Lime coagulant

Fig. 1. Turbidity and magnesium concentration versus pH during lime coagulation of Odra river water...

Experimental results show that turbidity removal by lime coagulation depends both on pH and on the quantity of magnesium precipitated in the form of magnesium hydroxide. Magnesium concentration exerts a significant influence determining... 

Until recently, the standard method for treatment of these wastes consisted of lime coagulation and flocculation with clarification by dissolved air flotation. Subsequently, the underflow is polished with sand or diatomaceous earth filtration. Finally, the sludge is dewatered by vacuum filtration. The effluent quality is variable and does not always meet discharge standards. 

Acrolein is destroyed by controlled burning in an incinerator. It is also destroyed by biodegradation after being treated with dilute NaOH or sodium bisulfite solution. Acrolein and other toxic pollutants from industrial laundry wastewater can be removed by treatment involving lime coagulation, carbon adsorption, and ultrafiltration (Van Gils and Pirbazari 1985). 

Sodium alumiaate is used ia the treatment of iadustrial and municipal water suppHes and the use of sodium alumiaate is approved ia the clarification of drinking water. The FDA approves the use of sodium alumiaate ia steam generation systems where the steam contacts food. One early use of sodium alumiaate was ia lime softening processes, where it iacreases the precipitation of ions contributing to hardness and improves suspended soHds removal from the treated water (17). Sodium alumiaate reacts with siHca to leave very low residual concentrations of siHca ia hot process water softeners. Sodium alumiaate is often used with other chemicals such as alum, ferric salts, clays, and polyelectrolytes, as a coagulant aid (18,19). 

Makeup. Makeup treatment depends extensively on the source water. Some steam systems use municipal water as a source. These systems may require dechlorination followed by reverse osmosis (qv) and ion exchange. Other systems use weUwater. In hard water areas, these systems include softening before further purification. Surface waters may require removal of suspended soHds by sedimentation (qv), coagulation, flocculation, and filtration. Calcium may be reduced by precipitation softening or lime softening. Organic contaminants can be removed by absorption on activated carbon. Details of makeup water treatment may be found in many handbooks (22—24) as well as in technical Hterature from water treatment chemical suppHers. 

Chemica.1 Remova.1. Phosphoms can be precipitated with lime to form Ca2(P0 2- The actual composition of the precipitate is a complex compound called apitate. Achieving minimum phosphoms concentrations requires a pH in excess of 10.5. Alum or iron will precipitate phosphoms as AIPO4 or FePO. This procedure is generally employed in conjunction with the activated sludge process, in which the coagulant is added at the end of the aeration basin or between the aeration basin and the final clarifier. 

Table 1 Hsts a number of common inorganic coagulants. Typical iron and aluminum coagulants are acid salts that lower the pH of the treated water by hydrolysis. Depending on initial raw water alkalinity and pH, an alkah such as lime or caustic must be added to counteract the pH depression of the primary coagulant. Iron and aluminum hydrolysis products play a significant role in the coagulation process, especially in cases in which low turbidity influent waters benefit from the presence of additional colHsion surface areas.

Polymeric coagulants do not affect pH therefore, the need for supplemental alkalinity, such as lime, caustic, or soda ash, is reduced or eliminated. Polymeric coagulants do not add to the total dissolved soHds concentration, eg, 1 ppm of alum adds 0.45 ppm of sulfate ion (expressed as CaCO ) the reduction ia sulfate can significantly extend the capacity of anion-exchange systems. 

Chemical precipitation can remove 95 percent of the suspended solids, up to 50 percent of the soluble organics and the bulk of the heavy metals in a wastewater. Removal of soluble organics is a function of the coagulant chemical, with iron salts yielding best results and lime the poorest. Metal removal is primarily a function of pH and the ionic state of the metal. Guidance is available from solubihty product data. 

Lime is somewhat different from the hydrolyzing coagulants. When added to wastewater it increases pH and reacts with the carbonate alkalinity to precipitate calcium carbonate. If sufficient lime is added to reach a high pH, approximately 10.5, magnesium hydroxide is also precipitated. This latter precipitation enhances clarification due to the flocculant nature of the Mg(OH)2. Excess calcium ions at high pH levels may be precipitated by the addition of soda ash. The preceding reactions are shown as follows ... 

The chemical formula for limestone is CaCOj and upon burning forms calcium oxide (CaO), which is known as burnt lime. Calcium oxide, when mixed with water, forms calcium hydroxide (Ca(OH)2). Calcium hydroxide is used to treat water as a coagulation aid along with aluminum sulfate. 

Rotary vertical shaft turbine units as shown in Figure 10 and other rotary or reciprocating equipment are other examples. Tapered flocculation may be obtained by varying reel or paddle size on horizontal common shaft units or by varying speed on units with separate shafts and drives. In applications other than coagulation with alum or iron salts, flocculation parameters may be quite different. Lime precipitates are granular and benefit little from prolonged flocculation. 

The nature of chemical coagulants are such that the macrofloc may possess certain charges for example lime (CaO), alum (A1203) and flocculating polyvalent cations cany positive charges, which interact with proteins. The interactions are simply illustrated in Figure 7.6. 

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