Arsenic removal in subsurface-flow constructed wetland planted with vetiver grasses | |
| Author | Chatchawal Singhakant |
| Call Number | AIT Diss. no.EV-09-07 |
| Subject(s) | Vetiver Constructed wetlands |
| 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 |
| Series Statement | Dissertation ; no. EV-09-07 |
| Abstract | This study aimed to determine the treatment performance and removal mechanisms of the subsurface-flow constructed wetland (SSF-CW) system planted with vetiver grasses (Chrysopogon zizanioides) to remove Arsenic (As)-rich synthetic wastewater at the concentration of 5 mg/L. In the first phase, experiments were carried out to compare the effects of different configurations of pilot-scale CW units; (i) deep-bed units (dpCW) at length to depth (L:D) ratio of 2 (each with dimension of 1mx2mx1m) and (ii) shallow-bed units (shCW) at L:D ratio of 8 (each with dimension of 1mx4mx0.5m); which were operated at hydraulic retention time (HRT) of 6, 9, and 12 days. Tracer study was undertaken by feeding lithium chloride (LiCl) as an impulse tracer in each CW unit to describe hydraulic patterns of the CW units. The results revealed that neither L:D ratio nor HRT had significant effect on the dispersion characteristics of CW units, but the higher effective volume ratio of the CW units was observed at the CW units with lower L:D ratio. The evapotranspiration rates in CW units operating at various HRTs and L:D ratios were found in the same magnitude which should not affect As removals. Based on the data obtained from the 117 -day experiments, it was apparent that dpCW units could achieve relatively high As removals of 52.9%, 59.2%, and 72.1 % at HRT of 6, 9, and 12 days, respectively. Analysis of As mass balance showed that only 0.2-0.4% of As input was taken up by vetiver grasses, whereas the major portion was retained in the CW bed (38.9-77.6%). The forms of As retention were determined by sequential fractionation which indicated As complexation with iron and manganese (hydr)oxides on the bed surface of 31-38% and As entrapment into the pores of bed materials of 42-52% of total fractions. No obvious difference of the As fractions between dpCW and shCW units could be observed. In addition, the field measurements of vetiver grass biomass could reveal that the adverse effect appeared only at the inlet zone for both dpCW andshCW units with the range of plant biomass reduction of about 41-81 %, whereas there was no dead plant throughout the experiments. In the second phase, experiments were carried out to compare the roles of vetiver grasses on As removals using CW units planted with vetiver grasses (CWplanted) and CW units without plants (CWunplanted). According to the data obtained from 147 days of experiment, it was apparent that CW planted units could remove As significantly higher than those of CWunplanted units at the range of 7-14%. Analysis of As mass balance in CW units revealed that only 0.5-1.0% of total As was found in vetiver grasses. Arsenic retained within the bed of the CW planted units (23.6-29.7 g) was significantly higher than those in the CWunplanted units (21.3-26.8 g), illustrating the effect of vetiver grasses on As accumulation in the CW units. Determination of As in different fractions in the CW bed suggested that the main mechanism for As retention was due mainly to As entrapment into the pores of bed materials (50-57% of the total fraction). However, fractions of As-hound in organic matters typically produced from plant roots decomposition, could enhance adsorption capacity of CW bed. In addition, organic sulfides produced from the root decomposition helped remove As through precipitation/co-precipitation process. In a contrary, As was leached out in the form of iron and manganese-bound complexes under reducing condition in those CW planted units. According to the aforementioned findings, the SSF-CWplanted with vetiver grasses should be used only as a pretreatment for As-rich wastewater. The improvement of As removal efficiencies could be further studied by planting with other As-tolerant plant species, installing porous materials in the CW bed, and adding some sulfate chemicals into the As-rich wastewater. In addition, isolation and identification of the microorganisms related to the As retained from the bed samples could be conducted to study the roles of microorganisms in As removal |
| Year | 2009 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. EV-09-07 |
| 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 (EV) |
| Chairperson(s) | Thammarat Koottatep |
| Examination Committee(s) | Jutamaad Satayavivad ;Preeda Parkpian ;Aramaki, Toshiya |
| Scholarship Donor(s) | Chulabhorn Research Institute ;Mahidol University ;Royal Thai Government Fellowship |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology - Chulaphorn Research Institute - Mahidol University, 2009 |