| The water from the rainwater collection cellar is the main source of drinking water in the northwestern villages of China.However,the water collection surface of the water cellar is vulnerable to environmental pollution on the ground.The water turbidity,color,and total bacterial colony of the rainwater collection cellar do not meet the sanitary standards for drinking water in China,which affects people’s health.The traditional water treatment process has the drawbacks of complicated operation,high energy consumption,and frequent cleaning and maintenance,resulting in difficulty widely used in rural areas of northwestern China.Gravity Driven Membrane system（GDM）is driven by the gravity of the water body itself and can operate for a long time without power,which has the advantages of low maintenance,low energy consumption,and easy operation.At present,the membrane materials of the existing GDM devices are mainly the flat membrane.But the membrane area is limited,the effluent flux is generally low,and it can’t guarantee the daily demand for drinking water.Therefore,in the process of GDM system operation,the GDM device must be optimally designed to obtain high effluent flux and take into account the other factors of the effluent quality requirements,the system cost,and the operating energy consumption.In this thesis,the influence of membrane material and membrane module design on the operation efficiency of GDM were studied.The membrane module with the best water purification effect was screened out.Furthermore,the flux change characteristics and water purification effect were investigated in long-term operation without cleaning and maintenance.Finally,the key influencing factors of effluent flux and the recovery mechanism of GDM operation efficiency by different cleaning methods were revealed.This study aimed to provide theoretical guidance and technical support for a long-term stable operation and maintenance mechanism of GDM and its application in rural areas.The main research contents and conclusions are as follows:（1）The cellar water quality was generally poor and not suitable for direct drinking via the field investigation and testing analysis of the cellar water in the northwestern rural areas.Turbidity,color,and microorganisms were the main characteristic pollutants in the cellar water in this area.（2）The influence of the ultrafiltration membrane characteristics on the operation effect of GDM system was tested from the viewpoint of membrane type and membrane material.The results showed that the water flux of GDM system has no difference in composed of different types and different materials of membrane filaments.Furthermore,the water quality of the effluent was in line with China’s drinking water quality standards.The hollow fiber membrane had better hydrophilicity compared with the flat membrane,which had a greater advantage in preventing irreversible pollution.After a comprehensive comparison,polyvinylidene fluoride and hollow fiber membrane were selected as the optimal membrane material and type for water purification,respectively.（3）The effect of membrane module design on the operational effectiveness of GDM systems was studied.The results showed that the filling density determines the effective filtration area of the membrane module,and a suitable filling density could help to improve the permeate flux and alleviate membrane contamination.The larger membrane pore size caused the larger membrane flux,but it was also easy to cause membrane pore blockage.The length of membrane filament was related to the critical flux.The longer the membrane filament,the more uneven the local flux distribution.By comparing the flux variation characteristics and pollutant removal effects of membrane modules with different design parameters,the optimal filling density of membrane modules was selected as 150 m~2/m~3,the suitable length as 120 mm,and the membrane pore diameter as 30 nm.（4）The flux variation characteristics and pollutant removal effectiveness of GDM under long-term operating conditions were studied.The screened membrane modules were operated continuously for 240 days,and the results showed that the flux stabilization phenomenon of GDM device was common.The long-term operation of GDM device could be divided into three stages:start-up period（0～8 d）,stable period（9～150 d）,and decline period（151～240 d）.The membrane flux could reach the steady state within 10 days,and the maximum steady operation time could reach 150 days with the average flux 8.85±0.74 LMH without cleaning.During the stable period,the effluent quality of GDM operation was good,and the average values of turbidity,color,TOC,and total bacterial colony in the effluent after purification were 0.14 NTU,7 Pt-Co,1.03 mg·L-1,and 66 CFU·m L-1,which all reached China’s Sanitary Standard for Drinking Water（GB5749-2006）.During the decline period,the membrane flux decreased to5.37 LMH,and the total bacterial colony in the GDM effluent exceeded the standard.Therefore,to restore the membrane flux and stabilize the water purification effect,it was recommended to clean and maintain the GDM after operation 150 days.（5）The effect of water quality factors and morphological characteristics of the filter cake layer on the effluent flux during the long-term operation of GDM were studied.The results showed that the influent water quality factors,including DO,p H,TOC,and total bacterial colony,were found as the key factors affecting the stable flux which showed different patterns in both the stable and decline periods.For dissolved oxygen,the effluent flux showed a positively and negatively correlated during the stable period and the decline period,respectively.The effluent flux was negatively correlated with TOC during the stable period and positively correlated during the decline period.For p H and total bacterial colony,the fluxes in the stable and decline periods decreased significantly with increasing p H and increased with the number of influent microorganisms.With the increase in operation time,the microbial community structure in the filter cake layer changed significantly,and Aspergillus and Bacillus became the dominant populations in the system.The filter cake layer was more likely to form a porous non-homogeneous structure,which contributed to the increase of flux.（6）The recovery rate of membrane flux by different cleaning methods after long-term operation of GDM and the operational efficiency of membrane modules after cleaning were studied.The results showed that different cleaning methods contributed to the recovery of membrane flux.The fluxes of membrane modules after surface cleaning,backwashing,and chemical cleaning were 13.1,32.1,and 36.9 LMH,resulting in about 32.3%,80.0%,and 91.0%of the initial membrane flux,respectively.The effluent flux level and water purification effect of the GDM system would be significantly improved after the membrane modules were cleaned.Therefore,according to the operation mode and operation time,the cleaning method of GDM could be flexibly selected and matched to achieve the purpose of restoring the effluent flux. |
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