Study On Nitrogen And Phosphorus Removal Characteristics Of Constructed Wetlands With Economic Plants Treating Rural Wastewater

Compared with biotechnology, constructed wetland which has the advantage of simple management, low investment and operation costs in realizing the function of nitrogen and phosphorus removal, is more popular in the rural sewage treatment.However, the economic value of traditional constructed wetland plants macrophytes is poor. It changes the original value of the land for rural areas where economic strength is weak and the agricultural production regarded as the main source of income. At the same time, most of the landscape plants wilt in winter and some evergreen plants go dormant under low temperature conditions, resulting pollutant purifying efficiency and landscape benefits reducing. Based on this situation, this paper combined aquatic plant filter bed, constructed wetland with soilless cultivation of vegetables technology and greenhouse vegetable technology and got aquatic vegetable filter bed-constructed wetlands planted with aquatic vegetables. After investigating nitrogen and phosphorus removal characteristics of the combined system for a year, we got the following conclusions:(1) In summer and autumn, the TN and TP removal rate of combined ecological system were in the range of 3.2～4.2 g/(m2·d) and 0.33～0.45 g/(m2·d), respectively. TN removal rate of spinach, zizania latifolia was around 14% lower than the macrophytes Thalia and Canna, and no significant difference occurred in TP removal rate. The aquatic vegetable filter system contributes 42.2% in TN removal and 44.6% in TP removal for the combined system, respectively. Nitrogen species and hydraulic load had great influence on nitrogen and phosphorus removal effect. The removal efficiency of TN, NH4+, TP increased to 72.9%,77.7% and 84.3% when the hydraulic load is reduced to 0.18 m3/(m2·d). The removal rate of NH4+ and TN were improved in different degrees when c (NO3-)/c (TN) increased from 0.38 to 0.71. As for the variation along the way, the removal of nutrients is more significant in front parts of the filter bed and subsurface constructed wetland.(2) In winter and spring, with the insulation effect of plastic greenhouse, the TN and TP removal rate of combined ecological system were in the range of 3.2～3.5 g/(m2·d) and 0.20～0.26 g/(m2· d), respectively. Compared with the purification efficiency in summer and autumn, the TN removal rate did not decreased significantly, but the TP removal rate has declined a greater degree caused by nearly saturated substrate conditions and limited light resulting in vegetable growth rate slowed down. The removal efficiency of TN, NH4+, TP increased to different extend when the hydraulic load is reduced to 0.18 m3/(m2·d) from 0.30 m3/(m2·d). The nitrogen species and carbon source were two main factors influencing the purification effect of nitrogen. With c (NO3-)/c (NH4+) varing from 1.20 to 1.84. the removal efficiency of NH4+ increased first and then decreased while TN removal efficiency increased first and then tends to be gentle. The average removal efficiency of TN increased from 42.1% to 70.2% when the ratio of influent carbon to nitrogen increased from 2.0 to 4.1. The concentrations of nutrients decreased along the path regularly, and nutrient removal is more significant in front parts of the filter bed and subsurface constructed wetland. What’s more, the law of diminishing along the path and front decline is more obvious.(3) As for the running effects of a year, a bunch of combined systems, which included spinach fiter bed- CW planted with spinach, spinach fiter bed-CW planted with zizania latifolia, tomato fiter bed- CW planted with Thalia dealbata in summer and autumn, as well as watercress fiter bed- CW planted with watercress、lachca sativa fiter bed- CW planted with water fennel、water fennel fiter bed- CW planted with lettuce set in a greenhouse in winter and spring, basically reached A level of first class stand, referring to the standard of Discharge standard of pollutants for municipal wastewater treatment plant (GB18918-2002) with the average hydraulic load of 0.24 m3/(m2·d) and the average influent concentrations of TN、 NH4+、TP reaching 27.4mg/L,10.8mg/L,2.0mg/L, respectively.At the same time, the TP of the first two systems mentioned successfully reached A level of first class stand, while that of the rest just reached B level of first class stand.The rise of TP removal efficiency partly depends on increasing the thickness of substrate, or coordinating the contradiction of the demand for hydraulic load when nitrogen and phosphorus meet the same discharge standard.(4) The vegetables harvested from aquatic vegetable filter bed-constructed wetlands, provided high content of Vitamin C and soluble sugar, in the range of 36.9～75.9mg/100g and 0.477～0.894g/100g,respectivaly. The nitrate content was in the range of 89.1～195.4mg/kg, far below the standard of 432mg/kg. Vegetables are safely edible without heavy metal pollution except Hg from spinach and Pb from lachca sativa over the limit value. TN accumulation of spinach, which means TN recovery, reached the peak of 73.0g/m2,19.8 g/m2 for TP as well. Zizania latifolia, water fennel and watercress were also well-performed in the recovery of nitrogen and phosphorus. Taking the spinach cultivated in summer and autumn, combined with watercress in winter and spring, for an example, the combined ecosystem yield was around RMB 25340 per 667m2, and the raw production benefit excluded the constructive fee for brick and concrete tank, plastic greenhouse used in winter and seed purchase cost.The results showed that the combined ecosystem which consisted of aquatic vegetable filetr bed-constructed wetland planted with vegetables for deep purification of decentralized domestic sewage which combined soilless cultivation and wastewater treatment,could achieve good effect and recover nitrogen and phosphorus resources effectively.The last but not the least, it produced significant economic benefits.