anaerobic-based sewage treatment plants
Ref # S2SMALL-27725
T. Bressani-Ribeiro1, E.M.F. Brandt2, V.R. Melo1, F.J. Bianchetti1, E.
McAdam3, C.R. Mota Filho, C.A.L. Chernicharo1*
1 Federal University of Minas Gerais, Department of Sanitary and Environmental Engineering, Av. Antônio Carlos 6.627, Campus Pampulha, 31.270-901 - Belo Horizonte - MG - Brazil.
2 Federal University of Juiz de Fora, Department of Sanitary and Environmental Engineering, Engineering College, Campus UFJF, 36036-330 - Juiz de Fora - MG - Brazil.
3 Cranfield Water Science Institute, Cranfield University, Bedfordshire, UK.
* Presenting Author: calemos@desa.ufmg.br
Abstract: Anaerobic-based sewage treatment plants (STPs) have an important role in the sanitation sector in Latin America. If well designed, built and operated, these STPs can effectively meet legal discharge standards. Nevertheless, there are commonly neglected problems whose solutions would bring significant environmental improvements. A few examples are the loss of dissolved methane in the treated effluent and incomplete biogas combustion in low-efficiency biogas burners, both associated with greenhouse gases emissions. The aim of this work was to identify the most suitable alternatives in terms of carbon emission minimization for the integrated management of biogas, diffuse emissions (waste gas) and sludge in small anaerobic-based STPs (10,000 Pe). Three different hypothetical scenarios were simulated to assess STPs carbon footprint, considering different strategies based on the most suitable options for the integrated management of by-products. Diffuse emissions were found to be the main source of carbon emissions from STPs, even when biogas is not used for thermal energy recovery. In order to effectively minimize GHG emissions from STPs, the use of biogas instead of LPG for cooking in the STP vicinity seems to be a key strategy.
Keywords: By-products management; biogas; UASB reactors; anaerobic sewage treatment, greenhouse gases
INTRODUCTION
The use of anaerobic technology in sewage treatment plants (STPs) has significantly expanded in the last decades and is now considered a consolidated technology in Latin America (Chernicharo et al. 2015). However, one of the major limitations for an even wider use of anaerobic-based reactors (e.g. UASB reactors) is the management of CH4 and H2S in the gas phase (biogas) and dissolved in the liquid phase (released as diffuse emissions).
Odorous emissions (caused mainly by hydrogen sulfide) are known to affect the quality of life of local communities and methane emissions significantly contribute to the carbon footprint of the treatment process.
The advantages of anaerobic treatment are considered important attributes for developing more sustainable environmental technologies, such as little (close to none) demand for fossil fuels, simple and robust technology, and potential for resources recovery. Nevertheless, solid (sludge and scum) and gaseous (biogas) by-products are usually disposed of in landfills or flared, respectively. Although these routes are accepted in Brazil, they are not the most appropriate ones, due to their associated negative impacts in the atmosphere, soil and groundwater. Efforts to recover by-products are still incipient, mainly in small, decentralized
STPs. When accomplished, they occur in a disjointed manner, usually addressing only a few of the possible alternatives.
The aim of this work was to assess the most suitable alternatives in terms of carbon emission minimization for the integrated management of biogas, diffuse emissions (waste gas) and sludge in small anaerobic-based STPs (10,000 Pe). Three different hypothetical scenarios were simulated to assess the STPs’ carbon footprint, considering different management strategies.
MATERIALS AND METHODS
The most suitable options for the integrated management of by-products generated in small STPs were drawn up in a schematic flow-sheet (Figure 1), based on results obtained in several studies carried out by our research group (Borges et al. 2005; Souza et al. 2011; Lobato et al.
2012; Glória et al. 2016; Brandt et al. 2016; Rosa et al. 2016). For the liquid phase treatment, this proposed flow-sheet comprises a first anaerobic stage (UASB reactor) followed by a compact or an extensive aerobic post-treatment system (e.g trickling filters, wetlands, land application or polishing ponds). It is also possible to implement a simplified degassing unit to desorb H2S and CH4 from the liquid phase (Glória et al. 2016), which would be followed by a biofilter for combined sulfide and methane abatement (Brandt et al.2016).
Figure 1. Flow-sheet proposal for integrated by-products management in decentralized anaerobic-based STP (10,000 Pe)
The two considered alternatives for the management of sludge from small STPs were sludge sanitizers or thermal drying beds. When sanitizers are used, an increase in the sludge solids content is not expected (Borges et al. 2005) and such an option could be applied where the treated sludge (bio-solid) is used in the immediate vicinity of the STP (long-distance transportation not required). It is important to emphasize that sludge management for the extensive natural post-treatment options (wetlands, land application and polishing ponds) can
be neglected. This can also be accomplished with the trickling filters by using sponge-based systems (Almeida et al. 2013).
Alternatives for biogas management comprise its use for cooking or water heating in households near the STPs. Biogas is burned when thermal energy recovery is not applied.
A recently developed tool called “Sulfide and Carbon Emission Avoidance and Energy Recovery in STPs” (Chernicharo et al. in press) was used to perform the carbon footprint assessment of an anaerobic-based 10,000 Pe STP, considering three different scenarios (Table 1). The first one (default scenario) was proposed based on the typical configuration of UASB-based sewage treatment plants implemented in developing countries. In this case, the produced biogas is directly burned without energy recovery, the excess sludge goes to drying beds and there is no downstream dissolved gas management.
The second proposed scenario considered the following uses of the produced biogas: 90% of the estimated production (in terms of annual volume) was considered for sludge sanitization (Borges et al.2005) and 10% directly flared. For the third simulated scenario, the same biogas proportion was considered for sludge sanitization purposes. However, the remaining biogas amount was equally divided for use in cooking and water heating for bathing in the vicinities of the STP.
Table 1– Proposed scenarios for simulation using the toolSulfide and Carbon Emission Avoidance and Energy Recovery in STPs
Gas phase / Solid phase
Proposed scenarios – Management alternatives
Scenario 1 (Default) Scenario 2 Scenario 3
Biogas Flaring and water heating in the STP vicinity
Diffuse emissions from settler compartment (waste gas)
Open air settler Open air settler Open air settler
Diffuse emissions from
Sludge Drying beds Drying beds Sludge sanitizer or thermal
drying beds
aAll simulated scenarios considered the liquid phase passing through an aerobic post-treatment. Such a condition does not influence the management of gaseous emissions.
The aforementioned developed tool enables the determination of the amount of sulfide and methane in three different emission compartments, considering UASB-based STPs: internal surface of the gas-solid-liquid (GSL) separator - biogas hood; external surface of the GSL separator - settler compartment; and exit of a degassing unit - removal and/or recovery dissolved gases released from the anaerobic effluent (see Figure 1). Therefore, the tool allows
which is then converted to CO2 equivalent, allowing the assessment of the carbon footprint of the STP. The main parameters and equations employed in the tool are described in Chernicharo et al. (in press).
RESULTS AND DISCUSSION
In decentralized STPs, capacity and skills for operation and maintenance are limited, mainly in developing countries. Therefore, highly mechanized and complex options for the recovery and management of by-products should be avoided. The proposed schematic flow-sheet (Figure 1) considers simple alternatives for energy recovery from biogas and for controlling gaseous emissions from the liquid phase. Also, a simpler UASB reactor with an open-air settler was considered. This allows for the development of a scum layer containing phototrophic microorganisms able to oxidize H2S (Garcia et al. 2015). Notwithstanding, the unclosed settler compartment leads to diffuse CH4emissions.
Scenario 1 (Default conditions)
The simulated default scenario took into account the typical flow-sheet for anaerobic-based STPs in developing countries, mainly in Latin America and Caribbean region (Chernicharo et al. 2015). In this case, the entire volume of the estimated biogas production is flared without recovery of the available energy (thermal) potential, contributing to the carbon footprint of the STP. Nevertheless, the most representative source of CO2 equivalent emissions for Scenario 1 was associated with the absence of diffuse emissions control. Dissolved methane in the UASB reactor effluent accounts for 54% of the total carbon footprint of the STP, which is equivalent to 544.2 tCO2,equiv.year-1(54.4 kgCO2,equiv.inhab-1.year-1), as shown in Figure 2.
Figure 2. Carbon footprint for the simulated Scenario 1
Scenario 2
The use of biogas for sludge treatment instead of flaring, as simulated in Scenario 2 (90% of the estimated biogas production for sanitization or thermal drying beds) consequently reduces by 90% the CO2 equivalent emissions associated with biogas. The use of heat for drying excess sludge allows the production of bio-solids (84% TS; Possetti et al. 2015) that can be used in the vicinities of the STP, directly contributing to additional GHG emissions reduction of ca. 0.5 tCO2,equiv.year-1 (5.4 kgCO2,equiv.inhab-1.year-1), due to lower consumption of fossil fuels for transport. It can be observed that the carbon footprint associated with diffuse emissions (settler compartment and effluent) for Scenario 2 is equal to that of Scenario 1.
That is expected, since there is no management of dissolved methane in the liquid phase.
Regarding the simulated solid and gaseous phase management alternatives in Scenario 2, the balance between emitted and avoided CO2 equivalents remains negative, as depicted in Figure 3. In this case, a total emission of 488.2 tCO2,equiv.year-1 (48.8 kgCO2,equiv.inhab-1.year-1) was estimated.
Figure 3. Carbon footprint for the simulated Scenario 2
Scenario 3
The use of biogas for cooking and water heating in the STP vicinity can avoid the emission of approximately 989.7 tCO2,equiv.year-1 (98.9 kgCO2,equiv.inhab-1.year-1) to the atmosphere, as shown in Figure 4. Such a carbon footprint reduction was mainly related to the replacement of 5% of LPG used in the STP vicinity (in a volumetric base) by the estimated biogas production. As previously stated for Scenario 2, the use of heat to sanitize excess sludge contributes directly to further GHG emissions reductions of ca. 0.5 tCO2,equiv.year-1 (0.05 kgCO2,equiv.inhab-1.year-1). In this case, biogas flaring would not be used. Simple desorption units (Glória et al. 2016) followed by sulfide and methane oxidation in biofilters (Brandt et al.
2016) can reduce the amount of dissolved gases in the final effluent, controlling odour nuisance and avoiding the emissions of approximately 164.8 tCO2,equiv.year-1 (16.5 kgCO2,equiv.inhab-1.year-1) to the atmosphere. It is important to highlight the possibility of complete carbon neutralization in Scenario 3, which is clearly demonstrated in Figure 4.
Figure 4. Carbon footprint for the simulated Scenario 3
CONCLUSIONS
x The developed tool “Sulfide and Carbon Emission Avoidance and Energy Recovery in STPs” allows a comparative characterization of the main sources of carbon emissions in anaerobic-based STPs. This can be useful for decision makers in trade off analyzes of investments in small STPs infrastructure.
x The control of diffusive emissions seems to be crucial to mitigating the carbon footprint of small anaerobic-based STPs. In order to effectively neutralize carbon emissions, the use of biogas instead of LPG for cooking in the STP vicinity provides significant reductions of GHG emissions.
ACKNOWLEDGMENTS
The authors would like to acknowledge the support obtained from the Prosperity Fund (United Kingdom) and the following Brazilian institutions: Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq; Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES; Fundação de Amparo à Pesquisa do Estado de Minas Gerais –FAPEMIG;Instituto Nacional de Ciência e Tecnologia em Estações Sustentáveis de Tratamento de Esgoto – INCT ETEs Sustentáveis (INCT Sustainable Sewage Treatment Plants).
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