Sludges from Natural Treatment Systems

A significant advantage for the natural wastewater treatment systems described in previous chapters is the minimal sludge production in comparison to mechanical treatment processes. Any major quantities of sludge are typically the result of preliminary treatments and not the natural process itself. The pond systems described in Chapter 4 are an exception in that, depending on the climate, sludge will accumulate at a gradual but significant rate, and its ultimate removal and disposal must be given consideration during design. In colder climates, studies have established that sludge accumulation proceeds at a faster rate, so removal may be required more than once over the design life of the pond. The results of investigations in Alaska and Utah (Schneiter et al., 1984) on sludge accumulation and composition in both facultative and partial-mix aerated lagoons are reported in Table 9.4 and Table 9.5.

A comparison of the values in Table 9.4 and Table 9.5 with those in Table 9.2 and Table 9.3 indicates that the pond sludges are similar to untreated primary sludges. The major difference is that the solids content, both total and volatile, is higher for most pond sludges than for primary sludge, and the fecal coliforms are significantly lower. This is reasonable in light of the very long detention time in ponds as compared with primary clarifiers. The long detention time allows for significant die-off of fecal coliforms and for some consolidation of the sludge solids. All four of the lagoons described in Table 9.4 and Table 9.5 are assumed to be located in cold climates. Pond systems in the southern half of the United States might expect lower accumulation rates than those indicated in Table 9.4.

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