The use of polyacrylamide to treat landfill leachate or garbage leachate wastewater is a common physical and chemical treatment method. It is mainly used as a coagulant or flocculant to strengthen the coagulation/flocculation process and remove colloids, suspended solids and some organic matter in the leachate.

The following is key information about using polyacrylamide to treat landfill leachate or garbage leachate wastewater:

Principle of action:

Charge neutralization and bridging:

Landfill leachate contains a large number of negatively charged colloidal particles, humus, microbial fragments, etc.

Polyacrylamide (PAM) is a high molecular polymer with many reactive groups on its molecular chain (depending on the type: anionic, cationic or nonionic).

Cationic PAM: Most commonly used for leachate treatment. The positively charged groups on its molecular chain can effectively neutralize negatively charged colloidal particles, reduce their Zeta potential, destroy the stability of the colloidal particles, and destabilize them.

Bridging effect: The long molecular chains of PAM can "bridge" between multiple destabilized colloidal particles or tiny flocs, connecting them together to form larger, denser, and easier to settle or float flocs (alum flowers).

Net catching and sweeping function: In the process of forming large flocs, it will wrap and sweep up small suspended solids and partially dissolved substances in the water.

Main application purposes:

Strengthen solid-liquid separation: Significantly improve the removal efficiency of suspended solids and colloids by coagulation sedimentation or air flotation process, and reduce turbidity and color.

Removal of part of organic matter: Remove organic matter that is combined with colloids, suspended matter or can be adsorbed by flocs, and reduces COD (mainly colloidal and insoluble COD).

Improve sludge dehydration performance: In the subsequent sludge treatment stage, PAM is also often used for sludge conditioning to improve sludge dehydration efficiency (lower moisture content).

As pretreatment: It is usually the pretreatment or biochemical post-treatment link in the entire leachate treatment process (such as: pretreatment + biochemical treatment + advanced treatment), aiming to reduce the load of subsequent treatment units (especially membrane treatment or biological treatment), prevent clogging, and improve overall treatment efficiency.

PAM type selection:

Cationic PAM: The most commonly used type for treating landfill leachate. Because colloidal particles in leachate are usually negatively charged, cationic PAM can effectively neutralize their charges and exert a strong bridging effect. The ionicity (charge density) needs to be considered when selecting. The more complex the leachate water quality and the higher the colloid concentration, the more cationic PAM with higher ionicity is usually required.

Anionic PAM: It may be used when the leachate mainly contains positively charged suspended solids (such as certain metal hydroxides), or it may be added after the positively charged floc formed by inorganic coagulants (such as PAC, PFS) to use its long chains for bridging. It is rarely used alone in leachate.

Non-ionic PAM: Insensitive to pH and salinity changes, but has weak charge neutralization ability. It is suitable for leachates with complex water quality conditions, large pH fluctuations or high salinity, or when the surface charge of the particles is close to neutral. There are relatively few applications.

Key: Laboratory tests or on-site pilot tests must be conducted to determine the most suitable PAM type, ionicity and optimal dosage. The composition of leachate is complex and variable (affected by landfill age, season, rainfall, etc.), and there is no universal choice.

Application notes:

Use with inorganic coagulants: PAM is usually not used alone, but in conjunction with inorganic coagulants (such as polyaluminum chloride, polyferric sulfate, ferrous sulfate, etc.). Generally, inorganic coagulant is added first for charge neutralization and preliminary flocculation, and then PAM is added for bridging to form large and dense alum flowers. The order and interval of dosing are important.

Dissolution and preparation: PAM is a solid particle or powder, which must be dissolved into a solution of a certain concentration (usually 0.1%-0.5%) before being added. The dissolution process requires slow and even stirring to avoid clumping ("fish eyes"). Insufficient dissolution will seriously affect the effect. Dissolved water should be clean tap water or recycled water.

Dosing point and mixing: The dosing point should be selected after the coagulation reaction is sufficient and before entering the sedimentation tank or flotation tank. Appropriate mixing conditions (such as pipe mixers, mechanical stirring) are required to quickly and evenly disperse the PAM solution, but without excessive shearing to destroy the formed flocs.

Optimization of dosage: Too little will not achieve the effect; too much is not only wasteful, but may also lead to colloid re-stabilization (charge reversal) or floc dispersion, making the effluent more turbid. The cost-effective dosage must be determined through experiments.

Water quality adaptability: pH value, temperature, salinity, organic matter type, etc. will all affect the effect of PAM. Leachate water quality fluctuates greatly and requires regular monitoring and adjustment.

Secondary pollution considerations: PAM itself has low toxicity, but its monomer acrylamide is neurotoxic. Products with low monomer residues that meet standards (such as drinking water grade or environmental certification) should be selected. The treated sludge contains PAM, and its final disposal (landfill, incineration, agricultural use, etc.) needs to consider the relevant environmental impacts.

Effects and limitations:

Advantages: Relatively simple operation; can quickly and effectively remove SS, turbidity, color and part of COD; improve sludge settling and dehydration performance; operating costs are relatively controllable (as pretreatment).

limitation:

It mainly removes colloidal and suspended pollutants, and has limited removal effect on dissolved small molecular organic matter, ammonia nitrogen, and heavy metal ions (unless precipitates are formed and adsorbed by flocs).

It cannot achieve complete purification and standard discharge of leachate, and must be used in conjunction with other processes (such as anaerobic/aerobic biological treatment, membrane separation, advanced oxidation, etc.).

A large amount of chemical sludge is produced, which increases the difficulty and cost of sludge treatment and disposal.

The consumption and effect of chemicals are greatly affected by fluctuations in water quality.

Summarize:

Polyacrylamide (especially cationic type) is an important coagulation aid/flocculant when treating landfill leachate. It has significant effects in strengthening coagulation sedimentation/air flotation, removing colloids and suspended solids, and improving sludge performance. But it is not an independent treatment technology, but serves as a pretreatment or intermediate treatment unit to serve the entire leachate treatment system. The key to its successful application lies in selecting the appropriate PAM type and ionicity, accurately controlling the dosage, optimizing the dosage point and mixing conditions, and effectively synergizing with inorganic coagulants and other treatment processes. In practical applications, rigorous experiments must be conducted to determine the optimal process parameters and pay attention to the sludge treatment problems they generate.