Biological Wastewater Treatment Systems

Treatment Wetlands Classification of wetlands

There are several types of natural wetlands such as swamps, fens, bogs, marshland, and tidal freshwater areas. Swamps and marshes have open flowing water

and are distinguished in terms of vegetation, soil type, and wild life. Mires such as fens and bogs are mainly subsurface wetlands with little open water. Bogs are isolated hydrological units that receive water only through precipitation, whereas fens have through flowing water. As water goes through such wetland areas it undergoes great chemical transformation. Both nutrients and elements like heavy metals that attach chemically (sorb) on solid surfaces are effectively removed such that water reaches a status corresponding to 'natural' water quality.

Constructed wetlands or treatment wetlands are usually built where natural wetland conditions can be found and are, therefore, to some extent modifications of a natural system. By introducing dams and canals, however, it is possible to provide proper water depth for carbon providing vegetation species, like common reed (Phragmites), and a separation of oxygen conditions. In some cases, wetlands can be built in clay strata or artificially sealed using clay even if there is no natural groundwater reaching the ground surface. Hence, leakage through infiltration is an essential problem that needs to be accounted for in the design.

Constructed wetlands are commonly divided into subsurface flow (SSF) and surface flow or free water surface (FWS) wetlands. Both types are used for treating domestic, municipal, and industrial wastewater. In particular, these systems can be useful for treating landfill leachates, agricultural runoff, and wastewater from minor communities. SSF wetlands are commonly used as a polishing step after conventional treatment plants for municipal wastewater. SSF systems are often favored in minor communities due to the soil cover of possibly contagious wastewater. Subsurface systems require separation of solid material in the wastewater before solute fractions are led into a sand filter or other soil layer in which phosphorus is removed through sorption to the particulate matrix and nitrogen to denitrification supported by soil bacteria (Figure 7).

Because of the large discharge capacity surface flow wetlands are usually preferred as polishing step for municipal wastewater. Phosphorus is generally effectively removed in the treatment plant, whereas nitrogen treatment requires longer detention times that are provided in the wetland.

Both FWS and SSF wetlands used for treating municipal and industrial wastewater are designed with an area of c. 5-10 m2 per person equivalent.

Functionality of FWS wetlands

Vegetation is important in surface flow wetlands to provide carbon for supporting denitrification, to offer host environment for biofilms that grow on stems, and to cause friction losses for flow water, which can be utilized to provide a beneficial flow pattern. Submersed vegetation also controls the oxygen level in the water.

Vegetation in a recently established wetland changes with time and this leads to a relatively long period (years) to approach equilibrium in the wetland ecosystem and the interacting treatment processes. In cold climates, the effectiveness of treatment wetlands also varies over the year, but there is a notable effect even during the winter.

The shores of wetland ponds in cold climates can be populated by reed sweet grass (Glyceria maxima L.), common reed (Phragmites australis L.), and cattail (Typha latifolia L.). Examples of submersed vegetation include slender waterweed (Elodea nuttallii L.), sago pondweed (Potamogeton pectinatus L.), coontail (Ceratophyllum demersum L.), and spiked watermilfoil (Myriophyllum spicatum L.). Coontail forms dense layers of vegetation that can be considered to be a porous medium for the water flow with a large inner surface available for biofilms.

A main role of treatment wetlands that are constructed as a polishing step after a conventional treatment plant is to remove nitrogen through denitrification in biofilms. Generally, the ordinary treatment process has included oxygenation of the water, which transforms most nitrogen fractions, like ammonium and nitrite, to nitrate before it enters the wetland. Biofilms grow both on vegetation stems and in the bottom sediments. Therefore, an important factor is the exchange rate for solute substances between flowing channels in the wetland with bottom sediments and vegetation zones. The potential for deni-trifications in the host environments for biofilms, like bottom sediments, is usually significantly higher than

Septic tanks sand filter

Figure 7 Typical subsurface treatment system for a single household including septic tanks and downstream sand filter.

Septic tanks sand filter

Figure 7 Typical subsurface treatment system for a single household including septic tanks and downstream sand filter.

actual rates on the scale of the entire system, because of the difficulty to arrange an effective contact between water and biofilms.

Functionality of SSF wetlands

The advantage of SSF wetlands is that the water is present below the ground surface, which decreases odors and the risk for public exposure for possible contagious bacteria. The construction usually includes a sand and/or gravel bed with supporting emergent vegetation such as cattail (Typha) and reeds (Phragmites). The systems are designed with aspect ratio (L:W) of about 15:1 and a flow velocity in the order of centimeters to decimeters per day.

A typical design layout for a single household is shown in Figure 7. A first step usually involves separation of coarse fractions of the wastewater in deposition basins, or, such as in this case, in septic tanks. This produces waste-water that can percolate and flow through the sand filter without rapidly clogging the pores of the filter and end its lifespan too fast.

The active processes include mechanical filtering of particulate (organic) matter in the porous material, sorption of phosphorus and heavy metals to the solid matrix, and nitrogen decomposing reactions caused by nitrifying and denitrifying bacteria in the upper soil layer. Good performance is commonly reported for the removal of biological oxygen demand (BOD), total suspended solids (TSS), phosphorus, and nitrogen.

WWTPs - The Activated Sludge Process General

Generally, WWT systems containing compartmentalized reactors (basins or tanks) for their performance often are termed a WWTP. In addition, the flow of wastewater through such systems is thoroughly controlled and optimized. The WWTP may consist of a mechanical, chemical, and biological step. In the mechanical step, heavy solid particles are allowed to settle at the bottom and light material floating on the water surface is removed. In the chemical step metal salts are added to precipitate phosphorus. Phosphorus removal can be performed at different stages in the treatment process: prior to, simultaneous with, or after the biological step and are hence called preprecipitation, coprecipitation, or postpre-cipitation, respectively. The biological step can be performed according to either of two basic principles. The reactor may contain solid surfaces to support bacterial growth and the development of a biofilm (trickling filters, rotating biological contactors, and fluidized beds) (Figure 1b). The other approach is to allow bacterial growth in the water body supported by natural occurring suspended solids (activated sludge process) (Figure 1a).

The activated sludge process can be designed as either a continuous flow system or as SBRs. Both systems normally include an aerated biological nutrient removal step followed by settlement of produced sludge. The difference between the systems is that in the continuous process these processes take place in two different reactors, whereas in the SBR process they occur sequentially in the same reactor.

Continuous flow systems and SBRs In the conventional continuous flow system primary treated wastewater is conveyed into an aerated basin (Figure 8a). The feed of wastewater and supply of compressed air can be done in many ways, from being introduced at one end leading to gradients of oxygen and substrate throughout the basin to being introduced at several points giving a more homogeneous environment. Moreover, during its way through the basin, the water may be led through more or less open compartments. In a completely mixed process, the water is also circulated within the basin. The effluent is led to a clarifier to allow particles to settle before the clear phase leaves the process. The settled excess sludge containing a viable biomass is removed and treated separately; however, some is recycled to reinoculate the process. This procedure will ascertain stable function of the unit. The whole concept is not unlike the continuous culturing of microorganisms in the laboratory or in many industrial processes.

The operation of one or more SBRs in a series consists of a sequence of fill-and-draw cycles. Each cycle typically consists of a number of separate operational phases of fill, react, settle, draw, and idle. Hence, after the react phase, that is, growth phase, the produced biomass is allowed to settle and the clear treated supernatant can be removed. The process resembles that of batch-culturing of bacteria in the laboratory.

The biological process

In the biological step of the activated sludge process, suitable mixing of the water is necessary to allow suspended solids, air, nutrients, and microorganisms to make intimate contact. Based on the mixing regimes, plug flow or completely mixed systems can be differentiated. Large volumes of air are blown into the reactor tanks from beneath to achieve effective mixing and support the aerobic microorganisms with sufficient oxygen for respiration. The concentration of dissolved oxygen (DO) should be kept at approximately 2mgl~ . As described above, the phase transitions in the system will support an optimal environment for microbial growth. During the react phase, three-dimensional aggregates of highly active microbial communities, called flocs, are formed (Figure 1a). Flocs typically are 100-500 mm in diameter. New microscopic techniques such as epifluorescence

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Figure 8 Examples of conventional activated sludge processes (a) without and (b) with nitrogen removal capacity, and (c) with capacity of both biological nitrogen removal and enhanced biological phosphorus removal.

Return sludge

Figure 8 Examples of conventional activated sludge processes (a) without and (b) with nitrogen removal capacity, and (c) with capacity of both biological nitrogen removal and enhanced biological phosphorus removal.

and CLSM in combination with image analysis have been used to analyze the aggregates of activated sludge. Generally, four main structures can be discriminated in the flocs: (1) active and inactive microbial cells, mainly bacteria, protozoa, and metazoa; (2) extracellular polymeric substances like carbohydrates and proteins; (3) inorganic particles (sand); and (4) water. From a technical point of view, the sludge properties are essential. The forming of dense flocs with good properties for settlement will provide good operational conditions. Filamentous bacteria will always be present in a healthy process which operates normally, and which shows no symptoms of problems with bulking or foaming. Several sludge indices have been suggested to describe and characterize the sludge property.

Occasionally the proportion of filamentous bacteria will increase causing flocs of loose structure that settle only slowly and compact poorly. This phenomenon called bulking leads to uncontrollable loss of solids, including, for example, the active nitrifying biomass. Though most filamentous bacteria are heterotrophs, they are shown to be extremely difficult to cultivate. Approximately ten types of filamentous microbes seem to be involved in most bulking events. Microthrix parvicella seems to be especially important. The bacterium is long and thin, and its coiled appearance makes it easy to distinguish by microscopy of activated sludge samples. Only metabolic characteristics of a few isolates have been reported and the results are not always concordant. Microthrix parvicella seems to be negatively affected by DO concentrations above >6mgl_1 but grows well at ~0.4mgl_ and should therefore be considered a microaerophile. It prefers a somewhat alkaline environment and optimum growth is reported at 25 °C, though some growth was still observed at 8 °C. The range of maximum growth rates (^max) reported is 0.38-1.44 d~ . The bacterium cannot utilize glucose but seems to prefer long-chained fatty acids like oleic acid. It can store intracellular PHA and lipids. No reliable control strategy exists for bulking caused by increased amounts of filamentous organisms in the activated sludge process. Based on the physiological properties of the bacterium, the following alteration of the process has been suggested to reduce its abundance: shorten the sludge retention time, increase the DO to >2mgl~\ removal of high lipid contents by flotation.

Another widespread problem also leading to solids separation problems is foaming. Stable foams will bring the sludge to the surface of the clarifier and carry-over of solids from the clarifier. The foam most often consists of a dense matrix of filamentous bacteria and air bubbles. Foaming may have several causes. Microthrix parvicella seems to be more hydrophobic than most other bacteria in the activated sludge process and are frequently associated with foaming problems. Another group of bacteria identified in activated sludge foams is mycolic acid-producing actinomycetes. The most commonly methods for controlling foaming are the same as those for controlling bulking problems. However, the magnitude of the problem has forced the development of both physical and chemical short-term measures to control these situations.

Nutrient removal capacity

A properly controlled activated sludge process can remove very effectively the content of organic carbon, and mineralize and nitrify nitrogen. Typically, the chemical oxygen demand (COD) and BOD removal capacities for municipal wastewater are higher than 85% and 95%, respectively. The reduction of carbon is due to aerobic respiration losses, removal ofsettled sludge produced by biomass growth, as well as flocculation of dissolved and particulate organic matter. In addition, some 20-30% each of influent phosphorus and nitrogen will be trapped in the settled sludge; however, most phosphorus and nitrogen will leave the system as dissolved phosphate and nitrate. Thus, the basic design of the activated sludge process is less effective in reducing nitrogen and phosphorus. By introducing chemical precipitation and combined nitrification-denitrification the total removal capacities for phosphorus and nitrogen may be improved to >90% and >70%, respectively.

The high amounts of sludge produced by activated sludge systems are problematic. Although sludge is a potential 'organic fertilizer', since it is rich in plant nutrients, due to the risk of occurrence of pathogens and chemical toxicants, such as heavy metals in the sludge, there are problems associated with recycling the sludge to arable land. Therefore, efforts are made to reduce the sludge production. Increasing the periods of aeration will lead to higher sludge residence time which will extend the periods of endogenous metabolism, that is, microbial consumption of internal cell material as well as mineralization of lysed cells and particulate matter. Application of aquatic predatory oligichaetes has been suggested as means to reduce excess sludge production. One common means to reduce the amounts of sludge from WWTPs is to treat the sludge in an anaerobic reactor to produce biogas (CO2 and CH4).

Enhanced nitrogen and phosphorus reduction

The combination of nitrification and denitrification has since long been known as an effective biological solution to achieve nitrogen removal in wastewater. The obvious way to arrange suitable environments for the two groups of bacteria is to connect an aerobic compartment or zone prior to an anoxic in a so-called post-denitrification process (Figure 8b). However, since most organic matter is consumed in the aerobic zone, this setup may experience low effects due to lack of easy available energy to the denitrifiers. A more effective solution can be to place the anoxic zone prior to the oxic zone and circulate the water between the two zones. In this design, called pre-denitrification, the denitrifiers will meet both anoxic conditions and fresh organic material from the influent. Another solution is to support the denitrification with an external organic energy source. Effective denitrification has been reported with, for example, acetate, ethanol, and methanol. The response to acetate and ethanol is immediate as these molecules are part of the normal metabolic pathways of organotrophic bacteria. For effective denitrification with methanol a long period of adaptation is needed, typically several months. Only a few slow-growing specialists, for example, Hyphomicrobium sp., can use one-carbon compounds (CH3OH) and the metabolic pathways are complex.

Recent developments in biological nitrogen removal techniques in combination with the discovery of novel bacteria have resulted in some new methods. By combining partial nitrification with the anammox process some nitrogen removal techniques have been set up that may consume lower resources (Figures 4b and 4e). In the partial nitrification process a shortcut is taken by preventing the oxidation of nitrite to nitrate by nitrite-oxidizing bacteria. Instead the nitrite is removed directly by hetero-trophic denitrification. In the single-reactor system for high ammonium removal over nitrite (SHARON), incomplete nitrification is achieved by use of the slower growth rate of nitrite oxidizers than ammonium oxidizers at higher temperatures (>26 °C). By applying higher hydraulic retention times, the nitrite oxidizer will be washed out. The nitrite thus accumulated can be removed by the anammox process in a succeeding reactor. In the anammox process nitrite is oxidized with ammonia as the electron donor. In the partial nitrification process, half the ammonium is converted into nitrite. One advantage with the process is that no extra organic energy is needed for the denitrification step. Another variation is to let nitrifiers oxidize ammonia to nitrate in a single reactor and consume oxygen to create the anoxic conditions needed by the anammox bacteria. This process is called CANON, the acronym for 'completely autotrophic nitrogen removal over nitrite'.

As both biological nitrogen removal and enhanced biological phosphorus removal need alternating cycles of aerobic and anoxic conditions, it seems logical to combine the two processes in the same WWTP. However, this is not as easy as it seems to be. In addition to alternating anoxic and aerobic regimes, the anoxic zone must be maintained completely anaerobic to provide fermentation end products like fatty acids to select for PAO bacteria. The level of nitrate in the anaerobic zone must be low; otherwise the heterotrophic denitrifiers will consume the organic molecules needed by the PAO bacteria. In the so-called three-stage PHOREDOX process, influent water is fed to an anaerobic reactor, and then conveyed to an anoxic reactor also fed with recycled activated sludge from the last aerobic reactor (Figure 8c). In this way less nitrate is returned with sludge from the clarifier to the head of the system. Thus, both phosphorus and nitrogen removal are accomplished by this design.

Regulation and simulation models

The activated sludge process does not only involve complex elements but also the influent wastewater characteristics vary temporarily. This emphasizes the need for thorough control and optimization to maintain and fine-tune the process performance. To describe the actual WWTP, a general model including the ensemble of an activated sludge model, hydraulic model, oxygen-transfer model, and sedimentation tank model can be used. The activated sludge model describes the biological reactions occurring in the process by a set of differential equations. In addition to use in control and optimization, a WWTP model can be used to simulate different scenarios for learning or to evaluate new alternatives for design.

Strengths and weaknesses of WWTP

In its basic design the activated sludge process has a high capacity to biologically oxidize carbon and nitrogen. In addition, this is achieved in comparable small units, that is, less space is needed, which most often is a prerequisite for WWT in urban areas. By modifying the design also high amounts of nitrogen and phosphorus can be removed by biological processes. The SBR process is both a stable and flexible activated sludge process. The biomass cannot be washed out and the possibility to handle shifts in organic and hydraulic loads is good. In addition, less equipment and operator attention are needed to maintain the SBR process.

WWT by the activated sludge process must be regarded as a highly technological process, that is, much knowledge and experience are needed to operate a system based on this technique. In the process design of activated sludge processes, much focus has been put into efficiency in nutrient removal. Although generally pathogens are acceptably removed, most WWTPs are not designed for treating pathogenic microorganisms. Moreover, the environmental selective pressure on the microbial communities probably leads to highly specialized ecosystems. Consequently, the treatment process may be sensitive to disturbances due to environmental variations such as sewage load and composition as well as influent toxicants. The costs for maintenance and care are high. The nitrogen removed from the system is left as gaseous emissions instead of using such a valuable plant nutrient in crop production. In addition, the plant-nutrient-rich sludge may contain heavy metals as well as anthropogenic organic pollutants that may pose a risk to the ecosystem and must therefore most often be deposited or possibly incinerated.

Finally, the activated sludge process most likely is a WWTP technique that will also prevail in the foreseeable future. Process designs are continuously evolving to meet the demands of upcoming wastewater types, improved performance, and less resource consumption.

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