During startup, why is influent wastewater preferred over primary effluent when sludge seed is not available?

The preference for influent wastewater over primary effluent during startup, particularly when sludge seed is unavailable, is primarily due to the greater populations of microorganisms that influent wastewater contains. Influents are the raw, untreated wastewater that enters a treatment facility, and they are rich in organic materials and various microbial communities. These microorganisms are crucial for initiating and supporting the biological processes that break down organic matter during treatment.

The diversity and abundance of microorganisms found in influent wastewater make it more advantageous for establishing a healthy biomass in the treatment system. This robust microbial population can rapidly adapt to the conditions inside the treatment facility and effectively commence the biochemical processes necessary for wastewater treatment.

In contrast, primary effluent has already undergone some level of treatment (such as sedimentation) and, therefore, tends to have fewer microorganisms compared to raw influent. The other choices relate to characteristics of influent wastewater that may not provide the primary reason for this preference during startup. For example, while influent may indeed be more concentrated or stimulate rapid growth, the key factor is the microbial population available in influent that can kickstart the treatment process effectively.