OPTIMISING THE ENERGY YIELD FROM ANAEROBIC DIGESTION THROUGH CALORIFIC VALUE ANALYSIS: CASE STUDIES FROM DAVYHULME AND SEAFIELD

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1 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 Aqua Enviro, 2 United Utilities, 3 Veolia Corresponding Author Tel ; rowlandminall@aquaenviro.co.uk; matthewsmyth@aquaenviro.co.uk 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 cooperative and responsive. Key Words Keywords: Calorific value; energy in sludge; methane yield; Sankey diagram; thermal hydrolysis.

2 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. Cow Manure Pig Manure Potato Waste Chicken Manure Brewery waste Green Clippings Grass silage Corn Silage Food Scraps Bakery Waste Fats and Grease Biogas yield m 3 per tonne of substrate 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

3 (CnHaOb) can be calculated on the basis of the chemical oxidation reaction, assuming a complete oxidation using Equation 1: C n H a O b (4n + a 2b)O 2 nco 2 + a 2 H 2 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: COD t (mgcodpermgc n H a b) = 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 Compound Classification Composition Theoretical COD (g COD/g material) Energy potential from 1kg of compound (kwh) Glucose Carbohydrate C6H12O Alanine Protein C3H8O Glycerine Trioleate Lipid C57H104O Chemical Oxygen Demand, Calorific Value & Volatile Solids COD provides a direct measure of the total energy available in a given material. Over the past decade and the growth in advanced anaerobic digestion facilities and specifically thermal hydrolysis, COD now appears within the performance test criteria. The COD requirement can be directly converted into calorific value. Table 2 : Typical VS destruction guarantee, COD & Calculated CV COD/VS VS Destruction (%) CV (kwh/kg. VS) CV (MJ/kg. VS) > >6.052 > < <5.34 <19.2 The higher the COD value (and therefore the total energy content of the sludge) the higher the required rate of volatile solids destruction. There is also likely to be a requirement to process a required tonnage or percentage of SAS (Surplus Activated Sludge), which could be as high as 100% (e.g. Cardiff THP-AD). Clearly this places an emphasis on understanding and accurately quantifying the energy content of the sludge. With respect to energy levels in sludge, Smith (2014) quotes 25 MJ/kg.DS for primary sludge and for SAS, whilst Mills (2015) states 19 MJ/kg for both primary and secondary (figures 2 & 3).

4 Figure 2: Properties of Sludge (Smith, 2014) Figure 3: Sludge characteristics (Mills, N. 2015) Taking the typical values, the energy content expressed in terms of volatile solids is 24.7 (Mills, 2015) and 35.7 for primary and for SAS (Smith, 2014). The difference between the values quoted is large with Smith values indicating a much higher energy content of primary sludge compared to Mills. For all typical values quoted the minimum energy content required to trigger the higher level of volatile solids destruction performance in table 2 is easily met. Table 3: Calorific value of sludges (adapted from Smith, 2014 & Mills, 2015) Parameter Mills Smith Primary & SAS Primary SAS Volatile solids % CV (MJ/kg DS) CV (MJ/kg VS) COD: VS

5 Analytical techniques for quantifying energy in sludge The dry and volatile solids tests are commonly used at digestion plants to measure digestion performance by quantifying the rate of volatile solids destruction. These tests do not however provide an indication of the energy content in the feed or in the digestate, for this either the COD or calorific value test must be used. Aqua Enviro was one of the first companies to investigate COD in sludges, the requirement for this arose from bench scale digestion studies for the Bran Sands and Howdon thermal hydrolysis schemes in COD testing is largely carried out by test and tube methods, whereby 2ml of sample is added to low (0-150 mg/l) or medium (0-1,500 mg/l) range cuvettes and 0.2 ml to the high range option (0-15,000 mg/l). Samples with expected values above this must therefore be diluted and for sludge cakes this can be 1:30, which means in effect that for the 0-15,000 range that just ml of sample goes into the vial. As a result, high dry solids samples and especially those that are challenging to homogenise (and pipette), a high degree of error could be expected with the results. Duplicate or triplicate testing can be employed to gain confidence in the test repeatability, however a degree of doubt over the absolute value remains. The use of the COD test as a measure of pollutant strength in high dry solids samples is in fact beyond the capabilities of the test, which is designed for wastewaters and natural water (SMEWW, 2006). Calorific value analysis offers an alternative solution. Benefits of this method include the larger sample volume, no dilution, able to handle high dry solids materials and the use of calibration fuels when testing to verify analytical equipment. Furthermore, the calorific value measured can be converted into COD, as such could be the stipulated method during performance tests where COD:VS is a requirement. Disadvantages of the CV test are that it is more expensive both for the initial outlay and the cost per test, the time taken to undertake the testing is longer and the equipment cannot be easily transported (an oxygen cylinder is required), dry solids apparatus or used in site conditions. CV testing may not therefore always be practical, but certainly when developing an energy mass balance for new schemes, for sites looking to optimise routes for its sludges, or for performance test monitoring, the CV test comes in to its own. Figure 4: Aqua Enviro CV equipment

6 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)

7 Whilst the energy remaining in the sludge may not be readily available through digestion (the hydraulic retention time at Seafield is long at ~30 days) it could be recovered through thermal processes, as for example is being pursued by Thames Water. Figure 7: Energy in the different sludge streams Mills (2015) notes that advanced digestion followed by dewatering, low temperature thermal drying and pyrolysis is the most efficient energy recovery route (figure 7).

8 Figure 8 : Energy recovery following THP, AD, Bucher dewatering and low temperature drying based upon feed sludge of 14 MJ/kg. DS (Mills, N. 2015) The difference in energy between primary sludge and SAS is highlighted when considered on a VS basis, however the relatively high inorganic content to the Seafield indigenous primary sludge offsets some of this benefit. Review of the contributing causes (including inlet works and chemical dosing) is recommended. It is also interesting to note the low CV of the Blackburn sludge, which by implication is lower in value when compared to others. Blackburn, unlike Seafield, East Calder or Whitburn is a filter works (figure 8). Humus sludges are known to dewater poorly and due to sloughing, storage and wasting of sludge are relative to activated sludge older and this could explain the lower energy content. A final point to note is that the proportion of volatile solids from the import sites is high at 83-85% considering that a 75:25 primary: SAS is target, review of the primary tank performance is recommended, the benefit in terms of total energy available may be sufficient to drive investment (if required) at these sites.

9 Figure 9 : Blackburn STW which incorporates filter beds for secondary treatment The energy content of the contributing input sludges (8,690 MWh from 1,740 TDS pcm) and of the digestate (4,650 MWh) are known, therefore energy in the biogas (converted through AD) is calculated as 4,048 MWh. Figure 10: Seafield Energy Balance This conversion of energy in the feed sludge to biogas is therefore 46.6%, 2.33 MWh/TDS or 234 Nm 3.CH4/TDS.

10 The target biogas yield at Seafield is m 3 /TDS. The methane content of the biogas is not measured; however, the CHP engine is set to operate at 60-65%; taking the midpoint of 62.5% the corresponding biogas yield from the CV data is 374 m 3 /TDS. Figure 11: Seafield KPI for biogas production Davyhulme SBAP The Davyhulme Sludge Balanced Asset Programme (SBAP) is a central sludge processing facility for United Utilities with a design annual throughput of 121,000 TDS and target for 2016 of 75,000. Sludge is delivered to the THP via two routes: 1. Liquid sludge from Davyhulme WwTW is thickened to around 25% dry solids using 4 dedicated centrifuges and transferred to a storage silo. 2. Sludge cake is imported to Davyhulme from 7 satellite sites operated by United Utilities. The cake (between 18% and 32% dry solids) is tipped into one of two cake reception conveyors and transferred to one of two storage silos.

11 Figure 12: Davyhulme SBAP feed composition on a dry solids proportional basis Not all of the import site sludges were available for the period of testing. Similarities with Seafield were seen, in that the calorific value of the sludge increased on a dry solids basis through digestion. Of the import sites Fleetwood had the highest CV (and therefore value), whilst Wigan was the lowest. Figure 13: Energy in the different sludge streams More information (compared to Seafield) was available to undertake a more detailed energy balance that accounts for biogas consumption to the boilers to drive the thermal hydrolysis process.

12 Figure 14: Davyhulme energy balance At the time of sampling (Sept/Oct 2016) an average of 11% (but as high as 16.3%) of the biogas produced was needed to drive the boilers. Under normal operation 3-4% is required, however, due to a backlog of SAS to be processed (37,000 m 3 ) to reduce the activated sludge plant operating mixed liquor suspended solids from 6,000 mg/l to its target) the methane yield was reduced, which led to a reduction in heat from the CHP and greater heat requirement from the biogas to the boiler. Figure 15: Proportion of biogas sent to boilers

13 This is quantified well in the energy balance for this time, which shows the energy yield before CHP & boilers as 2.2 MWh/TDS, equivalent to 222 Nm 3.CH4/TDS. Although the yield is variable over time the average, prior to this period, is 268 Nm 3.CH4/TDS (figure 16). Figure 16: Methane yield In this instance cost on a daily basis from additional SAS treatment exceeded 5k, excluding additional OPEX associated with sustaining endogenous respiration for the higher MLSS. Figure 17: The financial impact of dealing with a SAS backlog

14 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. 1-12).