Ultrasonication has been widely tested to improve the hydrolysis rate in anaerobic digestion of biosolids [1, 2]. Ultrasonication disrupts biosolids flocs and bacterial cells, releasing intracellular components, subsequently improving the rate of anaerobic degradation due to the solubilisation of the particulate matter, decreasing solid retention time (SRT) and improving the overall performance of anaerobic digestion [3]. The use of ultrasonication in the pretreatment of waste activated sludge (WAS) improved the operational reliability of anaerobic digesters, decreased odor generation and clogging problems, and enhanced sludge dewatering [4]. However, economical feasibility and durability due to erosion of the sonotrode as well as high energy inputs are major challenges that need to be resolved for the technology to spread [4]. Sludge characteristics such as type of sludge (primary solids, waste activated sludge or animal manure, etc.), total solids (TS) content and particle size could highly impact the disintegration efficiency and improve the overall economy of the process. Ultrasonication pretreatment studies found in the literature have focused mainly on WAS. While anaerobic digestion of hog manure is widely practiced, there has been sparse research on enhancing its hydrolysis. The main differences between hog manure and municipal biosolids, i.e. primary and waste activated sludge are: solids concentration, composition and heterogeneity. In general, the limiting step for the anaerobic digestion is the first step, hydrolysis, wherein the cell wall is broken and particulate
substrates are enzymatically hydrolyzed allowing the organic matter inside the cell to be available for biodegradation.
Hydrolysis is well documented to be a function of specific surface area among other variables [5]. Since hydrolysis is also a function of the ratio of biomass to particulate concentration (both of which are combined as volatile suspended solids), the rate of solubilisation depends on the nature and concentration of the particulates. Fibrous substrates such as hog manure will likely hydrolyze slower than WAS and primary sludges due to differences in particle size and the ratio of biomass to particulate substrates. Thus, pretreatment is required in order to achieve the release of lignocellulosic material and thus accelerate the degradation process by means of waste solubilisation. In the literature, there is a contradiction about the effect of TS content on disintegration efficiency. Akin et al. [6] studied WAS disintegration efficiency at various TS contents (2, 4 and 6%), specific energy (SE) inputs (up to 40000 kJ/kgTS) and ultrasonic densities (from 0.44 to 3.22 W/mL), and found that at constant TS content, the soluble chemical oxygen demand (SCOD) release showed an increasing trend with the increase in both specific energy input and ultrasonic density at all TS contents. However, at constant specific energy, the SCOD release decreased with the increase in initial TS content. This finding contradicts other studies that reported significant improvement in SCOD release with WAS for TS concentration in the 0.8 to 2.5% range [7, 8].
It is well known that sludge viscosity increases with solids concentration, with the critical concentration around 25 g/L or 2.5% TS content [9]. Ultrasonication efficiency is expected to decline with increasing viscosity due to resistance to energy flow, and theoretically increased TS concentrations are detrimental to ultrasonication, despite the lack of consensus on the critical solids concentrations.
Odor generation from biosolids is a significant global problem as it negatively affects natural environments. Laboratory tests have indicated that protein degradation, especially the bound protein, i.e. proteins that are physically adsorbed on the outer cell wall which can detach during high speed centrifugation, a very popular sludge dewatering technology, is the main precursor for the odor production in biosolids [10]. Proteins are hydrolysed by extracellular enzymes (proteases) into their constituent polypeptides and amino acids. Hydrogen sulfide (H2S) can be formed from the degradation of the sulfur containing amino acid such as cysteine,
leucine, tyrosine and methionine. The pathways for production of methyl mercaptan and hydrogen sulfide from protein are described by Higgins et al. [10]. Based on an extensive literature search, it can be concluded that the effect of ultrasonication on odor compounds precursors, especially bound protein needs more research since the very limited studies on protein solubilisation focused primarily on total and soluble protein measurements with no information on the critical bound proteins from an odor perspective. For instance, Wang et al. [11] examined protein release using WAS (TS content of 3%) at different ultrasonication densities (from 0.528 to 1.44 W/mL) and different ultrasonication times (from 5 to 30 min). The aforementioned authors investigated the protein in EPS, total protein and cell protein (difference between total protein and protein in EPS). Akin et al. [6] studied the effect of ultrasonication on protein release at different TS content.
The evaluation of ultrasonication efficiency in the literature is mostly based on the degree of disintegration (DD), which is the ratio between SCOD releases by ultrasonication divided by SCOD releases by chemical disintegration. It appears from the literature that there is no unique method for determining chemical disintegration. For instance, Kunz and Wagner [12] used 1 M NaOH in the ratio of 1:3.5 by volume at 20◦C for 22 h, while Muller and Pelletier [13] used 1 M
NaOH at a ratio of 1:2 by volume at 90◦C for 10 min, and Bougrier et al. [14] used 1 M NaOH at room temperature for 24 h. Additionally, the used techniques are time consuming and expensive [15].
The extensive literature reviewed above highlighted the challenges of applying ultrasonication to hog manure vis-a-vis WAS and primary sludges due to its characteristics such as fibrous versus excess biomass, particulate to biomass ratios, total solids concentrations well above the 2% - 3% for WAS and primary sludge leading to increase viscosity, and heterogeneity. Furthermore, it is apparent that despite the few studies on protein solubilization, the bound protein fraction implicated in odor generation has not been investigated.
Therefore, the overall objective of this study is to evaluate the impact of ultrasonication on solubilisation and anaerobic biodegradability of hog manure with high solid content and wide ranges of particle sizes, with particular emphasis on the effect of ultrasonication on proteins solubilisation, especially bound protein. Additionally, in this work, correlations between standardized and easy to measure solubilisation parameters and the laborious and expensive method of degree of disintegration will be presented.
3.2 Material and methods