Normally we throw away the water left over after sandayavandanam or pour it under a tree. Can I drink the water instead of throwing it away? Can it enhance my health? Also can others drink it?
Yes, you can consume it just like we consume Prasada of deities. It is supposed to enhance your health or health of anyone who consumes it.
Arghya must be offered before sunrise at noon and at sunset. Once there was a man called Idaikattu Siddha who grazed cattle. He said: "Kanamar konamar kandu kodu adugan pohutu par. " "Kanamal/r" means before you see the sun rise and "konamal/r" means when the sun is overhead and "kandu" is when you see the sun before sunset. These are the three times when you ought to offer arghya. "adu" means "niradu", bathe in the Ganga. "kan" here means "visit Setu" or have " have darsana of Setu". "Pohutu par"- by bathing in the Ganga and by visiting Setu your sins will be washed away. Here is mentioned the custom of going to Kasi, collecting Ganga water there and going to (Setu) Ramesvaram to perform the abhiseka of Ramanathasvamin there.
Only by the intense repetition of Gayatri shall we be able to master all the Vedic mantras. This japa of Gayatri and arghya must be performed everyday without fail. At least once in our lifetime we must bathe in Ganga and go on pilgrimage to Setu.
If a man has a high fever, people looking after him must pour into his mouth the water with which sandhyavandana has been performed. Today it seems all of us are suffering all the time from high fever! When you run a high temperature you have to take medicine; similarly Gayatri is essential to the self and its japa must not be given up at any time. It is more essential to your inner being than medicine is to your body. Sandyavandana must be performed without fail everyday. Gayatri-japa can be practised by all of us without much effort and without spending any money. All that you require is water. Sandyavandana is indeed an easy means to ensure your well being. So long as there is life in you, you must perform it.
Also, yes, others too can drink it as the above passage in bold clarifies.