Degradation and detoxification of cyanide in anaerobic processes | |
| Author | Amara Amornkaew |
| Call Number | AIT Diss. no. EV-99-4 |
| Subject(s) | Cyanides Sewage--Purification--Cyanide removal |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Technical Science, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Abstract | Tapioca starch is one of the important agro-based products in many parts of the world. Tapioca roots contain about 20-25% starch. The wastewater volume generated from starch extraction process can be anywhere between 20-60 m3 /ton depending upon efficacy of process employed. The wastewater is highly acidic; pH of combined wastewater is between 3.8 and 5.2 with COD as high as 25,000 mg/L. One of the most important problems relating to tapioca is its cyanide content. Cyanide occurs in tapioca in the form of linamarin and lotaustralin as natural defence mechanism. During starch processing, cyanide from tapioca is hydrolyzed and released into process water. As a result, the starch wastewater contains cyanide up to 10 mg/L. Since cyanide at such high levels is highly toxic to aquatic life, the wastewater needs to be treated before discharge. Due to its high COD concentration, anaerobic processes ideally suited for treatment of starch wastewater. This study accordingly deals with effect of cyanide toxicity and its degradation during batch and continuous methanogenesis. Investigations have been carried out to study perfo1mance of a continuous upflow anaerobic sludge blanket (UASB) reactor with synthetic and real wastewater containing cyanide. Cyanide degradation is investigated in batch methanogenesis using granule sludge from UASB reactor. Toxic effect of cyanide is measured qualitatively using Microtox Toxicity Test. Continuous Experiments: A laboratory scale acrylic reactor with 5.7-liter internal volume and 1 meter height was used for investigations. The reactor was seeded with seed sludge from anaerobic lagoon treating tapioca starch wastewater. The reactor startup was accomplished in about 3 months with synthetic wastewater without cyanide as feed. Steady state COD loading rate of about 50-52 kgCOD/m3 .day and gas productivity of 8.42 m3 /m3 .day was recorded with upflow velocity of 0.25 m/h. Cyanide contents in the feed were then increased to 1, 2, 4, 6, 8, 10, 15, 20, and 25 mg/Lin steps while maintaining COD loading rate constant at about 50 kgCOD/m3 .day. At each cyanide feed concentration the reactor was allowed to reach steady state as ascertained by constant cyanide and COD conversion and steady gas production rate. At all cyanide concentrations the reactor performance recovered to its original level. At lower concentration of cyanide in feed the time required for recovery of gas production rate was smaller. Typically for cyanide concentration of 4 mg/L in feed, the gas production rate recovered in 5 days. However, reactor needed higher recovery period as cyanide concentration in feed increased. At 25 mg/L of cyanide concentration in feed, the reactor required 15 days for complete recovery. Effect of operating parameters of UASB reactor such as upflow velocity, cyanide loading rate and COD loading rate on cyanide removal was also investigated. In all experiments, upflow velocity had the most profound effect on cyanide removal. Increase in upflow velocity resulted in reduced cyanide removal. At constant COD loading of 50 kgCOD/m3 .day and influent cyanide concentration of 25 mg/L, the cyanide removal decreased from about 94% to 76% when upflow velocity was increased from 0.15 m/h to 0.45 m/h. At constant COD loading of 50 kgCOD/m3 .day and constant CN- loading of 0.2 kgCN-/m3 .day, cyanide removal also decreased from about 91 % to 76% when upflow velocity was increased from 0.15 m/h to 0.45 m/h. These investigations suggested that upflow velocity less than 0.3 m/h was necessary to achieve cyanide removal higher than 85%. 111 Experiments on cyanide removal with real wastewater from tapioca starch factory containing upto 10 mg/L of cyanide also were conducted. The reactor yielded satisfactory cyanide removal of about 93% - 98% and gas productivity of 7.5 m3 /m3 .day with upflow velocity of 0.25 m/h and COD loading rate upto 50 kgCOD/m3 .day. Batch Experiments: Degradation of cyanide and its toxic effects on anaerobic biogranules were investigated in batch serum bottles. Batch anaerobic cultures were exposed to a range of initial cyanide concentrations from 1 to 100 mg/L for a period of 48-hour. Cyanide degradation rate increased with initial cyanide concentration in batch. Maximum cyanide degradation rate upto 35 - 40 mg/L.day was recorded. Activity of biogranules was affected due to the presence of cyanide. Methane production rate decreased with increase in initial cyanide concentration in batch. Methane production rate of 0.11 ml mr1 d- 1 was recorded for 10 mgCN-/L as against to 0.42 ml mr'd-1 for control containing no cyanide. Results from Microtox Toxicity Test confirmed degradation of cyanide occurring in batch tests. EC50 for exposure time of 5 minutes and 15 minutes increased with respect to batch time indicating reduction in cyanide contents and its toxicity. |
| Year | 1999 |
| 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, A. P. ; |
| Examination Committee(s) | Bhattacharya, S. C. ;S. Weesakul ;Dahl-Madsen, Karl Iver; |
| Scholarship Donor(s) | Rajamangala Institute of Technology Bangkok, Thailand ; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1999 |