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Odors are a major result of inadequately sized and mismanaged anaerobic lagoons. However, purple or pink colored lagoons, indicating the presence of phototrophic purple bacteria, are less likely to be an odor nuisance than are non–purple lagoons. Eight swine lagoons were studied to quantify critical parameters thought to allow purple lagoons to be a more reliable odor control alternative. Bacteriochlorophyll a (Bchl a), which indirectly measures the abundance of phototrophic bacteria, was greater in purple lagoons than in non–purple lagoons (P = 0.01). Oxidation–reduction potential (ORP) was less negative for purple lagoons than for non–purple lagoons in both spring (lagoon temperatures of 6.7°C to 8.8°C) and during summer (temperatures of 22°C to 25°C), indicating conditions favoring phototrophism (P = 0.04). Dissolved oxygen levels were near zero and light penetration was minimal in all lagoons. Average sulfide concentrations of all the lagoons were in the range of 1.6 to 6.5 mg/L, which is below the preferred range for purple sulfur bacteria (PSB) growth. Purple lagoons appeared to have lower concentrations of ammonia, alkalinity, chemical oxygen demand, and electrical conductivity among the lagoons studied. Copper and zinc concentrations of all lagoons were not in the range considered to be toxic for anaerobic bacteria. Calculated volatile solids loading rates did not explain differences in Bchl a levels in the lagoons.