What are the treatment methods for wastewater generated in starch production?

Currently, the commonly used treatment methods for starch wastewater both domestically and internationally include the following: sedimentation separation, chemical flocculation, simple aeration, biological treatment (activated sludge process, anaerobic biological treatment, biofilm process, biological ponds, etc.), membrane separation technology, and photosynthetic bacteria.

1.Sedimentation Separation: Starch is insoluble in cold water. Suspended solids in the wastewater can be directly settled out through physical sedimentation, typically using sedimentation tanks or ponds. Starch wastewater contains proteins, starch, sugars, and suspended solids, and belongs to a highly dispersed hydrophilic colloid system. Such colloids are generally stable. When anaerobic reactions occur in the tank, the resulting organic acids lower the pH of the wastewater, causing the colloidal proteins to form flocs and settle, thereby improving separation efficiency. The hydraulic retention time (HRT) of the sedimentation tank (pond) is approximately 2–7 days. For each ton of raw material, a sedimentation tank (pond) volume of 1.5–1.6 m³ is required. The purification efficiency of sedimentation tanks (ponds) is such that the SS removal rate is generally around 70%, with good performance reaching 90%, while the BOD₅ removal rate can reach 20–30%.

2.Chemical Flocculation: Natural sedimentation alone requires long treatment times and achieves low BOD₅ removal. If chemical agents are added using a chemical coagulation method, the stability of the colloids is disrupted, causing the dispersed organic matter to destabilize, coagulate, and form aggregated coarse particles that separate from the water. The coagulation–sedimentation method achieves better removal efficiency than physical sedimentation and requires shorter treatment times. Coagulation can remove larger-molecular-weight organic compounds, while smaller-molecular-weight organic compounds can be removed by activated carbon adsorption.

3.Simple Aeration: Wastewater is aerated for a short time using air or ozone-containing air. Purification is achieved through air oxidation, ozone oxidation, and stripping of volatile substances. This method is generally not used alone. In some small-scale potato starch plants in northern China, where production processes and seasonal conditions are not suitable for biological treatment, a combined process of sedimentation–separation followed by simple aeration can be used. Sedimentation–separation reduces the treatment load on the subsequent aeration step and also helps regulate and stabilize water quality and flow. The purpose of the subsequent simple aeration is to ensure that the treated effluent meets discharge standards, and it can also strip off organic acids generated during sedimentation, bringing the wastewater to a neutral pH.

4.Biological Treatment: Starch wastewater is a high-concentration organic wastewater containing no toxic substances and having good biodegradability. Therefore, the most commonly used treatment methods for starch wastewater both domestically and internationally are biological methods, including activated sludge, anaerobic biological treatment, biofilm processes, and biological stabilization ponds. Because starch wastewater has a high organic content, aerobic biological treatment requires high energy and treatment costs. In contrast, anaerobic biological treatment requires no oxygen supply and has lower treatment costs, making it widely applied. The following focuses on this treatment method. Anaerobic biological treatment refers to the process in which, under molecular oxygen‑free conditions, anaerobic microorganisms (including facultative bacteria) decompose various complex organic compounds in starch wastewater into methane, carbon dioxide, and other substances. At the same time, part of the organic matter is synthesized into bacterial cells, and through gas–solid separation, the wastewater is purified. In recent years, the upflow anaerobic sludge blanket (UASB) process has been the most rapidly developing and most widely applied anaerobic treatment for starch wastewater. In addition, processes such as the anaerobic contact process, anaerobic packed bed, anaerobic filter, vertical baffled anaerobic sludge bed (VBASB), and anaerobic baffled reactor (ABR) are also used.

5.Membrane Separation Technology: The membrane separation technologies used for treating starch wastewater are mainly reverse osmosis and ultrafiltration. Abroad, membrane separation technology is used to remove COD from potato starch wastewater and to concentrate and recover protein. Research has also been conducted in this area in China. The following introduces a simulation study on treating potato starch wastewater using ultrafiltration technology. The experiment used a two-stage hollow‑fiber ultrafiltration membrane module in series. The first stage used a polyacrylonitrile (PAN) membrane with a larger pore size and a molecular weight cut‑off (MWCO) of about 100,000 Da to remove large particles and part of the COD while allowing protein to pass through. The second stage used a polysulfone (PS) membrane with an MWCO of about 15,000 Da, mainly for protein concentration. Operating conditions: operating pressure 0.1 MPa, feed flow rate 70 L/h, room temperature, and the feed solution pH was adjusted to about 3.5 before ultrafiltration. The influent COD was 8175 mg/L. After ultrafiltration, the effluent COD was 3610 mg/L, giving a COD removal rate of 55.8%. Subsequent treatment technologies can follow to bring the wastewater up to standard. Fouled membranes were cleaned with 0.1 mol/L NaOH solution at 40°C, achieving a recovery rate of about 90%. When applying ultrafiltration technology, pretreatment of the feed solution is very important.

6.Photosynthetic Bacteria (PSB): Using photosynthetic bacteria to treat starch wastewater not only achieves high removal rates of organic pollutants but also requires low investment and small land area. Moreover, the bacterial sludge is non‑toxic to humans and animals and serves as a protein‑rich feed supplement. Therefore, the PSB method is a very promising treatment technology for purifying high‑concentration organic wastewater.

7.Resource Recovery Technologies for Starch Wastewater: Starch wastewater is rich in nutrients. Recovering useful substances from starch wastewater not only turns waste into treasure and enables comprehensive utilization but also reduces the cost of wastewater treatment. Research has been conducted in China on using starch wastewater to produce feed yeast and to extract amylase, among other areas.