Providing Innovative and Sustainable Solutions in Energy Efficient Surface Aerators
Comparison between Diffused Aeration & Mechanical Aeration System with Surface Aerators
We, as an environmentalist, think that it is our duty to present to you the Techno Economical Comparison between the aeration system with aerators, suggested by us and diffused aeration system with membranes. We are indicating this because we have replaced diffused aeration system at so many more places.
For any aeration system apart from oxygen transfer, mixing is also a must and what is missing in diffused aeration system is a mixing criteria. In the space between two membranes there is no activity at all and hence settling of solids (MLSS) occurs, which creates septic conditions at the bottom after couple of months working of the plant.
So you will notice that most of the diffused aeration systems installed in India are not yielding results.
Clients who had approached us for replacement of diffused aeration system are as below :-
- M/s. Gujarat Insecticides Ltd., Ankleshwar, – (Enclosure)
- M/s. Dr. Reddy’s Lab., – (Enclosure)
- M/s. Bajaj Processors Ltd., Ahmedabad. – (Enclosure)
And many more cases in India.
A guide to evaluating aeration’s – Low speed surface aeration v/s diffused aeration giving comparison of aeration’s costs, installation costs, maintenance costs from M/s. Philadelphia Mixers. Kindly refer – Enclosure
Diffused Aeration :-
In western countries, in Germany particularly, they place the membranes at 220 mm distance (Only) considering 500 mm long membrane can negotiate area of 1100 cm2 (0.11 m2), to take care of mixing criteria. This is clearly shown in Schumacher Germany & Otto System GmbH & Co. Langenhagen catalogues, copies of which are enclosed herewith. – Enclosure
Now, local representatives who place the membranes at 600 mm Or 1000 mm distance or more distance which is clearly seen in their coloured catalogue, copy of which is enclosed herewith. Since they consider 17.19 Sq. Ft. = 1.59 mt2 per diffuser, result in 17.19 Sq. Ft. / 2 Ft. length of membrane = 8.5 ft distance between two membranes (equivalent). This is too large distance, which will create lots of Dead Zones at bottom.
Due to this very less numbers of membranes are installed and HP of blowers also reduce to considerable low value. Hence, they claim that this system is more energy efficient. The scenario is different.
In India at several places we have replaced this system of diffused aeration system, comprising of membranes and blowers by our aerators.
For one particular case we have forwarded the enquiry to the manufacturers / dealers of diffused aeration system.
In the from one offer enclosed herewith, of Indian representatives of diffused aeration system, you will notice.
Calculations Of Surface Aerators :
|1||BOD / Day||2574 lbs.|
|2||O2 / Day||2574 X 2||5148 lbs.|
|3||O2 / Hr.||214.5 lbs / hr.||97.5 Kg / Hr.|
|4||BHP / Hr. / Kg O2||1.00|
|5||BHP of Surface Aerators||97.5 BHp|
Calculations As Per Diffused Aeration System :
|1||Number of Diffusers||204 Units.|
|2||Air Required||1016 cfm.|
|3||Air delivered at||5.8 psig.|
|4||BHP calculated||55 BHP|
You will notice three columns are there, A, B, & C.
|No. of Diffusers required||852.74||263.13||203.33|
|HP Motor required||175Hp||125Hp||60Hp|
|Diffuser Spacing (Sq. Ft./diffuser)||1.59 Ft2||5.16 Ft2||17.19 Ft2|
|Now dividing by length of diffuser (24”=2Ft)||1.59/2′||5.16/2′||17.19/2′ (length)|
|Centre of Diffusers (Placement) is||= 0.795′||2.58′||8.6′ (Centre)|
Generally they suggest Column C, with only 204 nos. of membranes placed at 2.62 mtrs ( 8.6 ft.) centre distance, with only 60 HP against aerator requirement of 90 HP.
But as per German catalogue of M/s. Schmacher, they must be at 220 mm centers to take care of Mixing Criteria, i.e. they suggest column A – Enclosure
i.e. membrane required = 852 Nos. x Rs. 3000/- (each) = Rs. 25,56,000/- + Price of 175 Hp Blower.
Now Blowers required would be 1 No. Required + 1 No. Standby = 175Hp + 175Hp = 2 Nos.
Total Cost of Membranes + PVC Piping cost + Cost of 2 Nos. Blowers with motors of 175Hp.
Connected Load = 175Hp + 175Hp = 350Hp.
While for Aerators – 4 Nos. of 25 Hp Slow Speed Floating Type Aerators would be sufficient.
Connected Power Would Be 4 Nos x 25 Hp = 100 Hp Only.
While as per Column “C” (suggested by them), only 204 Nos. membranes with 60 HP Blower. These figures look quite attractive. But the membranes offered are not as should have been adopted ideally for mixing criteria also.
This placement of membranes at 2.62 mtrs. (8.6 ft.) would definitely create the SEPTIC problem in aeration tank after few months of working – Enclosure
So it is very clear that, apart from Oxygen Transfer, if Mixing Criteria is to be taken care of (which is a MUST in any Aeration System), then installation cost of Diffused Aeration System is above three times compared to Mechanical Aeration System and from Energy point of view, the HP required is 1¾ times and so far as Connected Load is concerned, it would be three times.
The Zerox copy of ‘OPERATION FORUM’ magazine (*), published by WATER ENVIRONMENT FEDERATION, P.O. BOX 1663, BUFFALO NY 14205 0-9982, wherein you will notice the following in the one case study of Aeration System. – Enclosure
- Flow: 757 m3 / day
- BOD = 160 ppm
- Two Tanks of size 25 ft. x 15 ft x 10 ft. SWD (7.6 x 4.6 x 3M each) (combined tank dimensions 7.6 m x 9.2 m x 3 m = 210 m3 Vol.).
Kg of BOD / Day = 760 X 160 / 1000 = 122 Kg. So Kg of O2 / day = 2 X 122 = 244 Kg. / day, considering Oxygen requirement of 2 Kg of O2 / Kg of BOD.
So Hp of Aerator = 244 Kg of O2 / day / 24 X 1.12 = 9.0 HP say 10 HP in one combined Tank or 1 No. 5 Hp Aerator in each Tank. So total 2 Nos. of 5 Hp Aerators.
While you will notice the Hp used in :
- Coarse – bubble system is 38 HP (28 Kw)
- Fine – bubble system is 25 HP (19 Kw).
Which is nearly 4 times and 2 ½ times respectively higher than HP required in mechanical aeration system.
We are sure that this is due to the closer distance (about 220 mm) placement of the membrane is adopted in western countries. This is the exact site – experience report of Ashley water treatment plant, in Delaware country, Ohio. (*)
M/s. Schumacher Catalogue show closer distance of membranes. Kindly refer Enclosure.
We have noticed that one of the suppliers in India offers Disc Diffusers at the distance of 1125 mm and 975 mm distance which is certainly is not adopted in western countries.
Due to unsatisfactory performance of M/s. Metro Dye Chem – their own private CETP plant at Vatva, GIDC due to diffused aeration system, for their new plant at Umraya, near Baroda, where the supplier had ordered for Additional 4 Nos. of 25 HP slow speed floating type surface aerators of our Environ make, and are additionally installed in the aeration tank having already diffused aeration system. The zerox copy of the photograph of the same is enclosed. – Enclosure
So there are so many examples showing the failures of this system in India, because of the Wrong installations (larger distance between diffuser) of the system. In the beginning of the plant upto eight to nine months, it may work, but once the settlement of solids at bottom starts, due to development of Septic conditions, the system failure starts.
CASE STUDY : CETP, VATVA GIDC (AHMEDABAD)
In CETP, VATVA GIDC (AHMEDABAD) where Rs. 30 crore have been spent, the Diffused Aeration System is used and failed.
On going through the Data’s of effluent GIDC VATVA-CETP you will notice that even though considering 2 Kg of O2 Oxygen required per Kg of Bod. HP of aerators required will be 1320Hp. While existing blowers – 8 Nos. x (132Kw) 177Hp = 1416Hp are used. Therefore saving in Energy with surface mechanical aeration system is 100HP. Saving = 7% (about) with surface aerators.
- Flow = 16000 M3 / day, BOD = 1200 ppm
- Kg of BOD / Day = 16000 x 1200 / 1000 = 19200 Kg / day.
- Kg of O2 / day = 38000 Kg / Day.
- Hp of Aerators required = 38000 / 24 x 1.2 = 1320 Hp
So, even though claimed, there is No Saving In Energy with Diffused Aeration System.
Now, for equalization Tanks: Volume 16580 M3, Blowers = 3Nos. x 160 kw = 480 kw = 645 HP are installed.
Generally for Equalization, the energy required is 20 Watts/M3. So the total Horse Power required for Floating Mixer or Floating Aerators with Draft Tube required would be 16580 M3 x 0.02kw / M3 = 335 kw = 445HP.
So In spite Of Spending More Energy For Diffused Aeration System Compared To Aerators, They Are Facing Trouble Of Heavy Settlement On The Floor Level Of 60m Dia. Rcc Aeration Tank.
So We Do Not See Any Energy Saving In Diffused Aeration System, If Correctly Adopted.
There are so many cases of the failures of systems in India, which we have observed.
So before preferring the Diffused Aeration System to mechanical aerators systems, kindly think of Mixing Criteria requirement in Aeration, which requires close placement (200 to 220 mm.) of the membranes, otherwise there will be lots of Dead Pockets, settlement of the solids and creation of the septic condition would occur at the floor level of the aeration tank, if the distance is kept more than 220 to 250 mm. between two membranes.
This is written, just to share with you our three decades Site experience in the field of Effluent Treatment Plant as an equipment manufacturers.
ALSO THE AIR AT HIGHER TEMPERATURE IS COMING OUT FROM BLOWER WHICH RAISES THE TEMPERATURE OF THE EFFLUENT ABOVE 40o C SO
Secondly, we cannot avoid to compare from maintenance point of view – the MAINTENANCE of the MECHANICAL AERATION SYSTEM is much EASIER, ECONOMICAL and TIME SAVING compared to DIFFUSED AERATION SYSTEM.
The maintenance of the Mechanical Aeration System can be done on the platform of the Aeration tank (maintenance of the gearbox only) with disturbing the effluent in the aeration tank. While to maintain the Diffused Aeration System, you have to empty and clean the whole aeration tank then and then only you can install and maintain the Diffuseers at the bottom of the Aeration tank.