There are many biological methods for treating wastewater. Among them the most popular processes are suspended growth processes and the boifilm also known as attached growth . Both the above mentioned processes are used to treat wastewater by different mechanisms and variations in their principle of operation for removal of carbonaceous organic matter, nutrients (i.e. nitrogen and phosphorous) and production of biorenewable methane. Attached growth or fixed-film methods are biological treatment based. In these the microorganisms convert the organic substance in wastewater to air and cell tissues are attached to some inert medium. The inert media used in attached growth process is crushed trap rock, limestone granite, wood slats clinkers, , plastic tubes, hard coal materials and corrugated plastic section over which wastewater is distributed. Biological slime layer (i.e., zoogleal slime) develops on contact media as wastewater flows over it. Because of the concentration gradient, all the organic pollutants those are dissolved in wastewater are carried into the slime layer, results in organic oxidation. Removing organic pollutants is done by microorganisms which are present in biological slime film. Rotating biological contactors, Trickling filters, packed bed reactors are the examples for aerobic attached growth process. The examples of anaerobic attached growth are anaerobic packed and fluidized bed reactors.
Suspended growth processes are the biological treatment processes in which the microorganisms convert the organic matter, nutrients in wastewater to gases and rotating devices in the liquid will keep the cell tissues in suspension cells tissues. Examples for aerobic suspended growth process treatment are municipal and industrial wastewater. Anaerobic suspended growth processes are used when there is a need to treat high organic concentration industrial wastewaters and organic sludges. Activated sludge process, aerated lagoons, aerobic digestion are the examples for aerobic suspended growth processes and anaerobic digestion and anaerobic contact are the examples for anaerobic suspended growth processes. The schematics of both the attached growth and suspended growth processes for biological treatment of wastewater are shown in Figure 1. The objective of this essay is to compare and contrast both the attached, suspended growth processes for biological treatment of wastewater in terms of their microbial ecology and review their comparative advantages and disadvantages. In addition, the essay also elaborates and discusses how these processes can be adapted to (a) remove nutrients and (b) produce biorenewable methane from wastewater by emphasising the microbiological principles involved.
2. Comparison and Contrast of Microbial Ecology of Biological Wastewater Treatment - The attached growth / Suspended Growth Processes
The first basic difference between attached growth and fixed film systems is the relative motion of contact. In attached growth system media is at rest and the sewage flows over it where as in suspended system, sewage and contact media are in relative motion. In attached growth processes, a conservative microbial slime arrangement is considered. The waste water stream is applied on the air-renewable surface. This water stream has minerals and organic substrate. This substance is then metabolized by the Micro organisms in wastewater. This is done in order to increase their population by releasing some energy. In fact, it cannot be considered as an aerobic system, but a device to do aerobic treatment. facultative micro organisms are present in it so it can be considered facultative. They are Pseudomonas, Alcaligenes, Flavobacterium, and Micrococcus. This kind of Aerobic bacteria, like Bacillus, is mostly seen in the upper, aerobic slime surfaces (McKinney 1962). In case when an anaerobic zone or anoxic is created in thick slime layer, force anaerobe Desulfovibrio and sulfur-reducing bacteria are being removed from slime-medium interface (Rogovskaya & M. F. Lazareva 1962). At this case the microbial slime system is know how to grow odours and possibly sloughing takes places because of the production of gases in these interior slimes. Fungi are nothing but aerobic microorganisms existing in the aerobic zone of the slime. These too fester the untreated substrate in wastewater. The effectiveness of fungus is important under relatively low pH situation or with abnormal effluents from manufacturing factories because fungus is ineffective to bacteria for food under usual conditions of environmental. Algae increasing on the exterior of attached microbial slime are more often than not an unimportant factor of microorganism's population, incomplete to illuminated exteriors and they are clearly liberal to organic substances and elevated levels of carbon dioxide. though algae adds to oxygen in the wastewater, it has been stimulated with liability for bed blockage and also considered not easy from the operational position (Hawkes 1963). Protozoa are mainly minute animals with all shape from Phytomastigophora to Suctoria are controlling agents of bacterails population and they can't stable the waste. The ciliates(which are free-swimming) present at slime surface, while the followed ciliates attendance is main in the slime's lower regions. Advanced animals like insect larvae, worms, and snails nourish on the lower microorganisms forms in slime system. They also exist in higher aerobic areas. They help to keep bacterial population in a condition of soaring growth or speedy food use. Early learning on attached growth populations were intended at the direct of the nuisance organisms (filter flies) in dribbling filters by chlorination flooding, and using a variety of pesticides. A complete explanation of the organisms in close growth system is presented by Cooke (1959) and the list of a range of organisms can be found from Wang et al. (2009). Attached enlargement processes microbial slime which are short term preservation plans those should not perform as reduction devices which are effective for, S. paratyphi, Mycobacteria tuberculosis and S. typhosa and also for pathogenic protozoa like Entamoeba histolytica (Pierce 1978). For low organic loading, attached growth system does little nitrification because of the occurrence of Nitrosomonas and Nitrobacter. still, for high organic loading, nitrification may be absent or nominal in a trickling filter or similar system (Shammas & Wang 2009).
In contrast to the attached growth process, the suspended growth process maintains the microorganisms responsible for treatment within the suspension in flocs. In this process, there are several microorganisms involved to treat wastewater. The suspended growth flocs have bacterial cells, other microorganisms, organic and inorganic particles. The size of floc is 1-1000 mm. Figure 2 shows the major microorganisms in the suspended growth microbial centre of population. For dehydrogenase activity and the ATP analysis, the viable cells can report to 5-20% of total cells. a few authors said that active part of bacteria in suspended growth flocs is as small as 1-3% of the total. (Hanel 1988).
Suspended growth floc contains prokaryotic and eukaryotic microorganisms, which can be experimental with standard phase-contrast microscopy. The main genus in the flocs are Pseudomona,s Zooglea, , Flavobacterium, Alcaligenes, , Corynebacterium, Achromobacter Comomonas, Brevibacterium, Acinetobacter, Bacillus spp., and filamentous microorganisms. The population of bacteria decrease as the floc size increase results in less oxygen level in the flocs (Hanel, 1988). Anoxic zones happens within flocs, which is dependent on the accessibility of oxygen attention in the tank and these zones will vanish when the oxygen concentration crosses 4 mg/L.
The internal regions in big flocs favor the growth of anaerobic bacteria like as methanogens or SRB i.e.sulfate-reducing bacteria. Thus, pendant growth system can be a suitable and fitting seed substance for begining anaerobic reactors. Gram negative bacteria are main in suspended growth flocs. Hundreds of bacterial damage flourish in suspended growth but only small portion be able to be spoted by culture-based techniques. Bacteria oxidize the organic substance and change nutrient alteration and produce polymeric materials which facilitate the flocculation of microbial biomass. In aerobic pendant enlargement process, the total bacterial counts in around 108 CFU/mg of sludge. Suspended growth flocs too shelter autotrophic bacteria like Nitrosomonas and Nitrobacter nitrifiers, those change ammonium to nitrate. Phototrophic bacteria like the purple nonsulfur bacteria (Rhodospirillaceae), green sulfur bacteria are brought into being at much lower levels and they play a slight position in carbonaceous organic matter elimination in suspended growth processes. The suspended growth system does not typically favour the expansion of fungi, though few fungal filaments are seen in suspended growth flocs. The main genera found in suspended growth system are Geotrichum, Cephalosporium Penicillium, , Cladosporium, and Alternaria, which grow under exact conditions of low pH, nitrogen-deficient wastes and toxicity. Protozoa helps in reducing carbonaceous matter, pendant solids, and lot of bacteria, with pathogens (Curds & Hawkes 1983). There is an opposite relationship amid the amount of protozoa in mixed liquor and the carbonaceous matter and suspended solids concentration in suspended growth effluents. Changes in the protozoan community reflect the food-to-microorganisms (F/M) ratio, nitrification, sludge age, or dissolved oxygen level in the aeration tank. The composition of protozoan species of activated sludge indicates the carbonaceous matter removal efficiency of the procedure. for instance, the occurrence of big numbers of stalked ciliates and rotifers point to a low carbonaceous matter. The ecological series of microorganisms in suspended growth treatment system is shown in Figure 3. The ciliates (free, creeping, and stalked ciliates) are utilized for locomotion and for pushing into the mouth the food particles, which are most abundant protozoa in suspended growth systems. Stalked, Creeping ciliates ciliates are attached to the flocs. Such protozoa move by means of one or several flagella and take up food via the mouth or via absorption through their cell wall. The role of rotifers in suspended growth system is :
They help to remove suspended bacteria, other small particles and contribute to the clarification of wastewater. They are also capable of ingesting Cryptosporidium oocysts in wastewater and thus serve as vectors for the transmission of this parasite.
They contribute to floc formation by producing fecal pellets surrounded by mucus. The presence of rotifers at later stages of suspended growth system is because of the fact that these animals display a strong ciliary action that helps in feeding on reduced numbers of suspended bacteria.
3. Advantages and Disadvantages of Attached Growth with Suspended Growth Processes of Biological Treatment of Wastewater
The advantages of attached growth processes are low maintenance, low energy requirements, and, overall, less technology involved. These assets making them fit for wastewater treatment for small communities, as well as individual homes. In comparison with suspended growth process the main advantages of attached growth processes is easy operation, no bulking issues, and improved resurgence from shock loads (Metcalf & Eddy 2003). An attached growth process is very effective for biochemical oxygen demand (BOD) removal, nitrification, and denitrification. A disadvantage of attached growth processes are larger area, ineffective in cold weather, and create odor problems. An unprotected attached growth plant is weak to lower than freezing conditions and its recirculation limited during cold weather conditions. It is not much effective in the dealing of higher organic waste. Raw wastewater must be provided to primary treatment to remove the larger solids and floating debris, because these solids can clog the treatment system. The comparative advantages and disadvantages of attached growth with suspended growth processes of treatment of wastewater are illustrated in Table 1 and 2, respectively.
4. Nutrient Removal by Attached Growth and Suspended Growth Processes of Biological Wastewater Treatment
Nutrients (nitrogen and phosphorous) are be able to be removed from wastewater coming from biological means in the attached growth plus suspended growth processes. To remove nitrogen from, we must have to do nitrification followed by de-nitrification. Phosphorous can be removed from wastewater through assimilation of phosphorous in microbial cell.
Using nitrifying reactors nitrogen removal can be done in wastewater treatment using attached growth process based trickling. Experiments on nitrifying filters reveal that adsorption, desorption and de-nitrification are know to occur in attached growth bio-film to convert ammonium. At low organic loading the attached growth system does a quantity of nitrification since bacteria like Nitrosomonas and Nitrobacter are present but at higher organic loading nitrification in trickling filter is absent else nominal. The three-stage attached growth process is utilized for nitrification, carbonaceous oxidation and de-nitrification.
In case of suspended growth process, the flocs shelter autotrophic bacteria (Nitrosomonas, Nitrobacter), that change ammonium to nitrate. These type crop up in clusters plus they are in contact in pending growth flocs and in biofilms. Fungi are also able to do nitrification and denitrification in suspended growth process. Compensation of a fungi-based dealing system are their capability to take out nitrification in a sole step, plus they show great confrontation to inhibitory compounds. Molecular methods like aggressive PCR method showed that Nitrosomonas is ( AOB- ammonia oxidizing bacterium) 0.0033% of the whole bacterial populace and Nitrospira is ( NOB- nitrite oxidizing bacterium) 0.39% for NOB are present in suspended growth system. Features like ammonia or nitrite concentration, pH, temperature , oxygen concentration, , BOD5/TKN ratio, and the attendance of toxic chemicals control nitrification kinetics in suspended growth system (Metcalf and Eddy 2003). The growth of a nitrifying population in suspended growth system depends on the surplus rate of the sludge and, consequently, on the BOD load, MLSS, and mean cell preservation time. The growth of nitrifiers is lesser in sewage and consequently a extended sludge age is essential for ammonia changing to nitrate. If the age of sludge is more than 4 days, then we be expecting nitrification. There are two processes by which nitrification can be attained in suspended growth reactors:
(i) Combined carbon oxidation-nitrification (i.e. single-stage nitrification system): this course mechanises at elevated BOD5/TKN ratio and has a small populace of nitrifiers. The oxygen prerequisite is exerted by heterotrophs (Figure 4).
Nitrification should follow the denitrification to take away nitrogen from wastewater. The speed of denitrification is autonomous to nitrate concentration however depends on concentration of biomass and electron donor like methanol in wastewater. The schematic of single sludge denitrification system is shown in Figure 6. Some more efficient methods based on suspended growth system to remove nitrogen in wastewater are Bardenpho Process (Figure 7), Sharon-Anammox Process and Completely autotrophic nitrogen removal over nitrite process.
Biological phosphorous removal is done with the basic steps of an anaerobic zone then an aerobic zone. During the aerobic stage phosphorus uptake takes and in anaerobic stage it is released subsequently (Manning and Irvine 1985; Meganck and Faup 1988; USEPA 1987). The phosphorus removal processes is separated into mainstream with side stream processes. The alternating exposure to anaerobic conditions can be accomplished in the main biological treatment process. The most popular phosphorous removal systems based on suspended growth process like A/O Process, Bardenpho Process, and UCT Process are shown in Figure 8-10, respectively.
5. Production of Biorenewable Methane by Attached Growth and Suspended Growth Processes of Biological Treatment of Wastewater
Several attached growth and suspended growth processes like anaerobic digestion and microbial fuel cell are generally employed to produce biorenewable methane (CH4). A series of microbiological processes will takes place in anaerobic digestion tank to alter organic compounds to carbon dioxide , methane, and trim down the volatile solids by 35 to 60%. Bacteria and methanogens are the responsible species in stabilization of wastewater sludges and intended for the treatment of manufacturing units and urban wastewater. Anaerobic absorption produces less amount of sludge (3-20 times lesser than aerobic processes). Most of the power derived from substrate breaking down is seen in the final producti.e.CH4. merely 5% is changed into biomass in anaerobic conditions and 50% of organic carbon is converted to biomass under aerobic conditions, whereas. This biogas (mainly CH4) produced from anaerobic digestion has around ninety percent of the energy of rich value of 9000 kcal/m3, that is burned on spot to supply temperature for digesters. Furthermore, synergistic interactions between the various groups of microorganisms are implicated in anaerobic digestion of wastes. The overall reaction can be written as -
though a little fungus and protozoa that is anaerobic protozoa found in anaerobic digesters but bacteria and methanogens are certainly the main microorganisms. Huge quantities of facultative anaerobic bacteria like Bacteroides, Bifidobacterium, Clostridium, Lactobacillus and Streptococcus are utilizes in the hydrolysis and fermentation of organic compounds. Four types of microorganisms are implicated in the alteration of complex materials to simple molecules like carbon dioxide and methane. The process for methane production through anaerobic digestion is shown in Figure 11.
6. Conclusions
The attached and suspended growth processes are most widely used for biological treatment of urban wastewater with respective and comparative advantages and disadvantages. The victorious design and operation of attached and suspended growth processes for biological treatment of wastewater require an understanding of microbial ecology of each process, variety of microorganisms implicated the exact reactions that they perform plus the ecological factors that influence their recital, nutritional needs they have and reaction kinetics. In contrast to the microbial ecology of the suspended growth processes where the microorganisms responsible for treatment grow in suspension, the active bacteria in attached growth processes cling to some solid surface, natural or manmade, to perform the treatment. It can be inferred that the suspended growth processes are more preferable and advantageous for carbonaceous organic matter removal along with nutrient removal and production of biorenewable methane from urban wastewater in comparison to the attached growth processes.
