Which of the following methods is not typically associated with softening very hard water?

Coagulation is primarily used in water treatment for the purpose of removing suspended solids and impurities from water, primarily through the aggregation and settling of these particles. This process involves adding chemicals, known as coagulants, which help smaller particles clump together into larger ones that can be easily removed. While it's an essential part of water purification, coagulation does not mitigate the hardness of water, which is determined by the concentration of dissolved minerals, particularly calcium and magnesium ions.

In contrast, the other methods listed are specifically designed to address water hardness. Reverse osmosis operates by forcing water through a semi-permeable membrane, removing a wide range of dissolved solids including hardness-causing minerals. Ion exchange involves exchanging hard ions (like calcium and magnesium) with softer ions (typically sodium), effectively softening the water. Lime-soda treatment utilizes lime and soda ash to precipitate hardness-causing minerals out of the water, thus reducing its hardness.

So, in the context of this question, coagulation does not relate to the softening of hard water, making it the method that stands apart from the others that directly target water hardness.