Biotreatment of industrial effluents CHAPTER 3 – aerobic and anaerobic bioreactors

Biotreatment of industrial effluents CHAPTER 3 – aerobic and anaerobic bioreactors Biotreatment of industrial effluents CHAPTER 3 – aerobic and anaerobic bioreactors Biotreatment of industrial effluents CHAPTER 3 – aerobic and anaerobic bioreactors Biotreatment of industrial effluents CHAPTER 3 – aerobic and anaerobic bioreactors Biotreatment of industrial effluents CHAPTER 3 – aerobic and anaerobic bioreactors Biotreatment of industrial effluents CHAPTER 3 – aerobic and anaerobic bioreactors Biotreatment of industrial effluents CHAPTER 3 – aerobic and anaerobic bioreactors

Aerobic and Anaerobic

Bioreactors

Introduction

The aerobic biodegradation process can be represented by:

CxHy q- 02 q-(microorganisms/nutrients) ~ H20 + CO2 q- biomass The anaerobic bioprocess can be represented by:

CxHy q-(microorganisms/nutrients) ~ CO2 + CH4 q- biomass

Aerobic Degradation

Bacteria that thrive in oxygen-rich environments break down and digest waste The mixed aerobic microbial consortium uses the organic carbon present in the effluent as its carbon and energy source The complex organics finally get converted to microbial biomass (sludge)and carbon dioxide (CO2) Food here is limiting, which results in the microorganisms consuming their own protoplasm to obtain energy for cell maintenance reactions (endogenous respiration) Therefore, the biomass concentration continuously decreases until the energy content reaches a m i n i m u m so as to be considered bio- logically stable and suitable for disposal in the environment Sludges with

a specific oxygen uptake rate of less than or equal to 1 mg O2/h/g can be considered stabilized

D i g e s t i o n P a t h w a y

During this oxidation process, contaminants and pollutants are broken down into CO2, water, nitrates, sulfates, and biomass (microorganisms) In the conventional aerobic system, the substrate is used as a source of carbon and energy (Fig 3-1 ) It serves as an electron donor, resulting in bacterial growth The extent of degradation is correlated with the rate of oxygen consumption,

as well as the previous acclimation of the organism in the same substrate

19

20 Biotreatment of Industrial Effluents

I es 'ra"on I

IH End products|

2 ~ 002 I

04, S042- I

NH3 )

Oxygen +

@

I o,"es's 1

i I

I

(~ More microorganisms

FIGURE 3-1 Aerobic degradation pathway

Two enzymes primarily involved in the process are di- and mono-oxygenases The latter enzyme can act on both aromatic and aliphatic compounds, while the former can act only on aromatic compounds Another class of enzymes involved in aerobic degradation is the peroxidases, which have been receiving attention recently for their ability to degrade lignin

Anaerobic Degradation

In the anaerobic process, the complex organics are first broken down into a mixture of volatile fatty acids (VFAs), mostly acetic, propionic, and butyric acids This is achieved by "acidogens," a consortium of hydrolytic and aci- dogenic bacteria (Gottschalk, 1979) The VFAs are in turn converted to CO2 and methane by acetogenic (acetogens) and methanogenic (methanogens) bacteria, respectively (Zehnder et al., 1982)

H 2, CO 2

IM ethanogenic III A bacteria 1

utilizing CO 2 and H 2 ,)

l

CH 4 + H20

Complex organic molecules

l

I ' 1 Hydrolytic bacteria acidogens

Organic acids, neutral compounds

I Heteroacetogenic ,i 1

bacteria

Acetate

IM III B ethanogenic bacteria I utilizing CH3COOH )

l

CH 4 + 002

FIGURE 3-2 Anaerobic degradation pathway

A n a e r o b i c D i g e s t i o n P a t h w a y

Anaerobic digestion is a biological process in which organic matter is con- verted by several independent, consecutive, and parallel reactions In the absence of oxygen, close-knit communities of bacteria cooperate to form

a stable, self-regulating fermentation that transforms organic matter into

a mixture of methane and CO2 (Fig 3-2) The amount of methane gas produced varies with the amount of organic waste fed to the digester and the operating temperature Anaerobic digestion occurs in six main stages (Jeyaseelan, 1997):

9 Hydrolysis of complex organic biopolymers (proteins, carbohydrates, and lipids) into monomers (amino acids, sugars, long chain fatty acids) by hydrolytic bacteria (group I)(acidogens)

9 Fermentation of amino acids and sugars by hydrolytic bacteria (group I)

22 Biotreatment of Industrial Effluents

9 Anaerobic oxidation of volatile fatty acids and alcohols by heteroaceto- genic bacteria (group II)

9 Anaerobic oxidation of intermediary products such as volatile fatty acids

by heteroacetogenic bacteria (group II)

9 Conversion of hydrogen to methane by methanogenic bacteria utilizing hydrogen (group IIIA)

9 Conversion of acetate to methane by methanogenic bacteria utilizing acetate (group IIIB)

The hydrolysis of undissolved carbohydrates and proteins follows separate paths The heteroacetogenic bacteria grow in close association with the methanogenic bacteria during the final stages of the process The reason for this is that the conversion of the fermentation products by the heteroace- togens is thermodynamically possible only if the hydrogen concentration is kept sufficiently low This requires a close symbiotic relationship among the classes of bacteria

Comparison between Aerobic and Anaerobic

Degradation Pathways

While both aerobic as well as anaerobic degradation routes can equally remove complex organics from the effluents, the anaerobic route has an obvious advantage because it produces methane, a combustible biogas with

a reasonably good calorific value of 24 MJ/m 3 Aerobic treatment produces 2.4 kg CO2/kg COD, while an anaerobic process produces only 1 kg CO2/kg COD Sludge disposal is an important consideration since it represents about 60% of the total treatment cost The cost to dispose of the sludge produced

by an anaerobic plant is only 10% that of a corresponding aerobic plant Nutrient requirements are 20% lower for anaerobic plants than for aerobic plants, and the aeration process also involves possible volatilization of some

of the organic contaminants, turning water pollution into air pollution A comparison of electron acceptors, type of reaction, and metabolic byprod- ucts of aerobic and anaerobic processes is shown in Table 3-1 Oxygen is the only electron acceptor in the aerobic process that produces water and carbon dioxide as the products of reaction In the anaerobic process, several electron acceptors are possible, giving rise to several different metabolic byproducts; hence this is preferred for aromatic compounds

Several major differences exist between the aerobic and anaerobic degra- dation pathways of aromatic compounds They are listed in Table 3-2 (Jothimani et al., 2003)

Aerobic Reactors

The effectiveness of the design and operation of a biological treatment sys- tem depends on several parameters They include: amount of nutrients

TABLE 3-1

Electron Acceptors and Byproducts in Aerobic and Anaerobic Processes

Electron acceptor Type of reaction Metabolic byproduct

Oxygen

Nitrate (NO3)

Manganese (Mn 4+)

Ferric iron (Fe 3+)

Sulphate (SO 2- )

Carbon dioxide

Aerobic Anaerobic respiration Anaerobic

Anaerobic Anaerobic respiration Anaerobic respiration

Carbon dioxide, water Nitrogen gas, carbon dioxide Manganese (Mn 2+)

Ferrous iron (Fe 2+) Hydrogen sulfide Methane

TABLE 3-2

Comparison of Aerobic and Anaerobic Degradation of Aromatic Compounds

Channeling

Central intermediates

Ring attack

Central intermediates

Cleavage of the ring

+H20 , 2H,-2H, +CO2, q-CO 4

Benzoyl CoA, resorcino phloroglucinol

2 or 4 H + H20 Easy to reduce or hydrate Hydrolysis of 3-oxo compound

02 Catechol, proto catechuate gentisate

02 Easy to oxidize Oxygenolysis

available for the organism to grow, dissolved oxygen concentration (for aerobic treatment), food-to-microorganism ratio (this ratio applies to only activated sludge systems and is a measure of the a m o u n t of biomass avail- able to metabolize the influent organic loading to the aeration unit), pH, temperature, cell residence time, hydraulic loading rate (the length of time the organic constituents are in contact with the microorganisms), settling time (time for separating sludge from liquid), and degree of mixing

The design parameters for aerobic reactors are tabulated in Table 3-3 The gas-to-liquid mass transfer and bubble size governs the rate of the aerobic biodegradation process Hence, the reactor designs strive to achieve high gas transport rates and generate small air bubbles (see Chapter 4 for design) In the breeding of aerobic organisms, adequate amounts of dissolved oxygen m u s t

be ensured in the medium Since the solubility of oxygen in the m e d i u m is very low, it m u s t be continuously supplied and gas-to-liquid mass transfer should be maintained high A m i n i m a l critical concentration of dissolved oxygen m u s t be maintained in the substrate to keep the microorganisms active, and the values are in the range of 0.003 to 0.05 mmol/L, which is 0.1

to 10% of the oxygen solubility values in water But the presence of salts such

24 Biotreatment of Industrial Effluents

TABLE 3-3

Design Parameters for Aerobic Digestion

1 Retention time

Activated sludge only

Activated sludge with primary treatment

2 Solids loading

3 Air required

Activated sludge only

Activated sludge with primary treatment

4 Power required

15-20 days 20-25 days 1.6-3.2 kg VSS/m3.day

20-35 L/min.m 3 55-65 L/min.m 3 0.02-0.03 kW/m 3 VSS, volatile suspended solids

as NaC1 decreases oxygen solubility by a factor of 2 Microbes use oxygen for cell maintenance, respiratory oxidation for further growth (biosynthesis), and oxidation of substrates into related metabolic end products The oxygen uptake rate for bacteria and yeast are the highest (0.2 to 2 x 10 -3 kg/m3/s), followed by fungi (0.1 to 1) and the rest of the biocultures (0.01 to 0.001) The simplest and the cheapest design uses open lagoons and oxidation ponds, which are nothing but open pits where the effluent is stored and oxygen is either bubbled through the liquid medium using blowers or the liquid is churned using slowly rotating disks (Fig 3-3) The sludge that is generated settles to the bottom of the tank and is drained from time to time Oxygen transfer rates are low, and the residence time is generally on the order of 2 to 7 days Each disk is covered with a biological film that degrades dissolved organic constituents present in the wastewater As the disk slowly rotates, it carries a film of the wastewater into the air, where oxygen is available for aerobic biological decomposition The excess biomass produced disengages from the disk and falls into the trough Several contactors are often operated in series

Another common aerobic reactor design is the mechanically stirred tank reactor In this design, air is introduced from the bottom through a sparging arrangement Airlift reactors are reactors without any mechanical stirring arrangements for mixing The turbulence caused by the gas flow ensures adequate mixing of the liquid The inner draft tube is provided in the central section of the reactor (Fig 3-4) The introduction of the fluid (air or liquid) causes upward motion and results in circulatory flow in the entire reactor Because of the low liquid velocities, energy consumption is also low These reactors can be used for both free and immobilized cells The oxygen mass transfer coefficient in this reactor is high in comparison

to stirred tank reactors Deep shaft reactors are 50 to 150 m long and are made of concrete (Fig 3-5) They are buried underground and are used for

Submerged & suspended growth

Liquid overflow

Sludge drain

FIGURE 3-3 Rotating disk contactor

Gas out

t

"~o2 0 0

0

0 0

0

L o o L

0

I I o o

0 0

0

0

0 0

0 O I

0

0 O I

0

I Effluent in Air in

Liquid out

FIGURE 3-4 Airlift reactor for aerobic operation

26 Biotreatment of Industrial Effluents

Effluent liquid in

Air in

il Gas out

o o

o r ~

o

o

o

N/

Sludge out

Central shaft

J

FIGURE 3-5 Deep shaft reactor for aerobic operation

sewage treatment The air in this reactor is not introduced at the b o t t o m but

in the middle These reactors are also suitable for shear sensitive, foaming, and flocculating organisms

Other widely used aerobic reactors in wastewater treatment are packed bed or fixed bed bioreactors with attached biofilms (Fig 3-6) These reac- tors are widely used with immobilized cells Wastewater is pumped into the top of the reactor and made to flow downward through the packed bed

or sometimes vice versa Air is pumped from the bottom Microorganisms

in the aerated packed bed grow and degrade organic matter contained in the wastewater The disadvantages of packed beds are the change in the bed porosity and bed compaction with time, resulting in high pressure drop across the bed, and channeling due to turbulence in the bed Several modi- fications such as tapered beds to reduce the pressure drop across the length

of the reactor, inclined beds, horizontal beds, and rotary horizontal reactors have been tried with limited success

Bubble columns are slender, tall columns with a gas distributor at the

b o t t o m (Fig 3-7) The construction of bubble columns is very simple, and higher mass transfer coefficient can be achieved than with loop reactors These reactors can be as large as 5,000 m 3 Since they have broad residence time distribution and good dispersion properties, they can be used for aero- bic wastewater treatment The liquid column provides high pressure at the

Support/packing for growth of microorganisms

i i ~ i : ~ i

~ ~ : ~ :~ I ~ - ~ ~ i ~ i ~ I !!~li

!il lii i;lli ,

id

Air in

out

out

Effluent

in

FIGURE 3-6 Packed bed reactor for aerobic operation

reactor bottom, giving rise to increased oxygen solubility Hence, gas holdup

in such reactors is generally very high

An inverse fluidized bed is used in aerobic wastewater t r e a t m e n t (Fig 3-8), where the solid phase is an inert particle coated with a biofilm, the gas phase is oxygen/air, and the liquid phase is the wastewater that needs treating The bed of solids has a density lower than that of the liquid phase, but a fluidized state is created by the downward flow of the liquid The gas flows countercurrent to the liquid This mode of operation improves the mass transfer rate, reduces the attrition rate of solids, and helps the bed to refluidize easily after shutdown Low-concentration synthetic and munic- ipal wastewaters are treated at residence times ranging from 0.6 to 3 h in

an anaerobic inverse fluidized bed Sufficient care should be taken during start-up, when the biofilm is forming on the inert support

The slurry-phase bioreactor is a stirred tank in which soil is suspended

in water (greater than 40% solids) and mixed with air, microbial cells, and nutrients In the solid-phase bioreactor (also known as biopile), water is just sprinkled over the soil to adjust the soil moisture content; otherwise it is similar to a slurry-phase reactor Air and nutrients are fed through perforated pipes These reactors are a very cost-effective ex situ t r e a t m e n t if the biore- mediation time is not limiting Apart from slow degradation time, another

28 Biotreatment of Industrial Effluents

/ ~ ~ T ~ as out

Effluent in

0

0

o

0

o

Q

Air in

-~ Liquid out

FIGURE 3-7 Aerobic bubble column reactor

Air out Effluent waste i ~

9 / J

9 / / / 0

Air in

Particles (inert support with biofilm growth on top)

Liquid out

FIGURE 3-8 Inverse fluidized bioreactor (for aerobic operation}