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STRATEGIES TO ENHANCE SLUDGE PROCESSING THROUGH ANAEROBIC DIGESTION Irina Lukicheva Krishna Pagilla, Ph.D., P.E. Illinois Institute of Technology (IIT) 3201 S Dearborn Street Chicago, IL 60616 Movva Reddy, Ph. D., P.E. MPR Engineering Corp ABSTRACT Recently, there has been increasing number of wastewater treatment plants (WWTPs) that are planning to upgrade their conventional sludge treatment systems which produce Class B biosolids to produce Class A biosolids. This trend has triggered rapid growth of not only new sludge treatment technologies but also conventional treatment modification technologies. In the US, anaerobic digestion is the most widely used sludge treatment method at WWTPs. In majority of the cases the solids processing train (SPT) consists of thickening, mesophilic anaerobic digestion and dewatering. This type of conventional sludge treatment system only allows the production of Class B biosolids. The purpose of the work described here was to investigate feasible and sustainable options to enhance mesophilic anaerobic digestion to produce Class A biosolids while achieving the number of other objectives of solids processing at WWTPs. The work was based on the technical information on enhanced sludge treatment systems collected from recent publications, white papers, WWTPs, and technology vendors throughout the US and worldwide. The levels of Class A biosolids produced within the US as well as current trends in Class A biosolids practices were gathered from the survey of EPA biosolids coordinators from each state. All the identified technologies were analyzed in terms of the ability to retrofit the existing solids processing facilities as well as in terms of fulfilling the objectives of sludge digestion such as pathogens reduction, volatile solids destruction and biogas production. The main focus of the work was on the technologies that has been in operation in full scale in the US and that have been proven to achieve multiple objectives of solids processing. Carbon footprinting of each of the selected technologies for further consideration was determined to compare the selected systems in terms of their sustainability. Study demonstrated that currently, the most common way to produce Class A biosolids in the US is by implementing post-treatment of sludge by methods such as composting or thermal drying. Implementing any post-treatment process allows only Class A biosolids production, but it does not enhance the performance of anaerobic digestion. Thus, the current study concentrated on ways to enhance the existing anaerobic digestion either by using sludge pre-treatment before the conventional mesophilic anaerobic digestion, or by modification of the anaerobic digestion itself. All the obtained information from plants with the full-scale installations of identified technologies was ranked in terms of the technology's ability to achieve the listed goals of anaerobic digestion and compare the different systems performance. It was concluded that among the pre-treatment technologies, pre-pasteurization guarantees Class A production but does not necessarily enhances other goals of sludge digestion; thermal hydrolysis, although substantially improves the digestion, requires major changes in infrastructure of the plants that operate conventional anaerobic digestion; physical pre-treatment methods improve conventional

anaerobic digestion, relatively easy to retrofit, but do not enable Class A biosolids production. At the same time it was noted from the literature review, that physical pre-treatment methods provide flexibility to adapt to needs in changes in requirements for final utilization of biosolids. In case of modifications of anaerobic digestion TPAD was found to be a widely used technology but none of the surveyed plants were producing Class A biosolids at the time when the survey was conducted, instead, they were operating TPAD to enhance the digestion. In cases where TPAD was implemented to achieve Class A production, plants had technical difficulties of operating thermophilic stage in the batch mode. As for thermophilic anaerobic digestion, it was proven to produce Class A, but at all the plants new thermophilic digesters were built prior to implementation of the process. Alternative future sludge treatment configurations for a plant operating conventional SPT were determined based on all the observations during this study including technology assessment of existing full scale plants operating each of the systems, analysis of a typical conventional plant infrastructure including equipment age, average raw sludge and product sludge data. It was recommended that depending on the long term biosolids utilization goals, plants can utilize different selected technologies to achieve those pre-determined goals. In the first proposed configuration, OpenCEL ultrasound pre-treatment for waste activated sludge flow was recommended in case when plant's main goal is to solely improve the sludge digestion performance. EcoTherm pre-pasteurization was recommended for the next proposed configuration when part of the sludge flow is diverted to achieve Class A biosolids production, while the other part of the flow is modified to improve the sludge digestion performance. For the third and last proposed configuration TPAD in continuous operation mode was recommended to be implemented in stages for improved sludge digestion performance with the possibility of modification to sequential-batch operation mode for Class A biosolids production in the future. KEYWORDS Enhanced anaerobic digestion, Class A biosolids, biogas, retrofit of conventional anaerobic digestion INTRODUCTION Anaerobic digestion is the commonly used technology to stabilize sewage sludges. It allows the production of final product (biosolids) that can be beneficially used along with providing the opportunities for the plant energy recovery. Recently there is a growing trend among the wastewater treatment plants (WWTPs) to improve the performance of conventionally used mesophilic anaerobic digestion. The driving force to implementation of enhanced process depends on the specific goals of each plant in particular as well as on the existing plant infrastructure. Although any improvement in the conventional process leads to changes in the facilities, it is desired to minimize operational and design modifications. Goal of this work was to identify the technologies used worldwide to enhance the conventional anaerobic digestion and provide the list of most appropriate and practical solutions to satisfy various goals of plant upgrade. The work concentrated around the processes that can be retrofitted as a pre-treatment or modifications for the conventional anaerobic digestion and that have been implemented full-scale in the US.

Conventional anaerobic sludge stabilization Conventionally used solids processing train usually consists of thickening, stabilization by mesophilic anaerobic digestion and dewatering. The biosolids produced after the stabilization comply with the requirements of Class B biosolids and can be land applied (USEPA, 2003). The main focus of the work was to find the technologies that would require the modifications of only the digestion complex and would not involve the changes in thickening and dewatering operations. Goals of enhanced anaerobic digestion One of the main reasons leading the facilities to improve the digestion is to provide more advanced level of pathogen reduction and produce higher quality of biosolids. This is driven by growing public concern causing communities and municipalities to adopt restrictive regulation rules that essentially prohibit the land application of Class B biosolids (Godfree, 2005). In recent years there is growing interest in advanced anaerobic digestion technologies because they allow not only to produce biosolids end product of exceptional quality but also to provide all the following benefits of anaerobic digestion: Increased volatile solids reduction (VSR); Improved biogas production rates and biogas quality; Minimization of amount and volume of final product solids; Effective digesters capacity usage; Reduced vector attraction potential in the product sludge; Improved dewaterability of the product sludge; Diminished odor problems during and after processing; Reduced carbon footprint to the environment.

The most important goal of enhancing anaerobic digestion is the increased biogas production because it can substantially decrease plant operating costs. Larger amount of biogas produced due to higher VSR levels can be used for WWTP energy needs by co-generation and/or direct reuse of biogas. Energy produced from co-generation can cover not only the energy need for biosolids treatment but also a substantial part of energy consumed at the whole treatment facility. Energy recovery also allows for the plant to decrease its carbon footprint to the environment. Setting up the priorities of each of the goals of the sludge digestion according to plantspecific needs is a vital step in the process of selection of the alternatives for implementation on the full-scale. At the same time alternative technologies should be compared in terms of compliance with the goals of digestion listed above and evaluated by the degree of changes in existing sludge processing infrastructure needed to implement the new treatment alternative. The final choice of the technology depends not only on the project specific goals, but also on the subsequent cost estimations and future biosolids market evaluations. The last two criteria were not discussed in this work. In order to change the anaerobic stabilization process to comply with the goals listed above, the solids processing train can be modified by implementing pre-treatment prior to

anaerobic digestion or by the modification of the digestion itself. Post-treatment after the digestion can only change the quality of final product but not the performance of the stabilization process, therefore, post-treatment technologies were not investigated in this work. METHODOLOGY Technologies were evaluated based on the technical information dealing with enhanced sludge treatment systems collected from recent publications, white papers, WWTPs, and technology vendors throughout US and worldwide. The amounts of Class A biosolids produced within the US as well as current trends in Class A biosolids practices were gathered from the survey of EPA biosolids coordinators from each state. Enhanced digestion technologies were uniformly classified by their compliance with the goals of anaerobic digestion on a full-scale. This preliminary screening of all feasible technologies to improve anaerobic digestion allowed process selection for the upgrade of conventional sludge process. The selected alternatives were chosen according to priorities and goals plant might like to have. To further evaluate the performance of the selected technologies operational carbon footprint was calculated using a 30 mgd plant as the base case. RESULTS AND DISCUSSION Technologies evaluation In attempt to collect latest data on the Class A biosolids production in the US, survey of biosolids states coordinators was conducted as a preliminary study. Estimated number of facilities producing Class A biosolids by selected technologies in some of the US states is presented in Table 1. As it can be noticed, the most common technologies used are composting, heat drying (pelletizing), aerobic and alkaline stabilization. Composting and heat-drying are post-treatment to the digestion and they do not allow achieving any other goals of enhanced digestion besides the Class A biosolids production. Moreover, they require additional energy from the plant and thus, not included in this study. Conversion of the plant to aerobic stabilization or lime treatment requires major infrastructure changes for the plant that uses anaerobic digestion to stabilize sludge, and those processes were also not taken into consideration in this work. Only 3 plants producing Class A through enhanced anaerobic digestion were identified through the survey. The rest of the plants listed in the following Table 2 were identified through further literature review. Further detailed search of the processes from publications, surveys of plants and vendors in the US and from around the world revealed that to enhance the conventional anaerobic digestion treatment systems, several types of pre-treatment prior to anaerobic digestion or modifications of anaerobic digestion itself can be implemented.

Table 1 Distribution of facilities producing Class A biosolids in the US selected states State California Pennsylvania Ohio Wisconsin Utah Florida New Jersey Alaska Washington Georgia Colorado Montana Kansas Michigan Vermont Massachusetts Indiana Missouri Oregon Minnesota Louisiana N of compost facilities many 9 6 1 14 4 4 1 21 3 4 3 1 1 3 many some 2 3 n/a N of heat drying and pelletizing facilities many 4 3 1 n/a 8 1 3 2 1 some some 1 n/a 3 1 N of aerobic digestion facilities (including ATAD) 2 13 2 1 2 n/a some n/a n/a 4 4 N of chemical treatment facilities many 12 13 3 11 2 4 2 1 n/a some n/a n/a 4 4

Table 2 Class A advanced anaerobic digestion plants in the US Plant Name Hyperion WWTP, CA Terminal Island WWTP, CA Lakeland, FL Hemet WWTP, CA Tulsa WWTP, OK Carmel WWTP, IN ASA, VA Capacity 350 mgd 30 mgd 20 mgd 11 mgd 42 mgd 12 mgd 54 mgd Advanced Digestion Process Thermophilic AD Thermophilic AD TPAD EcoTherm pasteurization EcoTherm pasteurization BioPasteur pasteurization BioPasteur pasteurization

Pre-treatment methods listed in Table 3 help to pre-condition sludge prior to digestion. Depending on a type of pre-treatment, they either help to break down cell walls of the biomass enhancing the biodegradability of sludge for the further digestion, and/or pasteurize the sludge

improving pathogen destruction. Numerous pre-treatment technologies have been developed based on following methods: thermal methods - thermal hydrolysis, pasteurization, submerged combustion; chemical methods - oxidation, use of enzymes, alkaline/acid hydrolysis (not listed here), and physical methods - ultrasound, homogenizer, stirred ball mills, electronic pulse field, and microwaves. Table 3 Methods to Enhance Anaerobic Digestion TYPE OF TECHNOLOGY METHODOLOGY VENDOR NAME BioPasteur PRE-TREATMENT TO ANAEROBIC DIGESTION AlphaEnvir Pasteurization Pasteurization Thermal Submerged Combustion EcoTherm Enhanced Enzymatic Hydrolysis Hydrolysis Ultrasound Physical Microwaves Pulsed Electric Field Mechanical Disintegration MODIFICATIONS OF ANAEROBIC DIGESTION Thermophilic Anaerobic Digestion Temperature Phased Anaerobic Digestion (TPAD) Thermophilic Digestion (continuous feed) Thermophilic Digestion (feed/hold/withdraw) TPAD Acid-gas TPAD ODI 2PAD BioThelys Cambi Sonico n/a OpenCEL n/a n/a n/a

Columbus Biosolids Flow-Through Thermophilic Treatment

Modifications of anaerobic digestion (Table 3) usually involve introduction of thermophilic anaerobic digesters, whether it is thermophilic digestion by itself or in combination with mesophilic digestion (Temperature Phased Anaerobic Digestion (TPAD)). These methods of treatment, if they comply with the time and temperature requirements of EPA, are capable to produce Class A biosolids and are shown to improve the performance of the digestion itself.

Processes that were identified to be operating on full scale were analyzed by their performance in selected objectives of sludge stabilization (Table 4). The quantitative categories used in the comparison include solids loading rates of the digesters that show the effectiveness of digesters capacity usage, percentage of volatile solids reduction (VSR) within the system, Class A or Class B product, and biogas production. Biogas production at different plants is compared in terms of biogas production per mass of volatile solids that are fed to the digesters. Nonquantitative categories were also used to compare the processes and these include dewaterability of sludge, odor control issues and the ability of the process to retrofit mesophilic anaerobic digestion. To compare all the processes among each other all the data provided by plants, vendors and that was found in various publications was converted to common units, thus it does not give the exact value, but is intended for the illustration and comparison of the process performance. Table 4 Processes Performance in Selected Objectives of Anaerobic Sludge Stabilization Pathogen Destructio n Biogas Productio n (m3/kg VS fed) 0.25-0.62 >1.25 n/a n/a n/a >1.25 0.25-0.62 Degree of Changes in Existing Infrastructur e high n/a moderate moderate low high Reported Odor Problems

Process

VSR (%)

Dewaterabili ty of Sludge

Conventiona l anaerobic digestion Cambi BioThelys Bio Pasteur EcoTherm OpenCEL Ultrawave (ultrasound pretreatment) TPAD, continuous flow mode TPAD (ODI 2PAD) TPAD, feed/hold/ withdraw mode Thermophili c AD

Class B Class A Class A Class A Class A Class B n/a

40-50

>55

some none none n/a none none n/a

improved n/a n/a n/a same n/a

n/a 45-55 45-55

>55

40-45

Class B Class A

40-45 N/A

0.62-1.25 0.25-0.62

low high

some n/a

n/a n/a

Class B

45-55

0.62-1.25

moderate

n/a

n/a

Class A

>55

0.62-1.25

low

some

n/a

The performance of conventional anaerobic digestion solids processing is also included in the evaluation in order to demonstrate how well the enhanced anaerobic digestion plants perform in comparison with the conventional anaerobic digestion. The analysis of all the technologies demonstrated that the proven pre-treatment methods, such as thermal pre-pasteurization of sludge, comply with time and temperature requirements and guarantee Class A production. They are also relatively easy to retrofit to the existing plant. At the same time it is not proven that thermal pre-pasteurization of sludge helps to sufficiently increase biogas production and VSR. The thermal pre-treatment systems, based on operating plants experience, are not very flexible systems. Due to the fact that they do not noticeably enhance biogas production, their carbon footprint is comparable to the carbon footprint of the system anaerobic digestion followed by thermal drying. These might be the reasons why such systems at first were more developed in Europe, where land restrictions forced numerous facilities quickly switch to production of enhanced quality biosolids, and they were not very popular in the US. It is also the reason why recent new methods of pre-treatment, such as thermal or enzymatic hydrolysis are gaining more popularity all over the world and thermal prepasteurization systems are being phased out. Although retrofitting the plant to thermal or enzymatic hydrolysis usually involves major changes in existing infrastructure and high associated costs, in the long run hydrolysis is proven to sufficiently improve the digestion, increase the capacity of the plant and produce Class A product. Enzymatic hydrolysis as well as thermal hydrolysis might be a better solution for the plants that are planning to completely renew the thickening and digestion facilities due to old age or due to major expansions and increasing wastewater flows. The major disadvantage of both thermal and enzymatic hydrolysis processes is that there is no experience of working with either of them in the US, although some facilities (for example DCWASA) are already looking at implementing Cambi thermal hydrolysis at their plants. Among other pre-treatment technologies the electric pulse treatment looks like a promising technology that is improving the biodegradability of the sludge enhancing the digestion and is also very easy to retrofit (Banaszak, 2008). Ultrasound pre-treatment has not been implemented in the US and it is hard to conclude if the process helps to achieve the enhanced digestion in reality. The US plants have the experience with the modifications of anaerobic digestion with the pre-treatment systems, although continuous Class A biosolids production is known only for the thermophilic anaerobic digestion. Implementing thermophilic anaerobic digestion will more likely require building new thermophilic digesters or at least full refurbishment of the existing ones. The operation of the digesters in thermophilic mode also requires substantial heat supply and might cause odors. TPAD and its modifications if they comply with the time-temperature requirements of EPA are capable of producing Class A biosolids. In addition they are proven to enhance the performance of anaerobic digestion. On the other hand, the research showed that most facilities implementing TPAD either have difficulties in operation and not producing Class A product, or the main purpose of TPAD was to enhance the digestion and not to produce Class A. The rising interest in TPAD is driving more engineering companies to work on the process modifications

and improvements, which eventually should lead to proven techniques of retrofitting mesophilic anaerobic digestion to TPAD. One of the examples of such TPAD hybrid processes is Infilco Degremont's ODI 2PAD that has been granted the PFRP equivalency already. Choosing the appropriate alternative The decision making in choosing the process depends on the analysis of the performance of each process and how it helps to achieve all the objectives of enhance digestion simultaneously. Processes that has not yet been approved by EPA to produce Class A or processes that have been in operation can be considered. Although there may not be enough experience at a large scale that guarantees Class A biosolids production, if they are proven to enhance the digestion at a plant, they should also be taken into consideration with the possibility of Class A production in the future. As an alternative, plants that do not have a priority of Class A biosolids production at this point in time can enhance the digestion by implementing mechanical pre-treatment or phased digestion. By improving the other factors of digestion the plants would be able to operate more efficiently and generate more biogas. In case of changing regulations or other driving factors of biosolids production with the enhanced digestion, the plant would be able to recover sufficient amount of energy to treat part or the whole solids stream to Class A by implementing one of the post-digestion processes. Unfortunately, as the research shows, there still are not many technologies on the market that successfully operate on a full scale due to the fact that the development of those technologies started 5- 10 years ago. In order to demonstrate how the selected technologies can be retrofitted to the conventional anaerobic digestion plants, two case studies were taken into consideration. The technology selection for each of the case studies is based on the information from the full-scale operating experience and average sludge characteristics at the plants with conventional sludge. It is also assumed that sludge processing facilities such as thickening, dewatering and conventional mesophilic anaerobic digestion have substantial life cycle remaining. As it shown before, there are not many technologies on the market at this point in time that can be easily retrofitted into the existing conventional sludge processing train infrastructure and result in the enhanced digestion, especially in case when dealing with Class A biosolids digestion based processes. Even in case when the processes are implemented full-scale, the facilities tend to face technical difficulties operating them. At the same time, Class A biosolids production is proven to be achieved by the post-treatment processes that are widely used in the US. Implementing enhanced digestion pre-treatment technology that is relatively easy to retrofit can provide the plant with the additional source of energy required to operate the post-treatment. It also gives the plant the flexibility to decide on the amount of Class A and Class B biosolids produced in case of changing regulations or biosolids markets. Plant size is another important variable that can influence the selection of appropriate technology. In majority of full-scale installations of pre-treatment and modifications of anaerobic digestion the facilities sizes are below 50 mgd. Thus, there is no experience with the enhanced technologies operating at larger scale. In case of large scale plants the solids processing train can be divided into separate treatment trains where part of the treatment is used to produce Class A biosolids and the other part is used to produce more biogas. Such strategy also provides the flexibility to reflect the changes in biosolids regulations/markets while operating efficiently.

Table 5 demonstrates the alternatives suggested for two case studies of plants of different sizes according to possible goals of enhancing the digestion they might have. As it can be seen, the most promising technologies are chosen to be retrofitted are pasteurization (EcoTherm pretreatment), electric pulse pre-treatment (OpenCEL) and TPAD. Table 5 Case Studies for Upgrades of the Conventional Anaerobic Digestion Solids Processing Main goal of the plant upgrade Plant size Plant >50 mgd Enhancement of digestion performance (Class A only by means of post-treatment) OpenCEL pretreatment Class A production by modification of the existing digestion (either for the part or entire solids flow) EcoTherm pasteurization for part of the flow EcoTherm pasteurization Enhancement of digestion with possibility of Class A in the future TPAD for part of the flow first, later for entire flow TPAD first in continuous flow modification, later in sequential-batch

Plant <50 mgd

OpenCEL pretreatment

Case Study 1: Plant > 50 mgd Goal 1: enhancement of anaerobic digestion, Class A is not a main priority (separate post-digestion process is needed to produce Class A) OpenCEL for WAS and additional sludge (if applicable) was chosen for the retrofit of the facilities based on the following: 1. The process substantially enhances the digestion. The operation of OpenCEL at Mesa, AZ Northwest WRP showed improvement of VSR to 55-60%, increased biogas production and decreased digesters heating requirement. 2. There are no major changes in the infrastructure of the plant and no interruption in the plant operations. OpenCEL unit is installed in front of the digesters and it treats solids with the electric pulsed field in continuous mode so there is no need for additional tanks or reactors. The thickening facilities do not have to be changed. 3. Vendor (OpenCEL) is located in the US. 4. For a large scale plant currently there is no pre-treatment process on the market that was continuously in operation. Goal 2: Class A production for the part of the solids flow and enhanced digestion for the other part of the flow. EcoTherm is recommended for the part of sludge flow, such as additional sludge (if applicable) due to the following:

1. Guaranteed Class A production was demonstrated full-scale and it is EPA approved process to produce Class A. 2. The EcoTherm pasteurization or any other pre-pasteurization process is not found to be implemented anywhere at the scale more then 50 mgd. 3. Vendor (Ashbrook Simon Hartley) is located in the US, which guarantees easier access to technology. 4. Maximum usage of the existing facilities. Goal 3: the goal of the upgrade is an enhanced digestion with the possibility of Class A production. TPAD configuration is recommended for the upgrade of the facilities: 1. TPAD is being implemented at large scale plants either in continuous or batch configuration and there are more plants in design. 2. Based on the surveyed plants operation and on lab and pilot scale studies TPAD helps to enhance the digestion. 3. Process is flexible to changes in regulations and drivers of biosolids market. At the large scale plants the transfer to TPAD configuration can be done gradually. Only part of the solids flow can be treated in TPAD, and after the process is proven to work, the entire plant can be configured to TPAD. 4. The conventional anaerobic digestion infrastructure does not need to be completely changed prior to TPAD. This factor largely depends on the digesters configuration. Case Study 2: Plant < 50 mgd Goal 1: enhancement of anaerobic digestion, Class A is not a main priority (separate post-digestion process is needed to produce Class A) OpenCEL FP pre-treatment prior to digestion is recommended for an upgrade following the same reasoning as for the first case study. Goal 2: Class A production At the scale of plant size < 50 mgd, EcoTherm pre-pasteurization is recommended to achieve Class A biosolids production based on the factors described for the first case study, also taking into consideration that both plants currently operating EcoTherm are smaller than 50 mgd: Goal 3: the goal of the upgrade is an enhanced digestion with the possibility of Class A production TPAD is recommended in the continuous operation mode in the beginning, then switching to batch mode when the Class A is needed. TPAD is chosen due to the similar reasoning as for the plant larger than 50 mgd described previously.

Evaluation of carbon footprint for the selected alternatives Operational carbon footprint estimation for each of the technologies selected from the literature review is presented in Figure 1. As it can be noticed, the most sustainable options for plant upgrade were found to be TPAD and OpenCEL pre-treatment. This is most likely due to the fact that these processes although require higher energy demand at the same time enhance biogas production and improve energy recovery.

20000 15000

Conventional anaerobic digestion

EcoTherm

TPAD

OpenCEL

13625

10000 5000

12700

13000

10400

Input into carbon footprint

tCO2e

0

-5000 -10000 -15000 880 tCO2e 4105 tCO2e 0 tCO2e -760 tCO2e

-9520

-9520 -12700 -13760

Carbon footprint offsets

Net balance

Figure 1. Carbon Footprint Estimation of Selected Enhanced Digestion Technologies for a 30 mgd Plant CONCLUSIONS To determine which one of the alternative technologies is appropriate for implementation at a given plant, all the goals of anaerobic digestion should be evaluated according to plant's needs. The literature review identified only one technology that is proven in the US to continuously produce Class A (EcoTherm) and to be relatively easily retrofitted to the existing infrastructure of conventional anaerobic digestion plant. All the other methods of Class A production in the US usually involve post-treatment methods after anaerobic digestion. Implementation of the enhanced digestion can give a plant an opportunity to operate in a more efficient and sustainable way to stabilize sewage sludge. At the same time it can allow the plant to modify the process or implement additional treatment to produce Class A in the future. Based on all the observations several possible scenarios of enhancing the digestion for conventional anaerobic digestion plant were recommended. The recommendations should be based on the principle of maximum usage of the existing infrastructure, taking into consideration various goals of the plants upgrade.

In conclusion, more detailed analysis of heat and power requirements for each system at each of the plants should be performed in order to identify the most suitable option for a specific plant. Besides that, analysis should also include calculations of capital cost, carbon footprint associated with installations and operations/maintenance, estimation of social factors and economic benefits of each of the alternative systems. SELECTED REFERENCES Banaszak, J., and Rittman, B. 2008. The technology behind OpenCELTM. Vendor report and presentation. Godfree Alan, and Farrel, J. 2005. Processes for managing pathogens. J. Environ. Qual. 34 (1), pp. 105-113. USEPA. 2003. Control of pathogens and vector attraction in sewage sludge.

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