Wastewater treatment from hexavalent chromium and other heavy metals
The purification and safe disposal (neutralization) of industrial waste water effluents represent a very important ecological problem. The principal problem or requirement involves reducing the concentration of harmful impurities to concentration levels below their maximum allowable concentration limits. In fulfilling this requirement the amount of fresh water consumption needed to maintain the water level in the circulating or recycled water supply system is also reduced, and the operating life of production equipment or lines is concomitantly prolonged as a consequence of decreased corrosion and sediment formation or precipitation (scaling).
In many cases the use of traditional methods, such as settling or sedimentation tanks and ponds, or flotation devices, are not effective or efficient enough for operation, and so more advanced, progressive, and forced methods must be employed, such as treatment by Magnetic Vortex Activator AVS-100 (electromagnetic unit with velocity layer), for instance. The effect of electrophysical factors (electron acceleration, etc.) on contaminated water leads to rapid decomposition of many chemical impurities or contaminants to give inactive (non-harmful) substances.
-
- Intensifier of Technological Process AVS-100. General view
Designed for last few years electromagnetic units with velocity layer have a wide usage today. These machines can be use by the different branches of industry for intensification of technological (chemical and physical) processes and even for treatment (sterilization, neutralization) of waste water.
From the large quantity of waste water types usually occurs waste water contaminated by chromic compound, fluorine, arsenic, phenol and other harmful and organic impurities. Such waste waters are characterized for the many of machine-building plants.
We have investigated the characteristics of waste water samples before (table 1) and after (table 2) treatment by AVS-100.
Table 1
Characteristics and content of industrial waste water
|
Parameters |
Machine-building plants impurity concentration mg/dm3 |
|
|
food industry |
Plants of other sectors |
|
| pH effluent |
2-6 |
2-6,5 |
| Cr6+ |
10-250 |
20-1000 |
| Fe2+ |
50-150 |
10-200 |
| Cu2+ |
30-120 |
15-150 |
| Zn2+ |
20-150 |
20-170 |
| Ni2+ |
up to 180 |
10-190 |
| Cd2+ |
5-100 |
15-150 |
| Weighted components |
up to 300 |
up to 350 |
We have investigated the treatment effectiveness by using Magnetic Vortex Activator AVS-100 from two methods: Ith method – sterilization of chromium-containing wastewater by Cr6+ up to Cr3+ and using the reagent (sulfuric acid iron); IIth – combined neutralization and treatment from the heavy metals ions of chromium-containing wastewater and acid-base wastewater.
Intensifier of Technological Process AVS-100. General view
The process of chromium-containing waste water purification is as follows. The waste waters which contain the specific hexavalent chromium concentration at the rate of 10-15 m3/h from the flow-equalization basin enter the AVS-100. At the same time from the consumption tank by dosing pump to the AVS-100 deliver project quantity of ferrous [iron] sulfate solution (concentration 30-60 g/dm3). At the AVS-100 carry out the waste water treatment by FeSO4 solution, as result occurred regeneration of Cr6+ to the value Cr3+. Completeness of chromium regeneration was verified by the proximate analysis using the diphenilcarbazide and also by the colorimetric method. Sterilized waste waters entered to the neutralization and purification from trivalent chromium. During the research we was changed the consumption of ferrous [iron] sulfate from 80 to 100 % depending of: its stoichiometric consumption; pH environment from 2 to 4 during the regeneration of Cr6+ in the acid environment and from 7,5 to 9,0 in the alkaline condition. To alkalize the wastewaters we used the lime milk solution. Ferromagnetic particles are made from the steel DIN (WNr), mass is 150-250 g, diameter is 2,0-2,5 mm and l/d=10. Duration of treatment at the vortex layer is 0,5-1,5 c. Basic concentration of Cr6+ was changing from 100 to 1000 mg/dm3.
Another sterilization results of chromium-containing waste water by AVS-100
|
Basic characteristic of waste water |
Consumption of FeSO4, |
Cr6+ content after treatment, mg/l |
|
|
pH value |
Amount of Cr, mg/l
|
||
|
0,5 |
460 |
100 |
– |
|
0,5 |
43 |
100 |
– |
|
0,5 |
460 |
90 |
– |
|
5 |
83 |
100 |
– |
|
5 |
83 |
90 |
– |
|
5 |
83 |
80 |
0,3 |
|
0,8 |
76,5 |
100 |
– |
|
0,8 |
76,5 |
90 |
– |
|
0,8 |
2200 |
100 |
– |
|
4 |
103 |
100 |
– |
|
4 |
103 |
90 |
– |
|
4,5 |
1100 |
100 |
– |
|
4,5 |
1100 |
90 |
– |
|
4,5 |
1100 |
80 |
0,5 |
The results of industrial testing of AVS-100 show the high quality of treatment from chromium and heavy metals (Fe, Ni, Zn, Cu, Cd) on enterprises which clean chromium-containing wastewater in acid and alkaline conditions. At the same time consumption of reagents in installation AVS-100 is 90-100% of from stoichiometric consumption. Treatment facilities and their operation with using vortex layer of ferromagnetic particles are much simpler and efficient which is confirmed by our experimental investigations. Consumption of reagent such as additive (Ca(OH)2, Na2CO3 in the regular methods of treatment is on level 115-120% and consumption of reducing agent (FeSO4) is on level 150-175%.
Results of industrial research of units AVS-100 represented in table 2.
Table 2
The sterilization results of chromium-containing wastewater by AVS-100
|
Basic concentration value Cr6+, mg/dm3 |
pH process |
Consumption of iron sulfate, |
Ferromagnetic element mass, g |
Residual Cr6+ content after purification, mg/dm3 |
|
100 |
2 |
100 |
150 |
0 |
|
90 |
0 |
|||
|
80 |
0,56 |
|||
|
100 |
4 |
90 |
150 |
0 |
|
80 |
0,9 |
|||
|
590 |
2 |
100 |
200 |
0 |
|
90 |
0 |
|||
|
80 |
0,8 |
|||
|
1000 |
2,5 |
100 |
200 |
0 |
|
90 |
0,11 |
|||
|
80 |
1,1 |
|||
|
200 |
7,5 |
100 |
150 |
0,012 |
|
200 |
9,0 |
100 |
150 |
0 |
|
90 |
0,05 |
|||
|
80 |
0,98 |
|||
|
750 |
7,5-8,5 |
90 |
200 |
0,1-0,01 |
Neutralization of acid-base, sterilized, chromium-containing wastewaters and metal hydroxide formation was testing at the same installation by using AVS-100. As reagent was used 5-10% lime milk solution. Value of pH process was changing in the range of 7,5-9,0. Results of testing represented in table 3.
Table 3
Results of neutralization and treatment of wastewater from heavy metals ions by using industrial installation and AVS-100.
|
Basic metal concentration, mg/dm3 |
pH process |
Consumption of Ca(OH)2, |
Ferromagnetic element mass, g |
Residual metal content, mg/dm3 |
| Fe2+;3+= 130,0 |
7,5 |
90,0 |
200, |
Fe2+;3+ – 0 |
| Cu2+= 50,0 | Cu2+ – 0,12 | |||
| Zn2+= 45,0 | Zn2+ – 0,063 | |||
| Cd2+= 10,0 | Cd2+ – 0,07 | |||
| Cr3+= 120,0 | Cr3+ – 0 | |||
| Fe2+;3+= 170,0 |
8,5 |
100,0 |
150 |
Fe2+;3+ – 0 |
| Cu2+= 40,0 | Cu2+ – 0,018 | |||
| Zn2+= 28,0 | Zn2+ – 0 | |||
| Cd2+= 5,5 | Cd2+ – 0,011 | |||
| Cr3+= 100,0 | Cr3+ – 0 | |||
| Fe2+;3+= 250,0 |
8,7 |
100,0 |
200 |
Fe2+;3+ – 0 |
| Cu2+= 65,0 | Cu2+ – trace | |||
| Zn2+= 35,0 | Zn2+ – trace | |||
| Cd2+= 2505 | Cd2+ – 0 | |||
| Cr3+= 350,0 | Cr3+ – 0 |
At the same time we investigated method of industrial treatment using the mechanical mixing by flowmaker and also by using barbotage of air. By industrial technology consumption of lime milk is on a level 115-120% from stoichiometric consumption. Mixing time of wastewater with reagent is 15-20 min.
The following figures show comparative dependence effectiveness of wastewater treatment from heavy metals and also clarification of wastewater in the settling tank by using AVS-100 and reactors with the mixing machine.
Installation AVS-100 permit to provide the treatment quality below the maximum permissible concentration, reduce the consumption of reagent in 1,5 – 2 times and consumption of energy in 2 times, reduce the working area in 10-15%, it gives high economic benefits.We investigated the process of reclamation of chromium by regular reagent method using the barbotage by air with processing time 15-20 minutes to compare effectiveness of wastewater treatment from chromium in industrial conditions.