Efficiencies and mechanism of enhanced coagulation pre-treatment on domestic sewage with PAC-HCA compound

The rapidly rising generation of wastewater is an inevitable consequence of human activities due to urbanization, industrialization, and population growth (Hanjra et al. 2012). In this context, the number and scale of urban domestic sewage treatment plants are expanding, and the treatment process is becoming more mature (Fan et al. 2020). Pretreatment is a step set up according to the water quality requirements of the subsequent treatment process before sewage enters the traditional biochemical treatment, and it plays an important role in the sewage treatment process. Nowadays, many scholars have studied the pretreatment methods of domestic sewage. Ma et al. found that reverse osmosis pretreatment could effectively enrich organic and inorganic substances in water samples to enable a more effective and sensitive toxicity evaluation (Ma et al. 2013). Ezra et al. investigated the effect of membrane filtration pretreatment of domestic sewage by chemical enhanced primary treatment and found that the effluent was lower in quality with regard to total organic content (TOC) (45 vs. 16 mg/L) and suspended solids (SS) (21 vs. 8 mg/L), but had a much lower phosphate concentration (0.1 vs. 4.2 mg/L) in comparison to biologically treated effluent from a sequential batch reactor (SBR). (Even-Ezra et al. 2011). Sawai et al. estimated that the capital cost and treatment fees of applicability of pretreatment using subcritical water liquefaction were cost-competitive compared to conventional treatments (Sawai et al. 2013).

Coagulation is an effective and low-cost pre-treatment technology to improve the removal rate of SS and organic matter in the first-grade treatment unit of domestic sewage (Bu et al. 2019; Pramanik et al. 2019). Coagulation occurs in an initially dispersed mixed colloidal system, where differences in the rates of coagulation of various species are of sufficient magnitude that one species may separate, leaving the other in suspension after a certain period (Zhao et al. 2021). EC technology is to strengthen the treatment by increasing the dosage of coagulant and controlling the pH value (Lin et al. 2020; Priyatharishini & Mokhtar 2021), hydraulic condition (Rong et al. 2013; Lin et al. 2019), temperature (Nadella et al. 2020), and other coagulation conditions applicable to coagulant. In recent years, the application of EC has been gradually transferred from the initial water supply to the treatment of sewage and has generally achieved a good effect. Smotraiev et al. compared the treatment effect of pre-polymerized zirconium and traditional aluminium prosthetic on domestic water and found that the maximum rates of total suspended solids (TSS), TU, and OP removal by the zirconium and aluminium coagulants were 92.5% and 91.0%, 97.1% and 96.4%, and 97.6 and 93%, respectively (Smotraiev et al. 2022). Moringa oleifera at the optimal dosage of 600 mg/L showed results statistically equal to the use of alum (200 mg/L), achieving bacterial load, turbidity, and apparent color removal higher than 99, 92, and 66%, respectively (Vega Andrade et al. 2021). As one of the most commonly used coagulants, polyaluminum chloride (PAC) has been widely used in coagulation because of its low price and strong adsorption electricity neutralization ability (Shen et al. 2017). However, PAC has the disadvantages of lower molecular weight and poor ability to realize bonding and bridging (Sudoh et al. 2015; Zhang et al. 2018). HCA is a kind of polymer organic coagulant and widely used in the field of drinking water purification with the characteristics of good water solubility, high positive charge density, easy to control molecular weight, high efficiency, and non-toxic (Bolto & Gregory 2007; Wang et al. 2008; Sun et al. 2011; Xu & Gao 2012). Based on the characteristics of Poly dimethyl diallyl propyl ammonium chloride (HCA) and PAC, HCA has been combined with PAC to improve natural organic matter removal and the size and settling ability of flocs due to its positive charge and adsorption and bridging abilities in previous studies. Shen et al. found that coagulation using PAC-HCA with a viscosity range from 0.99 dL/g to 1.86 dL/g can significantly reduce membrane fouling, leading to increasing water fluxes from 0.1170 to 0.4906 in.the ultrafiltration process (Shen et al. 2017). Yang et al. investigated the relationship between removal efficiency, floc operational parameters, and residual Al species when PAC-HCA was applied in coagulation of a reservoir water (Yang et al. 2013). These studies all showed that PAC-HCA had a good performance in coagulation treatment.

However, there are few reports on the application of PAC-HCA in the pretreatment of domestic sewage. To improve the pretreatment effect of domestic sewage, HCA and PAC are combined to achieve the purpose of efficient and economic treatment in this study. Furthermore, HCA and PAC coagulants are used to make compound coagulants according to the proportion of their net content, and the treatment efficiencies and mechanism of compound coagulants on domestic sewage are studied. These experiments will provide a reference value for the application of coagulants in the pretreatment of domestic sewage in practical engineering.

The influent of the reactor was domestic sewage from Lanzhou Jiaotong University, which was mixed sewage from flushing, canteen drainage, toilet, and bath. Due to intermittent discharge of sewage, water quality and quantity change greatly. A total of 12 groups of water samples were sampled at different time periods within 3 days, and the specific inlet water quality during the test is shown in Table 1.

Influent and effluent detection indexes include SS, TP, and COD. SS and TP concentrations were determined using normative methods and the COD concentration was determined using DRB200 COD digestion apparatus and HACH DR5000 (Zhang et al. 2021); ZR4-2 portable agitator (Coagulation); Nano-zs90 Zeta potentiometer (Zeta potential); Cfx-1001 digital microscope(Microscopic observation); HACH-2100p (SS); 721- spectrophotometer; PHS-25 acidity meter (pH).

The flocs formed after coagulation were slowly moved to the glass surface dish, and the flocs formed were photographed continuously with a microscope (amplification ratio parameter: 0.25). After that, CFX-1001 was used to analyze the image, and the data were processed according to lnA = DrlnP + lnK by measuring the circumference P and area A of the image based on the fractal theory and practical experience (Kostoglou & Konstandopoulos 2001). The obtained lnA and lnK were plotted as horizontal and vertical values respectively, and the slope of the obtained line was the fractal dimension Dr (K was a constant).

Zeta potential is an important indicator of the stability of a colloid or suspension. After the coagulant is added to the water, many positively charged hydrolyzed polymeric cations of aluminum or iron are formed, which reduce the zeta potential of the colloid or suspended particles, destroy the stability and settle down by neutralizing the adsorption and compressing the double electric layer. The determination of Zeta potential can provide some reference for the analysis of the coagulation mechanism (Henderson et al. 2008). Zeta potential was measured by Nano-ZS90 Zeta potentiometer. A small amount of the supernatant obtained from the coagulation experiment was transferred to the Special U-shaped tube of the Zeta potentiometer to measure the Zeta potential of the water sample. The above steps were repeated 3 times and the average value was taken (López-Maldonado et al. 2014; Meraz et al. 2016).

As can be seen from Figure 4, with the gradual increase of the ratio of input, the removal rate of all indicators shows an overall upward trend. When the dosage ratio was 25:1, the removal rates reached the highest, and the removal rates of COD, TP, and SS reached 52.79%, 82.15%, and 99.37%, respectively. Theoretically, PAC plays role of coagulation, charge neutralization, but HCA with quaternary ammonium groups is responsible for flocculation to form big flocs, which further enhances its performance (Zhang et al. 2020). At a pH close to raw water (7.5–8.5), HCA dissociates the polymer chain with a positive charge, and PAC also generates hydrolytic products with a positive charge, such as a hydroxyl complex, in the hydrolysis process. The combination of the two can better play the role of electrical neutralization, which is conducive to strengthening the removal of nutrients by coagulation (Yang et al. 2011; Shen et al. 2017). It can also be found that when the dosage ratio was greater than 1:1, the removal rates of SS and COD did not change much, but the removal ability of phosphorus was enhanced, which increased from 40.88% at 1:1 to 82.15% at 25:1. Presumably, PAC in the compound coagulant can hydrolyze to produce polynuclear hydroxyl complexes such as Al₂(OH)₂⁴⁺ and Al₁₃(OH)₁₃⁷⁺, which can give full play to the functions of compressing double electric layer, electric neutralization, and so on, and can remove part of insoluble phosphorus by combining with negatively charged suspended particles and colloid (Campinas et al. 2021). Colloids such as alumina hydroxide can also be produced during PAC hydrolysis, which can remove orthophosphates by adsorption coprecipitation (Wu et al. 2011). In addition, HCA has been dissolved in water to form a polymer chain, which is easier to make the unstable colloidal particles adhere to form flocs settlement. The combination of the two coagulants has realized the efficient removal of phosphorus in the water.

As shown in Figure 5, with the gradual increase of the initial pH value, the removal of SS and TP firstly increased and then decreased, and the COD removal rate showed an overall upward trend. If the initial pH value is too low or too high, the enhanced coagulation effect will be affected. When the pH value is 8.0, the treatment effect is better, and the removal rates of COD, TP, and SS are 50.23%, 88.67%, and 98.93% respectively. The reason for this result is that the hydrolytic products of PAC in an acidic environment are mainly low polymers such as Al(OH)²⁺, A1₂(OH)²⁺. With the increase in pH value, hydroxyl complex and Al(OH)₃ colloid become the main hydrolytic products of aluminum salt, and the low polymer form has good flocculation characteristics (Gao & Yue 2005). Moreover, HCA is easier to dissociate in an alkaline environment. The positively charged particles combine with negatively charged suspended solids and colloidal particles to form flocs after dissociation. Therefore, appropriately increasing the initial pH value is conducive to enhancing the removal effect of contaminants by the compound coagulant (Huang et al. 2015). When the initial pH value exceeds 8.0, the increase of OH⁻ concentration in the solution can further promote the dissociation of HCA, but the PAC hydrolysate changes to [Al (OH)₄]⁻ and other forms, which repels each other with SS and colloidal particles in water and is difficult to form flocs settlement, and the removal rate of SS and TP is reduced (Yang et al. 2011; Toor & Kim 2019). COD removal rate did not change significantly.

It can be seen from Figure 6 that the removal rates of COD, TP, and SS all showed a trend of first increasing and then decreasing with the increase of the dosage. When the dosage was 15 mL/L, the removal rates of each index reached the highest, corresponding to 44.63%, 89.85%, and 98.74%, respectively. HCA has a high positive charge density, which is superimposed with PAC surface positive charge to enhance the electric neutralization ability of the compound coagulant. The stronger the coagulation effect is, the higher the removal rate of SS, COD, and TP will be when increasing the dosage of PAC-HCA within a certain range. When the dosage exceeds 15 mL/L, the excess positive charge attaches to the colloidal surface with a negative charge, resulting in charge reversal, which leads to the phenomenon of restabilization of the destabilized colloidal and makes the coagulation effect worse.

Zeta potential represents the potential difference of the diffusion double layer on the surface of colloidal particles and is an important indicator to characterize the stability of the colloidal dispersion system (Pandey et al. 2019). The level of Zeta potential is closely related to the treatment effect of coagulant: when the absolute value is low, the removal effect of nutrients is better, and the closer the absolute value is to 0 mV, the more obvious the coagulation effect is (Gerzhova et al. 2016). To further analyze the action mechanism of the compound coagulant, experiments on different PAC-HCA dosages were carried out. SS removal rate was taken as the evaluation index, and the Zeta potential of the water sample was measured. The results are shown in Table 2.

From Table 2, with the increase of PAC-HCA dosage, the Zeta potential of the water sample after coagulation increased from −18.07 mV to 9.68 mV, while the absolute value of Zeta potential decreased first and then increased. Since both HCA and PAC are coagulants with a positive charge, increasing the dosage of coagulants can effectively reduce the Zeta potential on the colloidal surface within a certain range, and the electrostatic repulsion between colloidal ions decreases, making it easier to destabilize and form alum deposition. If the addition amount is too large, the colloid surface with excessive positive charge reaches a stable state again, which is not conducive to colloid condensation, and the SS removal rate decreases. When the dosage was 15 mL/L, the absolute value of Zeta potential reached the minimum value of 2.29 mv, and the corresponding SS removal rate reached the highest 98.74%. It can be inferred that the mechanism of SS removal by PAC-HCA enhanced coagulation is mainly electrical neutralization.

In summary, the results are summarized as follows:

• (1)

  The effects of the hydraulic condition, PAC:HCA dosage ratio, initial pH value, and dosage on PAC-HCA treatment of domestic sewage were investigated by single factor tests. When the hydraulic condition was G₁ = 200.0–265.0 s⁻¹, T₁ = 1.5 min, G₂ = 40.0 s⁻¹, T₂ = 5 min, the ratio of HCA was 25:1, pH was adjusted to 8.0 and dosage was 15 mL/L, the coagulation treatment effect was better, and the removal rates of SS, COD, and TP were 98.74%, 44.63%, and 89.85%, respectively.

• (2)

  The fractal dimension and Zeta potential analysis showed that the combination of HCA and PAC was beneficial to reduce the absolute charge on the surface of the colloid and forming uniform and dense flocs. The analysis of the coagulation mechanism of PAC-HCA was mainly electrical neutralization.

• (3)

  The dosage range of compound coagulant is wide, and it has a good removal effect on SS, COD, and TP in the range of 10–20 mL/L. The initial pH value has an obvious influence on the treatment effect of PAC-HCA, and the removal effect is better when the initial pH value is 8.0. The compound coagulant still has a good treatment effect when the pH value of raw water is not adjusted. The results have certain reference significance for the application of PAC-HCA coagulant in actual water treatment.

In this experiment, PAC-HCA has a high removal rate of SS and TP in domestic sewage under the optimal conditions, and a removal rate of nearly half of COD. If this method is applied to the pretreatment of biochemical treatment of domestic sewage, it can effectively reduce the load of subsequent biochemical treatment and reduce the cost of building construction. In addition, although the performance of PAC-HCA applying on domestic wastewater has been analyzed in this study, removal efficiency of nitrogen and the composition of the sludge were not systematically investigated. Subsequent studies can quantitatively analyze and investigate them.

The research was supported by the Open Foundation of Key Laboratory of Yellow River Water Environment in Gansu Province (20JR2RA0002), the National Natural Science Foundation of China (51568034), and the National Key Research and Development Program of China (2016YFC040070).

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

The authors declare there is no conflict.