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In the United States, some utilities provide lime softening to remove hardness from the drinking water. Lime softening generates residuals as part of the water treatment process, which are inert and non-hazardous. This report provides an environmental and economic evaluation of three lime residuals disposal alternatives: a registered disposal site, beneficially reusing the lime residuals in cement manufacturing, and recalcinating the residuals to reduce the utility’s lime dosage. This report also provides an overview of relevant non-economic factors utilities should consider when selecting a residuals management solution. Because the best alternative for any utility will be specific to that utility’s resources, conditions, and values, this report serves as a framework others might use when evaluating their lime residuals management alternatives. This report found the emissions due to recalcination and the emissions reduction from beneficially reusing the lime residuals in cement manufacturing are orders of magnitude larger than the transportation emissions associated with the registered disposal site. Notably, the emissions due to recalcination will likely be offset by a reduction in lime production, but shifting the emissions from the private sector to a utility might be undesirable. Of the three alternatives, recalcination has the highest capital costs due to the need to purchase a rotary kiln. Cement manufacturing has the highest operations and maintenance costs, and recalcination presents a net savings in operations and maintenance costs due to the chemical savings associated with reduced lime usage. Overall, recalcination is the most financially attractive option, but cement manufacturing provides the most value when considering the social cost of carbon dioxide emissions. Utilities should also consider non-economic factors when assessing lime residuals disposal alternatives, including public perception, managerial impacts, regulatory requirements, and the utility’s values and organizational structure, to assess the full impacts of the alternatives and maximize the likelihood of the selected alternative receiving approval by the public and the utility’s governing body

In the United States, some utilities provide lime softening to remove hardness from the drinking water. Lime softening generates residuals as part of the water treatment process, which are inert and non-hazardous. This report provides an environmental and economic evaluation of three lime residuals disposal alternatives: a registered disposal site, beneficially reusing the lime residuals in cement manufacturing, and recalcinating the residuals to reduce the utility’s lime dosage. This report also provides an overview of relevant non-economic factors utilities should consider when selecting a residuals management solution. Because the best alternative for any utility will be specific to that utility’s resources, conditions, and values, this report serves as a framework others might use when evaluating their lime residuals management alternatives. This report found the emissions due to recalcination and the emissions reduction from beneficially reusing the lime residuals in cement manufacturing are orders of magnitude larger than the transportation emissions associated with the registered disposal site. Notably, the emissions due to recalcination will likely be offset by a reduction in lime production, but shifting the emissions from the private sector to a utility might be undesirable. Of the three alternatives, recalcination has the highest capital costs due to the need to purchase a rotary kiln. Cement manufacturing has the highest operations and maintenance costs, and recalcination presents a net savings in operations and maintenance costs due to the chemical savings associated with reduced lime usage. Overall, recalcination is the most financially attractive option, but cement manufacturing provides the most value when considering the social cost of carbon dioxide emissions. Utilities should also consider non-economic factors when assessing lime residuals disposal alternatives, including public perception, managerial impacts, regulatory requirements, and the utility’s values and organizational structure, to assess the full impacts of the alternatives and maximize the likelihood of the selected alternative receiving approval by the public and the utility’s governing body

The goal of this research was to maintain excellent water quality at reduced chemical operations cost. Chemical usage data at the Austin water treatment plants were examined by identifying trends and investigating suspected inefficiencies. The investigation consisted in jar test experiments, plant-scale experiments, and equilibrium modeling. Lime and ferric sulfate were suspected to be added inefficiently with respect to cost while the other treatment chemicals were assessed to be added efficiently. Lime was investigated in greater depth than ferric sulfate because ferric sulfate was better characterized in its effect on finished water quality within the range of interest. The goal of lime addition is to remove hardness from the water by a process called lime softening. Hardness removal decreases corrosion in transmission lines and prevents deposition of unwanted solids in household appliances. Additionally, lime softening aids in particle removal and disinfection-by-product precursor reduction. The efficiency of lime addition was evaluated based on settled water pH and causticity goals, which serve as the operating parameters for the water treatment plants. The most efficient lime softening occurs when multiple softening goals are simultaneously achieved. First, the dissolved calcium concentration must achieve a minimum. Second, the dissolved magnesium concentration must be reduced by at least 10 mg/L as CaCO₃. Third, total alkalinity must be preserved at its maximum concentration while also achieving excellent hardness removal. Fourth, natural organic matter (NOM), which serves as a precursor for disinfection-by-products, must be removed sufficiently to achieve DBP reduction goals. Finally, the turbidity in the effluent from the settling basin must be below 2.0 NTU. Through the chemical investigation of lime based on existing scientific literature, computer modeling, jar test experiments, and full-scale testing, it was determined that the optimal condition operating condition for lime softening was a settled water pH range from 10.0 - 10.1.

The goal of this research was to maintain excellent water quality at reduced chemical operations cost. Chemical usage data at the Austin water treatment plants were examined by identifying trends and investigating suspected inefficiencies. The investigation consisted in jar test experiments, plant-scale experiments, and equilibrium modeling. Lime and ferric sulfate were suspected to be added inefficiently with respect to cost while the other treatment chemicals were assessed to be added efficiently. Lime was investigated in greater depth than ferric sulfate because ferric sulfate was better characterized in its effect on finished water quality within the range of interest. The goal of lime addition is to remove hardness from the water by a process called lime softening. Hardness removal decreases corrosion in transmission lines and prevents deposition of unwanted solids in household appliances. Additionally, lime softening aids in particle removal and disinfection-by-product precursor reduction. The efficiency of lime addition was evaluated based on settled water pH and causticity goals, which serve as the operating parameters for the water treatment plants. The most efficient lime softening occurs when multiple softening goals are simultaneously achieved. First, the dissolved calcium concentration must achieve a minimum. Second, the dissolved magnesium concentration must be reduced by at least 10 mg/L as CaCO₃. Third, total alkalinity must be preserved at its maximum concentration while also achieving excellent hardness removal. Fourth, natural organic matter (NOM), which serves as a precursor for disinfection-by-products, must be removed sufficiently to achieve DBP reduction goals. Finally, the turbidity in the effluent from the settling basin must be below 2.0 NTU. Through the chemical investigation of lime based on existing scientific literature, computer modeling, jar test experiments, and full-scale testing, it was determined that the optimal condition operating condition for lime softening was a settled water pH range from 10.0 - 10.1.

In this research, a number of new approaches to increase the removal of NOM during the lime-soda softening process were investigated. Specifically, softening sludge recycling in the presence and absence of cationic polyelectrolytes, MgCl2 addition in the presence and absence of softening sludge, and the role of high magnesium lime were all evaluated. Recycling of softening sludge during lime softening was found to effectively improve NOM removal. A 554 mg/L sludge dose added with the stoichiometric lime-soda ash dose removed 28% of NOM present in a raw water as compared to only 10% NOM removal in the absence of sludge. In addition, Hydrophobic/Hydrophilic fractionation analyses, together with SUVA measurements indicated that hydrophobic fractions of NOM were preferentially removed. Further experiments showed that sludge recycling did not significantly affect pH, turbidity, hardness and level of inorganic elements in treated waters. The presence of polyelectrolyte CPAM was found to increase the sorption capacity of calcite particles for Aldrich humic acid by three times. Based on this, softening sludge was coated with CPAM and recycled during jar tests. Results showed that sludge coated with low doses of CPAM can improve NOM removal to level significantly higher than regular softening. However, the NOM removal level was comparable to that achieved by recycling bare softening sludge. The use of high magnesium hydrated lime and the addition of magnesium were found to be promising techniques to increase Mg(OH)2 precipitation and consequently NOM removal during the softening process. DOC removal increased as available magnesium in hydrated lime increased. A dolomitic quick lime sample, slaked at ambient pressure, however, hindered NOM removal due to lack of available magnesium and subsequent coprecipitation. As high as 43% NOM removal was achieved in the presence of 0.312 mM MgCl2 and 457 mg/L softening sludge dose as compared to only 13% for regular softening. DBP precursors were removed as indicated by THM concentration measurements. All the techniques discussed here can be implemented without any modifications in existing facilities or addition of new treatment step and/or equipments.