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Construction : Cover Story | October 2015 | Source : Infrastructure Today

Cleaning the Ganges

The lack of infrastructure may be turned to an advantage, with the right combination of funding and enlightened planning.

We venerate Ganga and other rivers, recognising water is essential to life, yet they remain heavily polluted. Domestic waste accounts for 80 per cent of pollutants in Ganga. Some reach the river via sanitary sewers, some by direct dumping. A significant route by which domestic waste is entering Ganga and other rivers is through wet-weather flows. When it rains, street waste- including that from open defecationû is washed into the river, either directly or via storm sewers. In addition to human waste urban wet-weather flows are a cocktail of other substances, such as motor oil, rags and grit, which build up in our city streets. In rural areas wet-weather flows will carry pollutants such a fertiliser and pesticides.

The other 20 per cent of pollution in Ganga is industrial. It has been estimated that approximately 1 billion litres of untreated sewage are dumped into Ganga on a daily basis. The lack of infrastructure at the root of much river pollution may be turned to an advantage, with the right combination of funding and enlightened planning. Because so much of our infrastructure spend is on new-build projects, we have the opportunity to adopt state-of-the-art technology that has been time-tested in full-scale facilities. One such approach to treating wet-weather flows can be found in Springfield, Ohio, USA. The city recently added an enhanced high-rate treatment (EHRT) facility featuring the world´s largest compressible media filtration (CMF) system.

The innovative project has added significant peak wet-weather flow capacity- 380 million litres a day (ML/d)- to Springfield´s publicly owned treatment works, and serves to reduce untreated overflows into the local river. Moreover, the CMF technology is fully automated, does not require clarification chemicals, and its capital and operating costs are a fraction of those associated with an expansion using conventional storage, conveyance and treatment technologies. The lesson from Springfield: overflow treatment facilities that incorporate best-fit advances in technology can offer a cost-effective alternative to new storage and conveyance infrastructure. The Mad River flows through Springfield and is the largest cold water fishery in Ohio. Its watershed drains more than 1,550 sq km. About two-thirds of it is used as crop and pasture land, one-fifth for urban use.

Water quality in the Mad River watershed is affected by many sources, including excessive nutrients such as nitrogen and phosphorus from fertilisers, animal waste and sewage. Polluted run-off from urban and agricultural areas is a culprit as is excess siltation resulting from stream channelisation.

Springfield is served by a waste-water treatment plant that has an average design flow of 95 ML/d and provides treatment of dry-weather flows through conventional preliminary, primary, trickling filters and nitrifying activated sludge processes. The plant has overflow weirs that discharges excess wet-weather flows largely untreated to the Mad River. Most of Springfield´s collection system is separate sanitary sewers, but about 22 per cent of the system consists of combined sanitary and storm sewers, including 57 combined sewer overflow (CSO) outfalls. During wet weather events when infiltration and inflow (I/I) through the combined sewers exceeds capacity, discharges through the CSO outfalls go untreated into the river. CSOs impacting the Mad River were about 3.8 billion litres a year in 2000.

Springfield developed a long-term control plan (LTCP), to increase its wet-weather flow capacity and its control of untreated CSOs to the Mad River. The plan recommended the addition of EHRT facilities as part of a range of treatment plant improvements.

Dynamic influent characterisation and full-plant dynamic process modelling was conducted to predict performance and evaluate impacts to the existing liquid and biosolids treatment facilities during wet-weather events. Alternatives were evaluated for the new EHRT facilities. Equipment bids were solicited for solids contact high-rate clarification, ballasted flocculation and CMF. Conceptual facility designs for the technology alternatives were developed and evaluated for economic and non-economic factors. It was determined that lifecycle costs were within seven per cent of each other.

An on-site demonstration pilot unit was operated from October 2010 to June 2011. Approximately 150 tests were run on dry and wet-weather flows. In addition to filtration performance, effluent disinfection dose response was tested, using E. coli as an indicator. Results of the pilot confirmed the influent characteristics, process design criteria, and process performance during a variety of dry- and wet-weather conditions.

CMF uses a bed of synthetic fibre balls to capture influent suspended solids and colloidal particles. Two types are available in the United States: the Fuzzy Filter and the FlexFilter. The Fuzzy Filter is usually configured with influent flowing up through the media bed. While in filtration mode, a perforated plate compresses the media from the top using an electrically actuated screw drive. The FlexFilter operates similarly except that it uses a down-flow configuration, and the media bed is compressed transversely to the liquid flow through sidewall bladders using influent hydrostatic pressure instead of external mechanical or electrical actuators. The CMF system selected for Springfield was the WWETCO FlexFilter.

A key advantage of CMF is automation. Process control uses robust flow and level meters and timers without needing additional human operators. It also requires no chemicals for most applications.

Another advantage of CMF, and most high-rate filtration technologies, is that chemicals are generally not required for wet-weather treatment applications. High-rate filters like CMF also can serve as a safety net downstream of secondary clarifiers to allow existing biological facilities to be maximised during smaller wet-weather events with less risk of losing biomass. Springfield´s EHRT facilities have two structures: a wet-weather headworks for preliminary treatment, and a separate common wall structure for the CMF process, high-rate disinfection and effluent pumping. The structures boast a number of innovative design features. Construction began in August 2013, with commissioning in October 2014. Process control programming and functionality testing of the CMF system started with secondary effluent then switched to raw influent for process performance testing. Challenges inevitably arise when emerging technologies are integrated into existing facilities and Springfield was no exception. Issues included odour control, resolved by recirculating influent in the rock-box chamber, to keep lighter organics and solids suspended and flowing to the main headworks. The CMF cells in the facility are the largest of their kind, which required construction materials, fabrication details and installation procedures to be carefully scrutinized prior to installation. Adjustments to control programme settings and override protections were suggested after a significant first-flush load overwhelmed the initial system programming.

By June 2015, wet-weather events were sizeable enough to require operation of the EHRT facilities on 21 days. Flows from five of those events were small enough in magnitude and duration to be completely captured by the new facilities. Performance was consistent with design criteria and pilot results. The CMF system has achieved excellent turbidity and removal of suspended solids and associated organics.

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