Key to Reuse-Water Treatment: Why PAC/PAM Chemical Processes Should Be Avoided for Oil-Water Separation, and Pure-Physical Processes Preferred

Key to Reuse-Water Treatment: Why PAC/PAM Chemical Processes Should Be Used with Caution for Oil-Water Separation, and Pure-Physical Processes Are Preferred

In industrial wastewater-reuse systems, TDS (Total Dissolved Solids) is a measure of reuse-water quality,core key indicatorthat directly determines the operational stability, water-production efficiency, and equipment service life of subsequent deep-reuse processes such as reverse osmosis, ultrafiltration, and EDI. Compared with ordinary compliant-discharge wastewater, reuse water has strict low-limit requirements for TDS; any slight increase in salinity brings chain negative impacts to the entire reuse system. In the oil-water separation step of oily-wastewater pretreatment, the traditional PAC+PAM chemical coagulation process is one of the main causes of abnormal TDS rise in the effluent and a constraint on downstream reuse; therefore, for oil-water separation in reuse scenarios, pure-physical processes are the optimal choice.

I. First Understand: Why Does Reuse Water Strictly Control TDS?

TDS refers to the total content of all dissolved inorganic salts and a small amount of organic matter in water, intuitively reflecting the salinity level of the water body. The control logic for TDS in industrial reuse (circulating cooling-water reuse, production-process-water reuse, advanced reclaimed-water reuse, etc.) is very clear:The lower the TDS, the higher the reuse-water quality and the smaller the load on downstream membrane-treatment equipment。

If the effluent TDS after oil-water separation pretreatment is high, the subsequent deep-reuse process will face multiple difficulties: first, the osmotic pressure of the reverse-osmosis membrane rises sharply, equipment energy consumption soars, and water production drops; second, high-salinity water easily aggravates membrane-element scaling, fouling, and corrosion, greatly shortening the membrane-module replacement cycle and increasing O&M costs; third, high-TDS reuse water cannot meet the water standards of precision production and low-scaling circulating water, directly reducing the reuse rate and causing serious water-resource waste. It can be said that the stable operation of the reuse system has as its core premisestrict control of TDS increment from the pretreatment source。

II. PAC+PAM Coagulation Process: The “Hidden Increment Source” of TDS in Oil-Water Separation

PAC (polyaluminum chloride) and PAM (polyacrylamide) are commonly used coagulants in wastewater treatment. With their good flocculation-sedimentation effect and fast settling speed, they are widely used for removing suspended solids and floating oil from conventional oily wastewater. But this typical chemical-treatment process inherently hassalinity superposition and TDS risedefects, making it completely unsuitable for reuse-type water-treatment scenarios.

1. PAC directly introduces large amounts of soluble salts, raising the baseline TDS

PAC is an inorganic polymer coagulant whose core component is an aluminum-salt compound. After dissolving in water it undergoes hydrolysis, releasing large amounts of soluble inorganic salt ions such as aluminum ions, chloride ions, and hydroxy complexes. These ions cannot be removed by sedimentation or filtration and remain permanently in the water body, directly adding to the water-body TDS value.

In actual engineering, to ensure oil-water separation and suspended-solids removal, PAC must be continuously dosed at a fixed rate; even if the agent fully reacts, the inorganic-salt residues from hydrolysis will steadily increase water salinity; if the dosage is improperly controlled or the agent does not fully react, residual PAC stock solution will further aggravate TDS rise—every dosing is a continuous, cumulative addition to the water-body salinity.

2. PAM assists solubilization, indirectly aggravating TDS fluctuation

The accompanying PAM organic polymer flocculant, although it does not directly produce large amounts of inorganic salts, forms trace soluble organic colloids after dissolving and simultaneously changes the water-body ion balance, assisting in increasing the total dissolved-matter content. More importantly, PAM must be used with PAC; the chemical process's “dosing system” itself is an external-substance input that breaks the original water's baseline water-quality balance, so the effluent inevitably has a net TDS increment compared with the raw water.

3. The fatal impact of chemical-process TDS rise on downstream reuse

Ordinary compliant discharge does not require strict TDS control, so the drawbacks of the PAC+PAM process can be ignored; but reuse systems have extremely high requirements for zero or low TDS increment. The TDS increase generated in the pretreatment stage is entirely passed on to the downstream deep-treatment process: originally low-TDS raw water could easily achieve high-rate reuse through reverse osmosis, but after chemical coagulation treatment, the high-salinity influent doubles the membrane-system load, requiring frequent cleaning and maintenance and greatly reducing the qualified rate of reuse water production; in severe cases it directly causes the entire reuse system to fail to meet standards.

III. Pure-Physical Oil-Water Separation Process: Eliminating TDS Rise from the Source and Adapting to Reuse Needs

The pure-physical oil-water separation process is based ongravity separation, coalescing adsorption, mechanical interception, and flotation separationand other physical principles as its core, adds no chemical agents throughout, has no external ion or salt input, perfectly solves the pain point of TDS rise in chemical processes, and is the suitable process for reuse-water pretreatment.

1. Core advantage: zero TDS increment, preserving the raw-water quality baseline

The pure-physical process separates floating oil, emulsified oil, and suspended impurities from the water solely through physical action throughout, without changing the original ion composition and salinity of the water body; the effluent TDS is basically equal to the influent, with no secondary pollution or salt superposition, holding the core low-TDS requirement of reuse water from the pretreatment source and providing stable, low-load influent conditions for downstream membrane-reuse processes.

2. Stable treatment effect, adapting to various oily reuse-water scenarios

Mature pure-physical oil-water separation equipment can efficiently remove floating oil, dispersed oil, and most emulsified oil from water; the effluent oil content and suspended-solids indicators fully meet reuse-pretreatment standards. Compared with chemical processes that are greatly affected by dosage, water-quality fluctuation, and pH value, the physical process operates stably, has strong shock resistance, and will not experience water-quality non-compliance or abnormal TDS fluctuation due to working-condition changes.

3. Cost reduction and efficiency improvement, simplifying downstream O&M

The pure-physical process requires no purchase of PAC or PAM agents, greatly reducing chemical-consumable costs; at the same time, with no agent residue and no salinity accumulation, it effectively reduces the probability of scaling and fouling of downstream membrane equipment, extends membrane-module service life, and lowers equipment cleaning, replacement, and O&M costs; its long-term economic performance is far superior to the chemical coagulation process.

IV. Process-Selection Summary: Prioritize Pure Physics for Reuse Scenarios; Chemical Processes Limited to Discharge Scenarios

The PAC+PAM chemical-coagulation oil-water separation process is suitable forwastewater directly discharged in compliance, no reuse needed, and no TDS-control requirementscenarios, with the advantages of fast treatment speed and low equipment cost; but inindustrial wastewater reuse, advanced reclaimed-water treatment, and circulating-water reuseand other TDS-sensitive scenarios, the TDS-increment drawback of this process is infinitely amplified, severely restricting the operation of the reuse system.

Therefore, for all oily-wastewater treatment projects involving downstream reuse, process selection must adhere to the core principle:give priority to pure-physical oil-water separation processes to completely avoid the TDS-rise problem brought by chemical agents, safeguard reuse-water quality from the source, and improve the water-resource reuse rate and systemoperational stability。