Which treatment processes are considered secondary in nature?

The treatment processes classified as secondary in nature are primarily focused on the biological treatment of wastewater after the initial physical treatment has occurred. Secondary treatment aims to remove dissolved and suspended organic matter, primarily through biological processes where microorganisms break down the organic pollutants.

Trickling filters and wetlands are quintessential examples of secondary treatment methods. Trickling filters utilize a bed of media where microorganisms grow and colonize, treating wastewater as it trickles over the media, allowing for the breakdown of organic materials. Similarly, constructed wetlands use natural processes involving wetland vegetation and microorganisms to filter and treat wastewater through absorption and microbial action.

In contrast, choices such as septic tanks and leach fields represent primary treatment methods, which focus on the initial physical separation of solids from wastewater. Composting and anaerobic digestion can be part of solid waste management rather than a direct secondary treatment of wastewater. Activated sludge and membrane bioreactors are more advanced secondary treatment processes but are more specific in their methodologies, often involving aeration tanks and filtration for high-quality effluent. Thus, options A distinctly align with the defined traits of secondary treatment processes.