The condensate storage tank is not fitted with a breather valve. When the liquid level in the tank drops during discharge, a negative pressure builds up inside the tank, causing outside air to be drawn in through various gaps in the tank body, manholes, flanges, access hatches and pipe connections. As a result, various substances from the external environment are drawn into the tank, contaminating the evaporative condensate, which is originally of high purity. Firstly, there are solid particulates from the environment: dust in the air, dust stirred up from the floor, rust particles, silt and sand from the floor around the tank, and rust and scale flakes falling from the roof and steel structures. The suction force generated by the negative pressure draws these fine solid particles directly into the tank; once they enter the condensate, they cause an increase in suspended solids. If the equipment is located within a workshop, process dust generated by grinding and material feeding operations within the workshop will also be drawn in, and these suspended impurities will directly increase the turbidity of the condensate. Secondly, there are gaseous and volatile pollutants. Volatile acid and alkali mists in the workshop air, volatile organic compounds (VOCs) emanating from nearby wastewater ponds, and volatile components from various additives will enter the tank via the back-draft airflow and dissolve into the condensate. This can cause abnormal fluctuations in the condensate’s pH and an abnormal rise in COD. If the site includes electroplating or acid washing processes, trace amounts of metal aerosols may also be drawn in, leading to heavy metal ion contamination. Microorganisms constitute another major category; negative-pressure suction draws bacteria and mould spores from the air into the storage tank. Although the condensate is at a moderate temperature and contains few nutrients, prolonged static storage allows microorganisms to proliferate within the tank, forming a biofilm on the tank walls. This results in turbid condensate, the growth of flocculent biological sludge, and effluent bacterial counts exceeding regulatory limits. This problem becomes even more pronounced during the rainy season when humidity is high. Another risk that is easily overlooked on site is the risk of backflow from wastewater and standing water on the floor. When the negative pressure suction is strong and the storage tank inlets and vent pipes are close to the ground, wastewater spilled on the floor or standing water from floor cleaning may be entrained by the airflow and drawn into the storage tank. This is particularly true of workshop rinse water, which has a complex composition; if backflow occurs, the quality of the condensate will deteriorate immediately. Furthermore, dirt accumulated on tank flanges and manhole seals, as well as dust and grime lodged in gaps around the tank openings, can be dislodged and blown into the tank when high-velocity negative-pressure airflow passes through these gaps. Even if the interior of the tank itself is clean, contaminants accumulated in the sealing gaps will be carried into the water by the back-suction airflow. Characteristics of the operating conditions: During the tank filling phase, as the liquid level rises and the internal pressure increases, gas is forced outwards, preventing the suction of impurities; it is only during discharge—when the continuous inflow rate of condensate is lower than the outflow rate—that negative pressure forms inside the tank, leading to back-suction contamination. This is the root cause of the phenomenon observed at many sites where the quality of the condensate is initially acceptable but deteriorates as the tank liquid level continues to drop. This situation is prone to misinterpretation, as it is often mistakenly attributed to the evaporator unit carrying material that causes condensate contamination; in reality, the condensate produced by the evaporator itself meets specifications, and the contamination occurs in the downstream storage tank stage. The corresponding solution is that the storage tank must be fitted with a breather valve, preferably equipped with a dust-filtering component to filter dust and some aerosols from the air; the vent pipe outlet should be raised well above any standing water on the ground, and fitted with a structure to prevent the ingress of rainwater and debris; if the condensate is to be reused and high standards are required, a venting device with sterilising filtration can be selected; efforts should be made to avoid the storage tank remaining at a low liquid level for prolonged periods during discharge, thereby reducing the frequent generation of negative pressure within the tank. Relying solely on the tank’s seal is not a viable solution, as changes in the liquid level of a completely sealed tank will inevitably generate positive and negative pressures, and trace amounts of back-suction will still occur at the seams.