Nutrients and Energy Recovery from Ecological Sanitation System

Abstract

Human excreta are abundant waste streams whose safe disposal to the environment is a challenge in many developing countries due to inadequate sewerage system and poor faecal sludge management. The growing practice of resource recovery from human excreta could minimize the challenge of unsafe disposal thereby contributing to protection of the environment and human health. The overall aim of the study was therefore to recover nutrients from human urine and energy from human faeces. On nutrient recovery, the study focused on adsorption of ammonium nitrogen (NH4+-N) and phosphorous (P) in human urine using pineapple peel biochar (PPB) and lateritic soil (LS). Physicochemical properties of PPB, and LS were characterized by Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS), X-ray powder Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetric Analysis (TGA) to investigate the relationship of their properties with adsorption of NH4+-N and P. Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models were fitted to correlate the experimental equilibrium adsorption data with the coefficient of correlation (R2) used to determine the model that offered the best fit. The effect of contact time and initial concentration of NH4+-N and P on adsorption was evaluated. Factors influencing adsorption process such as the contact time, agitation speed, and adsorbent loading were optimised using Response Surface Methodology (RSM) in order to predict the optimum conditions for achieving the highest adsorption of NH4+-N and P. For adsorption of NH4+-N, the D-R isotherm model best described the behaviour of its adsorption on both PPB and LS based on the coefficient of correlation values. This model showed that the adsorption of NH4+-N on both samples was a physical process with PPB and LS having mean surface adsorption energies of 1.826×10-2, and 1.622×10-2 kJ/mol, respectively. The PPB exhibited a slightly higher adsorption capacity for NH4+-N (13.40 mg/g) than LS (10.73 mg/g) with the difference attributed to its higher contact surface area and porosity. The adsorbed NH4+-N on PPB at optimal conditions described by RSM of 60 contact time, 80 rpm agitation speed, and adsorbent loading of 0.1g, yielded experimental adsorption capacity of 36.42 mg/g, which agreed well with the predicted adsorption capacity of 36.80 mg/g. These values are good indicators for assessing the effectiveness of the materials for adsorption of NH4+-N from human urine. For adsorption of P, PPB exhibited a tendency to adsorb and gradually release P into the human urine thereby not attaining equilibrium. LS on the other hand adsorbed P upto equilibrium. The P adsorption data on LS fitted the Langmuir model best (R2 = 0.984) compared to the Freundlich model (R2 = 0.966). The mean surface adsorption energy of 1.313 × 10-2 kJ/mol obtained from the D-R model (R2 = 0.858) indicated that the adsorption of P on LS occurred through weak forces of interaction. The amount of adsorbed P on LS at equilibrium (qe) increased with the initial concentration of urine as well as with the contact time. The Langmuir maximum adsorption of P on LS was found to be 45.25 mg/g. The RSM demonstrated that contact time 30 min, agitation speed 80 rpm, and adsorbent loading 0.1 g in 25 mL of urine were optimum for highest adsorption of P (112.80 mg/g). The results show that LS is a promising inexpensive adsorbent for effective removal and recovery of P from human urine. On energy recovery, the study examined the kinetic analysis of the thermal decomposition of faecal char, sawdust char and their blend using thermogravimetric analysis (TGA) under air to assess their reaction rates and thermal stability as potential sources of fuel. The kinetic parameters during the degradation were tested by combining the Kissinger-Akahira-Sunose (KAS), and Flynn-Wall-Ozawa (FWO) iso-conversional methods. The TGA and deconvoluted DTG curves indicated moisture release followed by devolatilization of hemicellulose, cellulose, and lignin between ~300 and 450 °C, before ash formation. Both KAS and FWO methods described the kinetic processes realistically and yielded similar activation energy values. The lowest activation energy required to initiate degradation as calculated by KAS were found to be 80.4, 80.0, and 86.4 kJ/mol for faecal char, sawdust char, and blend, respectively. On the other hand, the activation energy for the whole conversion range was 103.7, 108.7, and 104.8 kJ/mol for faecal char, sawdust char, and blend, respectively. These results suggest that the compositional differences between the samples translated to variability in the weight loss rates, shapes of decomposition peaks, and parallel, competitive, and complex reaction schemes resulting to irregular trend in the activation energy. Furthermore, the study on energy recovery compared the heating properties and toxic flue gases namely; carbon monoxide (CO), nitric oxide (NO), and hydrogen sulphide (H2S), emitted during combustion of charcoal and co-combustion (50:50 wt. %) of charcoal with briquettes densified from human faecal char, sawdust char, and molasses. The physicochemical properties (fixed carbon, volatile matter, moisture content, ash content, and gross calorific value) of the briquettes and charcoal was determined and characterization of the flue gases (concentration, oxygen level, combustion temperature) conducted using E8500P industrial integrated emissions system combustion gas analyser. It was observed that combustion of charcoal did not emit NO, however the concentration of the CO was above the critical short-term limits of 35 ppm. The level of emission of the CO and H2S was above the short-term exposure limits of 35 and 0.005 ppm, respectively during co-combustion whereas NO concentration was below dangerous exposure levels of 100 ppm throughout the combustion period. Co-combustion resulted into release of higher heat energy as evidenced by the flue gas temperatures reaching upto 475 °C compared to 222 °C during charcoal combustion. The gross calorific value for briquettes was 19.8 MJ/kg which was comparable to those reported for fuel wood although lower than 25.7 MJ/kg reported for charcoal. These results suggest that co-combustion of charcoal with briquettes densified from faecal and sawdust char is a promising alternative approach to generate safe and sufficient heat energy for indoor use, reduce deforestation, and mitigate unsafe faecal waste disposal issues. In conclusion, this research demonstrates that nutrients can be captured from human urine to produce enriched biomass that can be used as slow-release fertilizers. The study findings also demonstrated that human faeces are a potential biomass for production of safe solid fuels. Hence, nutrients and energy recovery from human excreta could mitigate the environmental and health impacts associated with their unsafe disposal into the environment.