Under the Sink or Under the counter water purifier buying Guide. Water purifier is essential for every household. However, many of them do not have enough space to be placed in the kitchen or can sometimes change the look of your kitchen. In such a situation, an under-sink RO is the best option for you as it can be easily placed in your kitchen to save space and avoid making any changes in the kitchen. Like other purifiers, submersible RO purifiers are equipped with various purification processes that remove various kinds of impurities from the water and make it safe for use. The tap of the water purifier can be directly connected to the washbasin so that purified water can be used not only for drinking but also for cooking.