One of the largest wastewater treatment plants in the Paris conurbation (240,000 m(3)/d) has been studied over several years in order to provide technical and economical information about biological treatment by biofiltration. Biofiltration systems are processes in which carbon and nitrogen pollution of wastewater are treated by ascendant flow through immersed fixed cultures. This paper, focused on technical information, aims: (1) to compare performances of the three biological treatment layouts currently used in biofiltration systems: upstream denitrification (UD), downstream denitrification (DD) and combined upstream-downstream denitrification (U-DD) layouts; and (2) to describe in detail each treatment step. Our study has shown that more than 90% of the carbon and ammoniacal pollution is removed during biological treatment, whatever the layout used. Nitrate, produced during nitrification, is then reduced to atmospheric nitrogen. This reduction is more extensive when the denitrification stage occurs downstream from the treatment (DD layout with methanol addition), whereas it is only partial when it is inserted upstream from the treatment (UD layout - use of endogenous carbonaceous substrate). So, the UD layout leads to a nitrate concentration that exceeds the regulatory threshold in the effluent, and the treatment must be supplemented with a post-denitrification step (U-DD layout). Our work has also shown that the optimal ammonium-loading rate is about 1.1-1.2 kg N-NH(4)(+) per m(3) media (polystyrene) and day. For denitrification, the optimal nitrate-loading rate is about 2.5 kg N per m(3) media (expanded clay) and day in the case of DD with methanol, and is about 0.25 kg N-NO(3)(-) per m(3) media and day in the case of UD with exogenous carbonaceous substrate.
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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