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For Biological Treatment of such oily wastewater as car wash effluent or hotel/restaurant effluent primary biological treatment is recommended between 48-72 hours.
With that consideration two underground holding tanks with 1.5 x ADF capacity is provided with Aspirating Aerator and Micro Bubble Generator (MBG) fitted in both the tanks.
- Aeration through Aspirating Aerator is provided to these two tanks.
- MBG induced micro air bubbles help in oil floatation among many other benefits and are operated either by manually switching connections or by automated timer operated controller and solenoid valves.
This treatment is to be followed by a clarifier followed by filtration (as in the schematics for Biological Treatment) or with further chemical treatment (as shown in the chemical process schematics) followed by filtration or with secondary bio-algal treatment in an underground tank capacity of 1 x ADF (single day holding capacity) before filtration.
Membrane Bio Reactor (MBR) can also be used for filtration instead of three stage filtration system as shown in the schematic diagram.
After Flocculalation
During Settling
After Flocculation in the Chemical Reaction Chamber
Filtration Unit Set Up
Filtration Unit with Chemical Reaction Chamber Set Up
Sample Source | pH | Chemical Oxygen Demand(mg/L) | Total Solids (mg/L) | Total Suspended Solids (mg/L) | Oil and Grease(mg/L) |
Heavy Vehicles | 6.4- 7.0 | 255-445 | 1200- 6000 | 400–2000 | 400-800 |
Light Vehicles | 7.1- 7.6 | 227-378 | 800- 1500 | 600–750 | 150- 700 |
Composite Samples | 6.2- 6.6 | 280-360 | 1800- 2500 | 1500-2000 | 300– 700 |
Subsequently, treatment of the wastewater with acclimated suspended biomass (activated sludge) resulted about 18 – 68% removal efficiency for initial oil and grease concentration of 300 – 600 mg/L under the batch period of 18 – 30 hours.
Biological treatment of automobile service station wastewater is feasible under batch mode, but it would take at least 30 hours to attain the removal efficiency of 70% or more for a biomass concentration of 3000 mg/L and at a (F/M) ratio of 0.1.Some scientific backdrop references from academic studies on the subject of treating oily Effluent:
Kurian and Natarajan (1997) studied 40 samples from 10 automobile service stations and found total suspended solids, BOD 5, COD and oil and grease concentration in the range of 610– 4950 mg/L, 75 – 570 mg/L, 270 – 1640 mg/L and 14 – 420 mg/L respectively.
Characterization of Wastewater Different parameters like pH, Total solid, Total suspended solid, COD and Oil and Grease of the sample were measured from samples collected from other similar sources of car washing facilities.
Chemical treatment of the representative sample of such wastewater with alum and chitosan resulted in the effluent quality, satisfactory for disposal into the municipal sewer.
In another instance the researchers had the composite oily wastewater first treated by using coagulants like alum, PAC, FeSO4 and CaCl2 .
The results showed that removal is feasible for initial oil concentration in the range of 300 –600 mg/L for the alum dose of 100 -400 mg/L, alum + Bentonite dose of 20 -250 mg/L and FeSO4 dose of 50 - 200 mg/L.
Ahmed et al. (2005) carried out a study with Palm oil mill effluent (POME) containing about 4000 mg/l of residual oil using three types of adsorbents – Chitosan, Activated carbon and Bentonite.
Chitosan showed the best removal compared to the other adsorbents.Chitosan, activated carbon and Bentonite at the dosages of 0.5 gm, 8.0 gm and 10 g/L respectively, had successfully removed 99% of residual oil from POME.
Following are the sequences and process for the chemical treatment of the effluent where all the doses mentioned are from a typical case.
(For any specific project a Jar Test is recommended to determine exact doses.
1. Add Bentonite @ 100-500 gram per m3/ or up to the point when floating oil has disappeared after through mixing.
2. Add Alum solution/Poly-Aluminum Chloride (PAC) slowly till coagulated particles show pin flocks. Test a small batch in a beaker to determine the exact dose.
3. Add Chitosan @ 1 ppm (1gm/CUM) if oil is present in the raw effluent and or Polyelectrolyte (such as the one supplied by SNF, Ashland-Hercules or equivalent) @ 10-50 ppm, till the pin flocks have turned into larger flocks and are clearly visible. If need be, small batch test should be conducted to determine the dose.
4. At this point as the flocks will have clearly formed which means the coagulation is complete. So it is time for flocculation to begin.
5. Check the pH. If it is in the acidic side add pre-diluted lime/caustic solution till the pH reaches 6.5-7, while stirring well.
6. Till this point if you are using a mechanical stirrer use it at 120-150 rpm for 10-15 min.
7. Add pre-diluted anionic PAM (Polyacrylamide) in pre-diluted solution @ 2-10 ppm (2 gm to 10 gm of PAM powder/m3) up to the point when all the flocks have come together and formed larger and quickly settling flock.
8. In case of ready made PAM solution is used, the dose will be accordingly to the situation and strength of the preparation.
9. During this flocculation phase use the stirrer at 10-20 rpm for 2-5 min.
10. Stop all agitation and let the flocks settle down.
11. After the larger flocks settle down at the bottom, drain them out through the bottom port.
12. Start filtration of the remaining supernatant or take it to secondary biological treatment prior to filtration as may be the design according to requirement.