Changes in sewage sludge carbon forms along a treatment stream

Abstract

The behaviour and fate of macronutrients and pollutants in sewage sludge applied to the land are affected by the chemical composition of the sludge organic matter, which in turn is influenced by both sewage source and by sewage treatment processes. In this study, ¹³C nuclear magnetic resonance (NMR) spectroscopy was used to characterise the organic matter of sludges collected at three different points along the treatment stream of a municipal sewage works with a domestic catchment. Sludge at the first point, an undigested liquid (UL) sludge, had a substantially different composition to the anaerobically digested (AD) and dewatered sludge cake (DC) materials, which were similar to each other. In particular, the UL sludge contained more alkyl C than the AD or DC sludges. All three sludges were found to contain mobile alkyl C that is poorly observed using the cross polarisation (CP) technique, necessitating the use of the less sensitive, but more quantitatively reliable direct polarisation (DP) technique to obtain accurate distributions of C types.

Introduction

The application and recycling of organic wastes to land is arguably the most environmentally sustainable way to dispose of such materials. In the UK over 100 million tonnes (fresh weight) of organic material a year is recycled in this way of which 820 000 tonnes is sewage sludge (Water UK, 2006). There are significant agronomic benefits from the application of organic wastes to land, including improved soil structure, water retention, drainage and nutrient availability (Düring and Gäth, 2002). However, these benefits must be balanced against the potentially detrimental environmental effects of the application of excessive nutrients, contaminants (such as metals and persistent organic pollutants) and pathogenic organisms (Düring and Gäth, 2002).

The rate of decomposition of sewage sludge organic matter following addition to soil is an important consideration in assessing potential risks. Some consequences of organic matter decomposition are clear, such as the release of N (Smith et al., 1998a, Smith et al., 1998b, Smith et al., 1998c) and P (Smith et al., 2006). However, there are other, indirect, consequences of organic matter decomposition, such as a decreasing capacity to sorb heavy metal pollutants (Chang et al., 1997, Antoniadis et al., 2007).

The rate of organic matter decomposition in the field is a function of both environmental conditions and the initial composition of the sludge, which, in turn, is influenced by its source (e.g. domestic versus industrial catchments) and the treatment process. For example, Rowell et al. (2001) reported differences in C mineralisation of sludges both between application sites and between different sludges at the same site, and Smith et al., 1998b, Smith et al., 1998c attributed differences in N mineralisation rate between sludges to differences in their organic matter composition.

Solid-state ¹³C NMR spectroscopy has been widely used to identify differences in organic matter chemistry between sludges and to follow changes in chemistry over time in the field (Pfeffer et al., 1984, Piotrowski et al., 1984, Leinweber et al., 1996, Ayuso et al., 1997, Hsiao and Lo, 2001, Rowell et al., 2001, Stacey et al., 2001, Smernik et al., 2003a, Smernik et al., 2003b, Smernik et al., 2004). Using this technique, it has become clear that the composition of sewage sludge organic matter is quite variable (Piotrowski et al., 1984, Rowell et al., 2001, Stacey et al., 2001, Smernik et al., 2003a), but it is usually richer in alkyl C than soil organic matter (Leinweber et al., 1996, Hsiao and Lo, 2001, Stacey et al., 2001, Smernik et al., 2003a). The variability of sludge organic matter is probably a function of both its source and the treatment processes used, but few studies have attempted to separate out these influences (Pfeffer et al., 1984). A further complication is that the quantitative reliability of the most commonly used NMR technique, cross polarisation (CP), can be compromised by the presence in sludges of either paramagnetic species (Pfeffer et al., 1984, Smernik et al., 2003a) or lipids with a high degree of molecular mobility (Smernik et al., 2003a).

This study uses a range of sophisticated solid-state ¹³C NMR techniques to quantitatively characterise the organic C of three sewage sludges sourced from different points along a single treatment process stream. The aim is to identify clearly how organic matter varies along a treatment stream in the absence of differences in source material.