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.
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.
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.
Oil and Grease