Negral L., Marañón E., Fernández-Nava Y., Castrillón L., University Institute of Industrial Technology of Asturias, Spain
(free)Abstract Biodegradability tests were performed on sewage sludge to assess the effect of conditioning additives (FeCl3 and cationic polyelectrolytes) on the methane yield. In addition, enzymic hydrolysis pretreatment (42 ºC, 48 hours), called “Inverted Phase Fermentation” (IPF), was also applied to the sludge as a pretreatment and biodegradability tests were performed on the concentrated solid phase obtained. The specific methane potential (SMP) of untreated (raw) sludge was 246 LCH4/kgVSo. The addition of high amounts of FeCl3 (5.6 g/L sludge) caused a minor change in the SMP, yielding 242 LCH4/kgVSo. When a cationic polyelectrolyte was also added, increases of 11% and 25% in SMP were obtained depending on the flocculant. When applying IPF as pretreatment, the concentrated solid phase produced a 20% increase in SMP if the sludge did not contain FeCl3, whereas a decrease of 6.8% was obtained if the sludge contained this coagulant. As IPF concentrates solids in an upper thickened layer, the addition of a cationic polyelectrolyte to the sludge inhibited this pretreatment.
Keywords Sewage sludge, biochemical methane potential, inverted phase fermentation, FeCl3, cationic polyelectrolyte
Introduction The advantage of Biochemical Methane Potential (BMP) tests resides in the rapid information obtained regarding the biodegradability of a substrate. As these tests are performed under controlled conditions, they provide sludge managers with relevant information to implement appropriate strategies for sludge digestion. Although BMP tests do not reach the specific methane production (SMP) of continuous digestion (Davidsson et al., 2007), they do not require a long time to achieve the optimum Hydraulic Retention Time (HRT).
However, some relevant aspects of BMP should be noted:
1. The ratio VSinoculum/VSsubstrate determines the feasibility of the test and the kinetics of biodegradation, its optimal value depending on the substrate and the inoculum (Raposo et al., 2006; Tomei et al., 2008; Kameswari et al., 2012).
2. Any inoculum adapted to digest a substrate is preferable to non-adapted inoculum for the same substrate. Acclimated microbiological cultures show the ability to overcome potential inhibitions (e.g. the presence of toxicants) (Chen et al., 2008).
When anaerobic digestion is performed on site, toxic xenobiotics usually come from the wastewater content. However, if anaerobic digestion is to be carried out off site, conditioning agents may be present in concentrations high enough to suppose the depletion of metabolic activity. This situation would lead to different BMP with the corresponding economic imbalance.
It is well known that raw sewage sludge presents poor SMP. This is due to the cellular occlusion of nutrients and the free occurrence of excessively high molecular weight species in the medium. The consequence is the limitation of the kinetics of methane production, in line with the idea of considering hydrolysis as the rate-limiting step of biomethanization (Skiadas et al., 2005). Effort thus focuses on enhancing this step, which is usually achieved by implementing pretreatments prior to anaerobic digestion. Pretreatments can not only accelerate the conversion of macromolecules into more edible substrates for bacteria strains, but also usually increase the amount of methane produced (Carrère et al., 2010). Müller (2001) ranked several pretreatments of sewage sludge according to operating behaviour and the subsequent sludge treatment processes. The Enzymic Hydrolysis pretreatment “Inverted Phase Fermentation, IPF” (Le et al., 2008) presents the advantage of enhanced solubilisation of the sludge coupled to its thickening without the need for reagents. In addition, this form of hydrolysis supposes a reduction in Escherichia coli of up to 99.9%.
This paper presents the results of BMP tests on raw sludge, sludge with conditioning agents (FeCl3 and the cationic polyelectrolyte “Chemifloc CH50” or “Chemifloc CH80”), sludge pretreated by IPF, and sludge pre-treated by IPF and FeCl3.