Carla Petrongolo Log
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Carla Petrongolo
Date Submitted: December 5, 2009

Waste Water Treatment: My means of an Upflow Anaerobic Sludge Blanket

Abstract:
An upflow anaerobic sludge blanket, UASB, reactor is designed to digest organic compounds under anaerobic conditions. This is seen to have a better yield of biogas when the system is kept at a thermophilic temperature of about 55 oC and at a pH ranging from 6.2 to 8.0. These digesters are not limited to the UASB design, but are seen to have very positive results for the largest variety of waste water. The yield of the digestion is biogas, mainly methane, which when burned is a source of energy. This particular reactor is so efficient because of its constant movement due to an upward flow of waste water through an anaerobic sludge blanket. Once the stability of the system is achieved the digestion is profitable as long as the systems environment is maintained.

Reason for Treatment of Waste Water:
Waste water treatment has become very popular in industry. Each country has requirements for the amount of the chemical and biological oxygen demands (COD /BOD) for waste of all types and the more polluted the waste the more money a company must pay to dispose of it. For example, in the European Union one can have a maximum of 125 mg O2l-1 to be able to dispose waste. (1) By decreasing the amount of chemical oxygen demand, the waste water is then easier and cheaper to dispose. There are many ways to treat the water to achieve this. One approach is by anaerobic means which is more energy efficient and produces less sludge than an aerobic system. The most important qualities of an efficient system are to have minimal sludge output and to have the most cost effective system possible. The least amount of energy expended is desired and also for the system to be fed a very low amount of nutrients to function properly. Since the systems run with live biomass to have chemical reactions reducing COD, they output energy. This energy is utilized and is a valuable product of digestion.

UASB Reactor:
These reactors come in a variety of designs. The most efficient for the widest array of waste water is the upflow anaerobic sludge blanket (UASB). In this reactor organic matter is broken down into biogas. This hybrid system is used for disposal of various waste waters from pharmaceutical to alcoholic beverage industries. There are many other factors such as the influent the system is fed to the hydraulic rate of the system. One important factor is the temperature at which the system is run. A thermophilic system, 50 oC -60oC, is considered to be more efficient since it increases the breakdown of organic solids, the energy output through gas production, and the separation of liquid and solid within system. (13) High temperatures will kill certain pathogens in the reactor, also called a digester. High temperatures also reduce reaction time, which leads to a decrease in digester volume. (13) Since reactors are easily disturbed, the temperature should remain constant throughout the digestion process. (14)

An upflow anaerobic sludge blanket has a specific design to be a proficient system. This system is run in the absence of oxygen and sole purpose is to breakdown biodegradable substances by means of microorganisms reducing the COD/BOD by as much as 90%. (1) The reactor itself is made up of complex mechanics within a large tank structure. A blanket of granular sludge containing microorganisms is suspended within the large tank. These microorganisms vary depending on the specific waste water and will need time to acclimate before the system will be efficient. When first starting a system, the granules in the sludge blanket are placed in the reactor from a previous system that is somewhat close in relation. For example if sludge granules from a one beer brewery could be used in another, it would not take much to acclimate the granules to the new waste water. (4)

The waste water is fed into the reactor in deliberately spaced inlets in an upward flow through the blanket of sludge. The system is in constant motion due to this upward current. This is where the microorganisms in the sludge process the waste water. The upward flow is crucial since it increases the filterability of the particles.(4) The direction of flow is opposite of gravity creating an optimum condition to improve aggregation. At three months the sludge granules have formed with a thick coding of bacteria. This live system acts much like any other living organism. The microorganisms that have not adapted by this point will die and most likely be washed out of the reactor. In the sludge bed the biggest granules fall to the bottom and the very small granules get washed out, so the sludge ‘wants’ to be as dense as possible.

There are many theories for the formation of the sludge granules. The most respected one was discovered by Dr. W. Wiegant. His “spaghetti” theory describes the formation of microorganisms by first dispersing the methanogens and then they become entangled. (6) They, as a result, flocculate due to their entanglement. There is then a formation of fungal pellets which can be described as spaghetti balls, giving this theory its name. The granules attach to other anaerobic microorganisms forming larger pellets. The pellets will gain more substance as the reactor matures reaching 0.5 mm to 2 mm in diameter. There is an organic breakdown when the biomass comes in contact with the water’s substrates. In this system lie anaerobic bacteria. This bacterium in the absence of oxygen is what produces the biogas as a waste product. (14) There are three basic steps of the digestion process that occur. First, organic substances are converted by microorganisms in the sludge to produce a subsequent organism which then form organic acids. These organic acids are used by methane producing anaerobic bacteria to finally decompose the substrate. (14)

The hydrodynamics of the reactor have a great effect on the activity and formation of granules. A study done by Y. Arcand and fellow researchers examined the effects of the upflow velocity along with the organic loading rate. With an increased loading rate the sludge beds were fluidized. With an increase in spatial gradient the bacteria had more room to grow and increase surface area, which induced the pellets to mature. This study also concluded that with an increased OLR the COD decreased but this did not have an increased effect of the granules size. This shows that the major effects on the granule growth is attributed to the speed at which the waste substrate in passing through the sludge bed and the need to increase surface area achieved by this fluidization. With an upward velocity increase the acidogenic activity is decreased. (9) This is necessary to ensure a healthy stable pH. The activity of other compounds such as acetate and formate are increased with a higher OLR. With compounds such as acetate, the anaerobic bacteria size was amplified whereas the acidic compounds reduced the pellet size. (9) This study shows the dynamic conditions to achieve maximum digestion from the sludge blanket, also called biomass.

Biomass is carbon-bound solar energy. Due to the decrease of organic substances caused by repeated catabolism of dead biomass the result is biogas. (12) Biogas consists of methane, carbon dioxide, and other small amounts of gasses such as hydrogen and nitrogen. (13, 14) There is 50-80% methane which is the main source of the energy when burned that is utilized. In many cases it has a large enough biogas output to give power to either run the reactor itself or is used as a source of power for other things. The amount of biogas produced depends on the nutrients fed into the system as well as how the reactor is managed. (14) When the biogas is burned about a cubic foot of biogas yields 10 British thermal units (Btu), which is a unit of energy. For instance, “biogas composed of 65% methane yields 650 Btu per cubic foot (5,857 kcal/cubic meter)”(14). This is different for each digester, but the result is profitable for all flourishing systems.

Since microorganisms need to adapt to the specific waste water, it takes about three months to be in a mature state proved in various studies. For example, Ganesh and fellow researchers’ study on dairy waste water shows that before 3 months the system was unstable and the granules were not fully matured. After this period there was not much variation in data such as COD and VFA readings, which will be described further. Once the system reaches this level of maturity it is in full function and as long as it is well monitored there should not be any problems. (7) The rate at which the influent is added, the hydraulic load, is slowly increased during the time of adaptation. The hydraulic retention time, HRT, is the volume of the reactor divided by the influent flow rate. The HRT is what is calculated to determine the time a compound stays in the reactor and to know how often the system needs influent, also called feed. Note that while the influent and effluent are being pumped in and out the waste water and sludge are within the tank giving off biogas as a byproduct of its waste. The HRT is increased as the system stabilizes. (2)

The amount of nutrients added becomes less diluted as the biomass adjusts. COD is gradually increased in the feed which also usually contains a like component, substrate, with the waste water for the system. For example, in the dairy industry, a waste water feed would contain milk products. The feed also has necessary nutrients such as protein and fat. (7) Another example is for beer brewery waste water, the feed would consist of protein, vegetable oil, sodium bicarbonate, cellulose, FeCl, micronutrients, and beer. (17) Sodium bicarbonate is added to help regulate the pH. This is needed for example when too much nitrogen is added into the reactor. The carbon nitrogen ratio should not exceed 30/1 or the digestion process is at risk. (14) All of these additives are important because they give the biomass proper nutrients and keep them satisfied so they perform efficiently.

When a system begins to decline in performance one of the first steps of recovery is diluting the feed to flush out any diseased granules. The system also must be letting out waste as it takes in nutrients. The effluent usually contains “residual degradable and non- or slowly biodegradable influent substrate; soluble microbial products, and intermediate products such as volatile fatty acids, VFA” (1). It also is “rich in nutrients such as ammonium, potassium, and phosphorous” to name a few (14). Even though it is crucial to test the effluent to maintain a healthy reactor, it is also can be used as a source of nutrients for soil. (14) It is very important that the least amount of sludge is lost during the effluent process as possible or the system will eventually not have enough sludge granules to keep the digester running. Since the sludge granules settle quickly, the UASB reactor does not have a problem doing this once it reaches a prime state. This also allows for a high hydraulic load in the reactor. There must be constant attention on the well being of the reactor. Tests must be run to ensure stability and they are performed on the effluent, which is continuously pumped out of the UASB reactor.

The pH of the system and the biogas output readings must be taken daily. A pH outside of the range 6.2 to 8.0 could be the first detection to a serious problem, for example if too much nitrogen has been added through feed. The biomass, specifically anaerobic bacteria, will not work at an extreme pH. (4) The biogas reading is also an immediate indication of a problem since this is the energy output of the system. There is a different expected output for various kinds of waste water, but by keeping a record there should be a consistency. The biogas readings will increase with maturity and will eventually level out after three months. If for instance the reading dramatically drops there is clearly a problem in the reactor. Besides these two easy tests, other simple assays are performed on the effluent.

A measure of the volatile fatty acids and chemical oxygen demand is also recommended to be performed daily. Volatile fatty acids, VFA, are short chain fatty acids with six carbons or less. They are used as a source of energy in the body and the microorganisms in the sludge blanket use them in the same way. If there is a great measured amount of VFA then the system is healthy. This means that the waste water’s substrates are properly being broken down and converted to a biogas byproduct. The COD of the system is easily measured by means of spectrometry. This is done to make sure that the biomass is doing their job and decreasing the chemical oxygen demand. The COD corresponds with the biological oxygen demand so as long as these numbers are not alarmingly high the system is working. There are some cases where waste water will not respond well to the reactor. If this is the case the COD will be higher than what is approved by regulations and will need further special treatment outside of the UASB like solid/liquid separation or aerobic polishing. (1) Even with a good VFA and COD result, there must be further testing to determine the loading and removal rate to achieve and maintain a stable system.

Volatile and total suspended solids, VSS/TSS, reading must be taken to determine whether the specific loading rate, SLR, and substrate removal rate, SRR, are sufficient. (9, 11) This is done by simply collecting effluent samples and weighing it at various temperatures. As a result, organic and inorganic solids will burn off at various temperatures, determining the amount of VSS and TSS present. Since the reactor is constantly in motion the effluent should have the positive and consistent result of suspended solids within. This allows one to detect a loss of sludge if for some reason any was washed out. Usually once the reactor reaches a stable state the only way it will become inefficient is by human error. For example, if the influent in incorrectly prepared and/or added or a power outage stops the constant flow. When this occurs sometimes damage control will work by purposely flushing out the system and adding new sludge granules. Flushing out the system consists of diluting the feed to remove infected granules. In extreme cases new sludge granules are added from a UASB reactor of similar substrate. This unfortunately is a set back and will take time to build up dense granules with a high COD removal. (9)

In the dairy waste water assay the reactor was started with 300 mg/L of COD in influent for 5 days to get the granules adjusted to the dairy waste water substrate. After this point HRT was 24 hours and the reactor was continuously fed. The initial organic loading rate, OLR, was 0.3 kg COD m-3d-1. After two weeks the OLR was increased to 0.45 kg COD m-3d-1. When the system reached 60% COD removal the OLR was increased further until it reached 4.8 kg COD m-3d-1. The HRT was at this point 6 hours so the OLR came down to 3.6 kg COD m-3d-1, to prevent wash out of sludge since the system was moving through so quickly. The reactor at 3 months what at a stable mature state and the COD removal bounced between 70% and 80%. The biogas generated from this study increased with OLR and reached 1 m3 per m3 of the reactor’s volume. (7) This particular waste already had a low concentration of COD and this study was successful in treatment and reduced the COD a great amount to allow for easy disposal of waste and generating a sufficient amount of energy.

A study was done by D. Sreekanth and team on bulk pharmaceutical waste water which would have a greater amount of initial COD compared to the dairy assay above due to the organic contents. (2) The study was done to determine the most efficient OLR with the greatest COD and BOD decrease. A reactor was monitored over time with a glucose based feed. The OLR started at 1 kg COD m-3d-1. The difference between 2 to 11 kg COD m-3d-1 was examined. The maximum OLR able to be used successfully was 9 kg COD m-3d-1. At this organic loading rate the COD reduction was 65%-75%, and the BOD reduction was 80%-94%. This was seen with a biogas containing 60-70% of methane. This is good since this particular waste water contained a high concentration of organic compounds. This reactor also proved to be very strong due to its ability to withstand shock loads. Even with increased OLR above the maximum it only took 4-5 days to recover and stabilize. Also through GC-MS analysis hazardous materials were identified in the waste water and all but one were eliminated through the UASB. (2)

Conclusion:
These studies conclude that a UASB reactor will work for a variety of waste water. The COD/BOD reduction is crucial for the safety of the environment. This digestion reaction is dependent on the organic breakdown performed by the anaerobic bacteria within the sludge blanket of the reactor. The design of the UASB reactor is what sets it apart from other systems with its upward flow through the sludge blanket. The sludge granules must adjust before the digestion process will occur to its full capability. The pellets build up with help of gradual increase of OLR and upward velocity of waste water. Once stabilized, the bacterium must be kept in a specific environment. UASB reactors function best under thermophilic conditions and at a pH range of 6.2 to 8.0. The system must be monitored at all times. A VFA reading and biogas index will indicate efficiency of organic digestion. As a result the waste water should have anywhere from a 70%-90% decrease in COD/BOD so fulfill requirements by law.


[The first DOI link in ref 1 is fine but all references that have "sciencedirect" in the URL are not acceptable - I also recommend getting rid of the inconsistent formatting JCB]
References
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2) Sreekanth D., D. Sivaramakrishna , V. Himabindu, and Y. Anjaneyulu. “Thermophilic treatment of bulk drug pharmaceutical industrial wastewaters by using hybrid up flow anaerobic sludge blanket reactor.” Centre for Environment, Institute of Science and Technology, Jawaharlal Nehru Technological University, Kukatpally, Hyderabad 500 085, Andhra Pradesh, India. August 2008. PDF
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PDF
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