SOLUTIONS MANUAL FOR

SOLUTIONS MANUAL FOR

by ENVIRONMENTAL

MICROBIOLOGY FOR

ENGINEERS

Second Edition Volodymyr Ivanov 1 / 4

• Ivanov. Environmental Microbiology for Engineers , 2

nd ed.

Exams solutions Question 1 The carriers in the reactor are initially colonized at a density (Xo) of 4 g of dry biomass/m 2 of carrier surface. The biofilm density (X, g of dry biomass/m 2 of carrier surface) increases linearly with time (T, days of reactor operation) as follows:

X = X0 + (0.72  T)

The following information is also provided:

Qm = 0.15 g of COD consumed/m 2 of carrier surfacehr; Hopt = 100 m; k = 0.2; Density of wet biomass (d) = 1.04 g/cm 3 ; and Content of water in an intact microbial cell (W) = 75% by weight of the cell.(a) Calculate the biofilm thickness after 50 days of reactor operation.

(b) Calculate the specific biodegradation activity of the microbial biomass after 50 days of reactor operation.

(c) The reactor must cease operation and excess biomass must be washed off the carriers when the specific biodegradation activity of the biomass decreases to below 30% of its maximum value. When must the reactor be stopped for backwashing to be performed to clean the carrier surfaces?

(d) What is the minimum number of bacterial layers in the biofilm (B) at the time the reactor is stopped for backwashing? Assume that the microbial 2 / 4

community is represented by rod-shaped bacterial cells with a diameter (D) of 1m and an overall length (L) of 2m, and that the rods are arranged inlayers with the longitudinal axis of each cell aligned parallel to the carrier surface.

Solution 1 (a) X = X0 + 0.72T = 4g biomass/m 2

• 0.72·50 = 4+36.0 = 40.0 g biomass/m

2

H = [X/(1-W)d] = (40.0 g/m 2

) /(1-0.75) 1.0410

6 g/m 3

) = 153.810

-6 m = 153.8 m

(b) Q = Qm(1 - kH/Hopt) = 0.15(1 - 0.2153.8/100) = 0.10 g COD/m 2 of carrier surfacehr (c) 0.3Qm = Qm(1 - kH/Hopt) H = Hopt(1 -0.3Qm)/k = 100(1-0.3)/0.2 = 350 m X = H(1-W)d = 350 10 -6 m  (1-0.75) 1.0410 6 g/m 3 = 91 g/m 2

T = (X-X0)/0.72 = (91 - 4)/ 0.72 = 120.8 days The reactor must be stopped for the backwashing and cleaning up of the carrier surfaces after 121 days of continuous operation.(d) The thickness of one layer is the diameter of a cell (D). Therefore, B = H/D = 350 m/ 1 m = 350 microbial layers.

Question 2 Batch cultivation is used to grow Pseudomonas putida strain EERC285 in the laboratory.Data from this batch cultivation is used to design a large-scale reactor for the industrial cultivation of strain EERC285. The biomass produced in this industrial-scale cultivation is used to bioremediate groundwater polluted by BTEX (benzene, toluene, ethylbenzene and xylenes). Strain EERC285 is a rod-shaped bacterium. Each cell can be considered as having a cylindrical central portion and two hemispherical ends, with a diameter (D) of

• m and an overall length (L) of 3 m. The water content (W) in each intact cell is

75%, and the density (d) of each intact cell is 1.04 g/cm 3 . The parameters associated with

batch cultivation in the laboratory are as follows:

Working volume of the reactor (Vr) = 3 litres 3 / 4

Initial BTEX concentration (S0) = 369 mg/L Final BTEX concentration (Sf) = 12 mg/L Amount of evaporated BTEX (E) = 425 mg Overall duration of the process = 73 h Duration of lag phase = 8 h Duration of stationary phase = 38 h Initial concentration of EERC285 cells = 410 4 cells/ml Final concentration of EERC285 cells = 810 7 cells/ml

(a) Calculate the maximum specific growth rate (max) of Pseudomonas putida strain EERC285 in the laboratory reactor.(b) Calculate the yield of dry biomass (Y) of Pseudomonas putida strain EERC285 in the laboratory reactor.(c) Calculate the working volume of the industrial reactor (Vir) and time (Tb) for industrial batch cultivation to produce 25 kg of dry microbial biomass for use in

enhanced soil bioremediation. Assume the following:

Initial concentration of BTEX in the industrial reactor (Soir) = 600 mg/l Final concentration of BTEX in the industrial reactor (Sfir) = 1 mg/l Duration of lag phase (Tlag) = 14 h The inoculum represents 1% by weight of the final amount of biomass.The amount of BTEX evaporated is proportional to the initial BTEX concentration.(d) Calculate the quantity of biomass of strain EERC285 needed to bioremediate 300,000 m 3 of BTEX-polluted groundwater over a period of 50 days. The initial concentration of BTEX pollutant is 0.3 mg/l, the final concentration of pollutant must be less than 1 g/l, and the rate of BTEX biodegradation is 15 mg of BTEX/g of biomass·day.

Solution 2 (a) Duration of exponential phase (T) = 73h - 8 h - 38 h = 27 h max = ln (Nf/No)/ T= ln(810 7

/ 410

4 )/ 27h = ln(210 3 )/27 = (ln2 +3ln10)/27 = 0.282 h -1

(b) Y = Vcd (1-W)(Nf - N0)/[(S0-Sf) - E/Vr] .where Vc is the volume of one cell = D 3

/6 + (L-D)(D

2 /4) =

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