Water Environment Federation (WEF)

Comparing Gravimetric and Centrifugal Spin Data: Accuracy of Suspended Solids Results and Process Control Calculations


The Carbondale Wastewater Treatment Facility is located on the banks of the Roaring Fork River at the intersection of Highway 82 and Highway 133 on Colorado’s western slope. Samples for total suspended solids (TSS) were analyzed by gravimetric analysis with 45 mm filter papers and by centrifugal spin for three years. While there is a clear correlation
between centrifuge and gravimetric data, one cannot be used to accurately predict the other. Data for the past three years was sorted into groups based on the SVI value at the time the samples were collected. Each set of data points included a centrifugal percent spin, a gravimetric TSS concentration, and an SVI value. All SVI calculations are based on gravimetric data. Correlation coefficients were calculated by linear regression for each group of data. The degree of correlation between centrifugal and gravimetric data was dependent on the sludge settling characteristics. For the SVI range of 50 to 150 mL/g where the Carbondale WWTF operates the majority of the time, the correlation between gravimetric and spin data is the weakest suggesting that centrifuge spin data should not be relied upon for process control.

The WWTF has experienced regular seasonal cycles of sludge bulking. Bulking is defined as having an SVI that exceeds 200 mL/g. SVI data for the Carbondale WWTF was analyzed with a specialized statistical method called a cumulative sum (CUSUM) to determine if temperature might be one of the factors contributing to seasonal sludge bulking episodes. There is some evidence that activated sludge bacterial populations may be very sensitive to changes in temperature away from 15oC. CUSUM analysis suggests a correlation between SVI and changing water temperature. SVI could not be correlated to any other operating variable such as sludge age, food to microorganism ratio, or MLSS concentration.


Mixed Liquor Suspended Solids, Gravimetric, Total Suspended Solids, Centrifugal Spin, SVI, Temperature

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