Design And Running Characteristics Of Zhengzhou Nansanhuan Semi-underground WWTP

As an important part of urban municipal infrastructure,wastewater treatment plants(WWTP)play a very important role in conserving fresh water resources,controlling water pollution and improving living environment.Most of the WWTPs in the world now are constructed in the open air.However,urban land becomes increasingly expensive with the continuous expansion of urbanization.Thus,the underground WWTP would be the developing trend in super cities whose land sources are increasingly in shortage for its less pollution,less area occupied and insignificant influence on landscape.As the first semi-underground WWTP in the central plains of China,Zhengzhou Nansanhuan WWTP adopts pre-anoxic A2/O process with design scale of100,000m3/d,and its effluent quality can meet first class of A of < Discharge standard of pollutants for municipal wastewater treatmentplant >(GB18918-2002).By comparing with a similar aboveground WWTP in Zhengzhou,the engineering design,equipments installation,operation practice and construction cost of Nansanhuan semi-underground WWTP was analyzed to study the design and operation characteristics of the semi-underground WWTPs.The main conclusions are as follows:In the engineering design,due to the high degree of intensiveness of land use of semi-underground WWTP,horizontal flow sedimentation tank is preferred as secondary sedimentation tank for its compact structure.Besides,the high-efficient sedimentation tank that integrates flocculation,coagulation,precipitation,sludge discharge and reflux,etc is more suitable for the advanced treatment processes.In the design of the odor control system,while the above-ground WWTP collects only the odor of the grid,aeration grit chamber and the sludge storage tank,the semi-underground WWTP is required to implement full collection and treatment of all the odor in the structures located in the box and sludge dewatering section outside the box,which induces more than 10 times cost for biological deodorant input thanthe above-ground WWTP.In addition,the equipments installation and inspection are more complicated for most of the structures are arranged in the box,and the large scale equipments have to be installed through the holes for hoist.In addition,the high requirement of equipments material are necessary to avoid corrosion in underground.In operation practice aspect,more than 97% of assurance rate could be achieved in Nansanhuan WWTP except for TP whose assurance rate was 92% in the whole2016.It's worth noting that the 95% of assurance rate was remained during the winter.Contrastively,the assurance rate of TN dropped to 80% in the above-ground WWTP.Thus,the semi-underground WWTP possesses more stable nitrogen removal effect.In investment and costs,although the investment within-site and annual average cost is increased 100% and 24%,respectively than the contrastive above ground WWTP,it has 46% saving in occupied area.Thus,the semi-underground WWTP has a good environmental and social benefits.