If circumstances permit, use of layered multi-sand filter comprising both coarse sand and fine sand as filter materials fitted with automatic backwash self-cleansing device is desirable.

• 如情况许可,宜使用包括粗沙和幼沙作为过滤物料的层沙过滤器,而过滤器应设有自动反冲的洁净装置。