Continuously flow microwave pre-treatment for enhanced anaerobic biodegradability of dairy industry sludge

Wastewater and sludge pre-treatments can be divided three major groups: chemical processes, thermal processes and biological processes. Chemical treatments are efficient, but many times cause corrosion problem in wastewater line, in equipment and tanks, or generate undesirable side-products. Biological processes can be considered more environmentally friendly than chemical treatments, but need longer time and considered more sensitive for the change and fluctuation of process parameters (temperature, pH, ionic strength, presence of inhibiting or toxic components, homogeneity of the material matrix etc.). Heat treatments has numerous advantages over the other processes, such as no formation of undesirable byproduct, more flexibility to unsteady capacity, good controllability, high number of pre-designed and modular industrial scale equipment etc. Compared to the conventional heating microwave irradiation has higher heating rate, heating equipment operated by microwaves requires less space, and using well designed system and intermittent irradiation mode the energetic efficiency is higher than conventional method (Cuccurullo et al., 2013). 

In high frequency oscillating electromagnetic field, because of the fast change of polarity, the dipolar components of irradiated medium vibrated and oscillated, and, furthermore movement of ionic compounds or charged particles has also been occurred. These effects led to intramolecular friction resulted in internal heat generation. The electromagnetic energy interact directly with the components of materials, the medium absorb the energy and convert it into heat (Mawioo et al., 2016). Considering the specific heat generation mechanisms of microwave irradiation, in real and complex medium, such as the wastewater, composition, dielectric properties of compounds, intermolecular interaction, chemical structure of molecules, physicochemical state, temperature, viscosity and density of continuous phase and their possible alteration during irradiation, furthermore the frequency of electromagnetic field has also effect on the thermal efficiency of heating (Holtze et al., 2006). Another interesting phenomenon of microwave irradiation is the selective heating. If components of a system have different dielectric behavior (dielectric constant, and/or dielectric loss) during irradiation the heating rate of solely components is different. Therefor the temperature difference, occurred by different energy absorption, can led to thermal stress inside of the material structure, or inside of microbial cell, or tissues.

In wastewater and sludge organic matters are partially present in particular form. In many cases the conventionally used wastewater purification processes aim the coagulation and flocculation of soluble organic and inorganic pollutants, transform them insoluble, particulate form. During phase separation processes, these components can be concentrated into sludge. But if the further utilization of wastewater or sludge is planned, for example in anaerobic digestion process, increasing of organic matter solubility is expediential for enhanced organic matter removal efficiency, biogas production rate and higher biogas yield. Among sludge management options anaerobic digestion (AD) has been more and more popular at wastewater treatment plant. 

The main advantage of AD is that sludge stabilization and energy generation can be carried out simultaneously. The output of AD cannot be considered obviously ‘final waste’, because digestate has a good potential for agricultural utilization, as fertilizer. One of the main problem for application of AD in-line at wastewater treatment plant (WWTP) is the long retention time. On the one hand the slow degradation increase the capital and running costs, and on the other hand present bottleneck for the overall capacity of WWTPs. Therefore, beside the enhanced dewaterability and microbial stabilization of sludge, pre-treatments serve as intensification process for AD. Results from the comparison of conventional heating and microwave irradiation for wastewater and sludge pre-treatment are contradictionally. Detach of non-thermal effects of microwave from thermal effects is difficult, because mainly of high temperature ramp and hot-spot heating effects occurred during microwave irradiation (Sólyom et al., 2011). It can be noticed, that in practice the same condition for conventionaland microwave heating in real material matrix cannot be ensured.

Beyond above mentioned difficulties microwave irradiation has verified positive effect on organic matters of wastewater and sludge, which can be utilizable as pre-treatment followed by AD. Microwave has strong effect on microbial destruction, microwave process need significantly shorter time demand than needed for conventional heat treatments. Depending on the heating rate and final temperature during the processes, with the application of microwave pre-treatments higher disintegration degree can be achieved. Therefore the higher disintegration degree and higher organic matter solubility led to higher biogas yield in AD process. Beside absolute value of biogas production, the biogas production rate presents key issue to evaluate the efficiency of a pre-treatment method. Depending on the material characteristics and heating rate and final temperature, microwave pre-treatments are suitable to increase the degradation rate, therefore accelerate the biogas production (Yang et al., 2013).

Efficiency of microwave pre-treatments depends on the type and condition of anaerobic digestion. Thermophilic digestion suitable to achieve higher biogas product, the effect of pre-treatments are slightly than that of obtained for mesophilic temperature ranged AD tests. It was found that there was no significant difference between the effects of conventional and microwave heating method on biogas yield, but temperature ramp considered as the main influential process parameter in high total solid contented sludge cake processing followed by thermophile anaerobic digestion. On the other hand, beside the temperature ramp has significant effect on biodegradation rate, as well (Koupaie and Eskicioglu, 2016). But in Europe mesophilic AD process can be considered the commonly usable and economic method, therefore the pre-treatment efficiency need to be investigated at this temperature range.

Summarized the experiences of laboratory scale batch microwave wastewater and sludge pre-treatment can be concluded that preliminary results can be considered promising. But extend the investigation for implementation of continuously flow microwave pre-treatments is need to evaluate the applicability of method, find the main influential microwave related process parameters, model and optimization of the process to make suitable the scale up (Boldor et al., 2008). Another key issue for investigation of applicability of sludge and wastewater treatment operated by microwave heating is the temperature range. Considering the disintegration degree as control parameters for pre-treatment solely, pre-treatment over 100 °C cause significantly higher increments in organic matters solubilisations. In multicomponent medium, such as food industry wastewater and sludge, application of elevated pre-treatment temperature led to Maillard reactions, in which refractory compounds are formed from amino acids and reducing sugars, and other polymerization reactions are occurred in the presence of low molecular weighted, easily degradable intermediates (Shahriari et al., 2010). Therefore, change of biogas production due to microwave pre-treatment cannot be intrinsically estimated based on solubilisation ratio alone (Koupaie and Eckicioglu, 2015). Because of evaporation in open vessel system, and polymerization reactions, over a certain value of final temperature and/or temperature ramp during microwaving process the biogas potential of some part of organic matters decrease.