Suspensions, pretreatment coagulation

The purpose of this work was to optimize mixing, various pretreatment (coagulation and sedimentation) and filtration processes, utilizing artificial asbestos suspensions containing asbestos levels of the order of 5.10 ° to 10 f/1. At first, tests were carried out through a batch process to obtain the optimum conditions which were then applied to the pilot plant. Also, the pilot plant effectiveness was evaluated for asbestos fiber removal. 

Sometimes slow filtration is used without previous coagulation. This is generally practiced with water that does not ctmtain much suspended matter. If the water is loaded (periodically or permanently) with clay particles in suspension, pretreatment by coagulation-flocculation is necessary. Previous adequate oxidation of the water, in this case preozonization producing biodegradable and metabolizable orgairic derivatives issuing from disstdved substances, can be favorable because of the biochemical activity in slow filters. 

An important aspect of solid-liquid separation is the pretreatment or conditioning of the feed stream. Several treatments can be applied to the feed suspension including coagulation, flocculation, pH adjustment, freezing. 

Pretreatment of Suspensions. Another important aspect of soHd—Hquid separation is conditioning or pretreatment of the feed suspension to alter some important property of the suspension and improve the performance of a separator that follows. A conditioning effect is obtained using several processes such as coagulation and docculation, addition of inert filter aids, crystalliza tion, freezing, temperature or pH adjustment, thermal treatment, and aging. The first two operations are considered in more detail due to their importance and wide use. 

Overall, the LJF experiments indicated thatflux is very dependent on the structure of the deposit, which was determined by the solute or colloid characteristics and their interaction in suspension. Organic ryection is more reliable for tighter membranes (10 kDa) and flux is more stable. Coagulation pretreatment can improve flux and rfiction, however the... 

A large number of commercial coagulants and synthetic flocculants are available with which to pretreat suspensions. While newer products have displaced the use of more traditional chemicals in recent years, few have been eliminated completely from the marketplace. The more important and widely used pretreatment chemicals are described here. 

The discussion of chemical and physical pretreatments given in this chapter highlights the potential benefits (and pitfalls) of artificially changing the particle size present in a suspension. Chemical pretreatments in particular can enhance performance in all phases of a filter cycle and provide for improved filtration rates and reduced cake moistures. Some separation devices such as the deep cone thickener and belt filter press rely on the satisfactory performance of flocculants to function correctly and their use is a necessary prerequisite. The use of coagulants and flocculants with other separation devices must be assessed on an individual basis. Their introduction generally increases the operating cost of a separation which must be balanced by the potential improvements in separation characteristics. 

---