Hi just wondering if anyone has experience doing a rainwater harvesting system for their house use (gardening, flush toilets) Be it DIY, or professionally done with underground tanks and pumps.. seems to be quite normal in other countries like Malaysia. ? 

would like to know how the PE run the pipings.. and pros & cons etc 

In Singapore, technically any rain water harvesting systems need to be designed and signed off by a QP and approved by PUB. the legality of DIY systems is also a question mark as even though it is not illegal to harvest rain water but since PUB doesn't approve your installation, it can be deemed illegal. the main concern is mosquito breeding if the water collection tanks are not covered properly.

if you search online, one of our famous politicians has installed a rain water harvesting system for watering his garden years ago. Technically it is illegal to use tap water for gardening and watering the plants but hardly anyone follows it or even know that this law exists.

if you really want to have a system with underground tanks and use it for toilets, then it should be designed right from the start before you do your house renovation/construction as the pipes need to be installed and segregated from the potable water source. Normally the water pipes are all interconnected to the source from PUB. so if you want to do this after, you will have a lot of problems with the water pipes routing. Also, the rain water down pipes for your house should also be connected back to this tank else the norm is for the pipes to just drain to the house drain instead which will then lead to the public drains. the roof of the house should also be designed such that the surface water will be channeled to the rain water down pipes for maximum collection else they would just go to waste and back into the drains. one point to note is that roof gutters is banned so the channeling of rain water would be a challenge for your QP to direct most of the flow towards the rain water down pipes. You would be surprised at how much water can come down at one time from the roof when the rain water is channeled to just one area.

for me, I'm just going to buy those big industrial water containers and build a sand, gravel, charcoal filter with the necessary mosquito nettings in place to collect the rain water when it rains. while the water can't be used for flushing toilers, it would to be used for my gardening purpose.

Bear in mind that water that comes from the roof can be pretty dusty so use sufficiently large caliber pipes..

A pool is a nice place to keep all that water ?