Decentralized Wastewater Treatment Systems Research Articles

Special Issue on Wastewater Treatment and Reuse Technologies

Wastewater treatment allows for the safe disposal of municipal and industrial wastewater to protect public health and the ecosystem[...]

Decentralized Wastewater Systems in Bengaluru, India: Success or Failure?

Decentralized wastewater treatment and reuse (DWTRU) using small-scale on-site sewage treatment plants (STPs) is an attractive solution addressing the problems of water pollution and scarcity, especially in rapidly urbanizing cities in developing countries, where centralized infrastructure for wastewater treatment is inadequate. But decentralized systems face several challenges (economic feasibility, public acceptance) that need to be better understood. The city of Bengaluru in India provides an excellent opportunity to evaluate such systems. In 2004, in an effort to curb the alarming levels of pollution in its water bodies due to untreated sewage disposal, the environmental regulatory agency mandated apartment complexes above a certain size to install STPs and reuse 100% of their wastewater, resulting in the installation of more than 2200 on-site STPs till date. This study attempts to analyze the factors influencing the extent of treatment and reuse in such systems, through structured surveys of residential associations, STP experts and government officials. The results are analyzed using a framework that integrates the technology adoption literature with the monitoring and enforcement literature. The study indicates that, while no apartment complex is able to reuse 100% of its treated water, there exists significant variation across apartment complexes in the level of treatment and reuse (from partial to poor) due to a complex mix of economies of scale, the price of fresh water, the level of enforcement and awareness, and technological choices made under information asymmetry. Only apartments dependent on expensive tanker water supply had clear economic incentives to comply with the order. Yet many large complexes that depended on low-priced utility or borewell supply were partially compliant, owing partly to lower (although positive) costs, higher level of formal enforcement and perhaps greater environmental awareness. On the other hand, the high treatment cost pushed smaller complexes to curtail the operation of their STPs (and the lower levels of enforcement further worsened this), resulting in inadequate treated water quality and consequently low reuse levels. The study recommends relaxing the infeasible 100% reuse criterion, and raising the threshold size above which DWTRU should be mandated so as to reduce the cost burden and increase enforceability. Subsidies towards capital costs and enabling resale of treated water will enable wider adoption. DWTRU is an apparently attractive solution that however, requires judicious policy-making and implementation to succeed.

Sustainable decentralised wastewater treatment schemes in the context of Lobitos, Peru

The implementation of decentralised wastewater treatment systems, such as biodigesters, septic tanks and treatment ponds, provides opportunities for rural or remote communities to be self-reliant and avoid infrastructural connections to faraway urban areas. However, the effectiveness, sustainability and success of such systems is heavily tied to understanding the overall context (geographical, social, cultural, political and economic) in which they are installed, as well as the ease of their operation and maintenance in the long term. Shortcomings to addressing these aspects can lead to the failure of a project. Using the town of Lobitos, located in the Piura District on the northern coast of Peru, as the case study for this research, this paper is discussing and analysing the use of biodigesters as a more sustainable solution over larger municipal wastewater systems in the context of Lobitos. It explains reasons, such as community engagement, behind past failures of such systems and outlines lessons learned from a practitioner’s perspective. It concludes that addressing the local context as well as considering its impact throughout the project cycle, such as installation and future operation and maintenance, helps to ensure continued delivery of safe and sustainable wastewater treatment.

Removal of helminth eggs by centralized and decentralized wastewater treatment plants in South Africa and Lesotho: health implications for direct and indirect exposure to the effluents

Wastewater may contain contaminants harmful to human health; hence, there is the need for treatment before discharge. Centralized wastewater treatment systems are the favored treatment options globally, but these are not necessarily superior in reduction of pathogens as compared to decentralized wastewater treatment systems (collectively called DEWATS). This study was therefore undertaken to assess the soil-transmitted helminth (STH) and Taenia sp. egg reduction efficiency of selected anaerobic baffled reactors and planted gravel filters compared to centralized wastewater treatment plants in South Africa and Lesotho. The risk of ascariasis with exposure to effluents from the centralized wastewater treatment plants was also assessed using the quantitative microbial risk assessment (QMRA) approach. Eggs of Ascaris spp., hookworm, Trichuris spp., Taenia spp., and Toxocara spp. were commonly detected in the untreated wastewater. The DEWATS plants removed between 95 and 100% of the STH and Taenia sp. eggs, with centralized plants removing between 67 and 100%. Helminth egg concentrations in the final effluents from the centralized wastewater treatment plants were consistently higher than those in the WHO recommended guideline (≤ 1 helminth egg/L) for agricultural use resulting in higher risk of ascariasis. Therefore, in conclusion, DEWATS plants may be more efficient in reducing the concentration of helminth eggs in wastewater, resulting in lower risks of STH infections upon exposure.

Cost comparison of centralized and decentralized wastewater management systems using optimization model

There is a growing interest in decentralized wastewater management (DWWM) as a potential alternative to centralized wastewater management (CWWM) in developing countries. However, the comparative cost of CWWM and DWWM is not well understood. In this study, the cost of cluster-type DWWM is simulated and compared to the cost of CWWM in Alibag, India. A three-step model is built to simulate a broad range of potential DWWM configurations with varying number and layout of cluster subsystems. The considered DWWM scheme consists of cluster subsystems, that each uses simplified sewer and DEWATS (Decentralized Wastewater Treatment Systems). We consider CWWM that uses conventional sewer and an activated sludge plant. The results show that the cost of DWWM can vary significantly with the number and layout of the comprising cluster subsystems. The cost of DWWM increased nonlinearly with increasing number of comprising clusters, mainly due to the loss in the economies of scale for DEWATS. For configurations with the same number of comprising cluster subsystems, the cost of DWWM varied by ±5% around the mean, depending on the layout of the cluster subsystems. In comparison to CWWM, DWWM was of lower cost than CWWM when configured with fewer than 16 clusters in Alibag, with significantly less operation and maintenance requirement, but with higher capital and land requirement for construction. The study demonstrates that cluster-type DWWM using simplified sewer and DEWATS may be a cost-competitive alternative to CWWM, when carefully configured to lower the cost.

Sustainable decentralised wastewater treatment schemes in the context of Lobitos, Peru

© 2018 ICE Publishing. All Rights Reserved. The implementation of decentralised wastewater treatment systems, such as biodigesters, septic tanks and treatment ponds, provides opportunities for rural or remote communities to be self-reliant and avoid infrastructural connections to faraway urban areas. However, the effectiveness, sustainability and success of such systems is heavily tied to understanding the overall context (geographical, social, cultural, political and economic) in which they are installed, as well as the ease of their operation and maintenance in the long term. Shortcomings to addressing these aspects can lead to the failure of a project. Using the town of Lobitos, located in the Piura District on the northern coast of Peru, as the case study for this research, this paper is discussing and analysing the use of biodigesters as a more sustainable solution over larger municipal wastewater systems in the context of Lobitos. It explains reasons, such as community engagement, behind past failures of such systems and outlines lessons learned from a practitioner’s perspective. It concludes that addressing the local context as well as considering its impact throughout the project cycle, such as installation and future operation and maintenance, helps to ensure continued delivery of safe and sustainable wastewater treatment.

Low temperature effects on nitrification and nitrifier community structure in V-ASP for decentralized wastewater treatment and its improvement by bio-augmentation.

The vegetation-activated sludge process (V-ASP) has been proved to be an environment-friendly decentralized wastewater treatment system with extra esthetic function and less footprint. However, the effects of low temperature on the treatment performance of V-ASP and related improvement methods are rarely investigated, up to now. In this work, the effect of low temperature on nitrification in V-ASP was comprehensively investigated from overall nitrification performance, substrate utilization kinetics, functional enzymatic activities, and microbial community structure shift by comparison with conventional ASP. Bio-augmentation methods in terms of single-time nitrifier-enriched biomass dosage were employed to improve nitrification efficiency in bench- and full-scale systems. The experiment results demonstrated that the NH4+-N removal efficiency in V-ASP system decreased when the operational temperature decreased from 30 to 15°C, and the decreasing extent was rather smaller compared to ASP, as well as ammonium and nitrite oxidation rates and enzymatic activities, which indicated the V-ASP system possesses high resistance to low temperature. With direct dosage of 1.6mg nitrifier/gSS sludge, the nitrification efficiency in V-ASP was enhanced dramatically from below 50% to above 90%, implying that bio-augmentation was effective for V-ASP whose enzymatic activities and microbial communities were both also improved. The feasibility and effectiveness of bio-augmentation was further confirmed in a full-scale V-ASP system after a long-term experiment which is instructive for the practical application.

Using submersible fluorescence sensors to track the removal of organic matter in decentralized wastewater treatment systems (DEWATS) in real time.

Decentralized wastewater treatment systems (DEWATS) using anaerobic treatment are increasingly being considered for wastewater treatment with options for non-potable water reuse at the community scale. One challenge for ensuring performance and reliability of DEWATS is the lack of suitable on-site sensors to monitor failure or contamination events. In this study, the aim was to use in situ fluorescence sensors to track the performance of a DEWATS, consisting of an anaerobic baffled reactor (ABR) coupled to anaerobic filter (AF) and constructed wetland (CW) treatment processes. A submersible in situ fluorometer equipped with tryptophan (TRP) and chromophoric dissolved organic matter (CDOM) sensors was deployed in each chamber of the ABR-AF-CW system, and results showed that TRP fluorescence was preferentially removed over CDOM fluorescence throughout the system. Significant relationships between TRP fluorescence and chemical oxygen demand (COD) also suggested that TRP fluorescence could be used as a surrogate for COD and soluble COD concentrations. Strong agreement between results obtained from the 1D in situ fluorometer and those obtained from a 3D benchtop fluorometer lends further support to the use of in situ fluorescence sensors to track DEWATS performance.

Management challenges for a more decentralized treatment and reuse of domestic wastewater in metropolitan areas

Abstract In a case study located in suburban sectors of the metropolitan area of the Lerma Valley (Valle de Lerma), in the province of Salta (Argentina), 24 informal decentralized wastewater treatment systems (DWWTS) were evaluated. The analyzed systems had three general configurations: A, septic tank; B, septic tank combined with upflow anaerobic sludge blanket (UASB) reactor; C, septic tank combined with UASB and a final filtration step. Statistically significant differences (p < 0.05) were observed in effluent quality, measured as total coliforms, thermotolerant coliforms, and chemical oxygen demand (COD). Treatment A was the most inefficient, and was statistically different from B and C; there were no significant differences between the latter two. Thermotolerant coliform concentrations were high in all analyzed systems and did not comply with local discharge standards in soakaway pits or in the ground. The lack of a final disinfection step in these systems is thus a weakness that needs to be addressed. The formal inclusion of DWWTS in urban planning could reduce overall investment costs, as long as the best technologies are selected for each case. Incorporation of DWWTS in formal urban planning requires an open debate in which the social perspectives of all relevant users need to be considered.

Sludge disinfection using electrical thermal treatment: The role of ohmic heating

Electrical heating has been proposed as a potential method for pathogen inactivation in human waste sludge, especially in decentralized wastewater treatment systems. In this study, we investigated the heat production and E. coli inactivation in wastewater sludge using electrical thermal treatment. Various concentrations of NaCl and NH4Cl were tested as electrolyte to enhance conductivity in sludge mixtures. At same voltage input (18V), sludge treated with direct current (DC) exhibited slower ascent of temperature and lower energy efficiencies for heat production comparing to that using alternate current (AC). However, DC power showed better performance in E. coli inactivation due to electrochemical inactivation in addition to thermal inactivation. Greater than 6log10 removal of E. coli was demonstrated within 2h using 0.15M of NaCl as electrolyte by AC or DC power. The heat production in sludge was modeled using Maxwell–Eucken and effective medium theory based on the effective electrical conductivity in the two-phase (liquid and solid) sludge mixtures. The results showed that the water and heat loss is a critical consideration in modeling of sludge temperature using ohmic heating. The experimental data also suggested that the models are less applicable to DC power because the electrochemical reactions triggered by DC reduce the concentration of NH4+ and other ions that serve as electrolyte. The results of this study contribute to the development of engineering strategies for human waste sludge management.

Design of Wastewater Treatment Plant by Using Anaerobic Filter Process on Five Stars Hotel in Surabaya

Surabaya, Indonesia's second largest city and the capital of East Java Province, has been one of the most important and busiest trading city ports in Asia. As its potential, facility and geography advantages, Surabaya has great economical potential as example hotel is one of the most rapidly growth business in Surabaya. Hotel is a commercial establishment providing lodging, meals, and other guest services. By the existence of facility and human activity, hotel produced waste such as wastewater and solid waste. Hotel wastewater produced from hotel activity sourced from cooking residue, restroom, spa, swimming pool, and etc. In this study, wastewater produced is treated by wastewater treatment with anaerobic filter as the alternative. The parameters of wastewater used are COD, BOD, TSS and oil and grease. Characteristic of wastewater is based on secondary data from earlier experimental studies with five stars hotel as the objects. Calculation of dimension based on DEWATS module. According to the results of calculation, Anaerobic Filter is designed with 1 septic tank and 8 compartments. The dimension of Anaerobic Filter is 16m x 4,5m x 2,5m within dimension of septic tank is 6m x 4,5m x 2,5m; dimension of each compartments are 2m x 4,5m x 2,5m.

Improvement of phosphorus removal by calcium addition in the iron electrocoagulation process

Small-scale wastewater treatment plants (SWTPs) are widely used as decentralized wastewater treatment systems in sparsely populated areas of Japan. Iron electrolysis, an electrocoagulation technology, is installed in these SWTPs for phosphorus removal. Phosphorus can be removed via the formation of an insoluble compound containing phosphate and iron, such as FePO4; however, it was necessary to determine the conditions under which phosphorus can be effectively and stably removed in actual SWTPs. According to previous studies using iron compounds, improved phosphorus removal was obtained by Ca addition. It is therefore thought that calcium addition may also be effective in improving the phosphorus removal during iron electrolysis in SWTPs. It is also important to determine the chemical state of iron to understand the phosphorus removal mechanism during iron electrolysis. In this study, laboratory-scale batch experiments with the iron electrolysis method were conducted to investigate the effect of phosphorus removal using treated wastewater from actual SWTPs without or with Ca addition. The results indicated that the addition of Ca improved the phosphorus removal performance. Furthermore, phosphorus removal was inhibited in the presence of high dissolved organic carbon (DOC). The X-ray absorption fine structure measurements of the produced particulates in the experiments showed no substantial change in the chemical state of iron without or with Ca addition. The statistical analyses revealed the range of improving or inhibiting effects on phosphorus removal due to the Ca and DOC. Thus, the results of this study provided useful information pertaining to the influence of coexisting substances on phosphorus removal and the chemical state of iron in the produced particulates.

Infiltration studies on soakaways receiving effluents from single household UASB and septic tank reactors

Clogging tendency has been analyzed in sandy soil soakaways at pilot scale receiving either septic tank effluent (SUM1) or UASB effluent (SUM2) and also at laboratory scale (SUMB1 and SUMB2), in relation to Chemical Oxygen Demand (COD) and accumulated Suspended Solids (SS) concentrations. Clogging was also estimated, by modeling the predicted time for infiltration hydraulic failure. The laboratory results obtained with SUMB1 and SUMB2 confirmed the results obtained for SUM1 and SUM2, showing that soakaways built in sandy soils which receive effluent from septic tanks treating predominantly domestic wastewater tended to clog 58% faster than those receiving UASB effluent. The good correlation observed between the decrease in average infiltration rate and the time of operation suggests that the UASB reactor is a promising technological alternative to septic tanks as a pre‑treatment prior to effluent soil disposal for on‑site decentralized wastewater treatment systems.

Assessment of decentralized wastewater treatment systems in the rural area of Cuenca, Ecuador

The rapid urbanization of Cuenca, Ecuador, since about 1990 has led to efforts to build and operate decentralized domestic wastewater treatment systems in periurban and rural areas. The treatment efficiency of some of these is falling, however, and others are no longer working. ETAPA, the municipal institution in charge of water supply and sanitation, is evaluating them fully to identify common operating and maintenance difficulties, as well as deficiencies in design and/or construction. This paper presents an evaluation of the physical infrastructure and characterization of the treatment processes performed historically. The objective is to overcome the technical deficiencies and adopt a long-term, sustainable O&M plan.

Decentralized wastewater treatment systems: Efficiency and its estimated impact against onsite natural water pollution status. A Romanian case study

Abstract The paper proposes discussions of some concrete datasets resulted for three solutions of wastewaters management systems applied in case of a Romanian petroleum chemical company. These management options, differing especially in type (centralized or decentralized system) and wastewaters treatment system, were emphasized, mainly by real datasets on treatment performance, polluting species removals, risk of some residuals in treated effluents and natural watercourse nearby, but also environmental impact assessment, using the alternative methodology of global pollution index (I*GP). Interpreting datasets based on in-time analyzed physical-chemical quality indicators and river characteristics, it was estimated the standard quality classification of receiving watercourse as ‘medium’ (water quality indices, WQI = 55.58 − 67.47), affected more or less by treated effluents discharged in it. The environmental risks of some selected residuals in treated effluents and natural aquatic receptor were assessed by the risk quotient (RQ) which appreciated the risk of residuals toward aquatic life as no to low-medium level, with few exceptions. The impact assessment results conclude an admissible pollution status of natural water receptor due to company wastewaters mechanical–chemical–biological treatment plant activities, in decentralized (I*GP = 1.73) and centralized (I*GP = 1.83) system, but generating discomfort effects to aquatic life due to only mechanical–chemical treatment plant operating in decentralized system (I*GP = 2.205). Direct discharge of un-treated industrial wastewaters had been interdicted. These real results underline that decentralized systems are reliable, qualitatively efficient and costly effective, offering long-term solution, and that all wastewaters management strategies of petroleum chemical company had been adapted to specific on-site functioning situations.

Using Permeable Reactive Barriers to Remediate Heavy Metal-Contaminated Groundwater through a Laboratory Column Experiment

Recently, groundwater sources are being polluted by various activities such as agriculture, livestock, decentralized wastewater treatment systems and acid rain. Groundwater can also be polluted by landfill leachate, sewage, mine tailings, non-engineered deep well disposal of liquid waste and seepage from industrial waste lagoons. There are many studies reported contaminated groundwater remediation using Permeable Reactive Barrier systems (PRBs) and many countries happen to use this system to eliminate groundwater contaminants. This study reports the outcomes of the batch and the column test experiments conducted to evaluate the removal efficiency of four heavy metals: Cd(II), Cu(II), Fe(II) and Pb(II) using five locally available reactive materials (in Sri Lanka) with grain sizes less than or equal to 2.0 mm: Red Soil (RS), Laterite Soil (LS), Bangadeniya Soil (BS), Burnt Clay Tile (BCT) and Coconut Shell Biochar (CSB) as PRB materials. Seven columns (A-G) were filled with the reactive material of CSB (column E) and reactive material mixtures; RS + CSB (column A), LS + CSB (column B), BCT+CSB (column C), BS + CSB (column F), RS + LS (column G), with a weight ratio of 50:50 and RS + BCT + CSB (column D) with a weight ratio of 100/3:100/3:100/3. The results showed that the reactive materials filled in column A, B, D, F and G removed the metal concentrations, with a removal efficiency of over 90%, except Cd in the column with BCT + CSB mixture. Considering both the removal efficiency and the hydraulic conductivity of the columns, materials in column A, B, D and F are more effective than the others as PRB adsorbents for heavy metal remediation, while columns C and E have lower removal efficiency.

Is the Swedish wastewater sector ready for a transition to source separation?

Source separation of urine for recycling has been applied in small-scale and decentralized wastewater systems in Sweden for the past 25 years and for blackwater for pollution control even longer. The Swedish experience with source separating nutrient recycling systems is relatively well documented; however, few reports have specifically studied the potential for expansion of this practice. The aim of this study is to fill this knowledge gap by assessing the status of source-separating technologies in Sweden based on transition theory. This study uses a multi-level perspective to determine how ready the Swedish wastewater sector is for transitioning to alternative systems. Given the stability of the existing sewage wastewater regime, it seems unlikely that changes within the regime will lead to a quick and large-scale transition to source separation. Instead, the initiative must come from the niche itself, exploiting institutional cracks in the regime and opportunities from shifting trends in the landscape. If source separation is to be mainstreamed in Sweden, it will need to break into markets within the wastewater jurisdictions. In order to do so, further knowledge needs to be developed that will overcome glitches with immature technologies, uncertain legal conditions/status, investigate potential risks, and clearly define complementary system advantages. This may require the use of new perspectives that focus on holistic sustainable use of resources, including other nutrients than phosphorous, and taking into account global issues such as planetary boundaries and effects from climate change, such as water scarcity. This knowledge can then be used to establish guidelines, norms, and standards, as well as clarify the legislative structures that can support such a transition. There is also a strong need to improve knowledge dissemination regarding best-practices for implementing source-separation technologies and supporting organizational structures. Similarly, support for entrepreneurial activities within the niche needs to increase, not least through strengthening social networks and communication platforms.

Performance Analysis of Vertical Flow Constructed Wetland to Treat Domestic Wastewater using Two Different Filter Media and Canna as a Plant

Objectives: Investigation for low cost, small scale, decentralized waste water treatment system for small communities and suburbs was done in this study. Total passive system, artificially engineered to treat domestic wastewater by mimicking the natural water purifying processes of river, swamp and wetlands, gives us what is known as constructed wetland. Methods: In this paper, we carried out the design and installation of two laboratory scale vertical flow constructed wetland within the premises of Lovely Professional University sewage treatment Plant. Two separate systems of height 45 cm and average diameter 15 cm was created, one with gravel and sand as a media and other with gravel, sand and soil as a media. Both the wetland was planted with Canna sp as an excellent phytoremediating plant and was available easily in locality. This plant shows very high growth rate and has a large rhizome. The waste water was fed through the 1.5cm diameter pipe. Eight liter of pretreated wastewater from the university Sewage treatment plant was fed with a retention period of 2 days, feeding was done in such a way that the gravitation does all the work, as the feeding tank was placed higher above both the wetland. Feeding was control by the valve. Findings: Initial analysis of wastewater subjected to wetland shows dramatic reduction in the visible suspended solids and color. Dissolve oxygen was improved from 3.6±0.4mg/L to 5±.2mg/L in both the wetland system. pH in effluent from both the CW was in between 6.5-8.5, pH of influent was in between 9±0.4. BOD5 at 20oC of both the effluent form vertical flow CW was found to be below 30mg/L as against the 180-190mg/L of influent given to wetland, which was well within the range as per as Environment (protection) rules, 1986 of GOI for inland surface water discharge. Application/Improvement: Potable water is already a scare resource; even the water for irrigation is becoming scarce and not to forget the depleting ground water table. The findings of this study shows us a potential cheap method to treat wastewater generated, which can be used to met our agricultural water demand and can even be used to recharge ground water.

Potential of Decentralised Wastewater Treatment Systems Applicable to India

Appropriate sanitation facilities are still a challenge in many parts of the world, particularly in developing countries. With regard to almost 950 million people defecating in the open, the question arises whether the existing treatment facilities are sufficient to provide for a healthy sanitation in the world. This paper mainly emphasizes on developing countries (particularly, India) where cost is generally a very important parameter of judgment for choosing the appropriate system. This makes decentralized treatment systems much more suitable for installation as they are easier to build and operate, both financially and technically. This paper includes basic differences and fundamental explanations about the processes involved in different decentralized treatment systems and their comparison on the basis of installation cost carried out by using a technology ranking method. It is concluded that waste stabilisation ponds would be most cost effective solution from capital investment point of view. However, Multi Criteria Analysis (MCA) should be carried out for appropriate technology selection in different scenarios on the basis of different criteria. New developments in sanitation technologies can play an important role in selecting appropriate sanitation technology for a particular scenario.

Comparison of linear and nonlinear dimension reduction techniques for automated process monitoring of a decentralized wastewater treatment facility

Multivariate statistical methods for online process monitoring have been widely applied to chemical, biological, and engineered systems. While methods based on principal component analysis (PCA) are popular, more recently kernel PCA (KPCA) and locally linear embedding (LLE) have been utilized to better model nonlinear process data. Additionally, various forms of dynamic and adaptive monitoring schemes have been proposed to address time-varying features in these processes. In this analysis, we extend a common simulation study in order to account for autocorrelation and nonstationarity in process data and comprehensively compare the monitoring performances of static, dynamic, adaptive, and adaptive–dynamic versions of PCA, KPCA, and LLE. Furthermore, we evaluate a nonparametric method to set thresholds for monitoring statistics and compare results with the standard parametric approaches. We then apply these methods to real-world data collected from a decentralized wastewater treatment system during normal and abnormal operations. From the simulation study, adaptive–dynamic versions of all three methods generally improve results when the process is autocorrelated and nonstationary. In the case study, adaptive–dynamic versions of PCA, KPCA, and LLE all flag a strong system fault, but nonparametric thresholds considerably reduce the number of false alarms for all three methods under normal operating conditions.