Investigations on biochar production from biosolids via microwave assisted pyrolysis and its potential applications
Other Publication ResearchOnline@JCUAbstract
Biosolids are the treated solid fraction produced in a municipal wastewater treatment plant. Biosolids production has been increasing every year mainly because of: 1) population increase, 2) more access to a sewage network, in particular in developing countries, and 3) more restrictive environmental regulations. Land application is typically used to manage biosolids, but contaminant leaching from biosolids to soil and groundwater has restricted this approach. Currently in Europe, biosolids are mainly incinerated, but in Australia there are no incineration plants, so a new environmental and sustainable solution for biosolids management has to be established in the near future. These new approaches must be focused on biosolids recycling and by-products valorisation, mitigating environmental impact and resource depletion. This research studied microwave assisted pyrolysis (MWAP) as a methodology for biosolids treatment and investigated the potential applications of the final biochar. This study investigated the dielectric properties of biosolids and the impact of MWAP conditions on biosolids processing and biochar properties. The dielectric properties of compacted biosolids with varying moisture content were measured at different microwave frequencies using a network analyser. Results showed that as the moisture content increased in biosolids, the dielectric constant and dielectric loss factor both increased. Dry biosolids were almost transparent to microwaves, indicating that the ability to absorb microwave energy is very low. To overcome this problem, an addition of a microwave susceptor (activated carbon) was employed for the biosolids pyrolysis experiments. As single-mode microwave cavities have just one "hot spot", it is fundamental to place the biosolids sample in this area to maximize coupling between microwaves and the sample. Numerical simulations were carried out to assess the impact of biosolids load and moisture content on microwave field intensity and distribution. These simulations were done by using Inventor Professional 2015 software (AutoDesk Co., USA) and XFdtd software, version 7.5.1.3.r43518 (Remcom Co., USA) to solve the Maxwell's electromagnetic equations for the pre-defined materials and microwave feed ports conditions in XFdtd. Results showed that distribution and intensity of the microwave field were significantly affected by moisture content. As the moisture increases the intensity and distribution of microwave field decreased due to the adsorption of electromagnetic energy by the sample because water is a good microwave absorber. As the sample load increased, the electromagnetic field decreased, and its distribution was clearly affected due to field perturbation. Pyrolysis conditions affect product yield and distribution, and the properties of the final by-products. Of all the conditions, temperature is the pyrolysis process variable with the greater impact on by-products properties. In this study, pyrolysis temperature impacted on biochar yield, surface area and resultant functional groups present on the biochar. Biochar yield decreased while surface area increased at higher pyrolysis temperatures. The heavy metals of biosolids accumulated in the final biochar and the percentage of heavy metals increased as the biochar yield decreased. Microwave susceptors affected the heating rate and consequently the pyrolysis energy consumption, biochar yield and properties. Activated carbon was the best option for biosolids MWAP due to the lower energy consumption, and the biochar had the highest stability and surface area, which are fundamental requirements for biochar soil application. The biosolids biochar was successfully used for phosphorus and silver recovery from synthetic and real solutions. The phosphorus removal capacity by biochar was affected by the calcium content in biochar, contact time between biochar and phosphorus solution, pH and initial concentration of phosphorus of the aqueous solution. As the calcium content increased in biosolids, the phosphorus removal capacity increased due to more available calcium cations to complex with phosphate species. The phosphorus removal capacity of biochar (11.5% of Ca) was maximised at pH 4, corresponding to 147 mg-P/g of biochar. The main mechanism of phosphorus removal was precipitation, and the experimental data was well described by a pseudo second-order kinetic model and the isotherms followed the Langmuir model. Brushite (calcium hydrogen phosphate dehydrate: CaHPO₄·2H₂O) was the main precipitation product resultant from the combination of calcium cations with phosphate species. Silver is an emerging pollutant in wastewater treatment plants due to the wide use of this element as an anti-bacterial in final products. This element represents a serious threat to life even when present in very small amounts. This study showed that silver removal by biochar is a spontaneous process of physical adsorption. Silver removal capacity increased with initial silver concentration and temperature of the silver aqueous solution. Experimental data fitted was well described by the pseudo second-order kinetic model and intra-particle diffusion model. The final composite (Ag-biochar) after silver adsorption can be further used for methylene blue degradation and adsorption. The results of this study demonstrated that MWAP is an environmentally sustainable approach to deal with the anticipated increases in biosolids production. The final biochar can be used as adsorbent for contaminant removal, water pollution mitigation and resource recovery, thereby avoiding future natural resource depletion.
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217
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DOI
10.4225/28/5ac5acab2251c