Oxygen Transfer - RBCs

Transcription

1 Oxygen Transfer - RBCs

2 Why are we talking about RBCs? Lowest energy usage/net zero Simple to operate Small footprint Shallow excavation Competitive capital cost

3 Net Zero Energy Most efficient motors and controls ± 10% Internal energy generation ± 10 40% Most efficient process selection ± 25 65%

4 Net Zero Energy Process Selection Primary treatment 250kWh/mg reduction MBR 3,000 kwh/mg consumed Activated sludge 1,000 kwh/mg consumed IFAS/MMBR 1,200 kwh/mg consumed Oxidation tower 700 kwh/mg consumed RBC 300 kwh/mg consumed AD/Energy Recovery net 400 to 700 kwh/mg produced

5 RBC Design Issues Mechanical problems no longer a problem Process design overloaded first stage, odors, nuisance growth, nitrification

6 Overloaded First Stage

7 RBC Process Design Biological BOD and Nitrification Oxygen Transfer

8 RBC BOD Removal Kinetics S n = (-1 + ((1 + 4 x x (A d /Q) x S n-1 ) ^ 0.5) / (2 x x (A d /Q)) Where: S n = effluent soluble BOD5, mg/l A d = surface area of media, m2 Q = flow rate, m3/d S n-1 = influent soluble BOD5, mg/l

9 RBC Nitrification Kinetics NH3 n = NH3 n-1 (U max x (1-0.1 x (S n-1 x Q/A d ) Where: NH3 n = effluent ammonia, mg/l NH3 n-1 = influent ammonia, mg/l U max = ammonia conversion rate, gm/m2 day (assumed to be 1.5 gm/m2 day)

10 RBC Nitrification Kinetics NH3 n = NH3 n-1 U max x A d /Q Where: NH3 n = effluent ammonia, mg/l NH3 n-1 = influent ammonia, mg/l U max = ammonia conversion rate, gm/m2 day (assumed to be 1.5 gm N/m2 day)

11 RBC Nitrification Nitrification will occur if bulk liquid DO is > 2 mg/l Nitrification can occur from the first to the last stage if this condition is met Previous design approach was to design for soluble BOD in first stages and then nitrify

12 Schematic of RBC Surface

13 RBC Oxygen Transfer Dimensional Analysis d La A K = / 1 d A D d A 624. t d A A V Chavan, A and S. Mukherji, Dimensional analysis for modeling oxygen transfer in rotating biological contactor, J. Bioresource Tech.,

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15 Clean Water Testing

16 Test Unit at Pontiac, MI

17 Field Data Standard Oxygen Transfer Rate - Field v. Calc. Clean Water, Clean Disc Handy Township, & Date Disc Diam, ft. Disc, rpm Act. K La20,, min -1 Calc. K La20,, min -1 Act. kg/kwh K La20, Act./Calc. 2-Apr Apr Apr Apr Apr

18 Conclusions RBC most energy efficient biological treatment Oxygen transfer for RBC is predictable Predicting oxygen transfer allows efficient RBC sizing for nitrification Predicting oxygen transfer allows avoidance of nuisance growth problem

19 RBC Oxygen Transfer Questions?

20 Supplemental Slides

21 Variables Oxygen Transfer Eq n: K La is volumetric oxygen transfer, sec-1 ρ is density of water, gm/m3 A d is surface area of discs exposed to air, m2 μ is dynamic viscosity of water, gm/m-s D is diameter of the discs, m ω is rotational speed, rps At is surface area of tank, m2 δ is thickness of water film, m V is volume of liquid in tank, m3

22 Estimating Thickness of Water Film on Disc 1 δ may be estimated using the following equation. δ = 1.2 x 10-4 x v pz. 0.5 Where: v pz is the vertical component of velocity of the perimeter of the disc as it exits the water. 1 Zeevalkink, et al, 1979