Technical, economic and environmental assessment of sludge treatment wetlands

Sludge treatment wetlands (STW) emerge as a promising sustainable technology with low energy requirements and operational costs. In this study, technical, economic and environmental aspects of STW are investigated and compared with other alternatives for sludge management in small communities (<2000 population equivalent). The performance of full-scale STW was characterised during 2 years. Sludge dewatering increased total solids (TS) concentration by 25%, while sludge biodegradation lead to volatile solids around 45% TS and DRI_24h between 1.1 and 1.4 gO₂/kgTS h, suggesting a partial stabilisation of biosolids. In the economic and environmental assessment, four scenarios were considered for comparison: 1) STW with direct land application of biosolids, 2) STW with compost post-treatment, 3) centrifuge with compost post-treatment and 4) sludge transport to an intensive wastewater treatment plant. According to the results, STW with direct land application is the most cost-effective scenario, which is also characterised by the lowest environmental impact. The life cycle assessment highlights that global warming is a significant impact category in all scenarios, which is attributed to fossil fuel and electricity consumption; while greenhouse gas emissions from STW are insignificant. As a conclusion, STW are the most appropriate alternative for decentralised sludge management in small communities.     

SLUDGE TREATMENT AND DRYING REED BED SYSTEMS IN DENMARK

Sludge Reed beds have been used for dewatering (draining and evapotranspiration) and mineralisation of sludge in Denmark since 1988 when the first sludge processing system was introduced. Sludge from wastewater treatment plants (2,500-125,000 pe) is treated in sludge reed bed systems with 1–18 basins with loading rates of 25–2,200 tonnes dry solids/year for ten years. In 2002, approximately 95 systems were in operation. Dimensioning and design of reed bed systems depends on the sludge production rate, sludge type, quality and regional climate.
The maximum sludge loading rate is approximately 50–60 kg DS/m²/year. Loading cycles are related to the sludge type and the age of the sludge reed systems. The sludge residue will, after approximately ten years of operation, reach an approximate height of 1.2–1.5 metres with dry solids content of 30–40%. Experience has shown that the quality of the final product with respect to heavy metals, hazardous organic compounds and pathogen removal after ten years of treatment make it possible to recycle the biosolids to agriculture as an enhanced treated product.     

Quantification of greenhouse gas emissions from sludge treatment wetlands

Constructed wetlands are nowadays successfully employed as an alternative technology for wastewater and sewage sludge treatment. In these systems organic matter and nutrients are transformed and removed by a variety of microbial reaction and gaseous compounds such as methane (CH₄) and nitrous oxide (N₂O) may be released to the atmosphere. The aim of this work is to introduce a method to determine greenhouse gas emissions from sludge treatment wetlands (STW) and use the method in a full-scale system. Sampling and analysing techniques used to determine greenhouse gas emissions from croplands and natural wetlands were successfully adapted to the quantification of CH₄ and N₂O emissions from an STW. Gas emissions were measured using the static chamber technique in 9 points of the STW during 13 days. The spatial variation in the emission along the wetland did not follow some specific pattern found for the temporal variation in the fluxes. Emissions ranged from 10 to 5400 mgCH₄/m² d and from 20 to 950 mgN₂O/m² d, depending on the feeding events. The comparison between the CH₄ and N₂O emissions of different sludge management options shows that STW have the lowest atmospheric impact in terms of CO₂ equivalent emissions (Global warming potential with time horizon of 100 years): 17 kgCO₂eq/PE y for STW, 36 kgCO₂eq/PE y for centrifuge and 162 kgCO₂eq/PE y for untreated sludge transport, PE means Population Equivalent.     

三峡库区污水处理厂污泥处置现状研究

目前,重庆三峡库区共建有污水处理厂18座,日产污泥132．95t／d。污泥处理主要采用浓缩＋机械脱水的工艺,然后送至垃圾填埋场填埋。运行过程中,由于脱水污泥含水率较高（80％左右）,且未经稳定化处理,已经极大的影响了填埋场的正常运行。针对这种情况,本文提出：在有垃圾填埋场的情况下,通过机械脱水、石灰稳定化等处理后进行填埋,不具备卫生填埋条件的实行堆肥后资源化利用。     

Pressurised electro-osmotic dewatering of activated and anaerobically digested sludges: electrical variables analysis

Pressurised electro-osmotic dewatering (PEOD) of two sewage sludges (activated and anaerobically digested) was studied under constant electric current (C.C.) and constant voltage (C.V.) with a laboratory chamber simulating closely an industrial filter. The influence of sludge characteristics, process parameters, and electrode/filter cloth position was investigated. The next parameters were tested: 40 and 80 A/m², 20, 30, and 50 V—for digested sludge dewatering; and 20, 40 and 80 A/m², 20, 30, and 50 V—for activated sludge dewatering. Effects of filter cloth electric resistance and initial cake thickness were also investigated.The application of PEOD provides a gain of 12 points of dry solids content for the digested sludge (47.0% w/w) and for the activated sludge (31.7% w/w). In PEOD processed at C.C. or at C.V., the dewatering flow rate was similar for the same electric field intensity. In C.C. mode, both the electric resistance of cake and voltage increase, causing a temperature rise by ohmic effect. In C.V. mode, a current intensity peak was observed in the earlier dewatering period. Applying at first a constant current and later on a constant voltage, permitted to have better control of ohmic heating effect. The dewatering rate was not significantly affected by the presence of filter cloth on electrodes, but the use of a thin filter cloth reduced remarkably the energy consumption compared to a thicker one: 69% of reduction energy input at 45% w/w of dry solids content. The reduction of the initial cake thickness is advantageous to increase the final dry solids content.     

Musk fragrances, DEHP and heavy metals in a 20 years old sludge treatment reed bed system

The Sludge Treatment Reed Bed (STRB) technology is a cost-efficient and environmentally friendly technology to dewater and mineralize surplus sludge from conventional wastewater treatment systems. Primary and secondary liquid sludge is loaded onto the surface of the bed over several years, where it is dewatered, mineralized and turned into a biosolid with a high dry matter content for use as an organic fertilizer on agricultural land. We analysed the concentrations of five organic micropollutants (galaxolide, tonalide, cashmeran, celestolide and DEHP) and six heavy metals (Pb, Ni, Cu, Cd, Zn and Cr) in the accumulated sludge in a 20-year old STRB in Denmark in order to assess the degradation and fate of these contaminants in a STRB and the relation to sludge composition. The results showed that the deposited sludge was dewatered to reach a dry matter content of 29%, and that up to a third of the organic content of the sludge was mineralized. The concentrations of heavy metals generally increased with depth in the vertical sludge profile due to the dewatering and mineralization of organic matter, but in all cases the concentrations were below the European Union legal limits for agricultural land disposal. The concentrations of fragrances and DEHP ranged from 10 to 9000 ng g⁻¹ dry mass. The attenuation of hydrophobic micropollutants from the top to the bottom layer of the reed bed ranged from 40 to 98%, except for tonalide which increased significantly with sludge depth, and consequently showed an unusual depth distribution of the galaxolide/tonalide ratio. This unexpected pattern may reflect changes imposed by a long storage time and/or different composition of the fresh sludge in the past. The lack of a significant decreasing DEHP concentration with sludge age might indicate that this compound is very persistent in STRBs. In conclusion the STRB was a feasible technology for sludge treatment before its land disposal.     

影响污泥深度机械压力脱水因素的探讨

根据污泥处理处置技术在实际应用中存在的缺陷,指出污泥的深度脱水是污泥处理处置的关键环节。因此,污泥深度脱水技术面临着巨大的技术需求和市场需求。介绍了污泥深度脱水性能的表征方法,指出模拟恒压压滤脱水后泥饼的含水率能够较好地表征污泥深度脱水性能。同时,从污泥性质、调理剂预处理、过滤压力及操作、过滤介质等方面分析了污泥深度机械压力脱水的影响因素。     

污泥脱水、输送及储存系统的设计探讨与总结

污泥脱水、输送及储存系统是污泥处理处置工程的重要组成部分,以南宁某项目为例,介绍了污泥脱水工艺、进泥设备、加药系统及泥水分离阀的设计及优化建议;介绍了污泥输送螺旋、缓存料斗及料位检测的工程应用特点,提出一体化破拱装置,详述污泥输送泵及其故障处理方式;介绍了污泥储存系统的管道、入料口的设计要点,滑架破拱装置的设计优化及其液压系统热平衡分析等。该项目投入运营后,脱水污泥含水率降至80%,污泥的日输送量达到66.85 m³/d,污泥储存系统储存量达到200 m³,实现了污泥的减量化生产。最后对该工程进行技术经济分析,为同类项目提供参考。     

A hierarchical control approach for virtual coupling in metro trains

As an emerging technology, virtual coupling improves the efficiency and flexibility of metro services by forming multiple trains (units) as a virtually coupled train set (VCTS) without mechanical couplers. However, to realize the desired VCTS operation in practical metro services, a significant gap to be filled is that the implicit and nonlinear safety constraints are hard to be addressed in real-time control. Thus, this paper proposes a hierarchical control approach with a two-layer framework. In the upper layer, a trajectory planning method is designed, which addresses the safety constraints and prescribes a reference trajectory for each unit. In the lower layer, a model predictive control approach is constructed, by which each unit accurately tracks its reference trajectory in real time. Afterward, the proposed hierarchical control approach is employed to realize VCTS inter-station operation in field tests. The average tracking error of speed is around 8 cm/s and the stopping error is less than 12 cm. The following distance between two units in VCTS is reduced to less than 105 m at the speed of 80 km/h, which is a breakthrough in the development of VCTS.     

福州马尾青洲污水处理厂污泥处理技改设计及效果分析

介绍了青洲污水处理厂污泥处理技改的工艺设计及运行效果,设计中充分利用了原有污泥处理构筑物,通过技术比较选择厢式压滤技术作为主体工艺,污泥经泥药混合、进料、压榨、吹脱、卸料等阶段处理后,出泥含水率可稳定达到60％以下,并具备达到50％以下的能力。实际运行情况表明,厢式压滤技术运行效果稳定,处理成本低,投资省、建设方便,在中小城镇污水处理厂污泥处理中较为适用。     

Removal of personal care compounds from sewage sludge in reed bed container (lysimeter) studies — Effects of macrophytes

Sludge reed beds have been used for dewatering (draining and evapotranspiration) and mineralisation of sludge in Europe since 1988. Although reed beds are considered as a low cost and low contamination method in reducing volume, breaking down organic matter and increasing the density of sludge, it is not yet clear whether this enhanced biological treatment is suitable for degradation of organic micro-pollutants such as personal care products. Within this project the effect of biological sludge treatment in a reed bed on reducing the concentrations of the fragrances HHCB, AHTN, OTNE was studied as on the bactericide Triclosan. Additionally, the capacity of different macrophytes species to affect the treatment process was examined.Three different macrophyte species were compared: bulrush (Typha latifolia), reed (Phragmites australis) and reed canary grass (Phalaris arundinacea). They were planted into containers (lysimeters) with a size of 1 m × 1 m × 1 m which were filled with 20 cm gravel at the bottom and 50 cm sludge on top, into which the macrophytes were planted. During the twelve months experiment reduction of 20-30% for HHCB and AHTN, 70% for Triclosan and 70% for OTNE were determined under environmental conditions. The reduction is most likely due to degradation, since volatilization, uptake into plants and leaching are insignificant. No difference between the containers with different macrophyte species or the unplanted containers was observed. Considering the usual operation time of 10 years for reed beds, an assessment was made for the whole life time.     

A shallow lake remediation regime with Phragmites australis: Incorporating nutrient removal and water evapotranspiration

Shallow lake eutrophication has been an important issue of global water environment. Based on the simulation and field sampling experiments in Baiyangdian Lake, the largest shallow lake in North China, this study proposed a shallow lake remediation regime with Phragmites australis (reed) incorporating its opposite effects of nutrient removal and water evapotranspiration on water quality. The results of simulation experiments showed that both total nitrogen (TN) and phosphorus (TP) removal efficiencies increased with the increasing reed coverage. The TN removal efficiencies by reed aboveground uptake and rhizosphere denitrification were 11.2%, 13.8%, 22.6%, 28.4%, and 29.6% for the reed coverage of 20%, 40%, 60%, 80%, and 100%, respectively. Correspondingly, TP removal efficiencies by aboveground reed uptake were 1.4%, 2.5%, 4.4%, 7.4% and 7.9%, respectively. However, the water quality was best when the reed coverage was 60% (72 plants m⁻²). This was due to the fact that the concentration effect of reed evapotranspiration on nutrient increased with reed coverage. When the reed coverage was 100% (120 plants m⁻²), the evapotranspiration was approximately twice that without reeds. The field sampling results showed that the highest aboveground nutrient storages occurred in September. Thus, the proposed remediation regime for Baiyangdian Lake was that the reed coverage should be adjusted to 60%, and the aboveground biomass of reeds should be harvested in each September. With this remediation regime, TN and TP removal in Baiyangdian Lake were 117.8 and 4.0 g m⁻², respectively, and the corresponding removal efficiencies were estimated to be 49% and 8.5% after six years. This study suggests that reed is an effective plant for the remediation of shallow lake eutrophication, and its contrasting effects of nutrient removal and evapotranspiration on water quality should be considered for establishing the remediation regime in the future.     

Dewatering characteristics of sludge conditioned by surfactant with bioleaching pretreatment

In view of the merits of bioleaching and surfactant for sludge treatment, the possibility of surfactant with bioleaching pretreatment applied to improve sludge dewaterability was investigated in this work. The results showed that cetyl trimethyl ammonium chloride (CTAC) with bioleaching pretreatment was highly efficient in dewatering sludge. The optimal CTAC dosage and bioleaching pH for this combination were 120 mg/g (dry solids) and 3.55, respectively, under which the water content of filtrated cake was 68.94% and the specific resistance to filtration was 0.12 × 10¹³ m/kg with a reduction of 94.92%. Although CTAC and bioleaching both had positive effects on sludge dewatering, their combination was more efficient. The significant enhancement of sludge dewaterability was mainly contributed from the breakage and charge neutralization of sludge flocs, and the release of extracellular polymeric substances (EPS). In addition, the positive correlation between supernatant EPS concentration and sludge dewaterability was observed under experimental conditions.     

填埋场腾发封顶系统中的水分运移分析

 从腾发封顶系统工作机制出发,依据Penman公式从能量角度计算潜在腾发量,然后依次分析和求解植被截留量、土面蒸发量、植被蒸腾量和地表径流量,从而求得净入渗量;并将净入渗量作为条件边界,选取典型参数建立腾发封顶的非饱和渗流模型,分析得到封顶内的水分运移规律,并对其性能进行评价。分析结果表明,蒸发蒸腾对该类型封顶中的水分运移起着决定性作用,封顶中浅部土体含水量受降雨和腾发作用影响显著,底部含水量基本不发生变化,起蓄水–释水作用的主要是植被根系发育区。在所考虑的气候条件下,植被良好、根系深度50 cm、总厚度为120 cm的腾发封顶系统的累积透水量很小,完全能满足设计要求,其减小降雨入渗的性能优于传统压实黏土封顶。     

Electro-dewatering of wastewater sludge: influence of the operating conditions and their interactions effects

Electric field-assisted dewatering, also called electro-dewatering (EDW), is a technology in which a conventional dewatering mechanism such a pressure dewatering is combined with electrokinetic effects to realize an improved liquid/solids separation, to increase the final dry solids content and to accelerate the dewatering process with low energy consumption compared to thermal drying. The application of these additional fields can be applied to either or both dewatering stages (filtration and/or compression), or as a pre-or post-treatment of the dewatering process. In this study, the performance of the EDW on wastewater sludge was investigated. Experiments were carried out on a laboratory filtration/compression cell, provided with electrodes, in order to apply an electrical field. The chosen operating conditions pressure (200-1200 kPa) and voltage (10-50 V) are sufficient to remove a significant proportion of the water that cannot be removed using mechanical dewatering technologies alone. A response surface methodology (RSM) was used to evaluate the effects of the processing parameters of EDW on (i) the final dry solids content, which is a fundamental dewatering parameter and an excellent indicator of the extent of EDW and (ii) the energy consumption calculated for each additional mass of water removed. A two-factor central composite design was used to establish the optimum conditions for the EDW of wastewater sludge. Experiments showed that the use of an electric field combined with mechanical compression requires less than 10 and 25% of the theoretical thermal drying energy for the low and moderate voltages cases, respectively.     

Temperature and salt effects on settling velocity in granular sludge technology

Settling velocity is a crucial parameter in granular sludge technology. In this study the effects of temperature and salt concentrations on settling velocities of granular sludge particles were evaluated. A two-fold slower settling velocity for the same granules was observed when the temperature of water decreases from 40 °C to 5 °C. Settling velocities also decreased with increasing salt concentrations. Experiments showed that when granules were not pre-incubated in a solution with increased salt concentration, they initially floated. The time dependent increase in mass and hence in settling speed of a granule due to salt diffusion into the granule was dependent on the granule diameter. The time needed for full salt equilibrium with the bulk liquid took 1 min for small particles from the top of the sludge bed and up to 30 min for big granules from the bottom of the sludge bed. These results suggest that temperature and salt concentration are important parameters to consider in the design, start-up and operation of granular sludge reactors and monitoring of these parameters will aid in a better control of the sludge management in anaerobic and aerobic granular sludge technology. The observations also give an explanation for previous reports which were suggesting that a start-up of granular sludge reactors is more difficult at low temperatures.     

Temperature and salt effects on settling velocity in granular sludge technology

Settling velocity is a crucial parameter in granular sludge technology. In this study the effects of temperature and salt concentrations on settling velocities of granular sludge particles were evaluated. A two-fold slower settling velocity for the same granules where observed when the temperature of water decreases from 40 °C to 5 °C. Settling velocities also decreased with increasing salt concentrations. Experiments showed that when granules were not pre-incubated in a solution with increased salt concentration, they initially floated. The time dependent increase in mass and hence in settling speed of a granule due to salt diffusion into the granule was dependent on the granule diameter. The time needed for full salt equilibrium with the bulk liquid took 1 min for small particles from the top of the sludge bed and up to 30 min for big granules from the bottom of the sludge bed. These results suggest that temperature and salt concentration are important parameters to consider in the design, start-up and operation of granular sludge reactors and monitoring of these parameters will aid in a better control of the sludge management in anaerobic and aerobic granular sludge technology. The observations also give an explanation for previous reports which were suggesting that a start-up of granular sludge reactors is more difficult at low temperatures.     

Comparing steroid estrogen, and nonylphenol content across a range of European sewage plants with different treatment and management practices

The effluent of 17 sewage treatment works (STW) across Norway, Sweden, Finland, The Netherlands, Belgium, Germany, France and Switzerland was studied for the presence of estradiol (E2), estrone (E1), ethinylestradiol (EE2) and nonylphenol (NP). Treatment processes included primary and chemical treatment only, submerged aerated filter, oxidation ditch, activated sludge (AS) and combined trickling filter with activated sludge. The effluent strength ranged between 87 and 846 L/PE (population equivalent), the total hydraulic retention time (HRT) ranged between 4 and 120 h, sludge retention time (SRT) between 3 and 30 d, and water temperature ranged from 12 to 21 °C. The highest estrogen values were detected in the effluent of the STW which only used primary treatment (13 ng/L E2 and 35 ng/L E1) and on one occasion in one of the STW using the AS system (6.5 ng/L E2, 50.5 ng/L E1, but on three other occasions the concentrations in this STW were at least a factor of 6 lower). For the 16 STW employing secondary treatment E2 was only detected in the effluent of six works during the study period (average 0.7-5.7 ng/L). E1 was detected in the effluent of 13 of the same STW. The median value for E1 for the 16 STW with secondary treatment was 3.0 ng/L. EE2 was only detected in two STW (1.1, <0.8-2.8 ng/L). NP could be detected in the effluent of all 14 STW where this measurement was attempted, with a median of 0.31 μg/L and values ranging from 0.05 to 1.31 μg/L. A comparison of removal performance for E1 was carried out following prediction of the probable influent concentration. A weak but significant (α<5%) correlation between E1 removal and HRT or SRT was observed.     

垃圾填埋场封顶系统的设计与水分平衡

为了解决传统压实黏土封顶系统在干旱及半干旱地区存在干燥开裂的问题,三种新型的ET(Evapotranspiration)封顶系统的设计方案被提出了。其中,一种ET封顶系统设计方案的结构由上下各1m厚的植物生长土层和阻隔土层构成,另外两种ET封顶系统分别在中间与底部增设了一层生物阻隔层。在考虑降水与蒸发蒸腾循环补给的条件下,建立了水汽在多层非饱和土壤中迁移的一维数学模型。以1976年大连市全年的降水与蒸发蒸腾强度为边界条件,模拟了水汽在四种封顶系统中的迁移规律。计算结果表明,在传统压实黏土封顶系统中,由于压实黏土层具有极低的渗透性,致使整层土壤不能得到有效的水分补给;ET封顶系统中整个土层可以在降水的过程中有效地从边界得到补给,同时在蒸发蒸腾的条件下,把土层中的储水释放;ET封顶系统设计方案2中设置的生物阻隔层,不仅可以防止生物对阻隔土壤层的破坏,而且起到了排水作用。     

SBBGR technology for minimising excess sludge production in biological processes

This paper reports the results of an investigation aimed at evaluating the performance of an innovative technology (SBBGR system - Sequencing Batch Biofilter Granular Reactor), characterised by a low sludge production, for treating municipal wastewater at demonstrative scale. The results have shown that even at the maximum investigated organic load (i.e., 2.5 kg COD/m³ d), the plant removed 80% of COD, total suspended solids and nitrogen content with relative residual concentrations lower than the Italian limits for discharge into soil. The process was characterised by a very low sludge production (i.e., 0.12-0.14 kg TSS/kg COD_removed) ascribable to the high sludge age in the system (θc >120 d). Molecular in situ detection methods and microscopy staining procedures were employed in combination with the traditional measurements (oxygen uptake rate and total protein content) to evaluate both the microbial activity and composition, and the structure of the biomass. A stable presence of active bacterial populations (mainly Proteobacteria) was found within compact and dense aggregates.