本文描述的研究解决了小型水系统面临的问题，需要建造易于操作和维护的处理设施，同时生产出符合饮用水标准的水。本文回顾了最近水传播贾第虫病的暴发情况，并强调需要一种以上的屏障（即氯化）来阻止疾病传播。描述了快速和慢速砂滤的特点，讨论了它们作为保护公众健康的第二道屏障的优缺点。中试研究结果表明，当原水水质较高时，慢速砂滤优于以明矾或阳离子聚合物作为混凝剂的直接在线过滤。浊度、颗粒计数和总大肠菌群数据证实了这一结论。 藻类计数或藻类种群的替代测量对于判断原水是否可接受缓慢砂滤至关重要。对于水质较差的水，可以使用直接在线过滤，但该系统需要更多的操作技能和注意力。慢砂过滤和直接在线过滤在过滤器运行初期都表现出一段时间的过滤质量差，因此，如果贾第虫囊肿是一个问题，则需要一段过滤到浪费的时间。包括19个参考文献、表格和图表。

The research described in this article addresses the problems of small water systems faced with the need to construct treatment facilities that are easy to operate and maintain and yet produce water that meets drinking water standards. Recent outbreaks of waterborne giardiasis are reviewed, and the need for more than one barrier (ie, chlorination) to disease transmission is emphasized. Characteristics of rapid and slow sand filtration are described and their strengths and weaknesses as second barriers for protecting public health are discussed. Results of a pilot study indicate that when raw water quality is high, slow sand filtration outperforms direct in-line filtration operating with alum or cationic polymer as a coagulant. This conclusion was substantiated by turbidity, particle count, and total coliform data. Enumeration of algae or a surrogate measure of algal population is essential for judging the acceptability of a raw water for slow sand filtration. For waters of poorer quality, direct in-line filtration can be used, but the system requires substantially more operational skill and attention. Both slow sand filtration and direct in-line filtration exhibited a period of poor filtrate quality at the beginning of filter runs, and therefore require a filtering-to-waste period if Giardia cysts are a concern. Includes 19 references, tables, figures.