Solving CSO problems

By Martin Couture, William D. Sullivan and Daniel MacRitchie

Combined sewers are designed to carry both sanitary and stormwater flows in the same pipe, and when these collection systems over flow, a combined sewer overflow (CSO) event occurs.

Combined sewers are the only collection systems with an issued permit to cover their overflows. Since the late 1960s, the US Environmental Protection Agency (EPA) and other North American regulatory agencies have prohibited building this type of sewer; therefore combined sewers tend to be old and urban, with designs dating back 50 years or more. These systems frequently have capacity problems because they were designed for smaller populations.

Population growth and aging infrastructure strain the combined sewers' capacity, but so does the weather. During wet weather, the flow can easily exceed peak dry weather flow by 20 times. All these excessive wet weather flow conditions lead to overloads in the sewer system, hydraulic surcharge of pipes, flooding of basements and larger CSO events.

Wastewater treatment plants connected to combined sewers often become overburdened during CSO events, resulting in inefficient treatment or untreated sewage bypass directly discharged into rivers, streams or other waterways. Either situation poses a threat to water quality, aquatic life, human health and properties and also contributes to the biological, chemical and aesthetic pollution of the waterway.

Four main methods can be used, either separately or combined, to eliminate or reduce CSO problems: in-line retention storage, off-line retention storage, high-rate clarification treatment and sewer separation.

With in-line storage, designers refine and optimise flow control in the system, raising the overflow weirs wherever possible to enhance water retention volume. Flow control devices include orifices, preset gates, pumps and motorised gates. The key to selecting a quality flow control strategy is to minimise maintenance and energy consumption while getting precise and reliable control. USFilter's IHV vortex flow control devices perform control flows without relying on electricity or moving parts. Maintenance is minimal, which reduces operation costs.

Another in-line storage solution, which is gaining popularity, involves using computer systems to adjust set points in the collections system or to adapt the system during storm events. Called real time control, each segment of the collections system is optimised to generate the maximum possible water storage at any given time. This approach requires extensive operations and maintenance (O&M), in addition to sewer retrofit and rehabilitation. It relies heavily on electrical power and moving parts to operate.

Part of in-line storage involves getting maximum performance from each overflow weir structure in the system. Bending weirs, flap spring-loaded weirs and air-regulated siphons are three technologies that, depending on client needs, may also be combined with a screen. Delaying overflow until just before the pipe network reaches its maximum fill can usually maximise pipe storage. At that point, the water needs to be released at very high flows to protect the city from spills, and street and basement flooding. This is achieved by relying on small-footprint structures and equipment that will enhance the weir's flow capacity.

Screening at the overflow weir is more complicated because adding screens reduces weir capacity by introducing headloss in the process. Consequently, a low headloss feature that removes solids, such as found on USFilter's OS-LP and StormGuard™ overflow screens, is key to a good overflow screen.

Designers must consider off-line retention, partial retention and emergency treatment or high-rate clarification treatment if the existing collection system does not present enough in-line storage. The design team considers each site's needs to decide which solution to use. While off-line retention generates high capital costs and very low O&M costs, high-rate clarification treatment does just the opposite, reducing capital costs considerably and requiring intensive O&M and controls. Nonetheless, off-line retention and partial retention and treatment are more popular than ever.

A more direct approach to preventing overflows involves creating an overflow retention basin (ORB) to capture the necessary volume at the collection system's CSO point. Such storage facilities provide an efficient means of reducing CSO occurrences and limiting pollution discharges to receiving waterways. ORBs are widely used to mitigate sanitary sewer overflows and various problems, such as equalisation at plants and pump stations or plant emergency retention structures. Considerable settling always occurs in these basins, however, as the captured overflow waits for plant capacity to become available. Retention times may vary from a few hours to a couple of days, depending on conditions.

Once the storm event passes and plant/collection system capacity is restored, the stored volume of sewage and stormwater is released to the downstream wastewater treatment plant. However, settled solids remain in the tank, possibly leading to odour problems. Automated systems, such as USFilter's SFT tipping buckets, efficiently remove accumulated debris, sewage and sediment from the bottom of the ORB and alleviate odours. The SFT buckets require minimal maintenance, can clean flushways as long as 75 metres and generally use a volume of cleaning water that is equal to or less than 1% of the tank capacity.

The wastewater treatment plant in Rocky River, Ohio, installed 12 Hydrovex® SFT tipping buckets in early 2000. Each bucket, holding 5 m3 of water apiece, can quickly and efficiently clean the plant's 12,113-m3 ORB. The ORB acts as an emergency clarifier for the plant, maintaining maximum outflow quality even in the worst conditions.

High peak wet weather flows can cause problems within a wastewater collection system and treatment processes. These excess flows can result in CSOs or sanitary sewer overflows (SSOs), and may present major problems for a plant's biological processes. In such cases, advanced high-rate clarification treatment processes, such as USFilter's Actiflo® microsand ballasted high-rate clarification process, are being used to increase the total treatment system's ability to handle normal dry weather capacities while providing physical-chemical treatment of the storm flows as they occur.

The Actiflo process combines the conventional physical-chemical treatment process of coagulation, flocculation and sedimentation. A coagulant is added to wastewater entering the Actiflo process. From there, the coagulated water flows to the mixing tank where polymer and microsand are introduced. The floc, along with the polymer, attaches to the microsand in the maturation stage of the process. Wastewater then flows to the clarifier where the microsand ballasted flocs settle out of the water at a very high rate. The water finally passes through the lamellar plates for some added polishing and into the clarified water overflow troughs. In some instances, the clarified water is disinfected with ultraviolet (UV) or chlorination systems.

The settled sludge, loaded with microsand, is continuously pumped from the bottom of the clarifier and recycled through hydrocyclone concentrators that separate the microsand from the sludge. The separated sludge is then sent to waste while the microsand is recycled back into the process.

Sewer separation

Combined sewer systems are designed to convey sanitary sewage and stormwater through a single pipeline system to a sewage treatment plant. During excessive rainfall, the capacities of the pipelines and treatment plant are often exceeded, causing overflows at one or more "relief points" in the system. Unless properly controlled, excess stormwater flows into the sewer pipes. To avoid sewage backups, sanitary sewage and stormwater mixtures from CSO events are discharged directly into nearby rivers, lakes and harbours without treatment.

Sewer separation can resolve this problem. Sewer separation involves using the combined system as a storm sewer and building a new, separate sanitary sewer. Conversely, the existing system may be used as a sanitary sewer, and a new storm sewer can be built.

In October 2000, the Water and Sewer Commission in the US city of Lynn, Massachusetts, awarded USFilter a nine-year, US$ 48 million contract to design and build a system to mitigate CSO events. USFilter's proposed solution involved converting the existing combined stormwater and sanitary sewer system into a stormwater-only conveyance system and constructing a new sewer system to convey sanitary flow to the city's wastewater treatment facility. The project ensures that ocean outlets serve as stormwater outlets only and all sewage will travel in separate pipes to the treatment plant.

Building a new sanitary sewer system is saving the city an estimated US$ 100 million because sewerage pipes are smaller than stormwater pipes. USFilter is laying 36,576 m of pipe under city streets and reconnecting storm basins and home services. Construction began in 2001 and will be completed in 2006, three years ahead of schedule. Lynn is also now fully compliant with the 1972 Clean Water Act and a 2000 federal consent decree.

Wet weather issues are complex, but CSO overflow solutions do not have to be complex. Municipalities may choose from a variety of solutions that help regulate pollutant loads, safeguard the environment and comply with state and federal discharge regulations, all at a reasonable cost. Over the next twenty years, the number of communities that will need to address CSO events is expected to increase exponentially.

The Actiflo process has consistently exceeded state performance requirements for CSO treatment plants at the 10-mgd East Bremerton CSO treatment facility in the US state of Washington, according to John Poppe, wastewater manager at Bremerton's Public Works and Utilities. The process is at the heart of the 10-mgd East Bremerton CSO treatment facility, which has complied with state and federal CSO rules since coming online in December 2001. Consequently, the improved water quality has increased the potential for shellfish harvesting in Puget Sound.

The Bremerton facility can start up and achieving excellent performance in only 15 minutes. "The process is removing between 90 percent to 95 percent of TSS, 80 percent of BOD, and 85 percent of total phosphorous," reported Poppe. "Effluent turbidity levels are less than 3 NTU." Post-clarification, the treated effluent passes through medium-pressure, high-output UV disinfection before it is discharged into the Puget Sound.

While the typical design-loading rate in the clarifier is 147 m3/h depending on the application, the loading rate in the clarifier can reach as high as 196 m3/h.