Chapter_6C_Attached_Growth_processes.pptx

## Anaerobic Biology: A Comprehensive Overview

### Introduction to Anaerobic Biology

Anaerobic biology refers to the study of biological processes that occur in the absence of oxygen. This field encompasses the metabolism of microbes and other organisms that have adapted to thrive in environments devoid of oxygen. The importance of anaerobic biology extends beyond ecological niches; it plays significant roles in global biogeochemical cycles, waste management, bioenergy production, and even food processing.

### The Importance of Oxygen and Anaerobic Environments

Oxygen is a vital element for many organisms, particularly aerobic organisms, which rely on oxygen for energy production. However, numerous environments exist on Earth where oxygen is scarce or completely absent. These anaerobic environments can be found in:

- Waterlogged soils
- Deep ocean sediments
- Anaerobic digesters
- Landfills
- The guts of animals

In these environments, anaerobic microorganisms have evolved specialized metabolic pathways that allow them to utilize alternative electron acceptors such as nitrate, sulfate, carbon dioxide, or organic compounds.

### Types of Anaerobic Organisms

Anaerobic organisms can be classified into several categories based on their energy-generating pathways:

#### 1. **Obligate Anaerobes**
These organisms cannot survive in the presence of oxygen. They rely exclusively on anaerobic respiration or fermentation to generate energy. Examples include:

- **Clostridium spp.**: A genus of bacteria known for producing spores and causing diseases like tetanus and botulism.
- **Bacteroides spp.**: Commonly found in the human gut, involved in the digestion of complex carbohydrates.

#### 2. **Facultative Anaerobes**
These organisms can switch between aerobic and anaerobic respiration depending on the availability of oxygen. Examples include:

- **Escherichia coli**: A common gut bacterium that can ferment sugars in the absence of oxygen or respire aerobically when oxygen is available.
- **Saccharomyces cerevisiae**: Yeast that ferments sugars for ethanol production but can also carry out aerobic respiration.

#### 3. **Microaerophiles**
Microaerophiles require oxygen for their metabolism but at lower concentrations than what is available in the atmosphere. Examples include:

- **Helicobacter pylori**: A bacterium associated with gastric ulcers that thrives in low-oxygen environments.

#### 4. **AEROTOLERANT Anaerobes**
These organisms do not require oxygen for growth but can tolerate its presence. They mainly ferment substances to produce energy. Streptococcus spp. are good examples.

### Anaerobic Metabolism Pathways

Anaerobic metabolism differs significantly from aerobic metabolism primarily in the final electron acceptor used in respiration. The following pathways are common:

#### 1. **Fermentation**

Fermentation is an ATP-generating process that does not involve electron transport chains or external electron acceptors. During fermentation, orga

1. Chapter 6C attached growth treatment processes

2. Part 6C1

3. topics 3.1 Background of attached growth processes Historical evolution. Process description Mass transfer limitations. 3.2 Trickling filters Trickling filter classification and applications Design of physical facilities Process design considerations Nitrification design. 3.3 Rotating biological Contactors Process design considerations Physical facilities for RBC process RBC process design. 3.4 Combined aerobic treatment processes Trickling filter/activated sludge processes. Activated sludge with fixed film packing. Submerged attached growth processes. Attached growth denitrification processes.

4. Background of attached growth processes Generally grouped in to Non-submerged attached growth processes Suspended growth processes with fixed film packing Submerged attached growth aerobic processes

5. Advantages of attached growth processes over activated sludge Less energy required Simpler operation with no issues of mixed liquor inventory control and sludge wasting. No problem of bulking sludge in secondary clarifiers Better sludge thickening properties Less equipment maintenance needs Better recovery from shock toxic loads .

6. Disadvantages of attached growth processes Poorer effluent quality in terms of BOD and TSS concentrations. Greater sensitivity to lower temperature . Odor production. Uncontrolled solids sloughing Difficult to accomplish nitrogen and phosphorous removal with a single stage process like activated sludge. Effluent with higher turbidity. Combined processes try to take advantages of both processes in terms of energy savings and effluent quality.

7. Trickling filters The depth of the rock varies between 0.9 to 2.5 m averaging 1.8m. Rock filter beds are usually circular. And the liquid wastewater is distributed over the top of the bed by a rotary distributor . Modern trickling filters are constructed with plastic packing . Depth of plastic packing range from 4 to 12m . Under drains and support to under drains are important parts of the trickling filter.

8. Air can circulate through the under drains that collects the treated water. Solids are separated from the liquid in the sedimentation tank. Recirculation of the settled effluent is recycled to the trickling filter to reduce the strength of the incoming waste and to maintain enough wetting to keep the biological slime layer moist. The distributor arm is rotated by the force of the water or be electric drives. Primary clarification is necessary before rock filter with fine screens used prior to plastic filters. Screening is essential to reduce floating objects and fouling of the packing.

9. The biological community in the filter includes aerobic and facultative bacteria, fungi. Algae and protozoan. Higher animals such as worms, insect larvae and snails are also present. Facultative bacteria are predominating in trickling filters. Within the slime layer of adverse conditions, filamentous forms are found. Nitrification occurs in the lower depth of the filter. Fungi have greater role under low pH and certain industrial wastes. Fungi growth can be so rapid that the filter clogs and ventilation becomes restricted.

10. Algae do not stabilize waste but may add oxygen. They can also cause problem of filter clogging. Protozoa feed on the bio-film and maintain it at higher growth state. Snails can consume nitrification bacteria to the extent of reducing the rate of nitrification. The film layer thickness can be as much as 10mm. The film layer is aerobic on the outer layer and anaerobic to indogenous inside. Sloughing occurs when the endogenous respiration results in loss of ability to stick to the packing surface. When a rock filter sloughs, the effluent before settling will contain higher amounts of BOD and TSS than the applied wastewater. The mechanism of packing is hydraulic shear in high rate plastic filters while large scale spring-time sloughing occurs due to the activity of insect larvae.

11. Trickling filter classification Low-rate filters: are relatively simple, highly dependable device that produce an effluent of consistent quality. Only the top 0.6 – 1.2m depth has appreciable biological slime. The lower portion can be inhabited by nitrifying bacteria. Where topography permits gravity flow is a clear advantage. Odors can be a problem especially if the wastewater is stale/septic and if the weather is warm. Filters should not be created where odor will create a nuisance.

12. Intermediate and high rate filters They use either rock or plastic packing. The filters are usually circular and flow is continuous. Recirculation permits higher organic loadings, provides higher dosing rates to improve the liquid distribution and better control of slime thickness.\ Recirculation provides more oxygen in the influent wastewater flow and returns viable organisims . . Recirculation also prevents ponding and reduces the nuisance from flies and odor.

13. Roughing filters Roughing filters: Are a high rate type that treat higher strength wastes and prior to secondary treatment processes. They are mostly designed as plastic packing and achieve overall energy efficiency. Two-stage filters: Are employed for high strength wastewaters as well as to achieveve nitrification. Nitrification; Significant nitrification occurs only after the BOD concentration is appreciably reduced. The effluent BOD has to be less than 30 mg/L to initiate nitrification and less than 15 mg/L for complete nitrification.

14. Design of physical facilities Factors to be considered in the design of trickling filters include: Type and physical characteristic of the filter packing to be used. Dosing rate. Type and dosing characteristics of the distribution system. Configuration of the under drain system. Provision for adequate air flow. Settling tank design.

15. Filter packing Where available rock has the advantage of low cost. The ideal filter would be one that has higher surface area per unit volume, low cost, high durability, high porosity. Rock sizes are between 7.5 and 10 cm. Sodium sulfate test can be used to test the strength of the rock. The depth of the rock filter is usually on the order of 2m. Low void volume of rock limits the space available for air flow and increases the potential for plugging and flow short circuiting.

16. Molded plastic packings have the appearance of a honeycomb. The sheets usually have a corrugated surface for enhancing slime growth and retention time. Bio towers as deep as 12m have been constructed. The high hydraulic capacity, high void ratio And resistance to plugging can be used in a high rate filter. Plastic packing has the advantage of requiring less land area since they use higher loading rate. Plastic filters have a distinctly higher advantage over rock filter at a higher rates only. At low and moderate rates they have similar performance.

17. Dosing rate For higher rotational speeds, the dosing rate is lower. Past studies indicated that reducing the rotational speed improves filter performance. Lower rotational speed distribution besides improved performance, has the advantage of reducing the fly population, bio film thickness and odours . At higher dosing rate (==low rotational speed), the larger water volume applied per revolution: Provides greater wetting efficiency. Results in greater agitation which causes more solids to flush out of the packing Results in a thinner bio-film Helps to wash away fly eggs. The thinner bio-film creates more surface area and results in more aerobic bio-film.

18. A daily intermittent high dose (known as flushing dose) is used to control the bio-film thickness and solids inventory. A once per day high flushing dose and a lower sustained rate is recommended as a function of the BOD loading. The rotational speed is determined from the following formula: Where n = rotational speed , rev/min. Q = influent applied hydraulic loading rate, m 3 /m 2 .hr R = recycle ratio A = number of arms in rotary distributor assembly DR = Dosing rate, mm/pass of distributor arm

19. Part 6C2

20. Distribution system The flow-driven rotary distributor has traditionally been used because it was reliable and easy to maintain. A clearance of 15 cm – 22.5 cm should be allowed between the bottom of the distributor and the top of the bed. Distributors are manufactured with diameters up to 60m. Head loss through the distributor is in the range of 0.6 to 1.5m. A distributor should have qualities of ruggedness, ease of cleaning, maintain adequate rotational speed at large flows and corrosion resistance.

21. underdrains Under drains are usually of precast blocks of vitrified clay or fiber glass grating laid on a reinforced concrete sub block. The floor and under drains must have sufficient strength to support the packing, slime growth and the waste water. 1 to 5 % grade is provided to the floor and under drain block to the center or periphery. . The effluent channels are designed to produce a minimum velocity of 0.6 m/sec. At the average flow rate. Under drains may be open at both ends so that they may be inspected easily and flushed out if they become plugged. Under drain system for plastic packing consists of either a beam and a column or grating. .

22. Typical under drain for tower filter

23. Air flow For efficient treatment and to prevent odours adequate air flow is essential. In addition to natural draft, forced ventilation using low pressure fans provides more reliable and controlled air flow. In the case of natural draft, the driving force for air flow is the temperature difference between the ambient air and the air inside the pores. If the wastewater is colder than the ambient air, the pore air will be cold and the direction of flow will be downward. If the ambient air is colder than the wastewater, the flow will be upward. The latter is desirable from a mass transfer point of view: The partial pressure of oxygen is lowest in the region of highest oxygen demand. In many areas there are periods especially during the summer when essentially no air flow occurs through the trickling filters because temperature differentials are negligible.

24. Estimation of draft (Pressure from temperature differences) Where D air = Natural air draft, mm of water T c = cold temperature , k T h = Hot temperature , K Z = height of the filter, m

25. Forced air ventilation Provides a reliable supply of Oxygen. A down flow direction has an advantage of providing contact time for treating odorous compounds released at the top of the filter. Downward flow direction also supplies rich oxygen where the oxygen demand is highest (at the top). For application with extremely low air temperature, it may be necessary to restrict the flow of air through the filter to keep it from freezing.

26. Settling tanks The function of a settling tank is to provide a clarified effluent. Unlike activated sludge, the suspended solids in the clarifier is low and sludge recirculation is not necessary. Sludge from trickling filter settling ranks is sent to sludge processing facilities or returned to the primary clarifier to be settled with primary solids. Traditionally shallow clarifiers with high overflow rate resulted in poor filter performance.. Clarifier design should be similar to activated sludge with appropriate feed well size and depth, increased side water depth and similar hydraulic overflow rate.

27. Process design considerations Design of trickling filter is based on empirical formula due to the process complexity. Inadequate ventilation, poor clarifier design, inadequate protection from cold temperature and the dosing operation have been the source of poor performance of trickling filters. For trickling filters, quantifying the biomass is difficult including calculation of F/M ratio. The biofilm solids concentration may range from 40 to 100 g/L and the liquid does not uniformly flow over the entire packing surface area. Broader parameters such as volumetric organic loading and hydraulic application rates have been used as design and operating parameters to relate to treatment efficiency.

28. Bod removal design equations For single stage or first stage rock filter: Where E 1 = BOD removal efficiency for first stage filte at 20 C, including recirculation, percent. W 1 = BOD loading to filter , kg/day. V = Volume of filter packing , m 3 F = Recirculation factor. R = Recycle ratio = Q r / Q influent

29. BOD removal of second stage filter: Where E 2 = BOD removal efficiency for second stage filter at 20 C , percent. W 2 = BOD loading applied to the second stage filter , kg/day. E 1 = Fraction of BOD removal in the second stage filter kg/day. Temperature effect:

30. Solids production and mass transfer limitations Solids production will depend on the wastewater characteristics and the trickling filter loading. A lower organic load results in greater amount of BOD degraded, the biomass has longer SRT and less biomass is produced. At higher organic load treatment efficiency is reduced, odours may be produced and anaerobic activity may prevail. Oxygen transfer may become limiting at BOD loading rates of 400 to 500 mg/L (The process become Oxygen transfer controlled).

31. Effect of influent BOD/TKN on nitrification

32. Rotating biological contactors Consists of a serious of closely spaced circular disks of polystyrene or polyvinyl chloride that are submerged in wastewater and rotated through it. The cylindrical plastic disks are attached to a horizontal shaft and are provided at standard unit sizes of approximately 3.5m in diameter and 7.5m in length. The RBC unit is partially submerged in a tank containing the wastewater and the disks rotate slowly at about 1 to 1.6 revolutions per minute. Wastewater flows down through the disks and solids sloughing occurs. RBC systems require pretreatment or primary clarification of the fine screens and secondary clarification for liquids solids separation.

33. Rbc process design considerations RBC units must be staged in series with six or more stages for BOD removal. Stages can be accomplished by baffles or by using separate tanks in series. Disks may be oriented parallel or perpendicular to the direction of flow. Low enough BOD should be used in design to discourage the formation of sulfur (H 2 S) oxidizing bacteria that form hard to slough biomass apart from odour problem. RBC units can be designed to provide secondary or advanced level effluent comparable to other treatment processes. Disks can be made of high density polyethylene.

34. Combined aerobic treatment processes Trickling filter may be designed as a roughing filter to activated sludge. The trickling filter without a clarifier precedes the aeration tank. Such arrangement may be suitable for treating higher strength industrial wastewater. A bio-filter in which the return activated sludge is returned to the bio filter and the aeration tank is not supplied with oxygen is possible. This combined process is known as activated bio-filter (ABF). A short aeration basin may follow the filter (plastic filter is used) to improve the effluent quality.

35. Part 6C3

36. Series trickling filter-activated sludge process OBJECTIVES: To upgrade an existing activated sludge system To reduce the strength of waste water where industry and domestic wastewater is treated together. To protect a nitrification activated sludge from toxic and inhibitory substances. Both soluble and particulate BOD are removed in the bio filter.

37. Combined activated sludge -trickling filter with intermediate clarifier

38. Activated sludge with fixed film packing A fixed/suspended film provides greater biomass in the aeration tank. For an existing system the loading can be increased by providing a fixed/suspended film without increasing the load on secondary clarifier. Nitrification can occur at lower apparent SRT values. Submerged rotating biological contactors are included in the activated sludge where by the submergence is 85% and rotation can be driven by aeration or mechanically.

39. Suspended film packing

40. Submerged attached growth processes May be up flow or down flow fixed reactors or fluidized bed reactors. Oxygen is supplied by diffused aeration in to the packing or by being predisposed in to the influent wastewater. No clarification is used with aerobic submerged attached growth processes and excess solids from biomass growth and influent suspended solids are trapped in the system and must be periodically removed such as by backwashing. There are no sludge settling issues, space requirement is small and filtration produces high effluent quality.

41. Downflow submerged attached grwoth process

42. Upflow biological reactor

43. Nitrification- denitrification reactor

44. Fluidized bed reactors Waste water is fed upward to a bed of 0.5 to 0.l5mm sand or activated carbon. Bed depths are in the range of 3 to 4m. Upflow velocities are 30 to 36 m/hr. Effluent recirculation is necessary to provide the fluid velocity within the necessary detention time. For aerated system, recirculated effluent is passed through an oxygen tank to pre-dissolve oxygen. For detoxification, activated carbon is used to provide both adsorption ang biological degradation.

45. Schematics of fluidised bed reactor

46. Advantages of fluidized bed bioreactors Provides an extraordinary long SRT for microorganisms necessary to degrade the xenobiotic and toxic compounds. Shock loads or non biodegradable toxic compounds can be adsorbed on to the activated carbon. High quality effluent is produced low in TSS and COD concentration. The oxygenation method prevents stripping and emission of toxic organic compounds in to the atmosphere. The system operation is simple and reliable.

47. Attached growth denitrification process An exogenous carbon source is provided as an electron donor and oxidized using nitrate or nitrite as an electron acceptor. Both attached growth and suspended growth have been used for post anoxic denitrification . The design can be upward flow, downward flow or fluidised bed reactor. Head loss increases can be temporarily relieved by occasional surges. Backwashing is needed once solids are accumulated. To separate the sand from the biomass, the packing is removed and passed through a high shear pump.

48. Attached growth preanoxic denitrification processes Nitrate is provided in the recycle stream while organic material is provided from within the wastewater. The cost of methanol addition is eliminated since influent organic matter is used. The disadvantage is the effect of the nitrate recycle flow on the design and operating cost. In an alternative pre-anoxic denitrification system, the nitrified trickling filter effluent is recycled to provide nitrate for a suspended growth pre-anoxic reactor. An intermediate clarifier is used to separate the denitrifying mixed liquor and to provide return activated sludge to the anoxic tank.

49. Pre-anoxic denitrification forms

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