Nitrogen removal in fixed-bed submerged biofilters without backwashing

The paper reports the findings of four years of pilot-scale research on nit rogen removal in fixed-bed biofilters fed on real raw municipal wastewater. The plant was made of two fixed-bed biofilm reactors in series with an int ermediate settling tank from which excess biomass from the first stage was discharged. The first filter was used for carbon removal either with oxygen or nitrates. The second filter was used for nitrification. The average nit rification rate at 20 degrees C was 0.84 gNH(4)(+)-N m(-2)d(-1) with 5 mg 1 (-1) dissolved oxygen in the bulk liquid. Temperature dependence was calcul ated (r(n) = r(n,20 degrees) 1,05(T-20)). The influent organic load strongl y affected ammonia oxidation. If the organic loading exceeded 2.5 gCOD m(-2 ) d(-1) nitrification rate was reduced by 50%. Denitrification was performe d by recycling nitrates back from the second filter and by using sewage its elf as carbon source. Denitrification rate showed to be strongly dependent on temperature (r(d) = r(d.20 degrees) 1.11(T-20)) and on the recycle rate. Hydrolysis of the colloidal COD fraction showed a similar dependence on bo th temperature and recycle rate. Therefore, it has been concluded that the hydrolysis of finely dispersed COD particles can be the limiting step of de nitrification in the biofilter when real sewage is used as carbon source. ( C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.