Mechanical performance and capillary water absorption of sewage sludge ash concrete (SSAC)

Disposal of sewage sludge from waste water treatment plants is a serious environmental problem of increasing magnitude. Waste water treatment generates as much as 70 g of dry solids per capita per day. Although one of the disposal solutions for this waste is through incineration, still almost 30% of sludge solids remain as ash. This paper presents results related to reuse of sewage sludge ash in concrete. The sludge was characterised for chemical composition (X-ray flourescence analysis), crystalline phases (X-ray diffraction analysis) and pozzolanic activity. The effects of incineration on crystal phases of the dry sludge were investigated. Two water/cement (W/C) ratios (0.55 and 0.45) and three sludge ash percentages (5%, 10% and 20%) per cement mass were used as filler. The mechanical performance of sewage sludge ash concrete (SSAC) at different curing ages (3, 7, 28 and 90 days) was assessed by means of mechanical tests and capillary water absorption. Results show that sewage sludge ash leads to a reduction in density and mechanical strength and to an increase in capillary water absorption. Results also show that SSAC with 20% of sewage sludge ash and W/C = 0.45 has a 28 day compressive strength of almost 30 MPa. SSAC with a sludge ash contents of 5% and 10% has the same capillary water absorption coefficient as the control concrete; as for the concrete mixtures with 20% sludge ash content, the capillary water absorption is higher but in line with C20/25 strength class concretes performance.     

Characterization of MSWI fly ash through mineralogy and water extraction

This paper investigates the mineralogical characteristics of fresh, aged and hot water extracted MSWI fly ash for providing the baseline information of minerals stability which controls the released heavy metals into the environment. Quantitative determination of bulk phase abundance in the fresh fly ash by the XRD Rietveld refinement method provided composition levels for amorphous and crystalline phases such as potassium tetrachlorozincate (K₂ZnCl₄), gehlenite, halite, quartz, anhydrite, and feldspar. The minerals association in the fly ash is clearly unstable and subject to mineralogical reactions. The phases of K₂ZnCl₄, halite and anhydrite in the fresh fly ash were involved in hydration and dissolution/precipitation processes to form new minerals such as the Zn-bearing mineral gordaite, syngenite, gypsum and hydrocalumite. The solubility-controlling phases and extractability of heavy metals were examined in a Soxhlet hot water-extractor. Here the soluble salts were simply removed from fly ash while Ca-, Al-, Si- and SO₄²⁻-bearing hydrate minerals were precipitated from the extraction solution. Furthermore, a low release of heavy metals Zn, Pb and Cd in hot water was noticed, indicating a strong retention of the trace metals in the mineral phases remaining in the insoluble fly ash residues.     

A quantitative estimate of potential aluminium recovery from incineration bottom ashes

The recovery of ferrous and non ferrous metals from the bottom ashes is a common practice in the most part of Europe, both for the environmental advantages of their recycle and to avoid problems of swelling and expansion that metals can cause when bottom ashes are reused in concrete production or in road construction. This paper focuses on metal recovery from bottom ashes produced in Municipal Solid Waste (MSW) incinerators, with a particular focus on aluminium. A forecasting model was developed in order to evaluate the quantity of aluminium scraps recoverable from the bottom ashes. The model was applied to the Italian situation but its validity can be extended to other countries. Focusing on Italy, by applying conventional technologies for the separation of non-ferrous metals, the amount of aluminium potentially recoverable from bottom ashes is estimated in the range from 16,500 to 21,000 tonnes at the year 2015, and from 19,000 to 28,500 tonnes at 2020.     

Balancing durability and environmental impact in concrete combining low-grade recycled aggregates and mineral admixtures

This paper investigated the effect of various amounts of low-grade recycled aggregates in concrete containing mineral admixtures at three different water-binder ratios on mechanical and environmental performance. The balance between durability and environmental impact for a given strength level similar to normal-use concrete was also examined using analytic hierarchy process. Results showed that increasing the water-binder ratio and volume of recycled aggregates reduced compressive strength and increased air permeability and drying shrinkage relative to normal aggregates. However, compared to normal-use concrete similar or better performance could be achieved, which was attributed to improvement of low-grade recycled aggregate performance when combined with fly ash. Similarly, CO₂ emissions and volume of raw materials were lower than the normal-use concrete for all mixes. Concrete mixes with low air permeability and low CO₂ footprint had the best balance of durability and environmental impact, as decreasing raw material volume tended to more greatly reduce durability.     

基于二次铝灰的地聚反应稳固化垃圾飞灰

基于工业生产铝过程中回收的二次铝灰（SAD）的地质聚合反应,提出了一种稳固化城市生活垃圾焚烧飞灰（MSWIFA）的新方法,分析硅铝物质的量之比对飞灰中重金属浸出浓度及地聚物固化体力学性能的影响规律.结果表明,当硅铝物质的量之比小于2.5时,二次铝灰-SiO₂基固化体与偏高岭土-SiO₂基固化体中的重金属浸出浓度均随着硅铝物质的量之比的增加而逐渐降低,2种固化体的抗压强度随着硅铝物质的量之比的增加而增加;硅铝物质的量之比达到2.5时,重金属的浸出浓度与固化体的抗压强度均趋于稳定.XRD分析结果显示,偏高岭土-SiO₂基固化体中聚合物的种类与数量均略高于二次铝灰-SiO₂基固化体.但从重金属的浸出浓度与固化体的抗压强度来看,2类固化体对飞灰中重金属的稳固化效果的差别很小,二次铝灰加上部分硅基材料可以作为偏高岭土的替代品,用于稳固化飞灰重金属的地质聚合反应中.二次铝灰-SiO₂基固化体的抗压强度达到13.65MPa,具备一定的力学性能,可用于部分特定的建筑材料.     

Flow analysis of major and trace elements in residues from large-scale sewage sludge incineration

Increase of sewage sludge (SS) has led to the construction of more incineration plants, exacerbating to the production of SS incineration residues. However, few studies have considered the mass balance of elements in large-scale SS incineration plants, affecting the residues treatment and utilization. In this study, flow analysis was conducted for major and trace elements in the SS, the fly ash (sewage sludge ash, SSA) and bottom ash from two large-scale SS incineration plants. The elemental characteristics were compared with those of coal fly ash (CFA), and air pollution control residues from municipal solid waste incineration (MSWIA), as well as related criteria. The results showed that the most abundant major element in SSA was Si, ranging from 120 to 240 g/kg, followed by Al (76–348 g/kg), Ca (26–113 g/kg), Fe (35–80 g/kg), and P (26–104 g/kg), and the trace elements were mainly Zn, Ba, Cu, and Mn. Not all the major elements were derived from SS. Most trace elements in the SS incineration residues accounted for 82.4%–127% of those from SS, indicating that SS was the main source of trace elements. The partitioning of heavy metals in the SS incineration residues showed that electrostatic precipitator ash or cyclone ash with high production rates were the major pollutant sinks. The differences in some major and trace elements could be indicators to differentiate SSA from CFA and MSWIA. Compared with related land criteria, the pollutants in SSA should not be ignored during disposal and utilization.     

粉煤灰和活性污泥对好氧填埋渗滤液组分的影响

通过在好氧反应器中添加粉煤灰和活性污泥来加速固体废物的降解。实验结果表明,添加粉煤灰不但改善了氧气扩散,而且增强了反应器对各种污染物的去除能力,对渗滤液中COD、NH4+、NO3-的去除率分别达到75%、68%和65%以上;添加活性污泥引入了一定量的微生物,也有利于固体废物的降解,渗滤液中COD、NH4+、NO3-的去除率分别达到了90%、65%和77%。因此,通过添加粉煤灰和活性污泥来加速固体废物降解是可行的。     

粉煤灰合成NaA型沸石对重金属离子的吸附动力学

以粉煤灰为原料,采用两步法合成了单一沸石矿物种的NaA型沸石,对合成产物的结构和性能进行了表征.通过静态吸附实验,研究了NaA型沸石对水溶液中Cu(ΙΙ)、Cr(Ⅵ)和Zn(ΙΙ)离子的吸附特性,从动力学角度探讨了吸附机理.结果表明,在所研究的浓度和pH值条件下,NaA型沸石对3种重金属的吸附符合Langmuir等温吸附方程,静态饱和吸附量(Qm)分别为82.30,65.96,47.78mg/g;3种金属离子在水溶液中的存在方式和大小是影响它们吸附行为的主要因素,通过动边界模型推算表明:NaA型沸石对Cu(ΙΙ)和Zn(ΙΙ)离子的吸附过程的速度控制步骤为液膜扩散,对Cr(Ⅵ)离子的吸附过程的速度控制步骤为颗粒扩散,3种金属离子的吸附过程符合伪二级方程.     

Formation characteristics of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and coplanar PCBs in fly ash from a gasification–melting process of municipal solid waste

PCDDs/DF and Co-PCB (dioxin) formations were studied with ash from a newly developed gasification and melting process for municipal solid waste. Ash samples were heated in a laboratory-scale fixed-bed reactor. Emphasis was placed on the effects of the type and composition of ash, temperatures, gas residence time, and gaseous organic precursors. Investigations using macroscopic and homologue distribution analyses led to the following conclusion. The ash from the gasification–melting process had the ability to generate dioxins in flue gas. A possible carbon source is unburned carbon in the ash samples, although this was very low (less than 0.01%). An experimental result that the level of dioxins generated from preheated fly ash obtained from a conventional incinerator was much lower than that from nonheated fly ash supported this conclusion. Dioxin concentrations obviously showed temperature dependence and peaked at 350°C. Dioxins formed in a gasification–melting process ash were readily desorbed from the surface, probably because of the low carbon content of the ash. There was no experimental evidence that gaseous organic precursors fed to the reactor generated dioxins. Therefore, an organic precursor was not essential for the formation of dioxins. A good linear relationship obtained between PCDDs/DFs and gas residence time also supported the assumption. Received: February 14, 2000 / Accepted: June 30, 2000     

酸浸粉煤灰混凝法处理制革废水的研究

笔者研究了在热电厂粉煤灰中加入少量的鼓风炉铁泥和适量的助溶剂HS,在加热条件下用稀硫酸搅拌浸取2h后,制得集物理吸附和化学混凝为一体的混凝剂。这种混凝剂与聚硅酸铝铁(PSAF)絮凝剂配合处理COD为1500～2000mg/L制革废水,SS,COD,硫化物和铬的去除率分别为95%,93%,92%和88%。该法的显著特点是混凝沉降速度快,污泥体积小,处理废水费用低,此外还探讨了酸浸粉煤灰混凝剂对制革废水的混凝沉降机理。