Cambi Solidstream® : Thermal Hydrolysis As a Pre-Treatment for Dewatering to Further Reduce Operating Costs

WEFTEC 2017 Cambi SolidStream® : Thermal Hydrolysis as a pre-treatment for dewatering to further reduce operating costs Bill Barber1, Paal Jahre Nilsen2, Paul Christy1 1. Cambi, Inc., 279 Great Valley Parkway, Malvern, PA, 19382 2. Cambi AS, Skysstasjon 11A, NO-1383 Asker. 3. Bucknell University, Lewisburg, PA. Email: [email protected]; Email: [email protected] Email: [email protected] ABSTRACT Introduction Thermal hydrolysis has been successfully used for over 20 years as a pre-treatment to anaerobic digestion. It has allowed digesters to be operated at typically double their loading rates thus reducing the size of new build facilities, or doubling capacity of existing ones, and fundamentally improved dewatering. This improved dewatering has resulted in significantly lower Biosolids recycling costs and, if used, smaller energy requirements for downstream dryers and incinerators. However, looking through the literature reveals that the initial aim of thermal hydrolysis was not to improve the performance of anaerobic digestion, but rather to improve dewatering. Work between the 1950s and mid 1970s found that when raw undigested sludge was exposed to conditions of thermal hydrolysis, the resultant material dewatered routinely above 50% DS, and depending on sludge type, as high as 60% DS. Subsequent work has shown that, by reintroducing materials known to influence dewatering, downstream anaerobic digestion deteriorates the dewatering potential of the Biosolids. Although use of thermal hydrolysis with digestion improves dewatering by approximately 10% points compared with a case with no pre- treatment, the dewaterability would have been higher without the digestion process at all. Considering this, Cambi have developed a process known as SolidStream® whereby the thermal hydrolysis unit is installed downstream of the digestion plant immediately prior to dewatering. In this instance the digested sludge is dewatered hot, and the centrate, now high in biodegradable COD, is recycled to the digester inlet and digested. This paper describes full-scale operating data from Amperverband in Germany, where the technology is installed. Although the results from third party analysis exceed even those of thermal hydrolysis, there is a current challenge with demonstrating that the technology is aligned with the US EPA’s interpretation of Class A under the 503 regulations. INTRODUCTION Thermal hydrolysis of sewage sludge, which involves the application of heat at above autoclave temperature for a defined time period prior to anaerobic digestion (as shown in Figure 1a), is an established commercially available technology since the first full-scale plant in HIAS in 1995. The heat is typically provided by live steam injection at design temperature and concomitant pressure which is then rapidly released (exploded), although some configurations use standard heat exchange. At the time of writing, there are 75 facilities of which 39 are operating and the remaining are in various stages of design. In total, 1.65 million metric dry tons of sludge per year WEFTEC 2017 Proceedings Copyright © 2017 Water Environment Federation 5070 WEFTEC 2017 are, or will be, processed with thermal hydrolysis. Cambi specific information is summarized in Table 1. Table 1. Summary of Cambi thermal hydrolysis Installations 56 In operation without prolonged 47 or unplanned shut downs In design 9 Countries 20 Continents 5 Population equivalent served 62 million Equivalent sludge throughput 6,287 ton dry solids/d 30% - Veolia, Thames Water, United Utilities, Repeat customers Suez, Welsh Water, Northumbrian Water Installed plant sizes 11 – 450 ton dry solids/d Reactors in service 307 in 84 trains Projects in North America 10 Drivers for installation are geographically market driven but typically include: increased loading rates (to minimize size of new digestion plants, or maximize use of existing facilities); improved sludge cake dewaterability which reduces downstream transport and processing costs; increased production of renewable energy, and sterilization of sludge. The reported advantages and disadvantages are given in Table 2. Table 2. Advantages and Disadvantages of thermal hydrolysis Advantages Disadvantages Significantly improves the biodegradability of Parasitic energy demand with some activated sludge configurations (depends on process) Improves the biodegradability of primary Higher ammonia concentration than standard sludge digestion – although this is better suited for advanced nutrient removal Allows significantly higher loading rates Potential for and production of refractory resulting in smaller digestion plants material especially with food-waste Increases rate of biogas production Potential increase in polymer demand for dewatering Reduces sludge viscosity More complex than standard anaerobic digestion Improves sludge dewaterability on all Requires boilers dewatering systems Sterilizes sludge providing pathogen-free Sludge needs cooling prior to anaerobic biosolids digestion Reduces odor and pathogen regrowth from Requires centrifuge thickening to 16 – 18% dewatering DS WEFTEC 2017 Proceedings Copyright © 2017 Water Environment Federation 5071 WEFTEC 2017 Advantages Disadvantages Eliminates scum and foaming and produces Higher release of nutrients with potential for conditions which do not encourage foaming salt crystallization and subsequent maintenance issues and deterioration of dewaterability Minimizes inhibition due to hydrogen sulfide Significantly reduces downstream requirements for drying and other thermal processes Numerous sites successfully operating at full- scale Lowest carbon footprint when benchmarked against options with no thermal hydrolysis Although most emphasis has been given to use of the technology to improve biodegradability of sludge with process efficacy being inversely proportional to the initial biodegradability of the material (Wilson & Novak, 2009), thermal hydrolysis was originally seen as a means to improve sludge dewaterability (Lumb, 1940, 1951). Interest gathered pace in the early 1970s when significant improvements in dewaterability were correlated with heat application to various sludges (Everett, 1972). A few years later, the concept of applying the technology to improve the biodegradability of sewage sludge – mainly that produced from activated treatment well known to be poorly biodegradable (Rudolfs & Heisig, 1929) – was conceived (Haug, 1977). The main perceived disadvantage of thermal hydrolysis refers to the need for energy to provide the steam for the process. This has been addressed by the introduction of systems treating only waste activated sludge, or processing of digested sludge prior to a second stage of digestion, as shown in Figure 1b and 1c. In both these configurations energy demand is typically halved and is adequately met without auxiliary fuel. Another way of significantly reducing the energy demand of the process, is to exploit the technology’s ability to improve dewatering and install it downstream of digestion, but this time without a second stage of digestion. In this case, centrate from dewatering is returned to the digester to be re-digested. This is the concept behind the SolidStream® process developed by Cambi. WEFTEC 2017 Proceedings Copyright © 2017 Water Environment Federation 5072 WEFTEC 2017 1a) 1b) 1c) Figure 1. Configurations of thermal hydrolysis. 1a) Standard upstream thermal hydrolysis of both primary and waste activated sludge; 1b) thermal hydrolysis of only waste activated sludge; 1c) ITHP – Intermediate thermal hydrolysis. Thermal hydrolysis of digested sludge prior to re-digestion. WEFTEC 2017 Proceedings Copyright © 2017 Water Environment Federation 5073 WEFTEC 2017 PROCESS DESCRIPTION A schematic of Cambi’s SolidStream® process is given in Figure 2 below: 2a) 2b) Figure 2. a) components making up Cambi Solidstream®, b) positioning of process WEFTEC 2017 Proceedings Copyright © 2017 Water Environment Federation 5074 WEFTEC 2017 In principle, the process has many similarities with standard thermal hydrolysis, but has some fundamental differences. The system has been adapted specifically to handle high ash content sludges with different rheological properties and also to minimize pumping by use of a barometric egg which controls sludge flows under pressure. As with digestion pre ‐treatment, the digested sludge is thickened using centrifuges to approximately 16% dry solids. The ammonia containing centrate at this stage may be sent back to the head of the works for processing. The thickened sludge is then thermally hydrolyzed in a similar way to pre-digestion hydrolysis, although it is done at a different retention time. The hydrolyzed biosolids exiting is then transported using the barometric egg into a dewatering stage where it is dewatered hot at 212 °F. Hot dewatering has provided challenges and Cambi have also developed a specific handling system and biosolids cooler which is part of the scope of supply. The centrate from this stage, now freshly hydrolyzed and solubilized is recycled to the anaerobic digestion plant where it is converted to biogas. Depending on the quantity of polymer used, this reduces the hydraulic retention time of the digester by a couple of days. However, testing has shown that digesters with typical retention time of 15 to 20 days are not adversely influenced by the reduction in retention time. This process can be described as the opposite of recuperative thickening (Torpey & Melbinger, 1967) where the liquid rather than the solids is

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