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Recovering biodegradable carbon from a thermophilic aerobic digestion supernatant for biological nutrient removal

University of British Columbia

The biological nutrient removal (BNR) process usually requires external carbon supplements for enhanced phosphorus and nitrogen removal. It has become popular for full-scale wastewater treatment plants to implement carbon addition and optimization, to ensure best system performance. Thermophilic aerobic digestion (TAD) is operated at elevated temperatures to achieve sludge stabilization, volatile solids destruction, and pasteurization. Preliminary tests indicated that the volatile fatty acids (VFAs) accumulation in the TAD sludge supernatant, under a microaerated operation (system oxygen demand exceeds the supply), was a potential carbon source for BNR enhancement. A targeted degree of solids destruction efficiency can also be achieved under the microaerated operation, and the VFAs can be internally recovered for BNR enhancement purposes. The objectives of this study were to investigate the feasibility of using the TAD supernatant as a carbon source for BNR enhancement, and the potential impacts of the TAD supernatant addition on the system performance. Furthermore, due to the nature of VFA variance in TAD supernatant, TAD supernatant addition must be optimized in practice to obtain the benefits of carbon supplement and eliminate the potential nutrient overloading. A new control and monitoring technique was developed in this study using the headspace gaseous monitoring to estimate the VFA concentrations in TAD supernatant, and assess the BNR system performance. In this study, TAD supernatant was proven to be a potential carbon source for B NR enhancement in both batch and continuous feed studies. The VFAs in TAD supernatant resulted in comparable phosphorus release and denitrification. In addition, substrates other than the VFAs in the TAD supernatant were also found to be available for both P release and denitrification. The extra nutrient load (nitrogen and phosphorus) was significant, requiring mitigation and dosing optimization to reduce treatment system deterioration. Due to the feature of degradation during its storage, it was found that TAD supernatant should be added into the process train as fresh as possible, to maximize the VFA utilization and heat energy production. The "headspace carbon dioxide (C0₂) monitoring" method proposed in this study was proven feasible in estimating the VFA equivalent in the TAD supernatant. This C0₂ monitoring approach can be applied for the on-line TAD supernatant dosing optimization practice. The duration of C0₂ changes shown on the C0₂ profile (between the point of C0₂ starting to increase, and the point starting to decrease after the peak) of the phosphorus release and denitrification enhancement, due to the external carbon source addition, was defined as the "E Time" in this study. The duration of "E Time" was found to be proportional to the available carbon source concentration at the time of addition. A high accuracy in sodium acetate (NaAc) concentration estimation was also demonstrated in this study. In addition, the VFA equivalent in TAD supernatant was derived by comparing the "E Time" with a standard sodium acetate test. This headspace C0₂ monitoring can be potentially applied as a means of monitoring the efficiency and microorganism activity in a BNR process train. This "E Time" approach using the headspace C0₂ monitoring can be an attempt to replace the current oxygen utilization rate (OUR) method for readily biodegradable substrate determination. BNR operation can be benefited by this on-line monitoring to obtain the information of readily utilizable carbon concentration, optimized dosage control, and system performance. The headspace monitoring setup also prevents the sensor contacting with the sludge samples and saves the maintenance efforts.

Thesis/Dissertation

application/pdf

2009-10-08

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http://hdl.handle.net/2429/13754

Civil Engineering

University of British Columbia

UBCV

DSpace