Date of Award

Fall 12-2021

Document Type


Degree Name

Master of Science (MS)


Civil & Environmental Engineering


Environmental Engineering

Committee Director

Gary Schafran

Committee Member

Charles Bott

Committee Member

Peter Pommerenk


The Hampton Roads Sanitation District (HRSD) recently completed the first North American and center driven installation of the Hydrograv® Adapt Variable Height Secondary Clarifier Inlet (Adapt) at HRSD’s Nansemond Treatment Plant. This is a variable height inlet structure designed to decrease clarifier effluent turbidity and maintain low turbidity during high flow events. Low turbidity is achieved by feeding the secondary clarifier influent within the solids blanket during dry weather conditions and lifting the inlet structure during wet weather conditions to avoid disrupting the blanket. The Adapt clarifier was monitored alongside an identical fixed inlet clarifier to assess performance. Both clarifiers were monitored using online and manually sampled measurements of solids blanket thickness and effluent turbidity. Effluent orthophosphate was also monitored to detect and evaluate phosphorus release in both clarifiers. During initial operation, regular orthophosphate spikes were observed in the Adapt clarifier prior to inlet control optimization. Sludge blanket levels in the Adapt clarifier were consistently higher than levels in the fixed inlet clarifier, but this was later discovered to have been caused by dysfunctional manifold seals. Manual sampling completed during normal and stressed conditions indicate that the mean turbidity for the Adapt clarifier was less than that of the fixed inlet clarifier with a 95% level of confidence. The difference in means was only 0.2 to 0.4 NTU and may not result in improved performance when evaluated in the direct filtration pilot. During stress testing the combination of high loading and increased blanket heights from inadequate RAS pumping capabilities led to higher turbidities during the peak evening diurnal. Stress testing should be repeated with mechanical and programming adjustments to allow for additional RAS capacity, and manual turbidity and blanket readings should be collected regularly and over a long duration test period. Depth profiling confirmed a more defined separation of the clear water and sludge blanket in the Adapt compared to the fixed inlet clarifier. Higher nitrate and orthophosphate concentrations observed in the fixed inlet clarifier could have been a result of orthophosphate release from the settled sludge, unintended mixing, or uneven loading to the clarifiers.