Study of the Organic Fraction in Biosolids

Abstract

Biosolids are commonly used in agriculture in the European Union, especially in the Mediterranean countries. Because biosolids can act both as fertilizer and organic amendment, the application dose is based on the specific composition of the waste product according to the European legislation of sewage sludge. Organic compounds in biosolids are extremely complex depending on the use of flocculants and/or coadjuvants in wastewater treatments, and therefore the classification into labile, nonlabile, humic-like compounds, etc., categories is necessary to elucidate the potential dynamics in the soil–biosolid systems. In addition, long-term studies are needed to evaluate the effects of biosolids on soil properties, especially considering soil organic-matter pools. In this study, more than 390 biosolids samples were collected and analyzed from 80 different wastewater treatment plants in southern Spain, during the period 2001–2008 (accumulated equivalent population > 500,000 inhabitants 200 hm3 of wastewater treated). The biosolids samples were originated by aerobic or anaerobic stabilization using different wastewater treatments (64.46% by prolonged aeration, 29.75% by active sludge, 1.37% from percolator filters, 1.37% from biocylinders, and 3.03% from biodisks). Total organic carbon (TOC), total nitrogen (TN), water-soluble C and N, humic acid–like (HA), and fulvic acid–like (FA) compounds were determined in all the samples and correlated with the scale and type of the wastewater treatment and also with the biosolid stabilization procedure used. Considering all the scenarios, biosolids from low-scale treatment plants, active sludge wastewater system, and aerobically stabilization of biosolids seemed to be the most efficient for mineralization and therefore for agricultural purposes. However, it is also necessary to take into account the potential N volatilization from calcareous amended soils and thus, more research must be done to ascertain the biosolid behavior in agriculture.