|Other Abstract||Constructed wetlands (CW) are engineered systems that have been designed and constructed to utilize natural processes involving wetland vegetation, soils and the associated microbial assemblages to assist in treating wastewaters. As their high purification efficiency and relatively low capital investment and treatment cost, constructed wetlands wastewater treatment systems have been popular, and are being increasingly applied in many countries. However, there are many problems and challenges to the long-term safety of CW system. Substrate clogging is by far the biggest operational problem of constructed wetlands, which leads to permanent ponding and to anaerobic conditions within CW. Prolonged clogging can cause variation in hydraulic characteristics and malfunction of the system. The reasons for the clogging can be very complex and there is no simple cause-and-effect chain. Accumulation of organic solids is a typical feature in CW system treating domestic sewage and organic wastewater. The accumulated organic solids provide a long-term storage of carbon (C) and nutrients and a sustainable supply of C for microbial denitrification. However, they may clog wetland pore spaces, thus leading to a decline in wastewater retention time and the efficiency of nutrient removal from wastewaters.
Aimed to the long-term safety of a new CW, Integrated Vertical Flow Constructed Wetland (IVCW) and the mechanisms, parameters, consequences, and possible strategies of clogging are reviewed and the relationships with clogging phenomenon and accumulation of organic solids are discussed by characterize organic matter (OM) fractions collected from above and within substratum in the small sale plots (SSP), medium scale plot (MSP) and Guanqiao CW systems (GQS). Detailed investigations were carried out at pilot-scale constructed wetlands (PSCWs) using a variety of methods: e.g. soil physical-chemical investigations, spectrums analysis methods, and various analysis methods of drinking water and wastewater. The main contents included:
1.Clogging phenomenon and its research development are reviewed by summing up present references. Also, the index appraisal system and evaluation standard of long-term healthy and safety development of CW system are put forward. Some possible strategies and effective techniques are summarized.
2.Two analytical methods are selected and compared which are to determine the quantity of accumulated organic matters at different depth of CW substrates. Loss On Ignition (LOI) is suitable for the sandy soil which have higher organic matter quantities only. High errors are brought out with large grain gravels media. Calculated capacity react to potassium dichromate is more applicable to determine the quantity of accumulated organic matters at different depth of CW substrates. The ration of two methods are normalized by the equation of Y=1.497X (R=0.927).
3.The organic matter accumulation (OMA) at different depth in MSP and GQS is measured and organic matter distribution is discussed. The results show that organic matter is mainly accumulated at the surface and 0~50mm upper layer in IVCW chamber. The equation describing the relation between OMA and substrate depth is y＝7.2126e-0.1168x + 0.1675. Different positions where OMA take place results from the runtime of CW system and influent sewage composition. High organic loading result in more OMA occurs in down-flow chamber in early experiments and the impact of wetland vegetation (Rhizoma Acoori Graminei) seems change this trend.
4.A practicable isolation method of humic acid (HA) has been built up in accordance with open literature which describes the relative fractionation procedures of natural organic matter (NOM) in soil. Compared with one commercial product, two humic acids were extracted and prepared from different chamber substrate surface, and characterized by Elemental analysis, UV-Vis, FTIR, 3DEEM fluorescence spectra. Characteristic data of functional groups and flocculation values were also determined. The results showed the elemental compositions, UV-Vis spectra and FTIR spectra of the three humic acids were all similar to each other. Humic compounds from CW system are slightly low polycondensation extent and saturation of double bonds of C and H. It is suggested that lowly colloidal and amorphous materials with lower water holding and physical binding properties than Aldrich HA. While 3DEEM fluorescence spectra had obvious differences reflecting the differences of the structures and conformations of humic acids. This work indicates that the fluorescence characteristic can be applied to disclose differences in structure of HAs from different sources. Higher C/N and E4/E6 of HA from substrate of up-flow chamber than down-flow chamber applies that the special lignose spectra of aromatic and polysaccharide structures which have relative lower bio-degradation characteristic. Lower flocculation limits of HA from substrate of up-flow chamber shows that the OM is liable to react with the Ca2+、SO42- in influent of CW systems, and coagulative precipitation and deposition of CaCO3 or CaSO4 result in blocking of pore space in substrate.
5.Comparisons of removal rates of main organic contaminations between combination plots (Z2, Z7) and GQS show that the capability of removing CODCr、BOD5 in CW systems. The average CODCr removal rates of Z7 and Z2 are 18.7% and 49.2%. The average BOD5 removal rates of Z7 and Z2 are 35.7% and 68.8%. The main removal field of organic matter is located in the upper layer of down-flow chamber of IVCW. CODCr deceases along the direction of inflow. The substrate of CW has already become a major limiting factor on the degradation of OM.
6.The distribution of dissolved organic carbon (DOC) changes in different substrate layer shows the autochthonous OM release to a certain extent. Many OMs are transferred from the upper layer to the bottom layer with the high hydraulic loadings and vegetation of CW may contribute to the increase of DOC in bottom layer of substrate. The changes of Specific ultraviolet light absorbance（SUVA）in pore water at different layer of substrate imply the changes of the proportion of refractory fractions in plant litter and applied organic wastewater of outfluent of CW. Accumulation of organic solids includes some stable fractions which are consist of dead plants roots and stems.
7.The OMA in CWs have great influence on environmental behaviors of organic pollutants such as polycyclic aromatic hydrocarbons (PAHs). The photodegradation of the carcinogenic pollutant 2-naphthol in aqueous solution containing Aldrich humic acid (HA) and ferric ions (Fe(III)) under simulated solar irradiation was investigated. The photooxidation efficiencies were dependent on the pH values, light intensities and Fe(III)/HA concentration in the water, with higher efficiency at pHs 3-4, and 50 μmol l-1 Fe(III) with 20 mg l-1 HA under 250 W HPML. The initial rate of photooxidation increases with increasing the initial concentration of 2-naphthol from 10 μmol l-1 to 100 μmol l-1, while do not change at 50 and 100 μmol l-1. However, higher removal efficiency of 2-naphthol is achieved at its lower initial concentration of 10 μmol l-1, and initial rate of photooxidation is 0.193 μmol l-1min-1. Dissolved oxygen (DO) plays an important role in the system containing Fe(III)-HA complexes in which Fenton and photo-Fenton reactions were enhanced in the environment. Hydroxyl radicals produced in HA solution with or without ferric ions were determined by using benzene as free radical scavenger and phenol as scavenging products proportional to hydroxyl radicals. By using UV–Vis and excited fluorescence spectrum techniques, the main photooxidation products, which have higher absorption in the region of 240-340 nm, were found, and the mechanisms for the oxidative degradation is proposed.
8.Particular parameters of thedesign, operation and performance of six cases in design and application of constructed wetland are described carefully, all these application of IVCW to the rehabilitation of fresh water (lake and river) ecology、to control non-point pollution in urban surface runoff and to treat sewage in scenery spot are summarized, the widely use of IVCW proved that IVCW can run safely for a long time if sound design strategies and measures are taken to prevent substrate from clogging.|