• Environmental Engineering Research
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• v.16 no.2
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• pp.103-109
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• 2011

This work experimentally determined the effect of thermal and microwave pretreatments on the anaerobic digestion of mixtures of municipal primary and secondary sludges in semi-continuous mesophilic digesters at hydraulic retention times (HRT) of 20, 15, 10, 7, and 5 days. The ratio of soluble chemical oxygen demand (COD) to total COD in thermally pretreated and microwaved sludges at $80^{\circ}C$ was 2.7 and 3.2 times higher than that of raw sludge, respectively. The volatile solids (VS) and COD removal efficiencies in all three digesters fed with raw (control), thermally pretreated (TM), and microwaved (MW) sludges decreased as the HRT was reduced. The highest relative improvement in VS removal compared to the control occurred at the HRT of 5 days in the TM and MW (29 and 41% higher than the control, respectively). At this HRT, improvement in the COD removal efficiencies in the TM and MW compared to the control was 28 and 53%, respectively. Improvements in biogas production compared with the control increased in both the TM and MW as the HRT was reduced to 5 days. The relative improvement in daily biogas production compared to the control from the TM and MW was 33 and 53% higher than the control at the HRT of 5 days, respectively. The results show that microwave pretreatment is more effective than thermal pretreatment in increasing the solubilization degree and mesophilic anaerobic biodegradability of sewage sludge.

• Applied Chemistry for Engineering
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• v.24 no.3
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• pp.279-284
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• 2013

In this study, the feasibility of a single-stage thermophilic aerobic process for the treatment of high-strength food wastewater produced from the recycling process of food wastes was examined to substitute anaerobic digestion process. Also, the removal and stability of thermophilic aerobic process were assessed according to the changes of hydraulic retention times (HRTs) and organic loading rates (OLRs). When the OLR increased from 9.2 to $37.2kgCOD/m^3d$, a pH value in R1 (HRT : 5 d) significantly decreased to 5.0, due to the organic acid accumulation. On the other hand, the pH value in R2 (HRT : 10 d) was stable and R2 showed the high removal of COD, organic acid and lipid, even though the OLR increased from 4.6 to $18.6kgCOD/m^3d$. In R1, the COD loading rates for COD removal was suddenly dropped, as the COD loading rate increased from 18.6 to $28.4kgCOD/m^3d$. In contrast, R2 showed that the COD loading rates for COD removal increased with regard to increment in the loading rates of 3.61, 7.05, 9.43 and $12.2kgCOD/m^3d$, indicative of the high COD removal efficiency. Therefore, the results demonstrated that over 10-d HRT, the high concentration of raw food wastewater was efficiently treated in the single-stage thermophilic aerobic process.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.55 no.3
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• pp.328-337
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• 2022

This study investigated the removal performance of a foam fractionator under seawater conditions. The foam fractionator was tested using a 3×3×3 factorial design for operating conditions by combining different solids concentrations (SS; 1, 5, and 10 mg·L^-1), surface air velocities (SAV; 1.1, 1.5, and 2.1 cm·sec^-1), and hydraulic residence times (HRT; 1, 3, and 6 min) at 16℃. Performance parameters such as daily solids removal rate and efficiency were measured, and a multi-regression model equation was developed accordingly. The daily solids removal rate and removal efficiency varied with the experimental conditions and ranged from 0.14-2.33 g-solids·m^-3-air·day^-1 and 8.9-96.7 %, respectively. Overall, the daily solids removal rate increased with increasing SS and SAV and decreasing HRT, whereas the removal efficiency increased with increasing SAV and HRT and decreasing SS. The daily solids removal rate (g-solids·m^-3-air·day^-1) of the foam fractionator for SAV (cm·sec^-1), SS (mg·L^-1) and HRT (min) were described by the following multi-regression model: Daily solids removal rate [f(z)]=-0.118+0.422SAV+0.094HRT+0.141SS (r²=0.873).

• Journal of Korean Society of Environmental Engineers
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• v.27 no.1
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• pp.17-24
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• 2005

This study was carried out to investigate the removal efficiency of an intermittently aerated nonwoven fabric filter bioreactor fed continuously with domestic sewage. The hydraulic retention time(HRT) of the reactor was reduced from 12 hrs to 10 hrs to 8 hrs during an experimental period of 17 months. In order to search an optimum aeration/nonaeration time ratio for the nitrogen removal at each HRT, the cycle times of 3, 2 and 1 hr were tested at the aeration/nonaeration time ratio of 1. Then, the aeration/nonaeration time ratio was changed from 50 min/70 min to 40 min/80 min to 30 min/90 min at the cycle time of 2 hr which showed the best nitrogen removal. During the experimental period, the effluent SS concentration was always below 1.2 mg/L with more than 95% of BOD removal efficiency. The highest nitrogen removal of 90.1% was observed at the aeration/nonaeration time ratio of 40 min/80 min at the HRT of 10 hr. Oxidation-reduction potential could represent the degree of the nitrification and denitrification reaction in the reactor.

• Journal of Korean Society of Environmental Engineers
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• v.37 no.7
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• pp.418-425
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• 2015

Anaerobic Filters (AF) packed with porous ceramic floating media were operated at different operational conditions to identify the feasibility of the renewable bioenergy, methane production from swine wastewater and to verify the suitability of effluent from anaerobic filters for the subsequent biological nitrogen and phosphorus removal. Stepwise increase in organic loading rates (OLRs) or decrease in hydraulic retention times (HRTs) with influent TCOD concentration of 14,000 mg/L were utilized at mesophilic temperature. The maximum methane productivity of 1.74 volume of $CH_4$ per volume of reactor per day (v/v-d) was achieved at an hydraulic retention time (HRT) of 0.5 day (OLR 28 g TVS/L-d). Based on the biogas production, the highest total volatile solids (TVS) removal efficiency of 63% was obtained at an HRT of 3 days (OLR 4.67 g TVS/L-d), however based on the result from the effluent total chemical oxygen demand (TCOD) analysis, the highest TCOD removal efficiency of 75% was achieved. The effluent alkalinity concentration over the range of 2,050~2,980 mg/L as $CaCO_3$ at all operational conditions, could compensate the alkalinity destruction caused by nitrification. The effluent from the anaerobic filter operated under the HRT of 2 days showed the COD/TKN ratio of 15~35 and COD/TP ratio of 38~56. Therefore effluent C/N/P ratio is able to satisfy the optimum COD/TKN ratio of greater than 8.0 and COD/TP ratio of 33 for the subsequent biological nutrient removal.

• Environmental Engineering Research
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• v.21 no.1
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• pp.69-73
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• 2016

Generation of food waste is a serious issue that needs to be addressed worldwide. Developing suitable treatment methods while generating energy (methane) is a common practice for sustainable treatment of waste. In this study, methane generation by food waste was investigated in mesophilic and thermophilic regimes at various hydraulic retention times (HRTs) and organic loading rates (OLR). In temperature regimes, influent concentrations and HRTs ranged from 30 to 110 g COD/L and 18 to 30 days, respectively, which corresponding to an OLR of 1.0 to $6.1kg\;COD/m^3-d$. Better methane production and organic removal was observed under thermophilic conditions because of the enhanced hydrolysis of complex polymers and microbial activity at higher temperature. The peak methane productivities attained in thermophilic and mesophilic regimes were 1.30 and $0.99m^3/m^3-d$, respectively. The maximum methane yields were achieved at 50 g COD/L and HRT of 24 d in both cases, and the values were 264 and $221m^3/ton$ COD, respectively. The results of this study will facilitate the development of sustainable methane production technologies using food waste as a feedstock.

• Journal of Korea Society of Waste Management
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• v.35 no.8
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• pp.731-743
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• 2018

This study was performed to test the feasibility of thermophilic ($55^{\circ}$) co-digestion of municipal sewage sludge and food wastes. The management variables of co-digestion were the mixed ratios of municipal sewage sludge and food waste hydraulic retention times (HRTs). During the operation of thermophilic co-digestion, the reactor pH ranged from 7.0 to 7.5 and the reactor alkalinity remained above 3,200 to 4,000 mg/L as $CaCO_3$. The volatile fatty acids concentration increased as the HRT shortened from 20 days to 10 days and the mixture ratio increased to 1:4, but did not reach toxic levels for co-digestion of sewage sludge and food wastes. Methane productivity increased gradually as the organic loading rate increased. Maximum methane productivity reached 1.03v/v-d at an HRT of 10 days and at the mixture ratio of 1:4. The TVS removal efficiency decreased from 70.6% to 58.3% as the HRT shortened from 20 days to 10 days. TVS removal efficiency ranged from 57.0% to 77% during the entire operation. It is likely that thermophilic co-digestion of sewage sludge and food wastes is a very effective method both to environmentally treat food waste and to economically produce gas for energy.

• Journal of Korean Society of Environmental Engineers
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• v.30 no.2
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• pp.155-160
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• 2008

In this study, the nitrate removal efficiency was examined under a variety of operating conditions, such as different doses of the reducing agent, different electrode types, different HRTs(hydraulic retention times), and different current densities, using the multistep electrochemical process. The nitrate removal efficiency increased and the input energy decreased when the reducing agent was used, and almost no difference was found between the electrode types in terms of their nitrate removal efficiency and current efficiency. So that the Zn reducing agent could be recovered, though, the B-type electrode was chosen(step 1: Pt-Zn; step 2: Pt-Zn; step 3: Pt-Zn; step 4: Pt-Zn). HRT experiments were carried out on constant electric current density unrelated HRTs and various electric current density related HRTs: the constant amount of electric current per unit volume. As a result, HRT and the electric current density caused concentration polarization and the lack of an applied current. That is to say,the lower the HRT, the greater the decrease in concentration polarization and in the amount of applied current per unit volume. Therefore, optimal conditions were found through the experiments that were conducted on HRT and electric current density. When a spacer was installed in the process, the nitrate removal efficiency and energy efficiency increased even more because the diffusion likewise increased.

• Journal of Korean Society of Environmental Engineers
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• v.22 no.6
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• pp.1011-1019
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• 2000

The purpose of this study is to examine the basis of design condition of existing treatment processes, namely, Extended Aeration Process, Conventional Activated Process, High Rate Aeration Process and Modified Aeration Process, by clarifying the correlations of influent wastewater concentration, hydraulic retention time, food-to-microorganism ratio and sedimentation of sludge, as well as to ascertain the feasibility of design, regardless of the existing design condition. In particular, this study made a priority investigation of hydraulic retention time and sludge sedimentation, because sludge sedimentation is the main factor$^{1)}$ which determines the operating conditions of existing treatment processes. Therefore, it is generally known that in case exceeds the sphere of design presented for each treatment, sludge bulking may occur. The results of Lesperance's test$^{1)}$, which formed the basis of design, showed the sphere of loading without security of sludge sedimentation, as in Fig. 1. The reason for sludge bulking in a certain condition, as above, is due to failure in application of optimum loading corresponding to each retention time by employing a few operating condition, which proved to be consecutively workable after years of trials and failures by Lesperance, for test conditions. However, the result of this test showed that in case of proper maintenance of loading. sludge sedimentation can be ensured under 120 SVI. Therefore, this study suggested hydraulic retention time and its corresponding optimum loading, and identified the hydraulic retention time as a determinant of sludge sedimentation. And. on the basis of these findings, this study suggested the feasiblity of UHR(Ultra High Rate), a new operating process, exceeding several times the applicable loading value of High Rate Aeration Process under one hour retention time which has not yet applied to the existing treatment processes.

• Journal of Environmental Health Sciences
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• v.37 no.1
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• pp.64-72
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• 2011

This study was performed to assess the removal efficiency on nitrogen, phosphorus and organic carbon in wastewater by spatial separation and internal recycling in a modified oxidation ditch process (modified OD). The performances of the modified OD were evaluated via laboratory-scale experiments. The process was operated at hydraulic retention times of 6-48 hours and solid retention times of 17-38 days. We found that organic carbon removal efficiency increased after the modified OD operation period. T-N removal efficiency remained stable; average T-N concentration of effluent was 8.02 mg/l after modified OD operation. In contrast, T-P concentration of effluent was over 1 mg/l. Nitrogen and phosphorus removal efficiency of modified OD at HRT 12 hr were 83.1% and 74.1%, respectively. Also, maximum efficiency was found at SRTs from 20 to 30 days. T-N removal efficiency was 83.1% at a C/N ratio from 3.0 to 3.5. However, T-N removal efficiency decreased at C/N ratios over 3.5. Also, T-P removal efficiency increased with HRT at C/P ratios in the same condition. Maximum efficiency was 74.1% at a C/P ratio from 25 to 28. T-N removal efficiency was 79.2% and T-P removal efficiency was 65.3% after M4 mode operation (added to the internal recycle line connected to the anoxic reactor). The modified OD with spatial separation and internal recycling developed in this study is, therefore, believed to be an improvement for solving problems in the nutrient removal technologies.