Water Journal : Water Journal May 2011
resource recovery refereed paper technical features 96 MAY 2011 water secondary sludge is harder to digest as it contains large quantities of high molecular weight material (such as cell walls) for which the hydrolysis is rate limiting (Tong and McCarty, 1991). There is an increasing demand to remove phosphorus from wastewaters for environmental reasons. If instead of biological removal, iron or aluminium salts are used to precipitate the phosphorus, it ends up in sludge as an inert. This reduces volatile solids content, ultimately leading to reduced energy generation. Previous work has shown that calorific value of sludge is significantly lowered on a dry basis when ferric salts are added to it (Barber, 2007). Methane Content Biogas methane content during anaerobic digestion is dependent on the oxidation state of carbon within the material being digested as highlighted in the work of Gujer and Zehnder (1983) and can be theoretically determined by stoichiometry (McCarty, 1966). For sludge digestion the methane content is approximately 65%. In this study it was varied between 55% and 75%. Energy generation follows a linear relationship with 1% methane content equivalent to 12kW of electricity. The methane content in biogas will become increasingly significant due to growing interest in the co-digestion of other waste streams. From the work of Gujer and Zendher (1983), methanol and fats will generate biogases with higher methane contents (compared with sewage sludge), while sugary wastes, proteins and fruit juices will generate lower methane content biogases. Urea, with a carbon oxidation state of +4 will generate biogas comprised entirely of carbon dioxide. Sludge Type The energy generation from secondary sludge is significantly lower than that from primary at the same retention time, as shown in Figure 5. The figure shows that the dry solids in secondary sludge routinely generate less than half the energy of an equivalent amount in primary sludge. At 16 days' retention time, typical of the UK, secondary sludge generates approximately 40% of the energy recovery from primary sludge. In fact, modelling conducted during this study has shown sludge type to be the single most fundamental influence on energy generation from sewage digestion. This is consistent with findings by Winter and Pearce (2010), who determined gas yields for primary sludge in the region of two to three times those measured for secondary sludge, depending on the units of measurement referred to. In spite of this, the proportion of primary and secondary sludge typical of a sewage treatment works has historically evolved to meet wastewater quality targets with little or no consideration for the type or quantity of sludge produced. Ironically, increasingly strict wastewater drivers will encourage the production of more secondary sludge at the expense of primary. Ammonia removal via nitrification requires longer sludge ages during secondary treatment, which, as well as generating more secondary sludge (which is increasingly difficult to biodegrade), encourages the growth of filamentous organisms which have been found to stimulate foaming problems in downstream digesters (Speece, 2008). There has been a trend in recent years to use the energy within the sludge as a carbon source for nutrient removal. This is generally practiced by purposely fermenting a proportion of the primary sludge, or bypassing primary treatment altogether and starting with aeration, resulting in plants generating no primary sludge. Thus there will be a corresponding reduction in biogas. CHP (Cogeneration) Availability and Electrical Efficiency In this context, availability refers to the percentage of the time that the plant is available for use. This is dependent on a number of operational parameters such as: presence of contaminants, such as hydrogen sulphide and siloxanes, in the biogas; calorific value of the biogas and its fluctuation; number of engines; availability of critical spares, and volume of biogas holders. The impact of availability follows a straight line with a slope of 7.5kWe per percentage point increase in availability. Electrical efficiency also followed a linear relationship. However, the influence was found to be greater with a slope of 23kWe per percentage point increase in electrical efficiency. Modern engines are capable of generating a higher fraction of energy from a given amount of biogas, with the larger engines capable of achieving as high as 41% efficiency. However, this will come at the expense of the heat necessary to maintain mesophilic temperatures. Discussion As the vast majority of anaerobic digestion infrastructure has long asset life, replacing it with facilities designed to meet modern drivers may be neither viable nor economic. Therefore, in order to assist with targets for renewable energy and carbon footprint reduction, it is necessary to make better and/ or alternative use of existing facilities, irrespective of whether new infrastructure is installed or not. This work has looked at a number of variables with respect to increasing renewable energy generation using existing infrastructure, and a graphical summary of the results are shown in Figure 6. Digesting 10,000 tonnes of dry solids of sewage sludge can generate anything between 0.4 to 1MW of electricity depending on site-specific parameters. Figure 10 shows that the most influential parameter is the sludge itself, with higher proportions of primary sludge providing 0 0.2 0.4 0.6 0.8 1 1.2 All SAS At 30C CHP available 80% Baseline At6%DS Clean digesters At8%DS At 80% VS CHP available 90% All advanced MAD Electricity at 41% All primary Generation (MW) Less beneficial Highest benefits Figure 6: Influence of various parameters on energy generation from 10,000 tonnes dry solids of sewage sludge digested at 5% dry solids at 16 days. 0 0.2 0.4 0.6 0.8 1 1.2 0 20 40 60 80 100 Generation (MWe) HRT (days) Figure 5: Influence of sludge type on energy generation against retention time. Key: primary sludge (full); secondary sludge (dashed) lines.
Water Journal April 2011
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