low-flush toilet toilets pre-1994 Wiki low-flow toilets low-flow toilet 1.6-gallon maximum toilets 20 million gallons of water per year 1.6 gallons per flush چه نوع توالتی چه نوع توالتی average $90 per year low flow toilet federal law prohibited toilets using >6L/flush 3.5 gallons Low-flush Low flush-toiletten