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

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

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.

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.

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

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 extracted. Unlike pre ‐digestion thermal hydrolysis, another major difference is that no cooling is required. The hot centrate is mixed with incoming digester feed and heats it up towards the digester operating temperature. As the process is installed downstream of digestion, the thermal hydrolysis plant is now over a third smaller meaning that the full energy balance to provide the heat for the process can be met using high grade co s‐ genno auxiliary w aste heat. fuel Subsequentl demand or y,need there to idivert any biogas. The cake which is produced from the second dewatering stage, varies in dry solids depending on how much polymer is used using existing normal unadjusted centrifuge equipment. Typically cake reduction is between 55 and 65% compared to standard anaerobic digestion. The cake is cooled with air – as part of the technology scope of supply – and cooling air is scrubbed to remove odorous components.

The key behind the performance of SolidStream® is the amount of solubilization which occurs and the concentration of readily biodegradable COD which is returned to the digester. Ironically, the more polymer is used to improve capture rate, the less soluble COD is returned to the digester, and whilst the centrate is cleaner, the biogas production falls. Therefore, there is a balance between optimum polymer dose in dewatering and the COD which is allowed to return.

In numerous previous tests, the quantity of soluble COD captured for biogas production was determined, and the results are shown in Figure 3.

Figure 3. Capture of soluble COD with SolidStream®

The results are dependent on sludge type but also polymer dose as described above, and show a minimum return of biodegradable material of 20%, rising to over 60%. It is important to note that some of the plants tested in Figure 3 had sludge which had already been thermally hydrolyzed, therefore minimizing potential further solubilization. A combination of improved volatile solids destruction due to the return of this readily biodegradable centrate, and also improvements in sludge cake dewaterability mean that on average 60% less biosolids cake is produced than when compared to standard digestion. This was determined based on testing of sludge from over 20 facilities.

Therefore, compared with standard thermal hydrolysis, SolidStream® further enhances the benefits of thermal hydrolysis viz. overcoming the need for inherent energy, removing the need for a cooler whilst further improving biogas production and dewatering.

CASE STUDY AT AMPERVERBAND, GERMANY SolidStream® has been in operation in Amperverband, Germany for a little over two years. The plant processes 4900 tons dry solids annually with a high volatile solids content of 83%. The sludge make is typical of a European plant, and comprises 60% primary with 40% waste activated sludge, based on biological phosphorous removal.

Figure 4. Photograph of Amperverband Wastewater treatment works, Germany.

The plant has mesophilic anaerobic digestion with retention time of 22 days and managed a volatile solids reduction of approximately 50%. A little over 13,000 wet tons Biosolids cake were produced annually and these were transported at distance to an incinerator due to lack of land application in Germany. The average performance is shown in Figure 5. In 2014, after trialing continuous thermal hydrolysis only on the activated sludge fraction since 2007, the client engaged Cambi to install the SolidStream® process.

Figure 5 summarizes the plant performance at Amperverband before the installation of either thermal hydrolysis or SolidStream®. A great deal of optimization has occurred to balance the clients concerns regarding quality of centrate returned, and impacts on refractory COD in the effluent from the plant based on the tight consent limits in Germany. Although the plant has been in operation over two years, a third-party expert (Dr. Julia Kopp) was engaged to determine the influence of SolidStream® over a particular time period.

Figure 5. Average plant performance at Amperverband, prior to installation of either pre- or post- thermal hydrolysis.

To determine performance a wide variety of performance criteria were analysed based on digestion performance, biogas production, dewaterability, polymer consumption, and quality of the return centrate. Table 3 summarizes the main points:

Table 3. Comparative performance at Amperverband Wastewater treatment works WAS-only Cambi Parameter Unit Control Cambi SolidStream® COD load [t/d] 21.9 17.9 20.2 Digestion Performance VS destroyed [%] 51 65 71 Biogas yield [scf/lb DSfed] 7.48 8.20 9.44 Biogas yield [scf/pe/d] 0.8 1.0 1.4 Biogas production [sfcm] 104 150 Dewatering Dewaterability [%DS] 19.3 25 36.6 Poly dose [lb/t DS] 13.3 17.7 46.4 Cake production [wet t/yr] 13145 9454 5203 Centrate return COD concentration [mg/l] 527 660 1889 COD load [lb/d] 301 486 1224 Ammonia concentration [mg/l] 717 1011 1115 Ammonia load [lb/d] 323 522 723 Final effluent COD* [mg/l] 19 21 25 Total N [mg/l] 8.9 8.7 9.0 * Effluent discharge limit <33 mg/l

The results in Table 3 clearly demonstrate improved performance with thermal hydrolysis which is further enhanced by the SolidStream® configuration. Prior to installation of thermal hydrolysis, the plant performed relatively well at 51% volatile solids destruction at 22 days retention time, with Class B equivalent biosolids dewatering approximately 20% dry solids. This performance is typical for type of sludge and retention time, and is observed on numerous other facilities. Employment of WAS-only thermal hydrolysis saw an improvement in volatile solids destruction to 65%, which increased further to 71% with SolidStream®. Accordingly, gas production increased from 104 to 150 scfm with SolidStream® compared to the base-case with no pretreatment.

A major cost saving was observed with downstream biosolids processing. As previously mentioned, the biosolids are hauled at distance and then fed to an incinerator. With SolidStream® the cake quantity has reduced on average by 60%, whilst the dewatering has improved by approximately 18 percentage points. Figure 6 shows a typical cake pile, dry solids of which was 43% when photograph was taken.

Figure 6. Typical Biosolids cake from the SolidStream® process. Cake dry solids 43% from centrifuge

However, whilst the cake is drier, it has lower volatile solids contained within it and this could influence the incinerator energy balance. The energy content of the sludge was calculated using methods presented previously whereby a correlation was found between volatile solids content and calorific value which was within +/- 9% of measurements taken by bomb calorimetry (Barber, 2007). These results are as shown in Table 4.

Table 4. Calculated sludge energy values on a dry and wet basis WAS-only Cambi Sludge type Unit Control Cambi SolidStream® Dry basis [BTU/lb] 6980 6197 5725 Wet cake [BTU/lb] 1347 1549 2095

As expected, the energy value drops on a dry basis due to more volatile material being converted to biogas. Here the energy value drops by approximately 10 and 20% for WAS-only thermal hydrolysis and SolidStream® respectively. However, it is wet cake which is fed into the incinerator and now the dewaterability of the sludge becomes an influential parameter. Table 4 shows that despite having less volatile solids on a dry basis, the improved dewatering and concomitantly lower water content actually increases the energy value of the sludge making the energy balance around the incinerator better. This makes SolidStream® an excellent process to install upstream of incineration and drying. WAS-only pre-treatment increases energy content by approximately 15% compared to no pre-treatment, whilst SolidStream increases the energy content by over 55% compared to standard anaerobic digestion.

With respect to Amperverband, the improved volatile solids destruction and dewatering reduced annual cake production from 13,145 to 5,203 wet tons/year and this alone has resulted in an annual cost saving of over 515,000 euros.

As expected the COD levels in the centrate return are higher. With thermal hydrolysis upstream this is due to the increased loading rate, whilst with SolidStream® this is due to downstream solublization of the COD to be returned to the digestion for conversion to biogas. The COD concentration with SolidStream® is nearly 1900 mg/l compared with only circa. 530 mg/l with

standard digestion. This is equivalent to an increased COD return load of nearly 400%, however, due to the high biodegradability of the COD the effluent differs by 6 mg/l. Detailed studies are currently ongoing to understand and quantify the types of compounds which make up the COD from SolidStream® (Kolovos et al., 2016). As well as COD, ammonia increases as a function of improved biodegradability of the sludge. With standard thermal hydrolysis, it is a little over 1000 mg/l, which is lower than normal (Barber, 2016) due to the fact that only the waste activated sludge fraction is processed. Despite further improvement in volatile solids destruction, the ammonia concentration increases only slightly further to 1100 mg/l with SolidStream® and this can be explained by dilution effects caused by returning the volume of centrate to the digester. This reduces retention time from 22 days to between 17 and 18 days. Interestingly, no observable difference was noted in total nitrogen in the plant effluent which remained at approximately 9 mg/l. Figure 7 shows a summary of the performance at Amperverband.

Figure 7 demonstrates the energy self-sufficiency of the process regarding steam production. Typically steam requirements for SolidStream are 40% lower than standard thermal hydrolysis depending on sludge type. At Amperverband, the steam requirements for SolidStream® were approximately 660 lbs/hr. As Figure 7 shows, the co-generation generates 330 kW of high grade heat which can produce over 1000 lbs/hr steam which is significantly more than required by the system. Another impact of SolidStream® concerns the lack of cooling required, unlike traditional thermal hydrolysis where thermally hydrolyzed sludge needs to be cooled after treatment prior to anaerobic digestion. Without thermal hydrolysis, 18.9 MMBTU of energy is required (Figure 5) to heat up the sludge to mesophilic operating conditions. With SolidStream® the centrate is returned at 167°F where it is mixed in with the incoming sludge load to the digester. Now, due to the heated return, the digester heating requirements are significantly dropped to 9.6 MMBTU thereby freeing up heat which could be used elsewhere on the plant.

Figure 7. Average performance of SolidStream® at Amperverband over testing period

Table 5 summarizes the entire performance period of the facility.

Table 5. Performance of mesophilic digestion, WAS-only Cambi thermal hydrolysis and Cambi SolidStream® Increase in VS Cake System Date biogas Dewaterability destruction Production production Standard Mesophilic 2006 – 2008 52% 22% digestion WAS-only thermal 2008 – 2014 65% 25% 28-29% -28% hydrolysis SolidStream® 2015 – 2017 75% 44% 38-42% -60%

SolidStream® and Class A According to the EPA 503 regulations, several criteria must be met to achieve Class A biosolids:

1. the pathogen density of the biosolids must be below detectable limits; 2. vector attraction requirements must be met; 3. the Class A pathogen reduction must be ‘accomplished before or at the same time as vector attraction reduction, except for vector attraction reduction by alkali addition or drying’ (EPA, 2003).

Regarding the above definitions, SolidStream® meets pathogen density of biosolids requirements, however vector attraction occurs simultaneously and after Class A pathogen reduction, rather than before as mentioned in the regulations. Cambi have therefore engaged Bucknell University in ongoing work to determine ways that SolidStream® can achieve Class A status. CONCLUSIONS The following conclusions are made: • SolidStream® further improves the performance of thermal hydrolysis by: o Increasing volatile solids destruction and biogas production o Fundamentally improving dewaterability . Both of which result in a decrease in cake production of 60% or greater compared to standard anaerobic digestion o Reducing steam demand by approximately 40% making system self-sufficient for energy o Reducing and optimizing the size of thermal hydrolysis plant required o Removing the need for a cooler • However, the following factors need to be considered: o Reduction in anaerobic digestion retention time

o Production of refractory COD which may enter effluent o Potential increase in polymer demand (although this was noticed during the test period, many tests were conducted which showed this not to be the case, with it being possible to dewater the cake, albeit at lower dry solids, without polymer) o Potential concern regarding Class A status even though cake is sterilized and vector attraction occurs upstream due to improved volatile solids compared to base-line and even up-stream thermal hydrolysis • Regarding Amperverband, compared to mesophilic digestion: o Increase in volatile solids destruction from 51% to 70 – 75% o Increase in biogas production of between 40 and 50% o Improvement in dewatering by 18 percentage points from 19 to 37 – 42% dry solids o Decrease in cake production by 60%, with an average of 1 wet ton exiting the plant for every 1 dry ton entering digestion o Increase in effluent COD concentration from 19 to 25 mg/l o No influence on effluent total nitrogen which remained at approximately 9 mg/l o No cooler installed o Self-sufficient for steam energy requiring 663 lbs steam/hr compared to production of over 1000 lbs steam/hr from co-generation o Decrease in anaerobic digestion heating requirements from 18.9 to 9.6 MMBTU o Increase in electricity generation from 400 kW to nearly 600 kW o Increase in energy content of the biosolids cake by 55% to 2095 from 1357 MMBTU/lb o Savings in biosolids haulage of over 550,000 euro annually

ACKNOWLEDGEMENTS The authors thank Daniela Gerstner and the staff of Geiselbullach WWTP for their support and assistance in providing operational data and access to their facilities during this project.

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Kolovos, A., Kjorlaug O. and Nilsen, P.J. 2016 Development and assessment of a model for Cambi´s SolidStream process using “SUMO” WWTP simulation software, Proceedings of the 21st European Biosolids and Organic Resources Conference,15-16 November, Edinburgh, Scotland. Rudolfs, W., and Heisig, H. M. 1929 Digestion of activated sludge. Sewage Works Journal, 146-159. Torpey, W.N. and Melbinger, N.R., 1967. Reduction of digested sludge volume by controlled recirculation. Journal (Water pollution control federation), pp.1464-1474. Wilson, C. A., and Novak, J. T. 2009 Hydrolysis of macromolecular components of primary and secondary wastewater sludge by thermal hydrolytic pretreatment. Water Research, 43 (18), 4489-4498.