Summer is here, social distancing has been relaxed and many people are taking advantage of local lakes, rivers and streams to get closer to nature and enjoy being in - or just next to - the water. And while most people will plan for comfort and safety by remembering to bring sunscreen or a first aid kit, thoughts about the cleanliness of the water they are about to enjoy are often furthest from their minds. In many EU countries, this lack of apprehension is possible thanks to the professionalism of wastewater operators coupled with years of financial investments in wastewater treatment facilities. But for municipalities which operate combined sewer networks, one aspect of network management is still seen as a source of concern – sewage overflows during rainy weather. 

Combined Sewer Overflow Basins – Old School, But Still in Use

Combined sewer networks are designed to manage a certain amount of rainfall when transporting sewage to a wastewater treatment plant (WWTP). But when too much rain enters the network, it can quickly become overwhelmed and this mixture of rain and wastewater, if not given a place to go, will flood into homes or public spaces. To prevent this from happening, most combined sewer networks rely on specially constructed rain overflow basins or over-sized storage pipes to temporarily store this excess. However, these basins are limited in size and, upon reaching their storage capacity, must release untreated waste into the environment - usually into a river, lake or ocean. While known by many names in the industry, sewer overflows due to excess rainfall are generally referred to as combined sewer overflows (CSOs). In a 2016 publication, EUREAU estimated that there are over 650,000 CSO systems across Europe. Unfortunately, data on the frequency and duration of overflows that occur each year is difficult to obtain. Estimates range from 2% to 12% of total wastewater load can overflow into the environment during rainfall events. A study from Belgium showed that the CSOs in its network discharged between 5% to 6% of the annual flow to the WWTP into the environment. In regions where tourism and water sports are economically important, the need to monitor and reduce CSO events becomes more obvious. While many large cities or territories are moving to separated rainwater/sewer collection systems, these projects can take decades to complete and are very expensive. This approach is also not without its problems as even separated networks may still use the existing CSO basins. This can lead to older basins receiving higher pollution loads than they were originally designed to manage.

Climate Change and CSO Events

The impact of climate change on our environment is a hotly debated topic, but there is a generally accepted view in the EU that climate change will lead to winter months where less water is stored in the form of snow and instances of heavier rainstorms will become more frequent due to warmer temperatures; something that is more apparent in early Spring and Late fall. Research into this topic is still ongoing, but it is clear that climate change will place new pressures on municipalities who maintain and build CSO basins. One study, modelling the future performance of 30 CSOs located in southern Quebec, noted that while the mean annual CSO duration will increase only slightly (4.2%), CSO frequency and duration are set to increase in the months of May and October. As residents of large urban areas, like London, are already experiencing the effects of intense flooding and its impact on CSOs, people living next to recreational bodies of water should also take note of what the future might hold.

Wastewater professionals clearly understand the immediate impact of sewage being introduced into our recreational waters, but experts are now making the public aware of some additional environmental benefits linked to improved CSO performance. In larger cities, researchers have now noted that pollution from CSO events contributes to the increase in the production of greenhouse gasses in nearby wetlands. Another study recently demonstrated that some of the negative effects of climate change can be offset if receiving water quality is improved. The publishers of this study examined how improvements in stream water quality - specifically, reduced sewage overflows - can help offset the damage done to macroinvertebrates when water temperatures rises. From a regulatory perspective, the EU commissioned a 2019 review of the existing Urban Waste Water Treatment Directive which found that “storm water overflows place significant pressure on surface water bodies” and signaled that they will most likely make recommendations for new regulations to further reduce this problem.

Antibiotic-Resistant Bacteria and CSOs   

Unfortunately, sewage released into lakes and rivers often contains a wide range of pollutants: pesticides, biocides, domestic cleaning chemicals and personal hygiene products can all be found in sewage. Within this waste are also medications which are not fully broken down by the human body. CSO management can play an important part in reducing the volume of these substances that make their way into the environment during these rain events. One interesting study clearly confirmed that small, or low flow CSO events contained a very high concentration of E. Coli due to lower dilution levels. This study reinforces the notion that every effort should be made to take all CSO events, large and small, seriously. 

Smart Design Can Lower Costs

In areas where CSO basins are still needed, striking a balance between CSO basin size and performance is a difficult task for engineering bureaus who need to deliver a long-term, cost-effective solution. Unusual rainfall patterns - such as those experienced due to climate change – will cause problems as storage and peak flow management will become bigger problems. In order to determine the required volume of overflow storage in a network, new data and climate models will need to be taken into account. The good news is that engineering bureaus can take advantage of mature, Model Predictive Control technologies (MPC) to help them model the performance of a virtual sewer network before any construction begins. While the benefits of using Real-time Control systems (RTC) based on MPC concepts have been understood for years, in the past many engineering firms may not have considered this approach as it requires IT communications and programmable control systems – which include pumps, valves and other mechanical actuators. However, adoption of these technologies is on the increase and many wastewater operators will use these systems when planning new projects. MPC-based RTC systems have been shown to offer advantages in optimizing the performance of combined wastewater networks and modeling analysis shows that designing a sewer network with RTC systems in mind can lead to significant cost savings as CSO basin sizes  - a key factor in their construction cost - can be reduced without sacrificing performance or infringing on regulations or standards.

Moving Forward

In addition to smart design, the use of green roofs, home rainwater collection systems, porous road surfacing materials and green spaces can all contribute to the reduction of sewage overflows when a collective sewer network is in place. While the topic of climate change can be difficult to discuss with friends and family, keep in mind that  a 2017 Eurobarometer study showed that more than a third (36%) of EU citizens picked the pollution of rivers, lakes and groundwater as one of the four most important environmental issues in their lives. And to help you with your summer planning, please visit the European Environmental Agency’s excellent map rating the quality of bathing water by location. Enjoy your summer!