RESUMO O uso agrícola de lodo de esgoto e derivados no Estado de São Paulo é regulamentado por norma federal e por norma paulista, que recomendam que esses materiais quando incorporados a solos agrícolas devem apresentar no mínimo 30 % de degradação do carbono total. Para observar as frações de degradação de um lodo de esgoto e de um composto, produzido a partir do mesmo lodo juntamente com poda de árvore urbana triturada, realizaram-se dois ensaios de biodegradação de C. As doses dos resíduos foram adicionadas em frascos contendo 500 g de solo coletado na profundidade de 0,00-0,20 m de um Nitossolo Háplico Álico textura argilosa. No ensaio I, as doses de lodo (L) e composto (C) utilizadas consideraram percentuais da necessidade em N para a cultura da cana-de-açúcar, e expressas em Mg ha-1 de resíduo (base úmida): L1 21,2 (100 %); L2 42,4 (200 %); C1 69,4 (50 %); C2 138,9 (100 %); e C3 277,8 (200 %). O ensaio II foi realizado apenas com o lodo de esgoto, utilizando-se doses 5, 10, 15 e 20 vezes maiores que aquelas recomendadas no ensaio I: L3 120; L4 240; L5 360; e L6 480, simulando sucessivas aplicações do resíduo no solo. O C-CO2liberado foi quantificado por meio da medida de condutividade elétrica. O lodo de esgoto no ensaio I apresentou menores frações de degradação quando comparado ao composto orgânico, mas a taxa de decomposição do composto foi menor, provavelmente pela presença de substâncias recalcitrantes resultantes do processo de humificação. A fração de degradação dos resíduos no solo atingiu valor próximo a 30 % apenas para a dose de lodo de esgoto 20 vezes superior à dose exigida pela cultura, e assim não poderiam ser utilizados em solos agrícolas argilosos. Observou-se a necessidade de revisão desse valor adotado pela legislação paulista de uso de resíduos orgânicos em solos.

ABSTRACT The agricultural use of sewage sludge and derivatives is to be carried out according to requirements established by Brazilian federal legislation and by legislation of the State of São Paulo, which requires that these organic materials must have at least 30 % of the total organic carbon degraded when incorporated in agricultural soils. To observe the degradation rates of sewage sludge and of organic compost produced from the same sewage sludge mixed with ground material from tree pruning, we performed two respirometry tests according to the modified method of Cetesb (1999). The waste materials were added to bottles containing 500 g of soil collected from the 0.00-0.20 m layer of a Nitossolo Háplico Álico (Hapludox) soil. In Test I, for sewage sludge (L) and sewage sludge compost (C), application rates were calculated to provide percentages of N requirements for one sugarcane cycle, in Mg ha-1(wet basis) of waste: L1 21.2 (100 %); L2 42.4 (200 %); C1 69.4 (50 %); C2 138.9 (100 %); and C3 277.8 (200 %). Test II was conducted only with sewage sludge, at the rates of 5, 10, 15, and 20 times higher than those recommended in Test I: L3 120; L4 240; L5 360; and L6 480. The CO2-C released was quantified by measuring electrical conductivity. In Test I, the treatments with sewage sludge had lower degradation rates than treatments with sewage sludge compost, probably due to the presence of recalcitrant substances formed during the humification phase of the composting process. The 30 % degradation rate was attained only by the application rate that was 20 times higher than the rate recommended for a cycle of sugarcane, and thus these waste products could not be used in clayey agricultural soils. Therefore, it is necessary to review the values established by legislation in the State of São Paulo.

Sewage sludge (SS) has been widely used as organic fertilizer. However, its continuous use can cause imbalances in soil fertility as well as soil-water-plant system contamination. The study aimed to evaluate possible improvements in the chemical and microbiological characteristics of domestic SS, with low heavy metal contents and pathogens, through the composting process. Two composting piles were set up, based on an initial C/N ratio of 30:1, with successive layers of tree pruning waste and SS. The aeration of piles was performed by mechanical turnover when the temperature rose above 65 ºC. The piles were irrigated when the water content was less than 50 %. Composting was conducted for 120 days. Temperature, moisture content, pH, electrical conductivity (EC), carbon and nitrogen contents, and fecal coliforms were monitored during the composting. A reduction of 58 % in the EC of the compost (SSC) compared with SS was observed and the pH reduced from 7.8 to 6.6. There was an increase in the value of cation exchange capacity/carbon content (CEC/C) and carbon content. Total nitrogen remained constant and N-NO3- + N-NH4+ were immobilised in organic forms. The C/N ratio decreased from 25:1 to 12:1. Temperatures above 55 ºC were observed for 20 days. After 60 days of composting, fecal coliforms were reduced from 107 Most Probable Number per gram of total solids (MPN g−1) to 104 MPN g−1. I one pile the 103 MPN g−1 reached after 90 days in one pile; in another, there was recontamination from 105 to 106 MPN g−1. In SSC, helminth eggs were eliminated, making application sustainable for agriculture purposes.

A determinação da taxa de mineralização de nitrogênio (N) em solos tratados com resíduos é fundamental para definir as doses a serem aplicadas, fornecendo N às plantas na época adequada, sem perdas do elemento por lixiviação. Inúmeros estudos em solos tratados com resíduos utilizam o teste de mineralização de N sem lixiviação e a caracterização das formas mineralizadas pelo método volumétrico. Em razão da deficiente repetibilidade do teste e da baixa sensibilidade do método de determinação observada em estudos com lodo de esgoto e composto orgânico, adotaram-se o método pouco difundido de mineralização de N com lixiviação e a especificação das formas minerais de N, por meio de métodos espectrofotométricos. O teste foi realizado em colunas de PVC, com 30 mm de diâmetro, preenchidas com terra retirada da camada 0,20 m de um solo classificado como Nitossolo Háplico, álico, argiloso, misturada às seguintes doses de resíduos, em Mg ha-1 (base úmida): lodo de esgoto - L1: 3,6; L2: 7,2; e L3: 14,4; e composto de lodo de esgoto e material vegetal triturado proveniente da poda de árvores urbanas - C1: 7,2; C2: 14,4; e C3: 28,8. As colunas foram incubadas e lixiviadas periodicamente com solução 0,01 mol L-1 KCl e as formas minerais de N determinadas colorimetricamente. Maiores taxas de mineralização ocorreram nas menores doses aplicadas de lodo e composto, evidenciando efeito priming, indesejado à fertilidade de solos sob condições tropicais. Menores taxas ocorreram nas doses L2 e C2, com valores próximos àqueles sugeridos pela legislação, não suprindo a demanda de N almejada, indicando possível necessidade de revisão na norma vigente. De modo geral, as taxas de mineralização foram maiores para o composto orgânico, enquanto a velocidade de degradação do composto orgânico foi menor que para o lodo, revelando maior distribuição da disponibilidade de N para a cultura ao longo do tempo. O método de mineralização com lixiviação e a determinação das formas de N-minerais por colorimetria evidenciaram-se rápidos e eficientes; estudos com solos e resíduos diversos podem validá-los para seu uso em rotina.

The determination of the nitrogen (N) mineralization rate in waste-treated soils is important to calculate the residue rates to be applied, supplying plants with mineral N to at the appropriate time, without leaching losses. Most studies on residue-treated soils use the technique without leaching of mineral forms and the volumetric determination method. The repeatability of the test is however poor and the sensitivity of the determination method low, especially in soils amended with sewage sludge and organic compost. In this study we adopted the not very well-known leaching method, with determination of N-mineral forms by spectrophotometer methods. In columns filled with soil taken from a depth of 0.20 m of an alic, loamy soil Hapludox, the following residues doses (Mg ha-1 - wet basis) were incorporated: Sewage sludge - L 1 = 3.6; L2 = 7.2; L3 = 14.4; and Organic compost (sewage sludge + tree cuttings) - C1 = 7.2, C2 = 14.4, C3 = 28.8. Columns were incubated and periodically leached with a 0.01 mol-1 L KCl solution, and the N-mineral forms were determined colorimetrically. High mineralization rates at lower sludge and compost rates were observed, indicating a priming effect, which is undesirable in soil management under tropical conditions. Lower N-mineralization rates were observed at L2 and C2, close to those suggested by the Brazilian legislation that regulates sewage sludge application to agricultural soils, but falling short of the N requirement, while higher mineralization rates were observed at the lower and higher doses, suggesting that the current legislation should be reviewed. Generally, the N mineralization rates were higher for organic compost than for sewage sludge, while the degradation speed was lower for the organic compost, indicating a greater distribution of the available N forms over time. The mineralization leaching method with determination of mineral-N forms by colorimetry proved to be fast and efficient, and more studies should test different soils and organic wastes for validation for routine use.