I was recently reading an article called "What if Everybody Flushed their Toilets at the Same Time?" The article discusses sewage systems from our households to treatment centers and what would happen if everyone unanimously flushed their toilets.
Pipes from your home are small in diameter while those leading to neighborhood, town, and city treatment centers get progressively larger. Sewage leaving your home and arrives at the treatment centers after being pushed by a lift station. This is the first place waste would back up. Eventually, all of the waste would be forced back through the pipes and into our homes, resulting in backed up toilets, sinks, and even dishwashers. Not only would it back up in homes, but manholes in the streets would also overflow.
A solution mentioned in the article is low flow toilets. Low flow and dual flush toilets use a reduced amount of water when flushed and also save money on water and energy bills.
Source: How Stuff Works