The water reclamation, the greywater usage, and the desalination processes are kind of similar, but also kind of different.
The water reclamation is a process in which the wastewater through chemical and biological treatments is cleaned from the polluters, so that it can be returned into the environment and not cause any damage.
The greywater is a wastewater from the households and offices that doesn't have fecal contamination. This water is not suitable for drinking, but it is also not in a very bad state, so it is used for the toilet flushing, laundry washing, for irrigation.
The desalination is a process through which the saline water is stripped of the salt, in order to be suitable for drinking. This is not a widely used process, but some places like the countries on the Arabian Peninsula have to do it because there is not a single stream or river with freshwater with constant flow.
Oil-immersion objectives are limited to use with coverslip-prepared samples, since they don't have enough working distance to focus through the 1 mm or thicker plastic wall of a typical cell culture vessel.
