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Improving Anaerobic Digestion by Physico-Chemical Pretreatment

S. Prez-Elvira and F. Fdz-Polanco

Department of Chemical Engineering and Environmental Technology. University of Valladolid. (Spain). Email: ffp@iq.uva.es

Abstract Anaerobic digestion is the preferred option to stabilize sludge. However, the rate limiting step of solid hydrolysis make necessary to implement a pre-treatment unit prior to the digester. Among the different alternatives to improving the hydrolysis step (physico-chemical, biological,) the thermal option presents the advantage of using thermal energy, resulting in more cost-effective process. A process configuration combining thermal hydrolysis (TH) and anaerobic digestion (AD) of sludge was studied with the objective of analyzing the feasibility of the technology for full scale installations. The study was performed through pilot scale experiments and energy integration considerations, and a scheme of the most profitable option is presented: thermal hydrolysis unit fed with secondary sludge, anaerobic digestion of the hydrolysed sludge together with fresh primary sludge, and a cogeneration unit to produce green electricity and provide hot steam for the TH. From a technical and practical point of view, the process scheme proposed is considered to be feasible, and successful results were obtained feeding the TH unit at a concentration of 7-8% TS: better digesters mixing and performance, and better quality effluent (dewatering and sanitation). From an economic point of view, the key factors in the energy balance are: the recovery of heat from hot streams, and the concentration of sludge. The balances done show that the scheme proposed in this article has proven to need no additional energy input for the sludge hydrolysis, but for using all the biogas for electricity generation it is necessary to feed the TH unit with a 20%TS concentration of secondary sludge. However, this concentration is too high to be possible. For smaller concentrations, the electricity generation will decrease, still being the process autothermic. The study and balances here presented set the basis for the scale-up to a demonstration plant. Keywords: Sludge; anaerobic digestion; thermal hydrolysis; biogas; sludge properties

INTRODUCTION Owing to environmental, economic, social and legal factors, the treatment and disposal of excess sludge represents a bottleneck for wastewater treatment plants. In large wastewater treatment plants, primary and secondary sludge are stabilised applying anaerobic digestion, resulting in elimination of volatile solids and production of biogas. However, hydrolysis is the rate limiting step of biological degradation of sludge (Li and Noike, 1992; Shimizu et al, 1993), and therefore the process is slow, big digesters are used (> 20 day residence time) and large quantities of biosolids are produced (25-50% degradation of organic matter). By improving the hydrolysis step solid substrates are more accessible to anaerobic bacteria, accelerating the digestion, increasing the volume of biogas produced and decreasing the amount of solids to be disposed. The main physical pre-treatments founded at research or commercial level are: mechanical (high-pressure homogeneizers, ultrasounds), thermal, chemical or biological technologies (Weemaes and Verstraete, 1998; Kepp et al, 1999; Dohnyos et al, 1997; Chiu et al., 1997; Mukherjee and Levine, 1992; Prez-Elvira et al., 2006). Among the different alternatives, thermal pre-treatment has proven to be of great interest. The advantage of the combined TH+AD process is that the energy input needed for the hydrolysis process is thermal energy, and could be satisfied from the energy production of the own process, resulting in a energetically self-sufficient process. A combination of thermal hydrolysis and anaerobic digestion is widely investigated in literature from a laboratory-scale point of view, regarding disintegration and biodegradability (Li and Noike, 1992; Kepp et al., 1999; Kepp and Solheim, 2001; Fdz-Polanco F. et al., 2008). Very few articles evaluate some other properties of great importance from a full scale point of view (such as dewaterability, rheology and sanitation), and still fewer refer to the global process economics.

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The aim of this research was: 1) to check the technical feasibility of the process in a pilot plant, 2) to evaluate biogas production and VS removal in digesters fed with hydrolysed sludge, and also the end-product characteristics, 3) to use the experimental data as the basis of an economic study, combining a continuous thermal hydrolysis unit with a cogeneration device.

MATERIALS AND METHODS Pilot plant The pilot plant combining thermal hydrolysis and anaerobic digestion was operated for two years at the Municipal Wastewater Treatment Plant of Vic (Spain). Mixed sludge was directly fed to the thermal hydrolysis pilot plant shown in Figure 1, consisting of a feeding tank, a progressive cavity pump (Pmax=12 bar), a steam boiler, a 20 L total volume hydrolysis reactor (Vutile=10 L) connected to a flash tank (V=100 L) with outlet pipes for steam and hydrolyzed sludge Sludge is pumped into the digester by means of the pump, and the operation temperature (170C) is kept with direct steam injection from a boiler. The plant is operated in a batch mode, and is controlled with a data adquisition and control system that controls the reactor pressure and the residence time (30 min).

PRESSURE CONTROL VALVE

DECOMPRESSION VALVE

Processvapour

HYDROLYSIS REACTOR (20L)

FLASH TANK (100L)FEEDING

TANK (50L)

MIXED SLUDGE

FEEDING PUMP

DATA ACQUISITION AND CONTROL

HYDROLYSED SLUDGE

Figure 1: Thermal hydrolysis pilot plant

Hydrolysed sludge from the hydrolysis unit is cooled to ambient temperature and continuously fed to a mesophilic anaerobic reactor (200L) provided with sludge and biogas recirculations to assure mixing and avoid sludge setting inside the digesters. Biogas production is automatically recorded, and the digester performance and effluent characteristics are recorded from laboratory analysis (COD, VAF, pathogens, dewaterability and viscosity).

Mass and energy balance considerations A process configuration combining thermal hydrolysis (TH) and anaerobic digestion (AD) of sludge should take into account the global process economics. A first approach allows to conclude that in order to reduce the heating requirements: i) it is a better option to segregate primary from secondary sludge in order to treat in the thermal hydrolysis unit only the biological sludge (as primary sludge presents a high biodegradability, and therefore the energy needed to treat the primary sludge in the thermal unit is too high compared to the subsequent little increase in biogas), and ii) some kind of energy integration has to be considered. In fact, the economic advantage of a thermal process compared to other pre-treatment alternatives (physical,

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chemical,..) is the possibility of getting the energy needed for the thermal process from the own process, such as the recovery of heat from hot streams to pre-heat the sludge (like vapour produced in the flash, hot hydrolyzed sludge, exhaust gases and hot water from the gas engine), and the use of the biogas produced in the anaerobic digestion to generate the steam. The quantity of steam required per unit of sludge depends on: 1) the sludge concentration, 2) the equipment selected to get energy from the biogas produced (boiler or gas engine), and 3) the process configuration (recovery of heat from hot streams). The optimization of the three factors is the key to have a self-sufficient process in which no fuel is necessary to operate the process and the steam generated from biogas is enough in quantity and quality to fulfil the sludge heating requirements. This second approach shows that there is an input concentration that optimizes the process, and its value is different depending on the process configuration and the energy generation device. The objective of present researches on thermal hydrolysis is to get the optimum scheme (Figure 2): an energetically self-sufficient thermal hydrolysis plant using a CHP unit which generates green electricity from biogas. To date, there is no full-scale such this, and a proposal is discussed at the end of the article.

Figure 2: Optimum scheme: sludge segregation and use of gas engine

RESULTS Technical feasibility Three years research in laboratory scale, and fourteen months operation in pilot plant support the practical feasibility of the thermal hydrolysis process studied (Fdz-Polanco et al., 2007). As presented in Prez-Elvira et al. (2008), the thermal treatment of fresh mixed sludge (175C, 30 min) disintegrated the floc structures, increasing four times the SCOD fraction and favouring the water removal: 10% TS concentration could be achieved by centrifugation, without polymer conditioning. Moreover, after the hydrolysis the sludge viscosity becomed radically smaller, which favoured the digester mixing and, therefore, the performance. The combined benefits of cell lysis, solubilisation and rheology had also a positive effect on the sludge anaerobic digestibility and the rate of degradation: but 48% VS removal and 502 mL biogas/g VSfeed can be considered as a mean value. Comparing the performance of the pilot digester with the full scale anaerobic digester of the WWTP (fed with non-hydrolysed sludge), 40% more biogas was obtained in nearly half the time (12 days instead of 20 days), and sludge production decreased by nearly 30%. The digested sludge was pathogen free, and a 7% DS cake could be achieved by centrifugation without the use of polymer conditioners, being the solids recovery rate above 95%. This performance results support the technical feasibility and benefits of the combined process. A proper economic study is presented below to assess the viability of the technology.

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Case study to evaluate economic feasibility Mass and energy balances for a continuous process treating the sludge generated by a municipal wastewater treatment plant have been performed. Among the different process configurations based on Figure 2, the one considered for the present study is shown in Figures 3 and 4. As was discussed above, the most profitable scheme consists of a continuous thermal hydrolysis unit (to exploit the energy integration possibilities that are impossible to consider in a batch process) fed with secondary sludge (not including primary sludge), followed by the anaerobic digestion of the hydrolysed sludge together with fresh primary sludge. Calculation basis are presented in Table 1: Table 1: Calculation basis considered in mass and energy balances

Main figures 100,000 inhabitants Sludge production: 0.23 ton DS/h Secondary sludge: 0.10 ton VS/h Primary sludge: 0.06 ton VS/hour

Thermal hydrolysis Continuous operation: 365d/yr Operation conditions: 175C for 30 minutes

Anaerobic digestion Continuous operation: 365d/yr 55% VS degradation Biogas production: 315 L/ kg VS feed

Gas engine 39% electric yield 25% thermal yield (to exhaust gases) Exhaust gas boiler 64% efficiency of exhaust gases in boiler

Figure 3 presents the process integration scheme considered. The amount of steam needed in the thermal hydrolysis unit depends on the capability of recovering heat from hot streams, and therefore on the configuration of this unit. In the scheme selected, the steam needed for sludge heating is approximately 0.8 tonne of steam per tonne of DS. Figure 4 presents mass and energy balances for a self-sufficient process composed by the thermal hydrolysis unit, anaerobic digestion (of hydrolysed secondary sludge and fresh primary sludge), and a cogeneration unit (to recover energy from the biogas).

Figure 3: Process integration scheme considered for the thermal hydrolysis unit fed with secondary sludge 20%TS concentration

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Figure 4: Mass and energy balances for a self-sufficient process composed by TH (fig. 3), AD and cogeneration with a gas engine, for 20% TS concentration of secondary sludge to the TH process

The results of the balances show that considering a 100,000 inhabitants municipal wastewater treatment plant, and the configuration scheme presented, it is possible to get the sludge heating requirements (0.13 ton/h) from the energy liberated during electricity generation (84kW). Therefore, this scheme is energetically self-sufficient, and the whole biogas production (81Nm3/h) can be utilized for electricity generation (205 kW). This means that, from an economic point of view, the 34% increase in electricity generation with respect to a conventional anaerobic digestion, represents a surplus of 40 kW, with an associated value of 60.000 $/y.

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But apart from this economic benefit, previous laboratory studies have shown the beneficial changes of the sludge properties with respect to viscosity (digesters mixing and performance) and dewatering characteristics (centrifugation behavior improvement and polyelectrolyte savings). However, in this scheme a high input concentration (20%DS) is necessary in the thermal hydrolysis unit to have a self-sufficient process. This value is too high to be possible. The challenge now is to enhance this scheme, proposing another process configuration, technically feasible, which allows reducing the steam requirements and therefore the sludge concentration needed to assure a self-sufficient process. One idea is to burn part of the biogas in the boiler to generate the steam needed. In this second option, the energy generation from biogas is smaller (as a fraction is lost to produce steam), but allows to get an autothermic process. Figures 5 and 6 present mass and energy balances for a self-sufficient process considering a secondary sludge concentration at the thermal hydrolysis unit of 8% (which has proven to be viable from a technical point of view).

Figure 5: Process integration scheme considered for the thermal hydrolysis unit fed with secondary sludge 8%TS concentration

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Figure 6: Mass and energy balances for a self-sufficient process composed by TH (fig. 5), AD and cogeneration with a gas engine, for 8% TS concentration of secondary sludge to the TH process

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CONCLUSSIONS A process scheme combining anaerobic digestion and thermal hydrolysis was studied through pilot scale experiments and energy integration considerations. The scheme defined as more profitable consists of a thermal hydrolysis unit fed with 7% TS secondary sludge, followed by the anaerobic digestion of the hydrolysed sludge together with fresh primary sludge. A cogeneration unit is used to recover energy from the biogas. The technical feasibility of the system working with 7-8% TS sludge has been proven from the pilot plant performance. Laboratory studies show the beneficial changes of the sludge properties with respect to viscosity (digesters mixing and performance) and dewatering characteristics (sludge volume reduction). No operational problems have been observed. Energy considerations suggests that the scheme defined as more profitable consists of a thermal hydrolysis unit fed with 20% DS secondary sludge, followed by the anaerobic digestion of the hydrolysed sludge together with fresh primary sludge. Mass and energy balances show that the methane surplus obtained with the thermal hydrolysis pre-treatment can be totally used for electricity generation only when feeding the TH unit with secondary sludge above 20%TS. In this case, 200kW green electricity are generated, which represents a surplus of 24% with respect to the conventional process. However, for 8%TS concentration in the sludge feed (proven to be technically feasible), half the biogas has to be burned to get the steam needed for the process, and therefore less electricity is obtained compared to the conventional process. Some other benefits that support the economic feasibility of the technology are:

Smaller foot print of the digesters, as a consequence of working at a higher input (7-16%TS) concentration and less residence time (10-12 days).

Increase in biogas production by 24% with respect to the conventional anaerobic digestion. Reduced TS production in the final biowaste by 26%. The better dewaterability of the waste

drives to a 58% smaller volume of biowaste to disposal. Meets US EPA Class A criteria pathogen free.

AKNOWLEDGEMENTS This work was financially supported by the Spanish Ministerio de Industria, Turismo y Comercio (Project SOSTAQUA, CENIT 2007-2010, leaded by AGBAR). The Valladolid University research group is Grupo de Excelencia GR 76 de la Junta de Castilla y Len and member of the Consolider-Novedar framework (Programa Ingenio 2010, Ministerio de Educacin y Ciencia).

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