Wastewater contains volatile organic compounds (VOCs)

Low-temperature evaporation: the most concealed and easily overlooked points of fugitive emissions

1. Vacuum pump shaft seals and mechanical seals

This is the primary source of concealed leaks.

Organic compounds enter the vacuum pump via non-condensable gases; slight seepage occurs at the static sealing surfaces of shaft seals, causing gases to be drawn inwards under negative pressure and to escape outwards when the system is shut down and under positive pressure. As there is little odour and no visible smoke, yet continuous trace emissions occur, these are difficult to detect during routine inspections.

2. Flange gaskets, threaded joints and valve stuffing boxes in vacuum piping

The entire low-temperature evaporation piping system operates under negative pressure. Flange ageing, gasket deformation and minute gaps in valve stem packing result in:

Internal suction and minor external leakage during operation.

Daily temperature fluctuations cause gaskets to expand and contract, creating permanent micro-gaps.

Organic volatiles slowly diffuse through these gaps, constituting widespread, latent leakage.

3. Sealing surfaces of equipment manholes, sight glasses and access ports

Manhole and sight glass retaining rings and gaskets are subjected to prolonged negative pressure, temperature fluctuations and acid-alkali corrosion.

The gaskets age and harden, resulting in localised leaks where a tight seal cannot be achieved; organic vapours slowly escape through micro-gaps at the edges.

Although the exterior appears intact, there is in fact continuous, unorganised emissions occurring 24 hours a day.

4. Condenser vent / non-condensable gas discharge points (without collection or treatment)

Non-condensable gases contain high concentrations of volatile organic compounds;

many sites simply vent these gases without connecting to a tail gas collection system;

these points are hidden on rooftops or in corners, going unnoticed, and constitute a major source of fugitive emissions.

5. Vacuum-breaking and gas-replenishment ports, and breather valves during shutdown

During shutdown or start-up/shutdown transitions, the system breaks the vacuum to replenish gas,

causing the accumulated organic waste gases inside to be forced out in the opposite direction;

Breathing valves with aged one-way seals allow frequent, minute-scale gas leakage under slight negative pressure, resulting in persistent odour emissions.

6. Waste liquid feed tanks, equalisation tanks and equipment feed interfaces

Organic compounds continuously volatilise within the raw liquid tanks;

Minor leaks at feed pipe joints, tank vent ports and flanges

allow vapours to escape along the feed pathways into the workshop, often mistaken for ordinary workshop odours.

7. Vapour dead zones above demisters and maintenance blind flanges

Blind flanges, plugs and maintenance flanges on demisters at the top of vapour chambers,

which are constantly exposed to a humid environment of organic vapours, causing seals to age easily,

resulting in organic gases slowly seeping out from high-level dead zones and rising to accumulate at the top of the plant.

8. Flanges and sand holes in the casings of circulation and discharge pumps

Minor sand holes in cast pump casings and poor sealing of inlet and outlet flanges

allow organic volatiles to penetrate through micro-pores, dispersing at low levels. As they remain close to the ground, they are difficult to detect and accumulate over time, causing odours.

9. Low-level discharge points in cleaning, drainage and condensate discharge piping

Condensate discharge from vacuum lines, tank drainage and floor drains for cleaning,

where liquid seals are absent or have failed, allowing organic exhaust gases to flow back through the drainage pipes and escape uncontrolled via floor drains and trenches.