Pilot & Full Scall Investigation Of Pellet Flocculation Blanket Process Optimization

Accompany with the development of water treatment technology, the treatment process tend to miniaturization with more equipment devices. The pellet flocculation blanket(PFB) process which is characterized with high surface-loading, short residence time, small occupation area, easy erecting, flexible and wider applicability, and easy to enforce auto control, is maturing into an ideal choice of moden water treatment process. Most foregone pellet flocculation researches focused on the flocculation mechanism, the process operational conditions, the reactor designation, et al., but there leaves something to be desired in many aspects such as the optimization of operational conditions, the growth of pellet flocs, and the kinetic of pellet blanket, large numbers of researches must be done to perfect the pellet flocculation theory.This paper focused on the optimization of the process designation such as the blanket water distribution system, the agitating peddles, and the choice of pre-coagulation process, the establishment of the pellets growth model which was proved by subsequent pilot scale experiment, the effect of operational conditions on blanket characteristics, and the operation mode under low influent concentration. At the end of the investigation, theoretical results were identified through full scall experiment. The major achievements in this paper were expressed as following:(1) The optimization of the process designationIn this investigation, hydrolic calculation was applied to optimize the designation of raw water distribution system and the blanket agitator. The optimization results indicated that, to achieve well distributed water distribution, the orifice diameter or narrow slit width should declined gradually along with the downward direction. In the designation, the wetted cross-section was designed with cuniform sharp to generate microscale eddies. The hydrolic calculation of rotating agitator illustrated that, the width of agitator should declined gradually among the paddle radius derection according to the relationship of B=Cr-3, based on this theory, paddles fit for small scale and large scall reactors were designed respectively. Optimizing experiment indicated that, tubular coagulator with (G/Re)0.5T value of 10.87 achieved best results, so in this investigation, this kind of mixer was selected as the pre-coagulation device.(2) The establishment of pellet flocs growth modelIn this paper, by deducing collide frequency between pellet particles, effect factors to pellets effective densityρe was analyzed, the results indicated that, both the volume concentration Vf and the G value appeared direct proportional relationship withρe.Relevant experimental results identified these relationships. By analyzing forces act on the pellet, expression about pellet size was established as d=uw/2 (?) Kρe3(1-Vf)m , as well as the minimum size could be expressed as dmin=0.038μ(?)uw/2. Based onρe(?) the mass balance equation, the variation of Vf in an infinitesimal blanket was expressed by analyzing pellets growth in the blanket with height of dh, the relationship could beThe statistics analysis of the verification experiment indicated that, correlated experimental results could fit the model preferably, from which could achieve the calculation of blanket height.(3) The optimization of the PFB processBy considering G value, the basic eqation of pellet coagulation could be deduced as C= C0 exp(?);By kinetic analyzing of the flocs blanket, the minimum uw up-flow rate of the blanket could be established as uwmin=(?), as well as (?) the maximun upflow rate as uw=(?). The fit up-flow rate is in the interval between the minimun up-flow rate and the maximum value.The related pilot experiment identified above theories, as well as optimized the operational conditions. (4) The determination of operational conditions under low influent concentration with sludge recycleThis part of investigation is to determine optimal operational mode under low influent concentration. The pilot scale experimental results indicated that, the optimal position for sludge recycle located behind PAM dosage position and in front of the inlet, and along with the increasing of the sludge reflux ratio, effluent turbidity tend to decrease at first, and then increase. The experimental results also expressed that, the increasing of sludge reflux ratio contributed to the improvement of pellets sedimentability.The kinetic investigation about primary partical removing indicated that, under a certain operational condition, there exists optimal influent concentration under which could achieve lowest outlet turbidity; according to this thoery, the optimal sludge recycle rate could be expressed as:(5) Full scale experiment of PFB processAt the end of this paper, PFB process was applied to concentrate ferric flocs sludge in Tianjin Yangliuqing water treatment plant to inspect the applicability of theoretical results in this investigation. The full scale experimental results indicated that, by applying PFB process, under experimental condition of PAM dosage of 1.2 mg/L, agitating rotation velocity of 2-4 rpm, effluent turbidity, sludge moisture content and maximal upflow rate could be less than 10NTU, below 96% and 66.4 cm/min respectively with inlet SS ranged between 108 mg/L and 195 mg/LThe development and debugging of automatic control system supported by demonstration project was executed after the full-scale experiment. The collected data from the automatic controlled continuous running system indicated that, the control system could alter PAM dosage to the fitful value automaticly according to the change of inlet flow rate or raw water concentration, and could calculate the accumulative thicken sludge weight automatically according to the raw water concentration and inlet flow rate to achieve automaticsludge drainage.The cost-effectiveness analysis indicated that, under the experimental scale of 30m3/h, the operation cost per ton raw water is￥0.17, the annual running cost of the demonstration project is Y44200, and according to current situation of DWTP residues treatment development, the integrated high efficient treatment process developed in this project could provide high value of application and dissemination, the application of the technology could achieve obviously economic, social, and environmental benefit.