Ultrasonic pretreatment for anaerobic digestion of suspended and attached growth sludges

Anaerobic digestion (AD) is a proven technology for energy production from the stabilization and reduction of sewage waste. The AD and impact of ultrasonic pretreatment of four waste activated sludges (WASs) from conventional and three non-conventional municipal wastewater treatment plants were investigated. WAS from a conventional activated sludge (CAS) system, a rotating biological contactor (RBC), a lagoon, and a nitrifying moving-bed biofilm reactor (MBBR) were pretreated with ultrasonic energies of 800–6,550 kJ/kg total solids to illustrate the impact of sludge type and ultrasonic pretreatment on biogas production (BGP), solubilization, and digestion kinetics. The greatest increase in BGP over the control of pretreated sludge did not coincide consistently with greater sonication energy but occurred within a solubilization range of 2.9–7.4% degree of disintegration and are as follows: 5%± 3 biogas increase for CAS, 12%± 9 for lagoon, 15%± 2 for nitrifying MBBR, and 20%± 2 for RBC. The effect of sonication on digestion kinetics was inconclusive with the application of modified Gompertz, reaction curve, and first-order models to biogas production. These results illustrate the unique response of differing sludges to the same levels of sonication energies. doi: 10.2166/wqrj.2019.039 s://iwaponline.com/wqrj/article-pdf/54/4/265/602419/wqrjc0540265.pdf Peter Roebuck Kevin Kennedy Robert Delatolla (corresponding author) Department of Civil Engineering, University of Ottawa, 161 Louis Pasteur, Ottawa, ON, Canada K1N 6N5 E-mail: robert.delatolla@uottawa.ca


INTRODUCTION
Biological treatment is the conventional means of treating municipal wastewaters. However, the conversion of soluble substrates into biomass results in a stream of waste activated sludge (WAS) that requires further management. Anaerobic digestion (AD) is an important sludge management tool that is used to degrade biomass, reduce waste volume, and stabilize the organic, putrescible content of the sludge (Appels et al. a). As a by-product, AD produces a renewable, energy-rich biogas (BG) which can be used to reduce the energy requirements of the wastewater treatment processes of municipal plants (Shen et al. ).
AD is a multistage process involving hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Pavlostathis & Giraldo-Gomez ; Appels et al. a). A cost intensive detraction to AD is its long hydraulic retention time (HRT) relative to aerobic digestion of sludge, resulting in larger footprints and higher capital costs. The rate limiting phase for the AD of sludge generated from municipal waste-  (Wang et al. ). Sonicated sludge can also indirectly improve AD by enhancing the buffering capacity of anaerobic phases sensitive to acid accumulation through increased system alkalinity and formation of bicarbonate. Methanogenic biomass can also be improved by 45-140% with sonication densities of 0.18-0.52 W/mL (Mao & Show ). Ultrasonic pretreatment has been shown to unfavourably reduce the dewaterability of pre-digested sludge, yet enhance the dewaterability after AD, thereby reducing the cost of residual solids treatment (Sahinkaya & Sevimli ). In this work, we investigate the impact of sonication and AD on a conventional sludge and three alternative sludges that are not conventionally digested anaerobically. In particular, the impact of various energy levels of sonication on BGP and solubilization were quantified through soluble chemical oxygen demand (sCOD) analysis and bioassays.
Linear and non-linear regression models based on cumulative BGP were applied to compare and interpret digestion results. These investigations provide necessary information for the application of ultrasonic pretreatment to alternative sludges to determine potential viability and BG response of the AD of varying sludges.

Sludge sources
Sludge was collected from four municipal WWTPs operating suspended and attached growth, biofilm biological treatment technologies. The inoculum and four waste sludges were characterized for total chemical oxygen demand (tCOD), sCOD, total solids (TS), and volatile solids (VS) ( Table 1). The first source of sludge for this study was thickened WAS (TWAS) that was collected from the Robert O. Pickard Environmental Centre (ROPEC) conventional activated sludge (CAS) facility, located in Ottawa, Canada. The plant is designed to treat an average of 545,000 m 3 /day with a solid retention time (SRT) of 5-7 days. ROPEC was not operated to achieve nitrification.
The bacteriological seed for AD (inoculum) was the same for all biogas tests during the study. Inoculum was collected from the mesophilic anaerobic digestors of the ROPEC facility operating at a 48/52% mixture of primary sludge and TWAS with an SRT of 20 days. The second source of sludge in this study was harvested from the Water Pollution Control Plant in Wendover, Canada and was sludge  (1)): where P is the power (J), t is the duration of sonication (sec), v is the sample volume (L), and TS o is the initial total solids (g/L).
Samples were sonicated in 200 mL batches for 1, 2, 5, and 10 min which correspond to specific energies of 800 Samples were sonicated without temperature control to mimic conditions of full-scale applications. Temperature increases were uniform for all sludge types resulting in an 8 C increase for 1 min of sonication, 14 C increase for 2 min, 32 C increase for 5 min, and 50 C increase for 10 min.
Solubilization was quantified through the degree of sludge disintegration (DD) calculated as the ratio of sCOD increase after sonication to the total possible sCOD increase (Equation (2)). Sonication did not affect tCOD, and thus tCOD is consistent for each sludge type after sonication.
where sCOD is soluble COD, sCOD 0 is the initial soluble COD of untreated sludge, and tCOD is total COD.

Bioassays
The Quantity of produced BG was automatically logged every hour by the system.

Analytical methods
TS and VS were measured as per standard method 2540 (APHA ). Samples for tCOD analysis were homogenized and measured using HACH method 10212. To mitigate the potential interference of filamentous bacteria causing bias during filtration, sCOD was separated by centrifuging sludge samples at 8,000 g for 20 min and measured using HACH method 8000.

Data analysis
Three non-linear models for the estimation of performance parameters were compared to empirical data for BGP (Table 2). The following models were shown by Donoso-  (4)) is based on control principles by considering the process as a system receiving inputs and generating outputs to predict maximum gas production. A first-order (FO) kinetic model (Equation (5)   In this study, BG yield was measured as BG produced per mass of sCOD consumed during digestion. As DD increased and more sCOD was available for digestion, the yield decreased (Figure 1).    (Table 3).
Hence, variation in reported results of AD of sonicated WAS may be due to variations in treatment style, initial sCOD, sludge age, and influent concentrations that are unique to each source sludge studied (Wang et al. ).
This could explain the wide variation in BGP increases.

Modelling of BGP results
Three kinetic models previously used for methane production to describe the AD process and critical digestion performance parameters were tested against BGP normalized by mass of VS ( Figure 2) (Donoso-Bravo et al. ). Overall, the models showed strong correlation (r 2 ¼ 0. 920-0.999) with the data and are deemed useful for the accurate determination and comparison of design parameters. As an example, Figure 4 illustrates the results of the non-linear regression using the three models for 1 min sonication times. The complete results can be found in Table S2 (available with the online version of this paper). Non-linear regression results for 0, 2, 5, and 10 min sonications can be  Overall, excepting 1, 2, and 10 min sonication pretreatment of MBBR sludge, sonication increases the maximum rate of digestion over non-sonicated sludge in the other 13 cases, but not in a clearly definable pattern. Ideal sonication energy may be unique for each sludge type and dependent on system requirements. Designing a system for both the greatest increase in kinetics and greatest BGP may not be possible.

CONCLUSIONS
In this work, sonication pretreatment was proved to significantly increase tCOD biogas yield in mesophilic batch assays of the four sludge types tested. It had a greater impact on increasing BGP from biofilm type sludge (15-20% for MBBR and RBC) as compared to conventional, suspended growth technologies (5-12% for CAS and Lagoon). The different sludge types tested responded uniquely to the same levels of sonication energies. An optimal specific energy for the greatest production of BG was not found that coincided for all sludges. Instead, optimal specific energy was unique for each sludge, but the peak BGP for all sludges occurred within a small solubilization range of 2.9-7.4% DD. Sonication pretreatment exhibited significant BGP inhibition relative to sCOD, even at the lowest applied energy levels of 800 kJ/kg TS. In most cases, there was no significant difference (p < 0.05) in increased BGP between low (1 min, 800 kJ/kg TS) and high ( Overall, ultrasound impacted BGP, disintegration and digestion rates in ways that could not be easily correlated with the basic characterizations and models employed in this study.