From Cloudy to Clear: The Essential Role of Aluminum Sulfate in Coagulation & Flocculation
Table of Contents
What you will learn
📋 What You'll Learn
This guide walks you through from cloudy to clear: the essential role of aluminum sulfate in coagulation & flocculation with detailed instructions.
In the critical field of water treatment, transforming turbid, particle-laden source water into crystal-clear effluent is both a science and an art. The removal of microscopic suspended particles represents the essential first step toward achieving clarity, safety, and regulatory compliance. This transformation is accomplished through coagulation-flocculation, a robust chemical and physical process that has been the backbone of drinking water and industrial wastewater treatment for well over a century.
This definitive guide provides a deep, technical dive into the science behind this process, with a sharp focus on the industry's primary workhorse: Aluminum Sulfate (Al₂(SO₄)₃), commonly known as alum. Whether you are a municipal water plant operator optimizing your clarification train, an industrial engineer designing a new wastewater treatment system, a pond manager battling persistent turbidity, or a researcher conducting bench-scale jar testing, this guide provides the expert knowledge needed to select, dose, and optimize alum for superior results.
Why This Guide Matters
Coagulation-flocculation removes up to 99% of suspended solids, 90%+ of bacteria and protozoa, and significant percentages of dissolved organic carbon, heavy metals, and phosphorus. A properly optimized coagulation process is the single most important unit operation in a conventional water treatment plant. Understanding water purification chemistry at this level enables operators to deliver safer water at lower cost.
The Problem: Understanding Turbidity and Colloidal Suspensions
The cloudiness or haziness observed in raw water sources, known as turbidity, is caused by vast numbers of microscopic particles — clay, silt, organic matter (humic and fulvic acids), bacteria, algae, and other microorganisms — that are too small to settle under gravity alone. These are known as colloidal particles, typically ranging from 0.001 to 1 micrometer (micron) in diameter.
Their refusal to settle is due to a fundamental principle of surface chemistry. Most colloidal particles in natural water possess a negative surface charge, acquired through the ionization of surface functional groups and the preferential adsorption of anions from the surrounding water. This charge creates what scientists call an electrical double layer around each particle — a tightly bound inner layer (Stern layer) and a diffuse outer layer of counter-ions.
The key measurement here is zeta potential, the electrical potential at the shear plane of this double layer. When zeta potential is high (typically -25 mV to -40 mV for natural colloids), particles strongly repel each other, similar to pushing two same-sided magnets together. This electrostatic repulsion keeps particles in permanent suspension, rendering simple sedimentation and filtration ineffective for their removal.
Technical Insight: The DLVO Theory
Colloidal stability is explained by the DLVO theory (Derjaguin-Landau-Verwey-Overbeek), which describes the balance between attractive van der Waals forces and repulsive electrostatic forces. Coagulants like alum work by compressing the electrical double layer and reducing zeta potential to near zero, tipping the balance toward attraction and enabling particle aggregation. Understanding this theory is foundational to industrial chemical processes involving colloidal systems.
The Solution: The Four-Step Coagulation-Flocculation Process
Coagulation-flocculation overcomes electrostatic repulsion using a chemical coagulant. The most widely used and cost-effective coagulant worldwide is Aluminum Sulfate, selected by water treatment professionals for its proven efficacy, predictable behavior, wide availability, and favorable cost profile.
Coagulation (The Flash Mix)
This is the critical charge neutralization step. When Aluminum Sulfate is added to water, it rapidly dissolves and dissociates, releasing highly positive trivalent aluminum ions (Al³⁺). During a period of intense, rapid mixing (the "flash mix," typically 10-30 seconds at G-values of 300-1000 s¹), these positive ions neutralize the negative surface charge of colloidal particles. Simultaneously, the aluminum undergoes hydrolysis, forming a series of positively charged aluminum hydroxide species. With their repulsive zeta potential reduced to near zero, the destabilized particles can begin to aggregate.
Flocculation (The Slow Mix)
Immediately following coagulation, water enters a flocculation basin for gentle, slow mixing (typically 20-45 minutes at G-values of 20-80 s¹). During this stage, destabilized microparticles collide and aggregate through a process called perikinetic flocculation (Brownian motion) and orthokinetic flocculation (fluid shear). The aluminum ions react with alkalinity to form a gelatinous, sticky precipitate of aluminum hydroxide (Al(OH)₃), commonly called the "sweep floc." This precipitate entraps particles in its growing, web-like structure, forming larger, denser, and more visible clumps called macroflocs.
Sedimentation (Settling)
After flocculation, water flows into a large, calm sedimentation basin or clarifier. Here, velocity is dramatically reduced (surface loading rates of 0.5-1.0 gpm/ft²), allowing gravity to pull the heavy macrofloc particles to the bottom, forming a sludge blanket. Modern designs include tube settlers or plate settlers to improve capture efficiency. Properly formed alum floc settles at 1-5 mm/s, and the clarified supernatant leaving the basin typically shows 85-95% turbidity reduction.
Filtration (Polishing)
As a final polishing step, clarified water passes through filters — typically dual-media (anthracite/sand) or multimedia (anthracite/sand/garnet) filters operating at 2-6 gpm/ft². This removes any remaining non-settled floc and fine particles, producing crystal-clear, low-turbidity water (<0.3 NTU) ready for disinfection with sodium hypochlorite or other oxidants such as hydrogen peroxide.
The complete coagulation-flocculation-sedimentation-filtration train can reduce turbidity from 100+ NTU to <0.1 NTU, achieving >99.9% particle removal.
The Chemistry of Alum Hydrolysis: What Really Happens
Understanding the hydrolysis chemistry of aluminum sulfate is essential for optimizing coagulation. When alum dissolves in water, the Al³⁺ ion doesn't simply neutralize particles directly. Instead, it undergoes a complex, pH-dependent series of hydrolysis reactions that produce various aluminum species, each with different coagulation mechanisms.
Key Hydrolysis Reactions
The primary dissolution reaction is:
Al₂(SO₄)₃ · 14H₂O → 2Al³⁺ + 3SO₄²⁻ + 14H₂O
The aluminum ion then undergoes stepwise hydrolysis, consuming alkalinity and producing hydrogen ions (which lower pH):
- Al³⁺ + H₂O → Al(OH)²⁺ + H⁺ (charge neutralization species)
- Al³⁺ + 2H₂O → Al(OH)₂⁺ + 2H⁺
- Al³⁺ + 3H₂O → Al(OH)₃ (s) + 3H⁺ (sweep floc precipitate)
- Al³⁺ + 4H₂O → Al(OH)₄⁻ + 4H⁺ (soluble, formed at high pH — undesirable)
In practice, polymeric aluminum species such as Al₁₃O₄(OH)₂₄&sup7;⁺ also form and play a crucial role in charge neutralization. The dominant species — and therefore the dominant coagulation mechanism — depends primarily on pH and alum dose. This is why pH control is so critical to effective water treatment, and why operators sometimes use sodium hydroxide for pH adjustment.
Two Coagulation Mechanisms
Charge Neutralization dominates at lower alum doses and lower pH (5.5-6.5). Positively charged hydrolysis products directly neutralize negatively charged colloids. This mechanism produces small, dense floc and is most dose-efficient.
Sweep Flocculation dominates at higher alum doses and near-neutral pH (6.5-7.5). Massive amounts of amorphous Al(OH)₃ precipitate form, physically entrapping particles in their structure. This produces larger, faster-settling floc but consumes more chemical.
Aluminum Sulfate vs. Other Coagulants: A Head-to-Head Comparison
While aluminum sulfate is the most widely used coagulant globally, operators should understand how it compares to alternatives. The choice of coagulant depends on source water characteristics, treatment goals, regulatory requirements, and budget. Professionals working with industrial acids and chemicals will find this comparison particularly useful for process design.
| Parameter | Aluminum Sulfate (Alum) | Ferric Chloride (FeCl₃) | PAC (Poly Aluminum Chloride) | Ferric Sulfate (Fe₂(SO₄)₃) |
|---|---|---|---|---|
| Chemical Formula | Al₂(SO₄)₃·14H₂O | FeCl₃·6H₂O | Aln(OH)mCl(3n-m) | Fe₂(SO₄)₃·9H₂O |
| Optimal pH Range | 5.5 - 7.5 | 4.0 - 11.0 | 5.0 - 9.0 | 4.0 - 11.0 |
| Effective Temp Range | Reduced below 5°C | Effective in cold water | Excellent in cold water | Effective in cold water |
| Floc Characteristics | Light, moderate settling | Heavy, fast settling | Moderate density | Heavy, fast settling |
| Sludge Volume | Moderate | Lower | Lowest | Lower |
| Alkalinity Consumption | ~0.50 mg/mg as CaCO₃ | ~0.92 mg/mg as CaCO₃ | 0.15-0.25 mg/mg | ~0.75 mg/mg as CaCO₃ |
| Cost per Dry Ton | $200 - $350 (Lowest) | $350 - $600 | $500 - $900 | $350 - $550 |
| Phosphorus Removal | Good | Excellent | Good | Excellent |
| Color Removal | Good | Excellent | Good | Excellent |
| Corrosivity to Equipment | Moderate (acidic) | High (very acidic) | Low | High |
| Residual Concerns | Aluminum residuals | Iron residuals (color) | Low residuals | Iron residuals (color) |
| Best For | General purpose, low cost, moderate turbidity | Cold water, high color, phosphorus | Low alkalinity, wide pH range | Phosphorus removal, cold water |
Why Alum Remains the Top Choice
Despite the advantages of newer coagulants like PAC, aluminum sulfate remains the most widely used coagulant worldwide because of its unbeatable cost-effectiveness, predictable performance, extensive operational track record spanning 100+ years, and broad availability. For typical municipal surface water treatment with moderate turbidity and adequate alkalinity, alum delivers excellent results at the lowest chemical cost. Facilities managing cooling tower water treatment also rely on alum for its proven reliability.
Dosing Guide: Aluminum Sulfate by Source Water Type
Proper dosing is perhaps the single most important factor in coagulation performance. Under-dosing leaves particles destabilized but unsettled; over-dosing can cause charge reversal, restabilization, and wasted chemical. The following table provides starting-point dosing ranges based on source water characteristics. Always confirm optimal dose through jar testing.
| Source Water Type | Turbidity (NTU) | Typical Alum Dose (mg/L) | Expected pH Drop | Notes & Considerations |
|---|---|---|---|---|
| Clear Reservoir / Groundwater | 1 - 10 | 5 - 15 | 0.2 - 0.5 units | May need polymer coagulant aid; low suspended solids make floc formation difficult |
| Moderate Lake/River Water | 10 - 50 | 15 - 40 | 0.5 - 1.0 units | Most common municipal scenario; excellent alum performance range |
| Turbid River (Seasonal) | 50 - 200 | 30 - 75 | 1.0 - 1.5 units | Monitor alkalinity closely; may need lime or NaOH supplementation |
| Highly Turbid / Storm Event | 200 - 1000+ | 60 - 150 | 1.5 - 3.0+ units | Pre-sedimentation recommended; alkalinity supplementation almost always required |
| High Color / Organic Water | 5 - 30 (high color) | 40 - 80 | 1.0 - 2.0 units | Dose driven by color/TOC, not turbidity; lower pH (5.5-6.0) improves removal; consider peroxide pre-oxidation |
| Industrial Wastewater | Variable | 100 - 300+ | Variable | Jar testing essential; often combined with polymer flocculant; monitor metals discharge |
| Pond / Lake Clarification | 20 - 200 | 25 - 50 lbs/acre-ft | 0.5 - 2.0 units | Apply as slurry; ensure adequate buffering capacity; aerate during treatment |
Critical Dosing Rule: The Jar Test Is King
The dosing ranges above are starting points only. Every water source is unique, and optimal dose varies with temperature, pH, alkalinity, organic content, and seasonal changes. Before full-scale application, always perform a standard jar test (ASTM D2035) using your actual source water. Jar testing lets you visually observe floc formation, measure settled turbidity, and determine the dose that produces the best settled water quality at the lowest cost. This rigorous approach aligns with green chemistry principles by minimizing chemical waste.
Aluminum Sulfate Grades & Specifications
Aluminum sulfate is manufactured in several grades, each suited to different applications. Selecting the right grade ensures both performance and regulatory compliance. For laboratory work and quality control, choosing the correct purity grade is critical — similar to the precision required when working with any industrial-grade chemical.
| Grade | Al₂O₃ Content | Form | Key Specifications | Primary Applications | Alliance Product |
|---|---|---|---|---|---|
| Technical / Industrial Grade | 15.2 - 17.0% | Granular, Powder, or Liquid (48.5% solution) | Fe < 0.5%; Insoluble < 0.1%; pH 2.5-3.5 (1% sol.) | Municipal water treatment, wastewater, pond clarification, paper manufacturing | Shop Technical Grade |
| NSF/ANSI 60 Certified | 15.2 - 17.0% | Granular or Liquid | Meets NSF/ANSI Standard 60 for drinking water additives; heavy metals limits | Potable water treatment plants (required for SDWA compliance) | Contact Sales |
| ACS Reagent Grade (Hydrate) | ≥15.3% (as Al₂O₃) | Crystalline/Powder | ACS specification: Cl < 0.001%; Fe < 0.002%; Heavy metals < 5 ppm | Lab jar testing, analytical standards, QC, research | Shop ACS Grade |
| Iron-Free Grade | 15.5 - 17.0% | Granular or Liquid | Fe < 0.003%; specifically manufactured to minimize iron contamination | Paper/pulp production, textile sizing, cosmetics, specialty applications | Contact Sales |
| Liquid Alum (48.5% Solution) | 8.0 - 8.3% | Liquid (ready-to-feed) | Specific gravity 1.32-1.34; pH 2.0-2.5; ready for metering pump feed | Large-scale municipal/industrial plants with liquid feed systems | Contact Sales |
Critical Applications of Aluminum Sulfate
Municipal Drinking Water
The largest application globally. Public water systems use alum coagulation as the primary method for removing turbidity, pathogens, and disinfection byproduct precursors from raw surface water sources.
Industrial Wastewater
Removes suspended solids, heavy metals, phosphorus, oils, and emulsified contaminants from manufacturing discharge. Essential for NPDES permit compliance and environmental protection.
Pond & Lake Restoration
Highly effective at clearing turbid ponds and reducing internal phosphorus loading to prevent algal blooms. Used in lake restoration projects across North America.
Municipal and Industrial Water Treatment
Municipal drinking water treatment represents the single largest application of aluminum sulfate worldwide, with millions of tons consumed annually. In a conventional surface water treatment plant, the coagulation-flocculation-sedimentation-filtration train removes 99%+ of turbidity, 90-99% of bacteria and protozoa (including Giardia and Cryptosporidium), and 30-60% of dissolved organic carbon (DOC). The DOC removal is particularly important because organic matter reacts with chlorine-based disinfectants to form regulated disinfection byproducts (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs).
Industrial water treatment facilities, including those managing cooling tower blowdown, boiler feedwater pretreatment, and process water clarification, also rely heavily on alum coagulation for suspended solids removal.
Wastewater Treatment and Phosphorus Removal
In both municipal and industrial wastewater treatment, alum serves dual purposes: it removes suspended solids through coagulation and precipitates dissolved phosphorus as insoluble aluminum phosphate (AlPO₄). Phosphorus removal is critically important for preventing eutrophication — the nutrient-driven excessive growth of algae that depletes dissolved oxygen and devastates aquatic ecosystems. Many NPDES discharge permits now require total phosphorus below 0.5-1.0 mg/L, achievable with alum addition.
The stoichiometric ratio for phosphorus removal is approximately 1.0-2.0 moles of Al per mole of P (atomic ratio Al:P of ~2:1 by weight), but practical dosing is typically higher due to competing reactions. Proper dechlorination of effluent with sodium bisulfite may also be required before discharge.
Pond and Lake Clarification
Alum is highly effective at clearing muddy or cloudy ponds caused by suspended clay particles. The process binds the clay, allowing it to settle and restoring water clarity. In lake restoration, alum is applied not only to clarify the water column but also to create an "alum floc blanket" on the sediment surface that seals in phosphorus and prevents its release back into the water — a technique that can control algal blooms for 10-15 years.
How-To: Clarify a Muddy Pond with Alum
- Calculate Dosage: The typical dosage ranges from 25 to 50 lbs of Aluminum Sulfate per acre-foot of water. Measure your pond's surface area and average depth to calculate total volume.
- Test Alkalinity First: Ensure alkalinity is >50 mg/L as CaCO₃ before treatment. If alkalinity is low, pre-treat with sodium bicarbonate or hydrated lime to buffer pH.
- Create a Slurry: Mix alum with water in a large tank to create a pumpable liquid slurry for even distribution.
- Apply Evenly: Using a boat and pump, spray the slurry across the entire pond surface. For very small ponds, careful broadcasting from shore is acceptable.
- Aerate: Run aeration during and after application to ensure mixing and support aquatic life, as the process can temporarily lower dissolved oxygen.
- Monitor: The pond should clear significantly within 24-48 hours. Monitor pH for 72 hours to ensure it remains above 6.0.
Optimizing Performance: Critical Process Parameters
Achieving effective coagulation is a science that demands careful attention to multiple interrelated variables. Operators who understand and actively manage these parameters consistently produce better water quality with lower chemical costs. Proper documentation and process control align with laboratory and plant best practices.
pH: The Most Critical Variable
Alum works best in a pH range of 5.5 to 7.5, with the optimal sweet spot typically between 6.0 and 7.0. Outside this range, aluminum solubility increases dramatically — below pH 5.5, aluminum remains in the soluble Al³⁺ form, while above pH 8.0, it converts to soluble aluminate (Al(OH)₄⁻). Both conditions produce poor floc and elevated dissolved aluminum in the treated water. Operators routinely use sodium hydroxide (NaOH), lime (Ca(OH)₂), or soda ash (Na₂CO₃) to raise pH, or phosphoric acid and sulfuric acid to lower it.
Alkalinity: The Buffer System
The alum hydrolysis reaction consumes approximately 0.50 mg of alkalinity (as CaCO₃) per mg of alum added. If the source water does not have sufficient alkalinity to buffer this consumption, pH will crash below the optimal range, resulting in poor coagulation and elevated aluminum residuals. As a rule of thumb, maintain raw water alkalinity at a minimum of 1.5 times the alum dose (in mg/L as CaCO₃).
Mixing Energy: G-Values Matter
The velocity gradient ("G value") is critical and differs dramatically between the two stages:
- Flash Mix (Coagulation): G = 300-1000 s⁻¹ for 10-30 seconds. Must be rapid and turbulent to ensure instant, complete dispersion of coagulant throughout the water.
- Slow Mix (Flocculation): G = 20-80 s⁻¹ for 20-45 minutes. Must be gentle enough to allow floc growth without shearing apart the fragile macrofloc particles. Many plants use tapered flocculation with decreasing G-values through a series of chambers.
Temperature Effects
Cold water (<5°C) significantly impacts alum coagulation by increasing water viscosity (which slows particle settling and reduces collision frequency) and by slowing hydrolysis kinetics. In cold climates, operators often need to increase alum dose by 20-50%, add polymer coagulant aids, or consider switching to ferric-based coagulants or PAC, which are less temperature-sensitive.
Coagulant Aids and Polymers
Anionic or nonionic polymer flocculant aids (typically at 0.05-0.5 mg/L) are commonly added during flocculation to bridge between alum floc particles and create larger, denser, faster-settling macroflocs. This can reduce required alum dose by 20-40% and improve settled water quality significantly.
Troubleshooting Common Coagulation Problems
Even experienced operators encounter coagulation challenges. Below are the most common issues and their solutions. Maintaining detailed operational logs and following chemical safety protocols helps prevent many of these problems.
Problem: Poor Floc Formation / "Pin Floc"
- Likely Cause: Sub-optimal pH, insufficient mixing energy during flash mix, or under-dosing
- Solution: Verify and adjust pH to 6.0-7.0 range; check flash mixer speed; perform jar test to re-optimize dose
Problem: Floc Breakup in Flocculation Basin
- Likely Cause: Excessive mixing energy (G-value too high in flocculation stage)
- Solution: Reduce mixer speed; implement tapered flocculation; check for short-circuiting
Problem: Floating Floc / Rising Sludge
- Likely Cause: Gas bubble entrapment (from supersaturated dissolved gases or biological activity in sludge blanket)
- Solution: Increase sludge withdrawal frequency; add baffling; check for algal growth in clarifier
Problem: High Residual Aluminum in Filtered Water
- Likely Cause: pH outside optimal range (too high or too low); over-dosing; inadequate flocculation time
- Solution: Tighten pH control to 6.2-6.8; reduce dose if over-dosing; extend flocculation detention time; ensure filters are properly backwashed
Problem: Increased Turbidity After Storm Events
- Likely Cause: Sudden increase in raw water turbidity and change in particle characteristics
- Solution: Increase alum dose proportionally; perform rapid jar tests; activate standby coagulant feed system; consider pre-sedimentation
Safety Protocols for Handling Aluminum Sulfate
While aluminum sulfate is not classified among the most hazardous industrial chemicals, it demands proper handling, storage, and personal protective equipment to ensure worker safety and product integrity. Following comprehensive chemical safety guidelines is essential for any facility handling bulk chemicals.
Mandatory Safety Requirements
- Dust Inhalation: Fine powder or granular alum generates respirable dust that irritates the respiratory tract. Always wear a NIOSH-approved dust mask (N95 minimum) or half-face respirator with P100 cartridges when handling dry product, opening bags, or loading hoppers.
- Eye Protection: Alum dust and solutions are eye irritants. Wear splash-proof safety goggles or face shield. Emergency eyewash must be within 10 seconds of handling area.
- Skin Protection: Wear chemical-resistant gloves (nitrile or neoprene) and long sleeves. Prolonged skin contact with alum solutions (pH 2-3) can cause irritation and dermatitis.
- Corrosivity: Alum solutions are acidic (pH 2-3.5) and corrosive to carbon steel, aluminum, zinc, and galvanized metals. Store and handle using properly rated plastic (HDPE, polypropylene), fiberglass-reinforced plastic (FRP), or rubber-lined steel equipment.
- Storage: Store dry alum in a cool, dry, well-ventilated area away from moisture. Keep containers sealed to prevent clumping and caking. Follow your facility's chemical storage guidelines.
- Spill Response: Dry spills: sweep up and recover. Liquid spills: contain, neutralize with soda ash or lime, and absorb. Dispose in accordance with safe chemical disposal procedures and local regulations.
Environmental Considerations & Sustainability
As the water treatment industry increasingly embraces green chemistry principles, understanding the environmental footprint of coagulant use has become essential. Aluminum sulfate offers several environmental advantages over alternatives.
Sludge Management and Beneficial Reuse
Alum sludge — the residual from coagulation-flocculation — is classified as a non-hazardous industrial waste in most jurisdictions. After dewatering, it contains primarily aluminum hydroxide, captured sediment, and organic matter. Beneficial reuse options include:
- Soil Amendment: Alum sludge can improve soil structure and phosphorus retention in agricultural applications
- Biosolids Co-Processing: Blending with wastewater biosolids for co-composting or land application
- Construction Materials: Research into incorporation into bricks, cement, and lightweight aggregates
- Acid Recovery: Aluminum can be recovered from sludge using acid dissolution for reuse
Minimizing Chemical Footprint
Operators can reduce alum consumption and environmental impact by optimizing dose through real-time streaming current monitoring, using polymer aids to reduce primary coagulant demand, and implementing enhanced coagulation protocols that maximize removal per unit of chemical added. These practices reduce sludge volume, lower chemical costs, and support sustainability goals.
Buyer's Guide: Selecting the Right Product
Alliance Chemical offers premium-grade aluminum sulfate products for every application, from bench-scale laboratory jar testing to full-scale municipal treatment plants. Our technical-grade product meets the demanding specifications required for water treatment applications, while our ACS-grade product satisfies the stringent purity requirements of analytical laboratories.
Technical Grade Aluminum Sulfate
Standard industrial purity for municipal water treatment, wastewater treatment, pond clarification, and paper manufacturing. Available in granular and powder forms. Consistent quality, competitive pricing, reliable supply.
ACS Reagent Grade (Hydrate)
High-purity product meeting American Chemical Society specifications. Ideal for laboratory jar testing, creating analytical standards, quality control testing, and research applications requiring traceable purity.
Bulk & Custom Packaging
Available in 50-lb bags, drums, totes, and bulk delivery. Contact our water treatment specialists for custom packaging, volume pricing, and technical support for your specific application.
Achieve Crystal-Clear Water with Confidence
Effective coagulation-flocculation is the cornerstone of high-quality water treatment. Alliance Chemical provides premium-grade Aluminum Sulfate and a full suite of water treatment chemicals, backed by the technical expertise to support your most critical clarification challenges.
Shop Water Treatment Chemicals Consult Our ExpertsFrequently Asked Questions
How does aluminum sulfate (alum) work as a water coagulant?
Aluminum sulfate dissociates in water to release Al³⁺ ions, which react with alkalinity to form aluminum hydroxide (Al(OH)₃) floc. This sticky, gelatinous floc traps suspended particles, bacteria, and organic matter, which then settle out by gravity in clarifiers. Typical dosage is 10-50 mg/L depending on raw water turbidity.
What is the optimal pH range for aluminum sulfate coagulation?
Alum coagulation works best at pH 6.0-7.5, with optimal performance typically around pH 6.5. Below pH 5.5, insufficient hydroxide floc forms. Above pH 8.0, aluminum becomes soluble again as aluminate ions, reducing effectiveness. Lime or soda ash is often added to maintain alkalinity and buffer pH during alum dosing.
Can aluminum sulfate be used in swimming pool maintenance?
Yes, alum is excellent for pool clarification. Add 4 lbs per 10,000 gallons of pool water with the filter off and pump running on recirculate. After 12-24 hours, the settled floc is vacuumed to waste. This removes fine particles that pass through sand filters, clearing cloudy water that chlorination alone cannot fix.
How much aluminum sulfate should I use for garden soil acidification?
Apply 1-2 lbs of aluminum sulfate per 10 square feet to lower soil pH by about 0.5-1.0 units. For acid-loving plants like blueberries and azaleas, test soil pH first and apply in stages—retest after 2-3 months. Work alum into the top 6 inches of soil and water thoroughly. Excessive application can cause aluminum toxicity in plants.
