Norway rats, commonly known as sewer rats, are brown rats that live in sewer systems. They are most active at night as they search for food. They need water to survive, but the sewers provide that, especially since they are good swimmers. These rats can emerge from your toilet and run around your house until they find any sort of food. Even garbage or scraps in your kitchen sink can attract sewer rats.
Keep all the toilet lids in your home down whenever they are not in use.
Squirt some dishwashing liquid around the inside of the toilet bowl quickly, if you spot a rat in there. Close the lid and flush the toilet to get rid of the pest. The soap will make the porcelain slippery so the rat can't climb out and enter your house.
Put away all foodstuffs from your kitchen area so they don't attract sewer rats. Even fruit or loaves of bread on countertops can attract them. Don't leave pet food out either.
Pour a cup of household bleach into your kitchen sink drain, then add a kettle full of boiling water. Do this once or twice a month to clean bits of food out of the drain.
You can also pour a cup of baking soda and a cup of vinegar down the drain to clean it out. Rinse the drain with boiling water afterward. If the problem is recurrent, notify your city's health department.
Avoid disposing of cooking grease down the drains. Sewer rats carry germs and diseases.