摘要
随着人口的增加和城市化的加快,污泥作为城市污水处理的副产物,其排放量也相应增加。污泥的处理处置是令很多污水处理厂头痛的大事,其出路主要有填埋、焚烧等耗竭性处理及农业利用等资源化处理技术。由于环境压力、资源浪费的缘故,耗竭性处理技术逐渐减少或被禁止,而被认为最有潜力的污泥处理处置是对其进行土地利用处理,它不仅可以有效利用污泥中对植物有用的营养物质,使其重新参与生态系统的物质循环,而且费用低廉,消纳量也很大。无论是污泥直接经过简单处理施入农田,还是将污泥发酵作为有机原料,生产有机肥或有机无机复合肥,都不可避免地将对环境有害的重金属带入环境。因此需采取一定的控制措施将污泥中重金属予以分离除去。
在分离污泥重金属的化学、物理及生物学等诸多方法中,化学方法具有见效快、可操作性强等优点而被广泛使用。以前所选用的化学提取剂多集中在无机强酸、有机酸和螯合剂等单一物质上。对其复合去除效应则研究甚少,特别是利用表面活性剂来强化螯合剂去除污泥重金属的研究还未见报导。近来已陆续出现了利用表面活性剂与螯合剂的联合来修复土壤重金属污染的相关研究。根据土壤与污泥的相似性原理,可以利用螯合剂和表面活性剂来去除污泥中重金属。
本文首先评价污水处理系统中各种污泥重金属的安全性,并对化学方法去除污泥重金属的效果进行了评估,建立了化学方法的应用原则及评价指标体系。在此基础上,通过模拟试验,重点研究了螯合剂与表面活性剂单独或复合去除重金属的效果,分析了这几种化学物质对污泥重金属去除的主效应及复合效应,并考察了化学去除重金属对污泥氮素损失的影响和处理前后污泥重金属的形态变化,为城市污泥的农用资源化提供科学依据。
通过对重庆市唐家桥污水处理厂的生污泥、活性污泥、一次厌氧消化污泥、二次厌氧消化污泥中的重金属进行测试,并以国家标准GB4284—84为判定依据,采用单项评价与Nemerow指数综合评价方法对各类污泥的农用安全性进行了评价,发现污泥在污水处理流程中,其重金属有从生污泥、活性污泥、一次厌氧消化污泥、二次厌氧消化污泥逐渐富集浓缩的趋势。尽管各污泥中重金属含量均未超过农用标准,但占城市污泥绝大部分的二次消化污泥中的Cd已非常接近农用标准,其农用潜在风险加大,必须在污泥资源化前将其去除。
通过对现行分离去除污泥重金属的方法进行评估,发现化学去除污泥中重金属的方
法是可行的。优选污泥重金属化学去除的原则是最大限度地去除重金属,最大限度地保
全有用成分,最大限度地减少二次污染,最大限度地节约经济成本,以及最大限度方便
操作,据此可确定评价化学去除方法的指标体系是目的重金属去除率、植物养分损失率。
产水系数、投资系数与可操作系数。
模拟试验以上述4类污泥中产生量最大的二次厌氧消化污泥为材料,选择两种代表
性的重金属Cd(阳离子型赋存\Cr(酸根阴离子型赋存)为去除对象,考察资合剂柠
檬酸、EDTA及表面活性剂十二烷基磺酸钠(SDS)等三种化学物质对Cd、Cr的去除
效果。试验分A、B、C、D四组进行。A组为化学提取剂的单独淋滤试验,柠檬酸的
浓度设为 0*5,0二,0.2,0.3mol*,EDTA设为 0*25,0刀5,0.125,0.25mol刁,SDS
设为 0刀05,0刀1,0刀ZmolL”’,淋滤齐量设为 500ml kg”‘与 1000ml kg”l(即因波比分另
为2*与1:l);B组为化学提取剂的复合淋滤试验,分别固定柠檬酸、EDTA浓度为0.3
与 0.25mol-‘(A组中最佳浓度人然后分别与 0.005,0.of,0.02mol”’的 SDS进行复
合淋滤,在复合中又设混合淋滤(两种化学物质1:1混合液)和顺序淋滤(两种化学物
质先后淋滤人淋滤总剂量仍为 500ml kg’与 1000ml kg”’两个水平;C组为化学提取剂
的单独振荡试验,所用浓度与A组相同,振荡时间设0.5,1,2,4,sh;D组为化学
提取剂的复合振荡离心试验,分别固定柠檬酸、EDTA浓度为 0.3与 0.25mol”l仅组
中最佳浓度人然后分别与 0刀05,0.of,0刀Zmol LI的 SDS进行混合振荡或顺序振荡,
总的振荡时间仍设0.5,l,2,4,sh等水平,但顺序振荡中每种物质的振荡时间均减
半。
根据上述试验方案,淋滤试验采用内径为4cm、长为20cm的硬质塑料管,用尼龙
布固定底部,再加 Icm厚石英砂与一层尼龙布,装入” 风干的污泥,上面铺设尼龙
布及石英砂。装柱完后,以去离子水浸泡至污泥湿润,以 0.sml min”‘的流速连续淋滤,
测定淋出液Cd、Cr含量。振荡试验是将3g污泥样加人30ml提取液中,放入50ml离
心管恒温振荡至设定时间,然后以1500rpm离心smin,测定上清液Cd、Cr含量。上述
试验均重复2次。最后选取单独处理或复合处理中的效果较佳的刀个组合,测定去除
重金属后的氮素损失率,结果显示:
在化学物质单独处理中,总的趋势是螫合剂(柠檬酸、EDTA)要优于表面活性剂
SDS。在振荡中,Cd的最大去除率(64.769)要显著高于 Cr(57.25o),在淋滤中刚
好相反,Cd、Cr的最大去除率分别为10.82O和60.36O。柠檬酸与SDS复合,在淋滤
去除Cd、Cr?
Sludge, as by-product of sewage treatment plant, is discharged increasingly with population growth and urbanization. The fate of sludge includes exhausting and agro-utilization. The former such as landfilling, incineration are applied and forbidden decreasingly because of environmental pressure and resources waste. Oppositely, the agro-utilization is used increasingly, because that it not only effectively makes use of nutrients but also has low expenditure and large treatment amounts. Agro-utilization includes two methods, one is directly applying sludge to farmland after being simply treated, and the other is manufacturing organic or organic-inorganic complex fertilizers. These treatments have become problematic because heavy metals in sludge may enter into environment and potentially posing a threat, consequently, it is essential to remove heavy metals from sludge by means of physical, chemical and biological methods.
Chemical agents are being employed widely for its quickly effect and maneuverability. Many attentions have been paid to the leaching of chemical agents such as strong inorganic acids, organic acids and chelators in the past decades. However, there are little reports about combination agents, especially using surfactants to enhance remediation of chelators. The chelators-enhanced extraction of heavy metals from contaminated soils by surfactants has become increasingly interest. Considering the comparability of sludge to soil, it is feasible to extracting heavy metals from sludge by chelators and surfactants, too.
Firstly, the security of heavy metals was evaluated within four sorts of sludge, while the efficiency of removal of heavy metals by agents was assessed, and then the standards and the evaluation system of chemical method were suggested. Secondly, the simulated leaching and surging experiments were conducted to determine the removal of Cd, Cr from sludge by several agents solely or jointly. Finally, the nitrogen lost and forms of Cd, Cr were investigated contrastively among treated sludge, aiming to provide scientific basis for proper agro-utilization of sludge.
The raw sludge, activated sludge, I - anaerobically digested sludge and II - anaerobically digested sludge were sampled from the Tiangjiaqiao sewage plant in Chongqing. At the base of heavy metals' detection of several kinds of sludge, the security of sludge was assessed by
monomial index and Nemerow index according to GB4284-84. The result showed as follows: the heavy metals were enriched gradually in the order of II - anaerobically digested sludge > I -anaerobically digested sludge > activated sludge > raw sludge. Although the content of heavy metals was below the national standard, the cadmium in II- anaerobically digested sludge was nearly exceeding the standard. Therefore, it hampered agro-utilization of sludge, and it is essential to remove the heavy metals.
The compare of the way of removing heavy metals from the sludge showed that it is feasible to use agents. The aims of screening agents were described to remove heavy metals efficiently, to save useful matter, to lower cost, to operate easily. So the targets could be suggested as removal ratio of heavy metals, the lost ratio of nutrient, contemporary, wastewater amount, economic efficiency, and maneuverability index.
Citric acid (CTA), EDTA and sodium dodecyl sulfate (SDS) were used to extract cadmium(anionic) and chromium(anion) in II -anaerobically digested sludge. The experiments consisted of A, B, C, D groups. A group was leaching test by agents solely. Each sample was treated with three different extractors, namely, 0.05, 0.1,0.2 and 0.3mol L"1 CTA, 0.025, 0.05, 0.125 and 0.25mol L'1 EDTA, 0.005, 0.01 and 0.02 mol L'1 SDS. The leaching dose was arranged two levels (/ e, 500ml kg"1 and 1000ml kg"1); B group was was leaching test by agents jointly, in which 0.3 mol L"1 CTA and 0.25 mol I/'EDTA were mixed with 0.005, 0.1 and 0.2mol L"'SDS, respectively. The leaching way including mixing treatment (/ e, the volume rate of the two solution was 1:1) and the leachin
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