Generally, domestic equipment that is 8~ 10 years old is of very good quality. Most domestic equipment is about 5 years old, and new products need to be replaced at most 8 years (upgrading and maintenance are expensive, and accessories are difficult to buy, so it is better to replace them directly).
There are many standards for the construction of sewage treatment plants, including design specifications, construction acceptance standards and water quality discharge standards.
The content is too huge. From the planning point of view, you may use dozens of specifications. Simple examples are as follows:
Specifications and standard grades:
Code for Design of Outdoor Drainage (GB500 14-2006)
Technical Guidelines for Water Pollution Control Engineering (HJ_20 15-20 12)
Discharge Standard for Urban Sewage Treatment Plants GB 189 18-2002
Comprehensive wastewater discharge standard GB8978 1996