Preparation And Properties Of Roof Greening Substrate With Alum Sludge From Waterworks

The amount of sludge produced by water treatment plants in China is huge every year,and the main disposal method is still landfill.The treatment cost is high and the land resources are occupied,which is contrary to the concept of ecological civilization in the new era.Furthermore,the new type of urbanization strategy in china has resulted in a sharp increase in the urban population and a severe urban ecological environment.The gray roof area accounts for about one-fifth of the city area,which can be used to manage the rainwater and flood through the roof greening technology to promote the function of sponge city.Due to the large amount of substrate materials required for roof greening construction,alum sludge from waterworks which also urgently needs low-cost,harmless and resourceful disposal ways,could be used for the preparation of roof greening substrate.This win-win method is in line with the concept of"treating waste by waste and turning waste to treasure".This study was guided by the requirements of physicochemical properties of roof greening substrate and the concept of resource utilization.Herein,three kinds of materials were selected to prepare the planting substrate(alum sludge from waterworks,coconut chaff,and landscaping waste).Based on the comprehensive analysis of the physical and chemical properties of each material and through the orthogonal test design,the three materials were mixed according to a certain volume proportion to form an improved substrate.Nine indicators of the substrate sample were measured to assess the physical and chemical properties of the substrate(water saturation bulk density,non-capillary porosity,p H value,EC value),water permeability and holding performance(permeability coefficient,water absorption,water loss rate)and plant growth status(malonaldehyde content,total chlorophyll content).Then,a modular roof greening facility filled with 10cm deep substrate prepared with alum sludge(30%,volume fraction)was set up to investigate the functions of landscape,storage,purification and cooling,providing a reference for the construction of greening roof.The experimental results were listed as follows:(1)Physical and chemical properties of alum sludge from waterworksThe main inorganic elements of alum sludge are almost as same as that of natural soil except the content of Al is 3～4 times of that in natural soil.Due to hydrophilic clay,alum sludge has significant water swelling and water loss shrinkage characteristics.Under the condition of p H 7,the leaching of aluminum and heavy metals in sludge was very little,within the safety concentration range of relevant regulations,which can be regarded as a safe soil for resource utilization.Plants can take root and grow on the raw alum sludge,showing that aluminum and heavy metals have no toxic effect on plant growth.However,the sludge was dried and broken finally,with poor cohesiveness and not suitable for the plant growth.Therefore,In order to meet the requirements of roof greening specifications,alum sludge should be used with other light,breathable,water and fertilizer conservation materials such as peat,coconut chaff,humus and perlite etc.(2)The proportion of the roof greening substrate for plant growthThe improved substrate was prepared by three materials of alum sludge,coconut chaff and landscaping waste.The water saturation bulk density was less than 1300 kg/m~3,the non-capillary porosity was greater than 10%,p H value was about 7.3 and EC value was less than 2.0 m S/cm.This substrate met the requirements of roof greening specifications.Except for T4,T8,and T7 groups,where the substrate permeability coefficient exceeded 1.5mm/min,the other groups were in the optimal range of 0.7 to 1.5 mm/min.The average value of water absorption was 56.70 g/100cm~3.After 4 days,the average water loss rate was78.97%,which meant good water permeability and water retention.The average value of malondialdehyde and total chlorophyll contents of plant leaves were 1.36μmol/g and 0.516mg/L respectively,indicating the growth condition was good.Since the average precipitation in South China is generally 1200～2000mm,concentrated in April～October,with large intensity,the proportion of alum sludge could be increased to more than 50%.Comparatively,the average precipitation in North China is generally less than 800mm,mostly concentrated in June～September,with small intensity,the proportion of coconut bran could be enhanced to 30%～60%.For the plant fast growth,the proportion of organic matter such as garden greening waste can be increased(not more than 35%)to improve substrate nutrition.(3)Ecological effects of roof greening facilitiesThe substrate prepared by 33%alum sludge was used to fill the substrate layer of roof greening facilities,with a thickness of 10cm.Plant growth was in good condition and the coverage was over 70%for a long time.Through artificial rainfall simulation,in the downfall(43.70mm)and rainstorm(79.22mm),the rainwater holding rates were 78.26%and 40.67%,and the runoff lag times were 46.96min and 24.40min,respectively.The cooling efficiency was more than 20%.The removal rates of SS,COD and TP by the facility were between10.67%to 18.26%,-2.46%to 14.67%and 8.33%to 13.19%,respectively.In total,the facilities have significant effects on increasing urban greening rates,stagnating rainwater,and cooling.