Resumo O principal objetivo deste estudo foi determinar os efeitos das diferentes condições operacionais, tais como carga orgânica variável, diferentes idades do lodo e taxas de aeração, limitação das concentrações de oxigênio dissolvido (OD) e choque de carga de amônio (NH4 +), nas rotas de remoção do nitrogênio total (NT) e nas emissões de óxido nitroso (N2O) provenientes de um sistema de lodos ativados em escala de bancada. Idades do lodo reduzidas (5 dias) combinadas com baixas concentrações de OD (0,5 mg L-1) foram responsáveis por uma baixa eficiência de oxidação do NT Kjeldahl (NTK) e, consequentemente, negligenciáveis taxas de acúmulo de nitrito (NO2 -). Esses resultados sugerem uma perda na eficiência da nitrificação completa em razão do maior comprometimento do OD com à oxidação da matéria orgânica, com a maior parcela do N removida pela incorporação ao lodo excedente. O aumento da idade do lodo (de 5 para 10 dias) combinada com o aumento da concentração de OD para 1,0 mg L-1, levaram ao alcance das taxas máximas de oxidação do NTK e de acúmulo de NO2 -, o que representou uma condição ótima para ambos os processos de oxidação da matéria orgânica e nitrificação incompleta. Sob essas condições, a transferência de gás para a atmosfera tornou-se a rota preferencial de remoção do NT. No entanto, a contribuição do N2O foi estimada em até 5,6% da carga de NT afluente. Condições de stress (choque de carga de NH4 +) e aeração insuficiente podem causar períodos de limitação de OD e acúmulo de NO2 -, podendo levar assim à maiores emissões de N2O. Portanto, o controle adequado das concentrações de OD é o fator chave para evitar o acúmulo de NO2 - e, consequentemente, maiores emissões de N2O.

Abstract This study sought to determine the effects of different operating conditions, such as variable organic loading, different sludge retention times (SRTs) and airflow rates, limited dissolved oxygen (DO) concentrations and ammonium (NH4 +) shock loading on total nitrogen (TN) removal routes and nitrous oxide (N2O) emissions in a lab-scale activated sludge system. Short SRT (5 days) combined with very low DO levels (0.5 mg L-1) were responsible for lower TKN oxidation efficiencies and, consequently, negligible NO2 - accumulation rates. These results suggest that nitrification efficiency was hampered by the oxidation of organic matter, with a large part of TN removed by sludge waste process. As the SRT increased (from 5 to 10 days) and DO was set to 1.0 mg L-1, TKN oxidation rates and NO2 - accumulation reached their maxima, which are thought to be the optimal conditions for both organic matter oxidation and partial nitrification. Under these conditions, gas transfer to the atmosphere became the preferential route for TN removal instead of incorporation into the sludge waste. However, N2O contribution is estimated as less than 5.6% (with respect to TN in the influent). Insufficient aeration and stress conditions (such as NH4 + shock loading) can cause limited DO conditions and NO2 - accumulation, leading to higher amounts of emitted N2O. Therefore, the adequate control of DO concentrations is a key factor to avoid NO2 - accumulation and consequently high N2O emissions.

As concentrações de N2O dissolvido e os fluxos de N2O na interface água-ar (utilizando câmaras estáticas) foram determinados ao longo do Rio Paquequer e em alguns de seus tributários. As concentrações de N2O variaram de 0,32 a 4,7 μgN.L-1, e os fluxos de N2O, de <0,56 a 141 μgN.m-2 .h-1 . As menores concentrações e fluxos de N2O ocorreram nas cabeceiras do Rio Paquequer e do Córrego Príncipe, e a maiores, nos trechos urbanos e no Córrego Fischer. As concentrações de N2O dissolvido e os fluxos de N2O na interface água-ar foram positivamente correlacionados com a alta variabilidade nas concentrações de NH4+. Espera-se que a taxa de emissão de N2O, em massa de N, no trecho poluído do Rio Paquequer seja maior do que o valor estimado de 0,62 kgN.dia-1, uma vez que a transferência pela purga em razão da turbulência provocada por cascatas, cachoeiras e corredeiras não foi determinada.

Dissolved N2O concentrations and water-air fluxes (using floating chambers) were measured along the Paquequer River and in some tributaries. Concentrations ranged from 0.32 to 4.7 μgN.L-1 and fluxes from <0.56 to 141 μgN.m-2.h-1. The lowest N2O concentrations and fluxes were found in the headwaters of Paquequer river and in Príncipe brook, and the highest within the urban boundaries and in Fischer brook. Dissolved N2O concentrations and water-air fluxes were positively correlated with highly variable NH4+concentrations. Mass emission rate of N2O in the polluted stretch of Paquequer River is likely to be higher than the estimate 0.62 kgN.day-1 since transfer by turbulent degassing in falls, cascades and rapids were not determined.

Soil fluxes of N2O were determined over one year in montane tropical rainforest of southeastern Brazil with average annual rainfall of 2.8 m. Annual mean (± standard deviation) and median N2O fluxes were 3.0 ± 1.4 and 2.7 µg N m-2 h-1, respectively, is 5-10 times lower than mean values reported in literature for tropical rainforest soils in the Amazon basin. N2O fluxes varied spatially and seasonally, were about twice as high during summer as in winter, and significantly influenced by both monthly precipitation and soil temperature.

Amostras de MP2,5 e MP2,5-10 foram coletadas em São José dos Campos (São Paulo) de fevereiro de 2004 a fevereiro de 2005. Nesse período ocorreram duas longas estiagens, com durações de 25 e 38 dias consecutivos. As concentrações de MP foram maiores na segunda do que na primeira. Os dados foram divididos em dois grupos, um contendo resultados das medições efetuadas durante as duas estiagens (período seco) e outro contendo os dados do restante do ano (período regular). As concentrações médias do MP2,5 e MP2,5-10 foram respectivamente 23 ± 7,5 µg m-3 e 19 ± 7,9 µg m-3 no período seco e 14±7,9 µg m-3 e 13 ± 7,9 µg m-3 no período regular. Houve variação entre os dias da semana, com concentrações médias de MP maiores na quarta-feira (MP2,5) e quinta-feira (MP2,5-10) e menores no domingo. A variação mais acentuada ocorreu para o MP2,5-10, sugerindo uma relação com emissões veiculares. Durante os períodos seco e regular, as concentrações de SO4(2-) e NH4+ foram aproximadamente 5 vezes maiores no MP2,5 do que no MP2,5-10. No MP2,5, as concentrações iônicas foram maiores no período seco (exceto Cl-, Na+ e Mg2+), especialmente K+. Análise de agrupamentos e trajetórias reversas de massas de ar para o MP2,5 sugere influência continental de origem significativa para os íons NO3-, SO4(2-), NH4+ e K+.

PM2,5 and PM2,5-10 samples were collected at São José dos Campos (São Paulo) from February 2004 to February 2005. During this time there were two periods of drought, one lasting for 25 consecutive days and a second one for 38 days. The later exhibited higher PM concentrations values than the first one. Data were divided into two groups, one comprising all measurements carried out during the 63 days without rain (dry period) and a second one comprising the remaining data (regular period). The PM2,5 and PM2,5-10 average concentrations were respectively 23 ± 7.5 and 19 ± 7.9 µg m-3 during dry period and 14 ± 7.9 and 13 ± 7.9 µg m-3 during regular period. There was a day-of-week variation with highest average concentrations on Wednesday (PM2.5) and Thursday (PM2.5-10) and lowest on Sunday. The PM2,5-10 exhibited the most pronounced variation suggesting an association with vehicular emissions. During both dry and regular periods, average SO4-2 and NH4+ concentrations were about 5 times higher in the fine than in the coarse fraction. PM2,5 ionic concentrations, especially that of K+, were higher in the dry period compared to regular one (except for Cl-, Na+ and Mg2+). Hierarchical cluster analysis and backward air mass trajectories indicate that during the dry period NO3-, SO4(2-), NH4+ and K+ associated with PM2.5 are significantly influenced by continental sources.

Air samples of fine (PM2,5) and coarse (PM2,5-10) particulate matter were collected in São José dos Campos from February 2004 to February 2005. Average PM10 mass concentrations was 31.2 ± 14.0 μg m-3, half of which belonging to the PM2.5 fraction. Ammonium and SO4(2-) were predominantly found in the fine fraction. Average (NH4)2SO4 concentration was estimated to be about 2.9 μg m-3. Chloride, Na+ and NO3- were mostly associated with PM2,5-10. Chloride deficits with respect to sea-salt Cl/Na ratio were found in both size fractions.

Nitrous oxide emissions from an activated sludge plant which serves a research institute in Rio de Janeiro city were estimated from six unit processes (grit tank, sand trap, aeration tank, secondary settling tank, sludge recirculation line and aerobic digester sludge tank) and also from the plant effluent. Total estimated annual flux was 3.2 x 10(4) g N2O yr-1 of which about 90% was from the aeration tank. Emission factors estimated from population served, wastewater flow and nitrogen load (conversion ratio) were 13 g N2O person-1 yr-1, 9.0 x 10-5 g N2O Lwastewater-1 and 0.14%.

In Surface water concentrations of N2O were measured at 37 stations in Guanabara Bay and fluxes estimated across the air-sea interface. Concentrations averaged 8.2 ± 2.2 nmol L-1 and 90% of the stations showed supersaturation averaging 33%. N2O fluxes were estimated using a two-film model which is given by the product of the concentration difference across the film and the gas transfer coefficient (k w). Two parametrizations of k w were used which provided average fluxes of 0.3 and 3.0 µg N m-2 h-1. Flux measurements using floating chambers (not reported here) seem to agree with the upper limit of these estimates.

Wet and dry (dust) deposition was measured in the Serra dos Órgãos National Park. VWM pH was 5.3. Non-sea-salt (nss) SO4(2-) comprised 97% of total SO4(2-). The molar ratio [2(nss-SO4(2-)) + NO3-]/[NH4+ + H+] was 1.1, suggesting that pH is predominantly controlled by H2SO4, HNO3, and NH3. Wet deposition of NH4+, NO3-, and nss-SO4(2-) was respectively 0.59, 0.25, and 0.30 kmol ha-1 yr-1. Assuming that dry deposition of N can comprise 30-50% of its total (dry + wet) deposition, the latter is estimated to be 1.2-1.7 kmol ha-1 yr-1 (17-24 kg N ha-1 yr-1).

Wet (n = 21) and dry (n = 23) deposition samples were collected from March to September 2002 at the southern coast of Ilha Grande. Rainwater pH varied from 4.3 to 6.3 and ionic concentrations (in µmol L-1) decreased in the order Cl- > Na+ > Mg2+ > SO4(2-) > NO3- ~ NH4+ > K+ ~ H+ > Ca2+. Scavenging of sea-salt aerosols was the major source of Cl-, Na+, Mg2+, and Ca2+. Half of total SO4(2-) was non-sea-salt SO4(2-). Total deposition fluxes of NH4+, NO3-, and non-sea-salt SO4(2-) were respectively 9.3, 17.8, and 16.0 mol ha-1 month-1.

In the begining of April 2004, concentrations of NHx (NH3 + NH4+) were measured in surface waters of the Guanabara Bay. Concentrations varied from 2 to 143 mmol L-1. Ammonia exchange at the air-sea interface was quantified using a numerical model. No measurement of NH3 concentration in air (c air) was performed. Thus, calculations of NH3 flux were based on the assumptions of c air = 1 and 5 µg m-3. Fluxes were predominantly from the water to the atmosphere and varied from -20 to almost 3500 µg N m-2 h-1.