Research On Domestic Sewage Treatment With Terrace-Typed Constructed Wetland

The study on constructed wetland technology for waste water treatment began at the 70's,20th century. With the advantages of high removal capacity, simple construction, low investment, low maintenance and operation cost, simple operation and maintenance and so on, the technology is extensively researched and widely used in the areas relatively absent of capital and technician for wastewater treatment by now. However the nitrogen and phosphorous removal efficiency of the method is usually not high enough in the domestic sewage treatment process to meet the higher requirement of the effluent quality in the common constructed wetland. So, the aim of this paper is to design a terrace-typed constructed wetland (TTCW) system for the treatment of domestic sewage. The small scale and pilot scale experiments were conducted to examine the efficiency of the system.The results indicated that the CODcr, NH4+-N, TP removal efficiency were 86.52%, 80.5 % and 96.16%, respectively ,in the small scale system. The corresponding effluent concentrations were less than 30mg/L, 10mg/L and 0.1mg/L. The effluent quality meets the discharge standard of pollutants for municipal wastewater treatment plant (GB 18918-2002) partⅠ(B) .The NH4+-N, TN, TP, CODcr loading removal efficiency were 15～5.09g/(m2·d), 2.87～7.16g/(m2·d), 0.48～2.41g/(m2·d), 9.62～36.03 g/(m2·d), respectively, the corresponding removal efficiency were 47.82～81.20%, 45.7～74.9%, 80.42～96.72%, 54.82～86.37%, respectively, in the TTCW system. A common vertical flow constructed wetland (VFW) system was taken as a contrast. And the TTCW system'NH4+-N, TN, TP, CODcr loading removal efficiency were 0.13～0.69g/(m2·d), 0.41～1.39 g/(m2·d), 0.29～1.46 g/(m2·d), -0.07～2.39g/(m2·d) more than the VFW system'; the corresponding removal efficiency were3.3～11.2%, 2.7～9.7%, -3.94～9.74%, -0.2～7.4%, respectively. The quality of the TTCW effluent was better than the VFW's, especially for nitrogen and phosphorous removal.The effluent quality meets the integrated wastewater discharge standard (GB 8978-1996) partⅠ.With the addition of a surface flow constructed wetland (SFW), TTCW enhances the auto- reaeration process of the system, leading the increment of the dissolved oxygen (DO). The effluent concentration of DO was 0.25～2.38 mg/L and 0.21～1.67 mg/L in the pilot scale TTCW and the common VFW.The DO concentration was raised to 0.03～0.68 mg/L after water flew the SFW. The most probable number (MPN) and PCR-DGGE methods were employed to determine the number and distribution of nitrifying and denitrifying bacteria in the strata and the microorganism community structure of substrate in the TTCW system. The results showed that in the small scale system the numbers of nitrifying and denitrifying bacteria were1.5×104 MPN/g～4.2×106 MPN/g and 3.0×105 MPN/g～1.86×107 MPN/g ,respectively. The succession of bacterial species in the TTCW is changed along with the gradient of DO, and there are some characteristic species. The microorganism diversity indexes of the four pools are 2.47, 2.23, 2.18 and 2.05, which is clearly better than 2.13 and 2.11 of the vertical flow system. Generally, the purification effect of the TTCW is also better than the vertical flow one.The Vicia faba root tip cell micronucleus test was used to evaluate the safety performance of the TTCW effluent. The results showed that the mutagenicity of domestic sewage decreases largely after the wetland treatment. The TTCW effluent's grade of the safety performance is higher than the VFW's. And the TTCW effluent has little mutagenicity materials.