France extends Europe's largest WWTP

Europe's largest and oldest wastewater treatment plant, the Seine Aval near Paris, was failing to meet new European discharge standards. Natasha Wiseman reports on the construction of a new nitrification-denitrification unit, using Biostyr technology from French engineering group OTV France (Veolia Water), which will remove the nitrogen contained in the effluent passing through the plant and improve the quality of water returned to the river Seine

Modernisation of the Seine Aval wastewater treatment plant, located at Achères in the Yvelines department, near Paris, is part of a 15-year programme launched by Syndicat Interdépartemental pour l'Assainissement de l'Aggloméra-tion Parisienne (SIAAP), Greater Paris's wastewater treatment authority, and Seine Normandy water agency, in March 2000. Seine Aval treats the wastewater generated by over 75% of the population in the Paris region and the programme consists primarily of diversifying and optimising the region's treatment sites and reducing the nuisances generated by the facilities.

The goal for 2015 is to achieve full nitrification and denitrification of all the effluent from the Greater Paris area, at all times. The Seine Aval nitification project is vast and the flow of water from the plant will resemble a third tributary of the Seine. The scope of the project has justified an investment of e390 million over five years and commissioning of the new facilities will start, as planned, in November 2006.

After a performance specification tender, the contract for Seine Aval was awarded to a joint venture consortium, managed by OTV France (Veolia Water) in September 2002. The other partners are Degremont SAS, civil engineering contractors GTP GCS and Eiffage TP and architect Luc Weizmann.

Ecological goals
Originally commissioned in 1940, the Seine Aval plant still plays a key role in wastewater treatment for the Paris region. Today, its treatment capacity is 2,100,000m3/d, but this will be reduced over the coming years. On the other hand, the treatment efficiency and performance of the "specialist plants" that make up Seine Aval are being and will continue to be enhanced.

As a result, they will release increasingly healthy water into the Seine to support its flora and fauna. The benefits are already being seen: 35 difference species of fish were recently identified, compared with only two 15 years ago.

At the Achères site, modernisation involves building a treatment unit to deal specifically with nitrogenous pollution. The new nitrification-denitrification unit, which will help SIAAP achieve this goal.

The programme also includes:
  • 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, therefore, drop from 2,100,000m3/d to 1,700,000m3/d in 2007, and then to 1,500,000 m3/d by 2015.
  • Building a unit on the site that treats excess water in rainy weather and removes the majority of phosphorus during dry weather. This is the role of the Seine Aval clariflocculation unit, commissioned in 2004 and located upstream of the new nitrification-denitrification unit.
  • Controlling odours, noise and visual nuisance for nearby residents.
Nitrogen treatment
The nitrogen treatment process known as nitrification-denitrification, is based on the exploitation and use of a phenomenon found in nature. It 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 and 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. These involve mechanical, biological and physical-chemical processes, which are carried out in buildings spread over an area covering 800 hectares.

The process starts with pretreatment (screening, grit removal and de-oiling). Afterwards comes primary settling to remove suspended solids, and an initial biological treatment using activated sludge to deal with carbon-rich pollution.
The next phase is clariflocculation using OTV's Actiflo process. This performs two functions: it 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. Here, too, the way the process works 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, dewatering, and storage before landfill).

At present, only the carbon and phosphorus content of pollution is treated at the Achères plant. 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/s in dry weather, 45m3/s in wet weather and 52 m3/s during exceptional peak periods. Their function is to treat ammoniacal nitrogen in dry weather and refine the treatment of excess water in wet weather.

Located downstream of the Seine Aval clariflocculation unit, the nitrification unit will be installed in a three-story building, 300m long and 170m wide. The building will also house the sludge treatment, ventilation and deodorization, as well as office, maintenance and administrative space.

A separate building, 90m by 50m, will house the denitrification treatment facilities. The waterfall created by the 8m slope in the ground has facilitated the installation of an energy recovery turbine, which can produce upto 800kWh, enough to cover 10% of the new unit's energy consumption.

Architect Luc Weizmann has designed the plant with ergonomics in mind, that is, the focus is on the well-being of employees in the overall efficiency of the plant's systems. Factors taken into account include layout, signage, lighting, design of man-machine interfaces, people flow and equipment accessibility and safety.

Externally, the plant is designed to minimise its visual impact in the landscape. It is low-level and constructed of long-lasting materials.

Visitors will be able to follow a tour route enhanced by scenographic display with activities and commentaries to inform the public about wastewater treatment. Parkland at the riverside site has been enhanced and extended.

The Biostyr process at Seine Aval
For OTV, Biostyr is the product of almost 30 years in submerged biofiltration applications in municipal wastewater and is used in about 100 plants worldwide, but the unit in Seine Aval is the largest application. The Seine Aval Biostyr process will perform full nitrification of all effluent (transformation of ammoniacal nitrogen into nitrates), remove residual carbon pollution and retain suspended solids.

There are 84 filtration cells divided into six identical galleries with four Amacon pumps from German manufacturer KSB operating in each gallery.
  • Surface area per cell: 173m2 (10x17)
  • Total surface: 14,532m2
  • Material volume per cell: 605m3
  • Total material volume: 50,862m3
  • Filtration speed per cell: 6.6-12m3/h.
The effluent leaving Biostyr will then be partially denitrified (30%) in a complementary treatment facility located in an adjacent building. The residual sludge will be treated by flotation to achieve 4% dryness.

Subsequent centrifugation, using a D7LL centrifuge from Austrian manufacturer Andritz, will increase dryness to the required 6% level. It will be possible to process 15 tonnes/day of sludge at a rate of 250m3/h.

Operating principle
Biostyr is a fixed-film biological process. The unique feature of biofilters is that biological treatment and clarification can 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, thereby 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 modified-polystyrene media known as Biostyrene, Biostyr provides an excellent quality of water for a contact time of less than two hours and generates no odours.

The breakdown of carbon and nitrogen pollution is achieved by developing the biomass within the structure. It fixes onto the floating media, which is held in place by a roof containing nozzles.

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

Six key strengths of Biostyr
  • Very efficient treatment levels with a small footprint
  • No need for a final clarification stage as it is already included in the system
  • No risk of clogging because filters are backwashed daily
  • Odors reduced as only a small surface of water is exposed to the air
  • Quick response to changes in flow and effluent load
  • Maintenance facilitated by easy, secure access to nozzles, which are only in contact with treated water

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