Water Journal : Water Journal May 2011
biosolids refereed paper technical features 92 MAY 2011 water 3. Chemical pre-treatment: This involves chemical addition, normally acids or bases, but also possibly ozone, peroxide or other oxidants to enhance hydrolysis rates. Chemical pre-treatment methods have declined in popularity due to the high cost of chemicals, and potential environmental and product impacts. 4. Thermal pre-treatment or thermal hydrolysis (TPAD): Note that this does not include low temperature pre- treatment, which is classed as biological. This involves heating the sludge stream to a high temperature (>140°C) with very good results. In particular, material with poor degradability (<30%) can be enhanced to a very degradable material (>55%)) (Batstone, Tait et al., 2009). However, it is capital intensive, and has additional operational requirements due to the use of 6 bar steam. Biological pre-treatment (2) is emerging as a lead contender for low intensity applications, especially due to its low capital cost, use of low grade heat energy in the first stage, and ability (with elevated temperatures) to produce a Class A sludge. This pre-treatment option makes anaerobic digestion more applicable at smaller scale, as small- scale heaters and vessels can be used, and the improvement in performance is sufficient to operate on material with poorer degradability. Performance has been largely related to an increase in degradation rate, rather than extent (Ge, Jensen et al., 2010), but can still result in substantial improvements in performance. For example, Figure 3 shows a 20% increase in biogas production rates over a 24-hour period from TPAD processes using thermophilic pre-treatment and mesophilic pre-treatment (representative of conventional anaerobic digestion). Enzymic or acid phase digestion employs a similar concept, although interestingly, our work has found that depressed pH has a strong negative impact on final degradability, and indeed it is better to operate a pre-treatment stage at neutral pH (Ge, Jensen et al., 2010; Ge, Jensen et al., 2011). Degradability Rate vs Extent -- Performance in Real Systems One aspect that has so far received very little attention in evaluations is whether the rate or extent of degradation is improved. That is, is more food made available to the microbial community through chemical changes, or is the existing food just made easier to digest? This is an important concept, since a pre-treatment process that just increases rate could be substituted by a larger main digester (though with increased mixing energy and other issues). In general, high intensity processes such as thermal hydrolysis will increase both rate and extent, while low intensity processes such as mechanical or biological will just increase rate. This, however, can have a massive impact on eventual performance. This is demonstrated in Figure 3, which shows expected digester performance (validated against continuous digesters) for different TPAD pre-treatment conditions. TPAD has a much stronger impact on rate (represented in khyd) than on degradability extent. As can be seen, TPAD can increase digester performance at 15 days retention time from 30% VS destruction with no pre-treatment to 45% VS destruction with 65°C pre-treatment. The impact of pre-treatment is to flatten out the performance curve and hence make it less dependent on digester hydraulic retention time. To fully assess this, however, requires either copious continuous digester performance data, or conducting biochemical methane potential tests, which is a conservative test that can identify both speed and extent (Ge, Jensen et al., 2011). Conclusion While the changing paradigm of nutrient removal in wastewater treatment design goals has made it more challenging to apply sustainable sludge digestion Figure 2: Comparison of biogas production rates over a 24-hour period from a TPAD process comparing thermophilic-mesophilic and mesophilic-mesophilic processes (Ge, Jensen et al., 2010). Figure 3: Performance curves for performance vs. hydraulic retention time (HRT) in continuous feed-mixed conventional digesters fed with different materials, including untreated WAS, primary sludge (PS) and primary sludge pre-treated at different conditions (2 day HRT 50°C-70°C). Pre-treatment has a strong impact on rate (khyd), but less on extent (fd). TP = thermophilic pre-treatment; MP = mesophilic pre-treatment.
Water Journal April 2011
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