Oxygen Uptake Rate (OUR)

The MBR worked on a semi-batch operational condition regarding the influent feed of wastewater. The influent was not coming into the reactor at a constant flow, but rather as a semi batch mode. As the permeate production lowered the MBR level (reaching the low level setpoint), the influent pump turned on and brought approximately 200 L of influent wastewater into the MBR (that is, approximately 20% of the reactor's volume). Due to this particular condition, and considering the large amount of wastewater that was loaded in every feed "batch", the soluble substrate concentration in the MBR aerobic tank fluctuated.

The substrate concentration is related to the bacterial specific growth rate; and therefore, to the OUR.

Consequently, this condition was considered when performing the OUR determinations as follows.

The "active" OUR was determined by adding a specific volume of influent to the vessel where the OUR was being measured to obtain OUR values closer to the actual MBR operational conditions.

The OUR results are showed in Table 4.5; they are classified in different groups as follows:

Group 1: Operational conditions: Aeration source, Flow (Q), Hydraulic retention time (HRT), Solids retention time (SRT), Total suspended solids (TSS), Volatile suspended solids (VSS), Temperature, Dissolved oxygen (D.O)

Group 2: Substrate fractionation: Chemical oxygen demand (COD) influent (CODin), effluent (CODout), removed (CODremoved). Calculated ultimate BOD (UBOD(calc)), calculated

y = 0.0725x^1.1069

EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE

AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 81 biologic oxygen demand (BOD5(calc)) Unbiodegradable particulate (Unbio(calc)), Volumetric loading (applied and removed).

Group 3: Calculated oxygen flux (FOc) for: removed COD (FOc1(removedCOD)), Ultimate BOD (FOc2(UBOD)), biologic oxygen demand (FOc3(BOD5))

Group 4: Oxygen uptake rates normalized at 20ºC (OUR₂₀) calculated using: Removed COD (OUR201), Ultimate BOD (OUR202), Biological oxygen demand (OUR203). Measured OUR (OUR20 M1(measured)).

Group 5: Specific Oxygen uptake rates normalized at 20ºC (SOUR₂₀) calculated using: Removed COD (SOUR201), Ultimate BOD (SOUR202), Biological oxygen demand (SOUR203) and the measured OUR20 M1 (SOUR20 M1(measured)).

In Table 4.5, the operational points corresponding to set points IDs #9, #11, #13, and #14 are not reported because the COD was not determined for these set points.

Table 4.5 MBR Operational conditions and OUR values

Point ID 5 6 7 8 10 12 15

G1: Operational conditions

Aeration source Air O₂ gas O₂ gas O₂ gas O₂ gas O₂ gas O₂ gas

TSS g/L 7.60 8.57 10.36 14.54 12.21 18.70 22.79

VSS g/L 5.71 7.40 8.34 11.17 10.29 14.53 18.05

Q m³/d 3.50 3.50 3.50 3.50 5.26 4.80 4.10

Volume m³  0.85 0.85 0.85 0.85 0.85 0.73 0.73

SRT days 20.00 23.00 25.00 32.00 30.00 30.00 30.00

HRT h 5.86 5.86 5.86 5.86 3.90 3.67 4.30

Temp ºC 18.8 21.9 21.6 20.4 20.8 19.3 20.8

D.O. mg/L 1.5 1.9 1.9 1.7 1.73 1.71 1.79

G2: Substrate fractionation

COD in mg/L 611.25 697.97 614.50 917.20 1044.80 1141.40 1513.00

COD out mg/L 25.52 51.70 61.50 33.70 58.24 12.43 25.52

COD _(removed) mg/L 585.73 646.27 553.00 883.50 986.56 1128.97 1487.48

% 0.96 0.93 0.90 0.96 0.94 0.99 0.98

UBOD_(calc) mg/L 382.03 436.23 384.06 573.25 653.00 713.38 945.63

(Biodeg(soluble+particulate))

BOD5 (calc) mg/L 244.50 279.19 245.80 366.88 417.92 456.56 605.20

Unbio(Particulate) (calc) mg/L 203.70 210.04 168.94 310.25 333.56 415.60 541.86

Volumetric Loading 1 (applied) KgCOD/m³/d 2.51 2.86 2.52 3.76 6.44 7.46 8.45

Volumetric Loading  2 (removed) KgCOD/m³/d 2.40 2.65 2.27 3.62 6.08 7.38 8.31

G3: Daily oxygen flux

FOc 1 (Removed COD) KgO/d 2.01 2.21 1.96 2.91 4.80 4.92 5.63

FOc 2 (UBOD) KgO/d 1.46 1.63 1.49 2.04 3.39 3.32 3.77

FOc 3 (BOD5) KgO/d 1.08 1.19 1.10 1.46 2.40 2.33 2.60

G4: Oxygen uptake rates

OUR20 1 (removed COD) mg/L/h 90.4 118.0 103.2 145.0 243.5 266.2 332.2

OUR₂₀ 2 _(UBOD) mg/L/h 65.7 87.0 78.5 101.4 172.0 179.7 222.2

OUR₂₀ 3 _(BOD5) mg/L/h 48.7 63.8 58.2 72.4 121.7 126.3 153.2

OUR20 M1 (measured) mg/L/h 59.7 106.2 76.7 206.0 157.6 180.8 332.3

G5: Specific oxygen uptake rates

SOUR20 1 (removed COD) mg/g/h 15.8 16.0 12.4 13.0 23.7 18.3 18.4

SOUR₂₀ 2 _(UBOD) mg/g/h 11.5 11.8 9.4 9.1 16.7 12.4 12.3

SOUR203(BOD5) mg/g/h 8.5 8.6 7.0 6.5 11.8 8.7 8.5

SOUR₂₀ M1 (measured) mg/g/h 10.5 14.4 9.2 18.4 15.3 12.4 18.4

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AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 82 The theoretical OUR in order to achieve the observed organic load removal is shown in Figure 4.42 as OUR₂₀1(removed OUR). It corresponds to the theoretical mass of oxygen required to degrade the removed load at each operational point. However, that is not fully correct because not the entire influent load to the MBR is fully biodegradable and some removal happens via floc adsorption. Therefore, the theoretical OUR corresponding to the biodegradable fraction (soluble+particulate) of the influent COD is better described when considering the ultimate BOD (UBOD fraction) in the calculations. These OUR values are shown in Figure 4.42 and Table 4.5 as OUR202(UBOD). The influent wastewater contains both a soluble unbiodegradable (CODout) and a particulate unbiodegradable fraction (Unbio Particulate). The later can be calculated by subtracting the soluble unbiodegradable fraction (CODout) and the UBOD to the influent COD (COD_in). The theoretical OUR values corresponding to the biodegradable COD as previously determined are shown in Figure 4.42 as OUR20 UBOD.

The aforementioned calculation process is explained step by step for (Point ID#5) in the following example:

Influent wastewater fractionation:

In 1972, Metcalf proposed a simplified expression to estimate the ultimate BOD (UBOD) dividing the COD by 1.6 (Metcalf, 1972). That is roughly 62.5% of the influent COD for typical domestic wastewater as follows:

This UBOD value represents all of the biodegradable content of the influent wastewater including soluble and particulate material.

Since no biological oxygen demand (BOD5) measurements were performed, it was calculated based on a one year data series from HNP WWTP monitoring routines. This data series contained daily COD and BOD5 measures over one year that allowed establishing the influent BOD as an average reliable factor of 0.4 the influent COD value. In the case of Point ID#5:

0.4

0.4 611.25 / 244.5 /

(4.5)

Knowing the influent COD (the total incoming substrate), the effluent COD (the soluble unbiodegradable fraction) and also the UBOD (both the soluble and particulate biodegradable fraction) it is possible to calculate the unbiodegradable particulate fraction (Unbiopart) to complete the influent fractionation as follows:

611.25 25.52 382.03 203.7 /

(4.6)

EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE

AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 83 Daily oxygen flux:

The carbonaceous oxygen demand for BOD5 can be calculated using the following expression (Henze, 2008):

Oxygen demand from ammonia oxidation:

4.3 (4.8)

Where:

FONH4: Daily flux of oxygen (for NH4) [gO2/d]

Q: Influent flow [m³/d]

Nti: Influent total TKN [mgN/L]

The total oxygen demand can be found by adding the values obtained from equations 4.9 and 4.10, for Point ID#5 that is:

660.9 421.4 1082.3 1.08 (4.11)

Oxygen uptake rate:

EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE

AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 84

52.69 1.07 ^. 48.7

θ: 1.07 if T<20 θ: 1.05 if T>20

(4.13)

Normalized specific oxygen uptake rate:

48.7 / /

5.71 / 8.5

(4.14)

Figure 4.42 Calculated and measured Oxygen uptake rate (OUR20)

The OUR201(removed COD) and OUR202(UBOD) values represent the theoretical rate of oxygen that should have been consumed to fully degrade the substrate. Therefore, it is expected that the measured OUR curves fit into that range. The measured OUR test results (OUR20M1(measured))exhibited values between that range. That is, the MBR was working between this OUR range in order to achieve the reported COD effluent concentrations. The measured OUR values (at the different set points) ranged between approximately 60 and 180 mg/L-hr when the system was working at steady conditions; and even OUR values as high as 320 mg/L-hr were observed at the last set point ID#15 when the system was fed acetate. At this last set point, steady state operational conditions were not achieved in the MBR system.

The OUR value represent the behaviour of the MBR immediately after dosing the acetate.

0

EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE

AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 85 The SOUR values were calculated dividing the OURs by the corresponding VSS concentration. These and the measured SOUR curves are shown in Figure 4.43. The same nomenclature as explained for Table 4.5 and Figure 4.42 was used. The measured SOUR values followed the very same trends and values as the theoretical SOUR values. That is, as the load to the MBR system increased (and therefore, the MLSS increased), the OUR values increased proportionally to the increased load in the reactor. That is, the OUR values (at the reported range) increased proportionally to the MLSS and were not negatively affected by the increase on the MLSS. That is, the measured SOUR values were relatively similar to the expected (theoretical) SOUR values as show in Figure 4.43.

Figure 4.43 Specific Oxygen uptake rate (SOUR20) 5

7 9 11 13 15 17 19 21

4 5 6 7 8 9 10 11 12 13 14 15

(mg/g/h)

Point ID

SOUR ₂₀

SOUR20 2 (UBOD) SOUR20 (BOD5) SOUR20 M1 (measured)

EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE

AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 86