Phenolic compounds (phenols)

Phenolic compounds are a family of organic chemicals based on phenol (C₆H₅OH) — an aromatic ring bearing a hydroxyl group — and its derivatives. They are toxic to aquatic life, impart taste and odour to water at extremely low concentrations, and resist easy biodegradation. Because even trace amounts ruin drinking-water taste (especially after chlorination, which forms foul-tasting chlorophenols), the effluent limit is very tight: 1.0 mg/L for inland surface water.

Phenols arise from petroleum refining, coke ovens and coal processing, pulp and paper manufacture, resin and chemical production, and the thermal breakdown of organic matter. This last route is the key one for recyclers: pyrolysis of biomass, plastics and tyres generates phenolic compounds in the pyro-oil and aqueous condensate (wood and biomass pyrolysis condensate, in particular, is phenol-rich), making phenols a characteristic contaminant of pyrolysis wastewater.

The hazard is both ecological and practical: phenolic effluent is toxic to fish, hard to treat biologically, and at trace levels contaminates downstream water supplies with persistent taste and odour. An effluent with high phenols is also likely to fail the whole-effluent bio-assay test even if other parameters pass, given phenol's toxicity.

The practical control is to separate and treat the phenol-bearing streams specifically. Pyrolysis aqueous condensate should be segregated rather than mixed into general effluent; phenols are removed by advanced oxidation, activated-carbon adsorption, solvent extraction, or specialised biological treatment with acclimatised cultures (since ordinary activated sludge struggles with phenol). For pyrolysis operators, recognising that the aqueous by-product is phenol-rich — and budgeting for its dedicated treatment — avoids the common mistake of an ETP designed for general effluent that cannot meet the 1.0 mg/L phenol limit.