Measuring actual (operating) oxygenation capacities of pneumatic aerating equipment

Our Company has been engaged in development of mechanical aerating equipment designed to agitate and aerate biological WWTPs’ activated sludge tank (activation) ever since the year 1998. The major parameter followed-up (monitored) within aeration systems and equipment is the electric power consumption and overall economics of the operations, e.g. over a 10-year period.

Therefore in course of developing our equipment we have been forced to measure actual introduction of oxygen. Currently we have been equipped with special gauges to measure real - operating introduction of oxygen and hence oxygen recovery (yield) at pneumatic aeration systems.

Such measuring involves:

• Concerning the particular activated sludge tank (activation) while under operation you place special sampling bell (cone) on the surface level for the leaking air to be removed from the tank. Original air will gradually be displaced by air leaking from the tank.
• The instrument is there to measure oxygen (O2) concentration continually in the air leaking from the tank via the sampling bell that prevents air to enter inside the bell from the above-surface space since the bell is placed on activated sludge level.
• Measuring goes on in case continually found O2 concentration gets stable.
• Carry out this measuring in several points of activation complete with estimate amount of area and activation surface area of given intensity of leaking air.
• We recommend to conduct this measurement under the condition the contents of air dissolved in activated sludge tank is maintained at the value of e.g. 2 mg/l - i.e. the most frequently advised O2 concentration to ensure proper function and economical operations of WWTP.
• In addition the real overall amount of air supplied to activation will be measured. To measure it is simple. You bore a hole inside the supply piping and insert measuring probe here.
• Based on the difference between oxygen concentrations from the above-surface air plus the air escaping from activated sludge basin on one side and the total amount of air supplied to the activated sludge basin you determine the quantity of O2 that remains in water. That is, the actual operating oxygenation capacity of aeration equipment under the given concentration of dissolved oxygen will be specified.
• Applying this method the actual oxygen demand under the specific conditions at given WWTP will in fact be found.
• Use ammeter to measure total real power input of the aerating equipment (at fine-bubble systems of blowers). Dividing °C with this power input measured you calculate actual oxygen recovery (yield) under the given conditions at the given activation and specific aeration equipment.

Note:
As at October 10, 2003 our Company used the aforementioned method to measure the actual - operating °C and the recovery (yield) of fine-bubble aeration system at 4 large municipal WWTPs wiithin Slovakia. These involved aerating elements having been in operation for 1,5 to 4 years. Manufacturers of these aerating elements have come from Austria, Finland and Czech Republic. Water temperature ranged from 13°C to 18°C . The altitude moved from 250 m above sea level to 500 m above sea level. Air temperature was between 20°C and 28°C at the time of measuring. Nowhere the oxygen actual - operating yield (recovery) under the given conditions was measured by us higher than 1,5 kg O2/kWh.

That means:

• Actual – operating oxygen recovery (yield) for the given conditions at fine-bubble aerating systems shall be determined ranging between ca 2,0 to 3,0 kg O2/kWh. The yield actually measured by us under the given conditions was in fact at the level of about 50% of these figures as well as of those ones specified in the Basis of Design concerning these WWTPs.
• Real electric power consumption was therefore ca 2 times higher compared to that stated in Basis of Design.
• In such cases it becomes economically very beneficial to have these aerating elements or even the entire aeration equipment (systems) replaced with the new - in terms of energy - more efficient ones.
• It is likely the actual specific consumption of electric power of fine-bubble aeration systems is substantially higher than currently stated in general practice by the companies supplying and manufacturing this equipment.

Advantages of the given way of measuring:

• The given measuring system is objective, accurate and straight-type. It uses no formulas, intricate computations or chemical analyses, thus not affecting the measurements with certain errors.
• The gauges are of high accuracy and client can supervise the whole measuring process - i.e. check up O2 concentration values, readings showing the amount of air and power input.
• You can carry out the entire measuring process even at larger WWTP within 1 day with its results available instantaneously.
• The measurement system outlined above reflects the real situation within electric power consumption at a particular WWTP under the viable conditions for such WWTP to function. The idealized oxygen recovery parameter (or oxygenation capacity parameter) in clean water is basically a data saying nothing to the investor about el. power consumption foreseen in the future within his/her biological sludge treatment unit.

In order to enhance the efficiency of biological treatment it may prove very convenient to alter the activation system to the activation complete with sludge thickening. This system of activation - as outlined hereinafter - is protected by patent application. The principle is rather simple. By switching OFF and ON the aerating and stirring of activation alternatively you can achieve increased sludge concentration within activation, and through setting up intermittent aerobic and anoxic conditions the nitrification and denitrification will proceed simultaneously inside one single basin.

The process of sludge concentration increase is as follows:

• Switching OFF the aeration and agitation makes sludge sedimentation to occur instantly along with sludge thickening inside activation bottom. The fastest removal of DO (dissolved oxygen) happens at the tank bottom and with time running the borderline of O2 free zone moves to the surface against the layer of settling and thickening sludge.
• Where anoxic conditions occur denitrification starts to proceed. Activation mixture will be withdrawn at the surface so only the least concentrated mixture gets removed. The longer the period with no stirring and aerating, the greater increase of sludge concentration compared to its original state will be accomplished.
• We advise for instance 1 hour of aeration and 1 hour of thickening with no aeration and mixing. As a result the aeration equipment need overating ca 100%, and also the agitation efficiency must be higher compared to continual systems.
• To increase sludge concentration furthermore you may implement cycling of wastewater removal into the activated sludge tank and if required, the cycling of sludge circulation from secondary settling tank (clarifier) as well. In time of aerating and agitating the activation no wastewater shall be pumped to the activation and if needed, no circulating of sludge as well. Only after a short moment following the aeration and agitation being switched OFF the withdrawal of wastewater and circulation of sludge may be restored. At the same time the wastewater supply is placed on the opposite side of activation than the removal of activation mixture with treated water is, and no short flowing of wastewater straight to the secondary settling tank occurs. In this way the activation mixture gets displaced from activation surface.
  Consequently maximal sludge concentration within the activated sludge tank will be accomplished. Based on our practical experience this is the way to achieve constant increase of sludge concentration against the original state by up to 100%. That means where 4 kg/m3 has initially been reached on constant basis, the concentration now able to attain will go up to 8 kg/m3.
  However, one disadvantage is that detention basin must be installed at WWTP’s influent side, or secondary settling tank has to be overdesigned adequately. The most suitable aeration system for the given way of operating the activation would be mechanical aerating equipment with switching ON and OFF thereof even several times an hour makes no problem.
• Another option left and dealing with sludge thickening is the system where mechanical aerating equipment has no additional stirring capability. Only the upper section of the tank gets aerated and stirred whereas at the bottom part the sedimentation and thickeniing of sludge occurs. And only in the short run the entire tank gets agitated intensively due to another sufficiently rated stirring equipment. For example 3 hours with no agitation, 10 minutes of intensive mixing the entire tank. According to our experience, using PAMP enables sludge concentration to increase by roughly 30% against the initial conditions inside the tank over 4 m deep.
• Sludge thickening using this industrially patented principle is also feasible at circulating activated sludge processes complete with fine-bubble aeration systems. For instance aeration is switched ON continuously - O2 concentration will rise significantly within this section of the tank. Concerning tank section with no aeration and hence no stirring the sludge sedimentation and its thickening will occur. Stirring - that is boosted sufficiently - will turn ON in a cyclical way only, to stir on regular basis the whole volume of activated sludge depending on the need of nitrification and denitrification efficiencies.