Lime Softening In Water Treatment

29th  August, 2024.

In this post, we will see the concept of lime softening in water treatment.

Hard water is not useful for drinking or washing purposes. It can cause itches on human skin, create salty layers on clothes, cause scaling problems, does not lather well with soaps, and causes other types of similar problems. It is thus necessary to soften it for use.  One of the popular methods in water softening is to use lime soda or lime ash for this purpose. In this post, we will see the concept of lime softening in water treatment.

What is hardness and why is hard water problematic?

First of all, let us understand why we do not require hard water for usage. Hard water is that quality of water which has some hard minerals in it like calcium, iron, zinc and magnesium. The major minerals to be counted are calcium and magnesium. So, the hardness of water is calculated by seeing their proportion in water. Due to their composition, they do not dissolve in water easily. They seem scalar in nature, where they want to make contact with some other surfaces where such water is present. Hardness is derived due to underground facilities, where the groundwater makes contact with rocks and soil present inside. The minerals from these rocks, which are nothing but calcium and magnesium, get mixed in the water and make it hard. A most common example of you seeing hard water is when you notice white scums on shower heads for bathtubs. This indicates that the water is hard in nature. The hardness is measured in mg/L of CaCO3 (calcium carbonate, which is the sum of calcium and magnesium). Values of more than 150 mg/L indicate the presence of hard water. 

Hard water is problematic because it causes white dandruff like scums on utensils, water shower, bathtubs, appliances, glassware, accumulates inside water geyser parts and damages the pipelines or internal metallic parts, causes itches on human skin, and results in hair damage. So overall, the hardness of water has long term effects on human use in day to day lives. So, it is necessary to soften this water for further usage.

What does lime do in water treatment?

Before continuing on this point, let us understand a term which will be used frequently below - precipitation. Precipitation is water which is released from clouds in the form of snow, rain, or hail. It happens due to condensation inside clouds. This means the water is thrown away from them. This same term is used as chemical precipitation for treating hard water, where we are throwing the calcium carbonate away.

To precipitate calcium carbonate, it is required to increase the pH levels (concentration of hydrogen ions, deciding whether the water is acidic or basic or neutral) of water. The standard pH level of water is between 0-14. Increasing pH levels will start to chemically precipitate calcium carbonate from water, as the concentration of hydrogen is increased. Now, to increase the pH level in water (9.4 for calcium removal and 10.6 for magnesium removal), we use a process called lime softening. It is nothing but the addition of lime (calcium hydroxide or CaOH2) and soda ash (sodium carbonate or Na2CO3) in the water. Due to their addition, the precipitation occurs for calcium and magnesium, which are then easily removed by processes like sedimentation, or coagulation / flocculation. It is to be noted that it is not possible to completely remove calcium carbonate; it will remain somewhere around 50 to 85 mg/L, which is enough for usage. Lime is used for calcium and soda ash for magnesium. So, as the pH level for calcium was 9.5, if majorly calcium is present, then only lime addition is required. Otherwise, both the lime and soda ash need to be added in the water. 

How does the lime softening process work?

Let us see how the lime softening works. The process works in two types - single stage and double stage. 

In the single stage process, lime or soda ash addition is used only once. If we have only high levels of calcium carbonate in the water with more proportion of calcium and less proportion of magnesium, then we add either only lime or in some cases, lime in more proportion and soda ash in less proportion. Due to this, the pH of water increases to 10.6. Note that such pH levels also indicate the acidness present in water, which cannot be used for further treatment and processes. So, to bring the pH down again to a level where calcium carbonate will also precipitate (means somewhat in between where precipitation occurs and pH too remains under level), we add carbon dioxide to the water. This process is also called recarbonation, and due to the chemical composition, the pH is brought down to a level where both the tasks are achieved (around 8.6 where a small calcium carbonate proportion will be present). After this, the water is sent to next treatment stages where the lime sludge is sedimented, clarified or coagulated. This softens the overall water quality.

In the double stage process, the pH will first be increased to 10.6 and then recarbonated to only 9.5. Further, it will be again increased to a little extent and then recarbonated to 8.6. Basically, such treatment is used where there is equal proportion of both the calcium and magnesium, and we will require a higher amount of lime and soda ash for that.  

Some important points to remember in this cycle are:

1. Lime feeders and lime basin tanks must be properly maintained by trained professionals, because pH is a sensitive parameter, and any up-down in it can hamper the overall process.

2. Caustic soda can also be used in place of lime and soda ash, but it is very expensive. But the end result of caustic soda is far better, because sludge deposits are lower in this material (total dissolved solids or TDS level is increased in caustic soda usage).

3. The lime sludge generated after the softening process (where excess calcium carbonate is deposited below the tank and referred as sludge) must be treated further before disposing it off somewhere or using further for more secondary treatment processes.

4. Proper mixing stage must be achieved in lime basin tanks, so that the chemicals merge with each other properly and result in a reduced level of hardness.

5. If total hardness is less than or equal to total alkalinity, there is no non-carbonate hardness (only carbonate hardness). This will require only lime addition or a little amount of soda ash too. If total hardness is greater than total alkalinity, non-carbonate hardness equals the difference between total hardness and total alkalinity (and carbonate hardness equals total alkalinity). This will require both the lime and soda ash addition in good proportion ratios. 

6. Water from the lime stage is not directly passed to recarbonation stage; it is first sedimented or clarified after adequate mixing and settling time to remove lime sludge and then sent for minimizing the pH levels as discussed earlier. Carbonated water will then be passed to further filtration stages to soften the water more.

I have some general principles related to lime softening treatment. I have also not attempted to cover all the topics, as it can vary from case to case and there is no end to it.  Once you are familiar with this engineering, you can easily tackle all types of problems in it. 

Thank you for reading the post. I hope you liked it and will find a new way in this type of technology.