WATER TRETMENT: IN-LINE CLARIFICATION
In-line clarification is the removal of suspended solids through the addition of in-line coagulant followed by rapid filtration. This process is also referred to as in-line filtration, or contact filtration. The process removes suspended solids without the use of sedimentation basins. Coagulation may be achieved in in-line clarification by either of two methods:
· an inorganic aluminum or iron salt used alone or with a high molecular weight polymeric coagulant
· a strongly cationic organic polyelectrolyte
Because metal hydroxides form precipitates, only dual-media filters should be used with inorganic coagulant programs. Floc particles must be handled in filters with coarse-to-fine graded media to prevent rapid blinding of the filter and eliminate backwashing difficulties. Where a high molecular weight polymeric coagulant is used, feed rates of less than 0.1 ppm maximize solids removal by increasing floc size and promoting particle absorption within the filter. This filtration technique readily yields effluent turbidities of less than 0.5 NTU. The second method of coagulant pretreatment involves the use of a single chemical, a strongly charged cationic polyelectrolyte. This treatment forms no precipitation floc particles, and usually no floc formation is visible in the filter influent. Solids are removed within the bed by adsorption and by flocculation of colloidal matter directly onto the surface of the sand or anthracite media. The process may be visualized as seeding of the filter bed surfaces with positive cationic charges to produce a strong pull on the negatively charged particles. Because gelatinous hydroxide precipitates are not present in this process, single- media or upflow filters are suitable for poly-electrolyte clarification.
In-line clarification provides an excellent way to improve the efficiency of solids removal from turbid surface waters. Effluent turbidity levels of less than 1 NTU are common with this method.