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The Case for Staging Biological Reactors — Part II

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Background: In the earlier paper, “The Case for Staging Biological Reactors” (Albertson 2007), the historical review of the investigative work on staging of the bioreactor and supporting plant results, was in agreement with Chudoba's (1985, 1989) statements of the primary factors which control the growth of filamentous bulking organisms. Based on the extensive studies of Chudoba, his co-researchers, other investigators and the reduction of the 14 sets of annual plant results in the earlier paper, the conclusion was that the key factors which controlled bulking sludges in biological nutrient removal (BNR) plants were:

Staging, minimum of 7, preferably 10 or more stages in larger plants

Oxic zones, 4 – 6 stages

Bioselector initial contact zone F/M (ICZ F/M) = 3 to 5 kg BOD5/kg.d or the COD of 6 – 10 kg/kg.d

Dissolved oxygen(DO) of > 2 mg/L in oxic stages

Sludge Regeneration, oxic SRT (SRTox) > 3 to 4 days in BNR systems

Mixed liquor suspended solids (MLSS) > 3,000 mg/L

No inter-stage back-mixing, except for controlled flows of return activated sludge (RAS) and internal recycle (IR) for nitrate removal.

The data base for the initial paper was one year of daily data from 14 facilities which were summarized for the average and maximum month dilute SVIs (DSVI) results as a function of the number of bioreactor stages, ICZ F/M, MLSS and temperature. These parameters were the only independent variables which found to be significant from the multiple regression analysis of the complete data bank. The significance of the independent variables on the dependent variable DSVI varied from plant to plant. It was recognized that daily data may not be responsive to the effects of process variables controlling bulking and monthly averaging could be damping the individual effects of the independent variables.

The initial analysis was in full agreement with Chudoba's conclusions (1985, 1989). Most of, if not all, of the plants had sufficient oxygen transfer capacity in the oxic stages to assure maintaining 2 mg/L DO during the maximum loading periods. However, there was no assurance that the 2 mg/L DO was actually maintained. A low DO in the initial oxic zones will cause the DSVI to increase as it disrupts the natural bulking control mechanisms derived from staging of the bioreactor. The DO is possibly the most significant and often unknown factor in a plant which has periodic short-term increases in the DSVI. Most plants do not have continuous and reliable 24 hr/d DO measurements in the oxic zones.

In the earlier data review it was noted that there could be as much as 90 to 95% of the data with < 100 mL/g DSVI, but there were short term excursions up to 120 mL/gDSVI. The operating and cost advantages of not only a low, but also stable DSVI is well known to plant operators and process design engineers. Thus, if the reasons for the excursions can be defined, then there is the possibility of achieving less than 100 mL/g DSVI at all times. There are several facilities that meet this DSVI criterium.

Document Type: Research Article


Publication date: January 1, 2008

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