Oxygen Transfer Measurements at Surface Aerators in Waste Water as Basis for Energy Saving in Aeration

OXYGEN TRANSFER MEASUREMENTS

AT SURFACE AERATORS IN WASTE WATER

AS BASIS FOR ENERGY SAVING IN AERATION

Oxygen transfer measurements were carried out in order to compare surface aerators under operating conditions in waste water. A simple method was used without using chemicals like cobalt or sulfite. Results show incomplete mixing of basins at high biological activity that lead to mass transfer data not comparable with data of standardized fresh water tests, but different types of aerators could be compared under local operating conditions as basis for investment decisions.

1. BACKGROUND

Energy demand for aeration of wastewater during biological treatment is one of the most important operating cost factors at sewage treatment plants. Therefore increasing energy costs often are reasons for investments in energy saving measures, e.g. in aeration systems with higher efficiency. In order to compare different systems under local conditions and to check guaranteed performance parameters the measurement of oxygen transfer and energy consumption leads to efficiency parameters, e.g. the oxygen yield defined as amount of transferred oxygen per KWh consumed or the mass transfer coefficient.

Fine bubble aeration is known for a higher oxygen transfer efficiency than surface aeration, but equipment costs as well as maintenance costs for surface aerators are lower often. An economic evaluation of both systems often results in fine bubble aeration, but depending on individual conditions also surface aeration is chosen due to economic benefits. Therefore oxygen transfer measurements for surface aerators are still important.

2. METHODS OF OXYGEN TRANSFER MEASUREMENT

2.1. Standard methods

Transfer of oxygen from air to water depends on the aeration system, geometric and hydraulic properties of the basin, and properties of the water, e.g. dissolved and suspended substances or temperature.

Standardized oxygen transfer measurement methods (DWA 2001) use drinking water or rain water with their normally low concentration of dissolved and suspended substances in order to neglect the influence of water properties of transfer efficiency (see also norm EN 12255-15). Then at a given basin the transfer efficiency depends only on properties of the aeration system. Also biological oxygen consumption due to bacterial activities does not influence transfer measures in fresh water.

But this advantage of comparability leads to the disadvantage that efficiency at normal operation in wastewater is still unknown, but necessary for economic calculations. The relation of oxygen transfer data in fresh water to those in waste water is often named α–value and published in literature for various waste water, but often not precise enough for local conditions. Therefore further measurements in waste water are necessary to get efficiency data for local waste water. Also those methods in waste water are standardized (see above), but often complex and costly, e.g. due to off-gas analysis, oxygen, or nitrogen demand. Therefore, especially for tests at surface aerators, a simple version of transfer measurement in waste water was used.

2.2. Used method

The volumetric oxygen transfer coefficient kla is defined via the mass transfer rate

regarding eq. (1): ) (c c a k m& = l ∗− (1)

c*_{: saturation concentration of oxygen}

c: oxygen concentration

The oxygen mass balance for a completely mixed basin without influent and effluent containing waste water and suspended bacteria can be set as follows (2):

2 ) ( O la c c r k dt dc₌ ∗_{− − (2)}

On condition that oxygen consumption does not change with time integration of eq. (2) results in: t a k c a k r c c a k r c l l O l O ⋅ − = − − − − ∗ ∗ 0 2 2 ln (3) That condition is almost fulfilled if bacterial growth does not much depend on oxygen concentration, e.g. substantially above the kinetic saturation constant regarding oxygen.

If eq. (2) is applied to the end of a test when the oxygen concentration remains at the constant concentration c_e it results in eq (4):

a k r c c l O e= ∗− 2 (4)

Eq. (4) as expression for c* used in eq. (3) gives eq (5): t a k c c c c l e e _{= ⋅} − − = − 0 ln (5)

or the oxygen concentration during a test run (6):

t k e e c c e l c c_{= −( − )⋅} − ⋅ 0 (6)

The oxygen transfer coefficient k_la can be obtained via a linear fit of data using eq. (5) (old method) or via a non-linear fit using eq. (6) (new method).

The conversion of transfer coefficients to the reference temperature of 20°C eq. (7) is used (DWA 2007).

(7)

T: water temperature in °C

The oxygen transfer rate MO2 shows how much oxygen is transfered into the water in

the whole basin. It results from eq. (1):

) * ( 2 V ka c c M&O = B⋅ l − (8) VB: basin volume

Often in literature a standard oxygen transfer rate at an oxygen concentration of 0 mg/l is used and named different to eq. (8) as OC or in waste water αOC. It results from eq. (8) to:

* c a k V OC= B⋅α⋅ l ⋅ α (9) ) 20 ( 20 l 1,024 T la ka k _{= ⋅} −

Another way to obtain the oxygen transfer rate αOC results from eq. (8) and (9). Assuming that the transfer coefficient k_la is independent on oxygen concentration it can be taken from eq. (8) and put into eq. (9):

c c c M OC O − = * * 2 & α (10)

Under steady-state condiditions at the end of a test run it follows from eq. (1) and (2): bzw. 2 2 O O r m =& (11) respectively B O O r V M& 2 = 2⋅ (12) If oxygen consumption r_O2 is measured the oxygen transfer rate can be obtained from eq. (10) and (12): e B O V _c c _c r OC − ⋅ = * * 2 α (13)

For this evaluation of measured data a constant oxygen consumption in the whole basin is required. This is the case if a constant biomass concentration and a minimum oxygen concentration above kinetic saturation coefficient can be assumed. Measurements of oxygen consumption rate at a sewage treatment plant showed a constant rate down to 1 mg/L oxygen concentration.

This evaluation can be used for tests at high oxygen consumption rates with steady state oxygen concentration c_e substantially lower than saturation concentration c*. Otherwise the denominator in eq. (13) is small and inaccuracy of measured data result in high fluctuations of oxygen transfer rate. It might be used alternatively to evaluation via oxygen transfer coefficient (eq. (5) or (6)) if this is difficult due to small differences between start concentration c₀ and end concentration c_e.

3. RESULTS AND DISCUSSION

Test runs were carried out in a quadratic basin with a surface of app. 300 m2 and app. 4,00 m water depth of the sewage treatment plant Bremerhaven. Different surface aerators were used for numerous test runs in waste water from other aerated treatment basins. Three oxygen sensors were used at different positions for each test run.

0 1 2 3 4 5 6 7 8 0 10 20 30 40 50 60 70 time in min O xy gen c on cen tr at io n in m g/ L

Fig. 1. Test run at low oxygen transfer rate

One data set is shown in Fig. 1 from a test run with low oxygen transfer rate and biological oxygen consumption for which the evaluation with eq. (13) was applied because the requirement of steady-state conditions could be reached.

0 1 2 3 4 5 6 7 8 0 5 10 15 20 25 30 35 time in min sensor 1 sensor 2 sensor 3 O xyg en c on cen tr at io n in m g/ L

Fig. 2. Results of another test run of individual sensors

Other test runs like in Fig. 2 showed significant differences between three sensors positioned close to the surface (sensor 1), in the middle (sensor 2), and close to bottom (sensor 3) and also there is no strict constant concentration during the measure period.

Obviously the surface aerator could not reach a complete mixing in that test run which is a requirement for all evaluations as shown above. Oxygen consumption in the depth of the basin is too fast to be compensated by oxygen transport from surface.

Therefore standard process models cannot be used for data evaluation under those conditions to obtain standardized oxygen transfer rates or coefficients. But under similar conditions, in particular similar oxygen consumption rates, for various test runs results could be obtained to compare different aerators by using average data from all sensors or by comparing data only from one sensor using methods for completely mixed systems regarding eq. (5) or (6).

As widely known results show the influence of the relative position of surface aerators to water level, of aerator speed and of aerator type. At low speed and optimal position a specific oxygen transfer of 1,5 Kg O2 per KWh could be

measured with existing old surface aerators while new apparatus obtained up to 2,7 Kg O2 per KWh.

Uncomplete mixing of basins with surface aerators under conditions with high oxygen consumption rates has to be taken into consideration for olant design and operation because oxygen concentrations close to zero at the bottom of basins will lead to low reaction rates of aerobic processes.

ACKNOWLEDGMENT

Many thanks go to the staff of treatment plant Bremerhaven. Without their help and support this investigation could not have been done.

REFERENCES

DWA 2007: Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.: DWA-Method M209 „Messung der Sauerstoffzufuhr von Belüftungseinrichtungen in Belebungsanlagen in Reinwasser und in belebtem Schlamm“, GFA, Hennef 2007.