coagulation, and flocculation will be discussed along with some case studies.

The scope of water treatment technologies is ever-changing, and hence the future of coagulation and flocculation will also be discussed in the concluding paragraphs. In spite of the numerous benefits of these processes, it is plagued with problems of danger posed due to the presence of inorganic coagulants, sludge disposal, inefficient removal capabilities by different coagulants, as well as the difficulties faced when implementing lab-scale results to full-scale installations. Hence, the current recommendations available to improve the process will also be stated with future directions for helping to improve one of the most conventional treatment processes in water and wastewater treatment.

Suraj Babu Pillai died before publication of this work was completed.

Suraj Babu Pillai: deceased.