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ASSESSING THE APPLICABILITY OF THE 1D FLUX THEORY TO FULL-SCALE SECONDARY SETTLING TANK DESIGN WITH A 2D HYDRODYNAMIC MODEL

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The applicability of the 1D idealizedflux theory (1DFT) for design of secondary settling tanks (SSTs) is evaluated by comparing its predicted maximum solids loading rate (SLR) with that calculatedfrom the 2D hydrodynamic model SettlerCAD using as a basis four fill scale SST stress tests conducted on the new and old Darvi11 wastewater treatmentplant 35m diameter peripheral effluent drawoff SSTs with a Stamford baffle. The new SSTs have 4.1 m side water depths (SW), sloping bottoms and scraper sludge collection, while the old SSTs have a 2.5 m SWD, flat bottoms and hydraulic suction sludge collection. For the four tests, in which the SLRs were the same on the new and old SSTs and only Test 4 ended in failure, the test SLRs (including recycle) were 7.17, 6.05, 6.35 and 11.13 kgSS/(m2.h) respectively and the test to 1DFT maximum SLR ratios were 0.87, 0.96, 0.77 and 0.91 respectively.

The SettlerCAD predicted maximum surface overflow rates (SORs) and SLRs for the Darvill SSTs were determined by simulating the four stress tests on each of the new and old SSTs by incrementally increasing the influent flow rate until failure (effluent SS>50 mg/l) took place. The SettlerCAD maximum to 1DFT maximum SLR ratios were 0.86, 0.86, 0.89 and 0.88 for Tests 1 to 4 respectively on the new SSTs and 0.83, 0.86, 0.81 and 0.71 for Tests 1 to 4 on the old SSTs. It was notedfrom the SettlerCAD simulations that for the shallow old SSTs, the higher the hydraulic loading rates (HLRs) on the SSTs, i.e. the better the sludge settled and/or the lower the feed concentration, the lower the SettlerCAD to 1DFT maximum overflow rate and SLR ratios. However, this sensitivity to HLR was not exhibited by the deep new SSTs.

It is concluded that the 1DFT predicted maximum SORs and SLRs cannot be applied to full scale SSTs and that these maximum rates need to be reduced depending on the SST HLR and depth. Factors that influence the magnitude of this reduction have not been fully explored and so until greater clarity is obtained, the 25% reduction on the maximum SLR (I/0.80) recommended by Ekama and Marais (1986) appears a reasonable value to apply
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Document Type: Research Article

Publication date: 2001-01-01

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