Overview
Heavy metals dissolved in industrial wastewater pose significant environmental and regulatory risks. Unlike organic pollutants that can be degraded biologically, heavy metals persist in the environment indefinitely. Indonesian PROPER and AMDAL regulations specify strict discharge limits for heavy metals in industrial effluent; exceeding these limits carries substantial regulatory, financial, and reputational consequences.
Chemical precipitation is the standard and most cost-effective treatment technology for heavy metal removal. The dissolved metal ions are reacted with a precipitant chemical to form insoluble metal compounds such as hydroxides, sulfides, or organosulfur complexes, which are then removed from the wastewater by coagulation, flocculation, and sedimentation or filtration. The correct precipitant selection depends on the metals present, the required effluent limits, the wastewater matrix (pH, temperature, competing ions, chelants), and downstream sludge management considerations.
Chelated metals: Some industrial processes (electroplating, metal finishing, printed circuit board manufacturing) use chelating agents (EDTA, citrate, gluconate) that form soluble complexes with heavy metals and prevent them from precipitating at normal hydroxide precipitation pH. For chelated metal streams, conventional lime/NaOH precipitation will not achieve discharge limits. Organic sulfide precipitants (TMT-15, sodium dimethyldithiocarbamate) or advanced oxidation pretreatment to break down chelants is required. Always characterise the wastewater matrix before selecting a precipitant.
Precipitant Types
Applications
| Application | Key Metals | Recommended Treatment |
|---|---|---|
| Refinery wastewater, catalyst fines | Ni, V, Mo (from spent catalyst) | Hydroxide precipitation at pH 9.5–10.5; coagulation with ferric salt for final polishing |
| Petrochemical plant ETP | Cr, Zn, Cu (from heat exchangers, additives) | Cr⁶⁺ reduction with FeSO₄ first; then hydroxide precipitation; SDDC for chelated fractions |
| LNG plant produced water | Hg (from natural gas condensate) | Mercury is present in some Indonesian gas fields; TMT-15 achieves sub-µg/L mercury levels required for AMDAL discharge compliance |
| Mining / mineral processing | Pb, Cd, As, Zn | Hydroxide + sulfide polishing for arsenic; TMT-15 for Pb/Cd if chelants present |
| Metal surface treatment / electroplating | Cr, Ni, Cu, Zn, Cd (chelated) | Cr⁶⁺ reduction; SDDC or TMT-15 for chelated metals; staged precipitation at different pH set points for mixed-metal baths |
| Battery and electronics manufacturing | Ni, Co, Li, Cu | TMT-15 or SDDC preferred for complex metal matrices; hydroxide pre-treatment reduces reagent cost |
Petrochemical & LNG Applications
Heavy metal contamination in wastewater is a relevant and often overlooked challenge at upstream oil & gas, petrochemical, and LNG operations. While these industries are not "metal industries" in the conventional sense, their processes generate metal-contaminated streams through multiple mechanisms.
In refineries, crude oil contains naturally occurring metals particularly vanadium and nickel in heavy crude fractions that concentrate in the catalyst and in certain process streams. The effluent treatment plant receives wastewater containing catalyst fines, spent caustic solutions, and cooling water blowdown that may contain elevated nickel, vanadium, chromium (from stainless steel corrosion), zinc (from galvanised equipment), and copper (from heat exchanger corrosion). Meeting PROPER and AMDAL metal discharge limits requires a dedicated precipitation step in the ETP.
In LNG operations, mercury is a well-known and critical concern. Indonesian natural gas from several fields contains significant dissolved mercury that concentrates in LNG condensate and process condensate streams. Before these condensate streams can be treated and discharged, mercury must be removed to sub-nanogram per litre levels to comply with discharge limits and to protect downstream equipment (LNG heat exchangers, aluminium cold boxes are highly susceptible to liquid metal embrittlement by mercury). TMT-15 precipitation combined with activated carbon adsorption is the standard treatment train for mercury-containing produced water at LNG sites.