First stage of the French vertical flow constructed wetland system: experiments with the reduction of surface area and number of units
Small vertical-flow constructed wetland units comprising the first stage of the French system were studied in Brazil for the treatment of raw sewage. Planted and unplanted units and different feeding strategies were tested. In the first phase, hourly batches of a daily flow of 13 m 3 d −1 were applied over three alternating units, resulting in an average hydraulic loading rate (HLR) on the full system of 0.15 m 3 m −2 d −1 . A second phase, aimed at reducing land requirements, kept the same daily flow and batch frequency, but used two alternating units, resulting in a HLR on the full system of 0.22 m 3 m −2 d −1 . Removal efficiencies were very good when the system operated with three units, with mean values of 82% for biochemical oxygen demand (BOD), 81% for chemical oxygen demand (COD), 85% for total suspended solids (TSS) and 59% for NH 4 + -N. With two units, the equivalent values were 74% for BOD, 59% for COD, 67% for TSS and 51% for NH 4 + -N. There were significant differences in the median removal efficiencies of COD and TSS. No significant differences were found between planted and unplanted units for most constituents. In both phases, the overall good performance and the simplicity of the system make this treatment process a very attractive alternative for developing countries.
Performance of different substrates in constructed wetlands planted with E. crassipes treating low-strength sewage under subtropical conditions.
The present study aimed to assess removal potential of chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), total ammonia nitrogen (TAN), total phosphorus (TP) and acetylsalicylic acid (ASA) in synthetic wastewater simulating low-strength sewage by sequencing-batch mode constructed wetlands (CWs). Six CWs with three substrates (gravel, light expanded clay and clay bricks) and one CW of each substrate was planted with E. crassipes to verify the feasibility of using a floating macrophyte in CWs and verify the best optimized substrate. Results showed that the presence of E. crassipes enhanced the removal of COD for systems with gravel, increasing the removal efficiency from 37% in the unplanted system (CWG-U) to 60% in the planted system (CWG-P). The vegetated CW with clay bricks (CWB-P) presented the best performance for both TKN and TAN removal, with maximum removal efficiencies of 68% and 35%, respectively. Phosphorus was observed to be efficiently removed in systems with clay bricks, both planted (CWB-U) and unplanted (CWB-P), with mean removal efficiencies of 82% and 87%, respectively, probably via adsorption. It was also observed that after 296days of operation, no desorption or increase on phosphorus in effluent samples were observed, thus indicating that the material was not yet saturated and phosphorus probably presents a strong binding to the media. ASA removal efficiency varied from 34% to 92% in CWs, probably due to plant uptake through roots and microbial biodegradation. Plant direct uptake varied from 4 to 74% of the total nitrogen and from 26 to 71% of the total phosphorus removed in CWG-P, CWC-P and CWB-P. E. crassipes was able to uptake up to 4.19g of phosphorus in CWC-P and 11.84g of nitrogen in CWB-P. The findings on this study suggest that E. crassipes could be used in CWs and clay bricks could significantly enhance phosphorus removal capacity in CWs.
Adsorption and biological degradation of ammonium and sulfide on peat
Abstract A dynamic mathematical and numerical model has been applied to predict concentration profiles over time for adsorption and biological degradation of ammonium and sulfide in a perfusion column with recycle. The model successfully accounts for the high adsorptive capacity of peat (two thirds of ammonium added to the system at pH 6) which makes this medium a useful carrier for engineered biological systems such as biofilters and biotrickling filters. This characteristic of peat gives it significant buffering capacity to protect against surges in loading. Solid/liquid partition coefficients were determined experimentally for adsorption of ammonium and sulfide onto a peat packing under prevailing pH conditions. The dynamics of mass transfer for the same were shown to be at least as fast as the dynamics of transport by bulk flow and probably very much faster. This enabled mass transfer resistance to adsorption and biological degradation to be discounted as negligible. The rate of biological degradation of ammonium on peat was found to vary between 10 and 80 (mg NH 4 + ) (kg dry peat) −1  h −1 , depending on the alkalinity of the liquid phase. This translated into predicted biological growth rates (based on published yields) of 1 to 7 (mg dry cell mass) (kg dry peat) −1  h −1 . Measurement of biological degradation rates for sulfide were frustrated by competing chemical oxidation of sulfide by dissolved oxygen. It is suggested that aerobic sulfide removing biofilters and biotrickling filters may remove sulfide by a two step process involving initial chemical oxidation to elemental sulfur followed by slower biological oxidation to sulfate.