Reuse Water Starts at Pretreatment: The Hidden TDS Cost of PAC and PAM Dosing

In industrial wastewater-reuse systems, TDS (Total Dissolved Solids) is the indicator that quietly governs everything downstream. It 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 salinity, and a small increase propagates through the entire reuse chain.

Why Reuse Water Is Sensitive to TDS

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

When pretreatment lets TDS climb, three things happen in sequence. Osmotic pressure across the reverse-osmosis membrane rises sharply, equipment energy consumption soars and water production drops. High-salinity water aggravates membrane-element scaling, fouling and corrosion, greatly shortening the membrane-module replacement cycle and increasing O&M costs. And high-TDS reuse water cannot meet the water standards of precision production and low-scaling circulating water, so the reuse rate drops and water is wasted. Stable reuse operation depends on strict control of TDS increment from the pretreatment source.

How PAC Adds Salinity

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

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

How PAM Compounds It

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

For ordinary compliant discharge, TDS is not strictly controlled and the drawback can be ignored. Reuse systems require zero or very low TDS increment. Whatever the pretreatment stage adds passes entirely to the deep-treatment process: raw water that could have achieved high-rate reuse through reverse osmosis now doubles the membrane-system load, needs frequent cleaning and maintenance, and misses specification often enough to fail the reuse scheme altogether.

What a Pure-Physical Route Does Differently

The pure-physical oil-water separation process is built on gravity separation, coalescing adsorption, mechanical interception and flotation separation. No chemical agents are added throughout, so there is no external ion or salt input.

• Zero TDS increment. Floating oil, emulsified oil and suspended impurities are separated by physical action alone, without changing the original ion composition or salinity. Effluent TDS is basically equal to influent, with no secondary pollution and no salt superposition, and the low-TDS requirement of reuse water is held from the pretreatment source.
• Stable treatment effect. Mature pure-physical oil-water separation equipment efficiently removes floating oil, dispersed oil and most emulsified oil, and effluent oil content and suspended-solids indicators meet reuse-pretreatment standards. Compared with chemical processes that are heavily affected by dosage, water-quality fluctuation and pH value, the physical route operates stably and has strong shock resistance.
• Cost reduction and efficiency improvement. No PAC or PAM purchase, so chemical-consumable cost drops sharply. With no agent residue and no salinity accumulation, the probability of scaling and fouling in downstream membrane equipment falls, membrane-module service life extends, and cleaning, replacement and O&M costs decline. Long-term economics are far better than the chemical coagulation route.

What to Measure Before Choosing

The comparison only works with real numbers, so start by measuring raw-water TDS and the TDS of water leaving the existing oil-water separation step on the same day. The gap is the increment the current process is already adding, and it is usually larger than operators expect. Set that figure against the TDS window the downstream membrane supplier specifies; if the increment alone consumes most of the allowance, no amount of extra membrane cleaning will recover the design recovery rate. Where a chemical route is already installed and cannot be removed immediately, the practical first step is to cut PAC dose to the minimum that still holds oil removal, then re-measure.

Selection Rule

The PAC+PAM chemical-coagulation oil-water separation process suits wastewater that is directly discharged in compliance, needs no reuse and has no TDS-control requirement; there it offers fast treatment and low equipment cost. In industrial wastewater reuse, advanced reclaimed-water treatment and circulating-water reuse, the TDS-increment drawback is amplified until it restricts the operation of the reuse system.

So for every oily-wastewater project involving downstream reuse, process selection should follow one principle: give priority to pure-physical oil-water separation processes, completely avoid the TDS-rise problem that chemical agents bring, safeguard reuse-water quality from the source, and improve both the water-resource reuse rate and system operational stability.