Electricity and substitute natural gas generation from the conversion of wastewater treatment plant sludge

doi:10.1016/j.apenergy.2013.07.028

Applied Energy

Volume 113, January 2014, Pages 404-413

https://doi.org/10.1016/j.apenergy.2013.07.028 Get rights and content

Highlights

•
A comparative energy generation assessment from WwTP sludge was conducted for Chile.
•
Economic potential and generation cost of electricity and Bio-SNG were evaluated.
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The representative generation cost of electricity was estimated at $21.5 ct € {kW h}_{e}^{- 1}$ .
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The representative cost of Bio-SNG was estimated at 43 € MMBTU⁻¹.
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Electricity offers both economic and environmental benefits in a larger extent.

Abstract

In Chile, the energy that can potentially be obtained from the digestion of sludge generated from wastewater treatment processing (WwT) was calculated using a holistic approach. The different pathways of electricity generation via the direct combustion of biogas and upgraded biogas produced as bio-substitute natural gas (Bio-SNG) for injection into the gas grid were assessed and compared. Information such as the served population, WwT technology employed and geographical distribution of the sludge sources was gathered to estimate energy potential; additionally, technical and economic information was collected from the literature. Furthermore, economic modelling was employed for the purpose of comparing the two end-use alternatives.

The results were presented by using supply-cost curves and then integrated into a geographical information system (GIS), the latter of which shows the distribution of energy potential nationwide. A comparison with a reference market price of the corresponding secondary energy type, electricity or natural gas, was conducted to elucidate the economic attractiveness of the two assessed options. From the assessment, it was concluded that the economic potential for the injection of Bio-SNG into the grid is 19 MM Nm³ y⁻¹ at a representative generation cost of approximately 43 € MMBTU⁻¹, whereas biogas for the electricity generation pathway has an economic potential of 75 GW h_e y⁻¹ at a representative generation cost of $21.5 ct € {kWh}_{e}^{- 1}$ . In view of these results, it can be observed that the electricity generation pathway may offer more major economic and environmental benefits than the Bio-SNG pathway, despite both options being hardly competitive without subsidies. Additionally, it can be observed that the Bio-SNG route is not competitive in most cases, and it is economically attractive solely under restricted conditions, principally those established by plant capacity. Furthermore, steadily increasing electricity prices over time suggests that the biogas-to-electricity option will become progressively more competitive. These results can be considered as outcomes to elaborate a national macro-policy to tackle the sludge issue under a waste-to-energy approach.

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 h_th y⁻¹, whereas the geographical potential bordered 253 GW h_th y⁻¹. The technical limit was estimated at 83 GW h_e 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 $8.76 ct € {kW h}_{e}^{- 1}$ 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 $11.0 ct € {kW h}_{e}^{- 1}$ at WwTP facilities in Spain. Morin et al. (2011) [54] found an electricity cost of $7 ct € {kW h}_{e}^{- 1}$ 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.

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Electricity and substitute natural gas generation from the conversion of wastewater treatment plant sludge

Highlights

Abstract

Introduction

Section snippets

Motivations of the research

Methodological approach

Results

Discussion

Conclusions

Acknowledgement

Renew Energy

Desalination

Filtr Separat

Renew Sustain Energy Rev

Renew Sustain Energy Rev

Energy Policy

Renew Energy

Appl Therm Eng

Biomass Bioenergy

Energy Policy

Renew Energy

Environ Sci Policy

Resour Conserv Recycl

Waste Manage (Oxford)

Renew Sustain Energy Rev

Bioresour Technol

Waste Manage

Water Res

Renew Energy

Water Res

Biomass Bioenergy

Int J Hydrogen Energy

Energy Convers Manage

Appl Energy