Staying on the safe side with stormwater
Stormwater peak flows are a major problem for wastewater treatment plant owners and operators. With every treatment plant facing unique circumstances, the response is typically multi-layered and complex but key technologies are a critical part of the solution to reduce the impact of wet weather peak flows.
For municipalities and the owners of wastewater treatment plants, wet weather is a continuous problem. Wet weather flows are very dynamic and even relatively small amounts of rainfall can lead to very high flow rates. Treatment plants are typically designed to be optimized for dry weather flows and while they may have some additional capacity to manage up to double that flow, during periods of rainfall, flows into a treatment plant can often be upwards of five times the normal dry weather volume. Under these circumstances, the water flowing into a treatment plant may exceed both its full hydraulic capacity as well as its capability for the biological treatment of such high volumes of water.
This could easily result in the discharge of untreated water through Combined Sewer Overflows (CSOs) or, worse, Sanitary Sewer Overflows (SSOs). Both Combined Sewer Systems and Sanitary Sewer Systems are designed to accommodate some level of higher flows however both are prone to exceeding their capacity, especially during wet weather conditions resulting in an overflow event. With untreated wastewater entering the watershed or other receiving stream, significant health and environmental impacts can occur, for example through the presence of pathogens, impacts on aquatic life through excess biological oxygen demand (BOD) and eutrophication prompting harmful algal blooms.
Stormwater discharges are also a key source of pollution in the aquatic environment. For instance, although urban areas cover only 3 percent of the landmass of the United States, urban stormwater is estimated to be the primary source of pollution for 13 percent of assessed rivers.
Wet weather peak flows can also have a detrimental effect on the treatment plant itself. Where significant excess volumes of water pass through a facility this can effectively blow or wash out the crucial biological reserve within the system. This reduces the plant’s capacity to treat water and may reduce its performance for many weeks following the event.
Given the consequences of such discharges, regulatory authorities are continually evaluating policies to ensure that even in the most extreme wet weather events, discharge water quality meets permit requirements. These policies and restrictions are being imposed at both state and federal levels and although these rules don’t always coincide it is clear that the rules covering stormwater discharges are getting tougher over time. It’s a trend that is certain to continue, placing more pressure on owners and operators of wastewater treatment plants to address the limitations of their facilities in terms of peak wet weather flows.
The growing challenge of managing wet weather peak flows
At the same time as regulatory authorities clamp down on stormwater discharges, the challenge is becoming far bigger. Climate change is changing weather patterns and prompting more intense weather events. Given the longevity of most municipalities’ current infrastructure, the bulk of those sewer networks and wastewater treatment plants (WWTP) in place across North America were certainly not designed with those considerations in mind. What might have been a once in a 100-year event half a century ago may be occurring every few years in today’s climate.
Other factors are also making the wet weather discharge problem more difficult. For example, the sustained growth of urbanization is creating more impervious surfaces that increase peak flow rates. At the same time, many sewer networks and WWTP are already well into the later years of their design lifespan or even beyond. This not only leaves such installations vulnerable as a result of older designs and technologies but the nature of aging assets can also increase peak flow rates.
For example, the often extensive wastewater collection system may be failing and pipes can be fractured and cracked or have faulty seals and bad connections. Where that happens, sanitary sewers can receive large amounts of infiltration and inflow with additional water entering the network from saturated soil for example. This boosts flow rates but can also impact the total suspended solids in the wastewater. Indeed, not only can flows show huge variation in volumes but also the amount of solids making the implementation of any treatment solution more difficult.
All these problems are building even while the vast majority of North America Municipalities are facing resource constraints, making large-scale capital investment unlikely if not impossible. Even where resources are available, many municipalities are restricted in their plant envelope, which limits what is possible in respect of retrofitting additional treatment technologies and increasing peak capacity.
Addressing wet weather peak flows
There is no one size fits all solution to the challenge of storm flows. While the issue is complex, key challenges include the management of the volumetric flow in the collection system and the ability to successfully treat the variability in influent characteristics at the WWTP. It is clear that every wastewater treatment facility is different and faces unique conditions.
Consequently, each municipal authority and any specific plants within the jurisdiction will require a tailored approach to peak flow management to fully address discharge water quality and bring it to the required quality even in wet weather. Nonetheless, certain characteristics are desirable in any wet weather flow solution which are important if an optimized solution is to be achieved for owners and operators.
Many facilities will look at a combination of solutions. For example, to reduce the peaking factor of a wet weather event flow they will try to improve the aging infrastructure to a certain point to reduce influx and infiltration. There’s lots to consider when looking at improving a collection system. There is clearly a huge challenge in terms of costs and timeframes, for example, and there are potential impacts on surrounding buildings and businesses. Nonetheless, it is always part of a thorough evaluation to resolve the challenge of wet weather peak flows.
The potential use of additional storage capacity is another important characteristic that should be explored. It may be that a treatment plant has some existing tanks on-site that are no longer being used and that could be redirected to temporary water storage during wet weather events. Storage capacity may also exist within the wider collection system. This can also be deployed to hold rainwater and shave peak flows, or handle shorter duration events by simply storing excess water until it can be treated by the plant through normal processes.
A comprehensive solution
Technology also has a role to play in reaching a comprehensive solution to stormwater flows. BIOACTIFLO™, for instance, is a process designed for the treatment of stormwater to provide additional biological treatment when the need arises. A two-stage process with biological treatment followed by high-performance ballasted clarification, BIOACTIFLO™ is very efficient from a treatment perspective. Extremely compact and operationally flexible, many wastewater treatment plants have already deployed it as an additional treatment capability alongside other potential solutions to reduce the impact of peak flows. In the USA, for example, Tennessee’s Knoxville Utilities Board has installed a BIOACTIFLO™ system at its Fourth Creek and Kuwahee facilities. Similar approaches have been adopted at facilities in Texas, Maryland, Pennsylvania, California, Louisiana, and Illinois.
Such plants have taken advantage of the technology to minimize the installation costs at space-restricted sites and with limited resources. Ideally suited to take advantage of any available space or retrofitted on the existing footprint of an old clarifier or a concrete basin, BIOACTIFLO™ is a solution that can also be implemented very quickly.
Solving mother nature’s biggest water treatment challenge
It is evident that mother nature is unpredictable and even short-duration events can prompt the kind of flows that cause havoc within a sewer network and treatment facility. Climate change and other megatrends like urban expansion are making this problem steadily worse. Each facility is different, with its own needs and challenges and therefore the solution also looks different at every plant.
Understanding the weak points of the treatment facility and the steps that need to be taken to address that weakness is the key to an optimal solution to wet weather peak flows. The answer is more than a treatment technology, but rather an overall approach. Nonetheless, given its many advantages, technologies like BIOACTIFLO™ form a key part to any comprehensive solution to the wet weather peak flow challenge.
To Learn More, Watch Our Free Veolia Water Tech Talk on this Subject
Managing Peak Wet Weather Flows
Wet Weather flows can often overwhelm sewer networks and treatment facilities and proper management of these events is a challenge many municipalities face. This on-demand Tech Talk will discuss some of these challenges and highlight Veolia’s experience in supporting municipalities' efforts to manage peak wet weather flows.