Look up the WHO Drinking Water Guidelines.  They list all the limits, both for health and for taste.

The guideline you need depends on what you intend to use treated wastewater  for and I expect that intended use must have been considered before choice of treatment.

If treated for irrigation, check Nnadi et al (2014)

https://www.researchgate.net/publication/265965480_Stormwater_harvesting_for_irrigation_purposes_An_investigation_of_chemical_quality_of_water_recycled_in_pervious_pavement_system 

In addition to WHO, http://www.who.int/water_sanitation_health/wastewater/gsuww/en/ 

You may find the following useful:           

USEPA: https://www.watereuse.org/government-affairs/usepa-guidelines

Article Stormwater harvesting for irrigation purposes: An investigat...

Water quality index will not work for waste water due to its complexity.

Hello Satya,

For the wastewater quality index analysis, you can apply the Canadian Water Quality Index (CWQI) model. This is formulated for the surface and drinking waters. But the same model you can apply by changing the excursion values according to the concerned agency such as CPCB or WHO or any other source. 

The below gives you the details about the CWQI model.

http://www.env.gov.nl.ca/env/waterres/quality/background/cwqi.html

AND, I have applied the same model for one of my studies. This can help you.

https://www.researchgate.net/publication/222408851_Evaluation_of_the_potential_of_various_aquatic_eco-systems_in_harnessing_bioelectricity_through_benthic_fuel_cell_Effect_of_electrode_assembly_and_water_characteristics

Good luck

Article Evaluation of the potential of various aquatic eco-systems i...

It also depends on the kind of wastewater you wanted to reuse after treatment. For instance, if it is human waste or animal waste, pathogens removal need to be counted. In addition to above valuable comments, I would recommend to look for regulations for the specific location. 

Hi Satya

There is no index as it depends on what you want to do.

Here is a references related to your question for agriculture and aquifer recharge :

El Ayni F., Cherif S., Jrad A., Trabelsi-Ayadi M. (2011) : Impact of Treated Wastewater Reuse on Agriculture and Aquifer Recharge in a Coastal Area: Korba Case Study. Water Resources Management. 25(9) 2251-2265. 115. doi:10.1007/s11269-011-9805-2 

You can use the WHO wastewater reuse guidelines. It is a helpful. Remember that is just a guideline not a standard.

Dear

finding Effluent quality index eqi is my pHD thesis. you can email me for more guidance about this subject.

[email protected]