European Biosolids and Organic Resources Conference 15-16 November, Edinburgh, Scotland www.european-biosolids.com Organised by Aqua Enviro OPTIMISING THE ENERGY YIELD FROM ANAEROBIC DIGESTION THROUGH CALORIFIC VALUE ANALYSIS: CASE STUDIES FROM DAVYHULME AND SEAFIELD Smyth, M. 1 , Minall, R. 1 , Stead, T. 2 , Walker, J. 3 . 1 Aqua Enviro, 2 United Utilities, 3 Veolia Corresponding Author Tel. 01924242255; Email: [email protected]; [email protected]Abstract Over one third of the UK’s sewage sludges are pre-treated by Cambi’s thermal hydrolysis process, which precedes anaerobic digestion. Of the many installed plants, the investment in thermal hydrolysis will be realized over a 25-year period and therefore taking any opportunity to maximize methane production should be fully investigated to deliver the best possible return on investment. This paper looks at the role that calorific value analysis and energy balance modelling can play in optimizing digester throughput. It reviews historical methods for evaluating feedstock energy content, including COD: volatile solids. Samples, analysis (for calorific value) and data were taken from two advanced AD plants (Davyhulme SBAP and Seafield STC). This highlighted the energetic value of primary sludge compared to secondary activated and humus sludges; and that wastewater treatment plant operation at elevated mixed liquor concentrations not only reduces energy availability, but also leads to increased diversion rates of biogas to boiler operation rather than electrical output, post CHP. In addition, the analysis showed that the energy content of sludge increases through digestion, when expressed on a dry solids basis. This shows that for Seafield and Davyhulme THP-AD shorter chain carbohydrates are preferentially converted to methane over longer chain proteins and fats. Acknowledgements Thanks go to United Utilities and Veolia for providing samples for analysis, sharing data, being co- operative and responsive. Key Words Keywords: Calorific value; energy in sludge; methane yield; Sankey diagram; thermal hydrolysis.
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OPTIMISING THE ENERGY YIELD FROM ANAEROBIC DIGESTION …€¦ · CV (MJ/kg DS) 19 25 21 CV (MJ/kg VS) 24.7 35.7 26.25 COD: VS 1.93 2.79 2.05 . European Biosolids and Organic Resources
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European Biosolids and Organic Resources Conference
15-16 November, Edinburgh, Scotland
www.european-biosolids.com Organised by Aqua Enviro
OPTIMISING THE ENERGY YIELD FROM ANAEROBIC DIGESTION THROUGH
CALORIFIC VALUE ANALYSIS: CASE STUDIES FROM DAVYHULME AND
European Biosolids and Organic Resources Conference
15-16 November, Edinburgh, Scotland
www.european-biosolids.com Organised by Aqua Enviro
Introduction
The biogas energy that can be recovered through anaerobic digestion is dependent upon digester
operational factors including:
carbon: nitrogen
sludge yield
presence and availability of macro and micro nutrients
temperature
presence of toxic or inhibitory compounds
organic loading rate
hydraulic retention time
presence or absence of pre-treatment
The methane yield achieved is also dependent upon the stoichiometric energy potential of the feedstock. For sewage sludges this is commonly measured by the Chemical Oxygen Demand (COD) test, indeed the quantity of COD (more specifically the COD: volatile solids) is often included in the performance test criteria for advanced digestion plants. The suitability and accuracy of the COD test for samples containing solids is questionable (discussed latterly). Explored here is the opportunity to employ calorific value testing as a routine measure of energy in the feed sludges and as a tool to prioritise feedstocks for digestion.
Calorific value can be used to provide an absolute value for the energy content. Its worth is increased when combined with Biochemical Methane Potential Testing (BMP) as this provides a measure of the potential to convert the available energy into biogas.
Figure 1 : Typical expected biogas yield (m3) per wet tonne of substrate from a range of different sources (data taken from Yeatman (2007))
CV analysis of the digestate and/or dewatered cake can also be used to provide an information for further
energy recovery processes, including pyrolysis and gasification.
Energy in feedstocks
An initial assessment of the theoretical energy potential of different feedstocks can be determined from
the relative proportions of carbohydrates, proteins and lipids present. The COD of an organic compound
European Biosolids and Organic Resources Conference
15-16 November, Edinburgh, Scotland
www.european-biosolids.com Organised by Aqua Enviro
(CnHaOb) can be calculated on the basis of the chemical oxidation reaction, assuming a complete
oxidation using Equation 1:
𝐶𝑛𝐻𝑎𝑂𝑏 + 14 ⁄ (4𝑛 + 𝑎 − 2𝑏)𝑂2 → 𝑛𝐶𝑂2 + 𝑎
2⁄ 𝐻2 Equation 1.
This shows that 1 mol of organic matter demands ¼ (4n+a-2b) moles of O2 or 8(4n+a-2b) grammes of O2. The theoretical oxygen demand of organic material can therefore be expressed as:
𝐶𝑂𝐷𝑡(𝑚𝑔𝐶𝑂𝐷𝑝𝑒𝑟𝑚𝑔𝐶𝑛𝐻𝑎𝑏) =8(4𝑛+𝑎−2𝑏)
(12𝑛+𝑎+16𝑏) Equation 2.
Since1 kg of COD will yield 3.56 kWh of energy, therefore it is possible to assign a theoretical energy
potential to any material (table 1).
Table 1: Stoichiometric values of COD and energy potential
European Biosolids and Organic Resources Conference
15-16 November, Edinburgh, Scotland
www.european-biosolids.com Organised by Aqua Enviro
Seafield & Davyhulme
Sampling and analysis was undertaken at Seafield STC (Sludge Treatment Centre) and Davyhulme SBAP
(Sludge Balanced Asset Programme) to investigate how calorific value varies through the digestion
process, to undertake an energy mass balance and ultimately to assess whether or not CV testing can be
used as a tool to optimise the energy yield from AD.
Seafield STC processes 1,800 tonnes of dry solids (TDS) each month, of which 396 are imports from sites
in the Almond Valley (East Calder, Whitburn, Blackburn). The import sites aim for a primary: SAS ratio of
75: 25, but this is not measured. Monthly indigenous sludge production at Seafield is 1,404 TDS, SAS:
primary is measured at 70: 30.
Figure 5: Seafield STC (TDS pcm)
Calorific value, in terms of VS, showed thickened primary sludge (Seafield indigenous) to be the highest,
followed by the digestate. This may seem surprising as digestion converts organic matter into
predominantly carbon dioxide and methane and so energy is stripped from the sludge. However, what
remains is still rich in energy, compared to the THP feed and on a VS basis has increased through digestion.
Figure 6: Seafield sludge characteristics. (N.B. for the purpose of mass/energy balance calculation the calculated values for feed to the digester, rather than measured on a single date, are used in the remainder of the paper)
European Biosolids and Organic Resources Conference
15-16 November, Edinburgh, Scotland
www.european-biosolids.com Organised by Aqua Enviro
Conclusions
1. COD:VS ratios are employed in contracts for advanced digestion thermal hydrolysis schemes,
usually in relation to volatile solids destruction guarantees. Calorific value analysis as an
indicator of AD process efficiency offers a more accurate measure of the energy available in feed
sludges and also of digestate. Where BMP analysis is undertaken with CV analysis the energy
yield can be accurately quantified.
2. The energy content (on a dry solids basis) has been shown to increase through THP-AD, this
indicates that shorter chain carbohydrates are preferentially converted to methane over longer
chain proteins and fats.
3. The different energy content of sludges was highlighted with humus sludge being lowest, followed
by SAS and primary sludge being the highest. Organic content is however key and any site
where the primary sludge is 75% or lower should consider optimisation of grit removal.
References
Mills, N. (2015). Unlocking the Full Energy Potential of Sewage Sludge. University of Surrey & Thames
Water.
SMEWW (2006). Standard Methods for the Examination of Water and Wastewater. American Public
Health Association (APHA), the American Water Works Association (AWWA) and the Water Environment
Federation.
Smith, S. (2014). How activated sludge sludge has been transformed from a waste to a resource, and the implications of this for the future of the activated sludge process. In ed. Horan, NJ, Activated Sludge: Past, Present & Future. Aqua Enviro Technology Transfer. Yeatman C. (2007) Biogas Experiences and Ethanol Prospects . Oxford Farming Conference , (pp. 1-