Effects of different flocculants and environment on the flocculation effect of potato starch-processing wastewater

Starch is the most important component of human food and also an important industrial raw material. Currently, the main sources of starch are extracted from crops such as potatoes, wheat, and corn. The wastewater generated during starch processing is organic wastewater with high viscosity and suspension as well as high concentration, which is difficult to treat. At the same time, such wastewater contains a large amount of organic matter, which is prone to eutrophication of water bodies, and has become one of the current environmental problems faced by starch-processing enterprises (Nikolavcic & Svardal 2000; Wang et al. 2008).

At present, potato starch production and processing technology mainly includes Holland's Niwoba company's Howe potato starch processing technology, and Sweden's Alfa Laval company's Larsen potato starch processing technology. The potato starch-processing process is mainly divided into potato washing, grinding, potato residue separation, starch washing and refining, drying and packaging of finished products and other processing processes. The wastewater source of potato starch processing is mainly produced by three stages: raw material washing, juice separation and starch washing (Fang et al. 2011; Gunsriwiang et al. 2021; Liyan et al. 2021; Sinaei et al. 2021; Warrajareansri & Wongthanate 2021).

The wastewater produced by potato processing is a high-concentration organic wastewater, although the wastewater does not contain heavy metals and pathogenic bacteria and other substances that pose a threat to human health, and is rich in nitrogen, phosphorus, potassium and other nutrients necessary for the growth of crops (Liu et al. 2008). 1 ton of potato processing starch can produce 0.7–1.0 tons of high-concentration organic wastewater and 0.2 tons of wet fiber (11% dry matter). The high-concentration organic wastewater is mainly potato cytosol and starch washing water, with mainly nutrients such as fine fiber, potato B starch, protein, amino acids, organic acids, polysaccharides, potassium salts and other inorganic salts (Barampouti et al. 2005; Liu et al. 2008; Wu 2016; Dobbeleers et al. 2017).

In recent years, the research on the treatment of starch wastewater mainly focuses on the reuse of resources, the improvement of traditional starch wastewater treatment technology and the development of new starch wastewater treatment methods. Starch wastewater treatment mainly includes physical methods (sand filtration, magnetic flocculation, foam separation and membrane filtration) (Beatriz & Aguirre 2011; Taihua et al. 2014; Bosak et al. 2016; Du et al. 2019a), biological methods (aerobic biological treatment, anaerobic biological treatment and photosynthetic bacteria method) (Guo et al. 2008; Prachanurak et al. 2014; Tan et al. 2014) and physical and chemical methods (coagulation precipitation and flocculation precipitation method) (Guo et al. 2015; Wang et al. 2015).

Physical methods make it relatively simple to treat starch wastewater, without the need to maintain the environment required for biological processes. The operation cost is low, the operation is simple, and there is no need to invest a lot of upfront funds. However, a single physical method cannot be a good pretreatment of water bodies, and other methods should be combined. Biological methods can efficiently decompose organic pollutants and significantly reduce energy consumption, but their costs are high and they have higher requirements for the environment. The physical chemistry method is further improved on the basis of physical methods, and its processing capacity is stronger and more friendly to the environment. For starch wastewater, a single treatment technology makes it difficult to meet the discharge standards. The combined process integrates the advantages of different processes to treat starch wastewater more thoroughly. There are many ways to reuse starch wastewater, such as production of ester substances, protein recovery, production of vitamins and biopesticides, biological hydrogen production, and development of biological flocculants (Ndao et al. 2019; Ngoc et al. 2019; Wadjeam et al. 2019; Li et al. 2020; Zhang et al. 2020a; Sinaei et al. 2021).

Magalhaes used cassava starch wastewater as fertilizer to study the effects of different wastewater amounts on maize growth traits and nutrient element accumulation (Barreto & Magalhaes 2014; Magalhaes & Rolim 2014). The results showed that untreated starch wastewater can cause crop yield reduction, while aerobic wastewater can be effectively used for irrigation with no adverse effects on plants. Devereux used centrifugation to recover potato starch from potato processing wastewater. On an industrial scale, 15–28 g/L of soluble starch and about 10 g/L of insoluble starch can be recovered (Devereux et al. 2011). Moreover, starch wastewater is an ideal medium for microbial culture due to its high organic content, sufficient nitrogen, phosphorus and other nutrients, and low toxicity (Bond et al. 2002; Brar et al. 2005; Brar & Verma 2006; Yezza et al. 2006; Xu et al. 2014). Loss used corn starch wastewater as a carbon source, nitrogen source and wetting agent for oyster mushroom culture (Loss et al. 2009). Chang compared the culture medium made from sweet potato starch wastewater and the commercial medium in the culture of microbial insecticide Bacillus thuringiensis (Chang & Zhou 2008). The results showed that the number of live bacteria and spores of B. thuringiensis cultured in the medium made of starch wastewater was 72 and 107% higher than that in the commercial medium. The content of endotoxin produced by the bacteria and the efficacy of insecticide were obviously better than that in the commercial medium.

Due to the particularity of the production stage of starch processing wastewater, the wastewater not only contains more organic matter, but also contains a lot of sediment and potato skin. It not only contains more organic matter but also a lot of sediment and potato skin. Therefore, pretreatment should be carried out before the reuse of starch wastewater to make its water quality meet the requirements of the water quality for returning to the field, and the subsequent process reduces the land required for the absorption of processing wastewater, disinfection and sterilization, and reduces the odor, including removal of mud and sand, protein extraction, pH regulation, anaerobic fermentation, etc. (Yin et al. 1999; Cassini et al. 2010; Burghoff 2012; Yang & Shiyuan 2015). The pretreatment of potato starch wastewater returning to the field is different from a biochemical treatment, the purpose of which is to remove bulk matter or debris. The objective of the pretreatment of starch-processing wastewater is to use it safely or reduce the adverse effects after returning to the field, and the key point is to control nutrient balance. In this study, different flocculants were used to pretreat wastewater, and the influence of different flocculants and conditions on wastewater quality was investigated.

Flocculation and sedimentation methods refer to the use of flocculants, netting, adsorption and other physicochemical synergy, flocculable suspended matter, colloids and other substances in the wastewater coalescence into flocs. Through the solid–liquid separation equipment, the wastewater contains flocculable pollutants with the filter residue separation and removal. It is a low-cost water treatment technology; the flocculation treatment effect depends on the performance of the selected flocculants and different compound flocculants. The effectiveness of the flocculation treatment depends on the performance of the selected flocculant and the type of different compound flocculants. The flocculants currently used are mainly natural polymer flocculants (starch, derivatives of cellulose, etc.), synthetic organic polymer flocculants (polyacrylamide (PAM), etc.), as well as inorganic flocculants and compound flocculants (polymeric ferric sulfate, polymeric aluminum chloride (PAC)) (Deng et al. 2003; Guo et al. 2013; Wang et al. 2015; Joshi et al. 2017; Du et al. 2019a, 2019b).

As a cationic inorganic polymer flocculant, PAC has the advantages of fast settling speed, small dosage, good effect and wide application range. Compared with organic polymer flocculants, PAC has been successfully applied in the treatment of water supply, industrial wastewater and urban sewage (Sun et al. 2021; Rizvi et al. 2022). PAM is a kind of synthetic organic polymer flocculant. If organic flocculant acrylamide is used instead of an inorganic flocculant, the water purification capacity can be increased by more than 20%, even if the sedimentation tank is not modified. In sewage treatment, the use of PAM can increase the utilization rate of the water reuse cycle, and can also be used as sludge dewatering. It is used as an important formulation agent in industrial water treatment (Ma et al. 2018). PAM is considered a low toxic substance and is generally considered safe for use in a variety of applications. According to the U.S. Environmental Protection Agency (EPA), PAM is not considered a hazardous substance and does not pose a significant risk to human health or the environment. The residual acrylamide monomer in PAM industrial products has been clearly defined; the cation requirement is less than 0.1%, the anion in the drinking water industry is less than 0.02%, and the industrial water treatment is less than 0.05%. In addition to synthetic organic polymer flocculants such as PAM, natural organic polymer flocculants also have important applications in wastewater flocculation treatment, such as chitosan oligosaccharide (COS), chitosan (CS) and carboxymethyl chitosan (CMC) (Ma et al. 2018; De-Paz-Arroyo et al. 2023).

Barta selected three inorganic composite flocculants FeSO₄, FeCl₃ and ZnCl₂ for the flocculation treatment of potato starch-processing wastewater. The results showed that the dosing method, dosing ratio and pH of the three flocculants affected the treatment effect, and that protein recovery from potato starch wastewater could reach 30.7, 86.4 and 25.8%, respectively (Brta et al. 2010). Chunjie Du prepared magnetic seeds by sulfide-roasting method and used the magnetic flocculation process to remove ultrafine HAP particles from wastewater, and the turbidity, total phosphorus (TP), and organic matter content in wastewater were significantly reduced by the synergistic effect of PFS (30 mg/L) and magnetic seeds (7.5 g/L) (Du et al. 2019b). Magnetite and its modified products are widely used as aqueous adsorbents. However, in large-scale applications, additional costs are required to build the magnetic field, which will also affect the microbial activity if the subsequent treatment is by biological methods. Junyuan Guo prepared a biological flocculant and used it for potato starch wastewater treatment; the chemical oxygen demand (COD) removal rate was 52.4% and the turbidity removal rate was 81.7% at 30 mg/L and pH 7.5 (Guo et al. 2013).

Joshi isolated and screened Klebsiella pneumoniae NJ7 strains to produce biological flocculants and used them in the flocculation of starch-processing wastewater. The results show that the flocculant can reduce COD by 41% and turbidity by 90% (Joshi et al. 2017). From the current research direction, the use of a cheap medium to isolate and screen flocculating microorganisms is the research trend of this method. Zhisheng Liu modified the chitosan membrane with GTA as HTCC, which successfully improved the electropositivity and water solubility of chitosan and increased the flocculation effect (Zhisheng et al. 2023). However, because chitosan can also be used in high-precision equipment production, the price is expensive. A single application of organic polymer flocculants in wastewater pretreatment will make the treatment cost too high, so it is necessary to explore a new method of mixed use of multiple flocculants. It can not only reduce the cost, but also improve the water treatment capacity.

Magnetic flocculation is a new wastewater treatment technology. Compared with the traditional flocculation process, it has the advantages of large capacity, high efficiency, simple and compact equipment and small footprint. As a flocculated nucleus, magnetic powder forms a tight and solid floc under the action of magnetic force, which is not easy to break up, and has a good effect of removing turbidity, which can reduce the load of subsequent treatment and the project cost. Du et al. (2019b) and Li et al. (2014) prepared magnetic seeds by the vulcanization roasting method, and used the magnetic flocculation process to remove ultrafine HAP particles in wastewater. Under the synergistic action of PFS (30 mg/L) and magnetic seeds (7.5 g/L), the contents of turbidity, TP and organic matter in wastewater were significantly reduced.

At present, the first step of reusing potato starch wastewater is to flocculate it, but there are many kinds of flocculation methods, and each treatment method has its own advantages and disadvantages. Reasonable selection of wastewater pretreatment methods can not only greatly reduce the content of impurities in water, but also simplify the subsequent treatment process, and reduce production costs and carbon emissions. Therefore, this paper studies the traditional flocculation treatment and biological flocculation treatment, which are the most widely used in industry at present, and determines the most suitable flocculant and its working environment for sweet potato starch wastewater. The research results can provide a theoretical basis for industrial production.

The sweet potato starch wastewater in this experiment comes from the direct wastewater of a sweet potato starch production enterprise, and its influent characteristics are shown in Table 1. The pH of wastewater is acidic.

PAC solution, PAM solution, COS solution and CMC solution are configured as aqueous solutions with mass fractions of 1, 0.1, 0.6 and 0.6%, respectively. CS solution: Prepare 1% dilute hydrochloric acid, weigh 0.6 g chitosan, dissolve in 100 mL 1% dilute hydrochloric acid, stir until completely dissolved, and after 12 h swelling, obtain a solution with a mass fraction of 0.6%.

Magnetic powder selection: Fe₃O₄ powder, using 200–300 mesh screen for screening, and a particle size of 30–40 μm.

Take six beakers and put 500 mL of raw water into them, with 7% NaOH, and adjust the pH to 6, 7, 8, 9, 10, 11, add 1% PAC 3 mL, 0.1% PAM 0.5 mL and magnetic powder 30 mg, with a stirrer (85–2, Guansen Biotechnology, China) at 100 rpm. After 1 min, the beaker is placed in the magnet for 30 min, and due to the role of the magnetic field, flocs settle quickly. The supernatant was taken to determine total nitrogen (TN), TP and suspended solids (SS) under different pH conditions, and the removal rate was calculated to determine the better pH range for flocculation. The effect of pH on the magnetic flocculation of PAC and PAM was analyzed.

Take seven beakers and put them into 250 mL raw water sample, adjust the pH value to 5, 6, 7, 8, 9, 10, 11 with 10%HCl and 7% NaOH, add 0.6% COS, 2 mL CS and CMC, stir with agitators at 100 rpm for 1 min, and then stir at 20 rpm for 3 min. After standing for 30 min, SS, COD, TN and TP were measured with the supernatant, the removal rates were calculated, and the influence of pH on the flocculation effect of COS, CS and CMC was analyzed.

Take six beakers and put 500 mL of raw water into them, adjust the pH value to 10 with 7% NaOH, add 1% PAC for 3 mL, 0.1% PAM for 0.5 mL, respectively, add magnetic powder 10 mg, 20 mg, 30 mg, 40 mg, 50 mg, 60 mg, stirring with a stirrer at 100 rpm for 1 min, then place the beaker on the magnet for 30 min. The supernatant was taken to determine SS, TP and TN under different pH conditions, and the removal rate was calculated to determine the better dosage of magnetic powder.

Take six beakers and put 500 mL of raw water into the water sample, adjust the pH value to 10, add 1%PAC, 1 mL, 2 mL, 3 mL, 4 mL, 5 mL, 6 mL, respectively, to the wastewater, and add 30 mg of magnetic powder without PAM. After stirring at 100 rpm with an agitator for 1 min, the beaker was placed on the magnet for 30 min. SS, TN and TP of the wastewater were measured with the supernatant, and the removal rate was calculated. The flocculation effect of flocculant PAC and magnetic powder compound on the wastewater was investigated, and the influence of PAC dosage on the magnetic flocculation effect was analyzed.

Take five beakers and put 500 mL of raw water into the water sample, adjust the pH value to 10 and the dosage of PAC to 3 mL, add 0.1%PAM, 1, 2, 3, 4 and 5 mL into the wastewater, add 30 mg of magnetic powder, other conditions are the same (2.4.2), stir and settle. SS, TN and TP of wastewater were measured with supernatant, the removal rate was calculated, and the flocculation effect of flocculant PAC, coagulant aid PAM and magnetic powder was investigated. The influence of PAM dosage on the magnetic flocculation effect was analyzed.

Take six beakers and put 500 mL of raw water into them, adjust the pH value to 10, add 0.6%COS or 0.6%CMC 1, 2, 3, 4, 5 and 6 mL into the wastewater, stir with a mixer at 100 rpm for 1 min, and then stir at 20 rpm for 3 min. After standing for 30 min, SS, COD, TN and TP were measured with supernatant, the removal rates were calculated, and the influence of COS and CMC dosage on the flocculation effect was analyzed.

Take eight beakers and put 500 mL of raw water into the water sample, adjust the pH value to 9, add 0.6% CS 1, 2, 3, 4, 5, 6, 7 and 8 mL into the wastewater, stir with a mixer at 100 rpm for 1 min, and then stir at 20 rpm for 3 min. After standing for 30 min, SS, COD, TN and TP were measured with supernatant, the removal rate was calculated, and the influence of CS dosage on the flocculation effect was analyzed.

The dosage of PAC and PAM was 3 and 2 mL, respectively, the dosage of magnetic powder was 30 mg, the pH was adjusted to 10, and the dosage sequence of PAC, PAM and magnetic powder was changed. Other conditions were the same as above. After stirring and static settling, the superserum was taken to measure TN, TP and SS in different dosage sequences; calculate the removal rate and analyze the influence of dosage sequence on the flocculation effect.

Take six beakers and put 500 mL raw water sample into them, adjust the pH value to 10, add 3 mL PAC and 2 mL PAM, respectively, and add 30 mg magnetic powder. When stirring, add magnetic powder, PAC and PAM successively, and the rotational speeds were set to 50, 100, 150, 200, 250 and 300 rpm, respectively. After settling for 30 min, supernatants were taken to determine TN, TP and SS under the conditions of different stirring rates. After 30 min, the supernatant was taken to determine TN, TP and SS under different stirring rate conditions, and the removal rate was calculated to determine the better stirring rate.

Take six beakers and put 500 mL raw water sample into them, adjust the pH value to 10, add 3 mL PAC and 2 mL PAM, respectively, and 30 mg magnetic powder. When stirring, add magnetic powder, PAC and PAM successively, and stir and settle at a stirring rate of 150 rpm to conduct the experiment. The super serum is taken every 10 min. The sedimentation time was 10 ∼ 60 min, TN, TP and SS were measured, the removal rate was calculated, and the influence of different sedimentation times on the magnetic flocculation effect was analyzed.

Take six beakers and put 500 mL raw water sample into them, adjust the pH value of COS and CMC to 10, the dosage is 3 mL, the PH value of CS to 9, and the dosage is 2 mL to conduct the experiment. After stirring at 100 rpm for 1 min with a mixer, stir at 20 rpm for 3 min, and take the supernatant every 10 min. The sedimentation time was 10–60 min, TN, TP and SS were measured, the removal rates were calculated, and the influence of different sedimentation times on the bioflocculation effect was analyzed.

TN was determined by an alkaline potassium persulfate digestion spectrophotometer (UV-VIS spectrophotometer, T2600, Thermo, USA), TP was determined by an ammonium molybdate spectrophotometer (UV-VIS spectrophotometer, T2600, Thermo, USA), and SS was determined by the gravimetric method (Turbidimeter, 2100Q, USA Hash, USA). COD was determined by an online COD meter (UVM-4020, Shimadzu, Japan) using the ultraviolet absorption method.

where A effluent is the value corresponding to SS (mg/L), TP (mg/L), and TN (mg/L) after the flocculation of effluent, and A influent is the value corresponding to SS (mg/L), TP (mg/L), and TN (mg/L) before the flocculation of influent.

Under acidic environmental conditions, PAC exists in the form of hexa-coordinated hydrated aluminum ions Al₃ + (OH)6 with poor flocculation ability. While under neutral and alkaline environmental conditions, aluminum salts can undergo rapid hydrolysis to generate multinuclear polyhydroxy complexes, which can cause phosphate and colloidal particles in the wastewater to coalesce into huge and compact flocs, and gradually, settle down through adsorption bridging and tape netting of long-chain macromolecules (Chen et al. 2016; Benchamas et al. 2021). Therefore, SS and TP are greatly reduced. An acidic environment can promote the hydrolysis of PAM whereas alkaline environment hydrolysis is much slower. Under acidic conditions, nucleophilic addition occurs between water and protonated acylaminocarbonyl, after which ammonia (NH₃) is eliminated, and the acrylamide structural unit is hydrolyzed to an acrylic structural unit, so the flocculation ability becomes worse (Zheng et al. 2011). Therefore, when conventional flocculants are used, the pH should be 10.

According to the polymer bridging theory proposed by Ruehrwein and Ward, when a flocculant is added to wastewater, the flocculant polymer collides with the pollutant particles in the wastewater. Under the action of adsorptive electric neutralization, the functional groups on the flocculant molecules that are opposite to the charge of the pollutant particles attract each other. The rest of the molecular chains reach into the colloidal solution and have sweeping net trapping and adsorption bridging, forming floc and gradually settling down under the interaction of the three mechanisms (Zhang et al. 2020b). However, when the dosage of PAC was greater than 4 mL, the removal rate showed a decreasing trend. When the dosage of the flocculant was excessive, the colloidal particles were surrounded by a large number of polymers, resulting in insufficient surface activity of the particles during the adsorption effect, or the surface of the colloidal particles was saturated with flocculant molecules and there was no more adsorption space. In this case, the bridging ability of the polymer flocculant PAC was weakened or even disappeared. Furthermore, due to the steric hindrance effect, particles in wastewater tend to disperse, thus reducing the removal efficiency of SS, TP and TN (Sun et al. 2021). Therefore, the optimal dosage of PAC should be controlled at about 3 mL.

As can be seen from Figure 2(c), the removal rates of SS, TP and TN have reached 80.1, 84.31 and 36%, respectively, when the PAM dosage is 2 mL. This shows that a small amount of PAM can play the role of strengthening bridging flocculation and network flocculation through its longer chain-like molecules. It makes the flocs tightly bound and the particle settling speed is accelerated, which greatly improves the removal rate of TN.

From the above experimental results, it can be seen that better dosages of PAC, PAM and magnetic powder are 3 mL, 2 mL and 30 mg, respectively. The sequence of PAC, PAM and magnetic powder is changed to ①, ②, and ③, respectively. The experimental results of the influence of the sequence on the magnetic flocculation effect are shown in Table 2. From the table, it can be seen that the sequential addition of magnetic powder, PAC and PAM during stirring has better flocculation effect than the other two injection methods.

Magnetic powder (Fe₃O₄), as a kind of suspended particulate matter, can be uniformly dispersed in water when added during agitation, which increases the suspended particles in wastewater and also the probability of suspended particles colliding. It can enhance the ability to adsorb and roll up suspended matter and colloidal particles in water (Lipus et al. 2001; Han et al. 2020). In addition, under the action of the magnetic field, after adding PAC and PAM, the floc can be closely combined around the magnetic powder particles, shortening the precipitation time. The settlement is more complete than the other two dosing methods in the same time, and the flocculation effect is better.

Therefore, when traditional flocculants treat wastewater, the dosage of magnetic powder is 30 mg, PAC slurry is 4 mL, and that of PAM is 2 mL. In addition, choose to add magnetic powder first, then add PAC, and finally, add PAM.

From Figure 3(b), it can be seen that the sedimentation time has a great influence on the removal rates of SS, TN and TP in wastewater. This is because the process from the full hydrolysis of magnetic powder and flocculant, to the full contact with colloidal particles to form a large number of magnetic floc, and then to the full precipitation of floc need to take a certain amount of time. So, with the prolongation of flocculation and precipitation time, the pollutants' removal rate steadily increased, and the effect of removal reached stability, basically after flocculation and precipitation for 40 min. After 40 min of flocculation and sedimentation, the removal effect was basically stabilized, and when the flocculation and sedimentation time reached 1 hour, the SS, TP and TN were reduced by 95.32, 93.81 and 38.13%, respectively, so the flocculation time was selected as 60 min.

According to Figure 4(b), due to the cationic polyelectrolyte properties of CS, under acidic conditions, COD, SS, TN and TP increased instead of decreasing, especially COD. At this time, CS was dispersed and dissolved in wastewater, and due to the same charge, it was unable to produce adsorption with the increase in the pH value. The flocculation of the bilayer compression, adsorption of the electric neutrality, and the role of adsorption and bridging can be given full play to, when the pH value is in the range of 7–9 and the flocculation efficiency is the most obvious. When a variety of flocculation effects work together, the suspended particles and the colloidal particles begin to destabilize. From the experimental phenomenon, the floc volume is large and dense, and the effect of decolorization and turbidity removal is obvious (Zeng et al. 2008; Ma et al. 2018). When pH > 9, the SS removal rate continues to rise and COD and TN tend to stabilize; therefore, the better CS pH is also 9. The COD removal rate is 17%, the SS removal rate is 74%, the TN removal rate is 25%, and the TP removal rate is 43%.

CMC is prepared by carboxylation of CS. The introduction of the carboxyl group destroys the secondary structure of the CS molecule, reduces the crystallinity and forms an amorphous structure, which has better water solubility and a wider pH range compared with CS (Kurniasih et al. 2018). From Figure 4(c), it can be seen that pH has a greater influence on the treatment of sweet potato starch-processing wastewater by CMC. Although the introduction of positively charged carboxyl groups reduces its adsorption effect, it is still a cationic flocculant and contains amine groups in its molecular chain. Its large molecular structure of long straight chains makes it easy for flocculation to occur, such as adhesion and bridging with colloidal particles. As the pH value increases, the electro-neutralization effect becomes stronger. When the pH value is 10, the electro-neutralization and adsorption effects tend to saturate, which is the better pH of CMC. At this time, the COD removal rate is 25%, the SS removal rate is 42%, the TN removal rate is 27% and the TP removal rate is 40%.

As can be seen from Figure 4(d), the better pH values of COS, CS and CMC are 10, 9 and 10, respectively, with similar pH ranges but with differences in their flocculation ability. Under the suitable condition of pH 9, CS was effective in removing SS up to 74%, and its effect on COD, TN and TP removal was between COS and CMC. The flocculation effect of COS is poorer, probably because the molecular weight is smaller than the other two. The molecular chain is shorter, its ability to capture the turbidity is poor, while COS and CMC molecular weight and the molecular chain is longer. Under the same pH conditions, the colloidal particles and the suspended materials contact more, showing a strong flocculation activity. The better pH of CS was smaller than that of CMC, and the removal effect of SS was significantly better than that of CMC. Probably because CMC introduced carboxyl groups with a negative charge, which had the same charge as that of representative values, amino acids, etc. in starch wastewater, the adsorption of the electric neutralization effect was weakened. The removal of COD and TN by all three flocculants was not significant, probably because the dosage and settling time affected the destabilization of colloidal particles.

According to Figure 5(b), under the same pH condition, increasing the amount of CS can quickly and fully realize the flocculation and destabilization of colloidal particles, and increase the contact area with the colloidal particles. Inter-particle adsorption is made to exceed the force of its dispersion under the action of van der Waals force and hydrogen bonding to realize adsorption, bridging, flocculation and sedimentation. When the dosage is >2 mL, the phenomenon of ‘re-stabilization’ occurs and the colloidal particles are wrapped around by cationic flocculant molecules, resulting in the dispersion of the flocculant molecules and the colloidal particles as well as the increase of SS and TN concentrations. Therefore, the better dosage of CS should be controlled at 2 mL, when the removal rate of COD is 32%, SS is 55%, TN is 31% and TP is 75%. It can be seen that with the increase in the dosage, the SS, TP, TN and COD of the three biological flocculants showed different degrees of increase and then decrease, for COS and CMC. When the dosage is <3 mL, the removal rate of COD, SS, TP and TN basically showed an increasing trend, indicating that with the increase of the input cationic flocculant, the colloidal particles in the wastewater and its adsorption sites on the long chains of molecules are cross-linked with the colloidal particles, and the flocs are generated and settled under the action of hydrogen bonds and van der Waals forces. However, the removal rate showed an obvious decreasing trend when the dosage was greater than 3 mL. Therefore, the dosage of COS and CMC should be controlled at 3 mL. For CS, its better dosage should be controlled at 2 mL. When the dosage is greater than 2 mL, there is no obvious change in the SS removal rate, and the removal rates of COD, TN and TP start to decrease, which indicates that the CS dosage tends to saturate.

According to Figure 5(d), there was a significant difference in the efficacy of the three biological flocculants in wastewater treatment under better dosage conditions. The better dosage of COS and CMC was 3 mL, and the better dosage of CS was 2 mL. The SS removal rate was 55%, the TP removal rate was 75%, the COD removal rate was 32% and the TN removal rate was 31%, which were higher than the other two, and could achieve better removal effect with lower cost. On the one hand, the molecular weight of COS < CS < CMC, the molecular weight of CS is larger than that of COS, so there are more free amine groups on the molecular chain of CS, and more adsorption vacancies available. The molecular weight of CMC is much larger than that of CS, but the excessively long molecular chains reaching into the solution encountering the higher concentration of suspended substances easily lead to the overlapping of molecular chains and affect the adsorption effect (Hassimi et al. 2020). On the other hand, the amine group is the fundamental reason for the flocculation performance of biological flocculants, and the amount of amine group directly determines the flocculation potential of flocculants. COS is the product of CS degradation, and the free amine groups on the molecular chain are less than CS, while CMC is prepared by introducing carboxyl groups into CS. −COOH has a better adsorption effect on metal cations and a poor flocculation effect on negatively charged particles in wastewater; so the flocculation effect of CS is better than COS and CMC under the same settling time condition.

As can be seen from Figure 6(b), under the conditions of better pH and dosage of CS, in general, with the increase of settling time, CS was faster and more effective in removing SS and TP from sweet potato starch-processing wastewater. The flocculant molecules preferentially came into contact with the suspended particles of the large particles, and furthermore, they then reached to the inside of the colloid to realize the destabilized flocculation of the colloid. CS is a long-chain macromolecule with a complex structure, and prolonging the coagulation and sedimentation time is conducive to the full stretching of the long-chain macromolecule and its adsorption with negatively charged particles. After 40 min of sedimentation, the removal rate of SS and TP still had a certain upward trend, indicating that the suspended particles were still leaning toward the flocculant molecules to varying degrees or naturally settling. The removal rate of COD showed a decreasing trend, probably because the formation of flocs was looser and the sedimentation time was too long, and the flocculant molecules and the adhesion between the colloidal particles decreased (Guo et al. 2019; Li et al. 2020). In summary, the better settling time of CS should be controlled at about 40 min, with a COD removal rate of 43%, SS removal rate of 83%, TN removal rate of 40%, TP removal rate of 80%, and when the flocculation effect is better.

As can be seen from Figure 6(c), under the condition of better pH and dosage of CMC, its suitable settling time should be controlled at about 40 min. The settling efficiency shows the trend of being first small and then large, and finally, tends to be stable; the removal rates of COD, TN, SS and TP are similar in the first 30 min of the settling. This indicates that the CMC molecules can contact and react with the turbid body very quickly, and achieve the simultaneous flocculation of the macromolecule suspended particles and the colloidal particles. However, the molecular weight of CMC itself is too large and the molecular chain is long; the overlap between the chain segments will occur during the bridging process, which is not conducive to the full stretching of the long-chain molecular structure. It will produce a repulsive force between the flocculant molecules and the colloidal particles, and flocculation will be weakened, so the flocculation effect of SS, TP, COD, and TN is obviously not good. After a settling time of 30min, there is a gap between the removal rates of COD, TN, SS and TP, TN and COD tend to be stable, while SS and TP will further increase. This is because in addition to the adsorption of bio-flocculant bridging, physical adsorption of molecular chains and adsorption of electric neutralization, and suspended particles of wastewater with the increase in the settling time will appear as natural settlement. The presence of COD and other organic matter within the colloid suspended in the wastewater is difficult to settle naturally, which resulted in the gap in the removal efficiency. After a settling time of 40 min, the flocculation effect tends to stabilize and the pollutant removal rate increase tends to stabilize; therefore, at the better settling time of 40 min, the COD removal rate is 31%, SS removal rate is 55%, the TN removal rate is 32%, and the TP removal rate is 65%.

In summary, the better settling time of the three biological flocculants was about 40 min. By comparing the flocculation effects of COS, CS and CMC under better settling conditions, it was found that the three biological flocculants showed distinctly different flocculation effects under a better pH, dosage and settling time. The removal effects of CS on SS, TP, COD and TN were significantly higher than those of COS and CMC, especially the removal rates of SS and TP. The comparison of the flocculation effect of the three is in the order of CS > CMC > COS.

The three factors that have a greater influence on the flocculation effect of CS were selected, i.e., pH, dosage and settling time. According to the conclusions of the one-factor experiment on CS, the three levels of pH were determined to be 9, 10 and 11; the three levels of dosage were determined to be 1, 2 and 3 mL; and the three levels of settling time were determined to be 15, 30 and 45 min. The CS solution with a mass fraction of 0.6% was prepared in order to prepare the CS solution; 250 mL of sweet potato starch-processing wastewater was injected into nine 500 mL beakers, which were divided into three groups of three each, totaling nine groups. The stirring conditions were based on fast stirring for one minute and then slow stirring for 3 min to examine the removal rate of SS, TP, COD, and TN by CS, to calculate the values of K and k to determine the superior levels and combinations. The value of R was calculated to determine the primary and secondary order of the factors.

The experiment was carried out in accordance with the orthogonal table without considering the interaction between factors. The removal rates of COD, SS, TP and TN were taken as reference indicators. The measured data are shown in Table 3. The measured data are analyzed using the range analysis method, and the values of K, k and R are calculated (as shown in Tables 4 and 5).

After the measured data, the experimental results were analyzed using the comprehensive trade-off method. The experimental results of each index were compared and analyzed for the nature of multiple indexes. The primary and secondary orders of the influencing factors were determined first. The levels of the influencing factors were also determined. The optimal levels and combinations of CS bioflocculation of high-concentration starch wastewater were finally derived.

According to Tables 4 and 5, the optimal level and optimal combination are first determined by the value of ki. SS, TP, COD and TP removal rates are all the larger the better. Through the calculation of SS, TP, COD, TP, each removal rate of k value and the extreme deviation of R, it can be seen that the optimization conditions of each of the above indexes analyzed individually are inconsistent. It should be comprehensively weighed and considered in a comprehensive manner to determine the optimal process conditions.

For the A factor, the size of its effect on the removal rates of TP and TN is ranked first. At this time, TP takes A₂, TN takes A₃, and its effect on COD and SS ranked second, which is a secondary factor, so A takes A₂ or A₃. Take A₂, the TP removal rate A₂ than A₃ increased by 2.51%, the TN removal rate A₂ than A₃ decreased by 7.055%, the COD removal rate A₂ than A₃ increased by 0.94% and the SS removal rate A₂ increased by 1.776% compared to A₃. Therefore, the A factor is taken as A₂ i.e. the pH value is taken as 10.

For factor B, its effect on the COD removal rate is ranked first; therefore, factor B is taken as B₂, i.e., the dosage of CS is taken as 2 mL.

For the C factor, its effect on the SS removal rate ranked first, so the C factor was taken as C₃, i.e., CS flocculation and sedimentation time was 45 min.

In summary, the superior level and superior combination of the three-factor, three-level multi-indicator orthogonal test for the CS treatment of high-concentration sweet potato starch-processing wastewater were A₂B₂C₃.

The better pH of COS flocculant for treating sweet potato starch-processing wastewater was 10, the dosage was 3 mL, the settling time was 40 min, and its effluent COD removal rate was 21%, the TP removal rate was 50%, the TN removal rate was 25% and the SS removal rate was 37%. The better pH of the CS flocculant for treating sweet potato starch-processing wastewater was 9, the dosage was 2 mL and the settling time was 40 min. The effluent COD removal rate was 43%, the TP removal rate was 80%, the TN removal rate was 40%, and the SS removal rate was 83%. The better pH for the treatment of sweet potato starch-processing wastewater with CMC flocculant was 10, the dosage was 3 mL and the settling time was 40 min. The effluent COD removal rate was 31%, the TP removal rate was 65%, the TN removal rate was 32% and the SS removal rate was 55%.

Through comparison, it was found that compared with the other two biological flocculants, CS has a moderate molecular weight and the number of amine groups on the molecular weight is higher. It has a strong adsorption capacity for negatively charged colloids and suspended substances. At the same time, the molecular structure of the cationic type with a long straight chain can give full play to the role of adsorption and bridging in turbid liquids. From the experimental phenomenon, the floc formed after the CS flocculation treatment was larger and denser than COS. The turbidity of wastewater was clearer than that of CMC. In addition, the CS treatment of sweet potato starch-processing wastewater makes the better pH and better dosage smaller because the sweet potato starch-processing wastewater is acidic. So, the use of CS bio-flocculant can greatly reduce the cost of engineering applications. It can be seen that the CS flocculant treatment of sweet potato starch-processing wastewater has obvious advantages over the other two.

CS was identified as a better flocculant for treating sweet potato starch-processing wastewater through the comparison of single-factor experiments. Due to the examination of more flocculation factors and the complexity of the pollutants, the optimal combination of better levels could not be determined by single-factor experiments alone. Therefore, the optimization method needs to be further explored through orthogonal experiments to determine the optimal levels and combinations of CS for the treatment of starch wastewater.

In this study, the removal rates of TN, TP, SS, and COD in potato starch-processing wastewater were studied under different pH, dosage, stirring rates, and settling times of traditional flocculants and biological flocculants. Compared with traditional flocculants and other biological flocculants, chitosan has a faster flocculation rate and use cost. Specific conclusions are as follows:

• The use of traditional flocculants can remove SS and phosphorus components in sweet potato starch-processing wastewater, but the removal effect of nitrogen-containing components is obviously lower. The colloidal solution is still not completely destabilized, and nutrients such as proteins and polysaccharides are not completely adsorbed by the flocculants, and are not captured by the nets.

• The CS treatment of sweet potato starch-processing wastewater pH and dosage are smaller because the sweet potato starch-processing wastewater is acidic. The use of CS bio-flocculant can greatly reduce the cost of engineering applications; compared with the other two, there are obvious advantages.

• The removal of SS and TP from sweet potato starch-processing wastewater by traditional chemical flocculants was better than that of CS, but its flocculation time and settlement time were longer than that of CS. CS bio-flocculant has some advantages in TN and COD removal, which shows that CS is better for destabilizing the colloidal solution of sweet potato starch-processing wastewater.

• Experiments were carried out on high-concentration starch wastewater according to the superior level and superior combination conditions of the orthogonal test. It was found that under the better combination and conditions determined by the orthogonal test, the effluent of wastewater had a COD removal rate of 51%, a TP removal rate of 91%, a TN removal rate of 47%, and a SS removal rate of 95%, which was better than flocculation under better conditions of the one-factor experiment.

Thanks to Mr Wang for the technical support of this study.

N.S. contributed to methodology, data curation, investigation, formal analysis, writing – original draft, validation. X.L. did investigation, collected resources, contributed to writing – review and editing. J.W. contributed to writing – review and editing, and collected resources.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.