Hydrolysis of polyaluminum chloride prior to coagulation: Effects on coagulation behavior and implications for improving coagulation performance

The effects of polyaluminum chloride (PACl) hydrolysis prior to coagulation on both the coagulation zone and coagulation performance of a kaolin suspension were investigated by a novel jar test named the “reversed coagulation test”. The tests showed that PACl hydrolysis prior to coagulation decreased the performance of charge neutralization coagulation in the case of short-time slow mixing (10 min; G = 15 sec ?1 ) and increased the optimal dosage for charge neutralization and sweep coagulation. Moreover, the hydrolysis time had insignificant effects on the size and zeta potential of PACl precipitates and the residual turbidity of the raw water. However, PACl hydrolysis prior to coagulation and the size of PACl precipitates had a negligible effect on the performance of sweep coagulation.The results imply that, in practice, preparing a PACl solution with deionized water, rather than tap water or the outlet water from a wastewater treatment unit, can significantly save PACl consumption and improve the performance of charge neutralization coagulation,while preparing the PACl solution with tap or outlet water would not affect the performance of sweep coagulation. In addition, the optimal rapid mixing intensity appears to be deter-mined by a balance between the degree of coagulant hydrolysis before contacting the primary particles and the average size of flocs in the rapid mixing period. These results provide new insights into the role of PACl hydrolysis and will be useful for improving coagulation efficiency.     

Effects of pre-ozonation on organic matter removal by coagulation with IPF-PACI

Ozone plays an important role as a disinfectant and oxidant in potable water treatment practice and is increasingly being used as a pre-oxidant before coagulation. The purpose of this study is to obtain insight into the mechanisms that are operative in pre-ozonized coagulation. Effects ofpre-ozonation on organic matter removal during coagulation with IPF-PAC1 were investigated by using PDA （photometric disperse analysis）, apparent molecular weight distribution and chemical fractionation. The dynamic formation of flocs during coagulation process was detected. Changes of aquatic organic matter （AOM） structure resulted from the influence of pre-ozonation were evaluated. Results show that dosage of O3 and characteristics of AOM are two of the major factors influencing the performance of O3 on coagulation. No significant coagulation-aid effect of O3 was observed for all experiments using either A1C13 or PAC1. On the contrary, with the application of pre-ozonation, the coagulation efficiency of A1C13 was significantly deteriorated, reflected by the retardation of floc formation, and the removal decreases of turbidity, DOC, and UV254. However, if PACl was used instead of AlCl3, the adverse effects of pre-ozonation were mitigated obviously, particularly when the O3 dosage was less than 0.69 （mg O3/mg TOC）. The difference between removals of UV254, and DOC indicated that pre-ozonation greatly changed the molecular structure of AOM, but its capability of mineralization was not remarkable. Only 5% or so DOC was removed by pre-ozonation at 0.6--0.8 mg/L alone. Fractionation results showed that the organic products of pre-ozonation exhibited lower molecular weight and more hydrophilicity, which impaired the removal of DOC in the following coagulation process.     

城市景观水体混凝除藻过程对氮元素协同去除特性初探

以城市景观水体净化为目标,考察混凝除藻同时去除氮元素的特性和影响因素. 结果发现,水体内较高的DOC(溶解性有机质)含量会阻碍藻浊度的去除. 聚合氯化铝(PACl)以及PACl与硫酸铝(Alum)的复合剂(PACl+Alum)对藻浊度的去除效果均相对单独使用PACl或Alum较好,其中PACl+Alum对其他物质(如氮、DOC等)的去除效果也相对单独使用PACl或Alum更佳;在同样的最佳投药量时,低pH条件能获得更好的藻浊度去除率;对氮元素去除的进一步研究发现,DON(溶解性有机氮)的去除特征和过程与TN(总氮)非常一致,表明该水体TN的去除主体上是通过DON的去除实现的;低pH条件同样有利于氮和DOC的去除;但DOC与DON去除的不同步性,说明DOC中的DON成分和性质是非均一性的,是未来可进一步深入研究的重要方向.      

Ultrafiltration with in-line coagulation for the removal of natural humic acid and membrane fouling mechanism

Experimental and theoretical analysis were made on the natural humic acid removal and the membrane fouling of ultrafiltration （UF） with in-line coagulation. The results showed dissolved organic carbon （DOC） and UV254 removals by the UF with in-line coagulation at pH 7 were increased from 28% to 53% and 40% to 78% in comparison with direct UF treatment respectively. At the same time, the analysis of high performance liquid chromatography showed that UF with coagulation had significant improvement of removal of humic acid with molecular weights less than 6000 Da in particular. Compared to direct UF, the in-line coagulation UF also kept more constant permeate flux and very slight increase oftransmembrane pressure during a filtration circle. Two typical membrane fouling models were used by inducing two coefficients Kc and Kp corresponding to cake filtration model and pore narrowing model respectively. It was found that membrane fouling by pore-narrowing effect was effectively alleviated and that by cake-filtration was much decreased by in-line coagulation. Under the condition of coagulation prior to ultrafiltration at pH 7, the cake layer formed on the membrane surface became thicker, but the membrane filtration resistance was lower than that at pH 5 with the extension of operation time.     

Removal natural organic matter in typical south-China source water during enhanced coagulation of a with IPF-PACl

Systematic investigation on enhancing removal of natural organic matter （NOM） using inorganic polymer flocculant （IPF）, polyaluminum chloride（PACI） and polyacrylamide（PAM） was performed in a typical south-China source water. Enhanced coagulation and applying polymer flocculant-aid were compared through jar tests and pilot tests. Raw water and settled water were characterized and fractionated by resin adsorption. The results show that DOC composes major part of TOC. The DOC distribution keeps relatively stable all around the year with typical high amounts of the hydrophilic matter around 50%. The distribution between HoB, HoA and HoN varies and undergoes fluctuation with the year round. During the summer season, the HoN becomes gradually the major part in hydrophobic parts. PACI with the species being tailor-made shows little pH effect during coagulation. The enhanced coagulation dosage for PACI could be 4.5 mg/L for the typical source water. The highest TOC removal achieved 31%. To be economically, 3 mg/L dose is the optimum dosage. Although hydrophilic fractions of NOM of both treatment strategies are removed about 30%, NOM causing UV254 absorbance were well removed（about 90%）. Hydrophobic bases and acids fractions are much more removed under enhanced conditions. The hydrophilic fraction could be better removed using PAM, the polymer coagulant aid.     

预压力混凝沉淀除藻工艺中DOC变化规律研究

为了探讨预压力混凝沉淀除藻工艺是否会使藻细胞破裂,导致水中溶解性有机物(DOC)增加,进而优化压力作用方式,得到安全、高效的预压力除藻工艺,实验研究了预压力、预氧化技术处理含藻水时DOC的变化,以及水中相对分子质量分布,并研究了混凝沉淀后藻类、浊度、DOC的去除效果.结果表明,在0.5~0.8 MPa压力作用后,水中DOC没有增加,反而减少,且有机物相对分子质量减小.而高锰酸钾与次氯酸钠预氧化均会导致DOC增加,相对分子质量分布没有明显变化.预压力混凝沉淀后藻类去除率96.23%;DOC去除率29.11%,高于预氧化混凝沉淀工艺10%~30%.     

Preferential binding between intracellular organic matters and Al13 polymer to enhance coagulation performance

Coagulation is the best available method for removing intracellular organic matter (IOM), which is released from algae cells and is an important precursor to disinfection by-products in drinking water treatment. To gain insight into the best strategy to optimize IOM removal, the coagulation performance of two Al salts, i.e., aluminum chloride (AlCl₃) and polyaluminum chloride (PACl, containing 81.2% Al₁₃), was investigated to illuminate the effect of Al species distribution on IOM removal. PACl showed better removal efficiency than AlCl₃ with regard to the removal of turbidity and dissolved organic carbon (DOC), owing to the higher charge neutralization effect and greater stability of pre-formed Al₁₃ species. High pressure size exclusion chromatography analysis indicated that the superiority of PACl in DOC removal could be ascribed to the higher binding affinity between Al₁₃ polymer and the low and medium molecular weight (MW) fractions of IOM. The results of differential log-transformed absorbance at 254 and 350?nm indicated more significant formation of complexes between AlCl₃ and IOM, which benefits the removal of tryptophan-like proteins thereafter. Additionally, PACl showed more significant superiority compared to AlCl₃ in the removal of < 5?kDa and hydrophilic fractions, which are widely viewed as the most difficult to remove by coagulation. This study provides insight into the interactions between Al species and IOM, and advances the optimization of coagulation for the removal of IOM in eutrophic water.     

Advanced treatment of municipal wastewater by nanofiltration: Operational optimization and membrane fouling analysis

Municipal sewage from an oxidation ditch was treated for reuse by nanofiltration (NF) in this study. The NF performance was optimized, and its fouling characteristics after different operational durations (i.e., 48 and 169 hr) were analyzed to investigate the applicability of nanofiltration for water reuse. The optimum performance was achieved when transmembrane pressure = 12 bar, pH = 4 and flow rate = 8 L/min using a GE membrane. The permeate water quality could satisfy the requirements of water reclamation for different uses and local standards for water reuse in Beijing. Flux decline in the fouling experiments could be divided into a rapid flux decline and a quasi-steady state. The boundary flux theory was used to predict the evolution of permeate flux. The expected operational duration based on the 169-hr experiment was 392.6 hr which is 175% longer than that of the 48-hr one. High molecular weight (MW) protein-like substances were suggested to be the dominant foulants after an extended period based on the MW distribution and the fluorescence characteristics. The analyses of infrared spectra and extracellular polymeric substances revealed that the roles of both humic- and polysaccharide-like substances were diminished, while that of protein-like substances were strengthened in the contribution of membrane fouling with time prolonged. Inorganic salts were found to have marginally influence on membrane fouling. Additionally, alkali washing was more efficient at removing organic foulants in the long term, and a combination of water flushing and alkali washing was appropriate for NF fouling control in municipal sewage treatment.     

Treatment of phosphate-containing oily wastewater by coagulation and microfiltration

The oily wastewater generated from pretreatment unit of electrocoating industry contains oils, phosphate, organic solvents, and surfactants. In order to improve the removal efficiencies of phosphate and oils, to mitigate the membrane fouling, coagulation for ceramic membrane microfiltration of oily wastewater was performed. The results of filtration tests show that the membrane fouling decreased and the permeate flux and quality increased with coagulation as pretreatment. At the coagulant Ca （OH）2 dosage of 900 mg/L, the removal efficiency of phosphate was increased from 46.4% without coagulation to 99.6%; the removal of COD and oils were 97.0% and 99.8%, respectively. And the permeate flux was about 70% greater than that when Ca（OH）2 was not used. The permeate obtained from coagulation and microfiltration can be reused as make-up water, and the recommended operation conditions for pilot and industrial application are transmembrane pressure of 0.10 MPa and cross-flow velocity of 5 m/s. The comparison results show that 0.2 μm ZrO2 microfilter with coagulation could be used to perform the filtration rather than conventional ultrafilter, with very substantial gain in flux and removal efficiency of phosphate.     

Towards a better hydraulic cleaning strategy for ultrafiltration membrane fouling by humic acid: Effect of backwash water composition

As a routine measurement to alleviate membrane fouling, hydraulic cleaning is of great significance for the steady operation of ultrafiltration (UF) systems in water treatment processes. In this work, a comparative study was performed to investigate the effects of the composition of backwash water on the hydraulic cleaning performance of UF membranes fouled by humic acid (HA). Various types of backwash water, including UF permeate, Milli-Q water, NaCl solution, CaCl₂ solution and HA solution, were compared in terms of hydraulically irreversible fouling index, total surface tension and residual HA. The results indicated that Milli-Q water backwash was superior to UF permeate backwash in cleaning HA-fouled membranes, and the backwash water containing Na⁺ or HA outperformed Milli-Q water in alleviating HA fouling. On the contrary, the presence of Ca^2 + in backwash water significantly decreased the backwash efficiency. Moreover, Ca^2 + played an important role in foulant removal, and the residual HA content closely related to the residual Ca^2 + content. Mechanism analysis suggested that the backwash process may involve fouling layer swelling, ion exchange, electric double layer release and competitive complexation. Ion exchange and competitive complexation played significant roles in the efficient hydraulic cleaning associated with Na⁺ and HA, respectively.     

Groundwater arsenic removal by coagulation using ferric(III) sulfate and polyferric sulfate: A comparative and mechanistic study

Elevated arsenic (As) in groundwater poses a great threat to human health. Coagulation using mono- and poly-Fe salts is becoming one of the most cost-effective processes for groundwater As removal. However, a limitation comes from insufficient understanding of the As removal mechanism from groundwater matrices in the coagulation process, which is critical for groundwater treatment and residual solid disposal. Here, we overcame this hurdle by utilizing microscopic techniques to explore molecular As surface complexes on the freshly formed Fe flocs and compared ferric(III) sulfate (FS) and polyferric sulfate (PFS) performance, and finally provided a practical solution in As-geogenic areas. FS and PFS exhibited a similar As removal efficiency in coagulation and coagulation/filtration in a two-bucket system using 5 mg/L Ca(ClO)₂. By using the two-bucket system combining coagulation and sand filtration, 500 L of As-safe water (< 10 μg/L) was achieved during five treatment cycles by washing the sand layer after each cycle. Fe k-edge X-ray absorption near-edge structure (XANES) and As k-edge extended X-ray absorption fine structure (EXAFS) analysis of the solid residue indicated that As formed a bidentate binuclear complex on ferrihydrite, with no observation of scorodite or poorly-crystalline ferric arsenate. Such a stable surface complex is beneficial for As immobilization in the solid residue, as confirmed by the achievement of much lower leachate As (0.9 μg/L–0.487 mg/L) than the US EPA regulatory limit (5 mg/L). Finally, PFS is superior to FS because of its lower dose, much lower solid residue, and lower cost for As-safe drinking water.     

粉炭强化循环造粒流化床去除天然有机物的研究

强化天然有机物去除是饮用水处理工艺的重要目标.循环造粒流化床作为一种新型高效固液分离技术具有处理负荷高、水质适应性强的特点,将其与粉末活性炭（PAC）联用,探究对水库水中有机物的强化去除效果.研究发现当聚合氯化铝（PACl）与PAC同时投加,且PACl、聚丙烯酰胺（PAM）及PAC投加量分别为24mg/L、1.2mg/L、30mg/L时,流化床系统对进水浊度、UV₂₅₄、COD_Mn、DOC去除率分别为96.5%、72.0%、71.7%、61.0%.对进出水中的有机物进行三维荧光分析,发现流化床系统中结团造粒作用下出水的类富里酸和类腐殖酸的FI分别比进水降低了40.1%和43.0%;加入30mg/L PAC后,出水的类富里酸和类腐殖酸的的FI分别比原水降低了54.0%和55.3%.水库水中的有机物分子量主要在 < 1kDa和10~30kDa两个区间,分别占总有机物含量的37.1%和39.1%.循环造粒流化床系统中结团造粒作用主要去除10~30kDa区间的有机物,PAC吸附作用对 < 1kDa的有机物有很好的去除.     

Elevated CO2 facilitates C and N accumulation in a rice paddy ecosystem

Elevated CO₂ can stimulate wetland carbon (C) and nitrogen (N) exports through gaseous and dissolved pathways, however, the consequent influences on the C and N pools are still not fully known. Therefore, we set up a free-air CO₂ enrichment experiment in a paddy field in Eastern China. After five year fumigation, we studied C and N in the plant–water–soil system. The results showed: (1) elevated CO₂ stimulated rice aboveground biomass and N accumulations by 19.1% and 12.5%, respectively. (2) Elevated CO₂ significantly increased paddy soil TOC and TN contents by 12.5% and 15.5%, respectively in the 0–15 cm layer, and 22.7% and 26.0% in the 15–30 cm soil layer. (3) Averaged across the rice growing period, elevated CO₂ greatly increased TOC and TN contents in the surface water by 7.6% and 11.4%, respectively. (4) The TOC/TN ratio and natural δ¹⁵N value in the surface soil showed a decreasing trend under elevated CO₂. The above results indicate that elevated CO₂ can benefit C and N accumulation in paddy fields. Given the similarity between the paddies and natural wetlands, our results also suggest a great potential for long-term C and N accumulation in natural wetlands under future climate patterns.     

Performance and fouling mechanism of direct contact membrane distillation (DCMD) treating fermentation wastewater with high organic concentrations

In this study,direct contact membrane distillation(DCMD)was used for treating fermentation wastewater with high organic concentrations.DCMD performance characteristics including permeate flux,permeate water quality as well as membrane fouling were investigated systematically.Experimental results showed that,after 12 hr DCMD,the feed wastewater was concentrated by about a factor of 3.7 on a volumetric basis,with the permeate flux decreasing from the initial 8.7 L/m~2/hr to the final 4.3 L/m~2/hr due to membrane fouling;the protein concentration in the feed wastewater was increased by about 3.5 times and achieved a value of 6178 mg/L,which is suitable for reutilization.Although COD and TOC in permeate water increased continuously due to the transfer of volatile components from wastewater,organic rejection of over 95%was achieved in wastewater.GC–MS results suggested that the fermentation wastewater contained 128kinds of organics,in which 14 organics dominated.After 12 hr DCMD,not only volatile organics including trimethyl pyrazine,2-acetyl pyrrole,phenethyl alcohol and phenylacetic acid,but also non-volatile dibutyl phthalate was detected in permeate water due to membrane wetting.FT-IR and SEM–EDS results indicated that the deposits formed on the membrane inner surface mainly consisted of Ca,Mg,and amine,carboxylic acid and aromatic groups.The fouled membrane could be recovered,as most of the deposits could be removed using a HCl/Na OH chemical cleaning method.     

混凝-超滤联用技术制备自来水的试验研究

采用混凝+超滤联用技术以苏州太湖水为原水进行制备自来水的实验。结果表明,投加混凝剂能够有效降低中空纤维膜表面的污染,采用气压水反洗技术系统跨膜压差能够有效恢复,常规化学清洗能有效清除膜表面的污染物,长时间运行后膜丝完整性良好,无"断丝"现象。混凝+超滤联用能有效去除原水中的各种污染物,浊度去除率99.5%以上,TOC去除率45%～50%,UV254去除率43%～45%,总铁去除率90%以上,水的利用率在87%左右。产水水质达到或优于《生活饮用水卫生标准》GB5749-2005。     

光催化/活性炭/纳滤组合工艺处理二级出水及对膜污染的控制

污水的资源化与回用可作为城市和各类建筑的非传统水源,必须满足相关水质标准,其中有机物一直是广泛关注的重要指标.本文采用华北某市大型污水厂的二级出水,研究了二氧化钛光催化氧化(UV-TiO_2)和颗粒活性炭(GAC)技术对溶解性有机物的去除特性以及对纳滤(NF)的膜污染控制特性,构建了UV-TiO_2/GAC/NF组合工艺,评价了组合工艺的有机物去除效能和膜污染控制作用,解析了影响NF膜污染程度的有机物种类和相对分子质量分布特征.结果表明,UV-TiO_2和GAC技术均可不同程度地缓解NF膜的不可逆污染、降低膜通量衰减速率; UV-TiO_2/GAC联用的不可逆膜污染程度比单独UV-TiO_2或GAC技术分别降低了48. 7%或61. 4%.类蛋白质和类腐殖质等有机污染物是不可逆膜污染的主要组分;相对分子质量为小于3×10~3和30×10~3～100×10~3的有机物是造成纳滤膜通量下降的主要因素. UV-TiO_2/GAC/NF组合工艺的COD、DOC和UV_(254)去除率为45. 7%、74. 5%和89. 2%,出水中COD、DOC和UV_(254)等有机物指标均得到有效去除.研究成果为城市和各类建筑非传统水源的深度净化与多途径回用提供了技术支持.     

Cu2+在混凝中对富里酸氯化消毒副产物的影响

混凝实验采用硫酸铝作为混凝剂,以富里酸（FA）为目标物,考察Cu²⁺对浊度、溶解性有机碳（DOC）去除效果和氯化消毒副产物生成势（DBPsFP）的影响,并采用三维荧光光谱（3DEEM）和超滤膜分级对混凝出水进行表征.结果表明,Cu²⁺可以通过络合反应提高混凝过程对FA中亲水性组分和芳香结构DBPs前驱体的去除效果.在pH> 7时,可以使二氯乙酸生成势（DCAAFP）和三氯甲烷生成势（TCMFP）明显降低.Cu²⁺-混凝的作用更倾向于影响FA中分子量级分处于10~30kDa的物质,提高其对DOC和DBPs生成势的去除效果.     

Fatty acid fouling of forward osmosis membrane: Effects of pH, calcium, membrane orientation, initial permeate flux and foulant composition

Octanoic acid (OA) was selected to represent fatty acids in effluent organic matter (EOM). The effects of feed solution (FS) properties, membrane orientation and initial permeate flux on OA fouling in forward osmosis (FO) were investigated. The undissociated OA formed a cake layer quickly and caused the water flux to decline significantly in the initial 0.5 hr at unadjusted pH 3.56; while the fully dissociated OA behaved as an anionic surfactant and promoted the water permeation at an elevated pH of 9.00. Moreover, except at the initial stage, the sudden decline of water flux (meaning the occurrence of severe membrane fouling) occurred in two conditions: 1. 0.5 mmol/L Ca^2 +, active layer facing draw solution (AL-DS) and 1.5 mol/L NaCl (DS); 2. No Ca^2 +, active layer-facing FS (AL-FS) and 4 mol/L NaCl (DS). This demonstrated that cake layer compaction or pore blocking occurred only when enough foulants were absorbed into the membrane surface, and the water permeation was high enough to compact the deposit inside the porous substrate. Furthermore, bovine serum albumin (BSA) was selected as a co-foulant. The water flux of both co-foulants was between the fluxes obtained separately for the two foulants at pH 3.56, and larger than the two values at pH 9.00. This manifested that, at pH 3.56, BSA alleviated the effect of the cake layer caused by OA, and OA enhanced BSA fouling simultaneously; while at pH 9.00, the mutual effects of OA and BSA eased the membrane fouling.     

强化混凝去除微污染湖泊水浊度及TOC的研究

通过烧杯混凝试验和动态连续混凝试验,就混凝剂种类、投药量、pH值、水温等因素,研究强化混凝对水中浊度和TOC去除效果的影响。试验结果表明:PAC最佳投量为30mg/L,浊度去除率为90.19%,TOC去除率为38.2%,水温高于20℃时,聚合氯化铝对浊度和TOC去除率分别高达84.95%、33.18%以上。虽然pH值是影响有机物去除的主要参数,动态连续混凝试验表明,不改变pH值,强化混凝工艺依然能极大地改善出水水质,滤后水质浊度为0.1NTU,TOC为6.23mg/L。     

Application of a green coagulant with PACl in efficient purification of turbid water and its mechanism study

The applications of natural polymeric flocculants due to their green feature has been recently received much more attention. In this work, the combined usages of a cationic starch-based coagulant and polyaluminum chloride (PACl) were extensively evaluated for various addition sequences in the coagulation of both raw (surface water from the Jiuxiang River) and synthetic turbid water (two kaolin suspensions with different initial turbidities). Two typical cationic starch-based coagulants with different structures (St-G and St-E) were tried. In comparison to St-G, St-E and PACl used individually as well as St-G and St-E dosed after PACl, the combination of the starch-based coagulants fed before PACl showed higher turbidity removal efficiency, which featured not only less optimal doses of both inorganic and organic coagulants but also lower residual turbidity. On the basis of a detailed analysis of the particle size and its distribution in solution supernatants before and after coagulation by two starch-based coagulants and PACl, polymeric coagulants preferentially coagulate the small-sized colloids due to their distinct long-chain structures, but PACl preferentially coagulates the medium-sized ones. Thus, the medium-sized particles that were previously formed by the starch-based coagulants would be collectively and effectively removed by the subsequent addition of PACl. The addition sequence of the inorganic and organic coagulants in their combined usage is an important factor for improvement of the turbidity removal efficiency in practice.