Anaerobic co-digestion of livestock wastes with vegetable processing wastes: A statistical analysis

Abstract

Anaerobic digestion of livestock wastes with carbon rich residues was studied. Swine manure and poultry litter were selected as livestock waste, and vegetable processing waste was selected as the rich carbon source. A Central Composite Design (CCD) and Response Surface Methodology (RSM) were employed in designing experiments and determine individual and interactive effects over methane production and removal of volatile solids. In the case of swine manure co-digestion, an increase in vegetable processing waste resulted in higher volatile solids removal. However, without a proper substrate/biomass ratio, buffer capacity of swine manure was not able to avoid inhibitory effects associated with TVFA accumulation. Regarding co-digestion with poultry litter, substrate concentration determined VS removal achieved, above 80 g VS L⁻¹, NH₃ inhibition was detected. Statistical analysis allowed us to set initial conditions and parameters to achieve best outputs for real-scale plant operation and/or co-digestion mixtures design.

Introduction

Castilla y León is one of the most important pig and poultry producers in Spain. In 2008, pig production was 3.7 million and lying hens production was 47.5 million (MARM, 2010). The continuous development of intensive pig and poultry production in that region has lead to an increase of livestock wastes in small and located areas. Uncontrolled discharge of these wastes cause serious environmental, social and health problems, thus it is necessary to minimize the risks following the current legislation.

Anaerobic digestion is a large extended technique with several full biogas-plants under operation for organic solid waste treatment and energy recovery (Angelidaki et al., 2005, Bolzonella et al., 2006). Although composting and direct application to land as a substitute for inorganic fertilizers are also widely used treatments, specially when dealing with poultry litter (PL), the rise in environmental concerns associated with the production of energy and CO₂ mitigation policies has renewed interest in digestion technologies. In this context, anaerobic digestion has largely been studied in recent years as a suitable technique for the treatment of swine manure (SM) and PL allowing reduction in organic matter content and odors and producing energy (Flotats et al., 2009). Moreover, by means of anaerobic digestion, pathogens can be minimized and even removed (Sahlström, 2003).

Anaerobic digestion of SM and PL has been extensively studied often leading to low methane yields due to the high amount of water, fibers and nitrogen content of these wastes (Bujoczek et al., 2000, Magbanua et al., 2001). Furthermore, breakdown of proteins during anaerobic digestion raise ammonium concentration of the medium (Angelidaki and Ahring, 1993). This fact contributes to ammonia-mediated inhibition of the process. Depending on the adaptation degree of microbial population, unionized ammonia has been reported to inhibit methanogenesis at initial concentration of 100–1100 mg N L⁻¹ (Angelidaki and Ahring, 1993, de Baere et al., 1984, Nielsen and Angelidaki, 2008). Other relevant factors which may hinder the digestion process, and thus needing special consideration, are organic overloading caused by high concentration of total solids (TS) and inadequate substrate to biomass ratio. When employing SM as substrate, increasing methane production is attained with increasing TS concentration, until a certain threshold after which methane production decreases due to total volatile fatty acid (TVFA) accumulation from the acidogenesis phase. More specifically, Fischer et al. (1984) reported a maximum biogas yield at solid concentration of 70 g TS L⁻¹. Similar results have been suggested for PL digestion when different concentrations of solids were evaluated. Under mesophilic digestion of PL, Webb and Hawkes (1985) achieved a maximum biogas production at TS concentration of 40–60 g TS L⁻¹, while higher TS concentration led to a reactor performance failure due to the increase of free ammonia concentration.

The low carbon/nitrogen ratio (C/N) characterizing the above-mentioned wastes makes them suitable for co-digestion with other carbon-rich co-substrates. Hence, methane production of both substrates is enhanced by reaching a more balanced C/N ratio as well as decreasing potential ammonia or VFA-mediated inhibition. Livestock wastes provide the nitrogen necessary for cell growth and their high buffer capacity avoids pH drops. The carbon-rich co-substrate supplies organic matter improving methane yields and avoiding toxic ammonia concentrations (Murto et al., 2004, Álvarez et al., 2010).

The aim of this work was to study the effect of two operating parameters, namely the initial concentration of substrate and the proportion of vegetable processing waste mass added as co-substrate on the anaerobic co-digestion of SM and PL under batch conditions. A central composite design followed by response surface methodology was applied in order to determine the effect of both operating parameters over the methane yield and the volatile solids removal.