Tesis doctoral de Anna Pedescoll Albacar
Constructed wetlands are alternative systems to conventional wastewater treatment for small com- munities (up to 2000 pe). This is mainly due to that operation and maintenance costs are reduced. despite the advantages, the clogging of the bed is the most important trouble encountered by the managers of subsurface flow wetlands treatment systems. The solids accumulation in the interstitial spaces of the gravel over time causes the decrease of both, hydraulic conductivity and porosity. this leads in the development of preferential paths and short-circuiting of the water through the wetland, which converge in overland flow and contaminant removal efficiency decrease. Therefore, clogging is the main limiting factor of the life span of a subsurface flow constructed wetland. the strategies to reverse clogging imply important economic investments. Generally, the most widespread option is the replacement of gravel. Hence the need to measure realibly the degree of clogging of a system in order to identify the factors that influence this phenomenon and to search new configurations and strategies to delay its progress, thus operations needed to reverse it. therefore, the aims of this thesis must contribute to establish new design criteria and operation of horizontal subsurface flow constructed wetlands to minimize clogging processes without reducing the contaminant removal efficiency. Then, the objectives were to compare a method to measure hydraulic conductivity in situ, based on falling head permeameter, in order to determine the state of clogging of wetlands and its horizontal distribution; to determine the most suitable indicator to as- sess clogging in horizontal subsurface flow wetlands; to identifie new design and operation parame- ters likely to cause or encourage the clogging of the filter medium; and to analyse two new configu- rations in a pilot scale system in terms of contaminant removal and clogging development. among the different clogging indicators analysed (drenable porosity, hydraulic conductivity, accu- mulated solids and effective volume from a tracer test), hydraulic conductivity seems to be the best one. Firstly, because hydraulic conductivity provides information about the hydraulic behaviour inside the bed, and linked appropriately with the accumulated solids (a correlation of 74.5% was found between both indicators). Secondly, because its application in full-scale wetlands is more straightforward than other techniques. the falling head permeameter for hydraulic conductivity measure is a simple method to measure reliably in the range of values that can be found in a wetland (0-500 m/d). It is a repeatable method although the discrepancies with a constant head permeameter (both in lab measurements and in situ constant head method). However, these discrepancies are acceptable in the range of conduc- tivities measured. A potential relationship was found between the two methods (fallinghead = constanthead0.7821 ). This indicates that the falling head method is less sensitive to small changes of hydraulic conductivity. Although, its straightforward applicability makes it suitable for practical pur- poses. hydraulic conductivity measurements by falling head permeameter lead to obtain reliable results of the degree of clogging and its horizontal distribution in subsurface flow constructed wetlands. The analysis of horizontal distribution of hydraulic conductivity allows identify aspects of the design that enhances preferential flow paths. Thus, wastewater tends to flow through the less resistance path- ways. Wetlands with length/width ratio <1 could easily develop preferential paths and dead zones, while the design of wetlands with this ratio >1 force the water to flow occupying the entire width of the bed. Therefore, the hydraulic behaviour is improved and consequently, clogging develops more uniformly. Water distributors also affect the appearance of preferential paths, favoured by a hetero- geneous distribution at the inlet. Channels distributors are recommended instead of perforated pipes since they perform better and their maintenance is more comfortable. the type of gravel used as filter media can also be a factor involved in the clogging development, especially if breakable gravels are used. Calcareous materials (limestone) are less resistant to abra- sion than granitic ones. However, being composed by uniform and rounded aggregates, are prefer- able to granite, which its composition leads in angular and breakable particles. It is recommended to use gravels composed basically by quartz, due to their mechanical resistance properties, or gravels with values below 25% of los angeles abrasion test. it is strongly recommended the use of primary treatments in order to reduce the solids loading rate at the inlet of the wetlands (limiting this parameter to 10 g/m2 d), which can greatly reduce the devel- opment of clog matter. In this sense, the use of a hydrolytic up-flow sludge blanket (husb) reactor can effectively remove particulate matter from wastewater than conventional primary treatments (as settlers). The husb reactor produced effluents clearly more reduced, which can imply a slight loss in contaminant removal efficiencies. However, this could be compensated by a lower accumulation of solids in the inlet of the wetlands (up to 30% less), then delaying the clogging symptoms. alternating saturated and unsaturated phases in the operation of horizontal wetlands allow increase contaminant removal efficiencies. This is especially favoured in cold moths, when the ammonium removal efficiency can achieve a 50% higher than wetlands conventionally operated. This operation strategy produced slightly higher accumulations of sludge (10% more accumulated sludge) instead, but more uniformly distributed within the media. However, the higher sludge accumulation was compensated by lower plant growth (and therefore roots), then keeping hydraulic conductivity val- ues in the same range than conventional operation.
Datos académicos de la tesis doctoral «Clogging in horizontal subsurface flow constructed wetlands. mesures, design factors and prevention strategies«
- Título de la tesis: Clogging in horizontal subsurface flow constructed wetlands. mesures, design factors and prevention strategies
- Autor: Anna Pedescoll Albacar
- Universidad: Politécnica de catalunya
- Fecha de lectura de la tesis: 04/11/2010
Dirección y tribunal
- Director de la tesis
- Joan García Serrano
- Tribunal
- Presidente del tribunal: manel Poch espallargas
- Juan José Salas rodriguez (vocal)
- Carlos Arias (vocal)
- eloy Bécares mantecón (vocal)