Effects of carbon dioxide stripping on solids settling in pure oxygen WWTP Conference

Coro, E, Kundral, S, Mudragada, R et al. (2010). Effects of carbon dioxide stripping on solids settling in pure oxygen WWTP . 3683-3692. 10.1061/41114(371)374

cited authors

  • Coro, E; Kundral, S; Mudragada, R; Moncholi, M; Mora, N; Laha, S; Tansel, B

authors

abstract

  • The South District Waste Water Treatment Plant (SDWWTP) is a pure oxygen waste water treatment plant (WWTP). It consists of two parallel plants: Plant 1 has four circular secondary clarifiers in service receiving flows from four oxygenation trains serving southeast Miami Dade County; and Plant 2, with two circular clarifiers, receiving flows from Trains 5 and 7 services south west Miami Dade County and the South Dade landfill. The high ambient temperature in South Florida produces higher rates of microbial activity in the secondary clarifiers and gravity concentrators further depleting dissolved oxygen and leading to septic sludge conditions that upset plant performance. High levels of entrapped carbon dioxide (CO2) in the Mixed Liquor Suspended Solids (MLSS) have corrosive effects on the concrete in the oxygenation train. This is especially problematic in the last two stages of the six-stage oxygenation train. In order to produce settled sludge/underflow concentrations that can be pumped more effectively, it is necessary to improve sludge settling characteristics. Improved sludge settling characteristics help maintain steady plant operations by reducing the most common problems in WWTPs, i.e., sludge bulking and septic sludge. Therefore, this study looks at the effects of stripping CO2 as a means to improve solids settling in secondary clarifiers. Although the Sludge Volume Index (SVI) at the SDWWTP is good, this study hopes to demonstrate further improvement of SVI by removing entrapped CO2 in sludge, thereby improving its settling characteristics. CO2 will be stripped at the last stage of the oxygenation trains by aeration. Aeration also helps in increasing the dissolved oxygen levels and increasing pH while concomitantly removing CO2 and improving sludge settling. Plant 1 samples remained well settled for up to 6 hours (greater than sludge retention time in the secondary clarifier) after aeration indicating a promise of fresh sludge desired at the plant. However, for sludge samples from Plant 2, the sludge blanket inverted after about 2 hours indicating microbial activity likely interfered with settling. Further investigation revealed increased activity of nitrifying bacteria that consumed oxygen and released CO2 at the improved/higher pH of ∼7.2. The activity of nitrifying bacteria is generally negligible at pH values of ∼6.3 typically observed at the plant. More tests need to be performed to confirm this conclusion. © 2010 ASCE.

publication date

  • August 2, 2010

Digital Object Identifier (DOI)

International Standard Book Number (ISBN) 13

start page

  • 3683

end page

  • 3692