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The irrigation fields of Paris have been used for 100 years. Their soils mainly contain heavy metals in the topmost layer. Metals come from raw sewage as well as from digested sludge of biological treatment plants which have been diluted for years in raw water. Vegetables that are cultivated in the irrigation fields concentrate metals but their average contents, however, are lower than the recommended limit values. Some vegetables concentrate more specifically one type of metal. Corn seeds accumulate less metal than green vegetables. The SIAAP keeps operating irrigation fields by delivering clariflocculated water with a low metal content from the new Seine Centre plant, with the purpose of keeping some 2,000 ha of green zone in an otherwise heavily constructed area and to prevent a metal release from the soil should irrigation be interrupted. Maintaining irrigation fields also relieves the biological treatment plant and then contributes to preserve the quality of the Seine river, especially in summer.

In order to estimate N(2)O emissions from immersed biofilters during nitrogen removal in tertiary treatments at urban wastewater treatment plants (WWTPs), a fixed culture from the WWTP of "Seine Centre" (Paris conurbation) was subjected to lab-scale batch experiments under various conditions of oxygenation and a gradient of methanol addition. The results show that during nitrification, N(2)O emissions are positively related to oxygenation (R (2) = 0.99). However, compared to the rates of ammonium oxidation, the percentage of emitted N(2)O is greater when oxygenation is low (0.5-1 mgO(2) L(-1)), representing up to 1% of the oxidized ammonium (0.4% on average). During denitrification, the N(2)O emission reaches a significant peak when the quantity of methanol allows denitrification of between 66% and 88%. When methanol concentrations lead to a denitrification of close to 100%, the flows of N(2)O are much lower and represent on average 0.2% of the reduced nitrate. By considering these results, we can estimate, the emissions of N(2)O during nitrogen removal, at the "Seine Centre" WWTP, to approximately 38 kgN-N(2)O day(-1).

The wastewater plant Seine Aval (located in Acheres, France) treats 6 million PE (25 m3/s average flow, 570 MGD) with activated sludge. Following a research program called “Azote Acheres” which demonstrated that biofilters are an economical solution, the Paris Metropolitan Area Sewage Service (SIAPP) has tested 3 industrial prototypes during a period of more than 4 years. One of these is an up-flow biofilter using a floating filter bed made of expanded polystyrene beads. This specific process is called BIOSTYR and is used here as a nitrifying tertiary biofilter. The purpose of this paper is to show that the biofilter process achieves very low, steady state, effluent ammonia concentrations with different applied nitrogen loads (0.3 to 2.7 kg NTK/m3.d), even with low temperature and high applied carbonaceous load. During this 4 years period, a model has been developed to explain the hydraulic behavior of the filter. By measuring the operating conditions (temperature, loads), the head loss of the filter can be predicted. After this test period, the prototype has been operated by Seine Aval plant staff. The results obtained have confirmed the previous test results.

To comply with new effluent discharge standards of 10 mg TKN/l, different upgrading methods for a highly loaded activated sludge plant were explored. As a conclusion, demonstration units were tested to assess process feasibility and performance data of an innovative technology. The Achères Treatment plant of the city of Paris is currently being extended to purify a flow of about 2 700 000 m3/d, corresponding to 8 Million population equivalents. Conventional activated sludge, loaded at about 0.6 kg BOD/kg SS d, delivers an effluent of 30 mg/l for both BOD and SS. To achieve nitrification, a considerable multiplication of basin volume and clarifier area would be required. In the densely urbanised Paris area, insufficient space is available for a such an extension. Therefore, new technology for plant upgrading was tested on industrial scale. Biological aerated filters combine aerobic degradation of pollutants with physical retention of suspended solids in one reactor. A high concentration of active biomass can be retained in the packed bed, and nitrifying bacteria can be attached to the filter media. Removal efficiency becomes thus independent of clarification and sludge settling, and ammonia oxidation can be achieved without sludge age requirements. Four parallel units were installed on the Colombes research platform, handling a total flow of 3000 m3/d. An extensive demonstration test program was carried out over a period of five years to assess the feasibility and performances of the process in line with a conventional activated sludge plant. The limits of loading to achieve different residual ammonia concentrations were studied, and the influence of temperature on biological and hydraulic parameters was verified. Backwash requirements and residual values of carbonaceous and suspended matter were explored in dependence on influent values and filtration velocity. At 13 °C, an ammonia load of 0.5 kg N/m3 d was completely oxidized. A concentration of 20 mg/l N-NH4 can thus be totally converted with an empty bed contact time of 1 hour. The Arrhenius temperature coefficient for nitrification was measured as 1.05. Biodegradable carbonaceous and suspended matter was completely removed at filtration velocities higher than 4 m/h, yielding an effluent of less than 5 mg/l for both SS and BOD. Backwash frequency was less than once per day, and a maximum of 5 % of the filter flowrate was used for backwashing.

In order to preserve the River Seine against point discharge of secondary effluent, the purification plant of Achères is building a tertiary step to nitrify and polish secondary effluent by biofiltration, the dry weather flow of which reaches 2 100 000 m3/day. Two biofilter prototypes, Biofor and Biostyr, have been created to test their performances in a wide range of operating conditions, especially in the case of storm water. First results show that a removal of 0.8 kg N-NH4/m3.d, corresponding to a Global Oxygen Demand removal of 5.5 kg O2/m3.d, can be achieved by Biofor at a temperature of 14°C. The Biostyr prototype brought later into service has not yet enough results to draw clear conclusions. Nevertheless the first data during May 1993 (temperature 18.5°C) are also very promising.

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