Can I use a refractor telescope 5" aperture, 900mm focal length on an equatorial mount for terrestrial viewing.
Yes, potentially. There are a few limitations:
It will be easier to use in alt azimuth mode, which in practice means setting the polar axis to 90 (ie straight up). Not all equatorial mounts can do this.
With the standard astronomical diagonal, images will be inverted left to right. You can get diagonals for terrestrial viewing that will fix this. If you don't use a diagonal, the image will be upside down.
The field of view will be quite restricted if you use a typical eyepiece. For example a 20mm plossl would give a magnification of 45 and a field of view of about 1 degree. To get more useful field of view, you would need longer focal length eyepieces, with with a wide angle design. These are more expensive than ordinary eyepieces.
Unless your scope is an apo, you may see quite noticeable chromatic aberration, particularly at the edges of objects with a bright background (eg birds).
Supplemental to @DrChuck's thorough answer and in reference to advice there about the use of a standard astronomical diagonal leaving images inverted left to right, I'd like to point out that there are image erectors that can solve this problem.
Binoculars are two telescopes where each has either a pair of prisms or newer fancy prism to flip the image up-down and left-right so that it is the same orientation as what we see without them.
These have been available with refractors for a long time so that we don't have to turn star charts upside down so that they look the same as they do through the telescope. My old Unitron 2.4 inch (50 mm) (like the one below but smaller) came with a big circular object containing a pair of prisms just like the old binocular configuration, but fancier, smaller prism image erectors now exist.
Out with the old...
(cropped & annotated from here)
and in with the new.
(from here; 45° Amici roof prism erector)