Environmental assessment of sewage sludge recycling options and treatment processes in Tokyo

Tokyo has historically suffered from a shortage of final disposal sites for the treated sewage sludge. Given this situation, sludge recycling and incineration have been promoted to reduce the volume of treated sludge conveyed to the disposal site, and the recycling options have changed since the late 1990s. This study aims to revisit the sewage sludge treatment and recycling processes in Tokyo and to evaluate different recycling options (brick, aggregate, refuse derived fuel and slag) from the energy consumption perspective by clarifying the complex flow of treated sludge within Tokyo's 23 wards. The study also estimates environmental loads associated with the operation of the whole sludge management system in the area. The environmental loads include: (1) total energy consumption and (2) gas emissions (greenhouse and acidification gases). The estimation was carried out for the years 1995, 1997, 1999 and 2001, during which a drastic change in recycling options occurred. The results indicated that the production of refuse derived fuel was the most energy consuming recycling option while aggregate production is the least energy consuming. They also showed that despite the increasing sludge volume, the energy consumption associated with the operation of the whole system decreased during the period while the gas emissions increased.     

Comparative goal-oriented assessment of conventional and alternative sewage sludge treatment options

Phosphorous (P) is a limited and non-substitutable resource. Sewage sludge contains significant amounts of P and is therefore a widely applied fertilizer. Due to its organic and inorganic contaminants, sewage sludge is also combusted in industrial facilities as well as in waste incinerators. This study compares five common methods and one novel alternative based on a thermo-chemical process to treat and dispose of sewage sludge with regard to environmental impact, resource recovery, and materials dissipation. The comparison is based on material flow analysis, energy balances, selected LCA impact analysis, and statistical entropy analysis. This work shows that the novel technology combines both advantages of the established practices: organic and inorganic pollutants are either destroyed or removed from the P containing material, and the P returned to the soil exhibits high plant-availability. The novel method also has low emissions. The additional energy requirements should be reduced. However, with regards to sewage sludge P recovery is more important than energy recovery.     

超临界水氧化法处理含油污泥

采用间歇式超临界水氧化(SCWO)反应器处理采油过程产生的含油污泥,考察了反应参数对污泥COD去除率的影响。实验结果表明,当初始COD为1 000 mg/L、反应温度为420℃、反应时间为10 min,反应压力为24 MPa、溶液pH为10、过氧比为400%时,SCWO法对含油污泥的COD去除率为92.20%,收集液COD低于GB8978—1996规定的一级排放标准(100 mg/L)。     

The first commercial supercritical water oxidation sludge processing plant

Final disposal of sludge continues to be one of the more pressing problems for the wastewater treatment industry. Present regulations for municipal sludge have favored beneficial use, primarily in land application. However, several agencies and entities have warned of potential health risks associated with these methods. Hydrothermal oxidation provides an alternative method that addresses the health concerns associated with sludge disposal by completely converting all organic matter in the sludge to carbon dioxide, water, and other innocuous materials. A hydrothermal oxidation system using HydroProcessing, L.L.C.'s HydroSolids process has been installed at Harlingen, Texas to process up to 9.8 dry tons per day of sludge. Based on a literature review, this system is the largest hydrothermal oxidation system in the world, and the only one built specifically to process a sludge. Start up of Unit 1 of two units of the HTO system began in April 2001. Early results have indicated COD conversion rates in excess of 99.9%. Harlingen Waterworks System estimates that the HydroSolids system will cost less than other alternatives such as autothermal thermophilic aerobic digestion and more traditional forms of digestion that still require dewatering and final disposal. The Waterworks intends to generate income from the sale of energy in the form of hot water and the use of carbon dioxide from the HydroSolids process for neutralization of high pH industrial effluent. The Waterworks also expects to generate income from the treatment of septage and grease trap wastes.     

超临界水氧化处理煤气化生化污泥

采用超临界水氧化(SCWO)技术处理煤气化生化污泥,优化了处理工艺条件,考察了有机污染物和重金属的去除效果。实验结果表明,处理含水率为90%(w)的污泥的最佳工艺条件为:反应温度580℃、反应压力25MPa、氧化系数(初始反应加入的H_2O_2的摩尔数与理论上废水完全氧化所需的H2O2的摩尔数之比)4.0、反应时间2 min。SCWO处理后的气相产物为O2、CO2和少量N2,清洁环保,可直接排放或回收利用。液相产物中的主要有机污染物和重金属含量均大幅降低,出水达到GB 8978—1996《污水综合排放标准》,可直接排放或回用。固相残渣浸出液中重金属含量均低于GB 5085.3—2007《危险废物鉴别标准浸出毒性鉴别》,可直接进行填埋处理或资源化利用。     

Environmental and economic life cycle assessment for sewage sludge treatment processes in Japan

Life cycle assessment for sewage sludge treatment was carried out by estimating the environmental and economic impacts of the six alternative scenarios most often used in Japan: dewatering, composting, drying, incineration, incinerated ash melting and dewatered sludge melting, each with or without digestion. Three end-of-life treatments were also studied: landfilling, agricultural application and building material application. The results demonstrate that sewage sludge digestion can reduce the environmental load and cost through reduced dry matter volume. The global warming potential (GWP) generated from incineration and melting processes can be significantly reduced through the reuse of waste heat for electricity and/or heat generation. Equipment production in scenarios except dewatering has an important effect on GWP, whereas the contribution of construction is negligible. In addition, the results show that the dewatering scenario has the highest impact on land use and cost, the drying scenario has the highest impact on GWP and acidification, and the incinerated ash melting scenario has the highest impact on human toxicity due to re-emissions of heavy metals from incinerated ash in the melting unit process. On the contrary, the dewatering, composting and incineration scenarios generate the lowest impact on human toxicity, land use and acidification, respectively, and the incinerated ash melting scenario has the lowest impact on GWP and cost. Heavy metals released from atmospheric effluents generated the highest human toxicity impact, with the effect of dioxin emissions being significantly lower. This study proved that the dewatered sludge melting scenario is an environmentally optimal and economically affordable method.     

Environmental assessment of sewage sludge as secondary raw material in cement production--a case study in China

A life cycle assessment was carried out to estimate the environmental impact of sewage sludge as secondary raw material in cement production. To confirm and add credibility to the study, uncertainty analysis was conducted. Results showed the impact generated from respiratory inorganics, terrestrial ecotoxicity, global warming, and non-renewable energy categories had an important contribution to overall environmental impact, due to energy, clinker, and limestone production stages. Also, uncertainty analysis results showed the technology of sewage sludge as secondary raw material in cement production had little or no effect on changing the overall environmental potential impact generated from general cement production. Accordingly, using the technology of sewage sludge as secondary raw material in cement production is a good choice for reducing the pressure on the environment from dramatically increased sludge disposal. In addition, increasing electricity recovery rate, choosing natural gas fired electricity generation technology, and optimizing the raw material consumption in clinker production are highly recommended to reduce the adverse effects on the environment.     

Biotests for environmental quality assessment of composted sewage sludge

The quality of sewage sludge-based products, such as composts and growth media, is affected by the contamination of sewage sludge with, potentially, hundreds of different substances. Therefore, it is difficult to achieve the reliable environmental quality assessment of sewage sludge-based products solely based on chemical analysis. In the present work, we demonstrate the use of the kinetic luminescent bacteria test (ISO 21338) to evaluate acute toxicity and the Vitotox? test to monitor genotoxicity of sewage sludge and composted sewages sludge. In addition, endocrine-disrupting and dioxin-like activity was studied using yeast-cell-based assays. The relative contribution of industrial waste water treated at the Waste Water Treatment Plants led to elevated concentrations of polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and polychlorinated dibenzo-p-dioxins and -furans (PCDD/F) in sewage sludge. The effect of elevated amounts of organic contaminants could also be identified with biotests able to demonstrate higher acute toxicity, genotoxicity, and potential for endocrine-disruptive properties. Additional extraction steps in kinetic luminescent bacteria test with DMSO and hexane increased the level of toxicity detected. Composting in a pilot-scale efficiently reduced the amounts of linear alkylbenzensulphonates (LASs), nonylphenols and nonylphenolethoxylates (NPE/NPs) and PAH with relative removal efficiencies of 84%, 61% and 56%. In addition, decrease in acute toxicity, genotoxicity and endocrorine-disrupting and dioxin-like activity during composting could be detected. However, the biotests did have limitations in accessing the ecotoxicity of test media rich with organic matter, such as sewage sludge and compost, and effects of sample characteristics on biotest organisms must be acknowledged. The compost matrix itself, however, which contained a high amount of nutrients, bark, and peat, reduced the sensitivity of the genotoxicity tests and yeast bioreporter assays.     

Environmental & economic life cycle assessment of current & future sewage sludge to energy technologies

The UK Water Industry currently generates approximately 800 GW h pa of electrical energy from sewage sludge. Traditionally energy recovery from sewage sludge features Anaerobic Digestion (AD) with biogas utilisation in combined heat and power (CHP) systems. However, the industry is evolving and a number of developments that extract more energy from sludge are either being implemented or are nearing full scale demonstration. This study compared five technology configurations: 1 – conventional AD with CHP, 2 – Thermal Hydrolysis Process (THP) AD with CHP, 3 – THP AD with bio-methane grid injection, 4 – THP AD with CHP followed by drying of digested sludge for solid fuel production, 5 – THP AD followed by drying, pyrolysis of the digested sludge and use of the both the biogas and the pyrolysis gas in a CHP.The economic and environmental Life Cycle Assessment (LCA) found that both the post AD drying options performed well but the option used to create a solid fuel to displace coal (configuration 4) was the most sustainable solution economically and environmentally, closely followed by the pyrolysis configuration (5). Application of THP improves the financial and environmental performance compared with conventional AD. Producing bio-methane for grid injection (configuration 3) is attractive financially but has the worst environmental impact of all the scenarios, suggesting that the current UK financial incentive policy for bio-methane is not driving best environmental practice. It is clear that new and improving processes and technologies are enabling significant opportunities for further energy recovery from sludge; LCA provides tools for determining the best overall options for particular situations and allows innovation resources and investment to be focused accordingly.     

Experimental study of the bio-oil production from sewage sludge by supercritical conversion process

Environment-friendly treatment of sewage sludge has become tremendously important. Conversion of sewage sludge into energy products by environment-friendly conversion process, with its energy recovery and environmental benefits, is being paid significant attention. Direct liquefaction of sewage sludge into bio-oils with supercritical water (SCW) was therefore put forward in this study, as de-water usually requiring intensive energy input is not necessary in this direct liquefaction. Supercritical water may act as a strong solvent and also a reactant, as well as catalyst promoting reaction process. Experiments were carried out in a self designed high-pressure reaction system with varying operating conditions. Through orthogonal experiments, it was found that temperature and residence time dominated on bio-oil yield compared with other operating parameters. Temperature from 350 to 500 °C and reaction residence time of 0, 30, 60 min were accordingly investigated in details, respectively. Under supercritical conversion, the maximum bio-oil yield could achieve 39.73%, which was performed at 375 °C and 0 min reaction residence time. Meanwhile, function of supercritical water was concluded. Fuel property analysis showed the potential of bio-oil application as crude fuel.     

Polyethylene conversion by partial oxidation in supercritical water

This paper gives the results of partial oxidation experiments of polyethylene (PE) in supercritical water (SCW). The experiments were carried out at a reaction temperature of 693K and a reaction time of 30 min using 6 cm³ of a batch-type reactor. The loaded sample weight was 0.3 g and there was 2.52 g water (0.42 g/cm³). The ratio of oxygen atoms to carbon atoms was 0.3. The results show a significant CO formation in O₂–SCW, and the 1-alkene/n-alkane ratio in partial oxidation was higher than that in SCW pyrolysis. These results suggest the possibility of the hydrogenation of hydrocarbon through partial oxidation followed by a water–gas shift reaction. Received: July 19, 2000 / Accepted: September 28, 2000     

Co-digestion of grease trap sludge and sewage sludge

Redirection of organic waste, from landfilling or incineration, to biological treatment such as anaerobic digestion is of current interest in the Malmö-Copenhagen region. One type of waste that is expected to be suitable for anaerobic digestion is sludge from grease traps. Separate anaerobic digestion of this waste type and co-digestion with sewage sludge were evaluated. The methane potential was measured in batch laboratory tests, and the methane yield was determined in continuous pilot-scale digestion. Co-digestion of sludge from grease traps and sewage sludge was successfully performed both in laboratory batch and continuous pilot-scale digestion tests. The addition of grease trap sludge to sewage sludge digesters was seen to increase the methane yield of 9–27% when 10–30% of sludge from grease traps (on VS-basis) was added. It was also seen that the grease trap sludge increases the methane yield without increasing the sludge production. Single-substrate digestion of grease trap sludge gave high methane potentials in batch tests, but could not reach stable methane production in continuous digestion.     

Life cycle assessment of sewage sludge co-incineration in a coal-based power station

A life cycle assessment was conducted to evaluate the environmental and economic effects of sewage sludge co-incineration in a coal-fired power plant. The general approach employed by a coal-fired power plant was also assessed as control. Sewage sludge co-incineration technology causes greater environmental burden than does coal-based energy production technology because of the additional electricity consumption and wastewater treatment required for the pretreatment of sewage sludge, direct emissions from sludge incineration, and incinerated ash disposal processes. However, sewage sludge co-incineration presents higher economic benefits because of electricity subsidies and the income generating potential of sludge. Environmental assessment results indicate that sewage sludge co-incineration is unsuitable for mitigating the increasing pressure brought on by sewage sludge pollution. Reducing the overall environmental effect of sludge co-incineration power stations necessitates increasing net coal consumption efficiency, incinerated ash reuse rate, dedust system efficiency, and sludge water content rate.     

Municipal sewage sludge use in forests of the pacific northwest,U.S.A.: Environmental concerns

Forest applications of dewatered municipal sludge in Washington State, U.S.A. were monitored for heavy metals, pathogens, and nitrates. Heavy metals did not leach from the sludge in significant quantities and remained associated with the sludge for at least four years. Tree seedlings grown in sludge-amended soil and compost accumulated moderate metal levels, but growth was not adversely affected. Populus seedlings took up more metal than coniferous seedlings. Some coniferous seedlings grown directly in unamended sludge had reduced growth and developed symptoms of nutrient imbalances. Fecal coliform and total coliform levels in sludge were initially high, but decreased to background levels in 1.0–1.5 years, suggesting that site access should be limited for a time after application. Bacterial movement in the soil was limited and very few viable bacteria reached the groundwater. Aerosols may be generated during sludge applications. Nitrogen leaching is a major environmental concern or sludge application. Heavy applications resulted in high NO₃⁻ levels in soils and groundwater. A significant fraction of the nitrogen loss was in gaseous form. Sludge applications in forests can be environmentally sound if application rates are matched to site characteristics.     

Environmental effects of using clay bricks produced with sewage sludge: leachability and toxicity studies

Use of sewage sludge from wastewater treatment plants as a raw material for making clay bricks has been analyzed to be an option to dumping sludges into landfills. This alternative has been shown feasible and interesting due to the high rate of use of ceramic materials in the building sector. However, it meets with some environmental issues and some prejudices on the part of users. This work shows some leachability and toxicity tests (outgassing and offgassing) which demonstrate the environmental compatibility of these ceramic products to be used as building materials and even in deconstruction of the building once its useful life is ended.     

Supercritical water gasification of sewage sludge using bench-scale batch reactor: advantages and drawbacks

Sewage sludge, a byproduct of municipal wastewater treatment, was gasified by supercritical water using a bench-scale batch reactor. Configuration of bench-scale batch reactor and operation procedures are discussed in detail. Experience and challenges that arose during the experiment are also shared. Using the bench-scale reactor under the condition of 600 °C, 23 MPa, and a reaction time of 60 min without catalyst presence, a total gas yield of 9.8 mol/(kg-sewage sludge) was obtained. Furthermore, investigations on operational parameters were conducted. Extension of reaction time up to 60 min increased the gasification, reaching a plateau thereafter. Investigation on pressure indicated the superiority of supercritical pressure. The addition of Ni as a catalyst also promoted gasification, although inorganic salts and char seemed to cover the catalyst surface. With regard to the prospect of future operation at a municipal waste water treatment plant, the effect of operational parameters on heavy metal concentration in the liquid phase is also discussed.     

Incorporation of sewage sludge in clay brick and its characterization.

This study reports the use of sewage sludge generated from sewage treatment plant (STP) as raw material in a clay brick-making process. The physico-chemical and mineralogical characterization of the sewage sludge and clay were carried out in order to identify the major technological constraints and to define the sludge pretreatment requirements if necessary. Moreover, the effects on processing conditions and/or on changes of typical final characteristics are also evaluated. Bricks were produced with sewage sludge additions ranging from 10 to 40% by dry weight. The texture and finishing of the surface of sludge-amended clay bricks were rather poor. As for the physical and chemical properties, bricks with a sludge content of up to 40 wt.% were capable of meeting the relevant technical standards. However, bricks with more than 30 wt.% sludge addition are not recommended for use since they are brittle and easily broken even when handled gently. A tendency for a general degradation of brick properties with sludge additions was observed due to its refractory nature. Therefore, sludge bricks of this nature are only suitable for use as common bricks, which are normally not exposed to view, because of poor surface finishing.     

Mechanical properties of dewatered sewage sludge

The mechanical properties of dewatered, anaerobically digested sewage sludge were determined from soil laboratory tests. The sludge material is largely composed of organic clay sized-particles, a sizable fraction of which is in an active state of biological digestion which can continue over many years under field conditions. Moderately digested sludge material was found to have a typical specific gravity of solids value of 1.55, and loss on ignition (LOI) value of 70% dry mass. Strongly digested sludge, produced by digesting the liquid sludge further at 35 degrees C in the laboratory, was found to have a lower LOI value of 55% dry mass, and a higher specific gravity of solids value of about 1.72. The maximum dry density of 0.56 tonne/m3 for the dried sludge material was produced using standard Proctor compaction at roughly 85% moisture content (54% solids content). Air-dried, compacted sludge material was tested in quick-undrained triaxial compression and vane shear. Undrained shear strength-moisture content plots are presented. Shear strength values measured in triaxial compression and vane shear were consistent. The effective angle of shearing resistance (phi') was determined from consolidated-undrained, triaxial compression tests on pasteurized, normally consolidated samples of the sludge material. The mechanical properties of the sludge material changed with the level of sludge digestion. The phi' value increased from 32 degrees for moderately digested sludge, to 37 degrees for strongly digested sludge. The effective cohesion of the sludge material remained zero throughout. The shrinkage, swelling and adhesion properties of the sludge material were also studied. Significant shrinkage occurred as the compacted material dried. The sludge material lost its adhesion below about 95% moisture content (51% solids content). Re-hydration of the dry material caused the bulk volume to double.     

Reductive hydrothermal treatment of sewage sludge

The United States and the European Union each generate around 6900 million dry tons of sewage sludge annually. This is disposed of by land application, landfilling, incineration and other approaches. Reductive hydrothermal (HT) treatment refers here to simple aqueous systems heated and pressurized above 300 degrees C/100bar under anoxic and/or reducing conditions. The purpose of this study was to examine the HT treatment of municipal sewage sludge and infectious fecal microbial cultures with respect to waste volume reduction, biological sterilization, and the generation of usable hydrocarbon product mixtures. These endpoints from HT treatment also were compared to those from pyrolysis. HT at 400 degrees C/150bar transformed sewage sludge solids into complex gas phase (4%) and liquid (6%) hydrocarbon mixtures (approximately 11% combined yield), along with similar amounts (5%) of solid residues. HT products in the aqueous phase (e.g., alcohols) were present but not analysed. Viable mixed fecal cultures (10(9) colony forming units/mL) were completely sterilized by HT treatment, and a hydrocarbon mixture also was generated from the cells, but it was markedly different from that resulting from HT of the sludge. The hydrocarbon assemblage generated from the sludge included n-hydrocarbons (C(9)-C(20)) and alkyl substituted benzenes, phenols, and related compound series of higher mass (e.g., indanes, naphthalenes). Light aromatic parent compounds were significantly less abundant than their substituted C(1)-C(5) alkyl series and there was a paucity of N-, O- and S-heterocycles and polycyclic systems with more than three fused rings. This was different from the products of pyrolysis which were dominated by a relatively simple mixture of linear and branched hydrocarbons and their oxidized homologues (e.g., aldehydes).     

Evaluation of bangkok sewage sludge for possible agricultural use.

Bangkok (Thailand) covers more than 1500 km2 and has 10 million inhabitants. The disposal of wastewater is creating huge problems of pollution. The estimated amount of sewage sludge was estimated to be around 108 tonnes dry matter (DM) per day in 2005. In order to find a lasting way of disposal for this sewage sludge, the suitability of the sludge produced from three waste-water treatment plants for use as fertilizing material was investigated. Monthly samplings and analysis of sewage sludge from each plant showed that the composition of sludge varied according to the area of collection and period of sampling, and there was no link to rainfall cycle. Plant nutrient content was high (i.e. total N from 19 to 38 g kg(-1) DM) whereas organic matter content was low. The concentrations of heavy metals varied between sludge samples, and were sometimes higher than the E.U. or U.S. regulations for sewage sludge use in agriculture. Faecal coliforms were present in the sludge from one of the plants, indicating a possible contamination by night soil. In order to decrease this potentially pathogenic population the sewage sludge should be heated by composting. As the C/N ratio of sewage sludge was low (around 6) some organic by-products with high carbon content could be added as structural material to enhance the composting.