A comparison of product yields and inorganic content in process streams following thermal hydrolysis and hydrothermal processing of microalgae, manure and digestate
Introduction
Hydrothermal processing of biomass can be utilised as either a pre-treatment or for energy densification. Thermal hydrolysis is often used prior to anaerobic digestion at temperatures in the range 160–170 °C resulting in enhanced biogas yields. Hydrothermal carbonisation (HTC) is operated at 180–250 °C and pressure between 2 and 10 MPa, and produces a carbon-rich bio-coal (Mumme et al., 2011). Hydrothermal liquefaction (HTL) is operated at 280–370 °C and pressures ranging from 10 to 25 MPa and produces a synthetic bio-crude (Biller and Ross, 2011). Supercritical water gasification (SCWG) is operated at temperatures above 450 °C and pressure above the critical point of water (22 MPa) producing a syngas containing H2, CO2 and CH4 (Toor et al., 2011). There is a growing interest in the recovery of nutrients from wet wastes such as manures and bio-solids and hydrothermal processing has been proposed to facilitate the extraction of nitrogen, phosphorus and potassium from these materials (Biller et al., 2012, Heilmann et al., 2014). Concentrated animal feeding operations (CAFOs) such as dairy, swine and poultry produce significant amounts of manure which pose challenges for safe and effective disposal. Manures from piggeries, poultry and dairy farming are commonly applied to land as fertilizers or are processed by anaerobic digestion following which digestate can be applied to land.
Extensive research has focussed on the hydrothermal processing of wastes such as sewage sludge (Zhu et al., 2011, Xu et al., 2012) and to a lesser extent manures but most have focused on energy densification (He et al., 2001, Theegala and Midgett, 2012, Chen et al., 2014, Titirici et al., 2007, Funke and Ziegler, 2010, Berge et al., 2011). A number of studies have investigated the fate of phosphorus in either the solid product or the aqueous product. Heilmann et al. (2014) demonstrated that during the HTC of swine, diary and chicken manures, over 90% of the phosphorus was associated with the hydrochar precipitated as phosphate salts. Similarly, Dai et al. (2015), investigated the immobilisation of phosphorus (P) in hydrochar from diary manure at 200 °C and observed an increase in apatite P due to the high levels of calcium in the hydrochar. In this study, HTC was proposed as a method of manure management, reducing soluble P and reducing the risk of P loss to the environment. He et al. (2000) performed HTL of swine manure at temperatures between 275 and 350 °C and observed that the reaction conditions had little influence on the distribution of nitrogen, phosphorus and potassium species (NPK) which was mainly found in the aqueous product.
Hydrothermal liquefaction of microalgae have indicated that a large proportion of the nitrogen and the phosphorus in the feedstock are found in the aqueous phase (Yu et al., 2011, Yu et al., 2014), and highlighted that the fate of P is closely linked to the metal composition of the feedstock. Feedstock high in calcium or magnesium will favour precipitation of the P into the solid phase (Yu et al., 2014). This is in agreement with the immobilisation of P as described by Heilmann et al. (2014). A number of reports have shown that there are sufficient nutrients in the process waters following HTL and SCWG (Biller et al., 2012, Cherad et al., 2013, Lopez Barreiro et al., 2015, Tsukahara et al., 2001, Jena et al., 2011) and hydrothermal carbonisation of algae (Du et al., 2012) to cultivate fresh microalgae. López Barreiro et al. (2014) observe that the levels of ammonium in the process waters increase following gasification compared to liquefaction. The levels of phosphate recovery in the process water were found to vary with feedstock (López Barreiro et al., 2014) and once again are linked to the inorganic content of the feedstock.
Hydrothermal processing therefore has the potential for facilitating the recovery of nutrients although its extraction is feedstock dependent. To the authors’ knowledge, no study has previously compared the extraction of NPK from the same feedstock via all four hydrothermal processing routes using the same reactor conditions. This study investigates the fate of NPK in the process streams following thermal hydrolysis, HTC, HTL and SCWG of swine and chicken manure and compares this to digestate and microalgae.
Section snippets
Materials
The four biomass feedstocks used in this study were obtained from different sources. Chlorella vulgaris was obtained as a dry powder from a commercial source. The sewage sludge digestate was provided by OWS (Belgium). The poultry and swine manure were collected from the University of Leeds farm. The manure and the digestate were pre-dried in an oven at 60 °C for several days after which they were ground into powder using an Agate Tema barrel before characterisation. The samples in powder form
Characterisation of feedstock
The proximate and ultimate analyses of the four feedstock investigated are listed in Table 1. The results show that nitrogen content was highest for the microalgae (C. vulgaris) at 9.7 wt.% followed by the chicken manure (5.7 wt.%) and swine manure (3.0 wt.%) with the digestate containing the least (2.7 wt.%). C. vulgaris is well known to be rich in protein which is responsible for its high nitrogen content (Toor et al., 2011). In all the four feedstocks investigated, the proportion of sulphur was
Conclusions
The aqueous phase following thermal hydrolysis and hydrothermal processing contains significant levels of nitrogen (N), phosphorus (P) and potassium (K). The extraction of phosphorus is feedstock dependent and linked to the presence of inorganics such as Ca, Mg and Fe. Phosphorus is typically immobilised in the residue at higher temperature processing due to precipitation of phosphate salts. Microalgae and chicken manure release phosphorus more easily than swine manure and digestate. At lower
Acknowledgements
The authors would like to thank the Niger Delta Development Commission, Nigeria for financial support of Miss Ugochinyere Ekpo and the European Commission for financial support of the ‘Biorefine’ project via the ERDF Interreg IVb NWE region programme.
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