Can I empty clean water onto pitch?

hello ive just come away from my first trip out and emptied my wastehog in appropriate drain but my caravan also has an onboard water tank which was half full of clean water ,can i ask am i allowed to open tap and let it drain or must this go down waste water drain with dirty water ,? me and hubby nearly had a stand off regarding this ,so it will be interesting to know who was right , cheers mrs bc