Integrated treatment of tapioca processing industrial wastewater based on environmental bio-technology

Tapioca processing wastewater containing high COD (9,630-13,760 mg/L), BOD (7,280-11,510 mg/L), SS (450-1,850 mg/L), total nitrogen (291-355 mg/L) total phosphorus (39-73 mg/L) and low pH (3.4-4.6) are one of the major causes of severe pollution to receiving source in South Vietnam. Based on the characteristics of tapioca processing wastewater, biological treatment processes are selected for study. In which the UAF, UASB, aeration tank and pond system are in series. For this treatment technology, the UAF reactor is used as pre-treatment for reducing of SS. The SS removal efficiencies ranged from 67-82%, equivalent to influent SS decreased from 450-1,850 mg/L to 148-290 mg/L. The core of treatment system is the UASB-reactor, which is responsible for the removal of organic matter. The OLR of UASB-reactor can achievedto 42 kgCOD/m ³ .d with HRT8.3-8.4 hours. The influent COD decreased from 10,498-14,330 mg/L to 1,082-2,178 mg/L, equivalent with treatment efficiencies ranged from 82.5-90.7%. The aeration tank and pond system are used as post-treatment. They take responsibility for the remainder of organic matter, nitrogen and phosphorus removal. With the HRT around 6-8 hours for aeration tank and 6-7 days for pond system, the effluent COD, total nitrogen and total phosphorus ranged from 78-94 mg/L, 4.5-5.7 mg/L, 7.3-7.9 mg/L, respectively. In the UASB-reactor, the granular sludge was observed after 20 days of operating at OLRof 6 kgCOD/m ³ .d. The study also showed that nutrients and trace elements are necessary for tapioca wastewater treatment using UASB-reactor, but only for start-up period, when OLR lowerthan 20 kgCOD/m ³ .d. The UASB-reactor was able to stand a shock load as the COD concentration three times higher than normal, in 24 hrs stretches, without any damage and the completed recovery took 2-3 days.The investigateon cyanide toxicity showed that cyanide exerts a distinct toxicity, in particular on the hydrolysis process of tapioca starch particles. Hence, hydrolysis of these particles was completed within 2 days in the control experiment (using 2g starch particles and 2 gVSS/L in the experiment conducted at 30 ^o C), while at a high cyanide concentration of 120 mgCN ^- /L it took up to 6 days. But compared to the hydrolysis process, the acidification process is much less affected by cyanide; acidification always proceeded immediately after hydrolysis of the starch particles. The experiments of reduction the cyanide toxicity showed that sulphide ions do not reduce the toxicity of cyanide through the formation of thiocyanate, but instead reinforce the toxicity of cyanide. And the addition of Fe ²⁺ significantly reduces the toxicity of cyanide on the hydrolysis and methanogenesis through the formation of the ferro-cyanide complex, but it does not stimulate the degradation of cyanide.The conclusions of the research showed that the solution comprising the implementation of the AnWT-technology combined with aerobic post-treatment, clearly demonstrates the big benefit of the energy cost savings and resource conservation together with its feasibility to meet really extremely severe discharge standards. It also shows how strong returns can be generated for investors using CDM. The estimate shows that the required treatment system with a capacity of 2,000 m ³ /d will produce 4,200 - 6,200 m ³ biogas per day (>60 % methane gas), which is equivalent to 2,600 - 3,800 liters of FO per day. And energy cost savings to the plant are estimated at 850 - 1,260 US$ per day.