Sludge production from municipal wastewater treatment in sewage ...

Energy Sources, Part A: Recovery, Utilization, and Environmental Effects

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Sludge production from municipal wastewater treatment in sewage treatment plant Ayhan Demirbas, Gaber Edris & Walid M. Alalayah To cite this article: Ayhan Demirbas, Gaber Edris & Walid M. Alalayah (2017): Sludge production from municipal wastewater treatment in sewage treatment plant, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, DOI: 10.1080/15567036.2017.1283551 To link to this article: http://dx.doi.org/10.1080/15567036.2017.1283551

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Date: 17 April 2017, At: 14:45

ENERGY SOURCES, PART A: RECOVERY, UTILIZATION, AND ENVIRONMENTAL EFFECTS http://dx.doi.org/10.1080/15567036.2017.1283551

Sludge production from municipal wastewater treatment in sewage treatment plant Ayhan Demirbas, Gaber Edris, and Walid M. Alalayah Department of Chemical and Materials Engineering, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia ABSTRACT

Sewage sludge is obtained from wastewater treatment in sewage treatment plants. The sludge consists of two basic forms, sludge and secondary sludge, also known as activated sludge in the case of activated sludge process. Municipal sewage sludge (MSS) or only sewage can be a solid, semi-solid, or liquid muddy residue. It contains mainly proteins, sugars, detergents, phenols, and lipids and also includes toxic and hazardous organic and inorganic pollutants source. Sewage is a mixture of domestic and industrial wastes that contains above 99% water. It is produced by residential, institutional, commercial, and industrial establishments. Sludge is semi-solid slurry and can be produced as sewage sludge from wastewater treatment processes. The sludge consists of a wide range of harmful substances such as dioxins and furans, polychlorinated biphenyls, organochlorine pesticides, absorbed and extracted chlorine derivatives, polycyclic aromatic hydrocarbons, phenols and their derivatives, phthalate, and others. Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage. It includes physical, chemical, and biological processes to remove these contaminants and produce environmentally safe treated wastewater. The treatment is divided into three stages: pretreatment, primary treatment, and secondary treatment. In pretreatment, large solids and grit are removed by screening. In primary treatment, the water is left to stand so that solids can sink to the bottom and oil and grease can rise to the surface. In secondary treatment, the sludge is further treated in sludge digesters.

KEYWORDS

Municipal sewage sludge; sewage treatment; sludge; sludge treatment; wastewater treatment

Introduction Wastewater treatment (WWT) is the process of removing contaminants from wastewater. It includes physical, chemical, and biological processes to eliminate pollutants. Once treated, water can then be released back into nature (Dursun et al., 2004; Pollice et al., 2007; Bharathiraja et al., 2014). Sludge is a by-product of water and wastewater treatment operations. A by-product of sewage treatment is usually a semi-solid waste or slurry called sewage sludge that has to undergo further treatment before being suitable for disposal or land application (Demirbas et al., 2016). Sewage sludge is obtained from wastewater treatment in sewage treatment plants. The sludge consists of two basic forms, sludge and secondary sludge, also known as activated sludge in the case of activated sludge process. Sludge from biological treatment operations is sometimes referred to as wastewater biosolids. Wastewater sludge contains a variety of organic and inorganic compounds (Bharathiraja et al., 2014). The generation of municipal sewage sludge (MSS) has increased in parallel with rapid industrialization (Demirbas, 2003). CONTACT Ayhan Demirbas [email protected] Department of Chemical and Materials Engineering, Faculty of Engineering, King Abdulaziz University, POB 8020421589, Jeddah, Saudi Arabia. Color versions of one or more figures in the article can be found online at www.tandfonline.com/ueso. © 2017 Taylor & Francis Group, LLC

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Sludge is mostly water, which is removed from liquid wastewaters containing less solid matter. The primary sludge contains precipitated solids that occur during the primary treatment in the primary clarifiers. The secondary sludge separated in the secondary cleaners contains the sewage sludge purified from the secondary treatment bioreactors (Dumrul and Taskesen, 2017; Ak and Dumrul, 2017). Sewage contains above 99% water that is a mixture of domestic and industrial wastes. Sewage is produced by residential, institutional, commercial, and industrial establishments. Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage. It includes physical, chemical, and biological processes to remove these contaminants and produce environmentally safe treated wastewater. As nouns, the difference between sewage and sludge is that sludge is a generic term for solids separated from suspension in a liquid, while sewage is a suspension of water and solid waste, transported by sewers to be disposed of or processed. Sludge is semi-solid slurry and can be produced as sewage sludge from wastewater treatment processes or as a settled suspension obtained from conventional drinking water treatment and numerous other industrial processes.

Sewage treatment stages The term sewage treatment plant is nowadays often replaced with the term wastewater treatment plant. Sewage treatment is mainly divided into three stages: preliminary treatment or pretreatment, primary treatment, and secondary treatment (Monfort and Baleux, 1990; Randall and Sen, 1996; Logan and Regan, 2006; Kartal et al., 2010; Fu et al., 2012). Pretreatment or preliminary treatment Figure 1 shows a simple flow diagram of sewage treatment. In pretreatment, coarse solids (with a diameter of more than 2 cm) and grit (heavy solids) are removed by screening. These coarse materials are not included in biosolids. Preliminary treatment may include a sand or grit channel or chamber, where the velocity of the incoming sewage is adjusted to allow the settlement of sand, grit, stones, and broken glass. These coarse solids are removed because they may damage pumps and other equipment. The influent in sewage water passes through a bar screen to remove all large objects such as cans, rags, sticks, and plastic packets carried in the sewage stream. The solids are collected and later disposed in a landfill or incinerated. Bar screens or mesh screens of varying sizes may be used to optimize solids removal. Primary treatment In primary treatment grit (fine, hard solids), suspended solids and scum are removed in two stages: pre-aeration and sedimentation. The settled and floating materials are removed, and the remaining liquid may be discharged or subjected to secondary treatment. Grease and oil from the floating material can sometimes be recovered for saponification or biodiesel production. The wastewater is aerated by air pumped through perforated pipes near the floor of the tanks. This aeration makes the wastewater less dense, causing the grit to settle out. As the air jets are positioned such that the water is swirling as it moves down the tanks, the suspended solids are prevented from settling out. The air also provides dissolved oxygen for the bacteria to use later in the process, but the wastewater is not in these tanks long enough for bacterial action to occur in the process. The solids are removed from the bottom of the tanks by scrapers, and scum is washed off with water jets. The scum and solids are brought to a common collection point where they are combined to form sludge and sent off for secondary treatment.

ENERGY SOURCES, PART A: RECOVERY, UTILIZATION, AND ENVIRONMENTAL EFFECTS

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SEWAGE

Pretreatment Coarse solid Screening

Landfill

Aneorobic digestion

Grit

Activation tank

Landfill

Biogas Aneorobic digestion Sludge digester

Primary sedimentation tank Biogas

Biogas

Clarified effluent

Lagoons

Dewatering

Secondary sedimentation tank

Landfill

Effluent discharged

Sludge

Figure 1. Simple flow diagram of sewage treatment.

In the primary sedimentation stage, sewage flows through large tanks, commonly called presettling basins, primary sedimentation tanks, or primary clarifiers. The tanks are used to settle sludge, while grease and oils rise to the surface and are skimmed off. The solids are scraped off the bottom, and the scum is washed off with water jets. These two substances are combined to form sludge. The primary wastewater treatment involves gravity sedimentation of the screened wastewater for collapsed solids. Part of the event that is suspended in the waste stream is passed through the primary operation. The residue from this process is a concentrated suspension, called primary sludge, which is further treated to become a biosolid. Secondary treatment The secondary wastewater treatment is carried out through a biological process that removes the biodegradable material. This processing process uses microorganisms to consume dissolved and suspended organic matter, thus producing carbon dioxide and other by-products. The organic material also provides the nutrients necessary to maintain the populations of microorganisms. When the microorganisms are fed, the densities are increased, the process is placed at the bottom of the water reservoir, and the cleaned water is separated into a concentrated suspension called as secondary sludge, biological sludge, waste activated sludge, or vibration filter humus.

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Secondary treatment may require a separation process to remove the microorganisms from the treated water prior to discharge or tertiary treatment. The organic compounds within the sludge are converted to carboxylic acids and then finally to carbon dioxide by aerobic fermentation and methane and by anaerobic fermentation. Thus, obtained biogas is a valuable source of fuel. When the sludge leaves the digesters, it has undergone a 50% volume reduction. Secondary treatment is designed to substantially degrade the biological content of the sewage which is derived from human waste, food waste, soaps, and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. To be effective, the biota requires both oxygen and food to live. Some secondary treatment methods include a secondary clarifier to settle out and separate biological flock or filter material grown in the secondary treatment bioreactor. To use less space, treat difficult waste, and intermittent flows, a number of designs of hybrid treatment plants have been produced. Tertiary treatment When very high quality waste is required, such as direct discharge to the source of drinking water, third or further processing is applied. The solid residue collected by the tertiary treatment consists essentially of chemicals added to the final recovered raw waste before emptying and is therefore not included in the biosolids. Combined primer and secondary effectors constitute the majority of materials used for municipal biosolid production. Careful management throughout the entire treatment process allows plant operators to control solid ingredients, nutrients, and other components of biosolids. The purpose of tertiary treatment is to provide a final treatment stage to further improve the effluent quality before it is discharged to the receiving environment (sea, river, lake, wet lands, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called as effluent polishing. Fourth treatment stage Micropollutants such as pharmaceuticals, ingredients of household chemicals, chemicals used in small businesses or industries, environmental persistent pharmaceutical pollutant (EPPP), or pesticides may not be eliminated in the conventional treatment process (primary, secondary, and tertiary treatment) and therefore lead to water pollution. The micropollutants eliminate via a fourth treatment stage during sewage treatment; however, since those techniques are still costly, they are not yet applied on a regular basis.

Physical and chemical characteristics of municipal sewage sludge The physical characteristics of sludge play an important role in the operation of anaerobic digestion (Dursun et al., 2004; Pollice et al., 2007). The physical and chemical characteristics of municipal sewage sludge samples are given in Table 1 (Xu et al., 2012; Werle and Dudziak, 2014; Bharathiraja et al., 2014; Dumrul and Taskesen, 2017). The sludge consists of a wide range of harmful substances such as dioxins and furans, polychlorinated biphenyls, organochlorine pesticides, absorbed and extracted chlorine derivatives, polycyclic aromatic hydrocarbons, phenols and their derivatives, phthalate, and others (Berset and Holzer, 1999; Aznar et al., 2009; Xu et al., 2012, 2013). The carbohydrate-rich WSS can be used in biogas production as a more efficient material (Imu and Samuel, 2014). As it is seen in Table 1, the inorganic part of the sewage sludge is mainly the compounds of iron, phosphorus, calcium, aluminum, and sulfur, including traces of heavy metals such as zinc, chromium, mercury, lead, nickel, cadmium, and copper (Demirbas, 2007, 2008). Zinc, copper, and lead are present in high


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Table 1. Physical and chemical characteristics of municipal sewage sludge (MSS). Characteristic pH Bulk density (kg/L) Particle density (kg/L) Organic matter (g/kg) Ash (g/kg) Higher heating value (MJ/kg) Organic carbon (g/kg) Oxygen (g/kg) Nitrogen (g/kg) Hydrogen (g/kg) Iron (g/kg) Phosphorus (g/kg) Calcium (g/kg) Aluminum (g/kg) Sulfur (g/kg) Magnesium (g/kg) Barium (g/kg) Zinc (g/kg) Silicon (g/kg) Potassium (g/kg) Copper (g/kg) Chromium (g/kg) Nickel (g/kg) Manganese (g/kg) Lead (g/kg) Sodium (g/kg) Tin (g/kg)

7.1–8.2 1.26–2.56 2.40–2.56 418–592 345–440 11.3–14.2 205–403 185–219 45–49 40–46 24–38 22–30 21–29 16–22 11–17 3.2–4.8 2.8–4.2 2.4–3.6 2.2–2.7 1.2–1.6 0.7–1.2 0.5–0.9 0.3–0.5 0.1–0.2 0.1–0.3 0.1–0.2 0.1–0.2

Concentration values in dry mass basis.

quantities, but other heavy metal is found in traces levels (Karvelas et al., 2003; Marrero et al., 2004; Abad et al., 2005; Cai et al., 2007; Fuentes et al., 2008; Szymański et al., 2011; Li et al., 2012)Both phosphorus and potassium in the sewage sludge have a high fertilizer value. On the other hand, the sewage sludge contains the cementitious elements such as Ca, Si, Al, and Fe. Sewage ash mainly can be used for cement production.

Main types of sludge treatment processes Sludge is treated by various processes that can be used in various combinations. Following are the different types of sludge treatment processes with little detail. Main sludge treatment processes are given in Table 2. Sewage sludge is usually treated by one or several of the following treatment steps: lime stabilization, thickening, dewatering, drying, anaerobic digestion, or composting. The solids in the sludge contain nutrients of value to plants and humus-like material that can be used as fertilizer. There is no process that removes the need for purified sewage sludge. Toxic substances released into sewer from industrial processes are polluted by many muds from industrial or commercial areas. Increasing concentrations of such substances may make the mud suitable for agricultural use and may then need to be incinerated or landfilled. Sludge is the largest by-product from wastewater treatment plants, and its disposal is one of the most challenging environmental problems in wastewater treating processes. Before sludge can be disposed, it needs to be treated to a certain degree. The type of treatment needed depends on the disposal method proposed (Harhangi et al., 2012). There are principally three final disposal strategies for wastewater sludge and sludge components. Sludge and sludge components may be deposited on land, in the sea or to a certain extent in the air (Ødegaard et al., 2002). The activated sludge process is a process used to treat sewage and industrial wastewater using a biological flock of air and bacteria and protozoa (Demirbas and Cakmak, 1996). Activated sludge is

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Table 2. Main sludge treatment processes. Treatment process Reason for importance Sludge degritting The most effective method of sludge degritting is to apply centrifugal forces in a fluid system to ensure that the sand particles are separated from the organic sludge. Dewatering Dewatering is a physical unit operation aimed at reducing the moisture content of the sludge. Dewatering can be improved by chemical conditioning such as the addition of a polymer. Drying The purpose of drying the sludge is to make the water content suitable for processing the sludge or fertilizer by lowering the water content to less than 10% by evaporation. Drying lagoons The sludge drying lagoons, which are suitable only for digested sludge treatment, are composed of shallowgrounded basins surrounded by dykes. Filtration Vacuum filtration is used for dewatering water from raw and digested wastewater sludge. The filtration process consists of four basic steps: the polymer control zone, the gravity drainage zone for excess water, the low pressure, and the high pressure zones. Stabilization Sludge is stabilized to reduce its pathogenic content, to remove irritating odors from the surface, and to reduce or eliminate the potential for imperfections. The technologies used for stabilization include lime stabilization, heat treatment, aerobic digestion, anaerobic digestion, and composting. Blending The sludge is blended to form a uniform blend with the downstream operations and process. Thickening Thickening is an application of increasing the solids content of the sludge by removing some of the liquid content. Conditioning Conditioning contains the chemical or physical treatment of sludge to improve its dewatering characteristics. Chemical Chemical precipitation is the most common technology used to remove metal ions from solutions such as precipitation wastewater containing toxic metals. Heat treatment The process contains the treatment of sludge by heating in a pressure vessel. Composting Composting contains aerobic degradation of organic matter. The purpose of sludge composting is to stabilize biodegradable organic matter, to destroy pathogenic organisms, and to reduce the volume/amount of waste. Anaerobic The anaerobic digestion process involves the anaerobic reduction of the organic substance with biological digestion activity in the sludge. Sludge digestion Sludge digestion is a biological process in which organic solids are decomposed into stable substances. There are three types of sludge: primary sludge, secondary sludge, and digested settled sludge. Aerobic digestion The anaerobic digestion process involves the anaerobic reduction of the organic substance with biological activity in the sludge. Reuse as The solids in the sludge contain nutrients of value to plants and humus-like material that can be used as fertilizer fertilizer. Phosphorus and potassium in the sludge have a high fertilizer value. Activated The activated sludge process is a process used to treat sewage and industrial wastewater using a biological sludge flock of air and bacteria and protozoa. The purpose of the activated sludge process is oxidation of organic substances, oxidation of nitrogenous substances to ammonium and nitrogen, and removal of nutrients. Sulfur removal When sludge contains high amounts of sulfur compounds, problems may occur in operating such anaerobic digestion process. Sulfide may inhibit the methane formation, and it also forms hydrogen sulfide gas which is toxic and corrosive. Cement The sludge contains the cementitious elements such as Ca, Si, Al, and Fe. Sewage ash mainly can be used for production cement production.

the conversion of organic substances in microorganisms into carbon dioxide, water, and other inorganic compounds. Composting process involves aerobic degradation of organic matter. The objective of sludge composting is to biologically stabilize putrescible organics, destroy pathogenic organisms, and reduce the volume of waste (Bharathiraja et al., 2014). During composting organic material undergoes biological degradation, resulting in a 20–30% reduction of volatile solids (Henze, 2008). In composting, aerobic microorganisms convert much of the organic matter into carbon dioxide leaving a relatively stable odor-free substance which has some value as a fertilizer (Karvelas et al., 2003). The resulting end product is stable and may be used as a soil conditioner in agricultural applications. Aerobic composting is more commonly used than anaerobic composting (Demirbas et al., 2016). The major advantage of this is compost is a very good fertilizer, but it is not much used yet (Li et al., 2012). Phosphorus and potassium in the sewage sludge have a high fertilizer value (Demirbas, 2001). The anaerobic digestion process involves the anaerobic reduction of organic matter in the sludge by biological activity (Barwal and Chaudhary, 2014). Anaerobic digestion consists of two stages that


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occur simultaneously in digesting sludge. The first consists of hydrolysis of the high molecular weight organic compounds and conversion of organic acids by acid forming bacteria. The second stage is gasification of the organic acids to methane and carbon dioxide by the acid splitting methane forming bacteria (Demirbas et al., 2016). Aerobic digestion sludge treatment is similar to activated sludge treatment. The function of aerobic digestion is to stabilize the waste slurry solids through prolonged ventilation, thereby reducing biological oxygen demand (BOD) and eliminating volatile solids. Biodegradable material and microorganism include prolonged oxidation of cellular material in open tanks. During this time, the biological material is reduced by about half of its original amount (Demirbas et al., 2016). When sludge contains high amounts of sulfur compounds, problems may occur in operating such anaerobic digestion process (Demirbas, 2006; Demirbas et al., 2015; Demirbas, 2016). Sulfide may inhibit the methane formation, and it also forms hydrogen sulfide gas which is toxic and corrosive (Sarner, 1990). For this purpose, sulfur compounds in the sludge can be recovered and converted into elemental sulfur. Two separate anaerobic processes are used for this purpose. First, sulfurcontaining organic compounds are converted to sulfur compounds. The sulfide compounds are stripped from the mixture by stripping gas such as carbon dioxide and then converted into elemental sulfur in a basic adsorbent.

Conclusion Sewage sludge is obtained from wastewater treatment at sewage treatment plants. Sewerage is a mixture of domestic and industrial waste containing over 99% water and is formed by residential, institutional, commercial, and industrial establishments. It consists of a wide variety of harmful substances such as sludge, dioxins and furans, polychlorinated biphenyls, organochlorine insecticides, absorptive and derived chlorine derivatives, polycyclic aromatic hydrocarbons, phenols and their derivatives, phthalates, and others. Wastewater treatment is the process of removing wastewater primarily from domestic wastes. To remove these pollutants, physical, chemical, and biological processes are applied and produce environmentally safe treated wastewater. The treatment is divided into three main steps: pretreatment, primary treatment, and secondary treatment. The sludge accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. The sludge treatment methodology to use depends upon the amount of sludge generated and other site-specific conditions.

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