Returning the waste liquid from the acid wash directly to the feed tank does indeed alter the foaming characteristics of the material, significantly exacerbating foam entrainment and material carryover during the evaporation process. This is a hidden contributing factor that is easily overlooked on-site. Often, when the unit is restarted after an acid wash, foam suddenly increases, the demister load rises sharply, and the conductivity of the condensate water rises abnormally—yet an investigation of the feed composition reveals no changes. The root cause lies in the reintroduction of the acid wash waste liquid. First, consider the changes in pH and ionic composition. Inorganic acids—commonly nitric acid, citric acid, and aminosulfonic acid—are typically used for evaporator acid washing. In addition to residual acid, the acid washing waste liquid contains large amounts of dissolved scale, including silicates, calcium and magnesium ions, metal ions, and complexed cations generated during the acid washing process. When the waste liquid is returned to the feed tank, it directly lowers the overall pH of the feed solution. Many wastewater systems inherently contain weakly alkaline buffering components; a slight decrease in pH may not immediately result in visible foaming within the tank. However, under boiling conditions in the vacuum evaporation chamber, the foaming critical point shifts. In a slightly acidic environment, the surface tension of certain organic additives and residual surfactants changes, increasing the toughness of the foam film. The foam layer becomes denser and more stable, making it less likely to collapse on its own. Under the same feed rate and vacuum temperature conditions, the foam height inside the chamber will rise significantly, making it highly likely to exceed the liquid level boundary and be carried away by the vacuum airflow, thereby exacerbating carryover. Secondly, metal ions leached during acid washing act as foaming catalysts. After the scale layer dissolves, ions such as iron and aluminum enter the feed solution. Under boiling evaporation conditions, these form hydroxide colloids, which act as foam stabilizers, reinforcing the foam film and inhibiting foam collapse. Even if the original solution has weak foaming properties, the addition of these colloidal particles significantly extends the foam’s lifespan. Unlike foaming caused by ordinary surfactants, colloid-induced foam is fine, dense, and viscous. Simply adding defoamers becomes less effective in this case, and the amount of defoamer required increases significantly. Additionally, there are minute solid particles stripped off during acid washing. The acid washing process detaches and breaks down old scale layers on the surfaces of plates and chamber walls, producing a large amount of microscopic insoluble debris and silicoaluminate particles, which return to the feed tank along with the wash wastewater. These fine solid particles adsorb onto the foam liquid film, acting as solid-phase foam stabilizers that make the foam more resilient and less prone to collapse. Once the mixture enters the evaporation chamber and begins boiling, the foam layer becomes thick; even with a fully functional demister, the amount of entrained mist and foam will increase. It is also important to distinguish between two operational scenarios: if the acid washing waste liquid is returned to the tank without being fully neutralized, the short-term drop in pH will be significant, and the foam outbreak upon startup will be very noticeable; if it has undergone simple neutralization, the pH may appear to have returned to the original range to the naked eye, but the metal ions, colloids, and fine scale residues carried by the waste liquid still remain. This constitutes a latent change that does not cause immediate, violent foaming; instead, it manifests as intermittent increases in foam, accelerated clogging of the demister, intermittent deterioration in condensate water quality, and intermittent malfunctions, making troubleshooting more difficult. Additionally, there are secondary chain reactions: acid washing waste liquid introduces large amounts of calcium, magnesium, and silicon compounds. When recirculated into the feed solution, these raise the feed’s hardness and silicon content, causing the risk of scaling to rise again during the concentration stage. The repeated formation and peeling of scale layers continuously supply solid-phase particles, further intensifying the foaming tendency and creating a vicious cycle. At the same time, metal ions introduced by the acid washing process can alter conductivity readings, interfering with the automatic control logic for concentration and discharge. Many on-site practices, in an effort to conserve water, recycle all acid washing water directly into the feed tank without diverting it for separate treatment. While no obvious harm is apparent in the short term, after multiple acid washing cycles, the accumulated metal ions and colloids in the tank gradually build up, leading to increasingly frequent carryover issues. Accordingly, the recommended operational control approach is that, in principle, acid washing waste liquid should not be returned directly to the feed tank. Prioritize the separate collection of acid washing waste liquid for dedicated pretreatment; after neutralization and precipitation, determine whether it can be introduced into the system. If recovery is indeed necessary, strictly prohibit the return of high-acid waste liquid from the main acid washing reaction stage. Only limited recovery of low-acid rinse water from the later rinsing stage is permitted, and it must be fully neutralized. This must be accompanied by a precipitation process to separate and settle the leached metal hydroxides and suspended sludge before being fed into the feed tank. Before restarting the system after acid washing, drain as much residual cleaning solution from the chamber as possible; do not allow it to enter the evaporation process directly. During the production cycle following acid washing, closely monitor the foam conditions inside the chamber. Temporarily increase the defoamer dosage as a transitional measure and observe changes in condensate water quality to prevent continuous material leakage from contaminating downstream systems. Today at 16:59