Basic Wastewater Treatment Terminology: What Is a Flocculant?
The theoretical basis of flocculation is the "coalescence" theory: the positively (or negatively) charged groups of a flocculant approach the negatively (or positively) charged particles or colloids in water that are difficult to separate, lowering their zeta potential and destabilizing them. The aggregating property of the flocculant then gathers these particles together so that they can be separated by physical or chemical means.
Flocculants come in many varieties, from low-molecular to high-molecular and from single-component to composite types; the overall trend is toward low cost, practicality, non-toxicity and high efficiency. Inorganic flocculants are cheap but can adversely affect human health and the ecological environment. Organic polymer flocculants require lower dosage, produce less scum, have strong flocculation capability, form flocs that are easy to separate, and perform well in oil and suspended-solids removal; however, their residual monomers are "triple-hazard" substances (teratogenic, carcinogenic, mutagenic), which limits their scope of application. Bioflocculants cause no secondary pollution, are convenient to use, and have attractive application prospects; they may partially or fully replace traditional inorganic polymer and synthetic organic polymer flocculants in the future. Research and application of microbial flocculants are flourishing, and their characteristics and advantages point to broad prospects for the development of water treatment technology.
Simple inorganic polymer flocculants are mainly polymers of aluminum and iron salts, such as polyaluminum chloride (PAC), polyaluminum sulfate (PAS), polyferric chloride (PFC) and polyferric sulfate (PFS). The fundamental reason why inorganic polymer flocculants outperform other inorganic flocculants is that they provide large quantities of complexing ions that strongly adsorb colloidal particles; through adsorption, bridging and crosslinking they cause colloids to coagulate. Physicochemical changes occur at the same time: the charges on the surfaces of colloidal particles and suspended solids are neutralized, the zeta potential is reduced, and interparticle repulsion turns into attraction, destroying colloidal stability so that particles collide and form flocculated coagulation sediment. The resulting precipitate can reach a specific surface area of 200–1000 m²/g and is highly adsorptive.
Polysilicic flocculants (PSAA), which are simple to prepare, use widely available raw materials and cost little, are a new type of inorganic polymer flocculant. They show stronger oil removal capability in the treatment of produced water from heavy-oil oilfields and therefore have great development value and broad application prospects.
In polyferric phosphate chloride (PPFC), high-valence anions have a strong affinity for Fe³⁺ and considerably influence the hydrolysis of Fe³⁺: they participate in the complexation reactions of Fe³⁺ and bridge between iron atoms to form polynuclear complexes. This strengthens charge neutralization, adsorption and bridging toward negatively charged colloids such as diatomite in water; the participation of phosphate also increases the volume and density of the flocs, improving the flocculation effect.
Polymeric ferric-aluminum sulfate chloride (PAFCS) performs better than alum in drinking water and sewage treatment, and outperforms PAC in both oily wastewater and printing-and-dyeing wastewater, with better decolorization as well. Its flocs have a high specific gravity, form quickly, are easy to filter, and give a high water yield. Its raw materials all come from industrial waste residues, so the cost is low and it is well suited to industrial water treatment. The aluminum-iron copolymer composite flocculant belongs to this same product family: its raw materials, aluminum chloride and ferric chloride, are both cheap traditional inorganic flocculants with wide availability and simple production processes. A copolymer of aluminum and iron salts differs from a simple mixture of the two salts: it more effectively combines the advantages of PAC and FeCl₃ and enhances turbidity removal. Its effective iron-aluminum content (Al₂O₃ + Fe₂O₃) exceeds 22%, and the product is strongly hygroscopic. Studies show that even when the effective aluminum content of PAC is higher than the effective aluminum-iron content of PAFCS, PAFCS still gives better results in sewage treatment, and in oily and dyeing wastewater it surpasses PAC in both flocculation and decolorization. The influence of the iron content and its species distribution in Al-Fe copolymer flocculants on flocculation performance still requires further study; the pH of the copolymer is determined by the hydrolysis capacity of the PAC and FeCl₃ solutions and lies between those of the two mother liquors, depending on their respective contents.
Polyferric sulfate is a cationic inorganic polymer flocculant with multiple hydroxyl groups and a polynuclear structure. It can be mixed with water rapidly in any proportion and has a larger molecular weight than ordinary inorganic coagulants. When used as a water treatment agent it exhibits strong adsorption, bridging and coagulation–sedimentation performance: the flocs form quickly and grow large, resist breakage, re-flocculate well, filter rapidly after settling, and work over a wide pH range.
Polyaluminum chloride is an inorganic coagulant used mainly for drinking water treatment, municipal sewage treatment, and papermaking and dyeing wastewater treatment. It is inexpensive and has a wide market.
Inorganic flocculants have the advantages of economy and simple application, but they require large dosages, give relatively low flocculation efficiency, and suffer from high cost of use and strong corrosiveness. Organic polymer flocculants, developed only since the late 1960s, are a newer class of wastewater treatment agents. Compared with traditional flocculants they multiply the treatment efficiency at relatively low price and are trending toward mainstream status; with stable product quality, organic polymer flocculants already account for 30–60% of total flocculant production.
Grafting natural polymers such as starch, cellulose and chitosan with acrylamide yields polymers with good flocculation performance, sometimes combined with special properties. Products developed in China are mainly applied to wastewater treatment and sludge dewatering.
Inorganic-organic composite flocculants dominate the market with their variety and versatile performance. Their mechanism is mainly synergistic: the inorganic polymer component adsorbs impurities and suspended particles, forming flocs that gradually grow, while the organic polymer component, through its own bridging action and the active groups adsorbed on it, sweeps up other impurity particles and carries them down together. At the same time, the presence of inorganic salts neutralizes the surface charges of pollutants and promotes the flocculation of the organic polymer, greatly enhancing the overall effect. China has made great progress in the production and application of inorganic polymer flocculants; research on PAC and polyferric sulfate, the two most representative products, is among the most advanced in the world.
A polymeric iron-silicon flocculant is another example: when used to treat domestic sewage, its treatment performance and COD removal both exceed those of polymeric iron, with turbidity removal above 99%, decolorization of 65–70%, COD removal of about 70%, and removal of most of the ammonia nitrogen and all of the phosphorus in the sewage.
Commercial production of microbial flocculants abroad began in the 1990s. Because they cause no secondary pollution and are convenient to use, their application prospects are attractive. Rhodococcus erythropolis and the NOC-1 flocculant derived from it are the best microbial flocculants found so far, with very strong flocculating activity, widely used for treating livestock wastewater, bulking sludge and colored wastewater.
Researchers of microbial flocculants discovered long ago that some microorganisms such as yeasts and bacteria exhibit cell flocculation, but for a long time this was merely regarded as a method of cell enrichment. Only in the last decade or so has cell flocculation technology been widely applied in continuous fermentation and product separation as a simple, economical bioseparation technique.
The earliest flocculant-producing strain was isolated by Butterfield from activated sludge. In 1976, Nakamura et al. screened 19 flocculating microorganisms from molds, bacteria, actinomycetes and yeasts, among which the flocculant produced by Aspergillus souae AJ7002 showed the best performance.
In 1985, Takagi et al. studied PF101, a flocculant produced by Paecilomyces sp. I-1. PF101 showed excellent flocculation and decolorization toward Bacillus subtilis, E. coli, brewer's yeast, red blood cells, activated sludge, cellulose powder, activated carbon, diatomite, aluminum oxide and other substrates.
In 1986, Kurane et al. developed the microbial flocculant NOC-1 using Rhodococcus erythropolis. It exhibits excellent flocculation and decolorization toward E. coli, yeast, muddy water, river water, fly-ash water, activated carbon powder, bulking sludge, pulp wastewater and more, and is the best microbial flocculant discovered to date.
The molecular mass, molecular structure and shape, and the functional groups of a flocculant all influence its activity. Generally speaking, the larger the molecular weight, the higher the flocculating activity; linear molecules show high activity, while branching and crosslinking reduce flocculation. In the late culture period, the cell surface of the producing strain becomes more hydrophobic and the flocculant produced is more active. The surface structure and charge of the colloids in the treated water also affect the result: reports indicate that cations in the water, especially Ca²⁺ and Mg²⁺, effectively reduce the negative surface charge of colloids and promote "bridging". High concentrations of Ca²⁺ can also protect the flocculant from degradation enzymes. With their high efficiency, safety and environmental friendliness, microbial flocculants also hold great potential value in pharmaceuticals, food processing and bioproduct separation.
Flocculation–sedimentation uses an inorganic flocculant (such as aluminum sulfate) together with the anionic organic polymer flocculant polyacrylamide (PAM), prepared as aqueous solutions and dosed into the wastewater. Compression of the electrical double layer destabilizes the suspended particles, the colloids coalesce and grow into flocs ("alum flowers"), and once the flocs reach a certain size they settle out of the water phase under gravity, removing large amounts of suspended solids. To improve separation, a coagulant aid can be added in due time and amount. After treatment, the sewage generally meets discharge standards for color, chromium and suspended solids, and can be discharged or reused as injection water for oil recovery.
Flocculants are currently the most widely used chemicals in wastewater treatment, and flocculation is an indispensable key step in sewage treatment processes. By chemical composition they are classified as inorganic salt flocculants, organic polymer flocculants and microbial flocculants, and users can choose appropriately according to the nature of the wastewater. Combined with wastewater treatment equipment, flocculants make treatment more effective and provide a practical solution to difficult wastewater problems.
The application of flocculants in wastewater treatment has effectively raised treatment speed and markedly improved results. Today these agents are widely used in wastewater treatment across industries, ensuring that water treated by large-scale treatment equipment meets national discharge standards, effectively preventing the deterioration of water pollution and supporting the sustainable development of the ecological environment.
In practical engineering, several aspects deserve attention:
1. Treatment of high-concentration organic wastewater. Such wastewater mainly includes livestock wastewater and various food-processing and farm effluents, which normally receive flocculation pretreatment before biological treatment. Microbial flocculants achieve better flocculation than PAM, and it has been reported that combining a microbial flocculant with a small amount of PAM further improves the pretreatment of monosodium glutamate wastewater while significantly reducing the total chemical dosage.
Mixed efficient bacterial communities with flocculating and degrading functions, isolated from municipal domestic sewage, have been used to treat such sewage, achieving COD and BOD removal rates of up to 100%.
Because microbial flocculants are safe and non-toxic, they are increasingly being adopted for food wastewater treatment with satisfactory results. They can also be applied broadly to municipal sewage, hospital sewage, petrochemical wastewater, papermaking liquor, pharmaceutical wastewater and many other streams.
2. UV degradation of PAM. Ultraviolet radiation degrades PAM rapidly: 4 hours of intense irradiation can reduce its molecular weight from 18 million to 10 million, and oxidants in solution further accelerate the degradation. PAM degradation proceeds via a free-radical chain reaction, so any factor that generates free radicals accelerates it; the reaction of oxygen with iron produces radicals, as does UV light, and both should be avoided. Part of the performance loss of PAM solutions stems from changes in macromolecular conformation: the extended linear long chains contract into coiled spheres. PAM molecules carry many negatively charged groups that repel one another and keep the macromolecule extended, long, and with its active groups fully exposed, which favors bridging and gives good flocculation. However, when cations are present in quantity in the solution, they form an electrical double layer around the negative groups, weakening the mutual repulsion and causing the PAM macromolecules to coil. The higher the ion concentration, the stronger the effect. Divalent ions such as Ca²⁺ are not only strongly adsorbed by the negative groups but can even bridge two of them together, further enhancing coiling. This lowers the solution viscosity (coiled macromolecules give far lower viscosity than linear ones) and reduces the effective activity of the carboxyl groups in PAM, markedly weakening flocculation performance.
3. Hydrolysis versus copolymerization in PAM manufacture. Compared with the copolymerization route, products made by ordinary hydrolysis have a hydrolysis degree (HD) below 30%; theoretically, products with HD above 70% should be made by copolymerization, which imposes certain requirements on hydrolysis temperature and time and is prone to macromolecular degradation during hydrolysis. Ji Lanying et al. of Tianjin University studied the hydrolysis route with NaOH and Na₂CO₃ as hydrolyzing agents and found that NaOH not only accelerates hydrolysis but also deepens it: to obtain latex with low hydrolysis degree (<10%) naoh="" can="" be="" used="" while="" for="" medium="" hydrolysis="" degree="">10%) co-hydrolysis with NaOH and Na₂CO₃ is preferable, reaching a higher hydrolysis value in a shorter time. In recent years, high- and especially ultra-high-molecular-weight acrylic acid/acrylamide polymers have played an undisputed role in tertiary oil recovery. AA/AM copolymers made by copolymerization generally have lower molecular weight and poorer water solubility, so ultra-high-molecular-weight AA/AM copolymers are mostly made by hydrolysis. Ji Hongjian, Sun Zhanwei et al. established a latent initiator system for acrylamide solution polymerization and studied, in a carbonate polymerization system, the effects of adding different amounts of ammonia, urea and EDTA-2Na, as well as system pH, monomer concentration and water-bath polymerization temperature, on the molecular weight and solubility of the product, resolving the contradiction between high molecular weight and insoluble polymer formation. Li Xiaofu and Li Miangui combined heterogeneous hydrolysis with reverse suspension to synthesize an anionic polyacrylamide with molecular weight above 1×10⁷, studied the relations of hydrolysis degree with hydrolysis time, system pH, water/alcohol ratio and temperature, and obtained the reaction conditions for an ultra-high-molecular-weight instant-dissolving polyacrylamide.
