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Wastewater Treatment Techniques: Anaerobic Digestion

Introduction

Wastewater treatment techniques are necessities in the contemporary world. This is attributable to amplified proportions of wastes, which are resultant features of anthropogenic activities. Noticeably, anaerobic digestion is an organic procedure that utilizes anaerobic microbes to physically, degrade untreated constituents of wastes in the absence of oxygen (Guillermo & Matti 3). Evidently, methane gas is subject to rate limiting processes thus controlling anaerobic digestion.

Apparently, waste management design is decisive in the whole process. Furthermore, operations should concentrate on providing the most favorable conditions for methane production. Anaerobic digestion procedures stabilize biological sludge prior to dewatering and/or final disposal. Anaerobic digestion management procedures treat an array of manufacturing wastes. Evidently, the organic substance available in the substrate is biologically decayed to “CO2 and methane” in the deficiency of air (Deshai and Rune 1). The presence of sulfur compounds in the wastewater facilitates the creation of H2S.

Factors essential for anaerobic digesters functional capacity include environmental aspects such as “temperature, pH, nutrients, and toxicant concentrations” and the distinctiveness of the wastewater. Anaerobic digestion has minimal impact on some resources such as phosphorus and nitrogen present in wastewater. Temperature variations affect the anaerobic digestion process significantly. Increase in temperature escalates the reactions as evident in biochemical reactions and microbial growth, which takes place within the heat tolerance capacity of bacteria. Different microorganisms display varying development and metabolism rates in well-defined temperature levels (Nwabanne et al., 515). The different microorganisms respond to changes in heat differently. Consequently, ensuring a regular temperature for microorganisms functioning is vital in attaining such rates. Methane forming germs responds greatly to variations in temperature, upon comparison to other microorganisms available in the digesters since they have a subordinate growth rate (Xiguang, Rowena and Zhang et al., 63). The maximum digestion takes place in mesophilic temperature range, which is approximately 35oC, and thermophilic temperature range, which is roughly 55oC. “Thermopilic anaerobic digestion” (TAD) supports superior levels of methane availability, reduced fluid viscosity, reduced biomass formation, improved turnover of organic wastes from matter to biogas and higher pathogen destruction

Most microbes have shown a higher growth rate at neutral pH conditions. The pH levels affects metabolism by shifting the chemical balance of reactions involving enzymes and by destroying such enzymes. Methane forming microbes are highly sensitive to variations in pH thus values lower than 6.0 inhibit their performance (Demirer 206). The balance between carbon dioxide and bicarbonate ion with ammonium cations puts significant resistance to pH variations. When in aqueous form, carbon dioxide balances with carbonic acid; subsequently, it separates to produce H and BCO32+. Evidently, volatile acids also affect methanogenic bacteria though they have little impact when pH levels are approximately neutral (Lyberatos & Skiadas 64). Ammonia quickly forms in the digester through an association with protein nutrients thus producing ammonia with elevated toxicity. Consequently, ammonia toxicity levels receptive to pH assume an elevated status.

Concentration of sulphides to proportions higher than 200mg/L inhibits the action of methanogenic bacteria thus contributing to failure in the process. Addition of (Fe) lowers the concentration of the sulphides in suspension. Heavy metals at low concentration pose toxicity problems to many anaerobic digestion microorganisms. The iron’s excess toxicity does not pose problems to the reactors since only soluble metals have an effect. Lowering of the soluble absorption to non-toxic levels takes place through precipitating with the sulphides (Lyberatos & Skiadas 65).

Loading rate referring to “the mass of organic matter added per volume unit per time unit” also affects the performance of digesters. Extra feeding the reactors above their suitable organic loading rate reduces gas yields due to accrual of hindering fatty acids components in the digester. This requires the reduction of organic feeding to improve gas production and prevent systems malfunction due to burden (Lens, Zeeman & Lettinga 432).

Elevated level mixing in the digester ensures improved contact between microbes and wastes. This improves the bacterial population’s ability to acquire nutrients. Greater mixing distributes and disrupts microbes thus reducing their activity. In case of dual digesters, the feedstock is mixed together before putting in the digester to attain uniformity. The retention time required for degradation of organic materials also affects the reactors (Lens, Zeeman & Lettinga 433). The time, which substrates take, depends on the temperature and the composition of the wastes. Mesophilic heat requires that wastes treated takes roughly 25 days while thermophilic digester dissipate take approximately 14 days.

Phases of anaerobic digestion of wastewater treatment

The initial phase of hydrolysis is the procedure by which enzymes degrades huge polymers. The hydrolytic bacteria produce extracellular enzymes, which degrade “carbohydrates, cellulose, proteins, and fats” (Botheju & Bakke, 2).This enhances their availability for use by the acidogenic bacteria at the subsequent stage. Additionally, the proteins turn into amino acids.

Wastes availability, microbe’s group density, heat, and pH check the speed of hydrolysis. The next phase is acidogenic fermentation; with acetate being the main constituent. “Volatile fatty acids” are produced along with CO2 and H during fermentation. Acetogenesis is the separation or dissipation of capricious acids to acetate and H. This stage is characterized by the generation of “acetic acid from monomers” released in the previous stage and “volatile fatty acid” (VFAs) derived from the protein, fat and carbohydrate constituents of the wastewater. Carbon dioxide and hydrogen also constitute due to breakdown of carbohydrates, with the extra chance for the generation of methanol and other simple alcohols. The constitution of the various products generated is independent of the environmental conditions and specific microbe species available (Botheju & Bakke, 3). The next phase is methanogenesis, a process where “acetate, formaldehyde, H and CO2 are converted to CH4 and H20”. Obligate anaerobes whose augmentation pace is lesser than other microorganisms from the prior stages facilitate the process. Methane is generated from simple substrates such as “acetic acid, methanol or carbon dioxide and hydrogen” (Vavilin 23). Most significant of these constituents are acetic acid and acetate because they generate about 75% of total methane produced (Lens, Zeeman & Lettinga 432).

“Up flow Anaerobic Sludge Blanket
Diagrammatic representation of “Up flow Anaerobic Sludge Blanket”

“This system has its advantages and demerits as illustrated in the table below” (Von Sperling 11).

ADVANTAGES DISADVANTAGES
  1. The full-scale UASB sewage treatment scales can operate in both tropical and sub-tropical setting while the industrial UASB treatment (strong liquid waste) works in moderate to cold climates. This gives a broad array of its uses.
The foremost launch of UASB process is quite fragile and time-demanding procedure.
  1. It has the advantage of reducing the pollution potential and providing a substitute power supply in form of methane that is currently in high demand.
Maintenance of aggregates of active biomass as granules with excellent sedimentation features must be done constantly for successful operation of the UASBs.
  1. The UASB as an attached augmentation system has the advantage of increased biomass immobilization in industrial waste management as compared to the suspended growth systems like the anaerobic slush digestion and contact processes.
Surplus slush has must be discharged occasionally to uphold the fluidized region and avert the removal of biomass.
  1. The system unlike the floating growth scenario has elevated biomass levels thus shorter hydraulic preservation times.
  1. The system relatively has progression stability as compared to the suspended augmentation systems.
  1. Possible sustained formation of granules may make the UASB process more advantageous than other anaerobic processes.
  1. Unlike other anaerobic processes, the UASB has very elevated loading rates due to the high biomass levels and good settleability resulting in high solids retention in the reactor.
  1. Many UASB’s possess solids severance part used to confine suspended granules that might arise overtime. This devise element is only unique to the UASB systems.
  1. This anaerobic treatment scheme permits a significantly lower management expenses because of ability to function at low temperatures. This is a merit over the aerobic curing method.
  1. UASB reactor lacks the electromechanical apparatus, unlike prime settling tanks, thickeners, and digesters of usual activated slush systems.
  1. In this system, there is smaller number of different units to be installed; there lacks need for primary settling tanks, thickeners, and digesters, which are replaced by the UASB reactor.
  1. There is reduction in the consumption of chemical products for dewatering due to decreased sludge production and improved dewaterbility.
  1. The total volume of the units in this system is lower than the total quantity of the usual activated slush unit(primary sedimentation tank, activated sludge reactor, secondary settling tank, sludge thickener, sludge digester, and sludge dewatering ) (Von Sperling 11)

“Up flow Anaerobic Sludge Blanket” (UASB) is an elevated rate anaerobic digester for wastewater management. It was initiated in the 1970s for money-making purposes by Professor Lettinga at the “University of Wageningen”. The technology was at first applied in South Africa with the intention of addressing anaerobic concerns in starch wastewaters. Today, its application is broad as evident in the tropics and related regions. The technology majorly treats domestic wastewaters, high power recyclable manufacturing and agro – engineering liquid wastes. UASBs are resistant concrete structures with a small hydraulic maintenance time, of 6-12 hours. After screening and sand exclusion, the untreated wastewater distributes equally crossways to the bottom of the reactor. It then flows aloft throughout the mud cover. This ensures close association amid the dissipate water and the anaerobic microbes present in the mud.

The blanket assists in the anaerobic biochemical reactions and increases the effectiveness of “Biochemical Oxygen Demand” elimination in the reactor. The wastewater, collectively with some active sludge particles, subsequently rises through the reactor. Sludge suspension then reaches the part divider, which is the vital characteristic of this kind of anaerobic reactor. The divider separates the reactor into two components the lower assimilation zone and the higher settling zone. Wastewater mixed with sludge rises over the settling zone and its up flow speed reduces because of the externally leaning phase divider. This improves the flow area and the suspended slush particles stay out, primarily on the leaning sides of the phase divider (Matsui, Morimura & Wu, 1378). Consequently, the mass of the settled sludge particles outweighs the frictional strength. This ensures the sludge remain on the inclined surface and accumulates on the mud layer. The phase divider maintains a soaring concentration of sludge eliminated from the reactor. The UASBs are very efficient and biogas foam collects below the phase divider, and can be extracted simply and prepared for use (Nunes, Danielle & Eclésio Cavalcante 66).

Deflectors are used between the phase divider to ensure gas froth does not enter accumulation zone because they can prevent sedimentation. The phase dividers separate the reactor into two parts, which also separates the three phases, fluid, gas, and solid. The part for wastewater compilation is a straight sewer with standard notches (Clàudia, Benatti & Filho et al, 3). There is also a scum guard, which ensures no suspended solids leave the reactor. This arrangement is similar to the sedimentation in main and resultant anaerobic treatment. The UASBs produce sludge, which requires disposal. The UASBs require post aerobic treatment of the wastes treated in the system (Clàudia, Benatti & Filho et al, 3).

Schematic Diagram for UASB

Stabilization ponds also identified as “sewage lagoons or oxidation ponds” is a considerably economical wastewater treatment method. They are anthropogenic with a pre-designed breadth. There are three sorts of stabilization ponds “aerobic, anaerobic, and facultative ponds” (Nichols 47). Their application is on last biological procedure of processing wastewater or partly treated liquid waste by additional methods. Food processing wastes, oil refineries, and paper mills among others are examples of wastes treated by stabilization ponds. Aerobic ponds are considerably superficial ponds, 1-1.5 meters deep, regularly connected jointly in a chain. They contain microorganisms, algae, small bacteria, some upper plants, and fish. The photosynthetic actions of algae suit the necessities for O2 in aerobic ponds. The microbes ensure right aerobic state through the depth and stabilization of organics. Sometimes appropriate mechanical devices are used to make the ponds aerated. The mechanical method ensures availability of oxygen and enables mixing of the pond contents. Anaerobic ponds have organic processes taking place throughout its profundity, which vary from 8-15ft. The ponds have the capacity to deal with high BOD load. Facultative ponds have a depth of 3-5ft with varied microbial activities occurring in three zones along the depth. Top zone enables aerobic microorganisms to stabilize organic in the occurrence of algae (Nichols, 47). Anaerobic microbes dominate the lower zone degrading the accumulated solids. The center region is partially aerobic and anaerobic. Bio stabilization in this level can be improved by facultative collection of germs. The main benefit of stabilization ponds is evident in their cheap nature and ease of construction.

Trickling Filters are a non-natural bed filled with compacted stones or pebbles above which wastewater is trickled at a regular speed by way of a revolving distributer. The laid rocks are covered with dynamic biological film, which eliminates the soluble and colloidal organics from the trickling water. Non-original filtering media including plastic packing are used. This method is regularly applied in treating manufacturing wastes due to their capacity to oppose shocks loads. They also contain little sensitivity to poisonous resources (Fezzani & Chiekh 5). The method is commonly applied in “food processing wastes, dairy wastes, and brewery and distillery wastes”. It is vital to take the wastewater through the pretreatment including compound precipitation prior to its application to a trickling filter.

The main advantages of trickling filters are evident in their manageability hence more applicable in small communities; besides, they require stumpy upholding costs. They are dependable, and have the capacity to endure distress loads of noxious inputs. The key drawbacks also include easy clogging when loaded with high organic due to overgrowth of substance germs in biofilms; they also produce odor smell and clogging which reduces oxygen circulation hindering biofilm microorganisms (Gareth 302).

Imhoff tanks are comparatively simple anaerobic method used to treat wastewater before the arrival of the heated digesters. Currently, it is in use in small capacity. The tanks have two rectangular parts partially built underneath the ground. The wastewater is aligned through the elevated section, used as settling sink. The settled solid steadies out anaerobically on the lower compartment, which is regularly not heated. The stabilized sludge is eliminated underneath of the tank roughly twofold in a year. This ensures the sludge get a considerable time to stabilize (Wang, Joo-Hwa Tay and Tiong 775). The tanks are at times made with inlets and openings not considering the ends. This calls for reversing the running of water regularly so that sludge can stabilize well. The limitations of the imhoff tanks include, the configuration of scum and odor, foaming which takes place when the temperature in the tanks reduce to below 15oC. This causes unequilibrium because the bacteria forming volatile acids dominate the process leading to lowered methane gas production. The scum forms due to the gases present from anaerobic digestion and trapped by solids thus causing their floatation (Wang, Joo-Hwa Tay and Tiong 775). Improving the depth of the lower section solves this problem. When the depths are very low, foam will appear at elevated pressure, expands greatly while going up enhancing their movement until they escape. The problem of odor is reduced by achieving a balance between acid and gas formation (Gabriel Bitton 284).

Schematic Diagram for Imhoff Tanks

Rotating biological contactors are more innovative economical wastewater treatment for biologically curing water. It is an attached augmentation manner, which puts together positives of biological fixed film. The RBC is commonly used in treating household and manufacturing wastewater (Gareth 106). It has a directly spaced discs developed on a common horizontal shaft, and is partly sunken in a crescent tank receptor. Wastewater flows in excess of the reactor and allows the microbes to feed on its substrates and grow attached to the discs surfaces. The excess substratum is removed from the discs and separates from the water at the secondary settling tank. Some biomass remains as a suspension in the water.

“Anaerobic Fixed Film Reactor” (AFFR) has very little use on managing municipal dissipate water since its only efficient on weak on liquid bio wastes with little solid filling. It works through establishing a bacterial biofilm on a preset enlargement cover inside the digester. The advantages of this method include the fact that it requires short retention time, biomass absorption is high, there are low costs incurred, the procedure is also simple, it is also less sensitive to shock loads and toxicity and that it only requires partial stirring. “Anaerobic Baffled Reactor” (ABR) uses a horizontal flow of wastewater above the digester container. It is used for treating liquid bio wastes and it is complete with its gas recirculation and combination method. Anaerobic Baffled Reactor (ABR) used for great force and characteristically trade, bio dissipates with post processed biomass reused after dewatering to boost the solid retention time (Gareth 104).

Schematic Diagram for Anaerobic Baffled Reactor

“Fluidized or Expanded Bed Reactors” (FBR) is a method only useful in treating liquid bio wastes, which fluidize the digesters interior bacterial growth medium as they are reprocessed within the digester. “Continuously Stirred Tank Reactor” (CSTR) is used for treating wastewaters and slurries. It is efficient with the draw round devoid of the want to reprocess solids.

Advantages and Disadvantages of anaerobic Digestion

The positive attributes of organic wastewater management include production of low amount of solids, little energy consumption; furthermore, it requires minimal area. Other advantages are low costs incurred for construction, methane gas is produced and utilized as energy, and elevated organic loads can be fed at the same time, nutrient consumption is very low (Yerima, Ogunkoya & Sada-Maryam et al., 4147). Finally, the produced waste gas does not require treatment thus no need to separate colloids and dispersed solid particles before treatment. The disadvantages of anaerobic digestion incorporate problems of post treatment; thus, the microorganisms are likely to stop working with large amounts of compounds. In addition, the anaerobic process can be very slow in the deficiency of modified seed sludge. There a likelihood of generation of bad odor, although this is controllable, finally it is not possible to remove all the nitrogen, phosphorus, and pathogens (Powers et al., 1419).

Conclusion

Wastewater treatment is very important for healthy and habitable setting. Anaerobic digestion normally takes place in four stages, thus “hydrolysis, fermentation, acetogenesis, and methanogenesis”. People may utilize wastewater is subject to diverse stages (Batson, Angelidaki & Keller 66). The techniques for wastewater treatment have also evolved over a long period of time. Dissimilar types of management methods were identified as “UASB, Trickling filters, stabilization ponds, imhoff tanks, Rotating Biological Contactors, Fluidized, or Expanded Bed Reactors (FBR)” among others. Anaerobic digestion for wastewater treatment has made the recycling of sewage possible (Batson, Angelidaki & Keller 67). The anaerobic digestion used to treat wastewater has noteworthy advantages, which evidently prevail over the disadvantages. This makes it attractive, upon comparison to related methods of wastewater treatment. It is also important to outline that anaerobic wastewater management is easily applicable even in small scales. Conclusively, this process is paramount to the sustainability of the settings.

Works Cited

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