Electricity and substitute natural gas generation from the conversion of wastewater treatment plant sludge
Introduction
Adequate water and sanitation services are crucial for the protection of public health, the maintenance of basic conditions of living and the protection of biota and natural resources. Despite the outstanding advances in wastewater treatment technologies in recent decades, the universalisation of water and sanitation services remains a major challenge for the 21st century [1].
Under a modern perspective, a centralised municipal wastewater treatment (WwT) programme was set up in Chile, thanks to a large-scale water reform policy started in the late 1990s, leading toward the privatisation of this service sector, which was previously managed by the state. In parallel with this restructuring, emissions standards for municipal sewage discharge were developed when the General Environmental Law was enacted (1997). With the advent of these standards, water supply companies have the obligation to treat polluted water after discharging it into the surface-water environment for the purposes of preserving biota, avoiding detrimental effects, improving the value of tourism sites and protecting human health. According to the World’s Water Report [2], Chile has 922 billion cubic metres of total renewable freshwater. Furthermore, by 2010, 87% of the urban population was connected to wastewater treatment plants (WwTPs) [3], a share that is in line with OECD countries [4]. This figure is expected to reach 98% and then 99% by the present year (2013) and 2015 respectively (see Fig. 1).
WwT constitutes a set of physicochemical processes employed to remove pollutants, which can be physical, chemical or biological substances. WwT is normally divided into primary, secondary and tertiary treatment and is designed according to the environmental regulations governing the treated water. While primary systems (also known as mechanical treatment) entail the removal of suspended solids, floating materials and scum from raw sewage, commonly by sedimentation or flotation, secondary treatment (also known as biological treatment) aims to remove dissolved organic matter by anaerobic or aerobic biochemical processes. In tertiary systems (also called advanced treatment), the organic matter remaining after secondary treatment is removed, along with phosphorous and nitrogen, to control nutrient levels. Disinfection may subsequently be conducted eventually to meet the standards of effluent regulations.
As Fig. 2 shows, the most common primary treatment technology employed in Chile is sedimentation, which accounts for 5% of the total. In some particular cases, it is followed by disinfection, and this two-step treatment is sufficient to meet environmental regulations. The most heavily employed system in secondary treatment is activated sludge, which includes conventional activated sludge (CAS), extended aeration, oxidation ditches or sequential batch reactors, and makes up 54% of the total technology employed. The stabilisation pond is the second most commonly used technology in secondary treatment at 6% of the total and entails wastewater treatment in large surfaces, with or without aeration. The remaining 12% of the total number of running WwTPs are wastewater emissaries (outfalls), which collect wastewater and then dispose of it in the ocean. The introduction of tertiary systems is practically nonexistent, mainly as a consequence of current environmental observances.
Section snippets
Motivations of the research
Despite the advantages of WwT, processing inevitability generates sludge at a significant rate, creating a new environmental problem to address [5]. Although sludge has been traditionally handled as a waste management problem (WMP) in most EU countries, sludge landfilling has gradually decreased, as the trend of sludge reuse as fuel has gained value [6]. The same tendency can be observed in Chile, where sludge landfilling has faced increasingly strict regulations with which to comply [7], [8].
Methodological approach
The analysis of the potential for biogas generation was conducted by applying sequential limits. These boundaries were delineated as the physical limit, geographical limit, technical limit and economic limit, according to the definition propounded by Hoogwijk [22], [23] and Izquierdo et al. [24]. Each limit implied restrictions which were used to estimate the economic use of biogas.
Supply-cost curves [24] were built for the two assessed alternatives. In each case, the whole process chain was
Results
The theoretical potential of electricity generation, however irrelevant in practical terms, reached 359 GW hth y−1, whereas the geographical potential bordered 253 GW hth y−1. The technical limit was estimated at 83 GW he y−1, significantly lower than the two previous limits. In the biogas-to-BioSNG pathway, the theoretical potential reached 38 MM Nm3 y−1, whereas the geographical potential reached 27 MM Nm3 y−1, or nearly 71% of the maximum theoretical limit. The technical potential reached 24 MM Nm3 y−1,
Discussion
Tsagariks (2007) [53] reported an electricity cost of for a set of generators installed at a municipal WwTP located in Iraklio, Greece. Gómez et al. [19] estimated a minimum electricity generation cost of at WwTP facilities in Spain. Morin et al. (2011) [54] found an electricity cost of via biogas co-generation through the mono-digestion of 150,000 inhabitants’ municipal WwTP sludge in Quebec, Canada. These figures are in line with the calculated
Conclusions
This study has shown how the introduction of a technology for the controlling of an environmental issue, wastewater treatment specifically, resulted in the appearance of a new environmental problem (i.e. sludge). In this way, anaerobic digestion may offer a solution through a waste-to-energy approach. For the two state-of-the-art options to treat WwTP sludge, biogas-to-electricity and biogas-to-BioSNG, it was found that the economic limit heavily penalised the energy potentially available based
Acknowledgement
This research was made possible by a grant from the doctoral post-graduate programme Conicyt-DAAD Chile–Germany.
References (54)
- et al.
Optimization of biogas production from waste activated sludge through serial digestion
Renew Energy
(2012) - et al.
Options for sustainable sewage sludge management in small wastewater treatment plants on islands: the case of Crete
Desalination
(2010) Energy and power generation: maximising biogas yields from sludge
Filtr Separat
(2009)- et al.
Prospects for expanded utilization of biogas in Germany
Renew Sustain Energy Rev
(2010) - et al.
Biogas generation potential by anaerobic digestion for sustainable energy development in India
Renew Sustain Energy Rev
(2010) - et al.
The prospects for an expansion of biogas systems in Sweden. Incentives, barriers and potentials
Energy Policy
(2007) - et al.
Potential and cost of electricity generation from human and animal waste in Spain
Renew Energy
(2010) - et al.
Municipal solid waste and production of substitute natural gas and electricity as energy alternatives
Appl Therm Eng
(2013) - et al.
Potential of biomass energy out 2100, four IPCC SRES land-use scenarios
Biomass Bioenergy
(2005) - et al.
Supply-cost curves for geographically distributed renewable-energy resources
Energy Policy
(2010)
Biogas purification from anaerobic digestion in a wastewater treatment plant for biofuel production
Renew Energy
Potential world garbage and waste carbon sequestration
Environ Sci Policy
Environmental and economic assessment of sewage sludge handling options
Resour Conserv Recycl
The production, use and quality of sewage sludge in Denmark
Waste Manage (Oxford)
Utilization of sewage sludge in EU application of old and new methods – a review
Renew Sustain Energy Rev
Increased biogas production at wastewater treatment plants through co-digestion
Bioresour Technol
Co-digestion of grease trap sludge and sewage sludge
Waste Manage
Mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. Effect of pre-treatment at elevated temperature
Water Res
Evaluation of biogas production from different biomass wastes with/without hydrothermal pretreatment
Renew Energy
Technical, economic and environmental assessment of sludge treatment wetlands
Water Res
Techniques for transformation of biogas to biomethane
Biomass Bioenergy
MSW landfill biogas desulfurization
Int J Hydrogen Energy
Optimal number of energy generators for biogas utilization in wastewater treatment facility
Energy Convers Manage
Economic and environmental assessment on the energetic valorisation of organic material for a municipality in Quebec, Canada
Appl Energy
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