ORIGINAL PAPER Iron flocculation stimulates biogas production in Microthrix parvicella-spiked wastewater sludge S. K. Pradhan • E. Torvinen • H. M. P. Siljanen • M. Pessi • H. Heinonen-Tanski Received: 15 June 2013 / Revised: 1 September 2014 / Accepted: 8 December 2014 / Published online: 18 December 2014 Ó Islamic Azad University (IAU) 2014 Abstract Municipal wastewater sludge has been used for fertiliser and biogas production for several decades. Chemical compounds such as iron and aluminium are common coagulants used in wastewater treatment plants to remove suspended solids, phosphorus and micro-organ- isms. This laboratory study explores whether ferric chlo- ride (FeCl 3 as PIX-111) or aluminium chloride (AlCl 3 as PAX-18) flocculation could stimulate biogas production in wastewater sludge contaminated with Microthrix parvi- cella. In a fermentation process run in three replicates, cumulative methane production was in average about 25 % higher using the iron flocculated sludge than using the aluminium flocculated sludge; this difference was statisti- cally significant (P \ 0.05) in the subsequent runs of the semi-continuous process. In all runs, the iron flocculated sludge produced less (P \ 0.05) hydrogen sulphide in the biogas than the aluminium flocculated sludge. The numbers of M. parvicella stayed at the similar levels throughout the process. It is concluded that biogas production is higher and more stable with iron coagulant in comparison with aluminium coagulant, presumably due to the reduced for- mation of hydrogen sulphide. Thus, iron coagulants seem to be better than aluminium coagulants to stimulate the methane production process. Both coagulants significantly suppressed multiplication of M. parvicella in the biogas reactor, i.e. they did not evoke foaming in this experiment. Keywords Aluminium Á Biogas Á Iron Á Microthrix Á Wastewater Á Sludge Introduction Municipal sludge has often been treated by anaerobic mes- ophilic digestion prior to being spread on the land. The reasons are that this treatment procedure produces energy, reduces the volume of sludge and makes more convenient its further treatment. Iron (Fe 3? ) and aluminium (Al 3? ) salts are widely used as electro-coagulants to remove phosphorus, suspended solids (SS), enteric micro-organisms etc. in wastewater treatment (Morse et al. 1998; DeWolfe et al. 2003). These coagulants are cheap and have been demon- strated to be effective under a variety of conditions. Alu- minium-based coagulants have been reported to lead to the presence of relatively high residual aluminium concentra- tions in the treated water (Yang et al. 2010). Since aluminium is a toxic element for some aquatic organisms, the use of aluminium-based coagulants has raised concerns about its safety in water treatment (Rosseland et al. 1990). The micro-organism, Microthrix parvicella, can cause problems in wastewater treatment, especially in temperate climates and in treatment plants during low loads. In fact, problems with M. parvicella have been reported in many countries, e.g. Denmark, The Netherlands, Italy, Greece, Czech Republic, Germany, France and UK. Westlund et al. (1998) reported that the biogas production of all three large wastewater treatment plants (WWTP) in the greater Stockholm area has experienced serious foaming problems caused by M. parvicella. It is regarded to be one of the most serious disturbing filamentous micro-organisms in activated sludge of WWTP since it can cause massive foaming (Eikelboom et al. 1998; Noutsopoulos et al. 2007). S. K. Pradhan Á E. Torvinen Á H. M. P. Siljanen Á M. Pessi Á H. Heinonen-Tanski Department of Environmental Science, University of Eastern Finland, Kuopio, Finland S. K. Pradhan (&) International Water Management Institute, PMB, CT 112, Cantonments, Accra, Ghana e-mail: s.k.pradhan@cgiar.org 123 Int. J. Environ. Sci. Technol. (2015) 12:3039–3046 DOI 10.1007/s13762-014-0733-6