Composting of de-inking sludge from the recycled paper manufacturing industry Teresa Gea, Adriana Artola, Antoni Sa ´nchez * Escola Universita `ria Polite `cnica del Medi Ambient, Universitat Auto `noma de Barcelona, Rbla Pompeu Fabra 1, 08100-Mollet del Valle `s, Barcelona, Spain Received 26 January 2004; received in revised form 28 September 2004; accepted 30 September 2004 Available online 2 December 2004 Abstract Composting of two different types of sludge from the recycled paper manufacturing industry was carried out at laboratory scale. Physico-chemical sludge (PCS) from the de-inking process and biological sludge (BS) from the wastewater treatment plant were composted and co-composted with and without addition of a bulking material. Despite its poor initial characteristics (relatively high C/N ratio, low organic content and moisture), PCS showed excellent behaviour in the composting process, reaching and maintaining thermophilic temperatures for more than 7days at laboratory scale, and therefore complete hygienization. Pilot scale composting of PCS was also studied, and a respiratory quotient of 1.19 was obtained, indicating a full aerobic biological process. Respiration tests showed a complete stabilization of the material, with final values of the static respiration index in the range of 1.1mg O 2 gTOM 1 h 1 . Composting is proposed as a suitable technology for the effective recycling of this type of sludge from the recycled paper manufacturing industry. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: C/N ratio; Composting; Hygienization; Recycled paper manufacturing sludge; Respiratory quotient 1. Introduction In recent years, new legislation in the European Union and the United States has promoted the utiliza- tion of recycled fibres in newsprint. This fact, together with the implementation of source-separated waste paper collection programs, has changed the raw materi- als in the paper manufacturing industry. In Spain, some industries are solely accepting waste paper to transform it into recycled paper. Recycled paper industries remove inks, clay filters and coatings of used paper by a de-inking process and recycle the wood fibres by using physico-chemical treat- ments. However, some wood fibres are rejected from this process and constitute a sludge with some organic con- tent. Moreover, this type of industry usually generates biological sludge from the biological treatment of wastewater. Since the majority of sludge from paper manufactur- ing industries is landfilled or incinerated, alternative methods to treat this waste are being developed. Com- posting is one of the most promising technologies to treat paper sludge in a more economical way (Das et al., 2002a). It is defined as the biological decomposi- tion and stabilization of organic substrates, under controlled conditions (Haug, 1993). The composting process permits the hygienization of the product by reaching thermophilic temperatures and reducing mass and volume, which makes compost suitable for agricul- tural applications. Previous works have studied the feasibility of the composting of sludge from different pulp and paper manufacturing industries. Jokela et al. (1997) studied 0960-8524/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2004.09.025 * Corresponding author. Tel.: +34 93 5796784; fax: +34 93 5796785. E-mail address: asanchez@eupma.uab.es (A. Sa ´nchez). Bioresource Technology 96 (2005) 1161–1167