Secondary treatment (secondary treatment)

Secondary treatment is the biological stage of wastewater treatment, applied after primary (physical) treatment, in which micro-organisms break down the dissolved and colloidal organic matter that primary settling cannot remove. It is the core BOD-removal step of any conventional effluent or sewage treatment plant, and the stage that does the bulk of the pollution reduction for organic-rich effluent.

The common technologies are the activated sludge process (effluent is aerated with a suspended culture of bacteria that consume the organics, then settled), trickling filters and biofilters (effluent passes over media coated with a biofilm), sequencing batch reactors, and — relevant to the recycling world — anaerobic digestion as an anaerobic form of biological treatment. All rely on the same principle: cultured micro-organisms convert organic pollutants into biomass, CO₂ (and, in anaerobic systems, biogas) and water.

For recyclers, secondary treatment is essential for any effluent with significant BOD/organic load: CBG digestate liquor, plastic-washing water contaminated with food residues, and pre-processing washings of organic-laden waste. It is also frequently the regulatory prerequisite for sewer discharge — the relaxed public-sewer limits assume the municipal system provides secondary treatment, and where it does not, the recycler must provide secondary treatment to inland-surface-water standards itself.

The practical points are that secondary treatment removes BOD effectively but does not remove non-biodegradable COD, nutrients or trace toxics — those need tertiary treatment. The biological process is also living and sensitive: it can be poisoned by toxic shock loads (metals, solvents, high ammonia, biocides), so a recycler must protect the biology from the very toxics their feedstock may carry. For CBG operators, the anaerobic digester is itself a biological treatment system, and the same care against inhibition (ammonia, sulphide, toxic shock) that protects gas production also protects the treatment function. A well-run secondary stage is the backbone of organic-effluent compliance; a poisoned one fails everything downstream.