Nitrogen transformation and removal mechanisms in duckweed-based ponds | |
| Author | Lukkhana Benjawan |
| Call Number | AIT Diss. no.EV-07-06 |
| Subject(s) | Sewage--Purification--Nitrogen removal Duckweeds |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Environmental Engineering and Management Inter-University Program on Environmental Toxicology, Technology and Management |
| Publisher | Asian Institute of Technology |
| Abstract | It has been proven that Duckweed-Based Ponds (DWBPs) had a promising potential for nitrogen (N) removal in wastewater. Nevertheless, the DWBPs system has drawbacks on poor effluent quality and system failure due to duckweed die off. This study focuses the effects of effluent recirculation which should help improve and minimize the aforementioned problems, by undertaking the experiments using pilot-scale DWBPs consisting of two chambers, each with dimension of 0.90 x 1.95 x 0.80 m. and 0.90 x 1.95 x 0.70 m. (width x length x water depth). The objectives of this study are: (i) to determine the hydraulic characteristics; (ii) to investigate effects of hydraulic retention times (HRT) and effluent recirculation rates (ER); (iii) to identify N-removal mechanisms, N-mass balance and N-removal rates. Tracer study results showed that the pilot-scale DWBPs with the application of ER (Recirculated DWBPs) had the actual hydraulic retention time (HRTactual) longer than the theoretical hydraulic retention time (HRTn1). The dispersion number ranged from 0.12 to 0.16 indicating moderate wastewater dispersion in the pilot-scale DWBPs. PHASE I results revealed that the Recirculated DWBPs had overall treatment performance in term of TN, NH4-N and SS removal higher than those obtained from the Conventional DWBPs. Most of duckweeds in DWBPs units were apparently died off when operated in DWBPs units without effluent recirculation. Whereas well growing of duckweeds was observed in the system applied effluent recirculation. The reasons for duckweed die off should not because of ammonia toxicity (unionized NH3 concentration of 0.75 mg/L in DWBPs units), but likely due to low DO level (about 0.1-0.3 mg/L) resulting from OLR of 7.8-15.6 g COD/m2 .d. Based on N-mass balance, it could reveal that most of N was contributed to unaccounted fraction which is prostulated to be nitrification-denitrification and sedimentation. The HRT 16 days with ER 100% were selected as the optimum operating conditions which should enable relatively high N removal. In PHASE II experiments, it could be summarized that average removal efficiencies for TN, TKN and N&-N were 89±6%, 90±6% and 93±8%, respectively in the Recirculated DWBPs operating at TN loading 0.55 g/m2 .d. The removal efficiencies of TN, TKN and NH4-N were 75± 7%, 76±8% and 78±9%, respectively in the DWBPs units operated at TN loading of 1.50 g/m2 .d. Analysis of N-removal mechanism and N-mass balance in DWBPs units could reaffirm that i) effluent recirculation help promote favorable conditions for duckweed growth; ii) nitrification-denitrification (15.7% and 45.4% in DWBPs unit 1 and unit 2) and duckweed uptake (47.0% and 21.0%) play significant roles in N-removal; iii) nitrification-denitrification rates were increased with increasing of TN loading; iv) plant uptake rate is independent of the difference in TN loading likely because the duckweed has limited capacity to assimilate N; v) ammonia volatilization can contribute to less than 1 % of N-removal; vi) N-removal via sedimentation process likely depending on the amount of solid contents presented in the influent; vii) loss of N from the system (< 10%) was presumably due to algal and microbial biofilm uptake attached to DWBPs walls. The integrated kinetic models were developed for TN and NH4-N removal. The equations are identified as: (i) For Conventional DWBPs (a) TN removal: lnCe/Co= -0.074λ- 0.408 .t (b) NH4-N removal: ln Ce/Co = -0.070λ-0.219 .t (ii) For Recirculated DWBPs (a) TN removal: lnCe/Co= -0.074λ-0.408 .t (1.4) (b) NH4-N removal: lnCe/Co= -0.070λ-0.219 .t (1.4) The models were validated satisfactorily with the experimental data from the pilot-scale DWBPs. The predicted results were comparable to the observed effluents, and indicating the possibility of using for DWBPs design. |
| Year | 2007 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Energy and Climate Change (Former title: Department of Energy, Environment, and Climate Change (DEECC)) |
| Academic Program/FoS | Environmental Engineering and Management (EV) |
| Chairperson(s) | Thammarat Koottatep ;Skorn Mongkolsuk; |
| Examination Committee(s) | Chongrak Polprasert ;Preeda Parkpian ;Lee, Seung Hwan ;Kim, Youngchul ; |
| Scholarship Donor(s) | Chulabhorn Research Institute / Mahidol University / Asian Institute of Technology (CRI-MU-AIT) ; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology - Chulabhorn Research Institute - Mahidol University, 2007 |