Hi Max, you could also use a grout brush with long handle and microfiber floor cloths, save you having to go on your hands and knees if you are going to do it manually every month, if the grout lines are getting that soiled over a month they must be mopping the floor a few times a day probably with string cotton mops which are one of the most useless tools used within the cleaning industry, if its going to be a handy earner every month its best to say nothing, but there is a solution to stop this problem happening for your customer if it helps i can introduce you to a mopping system which can solve this problem. Tadgh