wastes are less of a problem than acid wastes but nevertheless often require treatment. If acidic streams are not available or are not adequate to neutralize alkaline wastes, sulfuric acid is commonly employed Based on the chlorine dose used for disinfection, the ph of the disinfected effluent will be lower than that allowed for reuse applications and for dispersal to the environment. In such cases neutralization is controlled by automatic instruments using a feedback loop, and the final effluent pH is corded required.The reaction rate is slow, instrumentation and control design must take this factor into account. erials. If the reagent chemicals ca e fed automatically in the form of s With sis of Sealing Potential h the increasing use that is being made of nanofiltrat reverse osmosis. an ase by a factor of up to 10 concentration increase occurs, it is often possible to exceed the solubility product of calcium carbonate and other scale-forming compounds. The formation of scale within the treatment module will cause a terioration in the performance ultimately leading to the failure of the membrane module. The tendency to develop calcium carbonate(CaCO3) scale during the advanced treatment of treated effluent can be approximated by calculating the Langelier saturation index(LSi)of the concentrate Scaling Control Usually, CaCO scale control can be achieved using one or more of the following methods: Reducing calcium concentration by ion exchange or lime softening Adding a scale inhibitor chemical(antiscalant) to increase the solubility of CaCO3 in the werini Because it is not possible to ct a priori the value of ph in water treated with reverse osmosis, it is usually necessary to conduct pilot-scale studies using the same modules that will be used in the full-scale installation Stabilizatio effluent that is demineralized with reverse osmosis will generally require pH and calcium carbonate adiustment ( stabilization) to prevent metallic corrosion, due to the contact of the demineralized water with metallic pipes and equipment Corrosion occurs because material from the solid is removed (solubilized) to satisfy the various solubility products. Demineralized water typically is stabilized by dding lime to adjust the LSl, using the procedure outlined above 6-8 Chemical Storage, Feeding Piping, And Control Systems The design of chemical precipitation operations involves not only the sizing of the various trait operations and processes but also the necessary appurtenances. Because of the corrosive nature of many of the chemicals used and the different forms in which they are available, special attention must be given to the lesign of chemical storage, feeding, piping, and control systems. The following discussion is intended to erve as an introduction to this subject In domestic wastewater-treatment systems, the chemicals employed can be in a solid, liquid, or gaseous Chemical leeders form. Coagulants in the dry solid for to solution or slurry form prior to introduction into th wastewater. Coagulants in the liquid form are usually delivered Liquid feeders feeders to introduction into t quid). typically used for disinfection purposes.are either dissolved in water before iniection or are injected directly into the wastewater. Fig 6-9 Classification of chemical-feed system Loss in weight The various types of feeders are classified on Fig. 6-9 nemical feeders are generally designed to proportioning. feeding chemical in proportion to the influent wastewater flowrate, and(2) constant feed. designed to deliver chemical at a fixed rate regardless of the influent flowrate Chemical Storage and Handling General information on the handling, storage, and feeding requirements for various chemicals is presented in Table 6-10. The specific storage facilities required will depend on the form in which the chemical is available locally. For small treatment plants the available forms are usually limited. A typical storage 6-176-17 wastes are less of a problem than acid wastes but nevertheless often require treatment. If acidic waste streams are not available or are not adequate to neutralize alkaline wastes, sulfuric acid is commonly employed. Based on the chlorine dose used for disinfection, the pH of the disinfected effluent will be lower than that allowed for reuse applications and for dispersal to the environment. In such cases, neutralization is controlled by automatic instruments using a feedback loop, and the final effluent pH is recorded. Depending on the sensitivity of the environment, two-stage neutralization may be required. The reagent chemicals can be fed automatically, in the form of solutions, slurries, or dry materials. If the reaction rate is slow, instrumentation and control design must take this factor into account. Analysis of Scaling Potential With the increasing use that is being made of nanofiltration, reverse osmosis, and electrodialysis in wastewater reuse applications, adjustment of the scaling characteristics of the effluent to be treated is important to avoid calcium carbonate and sulfate scale formation. Depending on the recovery rate, the concentration of salts can increase by a factor of up to 10 within the treatment module. When such a salt concentration increase occurs, it is often possible to exceed the solubility product of calcium carbonate and other scale-forming compounds. The formation of scale within the treatment module will cause a deterioration in the performance, ultimately leading to the failure of the membrane module. The tendency to develop calcium carbonate (CaCO3) scale during the advanced treatment of treated effluent can be approximated by calculating the Langelier saturation index (LSI) of the concentrate stream. Scaling Control Usually, CaCO3 scale control can be achieved using one or more of the following methods: . Acidifying to reduce pH and alkalinity . Reducing calcium concentration by ion exchange or lime softening . Adding a scale inhibitor chemical (antiscalant) to increase the apparent solubility of CaCO3 in the concentrate stream . Lowering the product recovery rate Because it is not possible to predict a priori the value of pH in water treated with reverse osmosis, it is usually necessary to conduct pilot-scale studies using the same modules that will be used in the full-scale installation. Stabilization Wastewater effluent that is demineralized with reverse osmosis will generally require pH and calcium carbonate adjustment (stabilization) to prevent metallic corrosion, due to the contact of the demineralized water with metallic pipes and equipment. Corrosion occurs because material from the solid is removed (solubilized) to satisfy the various solubility products. Demineralized water typically is stabilized by adding lime to adjust the LSI, using the procedure outlined above. 6-8 Chemical Storage, Feeding, Piping, And Control Systems The design of chemical precipitation operations involves not only the sizing of the various trait operations and processes but also the necessary appurtenances. Because of the corrosive nature of many of the chemicals used and the different forms in which they are available, special attention must be given to the design of chemical storage, feeding, piping, and control systems. The following discussion is intended to serve as an introduction to this subject. In domestic wastewater-treatment systems, the chemicals employed can be in a solid, liquid, or gaseous form. Coagulants in the dry solid form generally are converted to solution or slurry form prior to introduction into the wastewater. Coagulants in the liquid form are usually delivered to the plant in a concentrated form and have to be diluted prior to introduction into the wastewater. Chemicals in the gas form (generally stored as a liquid), typically used for disinfection purposes, are either dissolved in water before injection or are injected directly into the wastewater. Fig. 6-9 Classification of chemical-feed system The various types of feeders are classified on Fig. 6-9. Chemical feeders are generally designed to be (1) proportioning, feeding chemical in proportion to the influent wastewater flowrate, and (2) constant feed, designed to deliver chemical at a fixed rate regardless of the influent flowrate. Chemical Storage and Handling General information on the handling, storage, and feeding requirements for various chemicals is presented in Table 6-10. The specific storage facilities required will depend on the form in which the chemical is available locally. For small treatment plants the available forms are usually limited. A typical storage