Scholarly article on topic 'Environmental & economic life cycle assessment of current & future sewage sludge to energy technologies'

Environmental & economic life cycle assessment of current & future sewage sludge to energy technologies Academic research paper on "Environmental engineering"

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Abstract of research paper on Environmental engineering, author of scientific article — N. Mills, P. Pearce, J. Farrow, R.B. Thorpe, N.F. Kirkby

Abstract The UK Water Industry currently generates approximately 800GWh pa of electrical energy from sewage sludge. Traditionally energy recovery from sewage sludge features Anaerobic Digestion (AD) with biogas utilisation in combined heat and power (CHP) systems. However, the industry is evolving and a number of developments that extract more energy from sludge are either being implemented or are nearing full scale demonstration. This study compared five technology configurations: 1 – conventional AD with CHP, 2 – Thermal Hydrolysis Process (THP) AD with CHP, 3 – THP AD with bio-methane grid injection, 4 – THP AD with CHP followed by drying of digested sludge for solid fuel production, 5 – THP AD followed by drying, pyrolysis of the digested sludge and use of the both the biogas and the pyrolysis gas in a CHP. The economic and environmental Life Cycle Assessment (LCA) found that both the post AD drying options performed well but the option used to create a solid fuel to displace coal (configuration 4) was the most sustainable solution economically and environmentally, closely followed by the pyrolysis configuration (5). Application of THP improves the financial and environmental performance compared with conventional AD. Producing bio-methane for grid injection (configuration 3) is attractive financially but has the worst environmental impact of all the scenarios, suggesting that the current UK financial incentive policy for bio-methane is not driving best environmental practice. It is clear that new and improving processes and technologies are enabling significant opportunities for further energy recovery from sludge; LCA provides tools for determining the best overall options for particular situations and allows innovation resources and investment to be focused accordingly.

Academic research paper on topic "Environmental & economic life cycle assessment of current & future sewage sludge to energy technologies"

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Waste Management

journal homepage: www.elsevier.com/locate/wasman

Environmental & economic life cycle assessment of current & future sewage sludge to energy technologies q

N. Mills a'b'*, P. Pearce a, J. Farrow a, R.B. Thorpe b, N.F. Kirkbyb

a Innovation Department, Thames Water Utilities Limited, Reading, UK b Department of Chemical and Process Engineering, University of Surrey, Guildford, UK

ARTICLE INFO ABSTRACT

The UK Water Industry currently generates approximately 800 GW h pa of electrical energy from sewage sludge. Traditionally energy recovery from sewage sludge features Anaerobic Digestion (AD) with biogas utilisation in combined heat and power (CHP) systems. However, the industry is evolving and a number of developments that extract more energy from sludge are either being implemented or are nearing full scale demonstration. This study compared five technology configurations: 1 - conventional AD with CHP,

2 - Thermal Hydrolysis Process (THP) AD with CHP, 3 - THP AD with bio-methane grid injection, 4 - THP AD with CHP followed by drying of digested sludge for solid fuel production, 5 - THP AD followed by drying, pyrolysis of the digested sludge and use of the both the biogas and the pyrolysis gas in a CHP.

The economic and environmental Life Cycle Assessment (LCA) found that both the post AD drying options performed well but the option used to create a solid fuel to displace coal (configuration 4) was the most sustainable solution economically and environmentally, closely followed by the pyrolysis configuration (5). Application of THP improves the financial and environmental performance compared with conventional AD. Producing bio-methane for grid injection (configuration 3) is attractive financially but has the worst environmental impact of all the scenarios, suggesting that the current UK financial incentive policy for bio-methane is not driving best environmental practice. It is clear that new and improving processes and technologies are enabling significant opportunities for further energy recovery from sludge; LCA provides tools for determining the best overall options for particular situations and allows innovation resources and investment to be focused accordingly.

© 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

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Article history: Received 15 June 2013 Accepted 15 August 2013 Available online 20 September 2013

Keywords: Sewage sludge

Energy recovery technologies LCA

Economics

1. Introduction

The UK Water Industry currently generates approximately 800 GW h pa of electrical energy from sewage sludge, a renewable by-product from wastewater treatment. This recovery of energy has until recently been mainly conducted using two methods: Anaerobic Digestion (AD) and incineration with energy recovery, both developed and deployed with achieving efficiencies in sludge disposal as the main driver (Barber 2010; Davis 1996). Incineration has not been considered in this study, as in the UK it is in decline due to very high operating costs.

Over the past 10 years significant development has been made in Anaerobic Digestion processes, which improves energy yields from this renewable resource, sewage sludge, which the industry has in abundance. These advanced processes have and are now

q This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Corresponding author at: Innovation Department, Thames Water Utilities Limited, Reading, UK. Tel.: +4477476 42919.

E-mail address: nick.mills@thameswater.co.uk (N. Mills).

being implemented at large scale across the UK and within Thames Water (Riches et al., 2010). At the same time drying sludge post digestion has been common place across Europe for over 20 years, especially in Germany where dried sludge is frequently used in coal-fired power stations and cement kilns (Fytili and Zabaniotou 2008; Kelessidis and Stasinakis 2012; Werther and Ogada 1999). Advanced energy recovery processes such as syngas pyrolysis and gasification are fast becoming feasible options post drying (Cao and Pawtowski 2012).

Life Cycle Assessment (LCA) studies on sewage treatment options pre-date these technological developments and are also questionable in depth as they do not consider process configuration in sufficient detail (Margareta Lundin and Morrison 2002). Therefore, a new LCA has been conducted alongside economic studies to assess advanced Anaerobic Digestion configurations and post drying recovery options.

2. Sludge to energy techniques

A comparison between several processes has been made and these are listed below:

0956-053X/$ - see front matter © 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2013.08.024

Nomenclature

AD Anaerobic Digestion WID Waste Incineration Directive

CHP Combined Heat and Power GWP Global Warming Potential

THP Thermal Hydrolysis Process POCP Photo Ozone Creation Potential

LCA Life Cycle Assessment EP Eutrophication Potential

DS Dry Solids AP Acidification

SAS Surplus Activated Sludge ADP element Abiotic Depletion Of Elemental Resources

STW Sewage Treatment Works ADP fossil Abiotic Depletion Of Fossil Fuels

VSD Volatile Solids Destruction CRC Carbon Reduction Commitment

TDS Tonnes Dry Solids OpEx Operational Expenditure

GtG Gas to Grid (bio methane injection) CapEx Capital Expenditure

RO Renewable Obligation IRR Internal Rate of Return

RHI Renewable Heat Incentive OFWAT Office for Water Industry Regulation in the UK

LG Low Grade (heat - hot water)

HG High Grade (heat - steam)

CV Calorific Value

1. Conventional AD with Combined Heat and Power (CHP) and land recycling of the digestate to agriculture (Conv. AD CHP).

2. Thermal Hydrolysis Process (THP) AD with CHP and land recycling of the digestate to agriculture (THP AD CHP).

3. THP AD with bio-methane injection or Gas to Grid (GtG) and land recycling of the digestate to agriculture (THP AD GtG).

4. THP AD with CHP, drying for fuel (THP AD CHP + Drying for fuel).

5. THP AD with CHP, drying and pyrolysis with CHP (THP AD CHP + Drying, Pyrolysis and CHP).

2.1. Conventional AD

Currently the most widely used method of sludge treatment is AD which achieves the required "sterilisation" or pathogen kill to allow the sludge to be recycled to land, encouraged by the EU sewage sludge directive 86/278/EEC and governed by the Sludge Use in Agriculture Regulations 1989. AD has the added benefit of reducing the dry mass of sludge for disposal and producing a methane rich biogas which can be used as fuel. The most common variant is mesophilic AD; it is a complex biological process involving a diverse bacterial consortium (Appels et al. 2008). In a typical process, sludge is thickened then heated to 35-40 °C before entering the mixed digester tank, typical retention times range from 12 to 30 days. The final digestate is then dewatered to a cake of around 20%1 Dry Solids (DS) and transported off site, generally for recycling on agricultural land (Suh and Rousseaux 2002). Fig. 1 shows the energy flows for a typical configuration with a CHP unit (referenced to 1 kgDS/h).

2.2. Advanced AD - THP

AD is widespread and an effective sludge treatment technique for the water industry, but it has limitations, particularly coping with Secondary Activated Sludge (SAS). Generally, installations have poor energy recovery and require large assets that are very capital intensive. For this reason there are a number of process variations which have been developed and applied for the last 15 years. These all aim to improve the digestibility of sewage sludge, increasing the yield of gas and asset utilisation. The benefits of advanced AD (McNamara et al. 2012; Pickworth et al. 2006) can be summarised as:

• Increased biogas yields.

1 Using centrifuge dewaterer, 30% DS achievable with more advanced technologies.

• Increased Volatile Solids Destruction (VSD).

• Reduction in total solids mass when compared with conventional digestion.

• Process allows increased loading (i.e. throughput) in existing assets reducing capital costs.

• Enhanced dewatering characteristics reducing transport costs and increasing the quality of product for farmers.

The most developed and widely applied AD techniques are thermal and biological hydrolysis, as hydrolysis is typically the rate limiting step of AD. Thermal Hydrolysis Process (THP) is the most widespread and the technology of choice for Thames Water to achieve future generation and carbon mitigation targets (TWUL 2009).

THP involves using a high temperature (165 °C) and pressure (7 barg) for 30 min to disrupt and solubilise sludge before feeding it to a conventional digester. The process also homogenises the sludge so that it is more digestible resulting in increased methane production and a smaller volume of digestate (Kepp 2000). Across the world there are 23 full scale THP sites either in operation or construction that will process 445,000 Tonnes of Dry Solids (TDS) p.a. (Cambi 2010).

However, the increase in biogas yield does not necessarily result in an overall net increase in energy per tonne digested. THP demands an input of high grade heat and additional electrical energy, when compared with conventional AD. The high grade heat demand typically outweighs the additional heat available from a CHP unit burning the biogas produced. All of the THP installations in the UK currently require additional support fuel (typically natural gas) to maintain the process (Mills 2011). Fig. 2 shows the energy flows for a typical configuration (referenced to 1 kgDS/h).

2.3. Biogas utilisation

The biogas produced in AD has traditionally been utilised in spark ignition gas engines or dual fuel engines which convert 35-42% of the chemical energy into renewable electricity. A proportion of the waste heat from the exhaust gas and the water jacket is recovered for utilisation by the process thus justifying the label CHP (Hawkes 2011). In the UK this form of generation is incentivised to varying degrees under the Renewable Obligation (RO) Scheme which rewards generators of renewable energy with additional revenue.

A new UK practice, Gas to Grid (GtG) aims to clean up and inject all of the bio-methane produced in AD into the gas network and is financially supported under the Renewable Heat Incentive (RHI)

Fig. 1. Energy flows for conventional AD with CHP and land recycling (1 kgDS/h).

Fig. 2. Energy flows for THP AD with CHP and land recycling, CHP losses not shown (1 kgDS/h).

(DECC, 2011). A number of technologies are available to remove the carbon dioxide and hydrogen sulphide but water absorption is most commonly used in the UK, the resulting gas has a methane content of >99% (Ryckebosch et al. 2011 ). Once cleaned the bio-gas requires the addition of propane and odorant to be compliant with gas quality standards before final compression into the gas network (Greer and Diane, 2010; Starr et al., 2012). Fig. 3 shows the energy flows for a typical configuration (referenced to 1 kgDS/h).

A disadvantage of this process is that the heat required by the process (e.g. THP) is no longer supplied from a waste source and has to be supplied by either burning some of the biogas or purchasing supplementary natural gas, which is usually the preference on financial grounds as the biogas attracts a large incentive. GtG LCA studies are few, but Jury et al. finds biogas injection from energy crop fermentation to be environmentally competitive with natural

gas (Jury et al. 2010). CHP is the more widely used and produces electricity, which is a very versatile form of energy - easily transportable to point of use, and with many applications that can use it. However, generation efficiency of electricity is at best only 42%. GtG has much higher conversion efficiencies than CHP (>95%) but the relative environmental burden displacement is less for the displacement of natural gas by biogas than the displacement of electricity from fossil fuels by electricity from biogas. This study aims to compare GtG and CHP as there has been no published material.

2.4. Drying post AD

To access the considerable chemical energy remaining in the sludge after AD, the sludge can either be burnt or be dried to produce a solid fuel product (Flaga, 2005; Niu et al., 2013). However,

Fig. 3. Energy flows for THP AD with GtG and land recycling (1 kgDS/h) electricity input is not shown.

sludge drying in the UK has had a troubled past with several dust explosions and fires (HSE, 2011) and expensive operating costs (Bowen et al., 2010). These issues are mainly associated with direct drying equipment fuelled with fossil fuels, particularly the hot air drum dryer type which can create a lot of dust within a rotating drum with hot air at over 400 °C. This kind of dryer is inefficient and with current increased energy prices prohibitive economically. There are new drying technologies that are safe, efficient and able to utilise low grade waste heat. Low temperature belt dryers mainly developed in Germany or Spain are operational across Europe. The technology consists of a large perforated belt, inside a chamber where hot air (60-120 °C) heated with hot water is circulated up through the sludge which is extruded onto the belts at 20-40%DS; the dryer produces a 90%DS product.

2.4.1. Fuel production

This study has shown that it is technically feasible using a low temperature dryer to dry all of the digested sludge from a THP AD plant. If the THP digested sludge is dewatered to at least 42%DS it is feasible to dry the entire site output to 90%DS as there is sufficient waste low grade heat available from the CHP which is the rejected from the engine jacket at 80-100 °C (labelled 'LG Heat' on Fig. 4). Fig. 4 shows the energy flows for the configuration modelled as part of this study (referenced to 1 kgDS).

Within the EU dried sludge has been used as a fuel in coal fired power stations as the calorific value (CV) is benefical. Use of dried sludge as a fuel is a concept which Thames Water and others aspire to implement in the UK (Jones, 2008; Mills, 2012a). However, unlike in Germany where Regulations are more flexible, dried sludge would need either to be combusted in a Waste Incineration Directive (WID) compliant power station. Or be given 'end of waste' status, high concentrations of copper, zinc and ash relative to comparator fuels make this a challenge, but by controlling the feed sludge quality, by varying the blend ratio of primary and activated sludge, these issues could be resolved.

One of the potential draw-backs of deploying more advanced energy recovery processes (discussed in Section 2.4.2) is that they all essentially take place on the same site, in order to utilise process heat and avoid moving large quantities of wet sludge from site to site. However, some sites can have significant restrictions on land use, electricity export or planning restrictions. In some cases (as discussed in this section) site specific LCA may show it is more efficient to dry the fuel to 90% DS and then transport it to where it can be used most efficiently.

2.4.2. Advanced energy recovery

Once a dried product has been produced it opens up other utilisation options, such as pyrolysis and gasification technologies which have a high energy conversion efficiency (greater than 85%) to a syngas which can then be used in CHP units (Ray et al.

2012). Fig. 5 shows the energy flows for the configuration modelled as part of this study (referenced to 1 kgDS/h).

Combining AD, drying and pyrolysis has been explored by Cao and Pawlowski who concludes that maintaining AD as an initial recovery step leads to a more efficient overall energy recovery configuration (Cao and Pawlowski 2012). The only remaining byproduct from these processes is bio-char, which is environmentally stable and can be used beneficially as a soil conditioner or potentially as a source for mineral recovery. Under the right conditions pyrolysis has advantages over gasification producing a concentrated syngas which is as it more suitable for CHP (Bridgwater, 2012; Domínguez et al., 2006). In this scenario the waste heat from the pyrolysis CHP has not been utilised due to the added complexity of downstream reliance on upstream process heat. However, it is quite feasible to use the high grade exhaust heat to produce steam for the THP plant and displace natural gas support fuel.

3. Environmental life cycle analysis

Many studies in the past have conducted extensive LCA for sludge treatment techniques, but these have focused on traditional disposal routes for the wastewater treatment by-product (sludge) (Dalemo et al., 1997; Lundin et al., 2004; Sonesson et al., 1997; Suh and Rousseaux, 2002). These typically include land fill, compost, incineration and land application after conventional AD. The studies vary depending upon the country of origin. Lundin et al. reviewed many of these studies observes a common difference which depends upon whether the organisation considers sewage sludge as a waste or a resource, this remains a feature in papers that postdating this paper (Lundin et al., 2004). More recently there have been several Chinese studies, which have explored various off site recovery options for sludge as a fuel showing clear environmental and economic benefits for energy recovery (Liu et al., 2011, 2013; Niu et al., 2013). The study by Carballa et al. is most applicable to the area of interest here in that it compared AD pre-treatment methods (including THP) of sludge and kitchen waste, it was found that pressurisation and chemical treatment most effective. An issue with this LCA is that all the operational performance data is scaled from laboratory work conducted using 101 anaerobic digesters. An average size site would use 5000 m3 digesters so the accuracy of these scaled results would be considered questionable by the industry. The study also excluded any impact from sludge handling post digestion (Carballa et al. 2011).

3.1. Goal and scope

There is a need to conduct an LCA study that incorporates the advances in technology and subtleties in configuration especially those incorporating a drying step. The goal of this study is to evaluate the relative environmental and economic impact of the configurations

Fig. 4. Energy flows for THP AD, CHP and sludge drying for fuel production (1 kgDS/h) CHP losses not shown.

Fig. 5. Energy flows for THP AD, CHP and sludge drying and pyrolysis (1 kgDS/h) CHP Losses not shown.

to inform decision makers across the industry and to identify any inconsistencies or anomalies in policy. The functional unit used is the dry mass of sludge; Tonne Dry Solids (TDS). All sludge parameters and process assumptions are listed in Appendix A.

3.2. System boundaries

Fig. 1 shows the outline system boundary; it has been assumed that all process variants are assessed in operation only and the impact of construction and decommissioning are ignored as these emissions are likely to be insignificant in comparison (Carballa et al., 2011). The 'sludge to energy' process itself will consume energy (electricity & natural gas) and chemicals (e.g. poly-electrolyte) which are included. On site there will also be emissions to air from CHP engines and gas boilers which emissions are dominated by CO2, SO2 Particulates, CO and NOX emissions (Poeschl et al., 2012).

It is assumed that digested sludge is applied to agricultural land (this is the current practice in the UK for 60% of the UK's sludge (Andrews, 2008)) and is transported an average of 60 km. In addition to vehicle emissions, this activity will have air emissions (CH4 and N2O) associated with the biodegradation of sludge cake in the soil (Inubushi et al. 2000). The Nitrogen and Phosphorus (N&P) content of the recycled sludge will be a credit to the system because it displaces industrially made fertilisers in this case Urea and Triple Superphosphate.

Electricity produced from CHP credits the system by displacing grid-produced electricity. Biogas injected under the GtG option also credits the system by displacing the burden associated with producing the equivalent amount of natural gas. All assumptions used within the model are listed in Appendix A. Problems associated with heavy metals and other non-biological sludge contaminants have been discounted from the study.

3.3. Inventory

A commercial LCA package (GaBi) was used to construct a model for each of the 5 scenarios. Figs. 1-5 display high level summary sankey diagrams for the energy flows in each scenario (note that electricity, road fuel and consumables are not shown but included in these results). Table 1 shows the inventory for the main performance indicators which drive the life cycle impacts, grouped as energy outputs, inputs and digestate/char (see Fig. 6).

4. Results

4.1. Environmental life cycle analysis

The software used (GaBi) in this study can allow a number of different impacts to be analysed, for this study the following were deemed important:

1. GWP - Global Warming Potential (excluding biogenic) (kgCO2 -

Equiv.)

2. POCP - Photo Ozone Creation Potential (kg Ethene - Equiv.)

3. EP - Eutrophication Potential (kg Phosphate - Equiv.)

4. AP - Acidification (kgSO2 - Equiv.)

5. ADP element - Abiotic Depletion (elements kg Sb - Equiv.)

6. ADP fossil - Abiotic Depletion (fossil MJ)

Fig. 7 displays the normalised results for the six impacts calculated as part of the study; negative values are environmentally beneficial and positive values represent environmental burdens. The largest impact area is ADP fossil which is negative (beneficial), this is due to all the processes displacing fossil fuel use. Conventional AD performs better than THP (CHP & GtG) and the pyrolysis options, because it has relatively low parasitic energy and chemical demand. The drying to fuel scenario is best due to the direct displacement of hard coal. The GWP impacts follow a different trend and are discussed in detail later due to their regulatory and financial significance.

The next most significant emissions are 'local' (AP & POCP) and reveal a slightly different picture that suggests that the GtG scenario has the least impact, due to the low direct emissions associated with the production of bio-methane, compared with a CHP exhaust. Unsurprisingly the scenarios with CHP units have the largest impact, due to the exhaust emissions (Dust, CO, NOx, SO2 and VOCs), the pyrolysis option performs the worst of the five as it has the largest CHP output. ADP elements and EP are insignificant in comparison and are therefore not discussed further.

Using a normalisation method the net environmental impact is shown in Fig. 8, revealing that drying for fuel production is optimum followed by the pyrolysis option, the worst performer is the GtG scenario. THP with CHP has environmental benefits over conventional AD with CHP.

GWP is considered the most important impact to water companies, as it is a reportable output to the regulator OFWAT and it also costs millions of pounds annually in taxes such as the Carbon Reduction Commitment (CRC). Fig. 9 shows the results for GWP of the five scenarios described previously. The net GWP for each scenario is shown as the black column with a data label, the emissions have also been categorised into six key process steps to improve analysis, shown as discrete columns.

The results show that the move from conventional AD with CHP to THP is beneficial, despite the parasitic fuel requirements, mainly natural gas support fuel for steam generation. The GtG option performs very badly for two reasons: firstly, the beneficial impact of injecting bio-methane into the gas grid is not as great as displacing electricity, and, secondly, the process requires a large 'top up' with propane gas and natural gas to maintain the steam demand for the THP plant. The emissions of CH4 and N2O from recycled sludge on agricultural land are significant and dominate Fig. 9. The two

Table 1

Inventory of key performance indicators for 1 TDS feed.

Inventory Item Units Conv AD CHP THP AD CHP THP AD GtG THP AD CHP + drying for fuel THP AD CHP + drying, pyrolysis & CHP

Energy outputs

Electricity generation kWh 728 1020 - 1020 1820

Bio-methane kWh - - 3230 - -

Solid Fuel kWh - - - 2260 -

INPUTS

Electricity consumption kWh 135 179 199 210 443

Natural gas kWh 0 370 907 370 370a

Propane kWh - - 546 - -

Diesel kg 7.3 3.7 3.7 0.8 0.4

Polymer kg 9.2 14.0 14.0 14.0 14.0

digestate/char

Sludge disposal T 2.3 1.4 1.4 - -

N&P benefit kg 254/156 150/92 150/92 - -

Char kg - - - - 220

a Could be reduced if the heat balance was optimised between THP and the pyrolysis CHP.

Fig. 6. Overview of system boundaries.

future options which avoid land displacement therefore result in a net benefit in GWP. In addition both these options achieve improved energy recovery, especially the pyrolysis option which has approximately doubled the electrical output (a carbon intensive burden) of the process when compared with conventional AD.

It can be concluded from these results that the future technologies offer a considerable advantage over the existing techniques, especially when considering GWP. However, the pyrolysis option does this at the potential detriment to the local environment.

4.2. Economics

Previous work has extensively modelled the Operating Expenditure (OpEx) of each of the processes described in the LCA study (Mills 2012b). However, for various reasons Capital Expenditure (CapEx) models for these processes had not been built.

The content of Table 2 is the result of combining a number of sources of data to produce a cost estimate for various process

configurations on a typical site. These are generalised values, they are not site or project specific. Over-heads are estimated for this comparative study and are not necessarily representative of those used within Thames Water. Using common chemical engineering CapEx estimation techniques, the non-linear nature of CapEx can be normalised and calculated for each scenario with Eq. (1) (Sinnott and Towler, 2009).

CapEx = k x S x Exp.0.6 (1)

Using cost data at various scales (S) and an exponent value of 0.6 (average value for similar installations) a series of k-values were calculated (Table 2).

Using and adapting the data in Table 2 the total CapEx for each scenario was obtained; with the OpEx information from previous work (Mills 2012b), the economic feasibility of each process scenario was calculated. Table 3 summarises the financial situation and the resultant Internal Rate of Return (IRR) with and without

Fig. 7. Life cycle impacts normalised (CML2001 - November 2010) of the five energy recovery scenarios.

Fig. 8. Weighted net impact CML 2001 - December 07, experts IKP (Northern Europe) method.

government incentives for a 100TDS/day plant for each scenario, general financial assumptions can be found in Appendix A.

The results show that the GtG option has the best IRR followed by pyrolysis and then the drying for fuel scenario. However, when the incentives are removed the IRR becomes negative for the GtG option, which means the investor would not see a return on the investment within the operational life of the plant.

Conventional AD and THP appear to be comparable financially, this maybe the case but what is not apparent from financial analysis is the benefit bought from a superior sludge cake. The product is preferred by farmers and as such reduces disposal risk. In addition THP allows for much larger throughput on the same footprint, on urban treatment sites land is limited so conventional AD, with large anaerobic digesters with their associated large footprint, is simply not feasible.

4.3. Summary of results

Combining the various environmental and economic results presents a challenge for decision makers, it was therefore deemed sensible to combine these results. Table 4 shows the results of a scoring exercise where each scenario was ranked and scored

between 1 and 5, 5 being best for the following performance indicators:

• Net environmental impact, because this is the intellectually most satisfactory.

• GWP, because it is reportable to OFWAT, the UK water Regulato.

• IRR with incentives, because this represents commercial reality.

• IRR without incentives, because some of the current incentives seem out of proportion and therefore at risk of change.

This study has assessed the sustainability of these different scenarios as a balance of economic and environmental impact. However, the third remaining sustainability impact (Brundtland, 1987), societal, has not been explored in this case as it has been considered that the social impact is broadly neutral across all scenarios.

The results in Table 4 show THP AD with CHP has advantages over Conventional AD and supports the current shift from conventional AD to THP AD in the UK and the future strategy of many UK & International wastewater companies.

Most notable is that the drying post AD options are good solutions both environmentally and economically and these should be

Fig. 9. Global warming potential (exc biogenic) for 5 energy recovery options.

Table 2

Sludge to energy process - CapEx model.

Component CapEx (US$) Size Unit k-value (US$)

Pre-treatment and thickening 4,114,727 100 TDS/d 2S9,622

AD 8,9S8,09S 22,000 m3 22,221

THP 9,130,003 100 TDS/d S76,064

Dewatering and cake storage S,908,967 60 TDS/d S06,S48

Odour treatment 1,031,00S 100 TDS/d 41,969

CHP and electrical 8,S79,9S9 S000 kWe S1,773

Control and instrumentation 1,223,S73 100 TDS/d 77,202

General 3,149,081 100 TDS/d 198,694

Sub total 42,09S,410

Contractor management (20%) 8,419,082

Client overheads (10%) S,0S1,449

Total SS,S6S,941

Additional options (before contractor and client overheads)

GtG (CHP eq output 2.S kW/m3/h) 7,162,110 S000 kWe 43,217

Drying plant 4,838,0SS 3200 kgH2O/h 38,1S8

Pyrolysis 8,120,331 60 TDS/d 696,118

Pyrolysis CHP 7,S04,802 4000 kWe S1,773

Table З

Financial performance of each process scenario.

Scenario OpEx with Incentives (US$ pa) OpEx without Incentives (US$ pa) IRR with incentives (%) IRR without incentives (%)

ConvADCHP 6,902,150 4,582,575 10.60 4.05

THP AD CHP 8,486,250 5,714,075 12.75 5.98

THP AD GtG 10,975,550 2,828,750 18.92 -2.81

THP AD CHP + drying for fuel 10,070,350 7,298,175 14.39 8.48

THP AD CHP + drying, pyrolysis and CHP 14,143,750 8,429,675 17.46 7.64

Table 4

Combined ranking of scenarios.

Scenario Net impact GWP IRR with incentives IRR without incentives Combined total

Conv AD with CHP 2 2 1 2 7

THP AD with CHP 3 3 2 3 11

THP AD with GtG 1 1 S 1 8

THP AD, CHP & drying S S 3 S 18

THP AD, CHP, drying & pyrolysis with CHP 4 4 4 4 16

considered by the UK water industry as the next logical steps in unlocking the full energy and financial potential of sewage sludge.

GtG should probably be avoided due to the poor environmental performance and financial risk posed by proportionally high renewable incentives. These may be removed or adjusted before a project could be commissioned and accredited and therefore represents a large investment risk. Upgrading biogas to a bio-methane suitable for transport fuel might be a better solution, requiring fewer incentives due to the relatively high price of transport fuels and displacing a carbon intensive fuel would be more environmentally beneficial, this is commonly seen in the EU. However, there may be a point in the future where the electricity grid carbon intensity maybe reduced to a level where the production of bio-methane for grid injection would be favourable environmentally over the more traditional electricity production.

5. Conclusion

Table A2

AD & THP process assumptions.

Parameter

Thickening - electrical demand Thickening - polyelectrolyte

consumption Input:output - dry solids THP - electrical demand THP pre dewatering - polymer demand

THP pre dewatering input:output DS

THP steam demand @12barg AD electrical demand Dewatering - electrical demand

Dewatering - polymer demand Polymer GHG emissions

Source

60 kW h/TDS Estimate

2.2 kg/TDS Estimate

1%:5%DS Estimate

50 kW h/TDS Estimate

5 kg/TDS Estimate (50% increase)

5%:16.5%DS Estimate

1 kg/kgDS TW asset standard

40 kW h/TDS Estimate

50 kW h/TDS Estimate for Bucher

10 kg/TDS Estimate

2 kgCChe/T Estimate

1.05 kW h/m3 Venkatesh and Bratteb0

(2011)

The study has found that both the post AD drying options performed well but the option used to create a solid fuel to displace coal was the most sustainable solution economically and environmentally, closely followed by the pyrolysis configuration.

Application of THP improves the financial and environmental performance compared with conventional AD.

Producing bio-methane for grid injection is attractive financially but has the worst environmental impact of all the scenarios, suggesting that the current UK financial incentive policy for bio-methane is not driving best environmental practice. However, as the electricity grid is decarbonised there will come a point at which it is environmentally beneficial to produce bio-methane for grid injection instead of electricity some sensitivity analysis would be logical next step.

It is recommended that an enhanced pyrolysis option is explored that utilises the waste heat from the pyrolysis CHP in the THP plant.

It is clear that new and improving processes and technologies are enabling significant opportunities for further energy recovery from sludge; LCA provides tools for determining the best overall options for particular situations and allows innovation and investment resources to be focused accordingly.

Acknowledgements

Nick Mills is registered on an Engineering Doctorate Programme at the University of Surrey; financial support from the UK Engineering and Physical Sciences Research Council (EPSRC) and the 1851 Royal Commission is gratefully acknowledged. Support from the Thames Water, Innovation team and Manocher Ass-adi, Achame Shana, Paul Fountain and PE International is also gratefully noted.

Appendix A. Assumptions

See Tables A1-A8.

Table A3

Land recycling assumptions.

Parameter Source

Average journey distance 120 km (100 km Estimate

Average journey load and utilisation 221, 50% Estimate

N & P concentrations 5%, 3% Estimate

Land area for spreading 1 m3/kg wet Estimate

CH4 emissions 0.02 kg/TDS UKWIR

(2012)

N2O emissions 0.00011 kg/TDS (as above)

Table A4

CHP Assumptions.

Parameter Source

CV of biogas 23 MJ/Nm3 TW typical

CHP electrical efficiency 38% Estimate

CHP high grade heat efficiency 18% Estimate

CHP low grade heat efficiency 20% Estimate

Bio-gas leaks (CH4) 2% Estimate

CO2 (biotic) exhaust emissions 175 g/kW h Estimate (13% exhaust gas)

CO exhaust emissions 986 mg/ Borkowski (2007)

Dust exhaust emissions 164 mg/ Poeschl et al. (2012)

NOx exhaust emissions 821 mg/ Borkowski (2007)

NMVOCs emissions 136 mg/ Diesel_net (2006)

SO2 emissions 439 mg/ Borkowski (2007)

Table A5

Bio-methane injection assumptions.

Parameter Source

Electrical demand 0.05 kW h/m3 Estimate

Propane required 17% or Calculation to meet CV (delivered

0.101 kg/m3 by tanker)

CO2 (biotic) emissions 105 g/kW h 35% of biogas @ 1.9 kg/m3

CV of grid ready bio- 37 MJ/m3 NG (2005)

methane

Table A1

AD process specific assumptions.

Parameter Conv. AD. & CHP. THP AD. & CHP. THP AD. & GtG Source

Bio gas yield 300 m3/TDS 420 m3/TDS 420 m3/TDS Typical

Solids destruction 30% 40% 40% Typical

Cake dry Solids 30%DS 42%DS 42%DS Bucher Press

Table A6

Sludge drying and fuel assumptions.

Parameter Source

Electrical demand SS kW h/TDS Klein manufacturera

Low grade heat demand SS0 kW/TDS Klein manufacturera

Final dry solids 90%DS Estimate

Journey distance 100 km Estimate

Journey load and utilisation 221, S0% Estimate

CV of dried sludge 14 MJ/kgDS TW typical

CV of UK hard coal 36 MJ/kg GaBi database

a Heat demand of 7.0kWh/kgH2O from 42% to 90% and electrical 0.7k Wh/ kgH2O from 42% to 90%.

Table A7

Pyrolysis assumptions.

Parameter Source

Electrical demand 400 kW h/TDS Estimate

Bio-char mass 380 kg/TDS Estimate (TW ash content)

Syn-gas conversion 3S00 kW h/ Estimate (90% @ 14 MJ/

efficiency TDS kgDS)

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power cost

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RHI incentive value £6S/MW h DECC

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Dollar to pound exchange rate 1.SS US$/ www.xe.com, accessed 10/08/

Base case sludge cost £1S0/TDS Displacement of lime disposal

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