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This paper describes the development and validation of an equilibrium model of an industrial hot-lime-softening boiler-water-treatment unit for a large-scale nickel processing facility in which approximately 6.6 ML per hour of water is processed. In the industrial process, multiple water sources of varying quality are combined before the softening treatment, which makes control and optimization of the softening unit complicated and has brought about the necessity of a robust numerical model of water treatment. In this paper, the numerical thermodynamic and adsorption relations describing the softening treatment process are presented. Lime, magnesia, and soda ash additions are modeled. Emphasis has been placed on calcium, magnesium, and silica treatments as these are of most relevance to the industry. Jar tests described in this paper are used to determine adsorption relations, estimate statistical uncertainties, validate the model performance, and optimize the model parameters. Parameter estimations for equilibrium constants are undertaken and provide insights into the range of model validity and interactions between additions and softened water quality. Further jar testing is utilized to evaluate the effectiveness of using the model to numerically derive optimal chemical additions.

Synthetic musk fragrances are common personal care product additives and wastewater contaminants that are routinely detected in the environment. This study examines the presence eight synthetic musk fragrances (AHTN, HHCB, ATII, ADBI, AHMI, musk xylene, and musk ketone) in source water and the removal of these compounds as they flow through a Midwestern conventional drinking water plant with lime softening. The compounds were measured in water, waste sludge, and air throughout the plant. HHCB and AHTN were detected in 100% of the samples and at the highest concentrations. A mass balance on HHCB and AHTN was performed under warm and cold weather conditions. The total removal efficiency for HHCB and AHTN, which averaged between 67% to 89%, is dominated by adsorption to water softener sludge and its consequent removal by sludge wasting and media filtration. Volatilization, chlorine disinfection, and the disposal of backwash water play a minor role in the removal of both compounds. As a result of inefficient overall removal, HHCB and AHTN are a constant presence at low levels in finished drinking water.

In this research, a number of process modifications to the lime-soda softening process were examined, including utilization of high Mg-content lime, addition of MgCl 2 , and the recycling of softening sludge, in order to improve the removal of natural organic matter (NOM) and reduce the formation of disinfection byproducts (DBPs). Jar test results showed that dissolved organic carbon (DOC) removal increased and trihalomethane (THM) formation was reduced as the magnesium in hydrated lime increased, and was directly correlated with the amount of magnesium removed from the system. However, a dolomitic quick lime hydrated under atmospheric conditions resulted in less effective DOC removal due to a lack of available Mg, and subsequently, less co-precipitation of Mg(OH) 2 -NOM complexes. The addition of MgCl 2 to the raw water also increased DOC removal and reduced THM formation in both the presence and absence of softening sludge, with DOC removal increasing as softening sludge and magnesium dosages increased. As high as 43% removal of DOC was achieved at the stoichoimetric lime-soda ash dose in the presence of 457 mg/L sludge and 7.5 mg/L MgCl 2 , as compared to only 13% removal in the absence of sludge and MgCl 2 . The recycling of softening sludge had little or no effect on the hardness and the level of inorganic elements in treated water. The results presented here provide new approaches for improving DBP precursor removal during lime-soda softening without significantly increasing lime and soda ash dosage or the generation of waste sludge.

Chlorination of water containing organic compounds results in generation of undesired chlorinated hydrocarbons. Enhanced removal of precursors present in raw water is one of the viable options for effectively reducing the formation potential of undesired reaction products. Total organic carbon (TOC) and turbidity removal by Fe (III) coagulation under low pH, and coagulation under high pH with Fe (III) combined with lime softening were investigated for a lake water. The results of jar tests showed that low pH coagulation removed 42% TOC and 96% turbidity at an optimum pH of 6.3 when 20 mg/L Fe (III) was used as a coagulant. High pH coagulation with lime softening removed about 81% TOC and 97% turbidity at pH 10.3 when 9.5 mg/L Fe (III) was used as a primary coagulant.

Cooling tower blowdown can contain zinc and chromates which must be removed prior to discharge into the environment. Treatment alternatives include chemical reduction, ion exchange, and electrochemical reduction. A novel process involves lime softening with recycle of the treated water to the cooling water system. The best technology to utilize is a function of cooling-water quality. If makeup water is high in hardness, lime softening may be most appropriate. The ion exchange process functions best in water low in dissolved and suspended solids. The chemical and electrochemical processes can function well irrespective of cooling water chemistry.