Thermal Hydrolysis of Municipal sludge: Finding the Temperature Sweet Spot: A Review

Abstract

The increasing demand for a Thermal Hydrolysis Process (THP) to pretreat municipal sludge upstream Anaerobic Digestion (AD) opens the opportunity to further develop and optimise this technology. The optimal THP temperature remains unclear due to the production of refractory compounds at high temperature. A compilation of literature data was conducted to investigate the existence of a temperature sweet spot for the THP applied to municipal sludge. All related reports (n=43) were included. The THP temperature range impact was assessed in the range of 100°C -200°C on 4 AD and dewatering performance indicators (CH4 production, Volatile Solid Reduction (VSR), Dewaterability (DW) and filtrate quality). Other parameters potentially affecting the performance indicators were also considered. These parameters include the type of sewage sludge and operational conditions related to THP and AD. The impact of all parameters on performance indicators was evaluated with a Kruskal-Wallis statistical test. For THP temperature optimisation, a pairwise comparison, using a Wilcoxon test, was made. A temperature optimum in the [140-160]°C range was proposed. It seemed to minimize the production of refractory compounds, while maximising AD and dewatering performances. It is noteworthy that above 160°C, the concentration in refractory compounds and soluble COD increases sharply, thus leading to a potential deterioration of WWTP effluent quality.

Thermal Hydrolysis of Sewage Sludge: A Case Study of a WWTP in Burgos, Spain

“…TP biogas − TP biogas-st-gasom = 0 + TP biogas-eng (4) where (TP biogas-st-gasom , kW) is the thermal power of the gas stored in the gasometers.…”

“…THP consists of heating the sludge to high temperatures, generally in the range of 140 to 180 • C, with a treatment time span of about 30 to 60 min, applying high pressure (between 6 and 11 bars). There are more than 82 full-scale, THP-operating WWTPs [4]. During the thermal degradation of sludge, proteins are partially degraded, giving rise to the production of ammonia (NH 4 + ) and, thus, reaching high concentrations of this compound in the pretreated sludge and the digestate [5].…”

“…The higher volatile solid (VS) content of sludge fed into the digestion system and the higher degradation rate attained cause a further increase in the ammonia content (total ammonia nitrogen and free ammonia nitrogen) of the reactor liqueur [6]. Digesters treating thermal-hydrolysed sludge can reach values in the range of 2400 to 3500 mg N-NH 4 + /L [7,8]. The increase in the temperature of sludge during thermal pretreatment guarantees that biosolids obtained will be free of pathogens, with a stabilisation degree of about 60% (measured as COD conversion into biogas) [9].…”

Performance improvement and decolourization in the high‐solid thermophilic anaerobic digestion of thermally pretreated sewage sludge

“…The pursuit of enhanced degradation is the central reason for applying pretreatment technologies to sewage sludge. When thermal pretreatment is combined with the anaerobic digestion of sewage sludge, improvements in biogas production, organics degradation, digestion rates, dewaterability and energy balance are generally observed (Barber, 2020; Devos et al, 2021; Pilli et al, 2015). One of the drawbacks is the formation of persistent, coloured substances, which is disadvantageous from the viewpoint of wastewater standards and visual comfort (Barber 2020).…”

“…One of the drawbacks is the formation of persistent, coloured substances, which is disadvantageous from the viewpoint of wastewater standards and visual comfort (Barber 2020). The temperature optimum reported for thermal pretreatment was 170–175°C to satisfy both digestibility and dewaterability (Fernández‐Polanco et al, 2008; Haug et al, 1978), and a recent review proposed 140–160°C from the viewpoint of refractory compound formation in addition to the basic performance (Devos et al, 2021).…”