Experimental Study on High Temperature CSTR Medium Temperature UASB Two Stage Anaerobic Treatment of Cassava Alcohol Wastewater

The production of alcohol from cassava as raw material has a relatively broad market in provinces such as Guangxi, Guangdong, Hubei, and Jiangsu in China. In the conventional production process, about 12-15 tons of wastewater are discharged for every 1 ton of cassava alcohol produced. The effluent temperature of cassava alcohol waste residue is high, containing a large amount of organic compounds and suspended solids, with COD reaching 30-60 g/L and suspended solids reaching 20-30 g/L. The pH is low, making it a typical high concentration organic wastewater. If the wastewater cannot be treated stably and reliably, it will inevitably cause serious pollution to the environment. In recent years, the government has attached increasing importance to the treatment of alcohol waste liquid that seriously pollutes the environment. The second level standard for COD allowed to be discharged from the alcohol industry waste liquid is ≤ 300 mg/L, and the first level standard is ≤ 100 mg/L. Based on the characteristics of cassava alcohol wastewater, an anaerobic aerobic combined process can be used to treat cassava alcohol wastewater, and the stable operation of the anaerobic process is crucial for the entire treatment system.

Cassava alcohol residue contains a large amount of suspended solids with high concentration and viscosity, making direct solid-liquid separation difficult. And the separated residue is difficult to sell as feed due to its low protein content. Two stage anaerobic fermentation can be considered for the treatment of this type of wastewater. The first stage anaerobic reactor directly performs high-temperature whole grain fermentation to recover biogas while solving the problem of sludge disposal. The second stage anaerobic reactor treats the subsequent high concentration residual liquid. The author first investigated the characteristics of cassava alcohol wastewater, proposed a two-stage anaerobic process for treating cassava alcohol wastewater, and preliminarily explored the production cycle of cassava alcohol and the economic benefits of the two-stage anaerobic process, in order to provide reference for the process selection and design of cassava alcohol wastewater treatment.

1 Experimental section

1.1 Experimental wastewater

The cassava alcohol wastewater used in the experiment was taken from a cassava alcohol plant in Jiangsu Province. The wastewater was first treated and precipitated in a high-temperature anaerobic continuous flow stirred tank reactor (CSTR), and the supernatant was used as the influent for an upflow anaerobic sludge bed (UASB). To prevent changes in water quality, the water sample is stored in a refrigerator at 4 ℃ for future use.

1.2 Inoculation of sludge

The inoculated sludge was taken from the UASB granular sludge in the wastewater treatment station of the alcohol plant, with a VSS of 42 g/L. The inoculation sludge volumes for high-temperature CSTR and medium temperature UASB are 1 L and 0.5 L, respectively.

1.3 Experimental setup and operation

The first stage anaerobic reactor CSTR adopts an anaerobic fermentation tank with a total volume of 5 L and a working volume of 4 L. Using an electric stirrer for stirring at a speed of 200 r/min, heating in a water bath, and stabilizing the reactor temperature at (55 ± 1) ℃ through a self-control device. The second stage anaerobic reactor UASB operates continuously under load control, with a total volume of 2 L, including a reaction zone volume of 1.16 L, a reaction zone height of 280 mm, and an inner diameter of 70 mm. Wrap electric heating wires around the reactor wall and connect a temperature control device to control the temperature at (37 ± 1) ℃.

The two-stage anaerobic high-temperature CSTR medium temperature UASB reaction device is shown in Figure 1. Wastewater enters from the upper part of CSTR through a peristaltic pump, and the effluent enters the sedimentation tank. After the separation of mud and water, some of the effluent is used as the inlet for UASB. The sludge in the sedimentation tank is regularly returned to the CSTR with a return ratio of 1:1. The UASB inlet is stirred evenly by a magnetic stirrer and enters from the bottom of the reactor through a peristaltic pump. The effluent is separated into gas, liquid, and solid through a three-phase separator and then flows out through the outlet on the upper side of the reactor.