according to China's national economy industry classification and code (GB/4754-2111), the national economy industry classification is: a agriculture, forestry, animal husbandry and fishery; B mining industry; C manufacturing industry; D electricity, heat, gas and water production and supply industries; E construction industry; F wholesale and retail trade; G transportation, warehousing and postal services; H accommodation and catering industry; I information transmission, software and information technology services; J financial industry; K real estate industry; L leasing and business services; M scientific research and technical services; N Water conservancy, environment and public facilities management; O Resident services, repairs and other services; P education; Q health and social work; R Culture, sports and entertainment; S public management, social security and social organizations; T international organization

in which D4621 sewage treatment and recycling

refers to the treatment and disposal of sewage sludge and the reuse activities after purification

sewage treatment plants belong to this category.