According to one embodiment, a apparatus includes a controller configured to obtain values measured by a first sensor and a second sensor, receive pH target values at the entrance of a first reverse osmosis membrane and at the entrance of a second reverse osmosis membrane, and use a transfer function from the acid injection to pH at the entrance of the first reverse osmosis membrane, a transfer function from the alkaline injection to pH at the entrance of the first reverse osmosis membrane, a transfer function from the acid injection to pH at the entrance of the second reverse osmosis membrane, and a transfer function from the alkaline injection to pH at the entrance of the second reverse osmosis membrane, to control an injection rate of the acid injected from a first injection device and an injection rate of the alkali injected from a second injection device.