A giant cost lurking in aeration basins
The energy consumed by bubble aeration systems varies significantly based on a number of plant parameters. A few of the factors affecting the efficiency are: aeration tank depth and shape, type of diffusion equipment and the layout specified, the flow entering the process, and the required performance. However, one factor is common to all aeration systems: a massive amount of energy is required to run the process.
According to the US EPA, “the aeration system consumes approximately 50% to 65% of the net power demand for a typical activated sludge wastewater treatment plant.” Performance efficiency in this power hungry process is a significant determinant in the overall operating costs of a plant. A slight decrease in aeration efficiency can result in a big increase in the power that a plant consumes.
The arch enemy of aeration
The abrasive effects of grit on mechanical equipment such as pumps, bearings and conveyance systems etc. are well known. However, what is not so well known is the insidious impairment of grit on aeration systems and Return Activated Sludge Systems. A steady incursion of grit can smother the aeration system nozzles and take up valuable biological treatment space in the tank. This leads to reduced efficiency and a drop in effluent quality. To counter these effects operators are forced to run the blowers at increased power.
As aeration systems are the most power hungry process on a treatment plant, this increase makes a big impact on power consumption and therefore cost.
Grit adds density to the mixed liquor
Water has a density of 1,000 kg/m^{3}. (Metcalf & Eddy 1991a)
Grit has a density of 2,650 kg/m^{3} (Metcalf & Eddy 1991b)
Mixed liquor in an aeration lane therefore has a density with a product of these various fractions.
ρm = xρw + yρg
where
ρm = Density of Mixed Liquor (kg/m^{3})
ρw = Density of Water (kg/m^{3})
ρg = Density of Grit (kg/m^{3})
x = Volume fraction of water in mixed liquor
y = Volume fraction of grit in mixed liquor
It has been assumed that the density of the biomass found in aeration basins is constant over the range and therefore only has a marginal effect on the calculations. As the fraction of grit increases so does the resultant density of the mixed liquor to be aerated.
Volume Fraction |
| |
Grit | Water | Density (kg/m^{3}) |
0.00 | 1.00 | 1000.0 |
0.01 | 0.99 | 1016.5 |
0.02 | 0.98 | 1033.0 |
0.03 | 0.97 | 1049.5 |
0.04 | 0.96 | 1066.0 |
0.05 | 0.95 | 1082.5 |
This ignores the fraction of grit which has settled below the diffuser grid.
Increased liquor density increases required blower pressure
The pressure required for the air bubbles to rise through the water can be calculated based on the density of the liquid, such that the pressure (p, mbar) is a product of the diffuser’s depth (h, m) and the density of the mixed liquor (ρm, kg/m^{3})
p = ρmgh (Nave 2005)
g = acceleration of gravity (m/s^{2})
The pressure required to get the air bubble to the diffuser will not be affected by the grit content. This pressure will vary with the type of diffuser, and the pipe work configuration. For the sake of argument we will assume that this is 20% of the pressure when there is no grit in the tank.
Volume Fraction of Grit | Density of MLSS^{*} (kg/m^{3}) | Density of MLSS without grit (kg/m^{3}) | Increase in Pressure loss through Mixed Liquor (%) | Blower Pressure associated with Mixed Liquor (%) | Blower Pressure for Pipe work & Diffuser (%) | Increase in Pressure through System (%) |
0.00 | 1000.0 | 1000.0 | 0.00 | 80 | 20 | 0.00 |
0.01 | 1016.5 | 1000.0 | 1.65 | 80 | 20 | 1.32 |
0.02 | 1033.0 | 1000.0 | 3.30 | 80 | 20 | 2.64 |
0.03 | 1049.5 | 1000.0 | 4.95 | 80 | 20 | 3.96 |
0.04 | 1066.0 | 1000.0 | 6.60 | 80 | 20 | 5.28 |
0.05 | 1082.5 | 1000.0 | 8.25 | 80 | 20 | 6.60 |
* MLSS – Mixed Liquor Suspended Solids
Increasing blower pressure means increasing power
Most compressors operate on a polytropic path approaching adiabatic compression (Perry 1997a) the work done is therefore proportional to the difference in pressure from outlet to inlet.
kW_{ad} µ {(p_{2}/p_{1}) ^{(k-1)/k} - 1}
For air and other diatomic gases k = 1.39 to 1.41, and is often taken as 1.395 (Perry 1997b), applying this based on a starting p_{2} level of 0.6 barg above p_{1}, and applying the additional pressure losses gives the following:
Volume fraction of Grit | Increase in Pressure Loss through System (%) | Increase in Power Demand (%) |
0.00 | 0.00 | 0.00 |
0.01 | 1.32 | 1.12 |
0.02 | 2.64 | 2.24 |
0.03 | 3.96 | 3.36 |
0.04 | 5.28 | 4.47 |
0.05 | 6.60 | 5.57 |
More power means higher energy costs
If there is an increase in power demand then there is an additional operating cost in terms of kilowatts required to operate the aeration system, and of course each extra kilowatt costs more money.
Economic and energy saviours
Without an effective and efficient grit removal and capture system not only do you increase your power requirements significantly but also increase your maintenance commitment to all of your processes downstream. The correct application of a high performance grit system can not only reduce your lifetime energy costs but can help minimise the whole life costs of your capital equipment.
Paul Barter (CEng MBA MIChemE MCIWEM), Senior Product Specialist, Hydro International
References
- US EPA (1999) Wastewater Technology Fact Sheet – Fine Bubble Aeration
- (Metcalf & Eddy 1991a) Metcalf and Eddy Inc. (1991) Wastewater Engineering, Treatment, Disposal, Reuse 3rd ed., McGraw-Hill, New York. p1253
- (Metcalf & Eddy 1991b) Metcalf and Eddy Inc. (1991) Wastewater Engineering, Treatment, Disposal, Reuse 3rd ed., McGraw-Hill, New York. p457
- (Nave 2005) Nave R. (2005) Static Fluid Pressure [online] http://hyperphysics.phy-astr.gsu.edu/HBASE/pflu.html [accessed 27 July 2009]
- (Perry 1997a) Perry, R.H. and Green D.W. (1997) Perry’s Chemical Engineers’ Handbook 7th ed., McGraw-Hill, New York. p10-41
- (Perry 1997b) Perry, R.H. and Green D.W. (1997) Perry’s Chemical Engineers’ Handbook 7th ed., McGraw-Hill, New York. p10-41