A Silver Bullet for the Collect-Store-Treat Conundrum?

rain water flowing into storm sewer

Addressing Stormwater & Wastewater with Sustainable Membrane Technology

By Greg Baird

Population growth and the development of urban/urbanized areas are major contributors to the amount of pollutants in storm runoff as well as the volume and rate of runoff from impervious surfaces. Sewer discharge in runoff threatens our public health. Together, they can cause changes in hydrology and water quality that result in habitat modification and loss, increased flooding, decreased aquatic biological diversity, and increased sedimentation and erosion.

Urbanization from population growth with land use change and development sprawl has created an average increase of an additional 55 percent of runoff. This is coupled with a complex mix of infrastructure assets including more than 850,000 miles of public sewers, 500,000 miles of private lateral sewers, more than 16,000 wastewater treatment plants, an estimated 3.5 million miles of storm sewers, 270 million storm drains and 2.5 million stormwater treatment assets across the United States.

The U.S. wastewater industry has a $62 billion annual operation along with the $6 billion tunneling market sector that continues to struggle with traditional infrastructure design, treatment processes and climatic storm events. Even with the advancements in tunnel boring machines (TBM) and digital planning, storm retention and storage tunnels are expensive, ranging from $10 to $35 per gallon, and do not eliminate all overflows. They require maintenance, are not modular and still need to convey the polluted water to treatment plants.

In 2021, the American Society of Civil Engineers (ASCE) provided a national infrastructure grade of a “D+” for wastewater and a “D” for stormwater as 850 billion gallons of untreated sewage continues to impair nearly 600,000 miles of streams, rivers and ocean. These poor grades also consider the $9 billion in direct costs and loss of lives annually from urban flooding, the nation’s greatest natural hazard.

Stormwater runoff is generated from rain and snowmelt events that flow over land or impervious surfaces, such as paved streets, parking lots, and building rooftops, and does not soak into the ground. The runoff picks up pollutants like trash, chemicals, oils, and dirt/sediment that can harm our rivers, streams, lakes and coastal waters. Storm flows are overwhelming sewer systems and increases the pollutant load with untreated sewer discharge. Runoff conveyance systems have been part of cities for centuries, but they only reflected the purpose to remove water from roads and walkways as rapidly and efficiently as possible which then created downstream flooding events. Historically, water quality was not the first concern.

The pollutant load reductions required of facilities have been driven by the National Pollutant Discharge Elimination System (NPDES) permit requirements of the U.S. EPA’s Clean Water Act (CWA). The NPDES stormwater program regulates some stormwater discharges from three potential sources: municipal separate storm sewer systems (MS4s), construction activities and industrial activities.

The benefits of effective stormwater runoff management can include:

  • protection of wetlands and aquatic ecosystems,
  • improved quality of receiving waterbodies,
  • conservation of water resources,
  • protection of public health from untreated sewer discharges, and
  • flood control.

Specific SCMs might be assumed to remove a percent of pollutants, for example 85 percent removal of total suspended solids (TSS) within a stormwater wetland. Reducing the volume of runoff from impervious surfaces (e.g., using an infiltration device) might be assumed to capture the first flush of pollutants during a storm event.

effluent quality of a stormBLOX system.
Figure 1. The typical effluent quality of a stormBLOX system.

The current treatment process and options of ballasted clarification, compressed media filtration, cloth filtration, and chlorination simply are not able to instantly “first flush” treat storm flows allowing for untreated water to violate permit requirements.

Whether a first flush of contaminants occurs at the start of a rainfall event depends on the intensity of rainfall, the land use, and the specific pollutant, it is still important that any SCM be designed to treat as much of the runoff from the site as possible. In many situations, elevated discharges may occur later in an event associated with delayed periods of peak rainfall intensity.

New Membrane Materials and Technology for Treating Storm Flows

New technology and processes using silicon carbide (SiC) membranes is rapidly emerging and will begin to dominate the membrane market considering it is suitable in harsh environments and has outstanding mechanical, thermal and chemical robustness. SiC is has a superior hydrothermal stability with high water permeability and fouling resistance. SiC is an ultrafiltration flat plate membrane with a 0.1 micrometer pore size, SiC creates a physical barrier blocking solids, pathogens, heavy metals, and oil and grease. The SiC ultrafilter physically blocks 99+% bacteria and TSS.

A Simple Solution to a Very Complicated Problem

As an example, the stormBLOX system by Ovivo is a membrane-based treatment process consisting of the following key steps: influent screening, coagulant addition and filtration through SiC ultrafiltration membranes.

Figure 2

StormBLOX, as a modular system built into an existing treatment train, instantly treats right at the onset of a storm while physically blocking solids and pathogens from being discharged into waterways. It also meets all disinfection requirements without supplemental chemicals, preventing toxic by-products from harming aquatic life. This means instant treatment and no permit violation as the operator is very busy and should not have to worry about the treatment plant while facing the challenges of chlorination and dechlorination (see figure 2).

Where to Apply SiC Treatment Solutions:

The immediate need and application of this technology can be readily found with communities struggling with EPA consent decrees and administrative orders for combined sewer overflows (CSO facilities) and sanitary sewer overflows (SSO facilities). See figure 3.

Figure 3

The U.S. reports there are about 16,000 publicly owned treatment works (POTW) in the country. ASCE reported that 81 percent of these wastewater treatment plants (WWTP) are at capacity and 15 percent have reached or exceeded capacity. Today, building a new WWTP can cost around $12 million per 1 million gallon a day (MGD) for average flow. So, for a community with a population of 10,000, assuming an average indoor/outdoor water usage, a 1 MGD WWTP would be required.

Figure 4

SiC membrane systems should be incorporated into the treatment train of existing Conventional Activated Sludge (CAS) treatment plants with greater than 5Q peaking factors due to high Inflow and infiltration (I&I) where stormwater and groundwater enter the wastewater system (see figure 4).

Membrane Bioreactor (MBR) plants can also gain immediate benefits with greater than 3Q peaking factors with high I&I (see figure 5).

Figure 5

In urban settings where space is limited and the plant is landlocked, the SiC modular process for 1 MGD can be fit in a tight 85 to 100 sq-ft space. A treat and discharge scenario can include a retrofit when there is not additional capacity or ability to expand the original storage or retention footprint.

How it Works

During a Storm

  • Raw wastewater passes through the screens.
  • Coagulant is added and flashed mixed.
  • Supplemental alkalinity is added and flashed mixed (if required).
  • Wastewater is filtered by the SiC membranes by creating a slight vacuum to pull water through the microscopic membrane pores.
  • Permeate is collected for backwashing and cleaning in place.
  • Blowers scour the membranes during operation to remove solids from the membrane surface.
  • The membranes are backwashed with permeate water periodically.

After a Storm

  • Basins are emptied and tank floor is flushed clean by controlled water flusher
  • Clean in place (CIP) is performed on the membranes. Basins are emptied and membranes kept dry in between storms

The SiC membranes create a physical barrier, completely removing bacteria and from the incoming storm flow. The result is an effluent that meets all numerical coliform and total residual chlorine limits without supplemental disinfection. Other contaminants such as heavy metals, oil, and grease are also effectively removed by the SiC treatment process. Regardless of incoming flow and loading, the process always provides the same level of treatment.

At the end of the storm, the membranes and basins are cleaned and emptied in preparation for the next storm. In some cases, a system is used as a dual-purpose treatment system, some of the membrane basins serve as redundant treatment during dry weather.

Storm flows are highly variable and can increase by orders of magnitude in an instant. Since SiC is a physical treatment process, it can be easily ramped up, or ramped down, depending on the severity of the storm. As storm intensity, and subsequent flow, increases additional SiC units are brought online to match the incoming flow. The system is easily and quickly brought on/off-line depending on treatment demand at any single point in a storm.

As a sustainable treatment option, the additional benefits include:

  • Disinfection without chemicals (This is a huge safety and cost savings for large MGD plants)
  • Instantaneous treatment
  • Supplement secondary treatment
  • No disinfection by-products
  • Utilize as tertiary filtration system during dry weather–
  • Extremely small footprint, easily fits into existing infrastructure (about 1/3 of other technologies)
  • Easily placed off-line regardless of weather conditions
  • Ease of use and peace of mind for operators

Treated stormwater can additionally be used to address drought issues such as recharging aquifers and treating to secondary standards for reuse.

Pilots and Results

Wastewater treatment best management practices and Water Environment Federation (WEF) recommendations include conducting pilots to better understand operating procedures and expected outcomes especially if the community has several treatment facilities. The SiC membrane example used, stormBLOX has completed several pilot studies (below) that can be used. StormBLOX is also equipped with a digital tool for remote operation and monitoring in the office or in the field.

greg baird

Greg Baird is president of the Water Finance Research Foundation and a frequent contributor to WF&M. As a management consultant, he specializes in long-term water utility planning, infrastructure asset management and capital funding strategies for municipal utilities in the United States. He has served as a municipal finance officer in California and as the CFO of Colorado’s third-largest utility. 

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