Zinc removal from industrial discharge using thermophilic biological sulfate reduction with molasses as electron donor | |
| Author | Warounsak Liamleam |
| Call Number | AIT Diss. no.EV-07-05 |
| Subject(s) | Zinc Sewage--Purification--Heavy metals removal |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering in Envirorunental Engineering and Management |
| Publisher | Asian Institute of Technology |
| Abstract | Rayon industry as a textile industry uses large quantities of zinc in its manufacturing process. The industry produces large quantities of highly polluted wastewater, especially a wastewater stream from the spinning process, which contains sulfate and zinc as major pollutants. Sulfate in this wastewater is originated from the use of Na2SO4 and sulfuric acid, whereas zinc is used as one of the components in the spinning process. The wastewater discharged from this process has a flow rate of approximately 4,800 m3/d. From chemical analysis, the results revealed that sulfate is found in the range of 8,000- 10,000 mg/Land zinc is 110-300 mg/L, respectively. Moreover, pH and temperature were in the range of 1.0-1.5, 60-70°C, respectively. At present, zinc is removed as zinc hydroxide (Zn(OH)2) by conventional precipitation process using lime (Ca(OH)2), whereas sulfate is partially precipitated as CaSO4. It is obvious that the current treatment process generates bulky sludge requiring dewatering and is unstable due to pH change. On the other hand, biological sulfate reduction generates sulfide which can be used for zinc removal. The extremely low solubility of metal sulfide formed by precipitation allows the removal of heavy metals from the wastewater stream. The sludge produced is far denser and sulfate concentration can be lowered to potable water level. The production of sludge from sulfide precipitation is low as compared to hydroxide precipitation. This research aimed to investigate the feasibility of thermophilic (65°C) sulfate reduction process for zinc and sulfate removal from wastewater discharged from rayon industry. The study aimed to maximize sulfide production from the sulfate reduction process and optimize both sulfate and zinc removal. The experimental studies consisted of both laboratory scale and pilot scale investigations. In the laboratory scale experiment, biological sulfate reduction was accomplished under thermophilic (65°C) conditions in a 6.4-L UASB reactor, using molasses as an electron donor and carbon source. Sulfide-rich effluent from the reactor was used to precipitate zinc as zinc sulfide (ZnS) in a precipitation chamber. This experiment aimed primarily to investigate the effect of feed COD: sulfate ratio, which is the key factor for the operation of sulfate reduction process. The laboratory scale experiments were divided into two stages: Stage I synthetic wastewater as feed and Stage II rayon industrial wastewater as feed. The experimental results showed that the process achieved a sulfate conversion rate of 7.22 ± 1.91 g SO4/L.d at a COD: sulfate ratio of 1.5: 1 and 7.20 ± 2.27 g SO4/L.d at a COD: sulfate ratio of 3: 1 during Stage I and Stage II, respectively. At the end of Stage II operation, the sulfide production was achieved at a maximum level of 496.2 mg S/L at a COD: sulfate ratio of 3: 1. The overall performance of the laboratory scale UASB reactor showed high sulfate conversion at a COD: sulfate ratio of slightly higher than 0.67: 1. For zinc sulfide precipitation process, sulfide rich effluent was maintained according to the stoichiometric ratio, that is 32 g of sulfide is required to precipitate 64 g of zinc. The results showed that more than 98% of zinc was removed from the wastewater when it was operated at pH 7.0. The outcomes from the lab-scale investigation of biological sulfate reduction were applied at pilot scale experiments to treat sulfate-containing wastewater from rayon industry, Angthong province, Thailand. The system essentially consisted of precipitation unit and sulfate reduction unit. Molasses was used as electron donor and carbon source. Thus, sulfide from sulfate reduction is further used for metal precipitation. Lime (Ca(OH)2) and caustic soda (NaOH) was used to adjust the pH of the wastewater to be in the range of 7-8. With the pH adjustment, zinc and sulfate were partially removed through zinc hydroxide and calcium sulfate (Ca(SO)4). This method consumed large quantities of lime to raise the pH to the neutral range, whereas pH adjustment by NaOH requires less amount of NaOH. However, NaOH was not able to reduce sulfate concentration in the wastewater. Zinc removal from wastewater achieved more than 98% by precipitation with sulfide rich effluent from the UASB reactor. For sulfate reduction, the performance of the UASB reactor demonstrated that sulfate conversion rate was found at 2 ± 1 kg SO4/m3 .d, when the UASB reactor was operated at COD: sulfate ratio 2:1 and 1:1, respectively. When lime was used for pH adjustment, sulfate was partially removed by precipitation as CaSO4. Thus, effluent sulfate concentration was 2889 ± 556 mg/L. On the other hand, with pH adjustment by NaOH lesser sulfate reduction efficiency was achieved. Effluent sulfide was found at 254 ± 56 mg/Land 210 ± 89 mg/Lat COD: sulfate ratio 2:1 and 1:1, respectively. |
| Year | 2007 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Energy and Climate Change (Former title: Department of Energy, Environment, and Climate Change (DEECC)) |
| Academic Program/FoS | Environmental Engineering (EV) |
| Chairperson(s) | Annachhatre, Ajit P. ; |
| Examination Committee(s) | Visvanathan, C. ;Preeda Parkpian ;Athapol Noomhorm ;Lens, Piet ; |
| Scholarship Donor(s) | Japan ; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2007 |