According to the calculation of 30,000 people, each person's daily water consumption in the 120L, the coefficient of production of 0.85, the design of 3060 cubic meters / day of sewage treatment plant, if to achieve the first class A standard, the investment cost of about 6.5 million (not counting the cost of the land), the civil construction of 50 years, the depreciation of the equipment according to the 5-year calculation, the recovery of the cost of the general calculated in 15-20 years.
Of course this depends on the specific situation and program development