Nitrogen transformation during organic waste composting by the Rutgers system and its effects on pH, EC and maturity of the composting mixtures

The evolution of the different forms of nitrogen during the composting of s everal wastes was studied, as well as its relation to the pH, electrical co nductivity and parameters of maturity of the composts obtained. Four mixtur es were prepared from different organic materials. sewage sludge, municipal solid waste. brewery sludge, sorghum bagasse, cotton waste and pine bark. The evolution of the different forms of nitrogen during composting depended on the material which supplied the nitrogen to the mixtures and the organi c matter (OM) degradation rate during composting. The greatest concentratio n of ammonium was observed during the first weeks of composting, coinciding with the most intense period of OM degradation, and ammonium then decrease d gradually to reach final values of below 0.04%. The use of urea as a nitr ogen source in the mixtures led to high ammonium levels during the first we eks as a result of its rapid hydrolysis. The nitrification process began on ly when the temperature of the mixtures had dropped below 40 degreesC and i ts intensity depended on the quantity of ammonium present when the process began. The highest concentrations of NO3-N were always produced at the end of maturation, reaching values of 0.52%, 0.53%, 0.12% and 0.20% in the four mixtures studied. Nitrogen losses during composting depended on the materi als used and on the pH values of the mixtures. Mixtures with the highest li gnocellulose content showed the lowest losses (below 25%), while those cont aining municipal solid waste lost more than 40% of the initial content. Sta tistically significant correlations at a high probability level were found between the NO3-N concentration and pH and electrical conductivity, confirm ing that nitrification was responsible for the falling pH values and increa sing electrical conductivity. The ratio of NH4-N and NO3-N concentrations w as shown to be a clear indicator of the maturity of the mixtures during com posting, the final values of 0.08, 0.04, 0.16 and 0.11 for the four mixture s being equal to, or below the maximum value established as a maturity inde x in other materials. (C) 2001 Elsevier Science Ltd. All rights reserved.