The answer is yes. Rainwater can be collected for indoor use. However, this one must be treated and purified for drinking purposes.
How much water do we use? The Demand of water depends on the size of each household and the end-use of the water. It is estimated that the amount water used per person is 80-100 gallons per day.
Also, around 70% of this water is used indoors with the toilet using most of the water. However, this percentage estimation can change depending on outdoor usage which varies with the yard size.
Design: The design for rainwater harvesting systems range from very simple to complex depending on the end-use of the water. For drinking purposes, the water must be treated to obtain potable water. For other purposes, the water can be used as non-potable. Both cases are brieflly demonstrated in the following figures.
Design 1 shows the process it takes to obtain potable water. After the rainwater is stored, this one is pumped and filtered to ultraviolet water purification system to finally obtain potable water. Note that there are different types of purification systems.
Design 2 is a perfect example of how water does not need to be purified for indoor purposes. However, note that the water is exclusively for the toilets and the washing machine. Piping is required to connect the sytem with the house water line. The pressure pump is in charge of moving the water from the system to a point-of-use.