Biological treatment (biological treatment)

Biological treatment is the use of living micro-organisms — bacteria, fungi, protozoa — to break down the organic pollutants in wastewater, converting them into harmless end products plus microbial biomass. It is the heart of secondary treatment and the most cost-effective way to remove biodegradable organic load (BOD) from effluent, because the microbes do the work that would otherwise require expensive chemicals or energy.

Biological treatment comes in two fundamental modes. Aerobic treatment (activated sludge, trickling filters, aerated lagoons) uses oxygen-breathing microbes that convert organics to CO₂, water and biomass; it is fast and produces a clean effluent but consumes energy for aeration and generates surplus sludge. Anaerobic treatment (anaerobic digestion, UASB reactors) uses microbes that work without oxygen, converting organics to biogas (methane and CO₂) plus a smaller amount of biomass; it is energy-producing rather than energy-consuming and suits high-strength organic effluent.

For recyclers, biological treatment is most directly the foundation of the entire CBG/biogas business — anaerobic digestion is biological treatment that has been turned into a product-generating process, recovering energy (biogas) and fertiliser (digestate) from organic waste while simultaneously treating it. The same biology that produces the gas is treating the organic load. For other recyclers, aerobic biological treatment handles the BOD in plastic-washing and organic-contaminated effluent streams.

The practical lesson is that biological treatment is a cultivated living system that must be fed, balanced and protected. It needs the right balance of carbon, nitrogen and phosphorus (the C:N:P ratio the existing glossary covers), a suitable temperature and pH, and freedom from toxic shock — metals, solvents, high free ammonia, sulphide and biocides all poison the microbial culture. A recycler running any biological treatment, and especially a CBG operator, succeeds by keeping the biology healthy: protecting it from inhibitory substances is the difference between a treatment system (and, for CBG, a gas yield) that performs and one that collapses.