Physicochemical and biological characterisation of the sludge from the Romanian municipal wastewater treatment plants for assessment of its conformity level with the legal norms for agricultural use

Abstract

The primary and biological sludges from municipal wastewater treatment plants represent its main solid residue. Because of its high organic load and the content in nitrogen and phosphorus compounds, the sludge may be used in agriculture. In order to check the possibilities to use for application in agricultural lands, the anaerobic fermented sludge was characterised for the following physicochemical and bacteriological parameters: total N, P, heavy metals, fecal coliform. The analyses were conducted in compliance with the Romanian standard methods as follows: total N content varies between 36.7-44.6 g/kg d.s., from which NH4+–N represents ≤ 10%, and NO3––N does not exceed 0.9%; P expressed as P2O5 is 1.2-2 g/kg d.s., and K evaluated as oxide (K2O) presents values of 3-5 g/kg d.s; the analysed heavy metals have lower concentration values than the limits for a biological sludge used as a fertiliser in agriculture, in conformity with Directive 86/278 EEC; according to EPA, the municipal wastewater treatment plant residual sludge, treated by anaerobic fermentation, is included in B class sludge. Pathogens indicators analysed for this class of sludge are – the density of fecal coliform, indicator which can be analysed in the same manner as Escherichia coli, the most important pathogen agent from this category of microorganisms. According to the norms of EPA, a sludge which is going to be used as an agricultural fertiliser must contain a smaller number than 2 millions colonies formatting units (CFU) per gram of total solids in the moment of its use. The results were compared with the Norms stipulated by Decree 49/14 January/2004 for approval of the Technical norms concerning the protection of the environment and especially for the soil when the sludges are used in agriculture. The results show that the sludges from the Romanian Municipal WWTPs may be used as fertiliser for less sensitive culture like: textile plants and pasture field.