Flocs and Separation Processes

From an initial review, it is determined that: floc water content appears to affect the loading rate; floc sizes must be sufficiently large to permit transfer to a collection surface; flocs must be suitably conditioned to enable capture (normally achieved by effective coagulation); and the ‘bigger is better' approach is not necessarily appropriate in situations in which floc/deposit strength plays a controlling role in the removal process.

A detailed description of floc structure and properties follows, in which the authors examine how properties such as density and strength are connected.

The different processes involved in conventional water treatment have one factor in common: the incoming stream of flocs encounters an array of collectors. These ‘collectors' could be in the form of floc blanket clarifiers (FBCs), bubbles in the case of dissolved air flotation (DAF) or the packed bed in the case of filters. Each of these three processes is examined individually in terms of how properties such as floc size, density and strength affect the process dynamics. It is shown that these features come into play both individually and collectively and are sensitive to the floc fractal structure. It is argued that floc strength often plays a critical role in separation processes through its impact on floc size.

Recognising the benefits of densification, the authors provide specific advice on how densification can be achieved. Among the many options considered, it is noted that the most straightforward means of securing greater density or strength is by use of smaller flocs, which also benefits dewaterability.

The authors conclude that, due to the range of factors affecting floc behaviour, separation processes require to be adjusted by means of trial and error. However, they stress that several ground rules can be considered, including: the overarching implications of strength; attempting to work with as small a floc as possible; treatment practices which promote densification; and recognising that many operational problems stem from the floc water content.