Treat less, treat faster – a Seine decision

The Seine Aval wastewater treatment works near Paris is claimed to be the world's second largest. With the help of OTV France (VeoliaWater) and to improve the quality of water returned to the River Seine, the plant is adding a new nitrification-denitrification facility to remove nitrogen from effluent.

Seine aval, located at Achères near Paris treats the wastewater generated by more than 75% of the population in the Paris region. Having been commissioned in 1940, it is currently undergoing a major modernisation programme that includes the installation of a nitrification-denitrification unit so that the new wastewater treatment needs of a growing urban area can be met while complying fully with the new discharge standards.

Using Biostyr technology, the new unit will remove the nitrogen contained in the effluent passing through the plant, and improving the quality of water returned to the River Seine. Also, the project has involved a wide environmental agenda – nuisances generated by such facilities are being reduced, for instance.

Construction of the facility is being carried out by a consortium, led by OTV France (Veolia Water). It won the contract in September 2002 following a performance specification tender. The scope of the project has justified an investment of euros 390M over five years. Commissioning of the new facilities will start in November 2006.

The modernisation at the Seine Aval treatment works is part of a 15-year programme launched in 2000 by the Syndicat Interdépartemental pour l’Assainissement de l’Agglomeration Parisienne (SIAAP), project owner and wastewater treatment authority for the entire Paris region, and the Seine Normandy water agency.

Their aim is to achieve, by 2015, full nitrification and denitrification of all the effluent from the Greater Paris area at all times, taking into account both the demographic growth in the region and the new regulations covering discharges into the River Seine.

The goal for Seine Aval is “to treat less, treat better”.

The new site spreads over 35 hectares, needs 1,200 workers on site at the most crucial times, requires 150,000m3 of concrete, 17 high rise cranes, 700 tons of pipes, more than 300 kms of cables and 55,000m3 of filtering material.

OTV is overseeing the majority of the work. Its partners on the project are Degremont SAS, architect Luc Weizmann and civil engineering contractors GTP GCS and Eiffage TP.

The Seine Aval plant plays a key role in wastewater treatment for the Paris region – it has the capacity to treatment capacity is 2.1 million m3/day, although this will be reduced over the coming years.

Key objectives

The treatment efficiency and performance of the “specialist plants” that make up Seine Aval are being and will continue to be enhanced so that they release increasingly healthy water into the Seine to support its flora and fauna. The benefits are already being seen – about 35 different species of fish were recently identified, compared with only two 15 years ago.

For the Achères site, the modernisation includes building a treatment unit to deal specifically with nitrogenous pollution. The key objectives are to further improve the quality of treated water, and remove all the ammonia and some of the nitrates, before releasing it into the Seine This will be the role of the new nitrification / denitrification unit, which will include:

* Reducing the volume of water transported to the plant in dry weather by implementing a new treatment distribution system throughout the region. The volume of wastewater received at Seine Aval should drop to 1.7 million m3/day in 2007, and then to 1.5 million m3/day by 2015.

* Building a unit on the site that treats excess water in rainy weather and removes most of the phosphorus during dry weather. The role of the Seine Aval clariflocculation unit, commissioned in 2004 and located upstream of the new unit.

* Controlling odours, noise and visual nuisance for nearby residents.

Care has also been taken to minimise pollution caused by the work. High-performance, self-compacting concrete has been used so that construction work took less time and was less noisy. Waste was sorted and recycled on site, and pipes were installed to channel the site’s wastewater to the treatment plant.

Activity on the site was limited to working hours, five days a week and the lighting was dimmed after 8pm.

Treatment cycle

The Seine Aval Nitrification site’ size is commensurate with the operations expected: the flow of water from the plant will resemble a third tributary of the Seine, traveling through pipes the size of a two-lane tunnel. And the volume of water could fill an Olympic swimming pool downstream of the plant in 30 seconds.

When complete it will feature 84 nitrification basins, each measuring 173m2; 11 denitrification basins measuring 147m2 each; and a residual sludge thickening unit.

The nitrogen treatment process – called nitrification-denitrification – involves the biological oxidation of ammoniacal nitrogen into nitrates (nitrification phase), followed by the transformation of the nitrates into gaseous nitrogen, which is a natural component of the atmosphere (denitrification phase).

However, the quantity of nitrogen, due mainly to urine, is too large to be naturally “assimilated” into the environment. Without special treatment, it is responsible for the deterioration of rivers.

Nitrogen treatment only takes place at the end of the wastewater treatment cycle. The effluent goes through various treatment phases beforehand involving mechanical, biological and physical-chemical processes, which are carried out in buildings spread over an area covering 800 hectares.

The process starts with pre-treatment – screening, grit removal and de-oiling. After primary settling, to remove suspended solids, an initial biological treatment takes place using activated sludge to deal with carbon-rich pollution.

The next phase is clariflocculation using OTV’s Actiflo process. This removes phosphorus in dry weather and treats excess flows of storm water in wet weather.

Nitrification-denitrification, which targets ammoniacal nitrogen, is the final stage in wastewater treatment. This process also varies automatically, depending on variations in the weather.

In parallel with the wastewater treatment, there is a complete treatment train for the sludge produced by the various wastewater treatment phases – thickening, digestion, thermal treatment, de-watering, and storage before landfill.Currently, only the carbon and phosphorus content of pollution is treated at the Achères facility.

Once operational in 2007, the new facilities will extend the plant’s treatment capability to include treating nitrogenous pollution by applying full nitrification (100% of the volume) and partial denitrification (30% of the volume) to the influent stream.

The facilities have been sized to deal with a flow of 24m3/second in dry weather, 45m3/second in wet weather and 52m3/second during exceptional peak periods. Their function is to treat ammoniacal nitrogen in dry weather and refine the treatment of excess water in wet weather.

Downstream of the Seine Aval clariflocculation unit, the nitrification unit will be installed in a three-story building. The building will house the sludge treatment, ventilation and deodorisation, as well as office, maintenance and administrative space.

A separate building will house the denitrification treatment facilities. An energy recovery turbine will be installed beside the building.

Architecturally, the focus of the plant is on the well-being of employees. The overall efficiency of its systems led to the creation of a new component in project design – ergonomics – whose mission was to combat the risks of employee fatigue and discomfort, thereby maximising efficiency and productivity.

Blending in

Factors taken into account include the layout of the premises, signage, lighting, design of man-machine interfaces, people flow management, and handling equipment accessibility and safety.

SIAAP also wanted the plant to blend into the surrounding landscape and serve as an educational facility. As a result, OTV has taken care in the overall layout and choice of materials. Visitors to the plant will see for themselves that these goals have been achieved.

Sympathetic to nearby residents and wishing to enhance their living environment, SIAAP wanted to create a park on the Seine Aval site. A 45-hectare meadow has been formed, sloping down from the Seine Aval Nitrification unit to the banks of the River Seine.

Also, with this natural space between the Parc de Fromainville and the Jardin de Paris, 27,000 trees and shrubs are also being planted.

There will be a total of 85 hectares of parkland beside the river, giving the landscape a new breath of life.

The old tow path will also be refurbished so that people can enjoy a stroll along the left bank.

The Biostyr process

OTV’s Biostyr is used in about 100 plants worldwide. The unit in Seine Aval will double the total area covered by all of the other units in France.

The Seine Aval Biostyr process will perform full nitrification of all effluent, remove residual carbon pollution and retain suspended solids. The effluent leaving Biostyr will then be partially denitrified (30%) in a treatment facility located in an adjacent building.

The residual sludge will be treated by flotation to achieve 4% dryness. Subsequent centrifugation will increase dryness to the required 6% level.

Biostyr enables biological treatment and clarification to be carried out in a single structure as opposed to the separate structures needed for suspended growth systems. It consists of an upflow of the stream to be treated during the filtration phase. The cells are backwashed, avoiding the risk of clogging. The oxygen needed for the biological reactions is supplied by injecting a flow of processed air into the water supply.

By combining the upflow through the cells with small floating media – Biostyrene – Biostyr gives “excellent” water quality in under two hours. The breakdown of carbon and nitrogen pollution is achieved by developing the bio-mass within the structure. It fixes onto the floating media, which is held in place by a roof containing nozzles.

A feature of the Biostyr is that it combines biological reduction and clarification to remove suspended solids in a single structure. This dual action helps to make it an efficient and extremely compact process.

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