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Laboratory-scale biosolids dewaterability assessment for municipal water resource recovery facilities
Avila, Isaac
Avila, Isaac
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2019
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Dewatering is an important operational component in water resource recovery facilities (WRRFs) to reduce conveyance costs by volume reduction of sludge (wet solid residuals). Cake solids are the agglomerated solids product of dewatering. Cationic polymers are the typical chemical conditioner used to destabilize solids during dewatering to increase the rate and extent of water release from a solids slurry. Dewatering is a costly endeavor for Metro Wastewater Reclamation District, and process upgrades are expected to affect dewaterability. Laboratory-scale dewatering methods that provide cake solids and optimal polymer demand (OPD) would be highly valuable to utilities to estimate cake solids hauling cost, inform required mechanical upgrades, and associated polymer requirements. These data could be used to inform pre-design assessments for process upgrades. The effect of solids pre-pretreatment impacts on anaerobic digestion was investigated using four mechanistically different methods that provide data on cake solids: Drying Rate, Modified Centrifuge Technique (MCT), Pneumatic Press (Press), and Limit Dryness Test (LDT). Three different methods were used in conjunction with solids conditioning to determine the OPD: Charge Demand, CST, and LDT. The laboratory dewatering methods were used to evaluate cake solids and OPD from Test and Control anaerobic digesters. The Wilcoxon rank sum test was used to examine differences between methods and lab used. The combined cake solids results from all methods for individual phases were similar (10 to 23% standard deviation). All methods did show an increase in cake solids by a factor of 1.5 to 2.5 when thermal hydrolysis was used as a solids pretreatment process. OPD was at least 1.5 times higher for the test than the control when thermal hydrolysis pretreatment was applied to the secondary solids; however, the low number of samples analyzed did not meet the Power criteria for the Wilcoxon rank sum test to statistically evaluate differences. All methods tested in-house provided similar cake solids and polymer results as external laboratories. In addition, simpler methods provided similar outcomes as more time-consuming, involved tests. The laboratory-scale dewatering assessments showed differences in solids pretreatment processes only when cake solids increased by at least a factor of 1.5. Extending the laboratory-scale testing results a full-scale scenario analysis suggests that thermal hydrolysis pretreatment of secondary solids could reduce the annual cake solids hauled by 64,000 wet tonnes but at the expense of a 1.75 times increase in polymer demand. Thus, laboratory assessment of cake solids and polymer demand provides valuable information for the evaluation of potential process modifications.
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