Sewer Systems – Information in Motion:...

When planning a city’s future, we look at maps of transport, housing, green spaces and public amenities. Yet beneath these maps lies another network that provides valuable information about urban life today: the sewer system. Over time, data obtained from wastewater should play a greater role in urban and regional development. They can help us better understand residents’ needs and detect changes early enough for local authorities to respond.

In the article “Wastewater – A Mirror of Society”, I explored what we can learn from wastewater. Today I would like to take this idea a step further and ask what we should do with that knowledge. In my work, I keep coming back to the belief that research becomes more valuable when we can turn its findings into useful decisions.

Wastewater contains, for example, residues of medicines, drugs and their metabolites, as well as genetic material from microorganisms and chemicals from households and businesses. Analysing these substances makes it possible to monitor selected trends in the population connected to a particular sewer network. This provides a collective picture of a given area. However, the results do not reveal who specifically takes medication or how ill any individual may be, and detecting a substance alone does not explain its origin.

This spatial dimension is crucial for urban planning. The catchment area from which wastewater flows to a single treatment plant does not necessarily match a city’s administrative boundaries. It may include neighbouring municipalities, an industrial site or a hospital. It also captures people who commute to the city, study there or visit it. Interpreting the data therefore requires an understanding of both the sewer network and the daily rhythms of the area. Calculating figures solely on the basis of officially registered permanent residents can produce a misleading picture.

Consider a rapidly growing suburban area. New housing increases demand for transport, schools, healthcare and water infrastructure. Conventional operational data on flow rates and loads help assess the capacity of sewer systems and wastewater treatment plants. Targeted analyses can supplement these data with information about the nature of pollution and how it changes over time. I see such knowledge as a useful basis for decisions on infrastructure modernisation. It must, however, be assessed alongside demographic trends, planned construction and expert capacity calculations.

Another area is health. The COVID-19 pandemic, for example, showed that monitoring viral material in wastewater can complement healthcare data and alert us to rapid changes in the spread of infection. For local authorities, this information is particularly valuable in cooperation with public health services. It can support timely communication with residents and the preparation of services. However, the concentration of genetic material cannot be used to calculate the exact number of people who are ill, and a single result is not sufficient grounds for a major decision.

Data on drug use should be approached in a similar way. I see their greatest value in supporting prevention and accessible assistance. A long-term trend can prompt discussion among municipal authorities, healthcare professionals and outreach workers. However, wastewater data cannot reveal users’ ages or the exact number of people with drug dependence. During festivals or tourist weekends, visitors must be taken into account. Without this context, we might wrongly attribute the behaviour of people temporarily present in the area to local residents.

Sewer systems also provide information about a city’s environmental burden. The repeated detection of a particular pollutant may justify a more detailed investigation of its sources and possible ways to reduce it. When developing industrial zones or healthcare facilities, appropriate monitoring and necessary wastewater treatment measures should therefore be considered at an early stage. A chemical or biological signal alone, however, does not prove that a particular facility is responsible, nor does it prove that residents’ health has been harmed.

For these data to be useful, we must interpret them correctly. Concentrations are affected by water volume, rainfall, infiltration or other extraneous water entering the network, and the degradation of the monitored substance during transport. A lower concentration therefore does not necessarily mean lower consumption or a smaller problem. We need repeated sampling, flow data and comparable methods. For planning purposes, a reliably observed trend is generally more valuable than a single figure taken out of context.

It is equally important to recognise who is missing from the data. Residents who are not connected to the monitored sewer network are not adequately represented. In regional planning, findings from a wastewater treatment plant therefore cannot be extrapolated to an entire district or region unless sewerage coverage is 100%. Areas without the necessary infrastructure may in fact need special attention. A lack of data about them must not become a reason to overlook them when allocating investments.

I also consider the protection of privacy and trust essential. The smaller the area being monitored, the more sensitive the results may be. Publishing a finding without explanation can damage a neighbourhood’s or community’s reputation. The aim should be to improve residents’ living conditions. In practice, I would start with well-planned cooperation among local authorities, the water utility, researchers and public health experts. First, we need to define the question the city wants answered, then plan the sampling and evaluation methods. The agreement should also specify who will receive the results and what steps should follow. Mapping tools and artificial intelligence can help identify connections, but their outputs still require expert validation.

I would measure the success of this approach by whether the data supported tangible improvements: a more effective investment, more accessible prevention services or a faster response to a problem. Spatial planning should create the conditions for people to live well. Sewer systems provide information about that life that deserves a place at the table alongside urban planners, water management professionals and local authorities. I regard the responsible use of this information as one practical path towards better-functioning cities.

Author: Tomáš Mackuľak et al.