Nutrient Pollution and Wastewater Treatment Systems
Summary and Keywords
Since the industrial revolution, societies across the globe have observed significant urbanization and population growth. Newer technologies, industries, and manufacturing plants have evolved over the period to develop sophisticated infrastructures and amenities for mankind. To achieve this, communities have utilized and exploited natural resources, resulting in sustained environmental degradation and pollution. Among various adverse ecological effects, nutrient contamination in water is posing serious problems for the water bodies worldwide.
Nitrogen and phosphorus are the basic constituents for the growth and reproduction of living organisms and occur naturally in the soil, air, and water. However, human activities are affecting their natural cycles and causing excessive dumping into the surface and groundwater systems. Higher concentrations of nitrogen and phosphorus-based nutrients in water resources lead to eutrophication, reduction in sunlight, lower dissolved oxygen levels, changing rates of plant growth, reproduction patterns, and overall deterioration of water quality. Economically, this pollution can impact the fishing industry, recreational businesses, property values, and tourism. Also, using nutrient-polluted lakes or rivers as potable water sources may result in excess nitrates in drinking water, production of disinfection by-products, and associated health effects.
Nutrients contamination in water commonly originates from point and non-point sources. Point sources are the specific discharge locations, like wastewater treatment plants (WWTP), industries, and municipal waste systems; whereas, non-point sources are discrete dischargers, like agricultural lands and storm water runoffs. Compared to non-point sources, point sources are easier to identify, regulate, and treat. WWTPs receive sewage from domestic, business, and industrial settings. With growing pollution concerns, nutrients removal and recovery at treatment plants is gaining significant attention. Newer chemical and biological nutrient removal processes are emerging to treat wastewater. Nitrogen removal mainly involves nitrification-denitrification processes; whereas, phosphorus removal includes biological uptake, chemical precipitation, or filtration. In regards to non-point sources, authorities are encouraging best management practices to control pollution loads to waterways.
Governments are opting for novel strategies like source nutrient reduction schemes, bioremediation processes, stringent effluent limits, and nutrient trading programs. Source nutrient reduction strategies such as discouraging or banning use of phosphorus-rich detergents and selective chemicals, industrial pretreatment programs, and stormwater management programs can be effective by reducing nutrient loads to WWTPs. Bioremediation techniques such as riparian areas, natural and constructed wetlands, and treatment ponds can capture nutrients from agricultural lands or sewage treatment plant effluents. Nutrient trading programs allow purchase/sale of equivalent environmental credits between point and non-point nutrient dischargers to manage overall nutrient discharges in watersheds at lower costs.
Nutrient pollution impacts are quite evident and documented in many parts of the world. Governments and environmental organizations are undertaking several waterways remediation projects to improve water quality and restore aquatic ecosystems. Shrinking freshwater reserves and rising water demands are compelling communities to make efficient use of the available water resources. With smarter choices and useful strategies, nutrient pollution in the water can be contained to a reasonable extent. As responsible members of the community, it is important for us to understand this key environmental issue as well as to learn the current and future needs to alleviate this problem.
Industrial revolution, technological advances, urbanization, and population growth since the 19th century have evolved newer relationships between human beings and the environment. To develop infrastructures, products, and other commodities, mankind has exploited natural resources in different parts of the globe resulting in sustained ecological degradation. Over the years, anthropogenic activities have resulted in the release of undesired waste materials as pollution into the surrounding ambience. Common types of environmental pollutions include air, water, land, noise, thermal, light, and nuclear pollutions (Ahluwalia, 2015; Harrison, 2001). These discharges are harmful for human health and other living organisms. Pollution problems pose global threat to communities, with challenges getting more complex and serious with time. Hence it is important to understand the extent of damages these environmental contaminants can cause and identify methods to alleviate them.
Among many pollution types, water contamination is a key issue affecting civilization. Water is the basic component for the survival and wellbeing of life. However, pollution of waterbodies like rivers, lakes, seas, oceans, streams, and groundwater aquifers due to discharges from agricultural runoffs (with excess fertilizers), industrial effluents (with toxic chemicals), and domestic sewage (with human and animal wastes) are adversely impacting aquatic ecosystems and societies relying on them for drinking and utility water (Leng, 2009). Shrinking clean water and pristine freshwater reserves and rising water demands are compelling communities to make efficient use of the available water resources.
Chemicals entering waterways are taken up by aquatic organisms. Mercury, lead, copper, cadmium, chromium, and other metals accumulating in the water prove toxic for living beings (Förstner & Wittmann, 2012). Oil spills from ships or tankers into waterbodies may result in the mortality of aquatic organisms and seabirds (Garrott, Eberhardt, & Burn, 1993; Sackmann & Becker, 2015). In addition to human-induced activities, natural processes like soil erosion, minerals leaching from rocks, and organic matter decay can also contribute towards water pollution.
Water quality is a major challenge that humanity faces in the 21st century, with polluted waterways contributing to waterborne diseases and other health-related problems (Schwarzenbach, Egli, Hofstetter, Von Gunten, & Wehrli, 2010). A major topic that has gained significant attention in recent decades is nutrients pollution, resulting from nutrients accumulation in lakes, rivers, and other water reservoirs (Bennett, Carpenter, & Caraco, 2001; Leng, 2009). Wastewater treatment plants in urban settings and the agricultural sector are the major contributors of nutrients to waterbodies. An extensive group of researchers and practitioners are focusing their work towards understanding and alleviating nutrient pollution problems.
Aquatic Environment and Nutrients Cycle
Nutrients are the vital constituents required for growth, maintenance, and proper functioning of living organisms. Sixteen important elements have been recognized as necessary for plant growth, including non-minerals (carbon, hydrogen, and oxygen), mineral macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur), and mineral micronutrients such as boron, copper, iron, chloride, manganese, molybdenum, and zinc (Barker & Pilbeam, 2015; Römheld & Marschner, 1991). Nature’s cycle involves consumption, movement, exchange, and recycling of elements within the ecosystem. These transformations include biological, chemical, and geological processes involving both living and non-living constituents.
Aquatic plants typically need water, sunlight, and nutrients to grow and survive. Macronutrients (such as certain nitrogen and phosphorus-based compounds) are key constituents responsible for plant growth in aquatic environments. Nitrogen is needed for protein synthesis and is an important part of chlorophyll, while phosphorus assists with production of sugars and is an essential component of photosynthesis (Barker & Pilbeam, 2015). Floating or submerged aquatic plants obtain nutrients directly from water, whereas rooted plants receive nutrients from soils. Aquatic plants may also receive nutrients from plant detritus or decaying leaves.
Although nitrogen and phosphorus are essential for aquatic plants, excessive accumulation of nutrients in waterbodies can stimulate rapid growth of invasive biomass and microscopic floating plants in water (such as algae, water hyacinth, phytoplankton, or other aquatic plants). This phenomenon is commonly known as eutrophication process (Chislock, Doster, Zitomer, & Wilson, 2013; Diersing, 2009; Khan & Ansari, 2005). Eutrophication leads to water quality issues and interferes with its beneficial use. Figure 1 shows an example of excessive macrophyte growth in the Yakima River near Kiona, Washington.
Two types of eutrophication processes typically observed include natural and cultural. On the one hand, natural eutrophication results from the natural accumulation of nutrients, sediments, and organics within the basin over a longer period of time. On the other hand, the cultural eutrophication process involves excessive dumping of nitrogen and phosphorus into waterbodies from human activities (Ansari, Gill, Lanza, & Rast, 2011; Bennett, Carpenter, & Caraco, 2001). Mankind causes natural habitat alteration and artificial deposition of nutrients from sources like sewage treatment systems, industrial discharges, septic tanks, agricultural fertilizers, urban run-offs, and animal farming wastes (Chislock et al., 2013; Selman & Greenhalgh, 2010). Nutrient loadings to waterways depend upon types and amounts of human activities occurring within the watersheds. Typical pathways for movement of nutrients in the environment towards the waterways are depicted in Figure 2.
Several undesired effects of the eutrophication process include reduced sunlight, lower dissolved oxygen concentration, dead and decaying organic matter, increased anoxic areas, foul smells, reduction in biodiversity, changes to plant growth rates and reproduction patterns, death of fishes and other aquatic organisms, and water quality degradation (Carpenter, 2008; Diersing, 2009). Nutrients pollution can harm both freshwater and marine life. In severe eutrophic conditions, harmful algal blooms (HAB) produce natural toxins and are often associated with large-scale marine mortality events and shellfish poisoning (Anderson, Glibert, & Burkholder, 2002; Diersing, 2009). Figure 3 shows the harmful algal blooms in Lake Le-Auqa-Na, Illinois, in 2012.
Use of nutrient polluted water for domestic purposes may result in excess nitrate concentrations in the drinking water, production of disinfection by- products and associated health effects. To handle these problems, water treatment cost increases considerably for public water supplies (Leng, 2009; Reilly, Horne, & Miller, 1999). Excess nitrates in water affects mostly infants and causes disease commonly known as blue baby syndrome. Reviewing impacts on cost economics, nutrient pollution can cause recreational advisories, diminish tourism revenues, cause property devaluation, and affect fishing industries and associated businesses (Chislock et al., 2013; Nilsson & Gössling, 2013). Overall, remediation of polluted lakes and rivers is important to minimize environmental, ecological, social, and economic impacts.
Overview of Polluted Waterways
Many countries worldwide are facing unfavorable consequences of water quality degradation due to eutrophication, with documented case studies showing the extent and gravity of this problem. Governments and scientific organizations have undertaken projects to understand, monitor, and remediate these damaging environmental impacts. Knud-Hansen (1994) discusses that, by 1970, nearly 10,000 public lakes were affected by excessive human-influenced nutrient enrichment. Worldwide, waterways and lakes have suffered from various forms of control, manipulation, and pollution for the past 6,000 years (Nienhuis & Leuven, 2001). Arthington and Pusey (2003) describe how, since 1857, Australia has constructed many weirs, floodplain levee banks, large dams, and inter- and intra-basin water transfer schemes. Such flow regulations have resulted in hydrological changes in major rivers and are widely acknowledged as a major cause of the deterioration of many rivers and floodplain ecosystems. The scope of eutrophication is vast, and it is difficult to discuss all the affected waterbodies in the present article. To provide an overview, examples of case studies reporting nutrient pollution impacts are listed in Table 1.
Table 1. Case Studies Reporting Nutrients Pollution in Waterbodies
Case Study Examples
Lake Rotorua, New Zealand
Researchers discussed occurrence of algal blooms in Lake Rotorua and restoration program with key mitigation actions, including nutrient load targets for the catchment and alum dosing (Smith, Wood, McBride, Atalah, Hamilton, & Abell, 2016).
Barton Broad, Norfolk, UK
Study evaluated interacting effects of abiotic processes and biotic dynamics in explaining variations of phytoplankton biomass in a eutrophic shallow lake, Barton Broad, using a long-term data set (Lau & Lane, 2002).
Lake Udaysagar, Udaipur, India
Study discusses eutrophication in the lake due to discharges from city sewage, industrial wastes, and agricultural field run-off. Consequences included fish mortality, foul odor, blue-green coloration of water, and overall decrease in the recreational values (Vijayvergia, 2008).
Sete Sidades and Furnas Lakes, Portugal
Study involved analyzing long-term monitoring data on water quality for Sete Cidades and Furnas Lakes, to assess effect of policies on their eutrophic status (Cruz et al., 2015).
Erhai Lake, China
Spatial and temporal distributions of sediment microorganism populations as well as their role in the evolution of Erhai Lake eutrophication were addressed in this study (Zhang, Wang, Li, Zhao, & Qian, 2015).
Lake Arendsee, Germany
Study showed evidence of significant dissolved P loads from lacustrine groundwater discharge to Lake Arendsee, accounting for more than 50% of overall external P load, thus resulting in lake eutrophication (Meinikmann, Hupfer, & Lewandowski, 2015).
Lake Veluwe, Netherlands
Long-term dataset on the recovery from eutrophication of Lake Veluwe was analyzed. Researchers observed clear hysteresis in a number of ecosystem variables: the route to recovery differed significantly from the route that led to loss of clear water (Ibelings et al., 2007).
Lake Varese, Italy
Lake Varese observed deterioration in water quality since 1960s, due to direct discharge of untreated sewage, and was classified as hypertrophic. Study discussed series of external and internal remedial actions in subsequent years for recovery of lake water quality (Zaccara, Canziani, Roella, & Crosa, 2007).
High Arctic Meretta Lake, Canada
Meretta Lake, Canada is a polar lake that has been receiving sewage since 1949 via a series of watercourses and utilidors. The lake was still eutrophic in the 1990s; however, nutrient levels went down eventually and reached near “natural” background levels (Douglas & Smol, 2000).
Lake Mjøsa, Norway
Lake Mjøsa is a large and deep lake in southeastern Norway. Eutrophication symptoms peaked in the 1970s, which led to extensive measures for reducing phosphorus load and implementation of monitoring program (Hobaek et al., 2012).
Lake Kasumigaura (Japan), Lake Donghu (China) and Lake Okeechobee (USA)
Lakes have been heavily influenced by point and non-point source pollution and other human activities. Processes affecting nutrient dynamics included nitrogen fixation, light limitation due to re-suspended sediments, and intense grazing on algae by cultured fish (Havens et al., 2001).
Chesapeake Bay, USA
Review provided an integrated synthesis with timelines and evaluations of ecological responses to eutrophication in Chesapeake Bay, the largest estuary in the United States (Kemp et. al, 2005).
Coastal waters of Baltic Sea
Nutrients are discharged into Baltic Sea through riverine load, coastal point sources, atmospheric deposition, and nitrogen fixation. Eutrophication is a serious problem in the entire Baltic Sea area, whereas effects and consequences vary in different parts of the sea (Rönnberg & Bonsdorff, 2004).
Kharaa river basin, Mongolia
Study involved assessing water quality conditions in the Kharaa River basin in northern Mongolia. Nutrient and sediment-bound heavy metal contaminations on a sub-basin scale were evaluated. Nutrient levels showed a significant eutrophication potential (Hofmann, Rode, & Theuring, 2013).
Imbuaçu, Guaxindiba, Marimbondo and Brandoas Streams, Brazil
Four streams in the city of São Gonçalo were evaluated for their potential as sources of nutrients to Guanabara Bay. Streams revealed to be hypereutrophic, with severe limitation of primary production by nitrogen and high phosphate levels. Streams were considered inexorable sources of nutrients, enhancing severe eutrophication in Guanabara Bay (de Carvalho Aguiar, Neto & Rangel, 2011).
Coral Reef, Reunion Island, Indian Ocean
Study investigated variation of bioerosional processes in relation to disturbances of reefal communities due to eutrophication. La Saline fringing reef (Reunion Island) was subjected to nutrient inputs from the adjacent land. Bioerosion by grazers, microborers, and macroborers were measured during study (Chazottes, Le Campion-Alsumard, Peyrot-Clausade, & Cuet, 2002).
Regions such as the Inland Sea of Japan, the Black Sea, U.S. mainland estuaries (the Chesapeake Bay and the Albemarle-Pamlico Estuarine System), and Chinese coastal waters have observed large biomass blooms due to increased nutrient loadings, causing anoxia and harmful impacts on fisheries resources, ecosystems, human health, and recreation (Anderson, Glibert, & Burkholder, 2002). To understand the severity of eutrophication effects, satellite images from the U.S. Geologial Survey Landsat (Figure 4) compare Lake Erie (North America region) conditions during periods in June 2014 and August 2014. Algae blooms in the lake appear as green swirls on the water surface during August 2014.
Nutrients Point and Non-Point Sources
Nutrient sources causing eutrophication in the waterways are commonly classified as point and non-point (Puckett, 1995). Point sources are specific locations or facilities, whereas non-point sources are discrete discharges. Municipal and industrial wastewater dischargers, leaching waste disposal systems, leaking septic systems, and large construction sites, are some examples of definitive point sources. Industrial wastes and domestic sewage are major contributors to the total amount of phosphorus unloaded into lakes from human settlements (Leng, 2009; Smith, Joye, & Howarth, 2006).
Non-point sources are scattered and include agricultural runoffs, urban stormwater discharges, animal farms, pastures, precipitation, atmospheric deposition, drainage, seepage, erosion, or hydrologic modifications (Carpenter, Caraco, Correll, Howarth, Sharpley, & Smith, 1998; Lombardo, Grabow, Spooner, Line, Osmond, & Jennings, 2000). Studies predict that fertilizer consumption will continue to rise in the world, resulting in potentially increased nutrient loads to freshwater reservoirs (Bumb & Baanante, 1996; Glibert, Harrison, Heil, & Seitzinger, 2006). Compared to non-point sources, point sources are easier to monitor, treat, and regulate.
Wastewater Collection and Conveyance Systems
Human activities require water for daily consumption and use. Domestic uses by communities may include washing, cleaning, cooking, drinking, bathing, and restrooms. Offices, restaurants, industrial sectors, and manufacturing plants (such as food processing, chemical, metallurgical, mechanical, pharmaceutical, and many other industrial users) also consume significant amounts of water. Used water is discarded as wastewater by societies and channelized to common regional facilities for further treatment. Domestic wastewater is a combination of two major streams—gray water and black water. Gray water originates from sources like kitchen, sinks, laundry, and bath, whereas black water is generated from toilets (Brandes, 1978). Releasing this sewage to the environment without sufficient or no treatment can create unhygienic conditions, foster health risks, develop bad odors, spread diseases, and contaminate rivers, streams, and lakes (Chislock, Doster, Zitomer, & Wilson, 2013; Schwarzenbach, Egli, Hofstetter, Von Gunten, & Wehrli, 2010).
Over the years, communities have developed effective sewage collection and conveyance systems to capture, contain, and convey this polluted water to a centralized treatment facility called a wastewater treatment plant (WWTP) for the purposes of cleansing and returning treated water back to the environment or for reuse by the public. Conveyance systems consist of pump stations, pipelines, manholes, valves, meters, and other associated components. Sometimes, sewer systems are of a combined nature where a domestic sewage system also receives storm water runoffs. Sewers can also be gravity, pressurized, or vacuum-operated systems (Little, 2004). If communities are isolated from centralized WWTP, they might opt for clustered treatment systems serving small numbers of customers, or they might install private onsite septic systems (Massoud, Tarhini, & Nasr, 2009; Mbuligwe, 2005).
Wastewater quantities and characteristics highly depend upon the type of community as well as industries served. Pollutants in wastewater may result from water used, wastewater infrastructure materials (piping, plumbing, fixtures, and equipment), anthropogenic wastes (feces, urine, and perspiration), household practices and products, and industrial chemicals (Henze, 2008; Kimbrough, 2009; Sandvig et al., 2009). Biochemical oxygen demand (BOD), total suspended solids (TSS), nitrogen, phosphorus, pH, and dissolved oxygen (DO) are some of the common parameters used to characterize wastewaters (Henze, Harremoes, la Cour Jansen, & Arvin, 2001; Ramalho, 2012; Thomas, Théraulaz, Cerdà, Constant, & Quevauviller, 1997). Infiltration and inflows also contribute to the wastewater quantities and characteristics. Infiltration may include flows entering sewers via service connections, cracks, and joints (Ridgeway, 1976). Inflows include water from foundation, springs, stormwater run-offs, roofs and yard drains, cross connections from storm drains, manhole covers, combined sewer systems (Ellis, 2001).
Nutrients in sewer systems originate from domestic sewage, industrial wastes, and storm drainage sources. Industry chemicals, processed food, laundry detergents, fertilizers, cleaning products, cosmetics, shampoos, medicines and ointments, insecticides, rodenticides, feces, and urine may also contribute nitrogen and phosphorus to wastewater (Tjandraatmadja, Pollard, Sheedy, & Gozukara, 2010).
Wastewater Pretreatment Programs
Global industrialization has resulted in elevated levels of pollution. Industries need water for manufacturing processes and utility needs. Wastewater generated at industries has significantly different composition as compared to the typical domestic wastewater (Goronszy, Eckenfelder, & Froelich, 1992; Kim, Park, Kim, Lee, & Kim, 2003; Stepnowski, Siedlecka, Behrend, & Jastorff, 2002). Many times, WWTPs designed to treat domestic sewage are not capable of handling unconventional pollutants present in the industrial wastewater. Hence, if industrial wastewater enters the community sewer system without prior conditioning, it can interfere, inhibit, or disrupt WWTP operations. Upsets to treatment process can cause violations of effluent quality. Additionally, some unaffected chemicals during treatment process may eventually end up in rivers, lakes, and other waters via plant effluent discharges, causing fish kills and other harmful effects on the receiving waters and human health (Thronson & Quigg, 2008; United States Environmental Protection Agency, 2011; Van Hoof & Van San, 1981). Toxic pollutants can also accumulate in the sewage sludge, which when used as fertilizer or as soil conditioner for land use can create deleterious effects to food crops, recreational parks, and other applications (Giger, Brunner, & Schaffner, 1984; McBride, 1995).
To protect sewage treatment plants from these priority toxic pollutants, governments develop pretreatment programs that prescribe minimum industrial wastewater discharge quality entering the sewage collection and conveyance systems (Brenner, Belkin, & Abeliovich, 1994; Ongerth & DeWalle, 1980; United States Environmental Protection Agency, 2011). Major objectives of these pretreatment programs mainly include preventing entry of pollutants toxic to WWTP operations, avoiding entry of pollutants that can pass through treatment works untreated, enhancing prospects for the recycle and reclamation of treated effluent and sludge, ensuring worker health and safety from any toxic or reactive gases or vapors. With growing awareness, permitting agencies are making efforts to regulate unconventional pollutant loads (including nutrients) entering the WWTPs (Cooley, Hunter, Sheridan, & Simmler, 1982; Swift, Wilson, & Jacobsen, 2005; United States Environmental Protection Agency, 2011). For the effective implementation of pretreatment programs, aspects like public interests, government’s water quality goals, industry compliance targets, permitting processes, and WWTP’s operational needs should be understood properly. Identifying common grounds can benefit all the parties and satisfy community’s overall environmental objectives (Shack & Moore, 2014).
Wastewater Treatment and Nutrients Removal
Sewage treatment plants utilize physical, chemical, and biological processes for contaminants removal from wastewater. These treatment processes are categorized into three stages: primary, secondary, and tertiary (Rao, Senthilkumar, Byrne, & Feroz, 2012). Primary stage settles out heavy solids (like rags and debris), small inorganic grit and lighter materials (oil, fat, and grease) from the sewage. Equipment and processes used in this stage may include manual or mechanical screens, grit removal systems, sedimentation tanks, clarifiers, or air floatation systems. Heavy solids are removed as primary sludge and lighter floating constituents are collected as scum from the surface for further processing at the treatment plant. Secondary stage is the next step that removes suspended and dissolved solids from wastewater via biological processes like activated sludge process, contact stabilization, aerated lagoons, stabilization ponds, extended aeration systems, oxidation ditches, trickling filters, and similar technologies (Forster, 2003; Von Sperling, 2007). Organisms involved in the biological treatment processes may include bacteria, fungi, algae, protozoa, and metazoan (Henze, Harremoes, la Cour Jansen, & Arvin, 2001). Upon treatment, the microbial biomass needs to be separated from the processed wastewater as secondary sludge. Tertiary or advanced techniques may include micro-screening, ion exchange, reverse osmosis, nutrient removal processes, and disinfection using ozonation, chlorination, or ultraviolet radiations (Ramalho, 2012; Tchobanoglous & Burton, 1991). In some cases, primary and secondary stages are combined into one operation. Selection of WWTP treatment processes depend upon factors like influent characteristics, effluent permit requirements, degree of treatment, design flows, type of discharge stream, and costs involved. Finally, treated effluent is returned to nature via surface or groundwater discharges.
Sewage sludge is the by-product produced during wastewater processing. Sludge generated from primary treatment stage includes settleable heavy solids, whereas secondary process sludge mainly includes concentrated microbial biomass. Amount of sludge produced depends on wastewater characteristics, volume, and degree of treatment. Pathogens, heavy metals, micro-pollutants, and other hazardous substances may get concentrated in the sludge (Pathak, Dastidar, & Sreekrishnan, 2009; Strauch, 1991). Hence, it is important to properly treat and dispose of the stabilized sludge commonly called biosolids. Various sludge treatment and stabilization techniques include thickening, conditioning, dewatering, aerobic or anaerobic digestion, composting, and drying. Stabilized sludge from the treatment plant can be used as fertilizer, soil conditioner, disposed of in a landfill, or incinerated, with the ash disposed to landfill (United States Environmental Protection Agency, 2006).
Nutrient removal technologies implemented at the WWTPs are commonly categorized as (a) nitrogen removal processes, (b) phosphorus removal processes, and (c) combined nitrogen and phosphorus removal processes (United States Environmental Protection Agency, 2008). Nitrogen removal can be achieved via physico-chemical methods (like ion exchange, air stripping, and breakpoint chlorination) or biological processes. Biological nitrogen removal is a three-step process. The first step, called ammonification, is conversion of organic-nitrogen to ammonia. The second, called nitrification, step involves oxidation of ammonia-nitrogen to nitrate under aerobic conditions. The third step, called denitrification, is the conversation of nitrate to nitrogen gas under anoxic conditions and in the presence of organic carbon source (Peng & Zhu, 2006; United States Environmental Protection Agency, 2008; van Haandel & van der Lubbe, 2007). Effective biological nitrogen removal involves several factors, like adequate supply of carbon from internal or external sources, anoxic zones, sufficient alkalinity, appropriate sludge age, hydraulic retention time, dissolved oxygen, favorable temperatures, and proper recycle rates (Cao, Zhao, Sun, & Zhang, 2002; Pochana & Keller, 1999). Nitrogen forms typically present in the wastewater include ammonia-nitrogen, organic-nitrogen, nitrate, and nitrite (Lai & Lam, 1997; Raveh & Avnimelech, 1979). Influent and effluent nitrogen species concentrations also have major impacts on the technology feasibility and selection process. Table 2 identifies common nitrogen removal technologies.
Table 2: Common Wastewater Nutrient Removal Technologies
Anaerobic/anoxic/oxic (A2/O) process
Three-stage process consisting of anaerobic zone, anoxic zone, and aerobic zone with an internal recycle stream that returns nitrates from aerobic zone to anoxic zone. Return activated sludge is recycled to the head of the anaerobic zone (Smith, 2005; United States Environmental Protection Agency, 2007, 2008).
Bardenpho process (four-stage)
Adaptation of activated sludge process involving anoxic zone, followed by aerobic zone (with an internal recycle to the first anoxic zone), followed by second anoxic zone and aerobic zone (Smith, 2005; United States Environmental Protection Agency, 2007, 2008; Water Environment Federation, 2007).
Bio-augmentation batch enhanced (BABE)—nitrification
Technology involves a batch reactor (operated both aerobically and anoxically) fed with batches of return-activated sludge along with recycled sidestream to achieve nitrogen removal (Henze, 2008; United States Environmental Protection Agency, 2008).
Consists of two activated process tanks side by side with influent fed alternately to the systems, allowing anoxic and aerobic zones to form for nitrification and denitrification (Jördening & Winter, 2005; United States Environmental Protection Agency, 2008; Water Environment Federation, 2007).
Cyclically aerated activated sludge process
Programed activated-sludge system that involved turning aeration system on and off periodically, to achieve denitrification and nitrification in the same tank (United States Environmental Protection Agency, 2008; Wang, Shammas, & Hung, 2010).
Fixed film processes
Attached growth systems consisting of packed or suspended medium suitable for slow-growing bacteria involved in nitrification and denitrification processes (Smith, 2005; United States Environmental Protection Agency, 2008).
Process involves treatment of ammonia-rich recycled stream in a separate nitrification reactor before recycling to the plant headworks (Cheremisinoff & Davletshin, 2015; United States Environmental Protection Agency, 2008).
Integrated fixed-film activated sludge (IFAS) process
IFAS systems involve attached growth media (fixed or floating type) included in an activated sludge basin involving both nitrification and denitrification (United States Environmental Protection Agency, 2008; van Haandel & van der Lubbe, 2012; Water Environment Federation, 2007).
Process consists of anoxic and aerobic zones followed by membrane reactors that filter solids from mixed liquor, taking place of secondary clarifiers (United States Environmental Protection Agency, 2008; van Haandel & van der Lubbe, 2012).
Modified Ludzack-Ettinger process
A continuous flow process with initial anoxic stage followed by aerobic stage. Internal recycle carries nitrates and mixed liquor from aerobic to anoxic zone (Seviour & Blackall, 2012; United States Environmental Protection Agency, 2007, 2008; Water Environment Federation, 2007).
Modified Bardenpho process
Modified University of Cape Town (UCT) process
Process involves anaerobic zone followed by two anoxic zones and an aerobic zone upstream of the secondary clarifiers with internal nitrate recycle (Seviour & Blackall, 2012; United States Environmental Protection Agency, 2007, 2008).
Moving-bed biofilm reactor
Consists of small media (carriers) in an anoxic or aerobic zone that move freely in the process tank and allow attached growth of biomass to occur (Hahn, Hoffmann, & Odegaard, 2012; United States Environmental Protection Agency, 2007, 2008).
Process of nitritation where only nitrite is produced aerobically. Denitrifiers are then encouraged to convert the nitrite to nitrogen gas (Stamatelatou & Tsagarakis, 2015; United States Environmental Protection Agency, 2007, 2008).
Oxidation ditch processes
Rotating biological contactor (RBC) process
Countercurrent aeration process that achieves alternating anoxic-aerobic zones within same tank and provides nitrification and denitrification (Wang, Shammas, & Hung, 2010; United States Environmental Protection Agency, 2007, 2008).
Sequencing batch reactor
Sequenced suspended-growth batch process involving four-phases: fill phase, react phase with alternating aerobic and anoxic cycles, settle phase, and decant phase (United States Environmental Protection Agency, 2007, 2008).
Step-feed activated sludge process
Variation of conventional activated process with influent flow split to several feed locations, sludge stream recycle, and alternating anoxic and aerobic stages (Andrews, Briggs, & Jenkins, 1974; United States Environmental Protection Agency, 2007, 2008).
Phosphorus (P) removal can be achieved with chemical precipitation or biological uptake by microbial biomass at the treatment plants (Morse, Brett, Guy, & Lester, 1998). Chemical removal of phosphorus is achieved by treatment with trivalent metal cations (such as ferric or aluminum) and is precipitated in the form of ortho-phosphate. Phosphorus removal capacities may depend upon species present in water such as organically bound phosphorus, polyphosphate, and orthophosphate (Razali, Zhao, & Bruen, 2007). Factors affecting phosphorus removal include phosphorus load, metal-to-phosphorus ratio, chemical used, feed location, number of feed points, mixing needs, reaction time, pH, and alkalinity (Fukase, Shibata, & Miyaji, 1985; Szabó, Takács, Murthy, Daigger, Licskó, & Smith, 2008). Chemical treatment methods can achieve low effluent P concentrations, however, they also generate higher volumes of chemical sludge. To achieve lower concentrations, filtration processes (such as gravity filter, moving-bed filter, traveling bridge filter, and membrane filters) can be used for removing smaller precipitate particles.
Biological phosphorus removal processes involve encouraging the growth of phosphate-accumulating organisms (PAO). As active microbial biomass is wasted, PAO-contained phosphorus also gets removed (Welles, 2015). Important factors affecting biological phosphorus uptake process may include phosphate concentrations, phosphate species, temperature, retention time, dissolved oxygen, and reaction kinetics (Kuba, Smolders, Van Loosdrecht, & Heijnen, 1993; Østgaard, Christensson, Lie, Jönsson, & Welander, 1997; Powell, Shilton, Pratt, & Chisti, 2008). Some of the technologies for biological phosphorus removal include Fermentation, Anaerobic-Anoxic Process, Phoredox Process, and Oxidation Ditch (Barnard, 2014; Kuba et al., 1993; Wei et al, 2012). Use of chemical or biological process depends upon influent and effluent P concentrations, and capital and operational costs involved.
Although nitrogen and phosphorus are mostly removed in the mainstream wastewater treatment processes, nutrients also get accumulated in the waste sludge. When the digested sludge is dewatered, nutrient-rich reject water or side-stream is generated. It can be treated separately or recycled back to the head of plant for further processing (Jin, Ji, Xu, Xu, Chen, & Li, 2014). Side-stream treatment processes can treat this concentrated water and potentially reduce the burden on the main treatment process, improve nutrients recovery, and reduce overall energy and chemicals consumption costs (Coma, Rovira, Canals, & Colprim, 2015; Raj, Banu, Kaliappan, Yeom, & Kumar, 2013).
Nutrients Resource Recovery Systems
Phosphorus is an essential constituent for sustainable crop yields, and modern agricultural systems are relying on mined phosphate rocks, supply of which is expected to peak by around 2033 (Cordell, Schmid-Neseta, Whiteb, & Drangerta, 2009). Globally, the demand for quality phosphate rock is escalating due to increasing human population (Karunanithi et al., 2015). To offset a portion of agricultural fertilizer demand, the impetus for exploring and commercializing nutrients recovery technologies from available resources is growing.
Wastewater plants are being recognized as valuable resources of nutrients, energy, and water rather than merely treatment facilities (Mo & Zhang, 2013). The concept of nutrients resource recovery from sewage (normally rich in nitrogen and phosphorus) has gained momentum. Research and development activities are underway to accelerate advancement of sustainable and cost-effective solutions. Such nutrients enrichment, recovery and reuse for fertilizer industry initiative can create revenue generation opportunities for WWTPs as well as reduce pollution stress on the discharging waterbodies (De-Bashan & Bashan, 2004; Mo & Zhang, 2013).
In general, nutrients recovery process can be divided into three steps—accumulation, release, and extraction. Nutrients accumulation can be achieved with chemical precipitation, membrane separation, sorption, binding with magnetic particles, or via plants and microorganisms (algae and prokaryotic). Biochemical (anaerobic digestion and bioleaching) and thermochemical treatment processes can perform nutrients release. Nutrients extraction can be achieved via crystallization, gas-permeable membranes, liquid-gas stripping, and electrodialysis (Mehta, Khunjar, Nguyen, Tait, & Batstone, 2015). Phosphorus can be recovered from wastewater in the form of magnesium ammonium phosphate (MAP), also called struvite, or as calcium phosphate (Daigger, 2012; De-Bashan & Bashan, 2004). During biological treatment, the nitrogenous materials are accumulated in the sewage sludge. Fertilizer-grade ammonium sulfate can be produced from sludge digestion sidestreams carrying high ammonia concentration via stripping and adsorption (Daigger, 2012).
Strategies for Nutrient Pollution Reduction
For minimizing pollution loads to surface water and groundwater reservoirs, governments are devising newer policies and strategies aimed at both point and non-point sources (Antonio Ruiz‐Quintanilla, Bunge, Freeman‐Gallant, & Cohen‐Rosenthal, 1996; Mitsch et al., 2001). These schemes involve voluntary and regulatory initiatives to collectively control nutrient pollution to waterbodies in a systematic and cost-effective manner (Bosch, Cook, & Fuglie, 1995). Point source strategies typically revolve around technological applications for nutrients removal and/or recovery (McQuarrie, Rutt, Seda, & Haegh, 2004; Urgun-Demirtas, Pagilla, Kunetz, Sobanski, & Law, 2008). Implementation of industrial pretreatment programs, banning phosphate detergents use, stringent effluent limits for WWTPs, and bioremediation techniques are some of the strategies for accomplishing the goals.
Non-point sources like agriculture, farmlands, and storm water runoffs are comparatively difficult to monitor and regulate for nutrients discharges to waterbodies. Nutrient trading options, watershed nutrient discharge reduction initiatives and agricultural best management practices can reduce non-point source pollution (Greenhalgh & Selman, 2012; Ribaudo, Heimlich, Claassen, & Peters, 2001). Effective strategies can minimize runoffs and nutrient loads and can achieve sustained long-term solutions for improving and maintaining water quality in the affected lakes, rivers, streams, and oceans.
Wastewater Source Nutrients Reduction Initiatives
Past efforts to ban phosphate rich detergents or encourage use of low-P products for domestic purposes were intended to achieve source nutrient loads reduction (Hartig & Horvath, 1982; McGucken, 1989). As discussed in the industrial pretreatment programs section earlier, setting water quality limits for manufacturing plants and industries discharging into sewer systems can reduce the burden on downstream sewage treatment facilities. Phosphorus and nitrogen removal initiatives via industrial pretreatment can thus reduce nutrients entering treatment plants (Abma, Driessen, Haarhuis, & Van Loosdrecht, 2010; Yilmaz, Lemaire, Keller, & Yuan, 2008). McComas and McKinley (2008) discuss successful implementation of pollution prevention strategies for industrial users via the Minnesota Technical Assistance Program, which helped to reduce phosphorus, organic, and hydraulic loads to publicly owned wastewater treatment plants. Overall, such efforts can help smooth WWTP operations and alleviate nutrient pollution problems.
Point-Source Dischargers Nutrient Reduction
Regulatory authorities set effluent nutrient discharge limits for municipal and industrial wastewater treatment plants (with direct discharge to waterways) to reduce pollution, protect human health, and avoid adverse effects on the receiving streams. Development of water quality permits are typically based on criteria such as scientifically defensible data, shared understanding of pollutants sources in the watershed, treatment capabilities, and associated costs for controlling the aquatic environment (Clark, Neethling, Pramanik, Sandino, Stensel, & Tsuchihashi, 2013). Larger treatment plants tend to contribute more nutrient loads to waterways and might experience more stringent limits. To comply with the nutrient permit limits, WWTPs quite often need to go through upgrade process or improve existing treatment methods to accommodate nutrient removal capabilities. Selection of WWTP technology upgrades may depend on factors like capital construction costs, operations and maintenance (O &M) costs, degree of nutrient removal required, and technical feasibility. Past studies show that biological nutrient removal technologies have been successfully implemented at sewage treatment facilities in many countries (McQuarrie, Rutt, Seda, & Haegh, 2004; Urgun-Demirtas, Pagilla, Kunetz, Sobanski, & Law, 2008). Clustered and private treatment systems are smaller in size, handle low flows, and hence, contribute a comparatively smaller portion of nutrient loads to the watersheds. Such systems may get evaluated on a regional basis to identify their impacts on water pollution. Overall, regulatory effluent limits for point sources as well as associated implementation schedules may depend upon watershed water quality goals (Branosky, Jones, & Selman, 2011).
Non-Point Sources Nutrients Load Reduction
Agricultural land, storm water runoffs, and other non-point sectors are major contributors of nutrient loads to surface and groundwater. Two potential strategies for reducing nutrient loads from agricultural practices are reducing fertilizer application rates and filtering nutrients leaching off from the cropland with the help of wetlands (Ribaudo, Heimlich, Claassen, & Peters, 2001). Conservation programs, education of farmers, development of innovative farming technologies, fertilizer management, riparian zones, strip cropping, improvements in grazing practices, effective residue management, and soil erosion reduction can significantly cut down this pollution (Roberts, 2007).
Storm water discharges and runoffs in urban and rural areas are mostly seasonal. Conventional storm water runoffs often discharge directly into the rivers and streams, thus contributing nutrients and other pollutants to the waterbodies. Storm water management practices can minimize runoff volumes and their nutrient carrying capacities (Barbosa, Fernandes, & David, 2012). Decentralized stormwater management tools (like low impact development and water sensitive urban designs) can involve ponding, infiltration, and harvesting of water at the source, thus encouraging evaporation, evapotranspiration, groundwater recharge, and re-use, offering sustainable solutions for stormwater management and minimizing water pollution (Roy et al., 2008). Bio-retention techniques, permeable paving areas, infiltration basins, vegetated filter strip areas, erosion and sedimentation control, and stormwater treatment wetlands can be effective tools for managing nutrient loads from storm water runoffs (Brattebo & Booth, 2003; Mitsch, 2016; Stanley, 1996).
Nutrients Trading Programs
Nutrient trading is an innovative concept involving collaborative efforts between both point and non-point pollution sources to achieve overall nutrients and water quality goals for specific regions or watersheds. Such programs are being widely explored and increasingly implemented as a means of providing flexibility and to lower community pollution control costs (Greenhalgh & Selman, 2012). Nutrient trading involves transfer of nutrients reduction credits (mainly nitrogen and phosphorus) between buyers and sellers. Credits become available when a seller reduces its nutrients pollution load below allowable discharge amount to the receiving waterbody (Chesapeake Bay Program, 2001). Number of participants, nutrient loading limits, trading ratios, transaction costs, abatement costs, and enforcement costs are some of the considerations while developing the program (Hoag & Hughes-Popp, 1997).
Watershed Mapping Tools
Among various avenues, developing watershed maps and prioritizing them to manage and reduce nutrient discharges are important considerations. Ranking watersheds based on criteria such as runoff index, sediment production index, animal loading index, or chemical use index can be useful tools for pollution management (Hamlett, Miller, Day, Peterson, Baumer, & Russo, 1992). Watersheds releasing higher loads and proving critical for pollution can be identified to help governments effectively manage contamination issues and devise schemes to implement ecosystem restoration projects in phases. Developing and applying parameters like the Watershed Sustainability Index (WSI) can help analyze the overall health conditions and identify key issues needing improvements within the watersheds (Catano, Marchand, Staley, & Wang, 2009). Such indicators can provide information to establish baselines and understand the water nutrients reduction progress.
Waterways Remediation Processes
Remediating eutrophication problems can be a costly affair involving millions of dollars for individual lakes (Carpenter, 2008). Technologies and scientific methods have evolved to pursue ecosystem restoration processes in the affected streams, rivers, lakes, bays, and oceans. Biomanipulation, application of Phoslock™, transplantation of aquatic plants, sediment dredging, hydrophytes restoration, artificial floating islands, oxygenation using wind aerators, iron treatment for phosphorus immobilization, hypolimnetic aeration, riparian zones, constructed wetlands, lime or alum dozing to enhance phosphate removal, application of chemicals to reduce internal phosphorus loading from the sediments, artificial circulation of lake to prevent low winter oxygen concentrations and increase spring oxygen concentrations, and many other techniques have been developed and implemented to restore affected waterways (Gołdyn et al., 2013; Granéli, 1999; Ha et al., 2013; Nakamura & Mueller, 2008; Novak & Chambers, 2014).
Researchers and scientific organizations have published numerous case studies and reviews to help understand past and ongoing efforts towards restoration process, associated problems, and success stories. The National River Restoration Science Synthesis (NRRSS) database of over 37,000 projects was created to summarize restoration trends and assess project effectiveness in the United States (Follstad Shah, Dahm, Gloss, & Bernhardt, 2007). Several floodplain restoration or rehabilitation projects have been realized along large rivers in Europe and North America (Buijse et al., 2002; Ibelings et al., 2007).
A biomanipulation restoration study for Lake Shirakaba, Japan, in 2000, indicated reductions in algal biomass, increased transparency, and decline in total phosphorus concentrations, in addition to other observations (Ha et al., 2013). Phoslock™ application and transplantation of native, perennial macrophyte species were successful in reducing total phosphorus (TP) in the eutrophic Canning River and the Lower Vasse River in Western Australia (Novak & Chambers, 2014). An integrated ecological engineering application, involving multi-pond constructed wetlands (to treat external loadings) and an in situ purification system that consisted of sediment dredging, hydrophytes restoration, and artificial floating islands (to purify internal loading), was applied to treat eutrophic water of the Shuangqiao River (SQ River). The results showed reduction in total phosphorus and nitrogen in the river (Fang, Bao, Sima, Jiang, Zhu, & Tang, 2016). Restoration efforts for Durowskie Lake, strongly eutrophic with cyanobacterial water blooms, were initiated in 2009 using oxygenation of hypolimnetic waters with wind aerators, phosphorus immobilization using iron treatment, and biomanipulation measures. Results showed improvement in water quality and ecological conditions of the lake (Gołdyn et al., 2013). Overall, a significant amount of information is available for restoration projects implemented in different parts of the world showing ongoing efforts by environmental organizations and governments towards regaining ecological conditions and water quality.
Uncontrolled or unregulated nutrient discharges from point sources (like wastewater treatment plants) and non-point sources (like the agricultural sector) are creating far-reaching and long-term impacts on aquatic ecosystems. Water quality degradation, eutrophication, foul smells, reduced dissolved oxygen, aquatic life mortality, recreational property devaluation, and drinking water issues are the observed downsides of this pollution problem. Studies and reviews published by scientists and researchers have shown that implementation of nutrients removal technologies at point sources can potentially decrease nutrient loads to discharging waterbodies. Also, innovative nutrients recovery processes are providing opportunities for revenue generation at sewage treatment facilities. Best management practices, storm water management techniques, and nutrient trading programs further create possibilities of diminishing nutrients load discharges from non-point sources. Restoration of affected streams, rivers, lakes, bays, or oceans can be a lengthy process. Several past efforts show positive effects of remediation techniques, like biomanipulation, Phoslock application, constructed wetlands, and riparian zones for restoring water quality. Overall, containment of nutrients discharges from point and non-point sources is an important aspect for alleviating chronic pollution problems in the waterways. To manage multifaceted nutrient contamination issues, effective strategies and solutions need to be devised across the globe for ecosystem preservation and to sustain water quality in waterbodies that can be enjoyed by the present and future generations.
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