Cassava starch wastewater treatment

News

25/07/2024

According to statistics, the cassava starch industry discharges about 240 - 300 million m3 of wastewater into the environment/year. Cassava starch processing wastewater includes organic components such as starch, protein, cellulose, pectin, and sugar in fresh cassava tubers, which are the cause of high pollution to the wastewater stream of cassava starch factories.

Origin of cassava starch wastewater

Based on the cassava starch processing process, wastewater can be divided into 2 streams:

Waste stream 1: is wastewater after being sprayed into the cassava washing wheel to remove dirt and cassava peel. This type of wastewater has a low flow rate (about 2m3 of wastewater/ton of cassava), mainly containing substances that can settle quickly (cassava peel, soil, sand, ...). Therefore, this type of wastewater can pass through the screen, settle and then recycle the water in the washing stage. The part retained in the screen (cassava peel) after drying is used as fuel in production households.

Waste stream 2: is wastewater discharged during the cassava filtering process, this type of wastewater has a large flow rate (10m3 of wastewater/ton of cassava), has a high organic matter content, high suspended matter content, low pH, high cyanide content, sour smell, and opaque white color. These parameters are expressed through the suspended solids content SS, nutrients containing N, P, biological oxygen demand indicators BOD, chemical demand COD,... with very high concentrations.

How harmful is tapioca starch wastewater to the environment?

The pH of wastewater is too low, which will cause the self-cleaning ability of the receiving water source to be lost because the microorganisms naturally present in the water will inhibit their growth. In addition, when wastewater is acidic, it will be corrosive, causing an imbalance in cell metabolism, inhibiting the normal development of life processes.

Biodegradable organic matter content
Tapioca starch wastewater has a high organic content, when discharged into water sources, it will cause a decrease in the concentration of dissolved oxygen in the water because microorganisms use dissolved oxygen to decompose organic substances. Oxygen concentration below 50% saturation can affect the development of shrimp and fish. Reduced dissolved oxygen not only depletes aquatic resources but also reduces the self-cleaning ability of water sources, affecting the quality of water supplied for domestic and industrial use. BOD5 is related to determining the level of pollution of biodegradable components in wastewater and COD indicates the level of pollution of organic and inorganic substances contained in industrial wastewater. Pollution of organic substances leads to a decrease in the concentration of dissolved oxygen in water. Reduced dissolved oxygen will seriously affect the aquatic system, especially the microorganism system. When anaerobic decomposition occurs with a BOD5 content that is too high, it will cause a foul odor to the water source and kill the aquatic system, pollute the surrounding air and spread over a wide area by the wind.

High suspended matter content
Suspended solids make water cloudy or discolored, not only losing its aesthetic appearance but also reducing the depth of the illuminated water layer, affecting the photosynthesis of aquatic plants. On the other hand, when the sediment settles to the bottom of the water, it will cause anaerobic decomposition, causing a foul odor.
High nutrient content
High concentrations of N, P in water will cause eutrophication of water sources, affecting aquatic organisms in the water source, negatively affecting tourism and affecting the quality of water supply.
Biological wastewater treatment
Tapioca starch wastewater has a very high organic content, a large suspended solid content, with such wastewater characteristics, anaerobic methods should be used for treatment. However, the wastewater after anaerobic treatment needs to be treated by aerobic methods... to meet QCVN before being discharged into the receiving source.
Biogas tank:
Wastewater from starch processing will be led to the Biogas tank to treat organic compounds with high pollution concentrations, reducing pressure on the following works. Here, anaerobic microorganisms will decompose organic substances in wastewater into simple inorganic substances such as CH4, CO2, H2S. In this process, biogas is recovered to fuel the mold, run the boiler, cook, and generate electricity.
Organic substances →CH4 + CO2 +H2 + NH3 + H2S + new cells
When restarting the system, we can add anaerobic microorganisms to the biogas tank to increase the number of anaerobic microorganisms. Reduce COD, BOD, TSS of wastewater with high COD concentration. Increase the stability of the anaerobic system and the amount of methane.
Anoxic tank:

Wastewater after being regulated in terms of flow and waste concentration is pumped with a stable flow into the anoxic tank. Here, the process of reducing nitrogen into NO3- into gaseous nitrogen is carried out to achieve the permitted nitrogen standard.

The biological denitrification process involves the biological oxidation of many organic substances in wastewater using NO3- or NO2- as electron acceptors instead of oxygen, in conditions of no oxygen or limited DO (less than 2mg/l).

This process is carried out by a group of nitrifying bacteria that account for about 70-80% of the bacterial mass (activated sludge). The denitrification rate ranges from about 0.04 to 0.42g N-NO3-/g MLVSS. day, the larger the F/M ratio, the higher the reduction rate.
The tank is equipped with a submerged mixer to increase the contact of the biological sludge with the wastewater, thereby increasing the treatment efficiency.
NO3- → NO2- → NO → N2O (g) → N2
When restarting the system, Biofix Ammonia microorganisms must be added to promote the nitrogen treatment process in the anaerobic tank
Aeroten tank:
Wastewater after anaerobic treatment is led into the "Aerotank" to thoroughly treat organic substances. In the aerotank, the aerobic biological process takes place thanks to the microbial system maintained by the air blower. The amount of air supplied to the tank is for the purpose of: providing oxygen for aerobic microorganisms to convert dissolved organic substances into water and CO2. Mixing the wastewater and activated sludge evenly creates conditions for microorganisms to have good contact with the substrates that need to be treated. Releases gases that inhibit the life processes of microorganisms, these gases are produced during the process of microorganisms decomposing pollutants. Positively affects the reproduction process of microorganisms. The aerobic process is based on the principle that aerobic microorganisms decompose organic substances in the presence of dissolved oxygen according to the following equation:
Here, aerobic microorganisms (activated sludge) will decompose the remaining organic substances in wastewater into simple inorganic substances such as CO2, H2O,...

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