Substrate clogging at vertical flow constructed wetlands

Research project P 14173 Substrate clogging at vertical flow constructed for wetlands Raimund HABERL 06.03.2000 Constructed wetlands have been used for wastewater treatment in Europe and North America for more than two decades. The use of constructed wetlands provides a relatively simple and inexpensive solution for treatment of wastewater from small communities. Vertical flow systems which use sand as main layer represent the latest generation of subsurface flow constructed wetlands and are the state of the art for this technology for wastewater treatment in Europe. By far the biggest operational problem of constructed wetlands nowadays is the clogging of the filter surface of the vertical flow beds. The term "substrate clogging" summarises several processes which lead to a reduction of the infiltration capacity of the substrate surface. The substrate clogging leads to an extremely fast failure of the treatment performance of the system. The reason of this is the reduced oxygen supply in a clogged constructed wetland. To run constructed wetlands at high loading rates (which means low specific area requirement - high area requirement is the most important disadvantage of the system) without running into clogging problems in a long term view can therefore be considered as the research and development goal in the constructed wetland technology in Europe. The main reasons of clogging are accumulation of suspended solids and excess sludge production of the involved micro-organisms. Chemical precipitation and deposition in the pores, growth of rhizomes and roots which can block a part of the hydraulically active pore volume and the formation and accumulation of humic substances are also considered to play a role to a certain extent. All mechanisms lead to an inner and outer blockage of the filter substrate which therefore cannot be aerated as it would be necessary for wastewater treatment. The biggest problem is the quantification of the different factors. How much amounts each process to the clogging of the substrate? This of course is the basis of an optimisation of the clogging controlling factors like substrate selection, organic and suspended solids loading rates and filter feeding strategies. The project therefore is a detailed investigation on the clogging processes within the filter. The project focuses on measurements in the "black-box" itself using an innovative approach by combining methods of soil structure investigation (e.g. raster-electronic microscopy) with methods of microbiology (e.g. enzymatic tests) and methods of wastewater analysis. The investigations are made at 5 small scale constructed wetlands (1 ml surface area). Additional tests are done at laboratory scale columns and after the first year of research also on three selected full scale constructed wetlands which are under operation since at least four years. There are two categories of aims - on basic scientific research and on applied research. The basic scientific research is necessary to understand the processes within the black box "vertical flow constructed wetland". Only on this basis it is possible to establish and calibrate a model which describes the oxygen input and substrate clogging processes. The model acts as forecast and design too] especially for the designer of a constructed wetland. Furthermore a guideline on possibilities of a clogging free operation of vertical flow systems acts as tool for the operator of the constructed wetland.

Constructed wetlands have been used for wastewater treatment in Europe for more than 20 years. They are characterised by low operation and maintenance costs and a robust removal efficiency but require a large area. Subsurface flow constructed wetland with vertical flow and intermittent feeding represent the state of the art. Substrate clogging is by far the biggest operational problem of vertical flow constructed wetlands. The term "substrate clogging" summarises several processes which lead to reduction of the infiltration capacity of the substrate surface. The lower infiltration rate causes a reduced oxygen supply and further leads to a rapid failure of the treatment performance. It can be summarized that substrate clogging is mainly caused by organic matter load and not due to biomass growth. Therefore a good mechanical pre-treatment is essential for a long-term stable operation of subsurface flow constructed wetlands for wastewater treatment. Five pilot-scale vertical flow constructed wetlands (PSCW) with a surface area of 1 m were operated in parallel over a period of two years with different hydraulic and suspended solids loads. Two different sandy substrates were used (sand with a gravel size of 0.06/4 mm and 1/4 mm respectively). Soil physical and structural investigations, methods of microbiology, and of drinking water and wastewater analysis were combined to quantify the contribution of the single processes to the overall process of substrate clogging. During the investigations operation without clogging was possible for hydraulic loading rates less than 100 mm/d. Substrate clogging only occurred at higher loading rates. For the PSCWs with the 0.06/4 mm substrate clogging occurred after 2 month operation with a hydraulic load of 150 mm/d, whereas for the PSCWs with the 1/4 mm substrate it occurred after 1 month with 250 mm/d. A recovery time of 14 days was enough to guarantee the operation after clogging happened. A good correlation between the water content before a feeding and the loading of the PSCW could be found for both used substrates. For early detection of clogging the water content measurements showed the best results. For the upper layer of the PSCW with the 0.06/4 mm substrate (0 to 10 cm) already 2 weeks before clogging a linear rising in the minimal water content just before feeding can be observed. About 5 days just before clogging occurs the minimal water content rises exponentially. In the deeper layers an effect on the water content can be observed at a maximum of 5 days before clogging. For a better understanding of the transformation processes inside the "black box" constructed wetland a simulation tool was developed which is able to model the transformation and elimination processes of organic matter, nitrogen, and phosphorus. The simulation results for the PSCWs showed a good match to the measured data for both flow and reactive transport simulations. A good calibration of the flow model was possible due to the amount of data which were available to describe the hydraulic behaviour of the pilot-scale constructed wetlands. There is still a lot of research to be done to obtain a full calibration of the simulation tool and to make simulation a reliable tool for constructed wetland design. A calibrated simulation tool can then be used for the design of subsurface flow constructed wetlands and the maximum possible hydraulic loading rate (the minimum of the aeral demand) can be calculated, for which still a stable operation is possible.