| Sumario: | The efficacy of three septic tank additives and a control was assessed using a randomized complete-block design in a double-blind study described in this article. Sludge depth, scum thickness, and total solids were measured within 48 full-scale, functioning septic tanks. These tanks were distributed across three sites with low, intermediate, and high prior-maintenance levels. No significant, positive long-term additive treatment effects occurred across all maintenance levels at the α = .05 level. Separate analyses of variance, however, indicated that at the high prior-maintenance site, significant treatment effects occurred on sludge depth, scum thickness, and total solids. Sludge depths for the Liquid-Plumr and Rid-X additives were significantly reduced compared to the control at the high-maintenance site, but not at the low-maintenance site. Liquid-Plumr septic tank additive had significantly greater sludge depth at the intermediate-maintenance site, which is a negative impact. Sludge accumulation rates averaged 6.9 cm/yr. for 3,780-L (1,000 gallons) septic tanks.
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