Lime clarification

Granular bed filters are used in ten coil coating plants to remove residual solids from the clarifier effluent, and are considered to be tertiary or advanced wastewater treatment. Chemicals may be added upstream to enhance the solids removal. Pressure filtration is also used in this industry to reduce the solids concentration in clarifier effluent and to remove excess water from the clarifier sludge. Figure 7.4 shows a granular bed filter and Table 7.13 presents the heavy metal removal data of a lime clarification and filtration system. 

Eggleston, G., Monge, A., and Pepperman, A., Preheating and Incubation of Cane Juice Prior to Liming A Comparison of Intermediate and Cold Lime Clarification, J. Agric. Food Chem., 50, 484—490 (2002). 

Eggleston, G., Monge, A., and Ogier, B., Further Insights on the Factory Performance of Cold, Intermediate and Hot Lime Clarification Systems., Proc. Sugar Processing Research Conf, 348-365, 2002. 

The design data shown in Table 1 are to be used to design a crosscurrent ammonia-stripping tower. The wastewater to be treated is municipal sewage that has been treated by activated sludge followed by lime clarification. A 90% ammonia removal efficiency is desired. The packing is assumed to have the characteristics shown in Fig. 8. 

A simplified process used in smaller systems is in-line flocculation followed by pressure filtration. The simplified process produces water of lower quality than the lime clarification process but the equipment is smaller and simpler to operate (71). Experience with in-line filtration showed that optimal dosage of alum was rarely achieved due to fluctuating influent turbidity (72,73). 

One of the most innovative industrial uses of reverse osmosis is at the Petromin Refinery in Riyadh, Saudi Arabia. The refinery takes an unusable municipal wastewater, secondary effluent from the Riyadh sewage treatment plant, and by using lime clarification, filtration, reverse osmosis and ion exchange demineralization, it converts that useless waste into the entire process water requirements for the refinery. Figure 4.16 is the process flow schematic for the refinery water treatment plant. 

The refinery clarification equipment has the capability of adding any of the chemicals mentioned above. However, lime clarification was chosen as the method to be used. The combined secondary effluent and the plant return streams (5.64 MGD) are pumped from the surge ponds to the rapid mix basin in the clarifier. The rapid mix basin has two compartments in series and each compartment has a high speed mixer. Lime and sodium hydroxide are added to the first compartment and these are vigorously mixed with the secondary effluent in both compartments. As a result, the pH of the effluent from the rapid mix basin is raised to between 10.8 to 11.0. At this pH, much of the bicarbonate in the water reacts with the lime and forms an insoluble calcium carbonate and the magnesium in the water reacts with hydroxyl ions to form insoluble magnesium hydroxide. 

Figure 3.49 Simplified RO pre-treatment with MF instead of lime clarification. Source [90].

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). 

Unit Operations. The chemistries elaborated by all of these systems are described by seven unit operations (Fig. 5). The first six, the use of lime and carbon dioxide as clarification agents, were laid out during the first half of the twentieth century and only the appHcation technology has changed since, mainly from small batch processes designed to handle 1000 Hters in a few hours to continuous systems capable of processing up to 10,000 L/min. 

Juice Purification Chemistry. Lime in juice purification serves as a source of calcium, a source of alkalinity, and a source of calcium carbonate which serves as the clarification—filtration medium. 

Corrective Action Application An acidic groundwater at a Florida site (pH 2.5-3) required treatment. The groundwater was collected by extraction wells, pumped to an above-ground reactor, and neutralized with lime. In the course of neutralizing the waste stream, precipitates were formed which were removed by clarification and filtration prior to discharge. Sludges produced from the clarification and filtration steps were dewatered by a filter press. 

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 ... 

Supply of MU water for a medium-pressure (450 psig) WT boiler, from a surface water source with very variable suspended solids and hardness (sugar refinery, South Africa). The process used is a. carbonate removal using hot-lime precipitation softening coupled with silica adsorption by magnesia addition b. clarification in anthracite filters and c. cation ion-exchange for the balance of hardness removal. 

PHOSPHATATION A clarification process where phosphoric acid or a soluble phosphate is used with lime and heat. The impurities are removed by flocculation, flotation, and surface scraping. 

Removal of Heavy Metals from Coil Coating Wastewater by Lime Precipitation, Clarification, and Filtration... 

The process flow scheme consists of chromium reduction, lime precipitation, and clarification. 

This plant produces 130 m2/h of enameled steel and operates 3500 h/yr. It uses 0.0036 m3 water/m2 of product to coat the steel. Average process water flow is 0.144 m3/h for coating operations and 0.734 m3/h for metal preparation. The primary treatment in-place for process wastewater is clarification and settling. Other water treatment practices employed are pH adjustment with lime or acid, sludge applied to landfill, polyelectrolyte coagulation, and inorganic coagulation. 

This facility produces 360 m2/h of porcelain enameled aluminum for 4000 h/yr, and uses 0.038 m3 of process water/m2 of product coated. The mixed wastewater stream is treated by equalization (settling), pH adjustment (lime or acid), polyelectrolyte coagulation, clarification, and contractor removal of the resulting sludge prior to discharge to a surface stream. Process water flow for this production consists of 8.12 m3/h and 4.37 m3/h for surface preparation and coating operations, respectively.3 5... 

In addition to the heavy metals stated in Table 22.10, ferro- and ferricyanide are also part of the pollutants in the wastewater generated in a chrome pigment plant. These wastes are generally combined and treated through reduction, precipitation, equalization, and neutralization to be followed by clarification and filtration processes. Most of the heavy metals are precipitated using lime or caustic soda at specific pH. Chromium is reduced by S02 to a trivalent form, wherein it is precipitated as chromium hydroxide at specific pH. Sodium bisulfide is also employed to precipitate some of the metals at a low pH. The treated water is recycled for plant use while the sludge is sent to landfills (Figure 22.7). 

The basic unit operations/processes required for treating the acid pickling wastewater are (a) neutralization with NaOH and/or lime to increase the pH and (b) physicochemical methods, such as chemical coagulation, precipitation, clarification (sedimentation or DAF), and filtration to remove BOD5, COD, and iron. 

The second-stage (ammonia removal) effluent contained unacceptable levels of F and P and had to be subjected to third-stage lime treatment. This raises the pH from 8.5 to 11.4 and produces an effluent with concentrations of F and P equal to 25 and 2 mg/L, respectively. The hydraulic design parameters were a 15 min reaction time and a 265 gpd/ft (10.8 m /m /day) clarifier overflow rate. The resulting precipitated solids underflow concentration was 0.6% by wt. In all three stages, an anionic polymer was used to aid coagulation, solids settling, and effluent clarification. 

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