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The potential for recovery of nutrients from biomass by hydrothermal processing

Ekpo, Ugochinyere Ngozi (2016) The potential for recovery of nutrients from biomass by hydrothermal processing. PhD thesis, University of Leeds.

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Hydrothermal processing has received increased interest mostly in the area of waste conversion to higher density fuels. However in addition to energy generation from these materials, it has become a promising route for nutrient extraction and recovery from either the solid or aqueous products depending on the processing temperature. This research was carried out in phases with the aim to improve extraction of nitrogen and phosphorus from various nutrient-rich wet wastes and the potential to recover these nutrients by biological means or by adsorption unto biochar. The initial study was on various hydrothermal processing routes – thermal hydrolysis (TH), hydrothermal carbonisation (HTC), hydrothermal liquefaction (HTL) and supercritical water gasification (SCWG) at 170°C, 250°C, 350°C and 500°C respectively using high moisture and high nutrient feedstocks namely microalgae, digestate, swine manure and chicken manure. Experiments were conducted with 10:1 water: solid ratio and 1 hour residence time except for SCWG which was 30 minutes with 15:1 water: solid ratio. The fate of N, P and other inorganics from each feedstock during these processes were investigated as well as the product yields, composition of the aqueous products and solid products. The results indicate that feedstocks behave differently during hydrothermal processing with the composition and yields of by products depending largely on feedstock composition and processing temperature. Highest solid yields were obtained at lowest temperatures while highest gas yields were obtained at the highest temperature. HTL favour formation of highest biocrude. Aqueous products from lower temperatures contain higher P levels (mainly organic-P and less phosphate) and lower N levels (mainly organic-N and less NH3-N). At higher temperature it is vice versa. Extracted P depends on Ca, Mg and Fe in unprocessed feedstock. P is immobilised in solid product at higher temperatures. TOC in aqueous product decrease with increasing processing temperature. The effect of additives on nutrient extraction during low temperature processing –Thermal hydrolysis at 120°C and 170°C as well as HTC at 200°C and 250°C using different reagents - alkali (0.1M NaOH), mineral acid (0.1M H2SO4) and organic acids, (0.1M CH3COOH and 0.1M HCOOH) was investigated. All experiments were performed with 10:1 water: solid ratio in high pressure 600 mL Parr batch reactor for 1 hour. The nutrient mass flow balance during these processes, the composition of the aqueous product and solid products were investigated. The results indicate that TN is significantly affected by temperature rather than pH. NH3-N in aqueous product increases with increasing temperature while organic- N reduces. Phosphorus extraction is pH and temperature dependent and further enhanced with additives. Acidic conditions favour phosphorus extraction especially with H2SO4 at all temperatures; highest (94%) extracted using H2SO4 at 170°C and presents opportunity for nutrient recovery. Neutral or basic conditions immobilise P in hydrochar and offers potential route for manure management as P-loss is reduced in the environment. Mg, Na and K are mostly extracted into aqueous product, while Ca and P concentrate in solid product as temperature increases. Acidic conditions extracted higher levels of micronutrient compared to water or NaOH. Generally micronutrients were more in the solid for most additives except H2SO4 while Ni and Al were mostly in the solid products. Microwave pre-treatment of various feedstocks such as sewage sludge, microalgae, digestate and manures was performed with 15:1 water: solid ratio at 120°C for 15 minutes. Also the influence of additives on nutrient extraction from swine manure was also investigated. Results show that aqueous products contain significant levels of N and P; nitrogen mainly as organic-N rather than NH3-N for all feedstock while it was mainly as organic-P for sewage sludge and digestate and more as phosphate with microalgae and manures. High TP was extracted with acidic reagents rather than with water or NaOH. With most additives, N in aqueous product was mainly as organic-N than NH3-N while P in the aqueous product was mainly as phosphate rather than organic-P. Aqueous products contain most K and Na while the residues contain most Ca, Mg, P and micronutrients for most feedstocks. Acidic conditions most especially with H2SO4 extracted more Ca, Mg, P, Co, Mn, and Zn unlike neutral or alkaline conditions. In comparison with conventional heating, microwave heating generally extracted more nitrogen and phosphorus into the aqueous products. Biological recovery of nutrients using various SCWG aqueous waste streams showed significant autotrophic growth of Chlorella in the diluted aqueous products except for the aqueous product from SCWG of Chlorella diluted at 1:50. Chlorella was able to utilise ammonium as a source of nitrogen. Higher dilutions of 1:400 had insufficient nutrients to promote growth. Biomass obtained from 1:50 dilution of catalysed SCWG of S. latissimi, L.digitata, sewage sludge and 1:200 dilution of SCWG Chlorella were comparable with biomass obtained using the standard Bold’s Basal Media. The recovered biomass could be used as feedstock for biodiesel or lipid extraction. Physical recovery of phosphate from H2SO4 extracted aqueous products by adsorption using Mg modified biochar shows that phosphate adsorption is affected by concentration and pH. Highest adsorption was achieved with 250 mg/L while higher adsorption efficiency was achieved at pH 8 and 9 compared to pH 7, precipitating more calcium phosphate on the biochar. Phosphate adsorption did not occur at lower/acidic pH conditions. There was no likelihood of struvite precipitation as EDX analysis showed no additional nitrogen and Mg in the adsorbed chars.

Item Type: Thesis (PhD)
Keywords: Hydrothermal processing,thermal hydrolysis, HTC, HTL, SCWG, nutrient recycling, manures, sewage sludge, digestate, microalgae, microwave extraction
Academic Units: The University of Leeds > Faculty of Engineering (Leeds) > School of Chemical and Process Engineering (Leeds)
The University of Leeds > Faculty of Engineering (Leeds) > School of Chemical and Process Engineering (Leeds) > Energy and Resources Research Institute (Leeds)
Depositing User: Dr Ugochinyere N Ekpo
Date Deposited: 06 Jul 2016 09:29
Last Modified: 01 Aug 2018 00:18
URI: http://etheses.whiterose.ac.uk/id/eprint/13521

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