hi experts,
we are operating a wastewater treatment plant for an industrial park. Contaminants are mainly metals. here are some input parameters:
Input parameters                                Discharge standard
pH = 5                                                     5-9
Q = 3000 m3/day
COD = 160 mg/l                                        50
BOD = 40 mg/l                                          30
SS = 75 mg/l                                             50
As = 0.5 mg/l                                            0.05
Hg = 0.05 mg/l                                          0.005
Pb = 1 mg/l                                               0.1
Cd = 0.5 mg/l                                            0.005
Cr(III) = 4 mg/l                                         0.2
Cu = 5 mg/l                                               2
Zn = 5 mg/l                                                3
Ni = 4 mg/l                                                 0.2
Mn = 4 mg/l                                                0.5
Fe = 10 mg/l                                               1
Sn = 5 mg/l                                                0.2
Cl = 1000 mg/l                                            500
N-NH4 = 20 mg/l                                          5
Total N = 80 mg/l                                         15
P = 15 mg/l                                                  4
The process is as below:
EQ tank, neutralization tank, MBBR tank, rapid mix tank, sedimentation tank, disinfection tank.
Hydraulic detention time of sedimentation tank is about 1.5h, Surface loading rate is 1.7 m3/m2.h
Usually, because the BOD concentration rarely gets over 40 mg/l, plus high concentration of heavy metals, the microorganisms don't play any role in this system. Our priority is to eliminate the heavy metals so as to meet the discharge standard. Before, we used an agent called Super Jas Clean together with Caustic and Anionic Polyacrylamide (at Rapid mix tank) to reduce metals. All output parameters met the standard except for Nickel. After a long time trying, we changed from Super Jas Clean to Ferric Cloride. This time, Nickel passed. However Iron failed. We tried to manage the Ferric Chloride dose as well as pH, polymer dose, but nothing could help the situation. Effluent water still contains a little visible suspended solid. We suspected that it was remain ferric hydroxide caused these suspended solid as well as account for the excess of effluent Iron.
So what are we supposed to do now? Replace ferric  chloride by another agent or set up some sand filters right after the sedimentation tank?

Hi mr. Cooper,

maybe you should try some good type (requires trial& error) of PAX (Poly Aluminium Chloride)? Often, the results can be better then with Fe(III).
Besides that you do not seem to have a norm for Al ...
Get advice of the manufacturer because there are many different types of PAX and formulations.

How do you perform the N-removal?
also chlorides are over the norm unless you have a good trick to deal with this?
Best of luck with it!

DS

Dear all,
The following question is a little beyond the topic, but I still wanna ask. In Iron removal (treating underground water), we often use oxidation to turn Fe (II) into Fe(III) before precipitation by raising pH. What is the purpose of Fe(II) oxidation? Because as far as I know, we can also precipitate Fe (II) by raising pH. Therefore, why to use oxidation?