Discharge Standards for Port Tank Washing Wastewater

Tank washing wastewater is generated when a ship cleans its cargo tanks after unloading, typically to prepare for a different cargo or for maintenance. This wastewater may contain oil residues or residues of noxious liquid substances. Due to its high pollutant concentration and complex composition, its discharge is subject to strict controls under both Chinese national regulations and international conventions.

The Core Chinese Standard: GB 3552-2018

The primary regulatory framework for tank washing wastewater in China is the Discharge Standard for Water Pollutants from Ships (GB 3552-2018), which applies to all Chinese‑flagged vessels and foreign‑flagged vessels operating in Chinese waters. The standard adopts a clear classification: wash water containing cargo residues is defined as ship garbage. This classification is critical because it means such wastewater is subject to the discharge prohibitions applicable to garbage, rather than being treated as ordinary wastewater.

For oily tank washing wastewater — generated from cleaning crude oil or fuel oil tanks — the standard sets maximum permissible oil content limits. Within inland rivers and waters within 3 nautical miles of the nearest land, such wastewater must not be discharged directly into the water; instead, it must be collected and delivered to shore reception facilities, or treated on‑board by an oil‑water separator and discharged only while the vessel is underway.

Discharge of Noxious Liquid Substance Washings

For ships carrying bulk liquid dangerous goods, tank washing must be carried out at designated locations with adequate reception facilities. The discharge of wash water containing noxious liquid substances must simultaneously satisfy several conditions:

The discharge must take place at least 12 nautical miles from the nearest land and in waters at least 25 metres deep.

The discharge must occur while the ship is underway — for self‑propelled vessels, at a speed of no less than 7 knots; for non‑self‑propelled vessels, no less than 4 knots.

The discharge must be made through an underwater outlet below the waterline, at a rate not exceeding the maximum design discharge rate.

These requirements, which align with MARPOL Annex II, apply to Category Y and Z substances under the International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk. For Category X substances (the most hazardous category), a mandatory pre‑wash is required before the ship leaves the unloading port. The pre‑wash water must be discharged to a reception facility until sampling analysis confirms that the residue concentration falls below 0.1 %. Only after this threshold is met may water subsequently introduced into the tank be discharged to sea.

Special Provisions for Inland Waters

Inland waters are subject to even stricter controls. Wash water containing cargo residues is prohibited from being discharged into inland water bodies. Instead, it must be delivered to port reception facilities or authorised waste reception units. Any tank washing or cleaning operations within port waters require prior permission from the maritime authority. Ships that fail to report such operations to the maritime authority within 24 hours of each reception or transfer may be held liable for non‑compliance.