Dear Shirley آب} اقیانوس ها و دریا ها شور هستند و استفاده از آنها استلزم تصفیه کردن است که این امر نیازمند سرمایه گذاری برای احداث [تأسیسات] آب شیرین کن میباش} very nice sentence you make, first tink first better dont use آب in plural form because after this world use two plural word, and just آب word most of the time means آب‌ها. So about two word you have question : تأسیسات : means some stuff tools or something help people to do something build something for example : تأسیسات ساختمانی means some stuff in building help for example have warm water. تسهیلات : this world most of the time means budget or money from somewhere you can give, for example when someone borrowing money from bank said : تسهیلات بانکی I hope this answer helpful