Effect of microwave pretreatment on fate of antimicrobials and conventional pollutants during anaerobic sludge digestion and biosolids quality for land application

Sludge management in wastewater treatment plants is still one of the most pressing issues due to dramatic increases in sludge production, high sludge handling/disposal costs and increasing stringent regulatory restraints. There is a substantial effort to explore and develop cost-effective and sustainable strategies and technologies for reduction and minimization of excess sludge production all over the world. Agricultural use of stabilized municipal wastewater sludge is known as the most economically and environmentally accepted disposal option because of providing essential nutrients and organic matter back to the soil. Anaerobic digestion is one of the most widely used sludge stabilization methods. However, slow and limited solubilization of waste activated sludge during the hydrolysis step (the rate-limiting) is the bottleneck of conventional anaerobic digestion with longer retention time requirements and lower organic degradation efficiencies. To overcome this limitation, several studies have focused on different sludge pretreatment technologies to disintegrate the complex structure of sludge floc and enhance the rate of methane production in smaller bioreactors. As an effective thermal pretreatment method, microwave (MW) irradiation has become attractive by boosting anaerobic digestion of waste sludge, improving pathogen destruction and digestate dewaterability as well as having additional advantages (e.g. rapid heating, compactness, reduction in reaction times) over other sludge pretreatment options. Despite the technological advancements in anaerobic sludge digestion process (e.g. pretreatment prior to anaerobic digestion), presence of "contaminants of emerging concern" such as antimicrobials is creating a potential human and environment health threat against the beneficial reuse of biosolids. Triclosan (TCS: 5-chloro-2-(2,4-dichlorophenoxy)phenol) and triclocarban (TCC: 1-(4-chlorophenyl)-3-(3,4-dichlorophenyl)urea) are broad-spectrum polychlorinated aromatic antimicrobials, mostly used for disinfection and sanitation. They are commonly used as active ingredients in many personal care products (e.g. soap, shower gels, shampoos, toothpastes, cosmetics) as well as household consumables (e.g. plastic additives, carpets, textiles) in daily life. Partial degradation of triclosan and triclocarban during treatment processes is led to the formation of new transformation products in wastewater treatment plants. There has been an increasing concern about the high concentrations of these antimicrobials in sludge due to their high lipophilicity and poor biodegradability. When biosolids are applied to land, existing and potential endocrine-disrupting effects of triclosan and triclocarban and/or their transformation products are another area of concern. There is limited knowledge of the occurrence and behavior of antimicrobials in solid matrices, as most of the researches to date has focused on the identification and quantification of these compounds in aqueous matrices due to the complex composition of sludge and the limited analytical methods for solid matrices. Even though MW irradiation has been studied extensively on sludge disintegration and performance of anaerobic sludge digestion, no previous studies have been conducted to investigate the effectiveness of MW irradiation on the fate of multi-purpose antimicrobials particularly triclosan and triclocarban, during advanced anaerobic sludge digestion. The aim of this study is to investigate the effect of MW pretreatment on transformation of triclocarban and triclosan during anaerobic sludge digestion as well as performance of anaerobic digesters and biosolids quality for land application. The effects of MW final temperature and holding time configurations were studied employing by ten bench-scale anaerobic digesters fed with mixed sludge under thermophilic (55 ± 1°C) and mesophilic (35 ± 1°C) conditions at different sludge retention times (SRTs: 20, 12, and 6 days). MW irradiation was applied to the dewatered waste activated sludge at high solids concentration for minimizing energy consumption during pretreatment and maximizing net energy generation. The focus of the preliminary studies was to evaluate the effects of MW pretreatment intensity on the degree of disintegration/solubilization of four different mixed sludge samples. Dewatered sludge samples were irradiated to desired final temperatures of 80 and 160°C which represented the low- and high-temperature pretreatment conditions, respectively. After reaching the desired final temperatures at a constant heating ramp rate of 2.25°C/min, samples were held for 1 or 30 minutes which indicated fast and slow thermal pretreatment, respectively, to assess the effect of both MW pretreatment temperature and holding time. Un-pretreated or pretreated thickened waste activated sludge samples were mixed with raw fermented primary sludge to obtain the different mixed sludge samples before fed to digesters. Increasing MW final temperature and holding time enhanced the particulate chemical oxygen demand and biopolymers (i.e. protein, humic acids and sugar) solubilization of mixed feed sludge. MW final temperature had significantly higher effect on solubilization than holding time. Ten bench-scale anaerobic digesters (5 thermophilic and 5 mesophilic) were set-up with the inocula from existing bench-scale digesters utilizing the mixed municipal sludge from wastewater treatment plant located in Kelowna, British Columbia, Canada. Upon completion of acclimation stage, the digesters were operated in a semi-continuously fed mode (once a day, 7 days/week) at SRTs of 20, 12 and 6 days, under the corresponding organic loading rates of 1.45 ± 0.13, 2.50 ± 0.21 and 5.18 ± 0.31 g volatile solids/L/day, respectively. The operation of ten digesters was first started at a conventional SRT of 20 days and continued for a period of 67 days after the steady-state conditions established. Then, the SRT of each digester was reduced to 12 days, and finally reduced to 6 days, respectively. All digesters showed stable performance even at the lowest SRT indicating that increasing organic loading rates did not affect anaerobic digestion performance within the evaluated range with volatile solids removals above 45%. During the anaerobic digestion operations, daily specific biogas volume were in the range of 485 - 580 mL and 501 - 590 mL/g VSfed, respectively for thermophilic and mesophilic temperatures. The methane contents in all digesters' headspace were 70 ± 2, 68 ± 1 and 65 ± 1% for SRTs of 20, 12 and 6 days, respectively. Regarding biogas production, the final MW temperature had a statistically significant effect on biogas improvement for mesophilic temperature, whereas MW holding time played a statistically significant role under thermophilic temperature. At different MW temperatures, longer holding time increased volatile fatty acid (VFA) accumulation in thermophilic digesters at the shortest SRT of 6 days. No enhancement in biogas production (over the respective control) was observed from the thermophilic digesters which were pretreated for longer exposure durations (30 min). It is probable that the extreme pretreatment combinations applied to sludge can cause the formation of complex compounds which can be refractory & slightly inhibitory and/or hard to digest anaerobically. This condition may finally cause to generate less biogas potential/VFA accumulation even under relatively stable operation. On the other hand, under mesophilic temperature, methane formers in digester with pretreated at the most intensive condition (160°C-30 min) were still able to convert organics into biogas more effectively with low total VFAs concentrations compared to thermophilic counterparts. Up to 21% higher improvements in volatile solids removal was achieved by MW pretreatment followed by anaerobic digestion. Based on the improvement results in specific biogas yield and volatile solids removal, it can be concluded that anaerobic digestion systems combined with MW pretreatment were able to tolerate 15 - 20% higher loading rates compared to control digesters for the SRT of 6 days. This situation may allow full-scale digestion facilities to operate at higher loading rates with smaller digester volume requirements while reaching the same or better organic removal efficiency. The effect of MW pretreatment was evaluated on environmental concentrations of TCC, TCS and their transformation products in digester effluent streams (un-pretreated and MW-pretreated digestates) as well as digester influent streams (raw and MW-pretreated mixed sludge). All sludge samples were quantified in total (sorbed and aqueous) and aqueous (supernatant) phases using ultra-high performance liquid chromatography combined with an electrospray ionization probe and a triple quadrupole mass spectrometer (UHPLC-MS/MS). MW pretreatment enhanced the removal of persistent antimicrobial TCC from treatment sludge by 29 to 64%. However, neither MW final temperature nor duration had discernible impact on persistent TCS removal during sludge pretreatment. TCC and TCS showed different occurrence and behaviour patterns during the advanced anaerobic digestion coupled with MW pretreatment. Under both thermophilic and mesophilic conditions, the effluent concentrations of TCC in the advanced digesters fed with high-temperature pretreated mixed sludge (160°C), were significantly lower than that of in low-temperature (80°C), especially at SRT of 12 days. Reductive dehalogenation product (carbanilide) of TCC was detected and quantified for the first time during advanced anaerobic sludge digestion. On the other hand, the highest reduction in concentration of TCS was obtained from advanced digester, which was pretreated at high temperature (160°C) and long exposure duration (30 min), at SRT of 12 days, under thermophilic temperature. This research demonstrated that advanced anaerobic digestion using MW pretreatment could be used to remove environmental concentrations of TCC and TCS, which are commonly found in the sludge and biosolids. The effect of MW pretreatment was evaluated on digested biosolids quality which is associated with the presence of trace elements (heavy metals), pathogens and vectors before used as agricultural or reclaimed-land soil amendment. In terms of pathogen limits, thermophilic digesters produced Class A biosolids with less restriction for land application. Digestates from the mesophilic digesters was able to obtain Class B classification with pretreated digesters resulting in lower fecal coliform densities than the control. Almost all thermophilic & mesophilic digestates meet Class A criteria based on metal contents for land application. Acceptable vector attraction was also attained in all digesters with volatile solids removals more than 38%. An energy analysis was performed for the control (representing conventional digestion) and MW-pretreated digesters, in terms of net energy as electricity and heat to evaluate the feasibility of MW pretreatment for full-scale application. Sludge concentration subjected to MW pretreatment was found to have a substantial impact on total input energy. High-temperature MW pretreatment produced the high amount of extra heat with a combined heat and power system that can be utilized within/outside the plant. However energy recovered from the enhanced methane production was not sufficient to cover the electrical energy consumed by MW.