Condensate storage tanks are not equipped with breather valves. When the liquid level inside the tank drops and a negative pressure builds up in the tank cavity, outside air is drawn in through various gaps in the tank body—such as manholes, flange gaps, access ports, and drain ports—carrying various impurities with the airflow and directly contaminating the originally clean condensate. Sources of contamination include solid particulates, gaseous pollutants, microorganisms, and backflow of ground wastewater.

Dust, silt, and rust particles from the external environment are directly drawn into the tank by the negative-pressure airflow. Suspended rust in the workshop air, metal particles generated by equipment wear, dust falling from roofs and walls, and sand and gravel kicked up from the ground are carried by the airflow into the water, causing an increase in the suspended solids level of the condensate. If the storage tank is located outdoors, it may also draw in debris such as willow catkins, insects, and fallen leaves, which settle at the bottom of the tank to form a layer of sludge.

Gaseous and aerosol substances in the air are also drawn into the tank through backflow. Acid and alkali mists volatilizing around the workshop, volatile gases from chemicals, as well as small amounts of salt mist and organic exhaust gases escaping from low-temperature evaporation workshops are drawn in under negative pressure and dissolved into the condensate, altering its pH and conductivity. Even if the evaporation system itself achieves satisfactory separation efficiency, the salt mist drawn into the tank via backflow can cause the condensate’s conductivity to rise inexplicably. Many sites mistakenly attribute this to failed demisting in the evaporator, when the actual problem lies in contamination from the tank’s air intake.

Large quantities of microbial spores, mold, and bacteria carried by the air enter the tank. Since the condensate temperature is mild, creating a favorable aquatic environment, microorganisms proliferate extensively within the tank. After prolonged storage, a slimy biofilm forms on the tank walls and bottom, causing the water to become cloudy and develop a slight odor. The COD and microbial levels in the effluent exceed standards, affecting its subsequent reuse. This microbial growth accelerates significantly, especially during the rainy season when humidity is high.

Another risk that is often overlooked is the backflow of ground wastewater. Many storage tank overflow and drain pipes connect directly to ground-level drainage ditches. When a strong negative pressure forms inside the tank, wastewater from the drainage ditches and surface runoff can be sucked back into the tank through gaps in the piping. This wastewater has a complex composition, containing salts, chemicals, and sludge. Once backflow occurs, the quality of the condensate water will deteriorate suddenly, representing one of the most hazardous types of contamination.

Contamination caused by backflow has a distinctive characteristic: it is not always immediately apparent and primarily occurs when condensate is pumped out for discharge. When the liquid level remains stable and the pressure inside the tank is balanced, no air intake occurs; however, once the discharge pump is activated to pump water out, negative pressure builds up inside the tank, and air intake contamination begins. In many on-site inspections, condensate fresh from the evaporator meets all quality standards, but the water quality at the tank outlet deteriorates—the root cause being the lack of a breathing protection system.

It is important to note that the tank must not be left completely open to the atmosphere, as this would result in continuous exposure to contaminated air. The correct approach is to install a breather valve equipped with a dust filter at the air intake; in humid conditions, a condensation-resistant breather valve should be selected. Additionally, air barriers must be installed on overflow pipes and drain lines to prevent negative pressure backflow of wastewater and block the back-sucking of various impurities into the storage tank.