The way question is ...

The way question is presented is rather misleading .

At first , talking about reusing rainwater whereas rainfall is not consistent is tricky .  Of course rainwater in areas as Tunisia is considered rather clean and therefore minimal treatment is required .

On the other hand , use of treated wastewater after tertiary treatment mainly for agricultural use is already widely used in GCC countries with different degree and complexity (with subsequent varying cost) .

I have been deeply involved in three cases of water re-use in Creta Island , Greece , in Jabel Ali Plant , Dubai , UAE and Sulaibiya Plant in Kuwait .

The last one is designed for water reuse at potable level treatment so after conventional biological treatment , water passes through 60μ. disk filters , followed by UF and finally two or three stage RO treatment.

The last Plant produces clean chemically water H2O , which i have tried it myself with BOD @ 0  , TDSS below 100 , zero micro organisms etc.

Of course a cost of around  $0.7 USD is involved . This water is used for agriculture .

Therefore one should consider all available technologies and combine costs involved .

So for waste water effluent i gave above an indication , whilst for rainwater one should consider cost of collection in open reservoirs (small storage dams) , piping network to distribution areas or pumping stations for enriching water wells and so on .

For sure it is a MUST for those countries that they already face water shortage and hence water handling and management is of utmost importance .