Enhancement Of Conventional Treatment Processes For Diatom Removal In Water Plants

This thesis focused on filter clogging problem resulted from diatom algae inShenzhen, to investigate the diatom variations in the raw water and during theconventional treatment processes and related diatom removal efficiencies, to evaluatevarious enhanced treatment technologies on Synedra acus removal in order to obtain theoptimum parameters, thus providing technical support for the control of diatoms inShenzhen city.Site investigation was conducted to identify the diatoms distribution in the rawwater and in the treatment train during2012to2013. It was found that there were11species of diaton, including Synedra acus in the raw water. The cell densities of Synedraacus increased to the peak values of0.6-1.8×106cells L-1along with the elongated cellsizes of100-300μm, January to February and May to June were the high-risk period tothe water supply. The pre-oxidation together with conventional coagulation andsedimentation process accounted for more than65%of total diatom removalcontribution. More diatom were collected during sand filtration in January, which meansthe risk for filter blockage significantly raised.Lab tests and full-scale experiments were carried out for various enhancedtreatments including pre-oxidation, coagulation, sedimentation with oxidants,coagulants and auxiliary chemicals for Synedra acus removal. A full scale test wascondudcted with capacity of3.0×105m3d-1. The optimal dose of the coagulant of polyaluminium chloride was2mg L-1, pH value around8modified with lime concentrationof6mg L-1, the dosage of the auxiliary agent of Dimethyl Diallyl Ammonium Chlorideas0.1mg L-1, potassium permanganate as0.9mg L-1, chlorine dioxide as1.5mg L-1andtheir combinations. The diatom removal rate was above93.6%when pre-oxidation ofpotassium permanganate and chlorine dioxide, coagulant aid Dimethyl DiallylAmmonium Chloride were used.Special tests were designed for stainless steel screen, non-wovenfabricsoxidant-injected deep-bed filtration to alleviate Synedra acus clogging. It wasrecommended that screen cloth of150mesh to retain more than60%diatom, with theflux loss less than20%after24hours’ operation. While the operation time prolonged 2-7hours in the filtration column when injecting hydrogen peroxide into the clogginglayer of the filter bed. The results in this thesis has significance to improve theoperational performance in dealing with the explosive growth of diatom in China.