The design features and performance expectations for natural aquatic treatment units are summarized in Table 1.1. In all cases, the major treatment responses are due to the biological components. Aquatic systems are further subdivided in the process design chapters to distinguish between lagoon or pond systems. Chapter 4 discusses those that depend on microbial life and the lower forms of plants and animals, in contrast to the aquatic systems covered in Chapters 6 and 7 that also utilize the higher plants and animals. In most of the pond systems listed in Table 1.1, both performance and final water quality are dependent on the algae present in the system. Algae are functionally beneficial, providing oxygen to support other biological responses, and the algal-carbonate reactions discussed in Chapter 4 are the basis for effective nitrogen removal in ponds; however, algae can be difficult to remove. When stringent limits for suspended solids are required, alternatives to facultative ponds must be considered. For this purpose, controlled discharge systems were developed in which the treated waste-water is retained until the water quality in the pond and conditions in the receiving water are mutually compatible. The hyacinth ponds listed in Table 1.1 suppress algal growth in the pond because the plant leaves shade the surface and reduce the penetration of sunlight. The other forms of vegetation and animal life used in aquatic treatment units are described in Chapter 6 and Chapter 7.
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