Objective 04: Hot Lime-Soda Softening

Question 1

What are 3 ways that Hot Lime-Soda Softening differ from Cold Lime-Soda Softening?

Answer 1

1. Steam is used to heat the incoming water to above 100⁰C
2. Chemical reactions are almost instantaneous
3. Softer water is produced

Question 2

What functions can be combined in Hot-Lime Soda Softening?

Answer 2

1. Softening (reducing hardness to 10-30 ppm)
2. Silica removal
3. Deaeration
4. Filtration

Question 3

What kind of water is Hot Lime-Soda Softening?

Answer 3

Water with high hardness

Question 4

What appliances are associated with typical Hot Lime-Soda Softening

Answer 4

1. Pressure filters to filter out any carryover of sludge particles from the softener
2. Chemical tank and pump with an arrangement for proportioning the chemical to the raw water flow
3. Sludge recirculating pump
4. Filter backwash pump

Question 5

Describe the process of Hot Lime-Soda Softening

Answer 5

1. Raw water enters the top through a series of sprays, with the amount being controlled by the level in the softener
2. Steam enters at the top, where it heats and deaerates the incoming water
3. Chemicals enter at the top and are thoroughly contacted by the water sprays. Precipitates are formed immediately and form sludge.
4. The large diameter of the softener causes slow velocity & it takes about 1 hour for the water to travel
5. At the inverted cone the water turns upwards, while the sludge drops to the cone chamber below, forming a sludge bed
6. Some of the bottom sludge is periodically discharged to waste to control the sludge level. Some sludge is continuously recirculated to the top of the softener, which promotes coagulation and reduces the amount of chemicals required.
7. The softened water leaves the outlet and goes to the pressure filters.

Question 6

How are the pressure filters backwashed in Hot Lime-Soda Softening?

Answer 6

The filters are backwashed with water from the softener; the backwash water returns to the softener after leaving the filters.

Question 7

What governs the temperature of the Hot Lime-Soda Softening Process?

Answer 7

The steam pressure maintained inside the tank.

Question 8

What caution must be taken when operating Hot Lime-Soda Softening?

Answer 8

1. Control of lime soda softening requires frequent testing and control adjustments
2. Extra care is required to ensure sludge carryover with the treated water is kept to a minimum so that the filters don’t plug up quickly
3. Chemical feed system requires constant attention since lime tends to settle out on the inside of the piping and pumps