标题
Impact of aeration conditions on the removal of low concentrations of nitrogenin a tertiary partially aerated biological filter

The discharge of nitrogen compounds to soil and waters mayadversely affect water resources in several ways including contributionto eutrophication, depletion of oxygen, toxicity to aquaticenvironment and public health concerns. Nitrogen polluted surfacewaters including those from lakes and rivers often need to bepretreated prior to use in drinking water systems. Hence, removalof nitrogen (both reduced form such as ammonia and oxidizedform such as nitrate) from treated waters and surface waters forreuse has become an important need to protect public health andreduce ecological risk. In order to satisfy stringent regulations concerningnitrogen removal from wastewater in the European Union(Urban Wastewater Directive 91/271/EEC, 10 to 15 g N/m3) andUSA (total N = 3 g N/m3 in some regions), conventional secondarywastewater treatment systems need to be retrofitted or the treatmentscheme should be expanded with polishing treatment. Theresearch described here addresses nitrogen removal from surface waters and wastewater effluents using low maintenance and effectivebiological aerated filters.

Application of submerged biological filtration to remove residualloads (nutrients, trace elements and pathogens) as a tertiaryand/or polishing step can be a promising alternative to retrofitting(Tchobanoglous et al., 2003; Jeong et al., 2006; Schulz andMenningmann, 2008; Farabegoli et al., 2009; Jenssen et al.,2010). Submerged biological aerated filters (BAF), also known assubmerged aerobic biofilters (Schulz and Menningmann, 2008)present several advantages over other fixed-film reactors (e.g.rotating biological contactors (RBC) or trickling filters) includinga high concentration of active biomass, good control of excessbiomass, high sludge retention time (SRT) that enables degradationof complex compounds, better protection against toxic peaksby the biofilm, good efficiency of pollutant (carbon, nitrogen, phosphorousand pathogen) removal combined with a high filteringcapacity in a single-unit process, easy maintenance and operation,no need for sludge recycling and a final clarifier (Mendoza-Espinosaand Stephenson, 1999; Grady et al., 1999; Tchobanoglous et al.,2003; Hidaka and Tsuno, 2004; Schulz and Menningmann, 2008).The capital cost of adding BAF as tertiary/polishing treatment islower in comparison with construction of a new advanced treatmentsystem and its construction does not interfere with theoperation of existing reactors.

The main disadvantages are the risk of clogging and energy costsassociated with aeration and washing systems. Clogging may beminimized if solids could be properly removed upstream or byusing media with void ratio over 0.4 (Grady et al., 1999; Farabegoliet al., 2009). The energy costs may be reduced by optimizing washingcycles and using low air flow rate or intermittent aeration.This last procedure may also be useful to change the biochemicalenvironment in the filter from anaerobic to aerobic in order topromote simultaneously or alternately nitrification/denitrificationmechanisms.

The media used in BAF must have suitable specific surfacearea (500 m2/m3 to 2000 m2/m3) to allow a good biofilm development,and a particle diameter ranging from 1 to 4 mm to allowa void ratio adequate for a good hydraulic flow rate (Mendoza-Espinosa and Stephenson, 1999; Albuquerque, 2003; Schulz andMenningmann, 2008). Mendoza-Espinosa and Stephenson (1999)point out removal efficiencies for organics and nitrogen over 80% incompletely aerated BAF running with hydraulic loading rates (HLR)from 1 m/h to 10 m/h. A natural porous volcanic rock (puzzolane)presents suitable properties for application in bioreactors and hasbeen already tested in sequencing batch biofilter (Buitrón et al.,2004) for the removal of azo dye. Villaverde et al. (2000) have alsotested this material in completely aerated BAF, but for the removalof high ammonia concentrations (100 g N/m3). Operation air flowrates in BAF may range from 18 L/H to 200 L/h (He et al., 2007; Haet al., 2010).

In a previous paper (Albuquerque et al., 2009a) the performanceof a non-aerated lab-scale biofilter (filled with 2–4 mm ofpuzzolane particles) on the removal of organic matter (acetate),ammonia, nitrite and nitrate was analysed. In this study, the samereactor was converted to a partially aerated BAF and long-termoperating data were analysed in terms of the removal of low ammonianitrogen concentrations. Therefore, the aim of this study was toevaluate the effect of aeration on the performance of a puzzolanebasedBAF reactor to remove low concentrations of ammonianitrogen (0.3 g N/m3 to 30.5 g N/m3), which may constitute a usefulmitigation measure for pollution control and integrated watershedmanagement. Most of the previous works with BAF have useda total aerated filter to remove higher concentrations of ammonia(Mendoza-Espinosa and Stephenson, 1999; Villaverde et al.,2000; Stephenson et al., 2003; Garzon-Zuniga et al., 2005; Lei et al.,2009; Ha et al., 2010), normally from 25 g N/m3 to approximately650 g N/m3 (i.e., the reactors were used mainly as a secondary treatmentstep). BAFs reactor studied in this paper could be used forrestoration of ponds, lakes, and rivers polluted by cultural eutrophicationas well as pollution prevention by treating wastewatereffluents, which contain low nitrogen levels.