Possible Causes for Suspended Flocs of Sodium Methallyl Sulfonate (SMAS)/AM Copolymer in Printing Wastewater
- Excess negative charge balanceSodium Methallyl Sulfonate (SMAS) brings abundant anionic sulfonate groups. If dye particles carry strong negative charges, excessive electrostatic repulsion stops floc compacting and settling.
- Improper molecular weight of SMAS/AM copolymerToo low molecular weight cannot form long bridging chains to gather tiny dye colloids; overly high viscosity disperses flocs finely to stay suspended.
- Unsuitable pH environmentExtreme pH changes surface charge of dye pollutants and alters ionization degree of SMAS’s –SO₃⁻, breaking charge neutralization balance.
- Insufficient or mismatched inorganic coagulant dosageWithout proper Al/Fe salt pre-neutralization, SMAS copolymer only loosely attaches particles instead of condensing dense flocs.
- High surfactant and salinity interferencePrinting wastewater holds lots of surfactants and dissolved salts; they wrap flocs, increase water affinity and hinder sedimentation.
Corresponding Solutions
- Adjust dosing sequence: Add aluminium or ferric salt first to neutralize negative charges, then feed SMAS/AM copolymer for bridging.
- Switch to SMAS/AM product with moderate molecular weight; test different molecular grades via jar trials.
- Tune wastewater pH to neutral or weak alkaline (pH 7–8.5), the optimal range for SMAS copolymer flocculation.
- Reduce overdosage of SMAS/AM copolymer to cut surplus anionic repulsion force.
- Add a small amount of coagulant aid like activated silica to increase floc density and settling speed.
On-Site Issue: Flocs Form but Neither Settle nor Float – Remaining Suspended in Water
When using an SMAS/AM copolymer to treat printing and dyeing wastewater on-site, if flocs form but neither settle nor float—instead remaining suspended in the water—this is a typical indication of a mismatch between the flocculant and the wastewater system, or improper process parameters. Considering the dispersive properties of SMAS and the characteristics of printing and dyeing wastewater, the following is a systematic analysis and solution.
I. Core Problem Diagnosis
The phenomenon of “suspended flocs” fundamentally occurs because the floc density is close to that of water and the floc particle size is too small, preventing effective separation by either gravity settling or buoyancy.
It is important to note that SMAS itself has strong dispersive functionality. In oilfield chemical applications, SMAS utilizes its sulfonic acid group (-SO₃⁻) to ionize and generate a strong negative charge, stably dispersing particles in water through electrostatic repulsion, preventing aggregation and settling. This means:
- If the dosage is excessive, the SMAS/AM copolymer may not promote flocculation but instead exert its inherent dispersive effect, stabilizing pollutants in suspension.
- If the wastewater contains a high concentration of colloids, using the copolymer alone may not effectively neutralize charges.
Specifically, the possible causes fall into several categories:
II. Detailed Cause Analysis & Solutions
Cause 1: Excessive SMAS Dosage (Most Likely)
Analysis: The SMAS/AM copolymer contains sulfonic acid groups, giving it strong anionic character and dispersive function. When overdosed:
- After saturated adsorption of polymer chains on particle surfaces, excess molecules coat the flocs.
- Excess negative charge re-stabilizes the particles (restabilization phenomenon).
- Simultaneously, excess polymer increases water viscosity, further hindering settling.
Solution:
- Determine the optimal dosage via jar testing: set up 5-7 concentration gradients from low to high (suggested range 0.5-5 mg/L), observe floc morphology and settling performance.
- The effective dosage is generally the lowest concentration that produces large, dense flocs(near the isoelectric point).
- Tip: Take a suspended water sample and let it sit for 1 hour, then shake it. If the flocs sink after shaking (previously suspended), the issue is gas entrapment; if they remain suspended after shaking, overdosing is highly likely.
Cause 2: Lack of Inorganic Coagulant as a “Skeleton”
Analysis: SMAS/AM is an organic polymer flocculant, whose main mechanism is adsorption bridging. However, polymer-formed flocs are often loose and low in density, prone to suspension. The large number of negatively charged colloidal particles in printing and dyeing wastewater require charge neutralization for destabilization, which is difficult for organic polymers to achieve alone.
Solution:
- Use a combined process: First add an inorganic coagulant (e.g., PAC, polyferric sulfate PFS, aluminum sulfate), then add the SMAS/AM copolymer.
- Best practice: Add the inorganic coagulant with rapid mixing for 1-2 minutes, then add SMAS/AM with slow mixing for 3-5 minutes.
- Reference parameters: PAC dosage ~0.5 g/L, PAM-type dosage ~4 mg/L often yields good results.
- Add coagulant aids such as diatomaceous earth (~1 g/L) or lime to increase floc density and improve settling performance. Diatomaceous earth primarily acts via enmeshment and sweep flocculation.
Cause 3: pH Deviating from Optimal Range
Analysis: The pH of printing and dyeing wastewater fluctuates widely (often between 2-12). SMAS/AM copolymer may undergo conformational changes at extreme pH values, affecting flocculation performance. Inorganic coagulants are even more pH-sensitive:
- Optimal pH for aluminum salts: 6-8
- Optimal pH for iron salts: 8-10
Solution:
- Measure the wastewater pH on-site using a pH meter.
- If pH < 6, add lime or NaOH to adjust to 7-8.
- If pH > 9, add sulfuric or hydrochloric acid to adjust to 8-9.
- Lime serves the dual purpose of pH adjustment and increasing floc density.
Cause 4: Flocs Entrapping Microbubbles
Analysis: Flocs that have entrapped tiny gas bubbles become less dense than water, leading to suspension or flotation. Surfactants present in printing and dyeing wastewater easily generate foam.
Solution:
- Diagnostic method: Take a suspended water sample, let it sit for 1 hour, then shake vigorously. If the flocs sink after shaking, the original flocs had entrapped bubbles.
- Add defoaming measures or a defoamer before the flocculation tank.
- Check if the influent carries significant air bubbles (e.g., pump suction air leaks, drop aeration, etc.).
Cause 5: Improper Mixing Intensity
Analysis: Flocculation involves two key stages:
- Mixing stage: Requires high intensity (high G value) for rapid chemical dispersion.
- Flocculation stage: Requires low-intensity slow mixing to avoid shearing and breaking already-formed flocs.
If mixing is too strong during the flocculation stage, flocs are broken into fine particles, dramatically reducing settling performance.
Solution:
- Check the flocculation tank mixer speed: recommended <50 rpm during flocculation (or velocity gradient G = 20-50 s⁻¹).
- Recommended flocculation reaction time: 5-10 minutes.
- Avoid high-shear zones (e.g., pumping, valve throttling) after flocs have formed.
Cause 6: Low Water Temperature
Analysis: When water temperature drops below 10°C:
- Polymer chain extension and adsorption bridging ability decrease.
- Coagulant hydrolysis reaction rates slow down.
- Water viscosity increases, raising resistance.
Solution:
- Measure the water temperature; if <15°C, consider warming measures.
- Appropriately increase the flocculant dosage (by 20-50%).
III. Systematic Troubleshooting & Optimization Procedure
Follow these steps to quickly locate the problem:
| Step | Operation | Diagnosis/Conclusion |
|---|---|---|
| 1 | Perform a jar test on-site using a water sample, in parallel with field conditions | If jar test settles well but field does not → check field hydraulic conditions (mixing, short-circuiting) |
| 2 | Adjust SMAS/AM dosage with gradient tests | Find the optimal dosage point. If always suspended → consider adding inorganic coagulant |
| 3 | Measure raw water pH and temperature | If deviating from neutral range, adjust first |
| 4 | Check flocculation tank mixer operation | If flocculation stage speed is too high, reduce it |
| 5 | Perform the “shake test” to diagnose bubble issues | If flocs sink after shaking → add defoaming measures |
IV. Recommended Solutions Summary
Prioritize the following according to site conditions and urgency:
Immediate Execution (No additional equipment/chemicals)
- Optimize dosage: Use jar tests to find the optimal SMAS/AM dosage point.
- Check mixing intensity: Reduce flocculation stage speed to below 50 rpm.
- Shake test: Determine if bubble entrapment is the issue.
Short-term Improvements (Add simple measures)
- Combine with inorganic coagulant: Add PAC (0.3-0.8 g/L) first, then SMAS/AM.
- Add coagulant aids: Diatomaceous earth or lime (1-3 g/L).
- Adjust pH: Adjust to the 7-9 range.
Long-term Optimization
- Establish a water quality-dosage log to accumulate treatment parameters for different wastewater batches.
- Consider adding dissolved air flotation (DAF) equipment (if microbubble suspension is the issue), or install a hydrocyclone ahead of the settling tank for forced separation.
V. Comparison with Other Flocculants
| Flocculant Type | Advantages | Disadvantages |
|---|---|---|
| PAC | Good decolorization, strong adsorption | Loose, fragile flocs; difficult settling |
| PFS | Fast settling, dense flocs | High effluent color, corrosive |
| SMAS/AM Copolymer | Good dispersion stability, high salt tolerance | Alone can cause suspension (core issue in this case) |
| PAM | Strong bridging effect, low dosage | Requires combination with inorganic coagulant |
Recommended combination: PAC (~0.5 g/L) + optimized low dosage of SMAS/AM copolymer, plus diatomaceous earth (~1 g/L) as a skeleton if needed.
If the problem persists after these adjustments, please provide the following information for further targeted diagnosis:
- Wastewater pH, temperature, approximate COD
- Current specific SMAS/AM dosage
- Whether any other coagulant is currently being used and its dosage
- Flocculation tank type (mechanical mixing/hydraulic mixing) and retention time
