The role of compactibility in liquid-solid separation of wastewater sludges.

The dewaterability of sludges is generally evaluated by the measurement of specific resistance to filtration (SRF) and capillary suction time (CST). It is generally, but not truly, agreed that the lower figures of these parameters indicate the easiness of sludge dewatering. The biological sludges which have the lower particle size and EPS contents may be characterised as hard-to-filter sludges based on SRF and CST measures. However, cake solids concentration of such a kind of sludge can be unexpectedly higher when the centrifugation is used as the dewatering mechanism. This study introduces compactibility, which is cake solids concentration of sludges after centrifugation as a new measure of dewaterability of hard-to-filter sludges. The results of this study claim that although the filterability indexes (CST and SRF) are moderately high for hard-to-filter sludges, their compactibility might be high indicating particle packing characteristics.     

The role of carbohydrate and protein parts of extracellular polymeric substances on the dewaterability of biological sludges.

There are many factors affecting the biological sludge dewaterability such as particle size distribution, floc structure, extracellular polymeric substances (EPS), etc. In this research, the role of the protein and carbohydrate parts of EPS (EPS(carbohydrate), and EPS(protein)) on the dewaterability of biological sludges was investigated. The sludge EPS composition was altered by feeding the sludges of same origin, in different reactors, with synthetic media having carbon to nitrogen (C/N) ratios of 8, 19 and 30 (in terms of COD/NH3-N), respectively. EPS in sludge samples were extracted by a cation exchange resin (CER). The characteristics of EPS were investigated by analytical methods and by using FT-IR spectroscopy. The dewaterability of the sludges was determined in terms of filterability and compactibility. Filterability, as filterability constant (X), and compactibility, as cake solids concentration, of sludges were determined by using the capillary suction time (CST) test and the centrifugation, respectively. The floc structure of sludge samples was also observed microscopically. Filterability and compactibility of the sludge samples were improved considerably with the increasing carbohydrate part and the decreasing protein part of the sludge EPS. EPS(protein) was inversely related to the cake solids concentration, which might be explained by the water holding capacity of EPS(protein). Filterability and compactibility of sludges improved by the increase of the size and strength of the flocs.     

High-solids centrifuge is a boon and a curse for managing anaerobically digested biosolids.

High-solids centrifugation can reduce the cost of managing or disposing of anaerobically digested biosolids. High-solids centrifuges can increase relative cake solids by as much as 5% DS compared with other dewatering devices, such as belt filter presses, with a resulting 15-20% reduction in overall mass of hauled biosolids. Cost reductions can be similar (15-20%) or more, depending on the type of disposal or management involved. For example, the additional removal of water from the cake increases the energy content in the biosolids, thereby facilitating incineration or heat drying processes. For land application, the benefits are more mixed. As explained in this paper, increases in biosolids odours associated with high-solids centrifuges may increase digestion requirements and may compel producers to transport biosolids to more remote, distant sites, potentially increasing transportation costs. High-solids centrifuges shear anaerobically digested biosolids. The shear results in a net increase in labile protein, an odour precursor. Additionally, high-solids centrifugation also results in the inhibition of methanogenesis, a major mechanism for degradation of organosulphur odours. Therefore, the risks and benefits should both be weighed when considering high-solids centrifuges for land application of anaerobically digested biosolids.     

长沙市第八水厂排泥水脱水工艺设计

长沙市第八水厂排泥水处理工程包括第八水厂和第三水厂(总规模80万m3/d)的排泥水处理,处理流程为排泥→浓缩→贮泥→脱水。其脱水工艺具有不加药,处理后泥饼含固率≥45%,卸泥方便,自动化程度高等特点。介绍了不加药脱水系统主要处理单元设计、设备选型、运行步骤、工艺特点等。     

给水厂污泥脱水技术改造设计方案研究

针对以黄河水为原水的给水厂污泥进行了混凝及优选比阻调理试验,结果表明:当进行给水厂污泥脱水技术改造方案设计时,可以不经过混凝处理,直接利用电厂粉煤灰作为比阻调理剂。细粉煤灰的最佳投加率为20%,粗粉煤灰的最佳投加率为30%,使污泥比阻由1013数量级降至1010数量级,改善了污泥脱水性能,所得滤饼含水率仅为40%~50%,易于剥离,能耗低,所得滤液的色度、浊度、CODMn均达到可直接回用标准。当进行技改工程设计时,建议选用带式压滤机或板框压滤机,既适应粉煤灰调理剂,又可节约投资。     

Investigation and assessment of sludge pre-treatment processes.

The pre-treatment of sludges by disintegration will result in a number of changes in sludge properties. Floc destruction as well as cell disintegration will occur. This leads to an increase of soluble substances and fine particles. Furthermore, biochemical reactions may appear during or immediately after disintegration. The influence of disintegration of excess sludge on anaerobic digestion was studied in full scale. A stirred ball mill, an ultrasound disintegrator, a lysate centrifuge and ozone treatment were used. The results of the degradation process were compared to a reference system without pre-treatment. An enhancement of the degree of degradation of 7.4% to 20% was observed. The pollution of sludge water as well as the dewatering properties of the digested sludge were investigated. COD and ammonia in the sludge water were increased and a higher polymer demand was observed while the solid content after dewatering stayed almost unchanged. Based on these results the cost effectiveness has been assessed taking into account different conditions (size of WWTP, cost for disposal, etc.). Capital and energy costs are the main factors while the decrease in disposal costs due to the reduced amount of sludge is the main profit factor.     

水电工程砂石系统生产废水处理工艺优化研究

针对目前水电工程砂石加工系统生产废水主流工艺存在的问题,现场采集了白鹤滩水电站和乌东德水电站两处废水原样,利用数值模拟、现场及室内实验等多种方法对废水处理工艺主要流程进行了优化。研究表明:模拟预测两级石粉回收预处理总回收率可达60%左右,较单级回收率提高约50%;投加合适的药剂可大大改善废水絮凝沉降性能,当固体颗粒粒径较小时联合投加聚丙烯酰胺(PAM)和氧化钙(Ca O),静水沉速可提高约11倍。泥渣脱水调理剂在药剂类型和药量方面均与前端絮凝沉淀流程中所投加药剂具有一致性,而且"前端一次加药"工艺是可行的。将"隔膜压榨+进气穿流"工艺引入压滤脱水工艺中,单机工作循环时间减少67%的同时,泥饼含水率可降低至15%左右。     

Treatment of 14 sludge types from wastewater treatment plants using bench and pilot thermal hydrolysis

A total of 14 types of sludge from household sewage, mixture of domestic and industrial wastewater, and industrial and oil wastewater treatment plants were selected to evaluate the effectiveness and adaptability of thermal hydrolysis pre-treatment. Organic solubilization, dewatering improvement, volume reduction, high-strength filtrate biodegradation, and dewatered sludge incineration were investigated using bench and pilot thermal hydrolysis experiments (170 °C/60 min). Results showed that sludge types significantly affected the treatment effects. Organic content has a primary influence on thermal effects. The relationship between suspended solid (SS) solubilization and raw sludge organic content was linear with an R(2) of 0.73. The relationship between raw sludge organic content and treated sludge dewatering was linear with an R(2) of 0.86 and 0.65 for pilot and bench pre-treatments, respectively. Household and oil sludge possessed incineration possibilities with high heat value. Industrial and oil sludge filtrate was unsuitable for digestion to recover bioenergy.     

Comparison of different thickening methods for active biomass recycle for anaerobic digestion of wastewater sludge

The effect of returning solids to the digester, after one of three thickening processes, on volatile solids reduction (VSR) and gas production was investigated. Three different thickening methods were compared: centrifugation, flotation and gravitational sedimentation. The amount and activity of retained biomass in thickened recycled sludge affected the efficiency of digestion. Semi-continuous laboratory digesters were used to study the influence of thickening processes on thermophilic sludge digestion efficiency. Centrifugation was the most effective method used and caused an increase of VSR from 43% (control) up to 70% and gas generation from 0.40 to 0.44 L g(-1) VS. Flotation and gravitational sedimentation ways of thickening appeared to be less effective if compared with centrifugation. These methods increased VSR only by up to 65 and 51%, respectively and showed no significant increase of gas production. The dewatering capacity of digested sludge, as measured by its specific resistance to filtration, was essentially better for the sludge digested in the reactors with centrifugated and settled recycle. The VS concentration of recycle (g L(-1)), as reflecting the amount of retained biomass, appeared to be one of the most important factors influencing the efficiency of sludge digestion in the recycling technology.     

Anaerobic thermophilic digestion of sewage sludge with a thickened sludge recycle

The process of anaerobic thermophilic digestion of municipal wastewater sludge with a recycled part of thickened digested sludge, was studied in semi-continuous laboratory digesters. This modified recycling process resulted in increased solids retention time (SRT) with the same hydraulic retention time (HRT) as compared with traditional digestion without recycling. Increased SRT without increasing of HRT resulted in the enhancement of volatile substance reduction by up to 68% in the reactor with the recycling process compared with 34% in a control conventional reactor. Biogas production was intensified from 0.3 L/g of influent volatile solids (VS) in the control reactor up to 0.35 L/g VS. In addition, the recycling process improved the dewatering properties of digested sludge.     

Quantitative analysis of biological effect on membrane fouling in submerged membrane bioreactor.

The objective of this study is to investigate solids concentration and extracellular polymeric substance (EPS) effects on the membrane fouling in the submerged membrane bioreactor. The relationship between the solids retention time (SRT) and the amount of EPS is observed in three lab-scale MBRs. Additionally, the EPS effect on membrane fouling is quantified by calculating the specific cake resistance (alpha) using an unstirred batch cell test. By observing the sludge over a long period under various SRT scenarios, a wide range of EPS and membrane fouling data is obtained. These observations provide sufficient evidence of the functional relationship between SRT, EPS and alpha. As SRT decreases, the amount of EPS bound in sludge floc becomes higher in the high MLSS condition (> 5,000 mg/L). The amount of EPS in the sludge floc has positive influence on alpha. A sigmoid trend between EPS and alpha is observed and the functional relationship obtained by dimensional analysis is consistent with the experimental results.     

Process performance and change in sludge characteristics during anaerobic digestion of sewage sludge with ozonation.

A new process configuration combining anaerobic digestion with ozonation, and operated at long SRT, was studied with the objective of on-site reduction in sludge quantity and improving biogas recovery. The process performance with respect to solid reduction efficiency and other important process parameters like accumulation of inorganic solids, changes in sludge viscosity and dewatering characteristics were evaluated from the data of long term pilot scale continuous experiments conducted using a mixture of primary and secondary municipal sewage sludge. Due to sludge ozonation and long SRT, high VSS degradation efficiency of approximately 80% was achieved at a reactor solid concentration of 6.5%. A high fraction of inorganic solid (>50%) consisting mainly of acid insoluble and iron compounds was found to accumulate in the reactor. The high inorganic content accumulated in the digested sludge did not, however, contribute to the observed increase in sludge viscosity at high solid concentration. The sludge viscosity was largely found to depend on the organic solid concentration rather than the total solid content. Moreover, higher inorganic content in the digested sludge resulted in better sludge dewaterability. For a quick assessment of the economic feasibility of the new process, an economic index based on the unit cost of digested sludge disposal to unit electric cost is proposed.     

A new approach to improve sludge dewatering using a semi-continuous hydraulic press system.

In future, it will be necessary to incinerate an increasing proportion of the sludge produced by public wastewater treatment. In this context, equipment for more effective dewatering of sewage sludge is required. A hydraulic press system (Bucher HP) which has been used to date in the food industry could represent an alternative to the sludge dewatering systems currently used. The basic suitability of this press system was demonstrated in comprehensive industrial-scale trials at the Glarnerland STP (Switzerland). Dry solids contents of up to 43% were measured in the pressed sludge. Currently, the optimization of individual process parameters is being investigated in a pilot plant at the Schwelm STP operated by Wupperverband (Germany).     

Bench- and pilot-scale sludge electrodewatering in a diaphragm filter press.

Electrodewatering is a technique in which pressure dewatering is combined with electrokinetic effects to realize an improved solid/liquid separation and hence increased filter cake dry matter contents. In order to be energy efficient, it is shown that sludge should be dewatered by pressure dewatering to a high extent prior to electric field application, and a sufficient contact time for the electric field must be guaranteed. In order to realize these goals, a bench- and pilot-scale diaphragm filter press suited for electrodewatering were constructed for treatment of sewage and other types of sludges. It was shown that electrodewatering of sludge is a feasible technique, especially for biological sludge types. Other types of sludge are less suited for electrodewatering because of the restricted improvements that can be realized in cake dry matter content and the high electric energy consumption. Furthermore, it was shown in pilot-scale tests that the use of a diaphragm filter press with electrodewatering facilities was very well suited to deliver dry filter cakes of sewage sludge at a moderate energy consumption. Depending on local market prices for investment, operating and sludge disposal costs, this technology may therefore lead to important savings in the sludge management process.     

Improvement of the thickening and dewatering characteristics of activated sludge by electroflotation (EF).

The performances of electroflotation (EF) on the thickening of activated sludge were investigated using laboratory scale batch flotation reactors. Four activated sludges including bulking sludges were tested. After 30 minutes of EF operation, 57-84% of sludge volume reduction could be achieved by EF, while only about 1.5-14% could be obtained by gravity thickening for the same period. After thickening the effluent water quality in terms of TCOD, SS, and turbidity was improved by EF operation for all sludge samples. In addition, the EF thickened sludge showed much better dewaterability both in SRF and cake solid content. It is induced that the air bubbles entrapped in the thickened sludge play a key role in the observed dewaterbility improvement.     

Effects of potassium ion on sludge settling,dewatering and effluent properties

Potassium ions appear to play an important role in determining the nature of activated sludge flocs. Relative to sodium, the concentration of potassium ions in most industrial activated sludge is typically low. Laboratory and field studies were conducted to examine the influence of potassium on activated sludge properties. The concentration of potassium affected the concentration of readily extractable (slime) proteins in the floc and the proteins in the surrounding solution. In laboratory tests, an increase in this cation's concentration beyond nutrient requirements impeded sludge dewatering properties as measured by capillary suction time (CST) and specific resistance to filtration (SRF) and associated with an increase in soluble protein. An increase in effluent total organic carbon and effluent turbidity was observed at higher concentrations of this ion. Conversely, an increase in concentration of potassium ion improved the settling properties of sludge with low equivalent monovalent to divalent cation ratio.     

Full-scale application of anaerobic digestion process with partial ozonation of digested sludge.

For improving sludge digestion and biogas recovery, a new anaerobic digestion process combined with ozonation was tested at a full-scale unit for 2 years and its performance was compared with a simultaneously operated conventional anaerobic digestion process. The new process requires two essential modifications, which includes ozonation for enhancing the biological degradability of sludge organics and concentrating of solids in the digester through a solid/liquid separation for extension of SRT. These modifications resulted in high VSS degradation efficiency of ca. 88%, as much as 1.3 times of methane production and more than 70% reduction in dewatered sludge cake production. Owing to accumulation of inorganic solids in the digested sludge, water content of the dewatered sludge cake also reduced from 80% to 68%. An energy analysis suggested that no supplemental fuel was necessary for the subsequent incineration of the cake from the new process scheme. The process is suitable to apply to a low-loaded anaerobic digestion tank, where power production is used.     

Dewatering and disinfection of aerobic and anaerobic sludge using an electrokinetic (EK) system

The objectives of the study were to upgrade sewage sludge to Class A Exceptional Quality biosolids (as defined by US EPA) using an electrokinetics dewatering system. The pathogens monitored were Salmonella spp, and fecal coliforms (FC). Ten bench-scale electrokinetic cells were set up for the disinfection of the following sludges: primary, secondary (attached growth culture and suspended culture), and anaerobically digested sludge. A conditioning liquid was also added to five cells. Blower system to aid in dewatering and drying was used in in four EK cells. Sludge characteristics such as water content, volatile solids content, sulfate and chloride ions concentrations, FC and Salmonella spp. before and after the tests were monitored. The highest total solids content (98% TS) was achieved in the cell with the low voltage gradient, in the presence of the conditioner and with the blower system. An average reduction by 50% of volatile solids was observed. The highest, 11 log-reduction of Salmonella spp. was observed in a cell with anaerobically digested sludge. No fecal coliforms were observed in any of the cells after EK treatment.     

The role of SSVF and SSHF beds in concentrated wastewater treatment, design recommendation

The return flows of reject water from sewage sludge dewatering alter the activated sludge process in a conventional WWTP and increase TN concentration in the final effluent from WWTP. The objective of the investigation carried out was to consider the application of multistage treatment wetland (MTW) for the treatment of reject water from sewage sludge dewatering in a centrifuge (RWC). This paper aims to present the design and performance of each stage of the treatment as well as the efficiency of total MTW. The full scale pilot plant for RWC, consisting of two vertical flow beds (SS VF) working in series, followed by an horizontal flow bed (SS HF), was built in 2008. The applied configuration ensured a very high removal efficiency of principal pollutant (COD - 76.0% and NH4+-N - 93.6%). In the investigated facilities, the SS VF beds ensured an effective removal of nitrogen compounds, especially NH4+-N, whereas the decomposition of hardly degradable Org-N and COD took place in SS HF. This research illustrates that the MTW could be successfully applied for the treatment of RWC.