|Phytodepuration is a natural treatment technique that reproduces natural purification processes in a controlled environment. Phytodepuration systems have been developed since the 80’s above all in the US and in Central Europe. They are artificial little deep basins, often filled with inert material and fed with aquatic plants (macrophytes). These plants reproduce the natural purification processes typical of humid areas. The macrophytes can be floating, flooded or emerging. The systems can have superficial or sub-superficial streams and the sub-superficial stream can be horizontally or vertically oriented. Superficial streams support all the types of macrophytes, sub superficial one only the emerging macrophytes.|
The superficial stream systems are basins with an elongated shape having variable depth depending on the macrophytes used and on the level of treatment realized. Wastewater is pre-treated by grid filtration and primary sedimentation to remove the largest particles and reduce the concentration of suspended solid particles. The FWS systems are fed continuously and the wastewater entering the basin is in direct contact with the atmosphere. The upper layer of the stream is in an aerobic condition, while the deeper layers are in anaerobic conditions.
Superficial stream with floating macrophytes
These are basins of depth ranging from 0.1 to 0.6 m with an impermeable loamy soil bottom. The plants growing in these systems shadow the water under them, avoiding algae growth. The stream is then in an anaerobic condition, apart from the superficial area. The main limit of these systems is the high frequency of plant removal. This implies high labour costs and biomass disposal problems.
Superficial stream with flooding macrophytes
These are basins of a depth ranging from 0.1 to 1 m, depending on the wastewater features and on the vegetation adopted. The bottom of the basin is covered with various types of material to allow plant’s growth. In these systems the basin surface is almost completely in contact with ambient air, thus allowing the bacterial death caused by solar radiation in the upper stream layers. The purification efficiency is higher than in the systems with floating macrophytes, because of this double action of solar radiation on the surface and of the flooding plants on the bottom of the basin. Besides the biomass require less frequent removal and disposal.
Subsurface Flow systems (SSF)
These are the most applied phytodepuration methods in Europe. Inert filling material is put in the basin to support the growth of macrophytes roots. Its depth is generally between 0.2 and 0.8 m depending on type of macrophytes. These systems are applied more often than the FWS ones because they have a higher efficiency, so they require a smaller surface to provide good results. The purification efficiency is nearly the same during the whole year, because high thermal excursion is avoided by the presence of the medium. On the other side the positive effect of the solar radiation is not present and the oxygen exchange with the ambient air is blocked. To avoid the substrate occlusion a pre-treatment of the wastewater is needed. The frequency of vegetable mass substitution is low, thus avoiding a high biomass disposal.
Sub superficial horizontal flow systems (H-SSF)
In these systems wastewater is uniformly distributed on the whole length of the basin trough a pipe disposed on the ground at the entrance of the basin. Wastewater is fed in the system in a continuous flow and goes through the whole basin going out at its end. The filter medium is made of inert material of constant size. Filtration and biological degradation processes, carried out both by anaerobic and aerobic microorganisms, remove organic matter. Nitrogen is removed by nitrification, denitrification, ammonia volatilization and root adsorption. Metals and phosphorus are adsorbed on the inert material surface.
Sub superficial vertical flow systems (V-SSF)
These are similar to the H-SSF, but the wastewater is fed from the top through a web of pipes, which distributes it on the whole basin surface. Wastewater goes down the basin in a horizontal flow and is collected at its bottom. Basin filling is not continuous. Filling-emptying phases of the basin allow a higher oxygen concentration than in the H-SSF systems, increasing the efficiency of nitrification and organic material degradation. To have a continuous purification process a second filter bed is needed or a collecting and regulating pool can be introduced.
|I - Subsurface Flow systems (SSF) |
II - Leaching with floating macrophytes
III - Superficial stream with emerging macrophytes
IV - Floating macrophytes
V - Flooded macrophytes
The first experiences with FWS systems had been made in the US and North America. They show average efficiency of 71% for BOD5 removal, 77 % for COD removal, 54% for nitrogen and ammonia, 51% for phosphorous and 64% for suspended solids.
H-SSF systems show high efficiency for the removal of most of the pollutants and they are applied in Europe frequently. The average efficiency is about 82% for BOD5, 72% for COD, 43% for nitrogen, 38% for ammoniac nitrogen, and 35% for phosphorous. These values are low for the nitrogen due to the presence of anaerobic conditions, which are not favourable for the nitrification process. On inert material few Fe, Al and Ca ions are present, hindering phosphorous removal. H-SSF proved to be very efficient for suspended solids removal, reaching the 80%, and for microorganisms’ removal, reaching values higher than 90%.
V-SSF systems are quite new and not very spread yet. They however show very high organic matter degradation: 92% for BOD5, 87% for COD. The very efficient aeration of the filter medium and the alternation of aerobic and anaerobic conditions allow ammoniac nitrogen removal of 77%, nitrogen removal of 47% and phosphorous removal of 70%. Suspended solids average removal is about 90%, bacteria removal is higher than 99%.
Building and managing features
The construction of a phytodepuration system is quite simple. In H-SSF systems the filtration bed slope should be rather low: at the exit the wastewater stream and the filter bed should not be too distant, allowing the plants root to reach the wastewater. The basin bottom has to be impermeabilized. The ideal particle size for the medium is between 8 and 16 mm and it should be preliminarily washed in order to avoid the blocking of the bed pores due to fine particles. The basin banks have a little slope to facilitate maintenance operations and plants growth and to reduce erosion. They can be protected with geo-webs.