Issues regarding the lime softening of wastewater

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Industrial wastewater is water with high hardness and high alkalinity, with a total hardness of 1100 ppm, calcium hardness of about 950 ppm, and a total alkalinity of 1200 ppm. (All values are on a calcium carbonate basis.) Given the characteristics of this water, the total hardness and total alkalinity are fairly similar; therefore, I used a single-component lime softening process, and after softening, a flocculant was added to help with slurry formation. Finally, the total hardness in the supernatant was determined. The amount of lime added is: 28* (calcium hardness + 2* magnesium hardness + flocculant concentration + CO2 + excess amount). I used a higher dosage than the theoretical value, yet the total hardness remaining in the wastewater was reduced by only 20%, which is far below the 90% reduction mentioned in the books. I also tried the lime-sodium carbonate method, but it didn’t work well either. I did some research; I first prepared lime into a slurry with a concentration of around 20%, allowing the lime to digest first before adding it to the wastewater. But in the end, there was no significant change in the results. Lime softening method: After adding lime, the pH value of the wastewater generally remains below 9.0, with no significant increase. Additionally, I used NaOH as a softener, and its effect was better than that of lime, resulting in a reduction in total hardness of over 30%. It didn’t have the desired effect. Moreover, the NaOH method cannot reduce alkalinity. I tried using more than twice the normal amount of NaOH; as a result, the pH of the wastewater reached over 10.5, and the total hardness dropped to below 300. But such a high dosage results in excessive costs, which is not economical. I would like to ask experts: for water with such high hardness and high alkalinity, and where the hardness and alkalinity are not very different from each other, which treatment process is better? What other issues need to be considered during the softening process? Does anyone have the operating methods or procedures for softening processes using lime-based chemicals? I would appreciate it if you could share them; I’ll be sure to thank you generously. Thank you!