Experimental and Modeling Studies of Horizontal Subsurface Flow Constructed Wetlands Treating Domestic Wastewater

• Main Author: Mburu, Njenga
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2013
• Subjects: Aquatic plants; Biofilms; Biomass; Carbon; Costs; Decades; Developing countries; Effluents; Engineering; Environmental impact; Environmental science; Harvest; Horticulture; LDCs; Microorganisms; Nitrogen; Nutrient removal; Pathogens; Plant sciences; Pollutants; Pollution; Software; Urban areas; Vegetation; Water supply; Water treatment; Wetlands
• ISBN: 9798728215219

Sustainable sanitation and water pollution control calls for adoption of affordable and efficient wastewater treatment technologies. In the developing countries, characterized as they are by inadequate sanitation, the safe management of wastewater is not widespread. There is therefore a need for an appropriate technology that can reliably achieve acceptable effluent quality for discharge to the environment at minimal cost. Constructed wetland (CW) systems have been used as a cost effective alternative to conventional methods of wastewater treatment. However, the mechanistic understanding of the CW has not matured, while performance data that can guide design and operation of CW under tropical climate are scarce. This study was undertaken to explore the treatment of domestic wastewater with subsurface constructed wetlands, in order: 1) to provide performance data that can influence design and operation of CW under tropical conditions and, 2) to evaluate the processes involved with the transformation and degradation of organic matter and nutrients. In the study a pilot scale horizontal subsurface flow constructed wetland (HSSF-CW) was established in Kenya, and an existing dataset with experimental data from 16 wetland mescocosms operated under greenhouse conditions were obtained from the U.S.A., kindly shared by Montana State University. The data from the tropical pilot scale HSSF-CW was used to conduct a performance evaluation (with respect to organic matter, nutrients and suspended solids) (Chapter 3), a comparison (performance and economics) with a waste stabilization pond (Chapter 4) and a reactive transport simulation in the HSSF-CW (Chapter 5). The data from the wetland mescocosms were used to conduct a simulation of carbon, nitrogen and sulphur conversion in the batch-operated experimental wetland mesocosms with and without consideration of biofilm development within the wetland (Chapters 6 and 7). The pilot HSSF-CW consisted of three cells receiving a continuous feed of primary effluent from the outflow of a primary facultative pond. In two of the cells, the macrophyte Cyperus papyrus was planted, while the third cell acted as a control. The wetland cells were 7.5 m long and 3 m wide with vertical masonry sides, 0.95 m deep, and a concrete floor sloped at one percent. The cells were filled with granite type gravel to a depth of 0.6 m, ranging in size from 9-37 mm, with a porosity of 45 %.The experimental constructed wetlands were operated under controlled greenhouse conditions at Montana State University in Bozeman (Montana, USA). 16 subsurface constructed wetland mesocosms were constructed from polyvinyl chloride (PVC) pipes (60 cm in height × 20 cm in diameter) and filled to a depth of 50 cm with washed pea-gravel (0.3–1.3 cm in diameter). Four columns each were planted with Carex utriculata, Schoenoplectus acutus and Typha latifolia, while four were left unplanted as controls. A series of 20 days incubations with artificial wastewater was conducted over 20 months at temperatures ranging from 4 to 24 ◦C at 4 ◦C steps. A synthetic wastewater simulating secondary domestic effluent was used with mean influent concentrations of 470 mg/l COD, 44 mg/l N (27 Org- N, 17 NH4+-N), 8 mg/l PO43--N, and 14 mg/l SO42--S. Columns were gravity drained 3 days prior to each incubation and then again at the start of each incubation. Upon each emptying, columns were refilled from above with new wastewater. Sampling from all 16 columns occurred at days 0, 1, 3, 6, 9, 14 and 20 of each incubation and those subsamples were analysed afterwards for the constituents. The results of the study showed successful performance of the tropical HSSF-CW for the secondary treatment of domestic wastewater with respect to organic matter (BOD5 and COD) and TSS removal at an average mass removal efficiency between 58.9 and 74.9 %. Moderate (36 - 49 %) removal of nutrients (nitrogen and phosphorus) was recorded. A two days hydraulic retention time was found to be optimum for organic matter removal. The presence of the macrophyte enhanced the ability of the wetland to withstand higher organic and suspended solids loading. The land area requirement for secondary treatment (based on BOD5 removal) was estimated as 2.0 m2 per population equivalent (Chapter 3). A waste stabilization pond would need 3 times the area that would be required for the HSSF-CW to treat the same amount of wastewater under tropical conditions (chapter 4). The evaluation of the capital cost of HSSF-CW system showed that it is largely influenced by the size of the population served, local cost of land and the construction materials involved. Using a (mechanistic) numerical model that incorporates the growth of six microbial groups (heterotrophic, autotrophic nitrifying, fermenting, acetotrophic methanogenic, acetotrophic sulphate reducing and the sulphide oxidising bacteria) and the subsequent consumption of electron donors and acceptors, the influence of key operating and environmental conditions on the biochemical transformation and degradation processes for organic matter, nitrogen and sulphur in subsurface flow constructed wetlands was evaluated (Chapters 5, 6 and 7). Sorption processes were found to be important in simulating COD and ammonia removal in subsurface flow constructed wetlands. The rates of oxygen transfer by physical re-aeration and root oxygen transfer were found insufficient, indicating that organic matter in the wastewater was removed mainly by anaerobic processes. Indeed anaerobic reactions occurred over large areas of the simulated HSSF-CW and contributed (on average) to the majority (68%) of the COD removal, compared to aerobic (38%) and anoxic (1%) reactions in the tropical HSSF-CW (Chapter 5). Further a thin (predominantly anaerobic) biofilm in which no concentration gradients developed was simulated. The simulations suggest that incorporation of plant roots into the substrate of constructed wetlands enhances microbial populations related to the transformation and degradation of pollutants in constructed wetlands. Measured and simulated data demonstrate that the resultant effect on the wetland performance may not necessarily be related to temperature. This research contributed to performance data and getting a better mechanistic understanding about the factors influencing the performance of horizontal subsurface flow constructed wetland treating real domestic wastewater under tropical conditions. The findings obtained in this research may prove useful towards the wider application of the constructed wetland wastewater treatment technology and the optimization of full-scale HSSF-CW.