Bioresource Technology 179 (2015) 373–381

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Toward energy-neutral wastewater treatment: A high-rate contact stabilization process to maximally recover sewage organics Francis A. Meerburg a, Nico Boon a, Tim Van Winckel a, Jensen A.R. Vercamer a, Ingmar Nopens b, Siegfried E. Vlaeminck a,⇑ a b

Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, B-9000 Gent, Belgium BIOMATH, Department of Mathematical Modelling, Statistics and Bio-informatics, Ghent University, Coupure Links 653, B-9000 Gent, Belgium

h i g h l i g h t s  HiCS recovered energy from sewage organics with minimal losses through oxidation.  Energy recovery was highest at a ratio of contact to stabilization time of 0.14.  The sludge entered a state of endogenous respiration during stabilization.  This alternated with fast substrate removal within 5 min in the contact phase.

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Article history: Received 5 November 2014 Received in revised form 5 December 2014 Accepted 6 December 2014 Available online 19 December 2014 Keywords: AB-system Bio-flocculation F/M ratio Mainstream partial nitritation/anammox Respirometry

a b s t r a c t The conventional activated sludge process is widely used for wastewater treatment, but to progress toward energy self-sufficiency, the wastewater treatment scheme needs to radically improve energy balances. We developed a high-rate contact stabilization (HiCS) reactor system at high sludge-specific loading rates (>2 kg bCOD kg 1 TSS d 1) and low sludge retention times ( p), but did not significantly differ from one another. By combining the COD removal efficiency, the sludge yield and the anaerobic methane yield, the overall recovery of CODinfluent as CODCH4 could be derived. Whereas the HiCAS reactors performed better in terms of COD removal percentage and specific methane yield, it was found that the HiCS reactor at tc/ts 0.14 had the highest overall COD recovery percentage, with 34% of the incoming COD recovered as methane. This was 33% higher than the energy performance of the secondbest reactor; the HiCAS system at SRT 0.41 d. Comparison with values reported in literature shows that the COD recovery percentage of the HiCS system at tc/ts 0.14 was comparable to the 35% that was obtained in a high-rate membrane bioreactor (Akanyeti et al., 2010) and considerably higher than the typical values of around 25% in CAS systems (Cornel et al., 2011; Verstraete et al., 2009). The difference in energy recovery potential between the two HiCS reactors showed that contact and stabilization times have a major influence on the energy recovery of the HiCS system, and that these parameters should, together with sludge separation and SRT, be further optimized. Due to the variability in the observed yields, however, the differences in energy recovery potential between the reactors was not significant. This indicates that the performance of the HiCS process is at least equivalent to existing highrate systems but, given further optimization, potentially better.

3.5. Process kinetics of the HiCS system

Fig. 5. Kinetic characterization of the different phases of the in the HiCS reactors at (A) tc/ts 1 and (B) tc/ts 0.14. Exogenous SOUR is represented on the left axis; COD profiles on the right axis.

cell growth in this reactor, such as substrate storage or adsorption onto the existing sludge flocs. However, due to high variability throughout the reactor runs, differences in yield were found not to be statistically significant. 3.4. Energy recovery as methane The specific methane yield was determined for the reactors treating synthetic wastewater (Table 3). The highest conversion of sludge to methane was obtained in the HiCS reactor at tc/ts 0.14 and the HiCAS reactors at SRT 0.41 and 1.31 d. Recalculated to volumes of methane produced per gram of sludge, the specific methane yield for these reactors amounted to 402, 484 and 389 mL CH4 g 1 TSsludge fed, respectively. These values were higher

Respirometric experiments were performed to improve understanding of the process kinetics of the HiCS system, and provide perspectives for further optimization. Fig. 5 presents the evolution of the SOURexo, i.e., a measure of the rate of substrate oxidation to CO2, with the COD concentrations during one SBR cycle of the HiCS reactors treating synthetic wastewater. During stabilization, SOURexo levels dropped drastically after approximately 8 min in the HiCS reactor at tc/ts 0.14 and 12 min at tc/ts 1, marking the depletion of reserves of rapidly biodegradable COD. Only the sludge at tc/ts 0.14 was able to reach a nearendogenous state during stabilization, which means that a more pronounced substrate gradient was present in this reactor, and that selection pressures toward sorption and storage must have been considerably higher than at tc/ts 1. In both reactors, DO levels dropped to nearly zero after 5 min of contact time because of a fast aerobic metabolism. Since the contact phase was not aerated, DO levels remained low (

Toward energy-neutral wastewater treatment: a high-rate contact stabilization process to maximally recover sewage organics.

The conventional activated sludge process is widely used for wastewater treatment, but to progress toward energy self-sufficiency, the wastewater trea...
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