Impacts Of Intermediate Thermal Hydrolysis Pretreatment (iTHP) on Anaerobic Digestion, Dewatering, and EPA VAR Requirements

Introduction
Thermal hydrolysis pretreatment (THP) of sludges prior to anaerobic digestion utilizes high temperature and pressure (typically 165 oC, 6 bar, 30 min) to solubilize material and break down the solids which greatly reduces viscosities and enhances digestion and dewatering. The reduction in viscosity allows much higher loading rates and intensification of the anaerobic digestion process. In addition, the thermal treatment greatly exceeds the EPA time-temperature requirement for meeting Class A biosolids (EPA, 2003), and eliminates issues with reactivation and regrowth of pathogen indicator bacteria (Higgins et al., 2008). Typically, THP is performed prior to anaerobic digestion. The intermediate THP (iTHP) process can be employed in which the THP is applied after a primary anaerobic digestion step and before the secondary digestion process. The process flow diagram for conventional THP and the iTHP processes are shown in Figure 1. Intermediate THP has the advantage of reduced size of the THP process (and lower capital costs), plus the benefits of intensification and potentially improved dewatering as well as meeting Class A biosolids time-temperature requirement. Since much of the readily degradable material would have been removed in the first digestion step, it is possible that the volatile solids reduction (VSR) of the second digestion step after iTHP could be below the EPA vector attraction reduction (VAR) rule which requires a 38% VSR which '…must be met after or concurrent with pathogen reduction…' (EPA, 2003). As an alternative, the EPA allows compliance with the VAR requirement if the process can demonstrate '…less than 17% additional volatile solids loss during ben-scale anaerobic batch digestion of the sewage sludge for 40 additional days.' This is known as Option 2 in the VAR options.
Several studies have examined iTHP and shown the efficacy of the process to improve overall digestion by increasing biogas and VSR as well as reduce costs (Rus et al., 2017; Bjerg-Nielsen et al., 2017); however, limited research has been performed on meeting the EPA VAR as well as evaluating downstream impacts such as final dewatering. Objectives The objectives of this research were to investigate the impact of iTHP on digestion and the EPA VAR requirements as well on downstream processes.
Methods and Materials
Samples of dewatered cake after the first stage digestion (SRT = 25 d) were shipped to Bucknell University where they were thermally hydrolyzed (165 ºC, 6 bar, 40 min). The thermally hydrolyzed solids were fed to a laboratory scale digester with an active volume of 8 L, and an SRT of 10 days, see Figure 2. The target feed concentration was 10% TS. The anaerobic digesters were housed in a temperature-controlled room held at 38 oC, and were fed once per day (semi-continuous operation) to achieve the desired SRT. The digester performance was monitored on a regular basis for typical parameters such as pH, TS/VS, COD, alkalinity, gas production, etc. The digesters were operated until steady state conditions were achieved as determined by stability in the different operating parameters. Figure 3 shows the digester TS and VS concentrations during the study, and the steady state period was considered to be from days 40 through 70. The values of the different parameters during the steady state period were averaged to determine average steady state values. During the steady state period, waste samples from the digester were used to perform the 40 d additional VSR test as stipulated by the EPA VAR requirements. Over a four-week period, samples from the digester were placed in triplicate serum bottles and every 10 days the samples were analyzed for TS/VS, COD and ammonia until day 40. At the end of the study, the digestate was dewatered using a lab scale dewatering method.
Results and Discussion Digestion Performance.
Table 1 summarizes the results from the digester study. The average VSR of the with the THP and digestion processes was about 28% using the Van Kleek method. This is below the 38% requirement for VAR; however, the first digestion stage at the plant achieves an average VSR of around 50%, so this is additional VSR after the conventional digestion. When considering the addition VSR for the iTHP process, the total VSR through the entire process is about 63%. The overall gas production would be increased by over 25% by implementing the iTHP process compared to the conventional mesophilic digestion.
EPA VAR Requirements.
Although the total system meets the 38% VAR requirement, the EPA regulations stipulate for thermophilic process used to achieve Class A biosolids, the VAR step must occur either simultaneously or after the thermophilic treatment. In this case with iTHP, the VAR is occurring after the thermophilic treatment, and it does not meet the 38% VSR requirement, which is a bit of a gray area in the regulations. The 40 d additional VSR test can be used to definitively meet this rule as long as the additional VSR is less than 17%. Based on the four different additional VSR tests that were performed (each in triplicate), the average additional VSR was 5.8% with a standard deviation of 3.5%, which is well below the maximum value of 17%.
Dewatering.
A dewatering test was performed at the end of the study, and the average cake solids was 21.4%, which is on the lower end for typical THP sludges, but still an improvement of the 18% cake solids currently achieved at the plant. The digestate had a relatively high reactive phosphorus concentration which has been shown to decrease dewaterability (Higgins et al., 2004). Based on the cake solids and the additional VSR, the total mass of wet cake leaving the plant would be reduced by about 50%, resulting in significant savings from hauling costs. The cake odorant production during storage was quite low, with a peak total organic sulfur concentration of 55 ppmv, which is quite low.
Summary
Application of the iTHP process had several benefits: 1. increased overall VSR from about 50% to 63%; 2. increased biogas production by 25%; 3. meets EPA Class A time-temperature requirements and VAR requirements; 4. reduced the overall wet cake mass leaving the plant by about 50%. The full paper will include a more detailed discussion of the methods and results as well as an economic comparison of THP, iTHP and conventional mesophilic digestion.