Practical issues in design process

Though solid theories exist for MBR design as described in other parts of this chapter, some fundamental issues persist due to the following reasons.

Although knowing BOD is the starting point of the MBR design, its accuracy is often questionable due to a number of reasons.

  • Especially when large suspended solids exist, BOD can vary widely depending on how much solids are included in the sample. In addition, BOD varies depending on sampling timing. Although sampling can be performed multiple times with time interval, having some error is inevitable.
  • Typical BOD test starts at a DO of around 9.1 mg/L and desired to end at around 3-6 mg/L at 20 oC for the maximum accuracy. Therefore, samples are desired to be diluted in the BOD bottle to make the initial BOD at 3-6 mg/L. Therefore wastewater samples need to be diluted to make the sample BOD in the 3-6 mg/L range. If wastewater contains insoluble BOD, the accuracy of the BOD measurement is decided by how the dilution process is handled properly. For municipal wastewater, dilution factor is relatively low due to its low BOD, but still 30x – 100x dilution is required.
  • Nitrification may occur during the 5-day period although it supposedly does not occur, which exaggerates BOD.
  • Diversity of the species contained in the inoculums used to seed BOD bottles varies depending on the source of the inoculum. Lower diversity may end up lower BOD since some components in wastewater may not be decompose (Jouanneau et al., 2014)

Since the accuracy of BOD test is highly dependent on how the test is performed, comparing the data from different sources is not always straight forward. The accuracy issue in BOD test contributes to the unreasonably wide variability of COD to BOD ratio for municipal wastewater, which is reported at 1-4. The issues related with BOD are also summarized elsewhere.

  • Chromium based COD may provide a better accuracy due to its significantly lower dilution factor. The sample COD can be measured without dilution up to 1,000 ‚Äď 1,200 mg/L with a good accuracy, perhaps ¬Ī5%. One drawback of COD is it does not distinguish biodegradable and non-biodegradable substrate. As a result, if any industrial wastewater contains a high level non-biodegradable COD, the oxygen demand cannot be estimated based on COD value.
  • Wastewater characteristics are unique in every location especially for industrial wastewater. Depending on the wastewater characteristics, there might be significant differences in the biosolids yield, ratio of MLVSS/MLSS, mixed liquor characteristics that affect membrane fouling, etc.
  • Actual oxygen consumption of wastewater can be measured by respirometry tests, where relatively constant DO is supplying pure oxygen. In one method, the KOH solution contacting with the headspace of a reaction tube absorbs CO2produced from the tube while pure oxygen is sucked in to the tube through a water column to fill the headspace. The number of oxygen bubbles passing the water column are counted and converted to the mass. Although this method provides very realistic oxygen consumption, exact oxygen consumption is still not obtainable since SRT and the associated sludge yields are not identical to the real wastewater treatment. In addition, the small sample volume is responsible for the representativeness issue of the wastewater sample.
  • The raw data required to estimate daily, weekly, monthly flow rates are often not available in its complete form. Moreover, the accuracy of the flow data is questionable more often than not and there is no guarantee that the old flow rate is an indication of the future flow rate. The theories and the methods to estimate daily, weekly, and monthly flow rates are described elsewhere (Metcalf & Eddy, 2003),
  • It is hard to predict OTE with a high accuracy since it varies widely depending on diffuser layout, specific air flow rate per diffuser, biological condition, organic loading rate, etc. as discussed above. In turn, the estimated biological air demand includes a significant level of uncertainties.

The above issues create a significant gap between theory and practice. Design and engineering firms often fill up the gap by finding working solutions combining their own knowhow obtained from long-term experiences in a specific area basically based on trial-and-error.  For example, the total oxygen demand of municipal wastewater can be estimated by multiplying a factor to BOD as follow (Metcalf & Eddy, 2003).

1
where
O2,avg               Average oxygen demand (kg/day)
f                           Multiplication factor (-)
Qavg                  Average wastewater flow rate (m3/d)
S0                        Wastewater COD (kg/m3)
STKN,0               Wastewater TKN (kg/m3)

The multiplication factor, f, is decided based on experience and typically ranges 1.1-1.25 for municipal wastewater and may vary for industrial wastewater. Although the above semi-empirical equation does not reflect the oxygen demand changes due to SRT changes and is not sensitive to the existence of anoxic tanks, it is widely used especially for municipal system designs.

In practical situation, the uncertainties in oxygen demand calculation are covered by various multiplication factors:

  • a peaking factor of 1.5-2.5 are multiplied to the average oxygen demand
  • a safety factor of 1.1-1.2 is multiplied to be prepared for uncertainties in design
  • redundancies in aeration capacity are considered for future expansion in some cases

As a result, some degree of errors in air demand calculation can be covered at least for most of the time except the short-term peak flow.

 

© Seong Hoon Yoon