First, in case it's not clear, these are (indirect measurements of) concentrations not volumes. There is a strong seasonal effect here, but it also tells other stories based on variations outside of that. Still, the seasonal bit is not unexpected, but hugely significant.

Most respiratory viruses, like influenza and colds have greater prevalence during the local winter. We'd expect COVID to behave similarly. The reasons for that are still debated.

In wastewater collection system, I&I (Inflow & Infiltration) is a constant problem where excess water from the environment enters the waste treatment pipeline. It's costly because all that otherwise clean water has to get the full treatment. So, it's interesting to me, that, unless they have normalized for I&I somehow which is not apparent in their raw data, the true seasonal variation in concentration is actually understated.