Influence of trace element supplementation on anaerobic digestion of chicken manure: Linking process stability to methanogenic population dynamics

Abstract

The effect of trace elements supplementation in anaerobic digestion of chicken manure (CM) and their influence on methanogenic population dynamics were assessed at total ammonia nitrogen (TAN) concentrations as high as 5000 mg/L for 332 days. With addition of selenium (Se), the CH₄ yield increased 50% and reached to 0.27 ± 0.01 L/g of volatile solids (VS) added. The results of metagenomic analysis showed that Se supplementation improved the digestion stability and CH₄ production by increasing the activity and number of hydrogenotrophic Methanoculleus bourgensis. Due to the deficiency of trace elements other than Se, in course of time the CH₄ yield decreased and dropped to 0.13 L/g of VS added. After starting to add a trace element mix containing cobalt (Co), nickel (Ni), molybdenum (Mo), tungsten (W) and Se, another hydrogenotrophic methanogen, Methanobrevibacter became dominant in the digester. Accordingly, along with the conversion of propionate to acetate and consumption of acetate via syntrophic acetate oxidation, the CH₄ yield increased and reached to steady state at 0.32 ± 0.01 L/g of VS added.

Introduction

The chickens reared in large farms for their meat and eggs provide an inexpensive source of animal protein for humans (Lynch et al., 2013). Commercially, egg-laying hens and broilers (meat-chickens) are reared in separate farms. In average, daily amount of manure excreted by a chicken is estimated to be approximately 120 g for egg laying-hens and 80 g for broilers by wet weight (Bolan et al., 2010, Santos Dalólio et al., 2017). While broiler manure contains feces, bedding materials, feathers and spilled feed, manure of egg-layers is generally consists only of droppings collected on conveyor belts (Molaey et al., 2018).

Chicken manure contains valuable plant nutrients such as nitrogen, phosphorus, potassium, and some others (Bayrakdar et al., 2017a). However, use of raw chicken manure damages the plants and lead to various environmental problems such as eutrophication in surface waters, spread of pathogens, air pollution and greenhouse gas emissions (Kelleher et al., 2002).

Recently, significant improvements were achieved in poultry sector in Turkey in terms of hen number, production technologies and marketing. It has been reported that 273 million chickens were raised in large farms for their eggs and meat in 2014. With the increase in number of chickens, 7 million tons of CM are produced every year and raise the relevant environmental concerns (Sürmeli et al., 2017a). Because of the high CH₄ potential and nutrient rich digestate of chicken manure, anaerobic digestion (AD) is considered as the most favorable method to stabilize it with concurrent biogas production (Sürmeli et al., 2017b). However, the great amount of organic nitrogen in the form of undigested proteins and uric acid in chicken manure causes high ammonia concentrations in anaerobic digesters (Bolan et al., 2010). If the ammonia concentration exceeds the inhibition threshold, problems such as process instability and low CH₄ yield are encountered (Bujoczek et al., 2000).

Some recent studies revealed that at high ammonia concentrations, trace element addition enhances the stability of AD (Bayrakdar et al., 2017b, Molaey et al., 2018, Westerholm et al., 2015). Supplementation of anaerobic food waste digesters with trace elements such as Se and Co were found to be necessary for stable operation at elevated ammonia concentrations (Banks et al., 2012).

The CH₄ production at high TAN concentrations is attributed to syntrophic acetate oxidation followed by hydrogenotrophic methanogenesis (Westerholm et al., 2016, Westerholm et al., 2015). It is known that deficiency of essential trace elements leads to process instability at high TAN levels by depressing the syntrophic acetate oxidation (Plugge et al., 2009). Formate dehydrogenase (FDH) plays an important role in syntrophic acetate oxidation and is dependent on trace elements such as Se, Mo and W (Banks et al., 2012, Plugge et al., 2009). Furthermore, Co is required for coenzyme M methyl-transferase (Pobeheim et al., 2011) and Ni is the most important nutrient for carbon monoxide dehydrogenase (CODH) that known to be active on the syntrophic co-culture (Westerholm et al., 2015).

At high TAN concentrations the hydrogenotrophic Methanoculleus bourgensis and Methanobrevibacter were found to be the dominant methanogens in mesophilic anaerobic digester (Bayrakdar et al., 2017b). These methanogens were reported to be resistant to high TAN and VFAs concentrations (Maspolim et al., 2015, Maus et al., 2015).

In our former study, we recently found that Se supplementation significantly stimulated the CH₄ production by hydrogenotrophic Methanoculleus bourgensis in an anaerobic chicken manure digester (Molaey et al., 2018). However, further studies are needed to elucidate the relationship between the trace elements and methanogens at elevated TAN concentrations. Therefore, in this study a trace element mix consisting of Co, Mo, Ni, Se and W was added to an anaerobic CM digester and its effectiveness on ammonia inhibition was assessed by linking the process stability to methanogenic population dynamics.