Research On Transformation And Pollution Characteristics Of Nitrogen And Organics In Combined Sewer Sediments

In recent years, urban waterlogging is relatively common in China. Inaddition to short return periods of underground pipelines and increasing urbanimpervious pavement, the presence of the pipe sediments is also an importantfactor. Sewer sediments can r educe pipe flow ability and a variety of pollutantsin sediment re-occurrence into the water with the overflow of sewage into thereceiving water body causing combined sewer overflows, resulting in theincreasing burden of water purification. Overflow poll ution load to theconfluence system are more widely researched at home and abroad. Thedomestic studies mainly research the occurrence and pollution characteristics ofphosphorus and heavy metals in separate drainage system sediments.Reasearchon nitrogen and organic matters in combined sewer sediments in beijing rarelymentioned. Nitrogen is the main pollutants of eutrophication. The relationshipbetween nitrogen and organic matter is more closely. Study on nitrogen andorganic matter pollution characteris tics in combined sewer sediments has certainpractical significance.In this study, we investigated the second ring of Beijing, including the DrumTower area, Baizhifang bridge area, the official Park Park area, LongevityPark area, recording pipeline diameter, sediment thickness, pipe material,catchment area characteristics and other information of research area. Throughdata analysis, we can know that in addition to individual research point, morethan90%of the combined system pipes were varying de grees of depositionphenomenon.There was always combined sewage flows into the pipe.Thesediments are immersing in a long time so that a majority of mosquitoes appeardand smelly bad. Average sedimentation rate of residential district was27.4%and average deposition thickness was94.2mm. Average deposition rate of parkdistrict was9.3%and average deposition thickness was26.7mm. Compared tothe sediments of residential district, park district significantly reduced. Theaverage deposition rate and the ave rage sediment thickness was about one-thirdof residential district.According to the circumstances of research points, we selected twosediment sampling points of neighborhoods and parks. We also designed astraight piston sampler and a branch pipes samp ler according to the actualsituation. After collecting samples, we analysised and determined thesamples,obtaining ammonia, nitrate, TN, TOC content of sediments inresidential district was0.469~0.481mg/g,0.007~0.008mg/g,6.650~8.720mg/g,15.700~275.000mg/g. Also ammonia, nitrate, total nitrogen, TOC content ofthe sediments in park district was0.092~0.213mg/g,0.002~0.005mg/g,2.275~4.650mg/g,22.100~285.700mg/g.According to the principle of dynamic similarity, we builded miniaturemodels to simulate the transformation and migration of nitrogen and organics insediments under no runoff erosion, different rainfall intensity conditions.According to the tests, we found that the content of total organic carbonreducing about22%, ammonia nitrogen i ncreasing, nitrate nitrogen reducingunder the condition of no runoff erosion. Orthogonal experiments showed thatmicroorganisms in water and sediments both contributed to the wastewaterquality changes. Microorganisms in sediments played a major role on t hetransformation of nitrogen and organic compounds. The laboratory waterscouring velocity associated with the actual rainfall return period. Respectively,we set up the laboratory flow velocity of0.43m/s,0.84m/s,0.96m/s,1.32m/s,corresponding to the rainfall return period of0.25a、1a、1.5a、5a. We foundthat nitrogen and organic matter reflected the same trend under different flowing conditions. With the increasing of return period, flow velocity increased,organic matter degradation efficiency drope d, ammonia grew less, thedegradation of nitrate nitrogen was with small amplitude.