Black & Veatch's Frank Rogalla looks into the treatment of sewage overflows
The European guideline on urban wastewaters did not only fix more stringent discharge objectives for municipal effluents but also required the treatment of a substantial portion of rainwater discharged in the sewers.
This is logical since the better effluents are treated, the more aquatic life will develop, which is then sensitive to disturbance in its environment, such as an occasional rain event with a substantial peak loading. In the US, it is estimated that about 40,000 overflow events occur every year, leading to severe impairments of 13% of the rivers, 21% of the lakes and 45% of the estuaries.
Therefore new treatment methods are being developed to be able to handle the high and periodic flows. In the UK, it is common practice to install storm tanks to capture the high rain flows and return them for treatment in the main plant once the flow has subsided. Alternatively, some plants are designed for a wet weather capacity of up to six times the dry weather flows, even in the biological treatment system. In many other countries, only a hydraulic peak factor of two is designed into the WwTWs, leading to a new challenge to incorporate the EU objectives.
Experience suggests when effluent flows are above two times the average flow, the wastewater concentrations are significantly weaker than the average waste strength normally seen. As a result, a simpler treatment combinations than full biological treatment would be sufficient to meet the excess flow plant effluent requirements. To avoid oversizing the WwTW just for occasional peaks and reduce the space need for large rainwater storage tanks, high-rate technologies have been adapted to produce a high-quality effluent in a minimum space need, allowing to treat peak flows with minimum surface area:
- ballasted flocculation (Actiflo), which is a chemical process for enhanced reduction of suspended solids and biochemical oxidation demand (BOD),
- deep bed filtration (STS-Tetra), which takes advantage of the storage holding capacity of a gravel bed.
The ballasted flocculation process was first used for the treatment of highly-variable surface water, such as rivers exposed to silty run-off, and was then adapted to separate wastewater solids. Fundamentally, the process is very similar to conventional coagulation, flocculation, and sedimentation technology used in WwTWs – a coagulant is used for destabilisation of suspended materials entering the process and a flocculant aid polymer is added to aggregate the solids into larger masses.
The resulting floc is removed by settling. In ballasted flocculation, this process is enhanced through the addition of microsand as seed for the development of high-density flocs, which are weighted by the relatively high-density microsand, and more easily removed by settling. Microsand is fine sand similar to silica powder with a grain size around 100µm.
The benefit of the ballasted floc process is the ability to achieve good solids removal performance at a very high surface overflow rate. In addition, the process can be rapidly started and optimised, even with variations in flow and water quality. This process has been integrated into western Europe’s largest WwTW in Paris-Acheres, where a ballasted flocculation facility with a capacity of 25m3/s (2,000Mld) is serving simultaneously as both tertiary polishing during dry weather and as standby rainwater treatment during wet weather. In the US, the largest sanitary sewer overflow (SSO) ballasted flocculation facility has been installed in Lawrence, Kansas.
The local WwTW had to be expanded and improved to accommodate future growth up to the design year 2020 and maintain compliance with new regulatory requirements. The design of the system and the plant effluent will be required to meet the plant’s National Pollutant Discharge Elimination System (NPDES) permit, which includes for the combined flow a weekly average BOD of 45mg/l and a fecal coliform limit of 2,000FCU/100ml in winter. More stringent requirements are imposed on the biological stage as it has to meet a monthly average ammonia limit of 7mg N/l and summer colony counts of 200FCU/100ml. The main liquid treatment process is designed to treat 100Mld under dry weather conditions. But during wet weather events, based on a ten-year storm, the design 2020 peak flow to the plant is predicted to reach 250Mld. Consequently, the excess flow treatment facilities must handle 150Mld of excess flow, provided in two 50Mld ballasted flocculation units. In addition to the treatment basins, a flow splitter or screening facility, a disinfection basin and a chemical feed facility were constructed.
The flocculation process is protected by fine screens, which will remove rubbish, debris and rags from the process stream.
Each 50Mld treatment basin includes a coagulation chamber, an injection chamber, a flocculation (maturation) basin and a settling zone, all about 5m deep. The coagulant (ferric chloride) is added to the wastewater flow at the excess flow splitter or screening facility ahead of the excess flow treatment basin. A diffuser disperses the ferric chloride ahead of the coagulation chamber, where it remains for 48sec.
The coagulated water then enters the injection tank where it is mixed for 60sec with polymer and microsand. The majority of the floc then forms in the flocculation (maturation) tank for 180sec. Finally, the ballasted floc settles in the sedimentation basin with a unitary surface of 38m2, equipped with lamella plate settlers, giving a peak overflow rate of 80m3/m2/h. The clarified water then flows over the effluent weirs where it flows to the excess flow chlorine contact basin.
At the excess flow chlorine contact basin, the flow is disinfected by sodium hypochlorite and dechlorinated with sodium bisulfite. The flow is then blended with the WwTW’s effluent and flows to the Kansas River for direct discharge. The sand-sludge mixture collected at the bottom of the settling tank is withdrawn and pumped to a hydrocyclone. The separated microsand is concentrated, discharged through the bottom of the hydrocyclone and re-injected into the flocculation process.
The sludge separated by the hydrocyclone will flow by gravity back to the head of the plant to the influent pumping station to be treated by the main biological treatment process.
After satisfactory completion of mechanical start-up testing, field optimisation and process performance testing was conducted during summer 2003. The coagulant, ferric chloride and polymer will be dosed at 75-100mg/l and 0.75-1.0mg/l, respectively. It is expected that a microsand loss rate of 1-2mg/l will occur during operation. In addition, to alleviate foaming problems on falls downstream of the settlers, two treatment alternatives were developed.
Based on the full scale tests, it was determined that defoamer rates between 1.5-5mg/l, depending on the product, will control the foam. Since these are silica-based defoamers, the dosages should not impact the Lawrence discharge permit. Another pathway to treat combined and sanitary sewer overflows has been developed with the deep bed filter, which has been used in wastewater filtration since the 1960s.
It was later adapted to tertiary denitrification, growing a biofilm on the filter media when methanol is added. The simplicity of this process, designed without nozzles, screens or small orifices that are sensitive to clogging, allowed the expansion of the system to treat raw wastewater. With a filter depth of 1.5-2m and a media size of 2-3mm, the solids holding capacity has been demonstrated to yield above 5kg TSS/m2 per cycle.
Together with a special backwash regime, using high air but low water flows, the long filter cycles in tertiary applications lead to reduced return flow between 2-4% of the feed. When overflow rates of 10m/h are applied with an inlet solids load up to 100MgTSS/L, as is typical for storm waters, cycle times of five hours are expected, which still results only in return flows of 15%. To extend the filtration cycles, a combination of bumps (short counterflow degassing and speed back-washes (an abbreviated procedure of only 5min) can be used. Extensive pilot tests were conducted on storm sewer overflows at the Village Creek WwTW in Birmingham, Alabama.
At filter overflow rates of 25m3/m2/h, the results showed BOD influent between 50-100mg/l was consistently reduced to below 20mg/l, whereas influent solid averaging 100mg/l where removed to below 15mg/l. Compared to ballasted flocculation, no chemicals are needed to achieve this result, and the total footprint is comparable since no coagulation tanks are needed.
Following the successful pilot demonstration at Village Creek WwTW, the first stage of what is supposed to become the largest storm water treatment facility in the US was built there and started up in mid 2003. While the final peak flow will reach 1,400Mld, a facility of 270Mld is now under operation with just five filters on line.
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