Long-Term Adverse Effects of Directly Recirculating Acid and Alkaline Cleaning Waste Liquids Back into the Stock Solution Tank

I. Severe and Repeated pH Fluctuations Disrupt Stable Evaporation Conditions

Acid cleaning waste liquids are strongly acidic, while alkaline cleaning waste liquids are highly alkaline; when mixed with the stock solution, the overall pH shifts significantly:

Acidic Recirculation: Inhibits flocculation and the precipitation of dye/heavy metal hydroxides, causing metal ions to remain in solution. During evaporation, this leads to the easy formation of metal salt scale, accelerating fouling on heat exchange surfaces; simultaneously, it significantly reduces the system’s ability to suppress foaming through salt precipitation, resulting in excessive foaming and increased carryover of mist and foam.

Alkaline reflux: Calcium and magnesium ions rapidly form carbonate and hydroxide scale, causing frequent blockages in plates and nozzles, and doubling the frequency of fouling and pipe blockages.

While the pH of the feed solution remains relatively stable during routine production, each cleaning and reflux cycle creates severe acid-base shocks. This causes continuous fluctuations in vacuum and temperature control, resulting in erratic evaporation rates and persistently rising energy consumption.

II. Chemical Residues Continuously Alter Material Foaming and Adhesion Characteristics

Cleaning agents contain corrosion inhibitors, penetrants, surfactants, and chelating agents, which accumulate over time in the feed solution tank:

Surfactants continuously accumulate, causing foaming during evaporation to become the norm; foam is carried into vacuum lines and condensers, and salts adhere to heat exchange surfaces to form composite scale.

Long-term accumulation of chelating agents (such as EDTA and organophosphates) complexes calcium, magnesium, iron, and aluminum ions, causing all originally settleable impurities to become stably suspended. These cannot be precipitated and removed, entering the evaporator where they continuously adhere to the walls and clog spray nozzles.

Corrosion-inhibiting polymeric agents have an organic colloidal base; when concentrated, they form viscous sludge, exacerbating residue adhesion on drum walls and pipes, and significantly shortening cleaning cycles.

III. Graded Intensification of Equipment Corrosion Risks

The dry-wet interfaces of tanks, circulation piping, and heat exchanger cylinders frequently undergo alternating acid-base cycles. The passivation film on stainless steel is repeatedly damaged and repaired, leading to a significant increase in the rates of pitting and crevice corrosion, and making welds and flanges prone to leakage.

PTFE-lined equipment: Alternating acid and alkali penetrate the lining gaps; salts and chemicals accumulate in the interlayer, causing the lining to blister and delaminate under thermal cycling; carbon steel supports and outer casings accelerate rusting when exposed to acid mist and leaks.

Pumps, valves, and mechanical seals are frequently subjected to acid-alkali impact, leading to aging and cracking of sealing components, and an increase in shaft seal leakage failures.

IV. Imbalance in Salt Solubility, Irreversible and Worsening Scaling and Blockage

Acid washing introduces H⁺, dissolving existing scale on pipe walls and returning large amounts of calcium and magnesium ions to the original solution; subsequent normal evaporation causes them to precipitate again, creating a vicious cycle of “dissolution—re-entry—re-scaling,” resulting in increasingly thick scale layers.

Alkaline washing introduces OH⁻ and carbonate ions, which immediately react with hardness ions in the original solution to form microcrystalline precipitates. These fine suspended particles remain in suspension throughout the process, clogging nozzles and the narrow flow channels of plate-type condensers, rendering them completely unpassable even with water washing.

V. Permanent Deterioration of Condensate Water Quality, Limiting Reuse

Chemicals, metal ions, and organic additives enter the condensate water via secondary steam:

Condensate conductivity and COD continue to rise, containing trace acids, alkalis, and heavy metals, making it unsuitable for reuse in upstream production; discharging it increases treatment costs.

When condensate is mixed back into the feedwater, it further dilutes the salinity and introduces foaming agents, creating a secondary cycle of deterioration.

VI. Secondary Burden on the Upstream Biological Treatment System (if the feedwater originates from pre-treatment biological processes)

After recirculation of wash wastewater:

Acid-base shocks affect microbial activity in the biological treatment tank, leading to sludge disintegration and deteriorated effluent quality, while introducing more suspended solids and extracellular mucus into the evaporator.

Chelating agents and corrosion inhibitors are toxic to microorganisms; long-term accumulation inhibits degradation, resulting in persistently high COD levels in the wastewater and increased organic sludge in the evaporator.

VII. Shortened lifespan of automated control instruments and persistent measurement drift

pH electrodes and conductivity/level probes are repeatedly exposed to high-concentration acid and alkali chemicals, leading to corrosion of electrode coatings and the accumulation of salt deposits mixed with chemical colloids. This results in frequent signal drift, rapid calibration failure, and disruption of level, temperature, and vacuum interlock controls.