Electroplating wastewater is generated intermittently, and wastewater properties vary greatly across different production stages, including pre-cleaning wastewater, plating rinse water and concentrated plating spent liquor. High concentrations of heavy metals exist in most of these streams, while wastewater discharged from post-plating treatment generally contains relatively low heavy metal levels.

In routine electroplating wastewater treatment, all wastewater streams from different processes are mixed before subsequent treatment units. Since each process differs in water output and discharge timing, the mixed composite water fails to reflect actual process water quality characteristics accurately. This leads to a key problem: Online Water Quality Monitoring data cannot truly reflect the real heavy metal pollution status. One typical phenomenon is that the heavy metal concentration in treated effluent may appear higher than that of raw wastewater, especially for metals with low consumption volume.

In addition, electroplating sludge carries a certain quantity of heavy metals. For facilities equipped with biochemical treatment units, partial heavy metals adsorbed in sludge may be released during biochemical reactions, resulting in elevated heavy metal concentrations in biochemically treated effluent.

Recommended Monitoring & Sampling Practices for Operators

Sampling shall be carried out according to the full production cycle of electroplating lines. The **flow-proportional composite sampling method** is recommended: collect and mix samples from each process stream in proportion to their respective wastewater output volumes for integrated testing.

For wastewater systems with biochemical treatment facilities, the same proportional sampling method shall be adopted to collect samples at three sampling points: raw untreated wastewater, effluent from heavy metal removal units, and final biochemical effluent. This sampling scheme can fully reflect the overall heavy metal discharge profile and track heavy metal concentration changes at each treatment stage.

Adopting the above sampling approach enables monitoring data to objectively reflect the generation and removal of heavy metals, supporting reasonable evaluation of heavy metal concentration levels in electroplating wastewater. Meanwhile, appropriate Online Water Quality Heavy Metal Analyzers shall be selectively deployed to match actual on-site testing requirements during heavy metal detection.