The potato is one of the most widely consumed staple foods in the world. In industrial processing, washing is a particularly important step. For every kilo of potatoes, 2 to 3 litres of water are needed to clean the potatoes. In the process, potato starch also mixes into the wastewater.
Starch removal from potato washing wastewater, and how to treat the remaining wastewater
Process steps for starch removal from potato washing water
Starch removal from potato washing water is an essential step in the potato processing industry. The process not only helps to recover valuable starch but also ensures the proper disposal of wastewater. The procedure typically involves the following steps:
1. Screening: The potato washing water contains large solid particles, such as peels and other debris. A preliminary screening process uses a rotating drum screen or a vibrating screen to remove these solids. This step helps in reducing the load on subsequent treatment processes.
2. Sedimentation/Settling: The screened water is then transferred to a sedimentation tank, where the water is held for a specific time, allowing the starch to settle at the bottom due to gravity. The settled starch forms a dense slurry, which is then collected and sent for further processing or disposal. The remaining water, known as the supernatant, is still rich in dissolved starch and requires further treatment.
3. Hydrocyclone separation: The supernatant is passed through hydrocyclones, which use centrifugal forces starch removal. The water spirals inside the hydrocyclone, and the heavier starch particles move to the outer wall and then to the bottom, where they are collected. The clarified water flows out from the top.
4. Dissolved Air Flotation (DAF): In this process, air is dissolved in the water under high pressure and then released at atmospheric pressure. The released air forms tiny bubbles that attach to the remaining starch particles, causing them to float to the surface. A skimming mechanism leads to the starch removal while the treated water is discharged.
Characteristics of the wastewater after starch removal
The characteristics of potato processing wastewater after starch removal can vary depending on the specific processes used and the efficiency of the starch recovery system. However, some typical concentrations of key parameters in the effluent are as follows:
1. Chemical Oxygen Demand (COD): COD is a measure of the amount of oxygen required to chemically oxidize the organic matter in the wastewater. After starch removal, the remaining wastewater usually has a COD concentration ranging from 1,500 to 6,000 mg/L, although values can vary depending on the specific process.
2. Biochemical Oxygen Demand (BOD): BOD is a measure of the amount of oxygen required by microorganisms to biologically degrade the organic matter in the wastewater. BOD values for potato processing wastewater after starch separation typically range from 500 to 3,000 mg/L.
3. Total Suspended Solids (TSS): TSS refers to the mass of solid particles suspended in the wastewater. After starch separation, TSS concentrations can range from 200 to 2,000 mg/L.
4. Nitrogen: Nitrogen is an essential nutrient for plant and microbial growth, and it is present in the wastewater as organic nitrogen, ammonia, and nitrate/nitrite. The total nitrogen concentration in potato processing wastewater after starch removal can range from 50 to 250 mg/L.
5. Phosphorus: Like nitrogen, phosphorus is also an essential nutrient for plant and microbial growth. Phosphorus is typically present in the wastewater as orthophosphate or polyphosphate. The total phosphorus concentration in potato processing wastewater after starch separation can range from 10 to 100 mg/L.
These values are approximate and can vary based on factors such as the type of potatoes, the efficiency of the starch recovery system, and the specific processes employed. It is important to note that these concentrations are still relatively high and require further treatment before the wastewater can be discharged into the environment or reused for other purposes.
How the efficiency of the starch removal affects the wastewater composition
The efficiency of the starch separation process directly affects the composition of the wastewater. A more efficient starch removal system will recover a larger proportion of starch from the potato washing water, reducing the load of organic matter and suspended solids in the wastewater. Conversely, a less efficient system will result in higher concentrations of starch and other substances in the effluent, which can impact the subsequent treatment processes and the environment. Here’s how the efficiency of starch separation influences the wastewater composition:
1. Organic matter content: A higher starch separation efficiency reduces the amount of organic matter in the wastewater, as measured by parameters such as Chemical Oxygen Demand (COD) and Biochemical Oxygen Demand (BOD). This reduction in organic load can make subsequent wastewater treatment processes more effective and reduce the potential for oxygen depletion in receiving water bodies when the treated effluent is discharged.
2. Suspended solids: Efficient starch removal also reduces the concentration of Total Suspended Solids (TSS) in the wastewater. Lower TSS levels facilitate the subsequent treatment processes, such as sedimentation and biological treatment, and reduce the potential for water pollution.
3. Nutrient content: Starch separation efficiency can also impact the concentrations of nutrients like nitrogen and phosphorus in the wastewater. While the primary source of these nutrients is the potato cells and other organic matter, the presence of residual starch in the wastewater can contribute to higher nutrient levels. Efficient starch separation can help reduce nutrient concentrations, making it easier to meet discharge regulations and minimize the risk of eutrophication in the receiving water bodies.
4. Treatment requirements: A more efficient starch removal system will result in a lower organic load and reduced concentrations of suspended solids and nutrients in the wastewater. This reduction can make subsequent treatment processes more effective and may require less energy, chemicals, and overall resources for treatment. In some cases, it may even allow for the use of simpler and more cost-effective treatment methods.
In summary, the efficiency of the starch removal process plays a critical role in determining the composition of the wastewater. A more efficient system can lead to reduced pollutant concentrations, making it easier to treat the wastewater and minimize the environmental impact of the potato processing industry.
Cleaning the remaining wastewater
To clean the remaining wastewater, a combination of physical, chemical, and biological treatment processes is used:
1. Primary treatment: This process involves sedimentation and flotation techniques to remove suspended solids and some organic matter.
2. Secondary treatment: It includes biological processes, such as activated sludge, trickling filters, or rotating biological contactors. FBBR has been shown to be one of the most effective and reliable forms of biological treatment. Microorganisms break down the organic matter in the wastewater, reducing the BOD and COD levels. The process may also involve the addition of coagulants, such as alum or ferric chloride, to facilitate the precipitation of phosphorus.
3. Tertiary treatment: This step aims to remove nitrogen and further reduce phosphorus levels. Methods such as nitrification-denitrification or enhanced biological phosphorus removal (EBPR) are employed. Additional filtration, such as sand or membrane filtration, may be used to remove fine suspended solids.
4. Disinfection: The treated wastewater is disinfected using methods like chlorination, UV radiation, or ozonation to kill any remaining pathogens before discharge into the environment.
The cleaned wastewater can then be safely discharged into rivers, lakes, or other water bodies without causing harm to the environment. In some cases, it can be reused within the processing facility for non-potable purposes, such as irrigation or cooling.