Analysis Of PAC Coagulation Performance in Low-Temperature Environments: Effects Of Temperature Variations On Polyaluminum Chloride Treatment Performance And Optimization Schemes

Introduction

For water purification plants, municipal wastewater treatment plants, and industrial wastewater treatment enterprises, low winter temperatures almost always bring the same challenge-decreased coagulation efficiency, increased chemical dosing, slowed floc formation, and even fluctuations in effluent turbidity.

Many operators believe this is an unavoidable seasonal issue for winter and therefore only continuously increase the coagulant dosage to maintain treatment results. However, extensive engineering practice and operational data demonstrate that the real factor affecting treatment performance is not merely the drop in water temperature, but whether the reaction mechanism of the coagulant itself can adapt to low-temperature environments.

Compared to traditional alum, Polyaluminum Chloride (PAC), by virtue of its pre-polymerized structure and higher active aluminum content, can still maintain high coagulation efficiency under low-temperature conditions. Consequently, it has become the preferred coagulant for an increasing number of water plants and industrial wastewater treatment systems.

As a professional water treatment chemicals supplier, ECOLINK TECHNOLOGY has long served multiple industries including municipal water supply, industrial circulating water, mining wastewater, papermaking wastewater, printing and dyeing wastewater, and food processing. Based on the characteristics of winter water temperature variations across different regions, we have found that selecting the appropriate PAC coagulant and optimizing operational parameters can effectively reduce winter operating costs and improve year-round treatment stability.

This article will systematically analyze the mechanism through which temperature variations affect the coagulation process, highlight the performance advantages of PAC under low-temperature conditions, and share winter operational optimization recommendations to serve as a reference for engineering design and on-site operations.

Why Does Temperature Affect PAC Coagulation Efficiency?

Many people believe that low temperatures simply make the chemical "react slower." In fact, the impact of temperature on coagulation treatment stems from two completely distinct operational mechanisms.

Only by correctly distinguishing between these two mechanisms can truly effective optimization measures be identified, rather than blindly increasing the chemical dosage.

These two mechanisms are:

Chemical reaction kinetics (hydrolysis reaction)

Physical flocculation kinetics (changes in water viscosity)

Although both affect the coagulation efficiency, their root causes, degrees of impact, and solutions are entirely different.

The First Impact Mechanism: Temperature Changes the Chemical Reaction Velocity of the Coagulant

The first step of coagulation treatment is the hydrolysis reaction that occurs after the coagulant enters the water body, forming active aluminum species with charge-neutralization capabilities, thereby destabilizing colloidal particles and creating conditions for subsequent flocculation.

For traditional alum, this step relies almost entirely on on-site hydrolysis.

As the water temperature decreases, the hydrolysis reaction rate slows down significantly, leading to a reduction in the active aluminum species formed per unit of time. Therefore, the dosage must be increased to achieve the same treatment effect.

In other words, the lower the temperature, the more difficult the chemical reaction that alum needs to complete becomes.

In contrast, Polyaluminum Chloride (PAC) possesses a distinct working principle.

PAC is a pre-polymerized inorganic polymer coagulant. During its manufacturing process, a large number of stable polymeric aluminum structures have already been formed, the most representative of which is the polymeric aluminum active component with high charge density. These active components are already polymerized during the product manufacturing stage and do not need to rely entirely on on-site re-hydrolysis like traditional alum.

Therefore, even when entering a raw water environment with lower temperatures, PAC can still rapidly exert its charge-neutralization effect, minimizing the issue of insufficient active components caused by low temperatures.

This is also an important reason why more and more water supply systems in cold regions have begun to adopt high-basicity PAC to replace traditional coagulants.

For water purification plants, industrial circulating water systems, and wastewater treatment projects that require stable year-round operation, this pre-polymerization advantage means not only faster reaction speeds but also more stable treatment performance and lower operational fluctuations.

Why is PAC More Suitable for Winter Operations Than Traditional Alum?

Many operational personnel share this experience:

In summer, the difference in treatment performance between the two coagulants is not very noticeable, but once entering winter, the system performance gradually begins to pull apart.

The reason lies in the completely different ways their active components are formed.

Traditional alum needs to undergo a complete hydrolysis process, and its coagulation capacity is highly dependent on water temperature. When the raw water temperature drops below 10°C, the generation rate of active species decreases significantly, and the system usually requires an increased chemical dosage to maintain normal operations.

On the other hand, because PAC coagulant already possesses a pre-polymerized structure, its primary active ingredients can rapidly participate in the coagulation reaction upon entering the water body, rendering it relatively less affected by low temperatures.

For many northern regions, water purification plants, and mountain water supply projects, this means:

Smaller fluctuations in winter chemical dosing;

Effluent turbidity is easier to keep stable;

System operations are more continuous and reliable;

Chemical chemical costs are effectively controlled;

Operators do not need to frequently adjust dosing parameters.

This is also one of the key reasons why Poly Aluminium Chloride has experienced sustained growth in the global drinking water treatment and industrial wastewater treatment sectors in recent years.

ECOLINK TECHNOLOGY recommends that for projects operating in long-term low-temperature environments, priority should be given to evaluating the pre-polymerization degree and basicity of PAC products rather than focusing solely on product price. Appropriately selecting a high-quality polyaluminum chloride coagulant usually yields better comprehensive operating costs, rather than just lowering the purchase price per single ton of chemical.

Second Impact Mechanism Preview: Low Temperature Affects Not Only the Chemical Reaction, But More So Floc Formation

Even when an excellently performing PAC is selected, phenomena such as smaller flocs, slowed sedimentation velocities, and increased sedimentation tank loads may still occur during winter treatment processes.

These issues do not mean the PAC has failed; rather, they represent physical flocculation limitations caused by the increased viscosity of the water body due to low temperatures.

In other words, after the chemical coagulation has been successfully completed, what truly impacts the subsequent sedimentation efficiency is the speed at which particles collide, aggregate, and grow.

The Second Impact Mechanism: How Does Low Temperature Affect the Flocculation Process?

If it can be said that the coagulation reaction determines whether pollutants can lose their stability, then the flocculation stage determines whether these microscopic particles can further grow and rapidly settle.

Many water plants find in winter that even after switching to a better-performing polyaluminum chloride (PAC) and seeing improvements in coagulation results, the flocs in the sedimentation tank remain small and the settling speed is slow. This indicates that the problem does not stem entirely from the coagulant itself, but is constrained by the physical properties of the water body under low-temperature environments.

Unlike chemical reactions, the flocculation process relies primarily on particles continuously colliding, adsorbing, and aggregating to form larger flocs. When the water temperature drops, the viscosity of the water increases significantly, particle movement speeds decline, and collision opportunities decrease, causing the floc growth rate to slow down accordingly.

This physical phenomenon affects not only PAC coagulants but also other inorganic polymer coagulants. Therefore, it cannot be resolved purely by increasing the chemical dosage; instead, it needs to be optimized in conjunction with process operational parameters.

For systems utilizing Poly Aluminium Chloride for coagulation treatment, stable treatment results can still be achieved as long as operational conditions are reasonably adjusted.

Why Does Cold Water Make Flocs Smaller?

Under normal temperature conditions, the microscopic particles after coagulation will continuously collide and gradually form larger flocs, ultimately settling rapidly under their own weight.

However, when the water temperature drops, the following changes occur:

Water body viscosity increases;

Micro-particle diffusion velocity decreases;

Collision probability between particles reduces;

Floc growth time is prolonged;

The size of the final formed flocs is smaller;

Sedimentation velocity drops significantly.

Consequently, during winter operations, the following phenomena are frequently observed:

Loose and scattered flocs;

Fine particles floating on the surface of the sedimentation tank;

Effluent turbidity increases slightly;

Sedimentation time is prolonged.

It must be emphasized that this belongs to a typical physical limitation rather than a decline in the product quality of polyaluminum chloride.

Therefore, even when using an excellently performing high-basicity PAC, it is still necessary to appropriately optimize flocculation times and operational parameters according to winter working conditions.

ECOLINK TECHNOLOGY recommends that during the winter commissioning process, comprehensive consideration should be given to factors such as raw water temperature, agitation intensity, retention time, and PAM-assisted flocculation, rather than resolving the problem solely by increasing the PAC dosage. This approach is more beneficial for reducing comprehensive operating costs.

Performance Comparison of PAC and Alum at Different Temperatures

Extensive engineering applications show that as temperature drops, the performance gap between PAC and traditional alum becomes increasingly apparent.

The data below reflects the typical operational performance of the two coagulants across different temperature ranges.

From the above data, it can be seen that the temperature range where the gap truly widens occurs primarily below 10°C.

For traditional alum, as the hydrolysis reaction continuously slows down, its active aluminum species generation efficiency drops, which not only requires an increased chemical dosage but also easily leads to a decline in coagulation stability.

In comparison, because PAC coagulant possesses a pre-polymerized structure, it can still maintain high reaction activity in low-temperature environments, resulting in more stable overall performance.

This is precisely why an increasing number of water supply plants, municipal wastewater plants, and industrial wastewater treatment projects in cold regions currently adopt Aluminum Polychloride to replace traditional alum.

Why Are More and More Water Plants Choosing High-Basicity PAC?

In addition to the pre-polymerization advantage, the performance of high-basicity PAC in low-temperature environments is typically more stable.

High-basicity products generally possess a higher proportion of polymeric aluminum active components. These components can complete charge neutralization faster and improve the destabilization efficiency of colloidal particles, thereby maintaining better coagulation performance in winter.

For projects operating in cold regions over the long term, selecting a polyaluminum chloride coagulant with the appropriate basicity is often more economical than simply increasing the dosage.

The advantages of high-basicity PAC are particularly prominent in the following application scenarios:

Municipal drinking water treatment;

Surface water purification projects;

River water treatment;

Mining wastewater treatment;

Coal washing wastewater treatment;

Papermaking wastewater treatment;

Printing and dyeing wastewater treatment;

Industrial circulating water purification.

For systems requiring stable year-round operation, ECOLINK TECHNOLOGY recommends focusing priority on the basicity, Al₂O₃ content, and production process of the PAC product rather than using unit price alone as the procurement basis. High-quality industrial-grade PAC can generally reduce seasonal dosing fluctuations in winter and enhance year-round operational stability.

Can Further Optimization Be Done After Using PAC in Winter?

The answer is yes.

Although PAC can effectively reduce the impact of low temperatures on the coagulation reaction, the physical limitations brought by increased water viscosity still exist. Therefore, appropriate process optimization is still recommended for winter operations.

Practice has proven that by reasonably adjusting operational parameters, one can not only further enhance the low-temperature treatment effect of polyaluminum chloride but also reduce chemical waste, improve floc quality, and increase sedimentation efficiency.

How to Fully Leverage the Treatment Advantages of PAC in Winter? Four Operational Optimization Measures

For projects utilizing polyaluminum chloride (PAC) for coagulation treatment, even though it can maintain good chemical reaction activity in low-temperature environments, the physical limitations brought by increased water viscosity cannot be ignored. Therefore, during the winter operation process, in addition to reasonably selecting the PAC coagulant, it is also necessary to conduct targeted optimization in combination with process parameters to truly achieve stable, efficient, and low-cost water treatment results.

According to a large amount of engineering operation experience, the following four measures can effectively elevate the low-temperature treatment effect of PAC, reduce seasonal fluctuations, and improve year-round operational stability.

Optimization Measure I: Appropriately Extend Flocculation Time to Improve Floc Maturity

In low-temperature environments, due to the reduced collision frequency between particles, the floc formation speed slows down significantly. If operations are still carried out according to summer operational parameters, it frequently results in flocs entering the sedimentation tank before they have fully grown, thereby affecting sedimentation efficiency.

Therefore, it is recommended to appropriately extend the flocculation retention time in winter.

Engineering practice demonstrates that:

It is recommended to extend the flocculation time by 20% to 40% compared to summer.

After increasing the retention time, particles can have more opportunities for collision and aggregation, thereby forming larger and denser flocs, improving subsequent sedimentation efficiency.

For flocculation systems utilizing variable frequency control, the following adjustments can be made based on actual working conditions:

Appropriately reduce the flocculation tank agitation intensity (lower the G value);

Extend the flocculation reaction time;

Maintain a stable rapid mixing effect during the coagulation stage;

Avoid floc breakage caused by excessive shear force.

If the system cannot adjust the retention time, the same operational effect can also be achieved to a certain extent by lowering the treatment flow rate.

ECOLINK TECHNOLOGY recommends that under winter working conditions, priority should be given to optimizing process parameters before considering adjustments to the PAC dosage, which generally yields better economic returns.

Optimization Measure II: Using PAC in Combination with PAM Can Significantly Improve Low-Temperature Flocculation Effects

In cold seasons, relying solely on Polyaluminum Chloride (PAC) to complete coagulation treatment can achieve particle destabilization, but due to the decline in particle collision speed, floc growth remains restricted.

At this time, introducing an appropriate amount of Polyamide (PAM) as a coagulant aid can further enhance the flocculation effect.

Anionic PAM can, through the bridging action of its molecular chains, connect the microscopic flocs formed by PAC into larger-sized aggregates, thereby ameliorating the issues of fine flocs and slow sedimentation under low-temperature conditions.

The generally recommended dosing range is:

0.5–2.0 mg/L

Within this dosing range, most projects can noticeably improve:

Floc size;

Floc density;

Sedimentation velocity;

Effluent turbidity;

Solid-liquid separation efficiency.

It is worth noting that PAM cannot replace PAC; instead, it serves as an auxiliary flocculant, forming a complementary advantage with the PAC coagulant.

For high-turbidity water bodies such as mining wastewater, coal washing wastewater, papermaking wastewater, printing and dyeing wastewater, and municipal sewage, the combined application of PAC and PAM has become a mature and widely adopted solution.

As a professional Poly Aluminium Chloride supplier, ECOLINK TECHNOLOGY can provide PAC and PAM combination application recommendations based on different water quality characteristics, helping customers achieve more stable and economical operational results.

Optimization Measure III: Re-conduct Jar Tests According to Seasonal Changes

Many water plants still follow the dosing schemes established in summer, and upon entering winter, they merely increase the chemical dosage based on experience. This practice not only easily causes chemical waste but also makes it difficult to obtain the optimal treatment effect.

In fact, as the raw water temperature changes, the optimal dosage and flocculation conditions for PAC also vary.

Therefore, during the autumn and winter seasons when water temperatures continuously drop, a Jar Test should be re-conducted to re-determine the optimal operational parameters under current raw water conditions.

Focus priority on the following indicators:

Optimal PAC dosage;

Optimal PAM dosage;

Floc formation speed;

Floc size;

Sedimentation performance;

Effluent turbidity.

Particular attention must be paid to:

Do not directly apply the results of summer jar tests.

Because the dose-response curve formed under cold-water environments is not completely identical to that under warm-water conditions.

For enterprises pursuing long-term stable operation, regularly performing jar tests can not only reduce operating costs but also fully leverage the performance advantages of the polyaluminum chloride coagulant, increasing overall treatment efficiency.

Optimization Measure IV: Select High-Basicity PAC Suitable for Low-Temperature Environments

There are many types of PAC products on the market, and significant differences exist in production processes, basicity, and polymerization degrees among different products; hence, their low-temperature treatment effects also vary.

Engineering practice has proven that:

High-basicity PAC (70%–85%) typically performs better in low-temperature environments than low-basicity products.

This is because high-basicity PAC contains more pre-polymerized active components, which can still maintain high reaction activity under cold conditions, facilitating rapid completion of charge neutralization and improving coagulation efficiency.

If the currently used PAC exhibits the following issues in winter:

Significant increase in dosage;

Slow floc formation;

Increased effluent turbidity;

Decreased sedimentation effect;

In addition to checking operational parameters, it is also recommended to re-evaluate the basicity and quality indicators of the PAC product itself.

For water supply projects or industrial wastewater treatment projects that need to operate in cold regions for the long term, selecting high-quality industrial-grade PAC is often more economical and reliable than frequently increasing the chemical dosage.

ECOLINK TECHNOLOGY provides a variety of high-basicity PAC products with different specifications and can recommend more suitable product schemes for customers based on raw water quality, temperature variations, and industry characteristics.

Storage Management of PAC in Low-Temperature Environments Likewise Cannot Be Ignored

In addition to the operational process, storage conditions can also directly impact the utilization effect of PAC, especially in cold regions.

Liquid PAC may experience crystallization or increased viscosity in extremely low-temperature environments; thus, storage management is particularly crucial.

Recommendations under different temperature conditions are as follows:

Below −5°C to −10°C: Liquid PAC may begin to crystallize or its viscosity may increase significantly, with specific situations depending on product concentration and Al₂O₃ content.

Once crystallization occurs, it may block metering pumps, transmission pipelines, and chemical dosing systems. Therefore, storage tanks and delivery pipelines should adopt insulation measures, and appropriate heating devices are recommended in severely cold areas.

Powdered PAC is virtually unaffected by low-temperature storage, making it more suitable for projects in cold regions, remote mining areas, or locations with poor winter transportation conditions.

Furthermore, attention must also be paid to the storage conditions of liquid PAC in high-temperature environments. Long-term exposure to temperatures above 40°C may accelerate the hydrolysis reaction, affecting product stability. In a normal storage environment of 5–35°C, the product can generally maintain a stable shelf life of 6–12 months, and its Al₂O₃ content and basicity will not experience significant declines.

Therefore, for enterprises planning long-term storage or cross-season use of PAC, a sound warehouse management system should be established to ensure product performance remains stable.

Recommendations for Year-Round Seasonal Operational Management of PAC Systems

For water treatment systems using Polyaluminum Chloride (PAC) for coagulation treatment, operational parameters are not set in stone. Raw water temperature, water quality fluctuations, and seasonal changes will all affect the coagulation result. Therefore, it is recommended to establish a seasonal operational management mechanism rather than adopting a single dosing scheme throughout the entire year.

Combining extensive operational experience from water supply plants and industrial wastewater treatment projects, ECOLINK TECHNOLOGY recommends dynamically optimizing the PAC dosage, flocculation time, and auxiliary chemical usage according to seasonal changes to achieve stable and efficient operation year-round.

Establishing a seasonal operational management system can not only improve coagulation stability but also reduce chemical waste and lower year-round operating costs, which is especially applicable to municipal water supply, industrial wastewater treatment, and large circulating water systems.

Frequently Asked Questions (FAQ) Regarding PAC Low-Temperature Applications

1. Can PAC still be used normally when it is close to 0°C?

Yes.

Polyaluminum Chloride (PAC) can still maintain its coagulation capability when close to 0°C, but due to the increased water viscosity caused by low temperatures, the flocculation speed will decline, so the overall treatment efficiency will be somewhat lower compared to normal temperature conditions.

When the water temperature is below 5°C, it is recommended to:

Extend the flocculation time;

Use PAM as a coagulant aid in combination;

Re-optimize the PAC dosage according to the actual water temperature.

PAC does not have a fixed "failure temperature"; its treatment capacity weakens progressively as temperature drops, rather than suddenly losing its function.

2. Why is high-basicity PAC more suitable for low-temperature environments?

High-basicity PAC (70%–85%) possesses a higher proportion of pre-polymerized active aluminum components, so it can complete charge neutralization faster under low-temperature conditions, improving coagulation efficiency.

Compared to ordinary products, high-basicity PAC offers the following advantages:

Higher low-temperature activity;

Faster coagulation reaction speed;

More stable dosing requirement;

Quicker floc formation;

Smaller operational fluctuations in winter.

For drinking water plants, wastewater treatment plants, and industrial wastewater treatment projects in cold regions, high-basicity PAC is usually a more economical and reliable choice.

3. Does water temperature affect the storage life of PAC?

Yes, but the impact is primarily reflected in liquid PAC.

Below −5°C to −10°C: Liquid PAC may undergo crystallization or increased viscosity;

Above 40°C: It may accelerate product hydrolysis and shorten the storage period.

It is recommended to store liquid PAC in an environment of 5–35°C, under which the product can generally maintain a stable shelf life of 6–12 months, and its Al₂O₃ content and basicity will not undergo apparent changes.

Powdered PAC exhibits better storage stability and is more suitable for cold regions and long-distance transportation.

4. Is it always better to have a larger PAC dosage in winter?

No, it is not.

Many operators find that after the treatment effect declines in winter, their first reaction is to increase the PAC dosage. In reality, however, low temperature affects not only the coagulation reaction but also particle collisions and the flocculation process.

If one only continuously increases the chemical without simultaneously optimizing:

Flocculation time;

Agitation intensity;

Auxiliary PAM dosing;

Jar test parameters;

It is not only difficult to achieve the optimal effect but may also increase operating costs and even affect effluent water quality.

The scientific approach should be to comprehensively optimize the entire coagulation system by combining water temperature, water quality, and process conditions.

5. Must PAC and PAM be used together at the same time?

Not necessarily.

PAC itself can complete the coagulation treatment independently and can meet operational demands under most normal temperature working conditions.

However, in the following scenarios, it is recommended to use PAC and PAM in combination:

Winter low-temperature operations;

High-turbidity raw water;

Coal washing wastewater;

Mining wastewater;

Papermaking wastewater;

Printing and dyeing wastewater;

Sludge dewatering systems.

PAC is responsible for colloidal destabilization, while PAM is responsible for bridging flocculation. The cooperation of the two can yield larger flocs, faster sedimentation velocities, and better solid-liquid separation results.

Summary: Scientifically Selecting PAC so Low Temperature is No Longer a Water Treatment Problem

Temperature variations do affect coagulation treatment efficiency, but this does not mean that winter operations can only rely on increasing the chemical dosage to maintain performance.

From the perspective of coagulation mechanisms, low temperature primarily impacts treatment results through chemical reaction kinetics and physical flocculation kinetics. Compared to traditional alum, Polyaluminum Chloride (PAC) maintains higher activity in low-temperature environments due to its pre-polymerized structure, giving it a clear performance advantage below 10°C.

At the same time, the increased water viscosity brought by low temperatures still needs to be ameliorated through optimized operational management, including:

Extending the flocculation time by 20% to 40%;

Reasonably dosing 0.5–2.0 mg/L of PAM;

Performing regular jar tests;

Selecting 70%–85% high-basicity PAC;

Properly managing the low-temperature storage of liquid PAC.

By combining product optimization with process optimization, chemical consumption in winter can be effectively reduced, effluent stability enhanced, and high-efficiency operation achieved year-round.

As a professional water treatment solutions supplier, ECOLINK TECHNOLOGY is dedicated to providing high-quality Polyaluminum Chloride (PAC), Polyacrylamide (PAM), and matching technical support to global customers. Whether for drinking water purification, municipal wastewater treatment, or industrial wastewater treatment in mining, papermaking, printing and dyeing, chemical, and other fields, we can provide professional product selection recommendations and application schemes based on different raw water conditions, temperature variations, and process requirements, helping to lower comprehensive operating costs and elevate treatment efficiency.