Anish, the answer is not easy ...

Anish, the answer is not easy as it requires a variety of factors like treated effluent parameters, the extent of treatment desired, land area available, the increase in the capacity etc.  Best would be to engage an experienced and unbiased consultant to audit the existing set up and then, suggest a suitable upgradation. Many companies will offer solutions suited to what they offer as a system, which may not be the best for your situation. Again, the muncipal outfits may not want to go for solutions,, which may be better than the existing processes. All stakeholders need to buy in an optimum solution.