Wood Lath Filters in Waste-Water Treatment Plants

Laths of Douglas-fir, western hemlock, southern pine, and redwood containing sapwood plus heartwood were compared with those of presently used redwood heartwood as trickling filters in waste-treatment plants. In addition to tests on untreated samples, all types of laths, except redwood heartwood, were treated with chromated copper arsenate or pentachlorophenol as a preservative. After 4 years of exposure to waste-water, untreated and treated Douglas-fir and southern pine were equal in breaking strength to, or stronger than, redwood heartwood. Shallow soft rot developed on the surfaces of some laths, mostly pine and hemlock, but significant losses of strength have been uncommon. All of the laths remained in serviceable condition. Future testing after 10 years of exposure will indicate more clearly if some woods other than redwood heartwood might attain a desired 20-year life. A choice of species offers cost and procurement advantages for fabricators of wooden filters.

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