FWEA Focus—Ronald R. Cavalieri and Kevin Frank

FWEA FOCUS Wet Weather Management

Ronald R. Cavalieri, P.E., BCEE

President, FWEA

Coauthor: Kevin Frank

Water quality has continued to be a high-profile issue in the state of Florida by the governor, state legislature, news media, and the public at large. Several environmental bills have been passed by the legislature, including the Clean Waterways Act (SB 712) in 2020 and the Reclaimed Water Bill (SB 64) in 2021. The Clean Waterways Act addressed several environmental issues, including provisions specifically related to water quality improvement.

One of the major components of the Clean Waterways Act directed the Florida Department of Environmental Protection (FDEP) to develop requirements for the establishment of asset management programs and associated reporting for domestic wastewater collections systems.

With increased emphasis on mitigation of sanitary sewer overflows (SSOs), the FWEA Utility Council sought to change the emphasis away from increasing fines and penalties for SSOs that occur to promoting best management practices for operation, maintenance, assessment, and repair and/or replacement of infrastructure in order to improve resiliency, reduce SSOs, and ensure long–term sustainability.

While the emphasis on mitigating SSOs has to some extent been on helping to ensure that utilities are investing in their collection systems, another option (or part of the solution) is to increase the wet weather flow treatment capacity. Although the abatement of inflow and infiltration (I&I) will help to reduce wet weather flows in the collection system, the cost of identifying excessive I&I and rehabilitation can be significant and may not completely address the problem. Increasing the wet weather flow treatment capacity is another tool in the toolbox to help utilities mitigate SSOs.

An approach that utilities can take to help “ride out the storm” and increase flow through the wastewater treatment plant is using a step feed functionality and “solids-holdup” process.

The focus of this month’s column is a general description on this innovative approach.

Increasing Wet Weather Flow With the Solids-Holdup Process

Step feed functionality and the solidsholdup process can be particularly useful in handling high wet weather flows. For most plugflow aeration tanks, all secondary influent enters the aeration tank at the influent end and the mixed liquor suspended solids concentration (MLSS) is mostly homogeneous throughout the length of the reactor. The aeration tank effluent MLSS is then settled in secondary clarifiers, which is usually the process bottleneck that dictates wet weather flow capacity. Most secondary clarifiers can handle solids loading rates (SLRs) up to about 20 to 30 lbs/d/ft2 , depending upon the sludge settling quality.

A typical dry weather flow condition is illustrated in the first diagram of Figure 1, where the influent flow, MLSS, and SLR are respectively at 1 million gallons per day (mgd), 3,000 mg/L, and 20 lbs/d/ft2. When a wet weather flow event occurs, the influent flow can often peak up to two or three times the dry weather flow; if the peak flow reflects a three-fold increase, the SLR also increases three-fold. This is shown in the second diagram of Figure 1, where the influent flow is at 3 mgd and the SLR has increased to 60 lbs/d/ft2. Such a high SLR would likely cause the sludge blanket to rise and result in a major solids loss and possible effluent violation.