给水厂铝污泥作为屋顶绿化基质应用的关键问题分析

SEVERAL KEY ISSUES ON USING ALUM SLUDGE FROM WATERWORKS AS SUBSTRATE IN ROOF GREENING FACILITIES

屋顶绿化(roof greening)作为“海绵城市”生物滞留设施之一,是景观好、效果佳的城市雨水管控措施。使用给水厂铝污泥配制屋顶绿化基质,可缓解污泥填埋的用地压力,取得污泥资源化和减量化效果,进而阐明此类生物滞留设施应用中的主要关键问题:1)铝污泥是否存在铝和重金属的浸出;2)铝污泥是否能直接用作种植基质;3)铝污泥应如何处理并搭配其他基材制成轻型改良基质;4)以铝污泥改良基质制成的屋顶绿化设施对雨水的滞蓄净化效果。结果表明:在土壤pH约为7的景观应用上,铝污泥中铝和重金属的浸出量极少,对环境构成威胁甚微;铝污泥初期含水率为80%,性状不稳定,不宜直接作为屋顶绿化设施的种植基质使用,需干燥至60%以下;为满足屋顶绿化规范要求,还应搭配其他轻质、保水、保肥的基材(如椰糠、泥炭、腐殖土等)使用;在铝污泥(40%,体积分数,下同)、椰糠(35%)、珍珠岩(25%)的改良基质中栽种地被植物,植物生根率为100%,覆盖率长期维持75%以上,生长状况良好;在中雨标准(20 mm/h)和暴雨标准(45 mm/h)的降雨模拟中,设施雨水持蓄率分别达到89.5%和37.1%,对各类污染物有一定的去除效果,更弥补了传统生物滞留设施对磷去除效果较差的缺点。

As one of the bioretention facilities in sponge city,roof greening is an effective measure for urban rainwater management and control.Alum sludge from waterworks reused as one of components of green roof substrate can relieve the pressure of sludge landfill and improve sludge reduction and utilization.Through the study of alum sludge as planting substrate for roof greening,this paper aimed to clarify the key issues in the application of such bioretention facilities:1)whether leaching of aluminum and heavy metals took place in alum sludge;2)whether alum sludge could be directly used as planting substrates;3)whether alum sludge should be treated and mixed with other substrates to make light modified substrates;4)the effect of roof greening facilities made of alum sludge on rainwater storage and purification.The results showed that the leaching amount of Al and heavy metals in alum sludge was very small in landscape application with soil pH of about 7,hardly posed a threat to the environment.The initial water content of alum sludge was 80%,and its character was unstable.Therefore,it was not suitable to be used directly as the planting substrates of roof greening facility,which needed to be dried with water content of 60%below.In order to meet the requirements of roof greening specifications,other light,water-retaining and fertilizer-conserving materials such as coconut bran,peat and humus soil,etc,should be added.The mixture with the formula(volume ratio)of alum sludge(40%),coconut chaff(35%)and perlite(25%)was selected as the substrate for ground cover plant.The results revealed that this substrate was efficient for the plant growth,with the rooting rate of 100%and a coverage rate of 75%above for long time.The stormwater retention rate of the roof greening facility reached 89.5%and 37.1%in the simulated moderate rain(20 mm/h)and heavy rain(45 mm/h)respectively.The facility had a certain removal capacity on various pollutants,especially enhanced the phosphorus removal,comparing with traditional bioretention facilities.