Effect of Hydrothermal Pretreatment on Resource Recovery From Thickened Waste Activated Sludge

INTRODUCTION
The world fossil fuel reservoirs are depleting and there is need for renewable energy resources. Sewage sludge is rich in organic carbon, and it can be utilized as a sustainable source for the production of value-added products such as Volatile Fatty Acids (VFAs) and biogas (CH4 and CO2). Anaerobic digestion (AD) is a technology that not only treats the waste, but it also turns the waste to value added products such as VFAs and/or methane. In addition of using methane as bioenergy source, the VFAs have variety of applications such as the production of biodegradable plastics, generation of bioenergy, and carbon source for biological nutrient removal [1]. Researches have evidenced that sludge pre-treatment is a viable strategy to accelerate the hydrolysis process which is the rate limiting step of the AD process [2]. Different pre-treatment technologies such as thermal, chemical, biological, and mechanical have been used to improve the hydrolysis step [2–4]. Among them Hydrothermal pretreatment (HTP) is one of promising techniques. The main objective of this research is to investigate the application of HTP of thickened waste activated sludge (TWAS) on the VFAs and methane production. Furthermore, evaluating the application of HTP as a pre- and inter-treatment on fermentation and AD of TWAS was examined. Moreover, combining chemical pretreament (free nitrous acid (FNA), acid, and alkaline) and HTP on the anaerobic digestibility of TWAS was investigated.
MATERIALS AND METHODS
Different HTP conditions were applied to the TWAS both individually and in combination with chemical pretreatment. HTP was conducted using Parr 4848 Hydrothermal Reactor. For the FNA pretreatment, the pH of the sample was adjusted at 5.5 and the required nitrate was added to achieve 1.8 mg HNO2-N/L, then the sample was mixed for 24-hour at 25°C. The pH of the sample was adjusted to 3.0 and 10.0 3M HCL and 3M NaOH for acid and alkaline pretreatment, respectively, then the sample was mixed for 24-hour at 25°C. Semi-continuous fermentation tests were conducted at 37°C and hydraulic retention time of 3 days. The biochemical methane potential (BMP) tests were conducted at 37°C and were designed based on F/M ratio of 1 g-TCOD/g-VSS.
RESULTS AND DISCUSSION
Effect of HTP on TWAS solubilization and methane production: The results of this study showed that the COD solubilization increase from 27% at lowest HTP condition of '150°C-30min' reaching its maximum value of 49 % at '200°C-10min' and it was almost constant afterwards, see Fig. 1. It was observed that lower retention time combined with higher temperatures demonstrated higher solubilization emphasizing that the temperature is the dominating parameter in the HTP process. However, the effect of both temperature and retention time on sludge solubilization were statistically significant (p<0.005). Additionally, the methane production improved by 12%-40% for the pretreatment samples, see Fig. 1. The highest methane yield was achieved at 170°C-10min. When the pretreatment condition reached to 220°C-30min, the methane yield gets lower than the raw TWAS. This reduction in methane production can be caused by the formation of toxic refractory compounds [5]. The results showed that there was no correlation between the increase in SCOD and the methane yield.
Effect of combined chemical (FNA, acid, and alkaline) and HTP on TWAS solubilization and methane production: Combined pretreatments resulted in higher COD solubilization of 39% compared to the individual pretreatment (average 12% for chemical pretreatment and 36% for HTP), see Figure 2. Furthermore, alkaline pretreatment was the most effective one out of the chemical pretreatments with doubing the COD solubilization compared to the acid and FNA pretreatment. Furthermore, all pretreated samples resulted in higher methane yield, see Figure 2. The results also revealed that combined pretreatments were associated with higher percentages increase (36%-53%) in methane production compared to the individual pretreatment (7%-30%). The highest increase in methane production of 53% was achieved for FNA + HTP. A preliminary economic analysis was performed considering the cost of pretreatment and the increase in methane production and the saving in solid disposal. The costs for the pretreatments per ton TWAS were estimated to be $4.6, $0.21, $0.6, and $0.46 for HTP, acid, alkaline, and FNA pretreatments, respectively. The net revenue/loss ranged from -$1.35/ton TWAS to + $3.64/ton TWAS. The highest net revenue of + $3.64/ton TWAS was observed for the combined HTP and FNA pretreatment.  Evaluating the application of HTP as a pre- and inter-treatment on fermentation and AD: Two semi-continuous fermentations were used for raw and pretreated TWAS. The percentages solubilisation due to fermentation were calculated based on the data during the steady state. The results revealed that HTP only has better influence on solubilization compared to the fermentation only, 26% vs 17%. Moreover, combining HTP with fermentation with different configurations had similar effect in terms of solubilization, 44% for HTP plus fermentation and 45% for fermentation plus HTP. Furthermore, the percentage increase in the VFAs production for the HTP followed by fermentation was higher than that of the fermentation followed by HTP. Figure 3 shows the percentages increase in methane production compared to the raw TWAS for the different combinations of the HTP, fermentation, and thickening the samples. Percentage increase in methane production varied from 17% for the hydrothermally concentrate to 46% for the hydrothermally pretreated fermented. The fermented sample resulted in 34% increase in methane production compared to the raw sample, while the hydrothermally pretreated sample resulted in 22% increase in methane production compared to the raw sample.
Conclusions
The following conclusions can be drawn: - HTP improved the COD solubilization and the methane production significantly. The maximum solubilization of 49% was achieved at 200°C with 10min - The maximum methane production of 227 mLCH4/gTCODadded was achieved at 170°C with 10min. - Combining chemical and hydrothermal pretreatments improved methane yield but not the solubilization. - The highest methane yield of 275 mLCH4/gTCODadded was achieved for the combined pretreatment with FNA and HTP. - HTP increased the VFAs production by 40% compared to the raw TWAS. - There were no significant differences in the methane yield when HTP was applied prior to or after fermentation. - The percentage increase in methane production compared to the raw TWAS for the pretreatment or the inter-treatment were 46% and 44%, respectively.