Effect of Pre- and Post-AD-THP on Dewaterability, COD Solubilization, and Formation of Refractory Compounds

Cambi developed the Thermal Hydrolysis Process (THP) during the 1990's, based on ideas from Professor Halvard Ødegaard (Norwegian University of Science and Technology) and experiences reported from the Zimpro and Porteous processes developed decades prior. The Zimpro and the Porteous processes are sometimes referred to as Hydro-Thermal Carbonization (HTC) and were installed mainly to enhance dewatering. Unfortunately, these processes suffered from both design and operational challenges such as scaling in heat exchangers, odor release and reliability issues. Furthermore, the long holding times at high temperatures resulted in high concentrations of refractory compounds which are formed through caramelization of sugars and Maillard reactions. These reactions caused elevated concentrations of refractory Chemical Oxygen Demand (COD) and refractory Dissolved Organic Nitrogen (DON), and this became increasingly problematic with more stringent water emission requirements. Cambi developed its THP to operate at lower temperatures in order to form less refractory compounds. Most Cambi THP plants operate in the range from 140-165 °C and with 20 to 30 minutes retention time. HTC processes typically have somewhat longer retention times compared to THP and typically operate at or above 180 °C. The goal with the present work was to identify process conditions and develop a process design to improve dewatering properties while keeping formation or refractory compounds at low levels. All experiments carried out in the present work was on thermophilically digested mixed sludge which was treated in a lab-scale THP-pilot at temperatures between 140 and 200 °C for 30 or 60 minutes. Pre-AD-THP has now been widely adopted in several countries and many new plants have been successfully commissioned in North America. The main drivers are often intensification of the digestion process, improved dewaterability, increased biogas production and high-quality Class A biosolids with low odor. On average, the pre-AD-THP improves dewaterability by approximately 8 percentage points compared to conventional digestion. Full scale and laboratory studies shows that post-AD-THP improves dewaterability to a greater extent compared to pre-AD-THP. With post-AD-THP, the increased conversion of Volatile Solids (VS) into biogas and improved dewaterability can reduce the final cake volume by more than 60% compared to conventional digestion. Cambi´s post-AD-THP solution, Cambi SolidStream®, has been demonstrated both in lab scale and in full scale in Germany. The amounts of VS solubilized to soluble COD (sCOD) depends on the type of sludge and treatment conditions. In the present work, solubilization measurements, presented in Figure 2, shows that about 10 to 40 % of VS was solubilized. Previous dewatering experiments carried out by Cambi with a Bucher laboratory scale dewatering setup on sludge from the same plant shows that a final cake dryness in the range from 46 to 57 %DS was achieved after treatment at temperatures within the range from 140 to 212 °C and retention times between 30 and 120 minutes. The highest dryness levels were achieved with the most severe conditions. In SolidStream, most of the sCOD and some of the particulate COD (pCOD) will enter the centrate or filtrate which is returned to the anaerobic digester as shown in Figure 1. Refractory compounds formed through caramelization and Maillard reactions are difficult to degrade in biological processes and typically produce a yellow or brown color. The amounts of refractory compounds formed depends on the type of feedstock and increases with temperature and retention time. Some of the practical consequences for a WWTP are that refractory compounds are not converted into biogas, it may deteriorate the efficiency of UV-disinfection, reduce side stream nitrogen removal efficiency, and such compounds may end up in the final effluent if not removed. There is a link between the extent of formation of refractory compounds and color. In this work, color has been measured as APHA color of filtrate (0.45 µm) as shown in Figure 3. It should be highlighted that this work was carried out on digested sludge. Other work carried out on raw sludge typically shows a relatively larger increase in color already at lower temperatures. This could possibly be explained by the relatively higher concentrations of sugars and proteins in raw compared to digested sludges. While increased biogas production from centrate or filtrate returned to anaerobic digestion is a benefit with post-AD-THP, the main driver is typically improved dewaterability. Capillary Suction Time (CST) is a common measurement which to some extent can give indications on dewatering characteristics. Figure 4 shows the CST values measured on sludge treated at different process conditions. Measured CST decreased with more severe treatment conditions. As shown in Fig.3, color formation is somewhat limited at or below 165 °C and 30 minutes retention time, which are conditions typically used for THP while, it is more pronounced at typical HTC conditions which are at or above 180 °C and 30 min retention time. Furthermore, retention time has a somewhat limited influence on color formation below approximately 160 °C while the influence on CST, as shown in Fig.4, is large. This finding is relevant for the THP system shown in Figure 5. This system includes two energy recovery steps where temperature in the warmest pre-heating step and pressure reduction step is sufficiently high for desired reactions to take place. Cambi is currently delivering one post-AD-THP system in Norway which will be equipped with two energy recovery steps, and which is designed with flexibility for operating with reactor temperatures from about 140 up to 180 °C. Laboratory experiments on sludge from this plant shows that adding retention time in the warm pulper and flashtank helps to decrease polymer demand for final dewatering by around 70% compared to operation with only 40 minutes retention time at 180 °C and no holding time in pre-heating and pressure reduction steps. Results from this work shows that formation of colored compounds can be kept low at conditions typical for THP and that retention time has a large influence on dewatering properties when operating at low temperatures. A possible explanation for the relatively low color formation measured in the present work could be that sugar and protein concentrations are lower in digested compared to raw sludges. Cambi has developed a novel THP design which includes a cold pulper, hot pulper, reactors, hot flashtank and a cold flashtank. The additional retention time in the hot pulper and hot flashtank appears to provide substantial benefits with regards to dewaterability and polymer consumption while the impact on color formation appears to be limited in a post-AD-THP configuration.