Water Journal : Water Journal May 2011
refereed paper biosolids water MAY 2011 91 • Even poorer sludge dewaterability due to loss of the primary stream. • Decrease in biosolids degradability, both because of long sludge ages and loss of the primary stream. There is a strong case for beneficial reuse of biosolids in Australia. The use of biosolids in agriculture across Australia is being extensively researched by the National Biosolids Research Program (NBRP). Use of biosolids has been generally found to have either the same or a positive benefit compared to use of mineral fertilisers at comparable nitrogen loading rates (Warne, McLaughlin et al., 2009). This is becoming even more important as the value of nutrients and energy rises. Our calculations have indicated that as of 2011, 1 tonne of wet biosolids (12% solids) has a value of $17, evenly divided between nutrient value (N & P) and energy. The value of bound water is $1 per tonne. Unfortunately, this is minimal compared with transport costs of >$30 per tonne, and is even marginal compared with basic agricultural spreading costs of approximately $10 per tonne. Biosolids Stabilisation Processes Stabilised biosolids products that meet legislative requirements for agricultural use can be produced using a wide range of technologies. A summary of treatment processes and the different product qualities are shown in Table 1. They are split into technologies applicable at all sizes, technologies applicable at large scale only, and technologies that could be applied as either stand-alone or additional treatment options to the previous methods. All small-medium scale options are also available in large scale, but may have undesirable social impacts (e.g., open composting in large scale causes odour problems). While anaerobic digestion compares favourably with alternative treatment options shown in Table 1, it is considered as only being applicable at large scale. The reduction in highly degradable primary sludge streams, and the increase in poorly degradable waste-activated sludge streams, reduce the feasibility of conventional anaerobic digestion at modern N-removal wastewater treatment plants. At large scale this has been addressed through the development of advanced anaerobic technologies (e.g. thermal hydrolysis). However, these advances are currently impractical in small- medium scale systems for reasons such as: • Although civil construction costs scale reasonably well for anaerobic digestion, mechanical and electrical (e.g., flare, pumps, gas circulation system, gas storage, heating system) become proportionally much more expensive at smaller scale. • Smaller scale systems are more likely to be designed to operate at longer sludge ages, to provide for a more robust process (needing less attention) and reduce sludge production levels. • Process intensification (e.g., thermal pre-treatment) is capital intensive and can realistically only be applied in centralised (or very large scale) facilities. • Conventional internal combustion combined heat and power (CHP) cogeneration engines have a minimum size of approximately 250--500kW and cannot effectively operate at below 70% capacity. This means a system producing secondary sludge only would need a minimum of 100--200 tonnes per day at 12% solids. Loss of renewable energy as a product stream removes one of the motivations for anaerobic digestion. The current default treatment method for biosolids at small-medium scale is aerobic digestion, which produces a product with higher odour potential, and lower overall stability. Biosolids treated using anaerobic digestion have a lower odour potential. This is because anaerobic digestion effectively destroys volatile sulphur compounds present in biosolids (Murthy, Higgins et al., 2006). Emergence of Pre-treatment Options for Anaerobic Digestion As already stated, the movement towards enhanced biological nutrient removal and production of activated sludge rather than primary sludge has reduced degradability. There is a direct relationship between WAS sludge age and degradability (see Figure 1). WAS with an age of >15 days has poor degradability and, if applied to anaerobic digestion, is likely to result in a failed process, not producing enough energy to meet heating requirements, and requiring excessive mixing energy due to poor gas mixing and rheological characteristics. This has decreased the applicability of anaerobic digestion, which is the leading practical method to achieve net energy generation, while producing a high-quality final product. To enhance applicability of anaerobic digestion, pre-treatment methods are often applied, to increase the speed of degradation (hence allowing intensification) and/or increase the ultimate extent of degradation. The range of pre-treatment methods has increased significantly in the last 10 years, though most are new variations on older themes. Pre-treatment methods for anaerobic processes have been recently reviewed (Carrère, Dumas et al., 2010), and will be briefly summarised here. All pre-treatment methods attempt to enhance the first stage of anaerobic digestion by a variety of mechanisms, including chemical/thermal solubilisation, lysis and particle size reduction. The net effect is to either increase the rate or extent of degradation, such that specific gas production and VS destruction are improved. Note that these are inherently linked, and claims of an increase in one without a corresponding increase in the other should be regarded with scepticism. An increase in VS destruction should also result in an improvement in 'dewaterability' (Novak 2006) and better mixing characteristics, reducing costs. Pre-treatment methods can be divided into four key types: 1. Mechanical pre-treatment: These can include sonication, impact plate, homogenisation, grinding and lysis-centrifuge. Particularly, sonication is well represented in both commercial applications and in research articles (see Carrère et al., 2010 for further details). In full-scale implementation, energy input is normally of the order of 30kWh.m-3 treated, with moderate (+10%) increases in final VS destruction. 2. Biological pre-treatment: There is a wide range of processes which use a biological agent, but most use a short- term biological process (~2 day) prior to the main mesophilic digestion stage. This may involve higher temperature (temperature phased anaerobic digestion), or lower pH (acid-phase or enzymic digestion) that attempt to enhance the hydrolytic stage of anaerobic digestion. Performance is also further discussed overleaf, but improvements are similar to mechanical pre-treatment. Figure 1: Degradability vs. upstream sludge age, based on (Gossett & Belser, 1982) modified for Australian conditions.
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