Which of the following tests is used to evaluate an aerobic treatment plant for excessive solids?

The 30-minute settling test is specifically designed to evaluate the performance of an aerobic treatment plant by measuring the ability of sewage solids to settle out of the wastewater. In this context, excessive solids can lead to operational issues, such as reduced treatment efficiency and poor effluent quality. The test assesses how quickly and effectively the solid particles in the wastewater can settle to the bottom of the testing vessel under static conditions.

This information is essential for plant operators because significant settling ability indicates good separation of solids, leading to better effluent clarity and treatment outcomes. It serves as a straightforward assessment that identifies whether the treatment plant is operating within acceptable limits regarding solid accumulation, which is crucial for maintaining the overall health and effectiveness of the treatment process.

The other tests listed, while valuable for different evaluations, do not specifically focus on identifying excessive solids in the context of aerobic treatment systems. For instance, the pH test measures acidity or alkalinity, the biochemical oxygen demand assesses organic matter decomposition, and the soil absorption test evaluates how well the soil can absorb treated effluent. These tests serve distinct purposes and do not directly assess the settling characteristics of solids in the treatment plant as the settling test does.